diff --git a/frontend/src/api/inception.ts b/frontend/src/api/inception.ts
index ece42631..f520cc21 100644
--- a/frontend/src/api/inception.ts
+++ b/frontend/src/api/inception.ts
@@ -1,9 +1,14 @@
 /** Project inception (milestone 297): what a project was decided to inherit. */
 import { apiGet, apiPost } from "@/api/client";
+import type { ProjectPlatform } from "@/api/platforms";
 
 export interface InceptionChoices {
   design_system_id: number | null;
   seed_systems: boolean;
+  /** Platform slugs (milestone 463). null = not answered; a list is the
+   *  whole answer — members left out of it are recorded as "not this".
+   *  Absent on records decided before the question existed. */
+  platforms?: string[] | null;
 }
 
 export interface InceptionRecord {
@@ -17,16 +22,24 @@ export interface InceptionDefaults {
   design_system_id: number | null;
   design_systems: { id: number; title: string }[];
   systems: number;
+  platforms: { slug: string; name: string }[];
+  /** What detection (or an earlier answer) already says about the project. */
+  project_platforms: ProjectPlatform[];
 }
 
 export interface InceptionDecision {
   project_id: number;
   inception: InceptionRecord;
-  effects: { design_system_id: number | null; systems_seeded: string[] };
+  effects: {
+    design_system_id: number | null;
+    systems_seeded: string[];
+    /** The project's answers after the decision; null when left unstated. */
+    platforms: ProjectPlatform[] | null;
+  };
 }
 
 export const emptyChoices = (): InceptionChoices => ({
-  design_system_id: null, seed_systems: false,
+  design_system_id: null, seed_systems: false, platforms: null,
 });
 
 export const fetchInceptionDefaults = (projectId: number) =>
diff --git a/frontend/src/api/platforms.ts b/frontend/src/api/platforms.ts
new file mode 100644
index 00000000..8e27cd58
--- /dev/null
+++ b/frontend/src/api/platforms.ts
@@ -0,0 +1,77 @@
+/**
+ * Platforms — what a project is built on or ships as (milestone 463).
+ *
+ * The catalog is GLOBAL, like the canonical areas, and admin-written. A
+ * project's answer per platform is `declared` (a person said so), `detected`
+ * (a marker file in a bound repo said so) or `rejected` (a person said no —
+ * kept, so the next refresh cannot detect it back). Only declared and
+ * detected make a project a member, and membership is what decides which
+ * family ideas reach it.
+ */
+import { apiGet, apiPatch, apiPost, apiPut } from "@/api/client";
+
+export interface Platform {
+  id: number;
+  name: string;
+  /** The match key, and the name every door takes (stable across restores). */
+  slug: string;
+  description: string | null;
+  /** Repo-relative globs. A bare name matches a basename anywhere; one with
+   *  a slash matches the whole path. Empty = declare-only. */
+  markers: string[];
+  order_index: number;
+  created_at: string | null;
+  updated_at: string | null;
+}
+
+export type PlatformState = "declared" | "detected" | "rejected";
+/** What a person may set. `null` withdraws the answer. */
+export type SettablePlatformState = "declared" | "rejected" | null;
+
+export interface ProjectPlatform {
+  id: number;
+  slug: string;
+  name: string;
+  state: PlatformState;
+}
+
+export const MEMBER_STATES: PlatformState[] = ["declared", "detected"];
+
+export async function listPlatforms(): Promise<Platform[]> {
+  const data = await apiGet<{ platforms: Platform[] }>("/api/platforms");
+  return data.platforms;
+}
+
+/** Admin only. A name that reduces to an existing slug answers 409. */
+export async function createPlatform(data: {
+  name: string;
+  description?: string;
+  markers?: string[];
+}): Promise<Platform> {
+  return apiPost("/api/platforms", data);
+}
+
+export async function updatePlatform(
+  id: number,
+  data: Partial<{ name: string; description: string; order_index: number; markers: string[] }>,
+): Promise<Platform> {
+  return apiPatch(`/api/platforms/${id}`, data);
+}
+
+export async function fetchProjectPlatforms(projectId: number): Promise<ProjectPlatform[]> {
+  const data = await apiGet<{ project_platforms: ProjectPlatform[] }>(
+    `/api/projects/${projectId}/platforms`,
+  );
+  return data.project_platforms;
+}
+
+/** Only the slugs named change; the update applies whole or not at all. */
+export async function setProjectPlatforms(
+  projectId: number,
+  platforms: Record<string, SettablePlatformState>,
+): Promise<ProjectPlatform[]> {
+  const data = await apiPut<{ project_platforms: ProjectPlatform[] }>(
+    `/api/projects/${projectId}/platforms`, { platforms },
+  );
+  return data.project_platforms;
+}
diff --git a/frontend/src/components/InceptionCard.vue b/frontend/src/components/InceptionCard.vue
index 11929960..0d7b8ffa 100644
--- a/frontend/src/components/InceptionCard.vue
+++ b/frontend/src/components/InceptionCard.vue
@@ -6,10 +6,16 @@
  * second step and only emits the choices (the project does not exist yet);
  * mode="decide" sits on ProjectView for an undecided project, loads that
  * project's current defaults, and records the decision itself.
+ *
+ * Platforms (milestone 463) start UNANSWERED (null). Ticking any box makes
+ * the list the whole answer; in decide mode the boxes start from what
+ * detection already found, so recording confirms it.
  */
 import { computed, onMounted, ref, watch } from "vue";
 import { apiErrorMessage } from "@/api/client";
 import { fetchDesignSystems } from "@/api/designSystems";
+import { MEMBER_STATES } from "@/api/platforms";
+import { usePlatformsStore } from "@/stores/platforms";
 import {
   decideInception, emptyChoices, fetchInceptionDefaults,
   type InceptionChoices, type InceptionDecision, type InceptionDefaults,
@@ -29,6 +35,9 @@ const emit = defineEmits<{
 const local = ref<InceptionChoices>(props.choices ? { ...props.choices } : emptyChoices());
 const designSystems = ref<{ id: number; title: string }[]>([]);
 const systemsCount = ref(0);
+const platforms = ref<{ slug: string; name: string }[]>([]);
+const detected = ref<Set<string>>(new Set());
+const platformsStore = usePlatformsStore();
 const loading = ref(true);
 const saving = ref(false);
 const error = ref("");
@@ -46,11 +55,25 @@ async function load() {
       const d: InceptionDefaults = await fetchInceptionDefaults(props.projectId);
       designSystems.value = d.design_systems;
       systemsCount.value = d.systems;
+      platforms.value = d.platforms;
+      const members = d.project_platforms
+        .filter((p) => MEMBER_STATES.includes(p.state))
+        .map((p) => p.slug);
+      detected.value = new Set(
+        d.project_platforms.filter((p) => p.state === "detected").map((p) => p.slug),
+      );
       // Start from what stands today, so "record" without changes keeps it.
-      local.value = { design_system_id: d.design_system_id, seed_systems: false };
+      local.value = {
+        design_system_id: d.design_system_id,
+        seed_systems: false,
+        platforms: members.length ? members : null,
+      };
     } else {
-      const ds = await fetchDesignSystems();
+      const [ds, catalog] = await Promise.all([
+        fetchDesignSystems(), platformsStore.fetchCatalog(),
+      ]);
       designSystems.value = ds.design_systems.map((d) => ({ id: d.id, title: d.title }));
+      platforms.value = catalog.map((p) => ({ slug: p.slug, name: p.name }));
     }
   } catch (e: unknown) {
     error.value = apiErrorMessage(e, "Could not load what this project could inherit");
@@ -61,6 +84,22 @@ async function load() {
 
 const nothingToDecide = computed(() => !designSystems.value.length);
 
+function isChecked(slug: string): boolean {
+  return (local.value.platforms ?? []).includes(slug);
+}
+
+function togglePlatform(slug: string, on: boolean) {
+  const current = new Set(local.value.platforms ?? []);
+  if (on) current.add(slug);
+  else current.delete(slug);
+  local.value.platforms = [...current].sort();
+}
+
+/** Back to "not answered" — distinct from an empty list, which says "none". */
+function clearPlatforms() {
+  local.value.platforms = null;
+}
+
 async function record() {
   if (!props.projectId) return;
   saving.value = true;
@@ -105,6 +144,31 @@ onMounted(load);
           </span>
         </label>
       </div>
+      <div v-if="platforms.length" class="inception-group">
+        <h4>Platforms</h4>
+        <p class="inception-muted">
+          What it is built on or ships as. Ideas the family has proven for a
+          platform reach every project that is one.
+          <template v-if="local.platforms === null"> Not answered yet.</template>
+          <template v-else>
+            Anything left unticked is recorded as "not this project".
+            <button type="button" class="btn-ghost btn-sm" @click="clearPlatforms">Leave unanswered</button>
+          </template>
+        </p>
+        <div class="inception-platforms">
+          <label v-for="p in platforms" :key="p.slug" class="inception-choice">
+            <input
+              type="checkbox"
+              :checked="isChecked(p.slug)"
+              @change="togglePlatform(p.slug, ($event.target as HTMLInputElement).checked)"
+            />
+            <span>
+              {{ p.name }}
+              <em v-if="detected.has(p.slug)" class="inception-muted"> — detected</em>
+            </span>
+          </label>
+        </div>
+      </div>
       <p v-if="nothingToDecide" class="inception-muted">
         No design systems on this install yet — recording still settles the question.
       </p>
@@ -132,6 +196,11 @@ onMounted(load);
 .inception-group h4 { margin: 0 0 0.35rem; font-size: 0.9rem; font-weight: 500; }
 .inception-choice { display: flex; align-items: flex-start; gap: 0.5rem; font-size: 0.9rem; margin: 0.25rem 0; }
 .inception-choice input { margin-top: 0.2rem; accent-color: var(--fs-accent); }
+.inception-platforms {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(12rem, 1fr));
+  gap: 0 1rem;
+}
 .inception-select {
   padding: 0.45rem 0.7rem;
   border: 1px solid var(--fs-border-color);
diff --git a/frontend/src/components/ProjectFamilyTab.vue b/frontend/src/components/ProjectFamilyTab.vue
new file mode 100644
index 00000000..c76aef41
--- /dev/null
+++ b/frontend/src/components/ProjectFamilyTab.vue
@@ -0,0 +1,200 @@
+<script setup lang="ts">
+/**
+ * A project's place in the family (milestone 463): which platforms it is.
+ *
+ * Membership is what makes "does this family idea reach this project?" a
+ * lookup rather than a judgment — an idea is for some platforms, and it
+ * reaches every project that is one of them. So the answers here matter, and
+ * each kind of answer is shown as what it is:
+ *
+ * - Yes — a person said so.
+ * - Detected — a marker file in a bound repo said so. Detection only adds,
+ *   and only where nobody has answered; it never overrides a Yes or a No.
+ * - No — a person said this project is NOT that platform. Kept as a row, so
+ *   the next coverage refresh cannot detect it straight back.
+ * - No answer — detection may decide on the next refresh.
+ *
+ * Each change is saved as it is made: one platform, one request, applied
+ * whole or refused whole.
+ */
+import { computed, onMounted, ref, watch } from "vue";
+import { apiErrorMessage } from "@/api/client";
+import {
+  fetchProjectPlatforms, setProjectPlatforms,
+  type PlatformState, type ProjectPlatform, type SettablePlatformState,
+} from "@/api/platforms";
+import { usePlatformsStore } from "@/stores/platforms";
+
+const props = defineProps<{ projectId: number; canWrite: boolean }>();
+
+const platformsStore = usePlatformsStore();
+const answers = ref<ProjectPlatform[]>([]);
+const loading = ref(false);
+const failed = ref(false);
+const saving = ref<string | null>(null);
+const error = ref("");
+
+const LABELS: Record<PlatformState, string> = {
+  declared: "Yes",
+  detected: "Detected",
+  rejected: "No",
+};
+
+async function load() {
+  loading.value = true;
+  failed.value = false;
+  try {
+    const [, rows] = await Promise.all([
+      platformsStore.fetchCatalog(),
+      fetchProjectPlatforms(props.projectId),
+    ]);
+    answers.value = rows;
+  } catch {
+    // Said out loud: "couldn't load" must not read as "no platforms".
+    failed.value = true;
+  } finally {
+    loading.value = false;
+  }
+}
+
+const stateBySlug = computed(() => {
+  const out: Record<string, PlatformState> = {};
+  for (const a of answers.value) out[a.slug] = a.state;
+  return out;
+});
+
+const rows = computed(() =>
+  platformsStore.catalog.map((p) => ({
+    ...p,
+    state: stateBySlug.value[p.slug] ?? null,
+  })),
+);
+
+const memberCount = computed(
+  () => answers.value.filter((a) => a.state !== "rejected").length,
+);
+
+async function answer(slug: string, value: string) {
+  const state: SettablePlatformState = value === "" ? null : (value as SettablePlatformState);
+  saving.value = slug;
+  error.value = "";
+  try {
+    answers.value = await setProjectPlatforms(props.projectId, { [slug]: state });
+  } catch (e: unknown) {
+    error.value = apiErrorMessage(e, "Could not save that answer");
+  } finally {
+    saving.value = null;
+  }
+}
+
+onMounted(load);
+watch(() => props.projectId, load);
+</script>
+
+<template>
+  <div class="pft">
+    <header class="pft-head">
+      <h3 class="pft-title">Platforms</h3>
+      <p class="pft-muted">
+        What this project is built on or ships as. Ideas the family has proven
+        for a platform reach every project that is one, and each project
+        answers them — adopts, varies with a reason, or owes the work.
+      </p>
+    </header>
+
+    <p v-if="loading" class="pft-muted">Loading…</p>
+
+    <div v-else-if="failed" class="pft-note">
+      <strong>The platforms couldn't load.</strong>
+      <p>Nothing is known either way — this is a failure, not an empty answer.</p>
+      <button class="btn-secondary btn-sm" @click="load">Try again</button>
+    </div>
+
+    <p v-else-if="!rows.length" class="pft-muted">
+      This install has no platforms in its catalog yet. An administrator adds
+      them in Settings.
+    </p>
+
+    <template v-else>
+      <p class="pft-summary">
+        <template v-if="memberCount">
+          A member of {{ memberCount }} platform{{ memberCount === 1 ? "" : "s" }}.
+        </template>
+        <template v-else>
+          Not a member of any platform yet — bind a repo and refresh coverage
+          to detect them, or answer below.
+        </template>
+      </p>
+      <p v-if="error" class="error-msg">{{ error }}</p>
+      <ul class="pft-list">
+        <li v-for="p in rows" :key="p.id" class="pft-row">
+          <div class="pft-name">
+            <span>{{ p.name }}</span>
+            <span v-if="p.state" :class="['pft-tag', `pft-${p.state}`]">{{ LABELS[p.state] }}</span>
+            <span v-if="p.description" class="pft-desc">{{ p.description }}</span>
+          </div>
+          <select
+            v-if="canWrite"
+            class="pft-select"
+            :value="p.state === 'detected' ? 'detected' : (p.state ?? '')"
+            :disabled="saving === p.slug"
+            :aria-label="`Is this project ${p.name}?`"
+            @change="answer(p.slug, ($event.target as HTMLSelectElement).value)"
+          >
+            <option v-if="p.state === 'detected'" value="detected" disabled>Detected</option>
+            <option value="declared">Yes</option>
+            <option value="rejected">No</option>
+            <option value="">No answer</option>
+          </select>
+        </li>
+      </ul>
+    </template>
+  </div>
+</template>
+
+<style scoped>
+.pft { padding: 0.25rem 0; }
+.pft-head { margin-bottom: 1rem; }
+.pft-title { margin: 0 0 0.35rem; font-size: 1rem; }
+.pft-muted { color: var(--fs-text-tertiary); font-size: 0.875rem; margin: 0; }
+.pft-summary { color: var(--fs-text-secondary); font-size: 0.9rem; margin: 0 0 0.75rem; }
+.pft-note {
+  background: var(--fs-surface-raised);
+  border: 1px solid var(--fs-border-color);
+  border-radius: var(--fs-radius-md);
+  padding: 0.85rem 1rem;
+}
+.pft-note p { margin: 0.35rem 0 0.6rem; color: var(--fs-text-secondary); font-size: 0.875rem; }
+.pft-list { list-style: none; margin: 0; padding: 0; }
+.pft-row {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 1rem;
+  padding: 0.6rem 0;
+  border-bottom: 1px solid var(--fs-border-color);
+}
+.pft-name { display: flex; flex-wrap: wrap; align-items: baseline; gap: 0.25rem 0.5rem; min-width: 0; }
+.pft-desc { flex-basis: 100%; color: var(--fs-text-tertiary); font-size: 0.8rem; }
+.pft-tag {
+  font-size: 0.75rem;
+  padding: 0.05rem 0.45rem;
+  border-radius: var(--fs-radius-sm);
+  border: 1px solid var(--fs-border-color);
+  color: var(--fs-text-secondary);
+}
+.pft-declared { color: var(--fs-text-primary); }
+.pft-rejected { color: var(--fs-text-tertiary); text-decoration: line-through; }
+.pft-select {
+  padding: 0.35rem 0.6rem;
+  border: 1px solid var(--fs-border-color);
+  border-radius: var(--fs-radius-sm);
+  background: var(--fs-surface-page);
+  color: var(--fs-text-primary);
+  font-size: 0.85rem;
+  flex-shrink: 0;
+}
+@media (max-width: 640px) {
+  .pft-row { flex-direction: column; align-items: stretch; }
+}
+</style>
diff --git a/frontend/src/stores/platforms.ts b/frontend/src/stores/platforms.ts
new file mode 100644
index 00000000..531427f3
--- /dev/null
+++ b/frontend/src/stores/platforms.ts
@@ -0,0 +1,48 @@
+import { ref } from "vue";
+import { defineStore } from "pinia";
+import * as api from "@/api/platforms";
+import type { Platform } from "@/api/platforms";
+
+/**
+ * The global platform catalog (milestone 463). Shared by every project, so
+ * it is fetched once per session — as the canonical-area catalog is.
+ */
+export const usePlatformsStore = defineStore("platforms", () => {
+  const catalog = ref<Platform[]>([]);
+  const loaded = ref(false);
+  const loading = ref(false);
+
+  async function fetchCatalog(force = false) {
+    if (loaded.value && !force) return catalog.value;
+    loading.value = true;
+    try {
+      catalog.value = await api.listPlatforms();
+      loaded.value = true;
+    } catch {
+      // The catalog is a vocabulary for a form; an empty one degrades the
+      // form rather than failing the screen it sits on.
+      catalog.value = [];
+    } finally {
+      loading.value = false;
+    }
+    return catalog.value;
+  }
+
+  async function createEntry(data: { name: string; description?: string; markers?: string[] }) {
+    const entry = await api.createPlatform(data);
+    catalog.value.push(entry);
+    return entry;
+  }
+
+  async function updateEntry(
+    id: number,
+    data: Partial<{ name: string; description: string; order_index: number; markers: string[] }>,
+  ) {
+    const entry = await api.updatePlatform(id, data);
+    const idx = catalog.value.findIndex((p) => p.id === id);
+    if (idx >= 0) catalog.value[idx] = entry;
+    return entry;
+  }
+
+  return { catalog, loaded, loading, fetchCatalog, createEntry, updateEntry };
+});
diff --git a/frontend/src/views/ProjectView.vue b/frontend/src/views/ProjectView.vue
index ae9bdafc..48e0ef90 100644
--- a/frontend/src/views/ProjectView.vue
+++ b/frontend/src/views/ProjectView.vue
@@ -12,6 +12,8 @@ import KindBadge from "@/components/KindBadge.vue";
 import ProjectStatusBadge from "@/components/ProjectStatusBadge.vue";
 import type { TaskKind } from "@/types/note";
 import ProjectDesignTab from "@/components/ProjectDesignTab.vue";
+import ProjectFamilyTab from "@/components/ProjectFamilyTab.vue";
+import { canWriteRecord } from "@/utils/permission";
 import ProjectRulesTab from "@/components/rules/ProjectRulesTab.vue";
 import SystemsSection from "@/components/SystemsSection.vue";
 import InceptionCard from "@/components/InceptionCard.vue";
@@ -125,7 +127,7 @@ async function confirmStartPlanning() {
 const saving = ref(false);
 const error = ref<string | null>(null);
 
-const activeTab = ref<"tasks" | "notes" | "systems" | "rules" | "design">("tasks");
+const activeTab = ref<"tasks" | "notes" | "systems" | "rules" | "design" | "family">("tasks");
 
 const tasks = ref<NoteItem[]>([]);
 const notes = ref<NoteItem[]>([]);
@@ -747,6 +749,10 @@ async function confirmDelete() {
         Inheritance decided {{ fmtDate(project.inception.decided_at) }} via {{ project.inception.via }}
         · design system {{ project.inception.choices.design_system_id ? "#" + project.inception.choices.design_system_id : "none" }}
         <template v-if="project.inception.choices.seed_systems"> · Systems seeded</template>
+        <!-- What was said AT inception; the live answers are on the Family tab. -->
+        <template v-if="project.inception.choices.platforms?.length">
+          · platforms {{ project.inception.choices.platforms.join(", ") }}
+        </template>
       </p>
 
       <!-- Summary stat chips -->
@@ -945,6 +951,9 @@ async function confirmDelete() {
             <button :class="['tab-btn', { active: activeTab === 'design' }]" @click="activeTab = 'design'">
               Design
             </button>
+            <button :class="['tab-btn', { active: activeTab === 'family' }]" @click="activeTab = 'family'">
+              Family
+            </button>
           </div>
 
           <!-- Tasks tab — milestone-grouped kanban -->
@@ -1190,6 +1199,13 @@ async function confirmDelete() {
             :project-id="projectId"
             :design-system-id="project.design_system_id ?? null"
           />
+
+          <!-- Family tab (milestone 463): which platforms this project is. -->
+          <ProjectFamilyTab
+            v-if="activeTab === 'family'"
+            :project-id="projectId"
+            :can-write="canWriteRecord(project.permission)"
+          />
         </div>
       </div>
     </template>
diff --git a/frontend/src/views/SettingsView.vue b/frontend/src/views/SettingsView.vue
index 94875ae2..e994238b 100644
--- a/frontend/src/views/SettingsView.vue
+++ b/frontend/src/views/SettingsView.vue
@@ -4,6 +4,7 @@ import { useSettingsStore } from "@/stores/settings";
 import { useAuthStore } from "@/stores/auth";
 import { useToastStore } from "@/stores/toast";
 import { useCanonicalSystemsStore } from "@/stores/canonicalSystems";
+import { usePlatformsStore } from "@/stores/platforms";
 import { apiGet, apiPost, apiPut, apiDelete, listGroups, createGroup, deleteGroup, listGroupMembers, addGroupMember, removeGroupMember, searchUsers, listApiKeys, createApiKey as apiCreateApiKey, revokeApiKey as apiRevokeApiKey, getProfile, updateProfile, type ApiKeyEntry, type GroupEntry, type GroupMember, type UserSearchResult, type UserProfile, apiErrorMessage } from "@/api/client";
 import type { User } from "@/types/auth";
 import PaginationBar from "@/components/PaginationBar.vue";
@@ -71,6 +72,75 @@ async function saveArea() {
     savingArea.value = false;
   }
 }
+// ── Platforms (milestone 463) ───────────────────────────────────────────
+// The global list of what a project can be built on or ship as. Same split
+// as the areas: admin-only to write, readable by everyone. Markers are the
+// repo-relative globs that let a coverage refresh DETECT a platform; they
+// are edited one per line, since a glob may itself contain a comma.
+const platformsStore = usePlatformsStore();
+const newPlatformName = ref("");
+const newPlatformDescription = ref("");
+const newPlatformMarkers = ref("");
+const creatingPlatform = ref(false);
+const editingPlatformId = ref<number | null>(null);
+const editPlatformName = ref("");
+const editPlatformDescription = ref("");
+const editPlatformMarkers = ref("");
+const savingPlatform = ref(false);
+
+function markerLines(text: string): string[] {
+  return text.split("\n").map((m) => m.trim()).filter(Boolean);
+}
+
+async function createPlatform() {
+  const name = newPlatformName.value.trim();
+  if (!name || creatingPlatform.value) return;
+  creatingPlatform.value = true;
+  try {
+    await platformsStore.createEntry({
+      name,
+      description: newPlatformDescription.value.trim() || undefined,
+      markers: markerLines(newPlatformMarkers.value),
+    });
+    newPlatformName.value = "";
+    newPlatformDescription.value = "";
+    newPlatformMarkers.value = "";
+    toastStore.show("Platform added");
+  } catch (e) {
+    // A 409 names the existing platform the new name reduces to.
+    toastStore.show(apiErrorMessage(e, "Failed to add platform"), "error");
+  } finally {
+    creatingPlatform.value = false;
+  }
+}
+
+function startEditPlatform(id: number, name: string, description: string | null, markers: string[]) {
+  editingPlatformId.value = id;
+  editPlatformName.value = name;
+  editPlatformDescription.value = description ?? "";
+  editPlatformMarkers.value = markers.join("\n");
+}
+
+async function savePlatform() {
+  const id = editingPlatformId.value;
+  const name = editPlatformName.value.trim();
+  if (id == null || !name || savingPlatform.value) return;
+  savingPlatform.value = true;
+  try {
+    await platformsStore.updateEntry(id, {
+      name,
+      description: editPlatformDescription.value.trim(),
+      markers: markerLines(editPlatformMarkers.value),
+    });
+    editingPlatformId.value = null;
+    toastStore.show("Platform updated");
+  } catch (e) {
+    toastStore.show(apiErrorMessage(e, "Failed to update platform"), "error");
+  } finally {
+    savingPlatform.value = false;
+  }
+}
+
 const userTimezone = ref("");
 const savingTimezone = ref(false);
 const timezoneSaved = ref(false);
@@ -479,7 +549,7 @@ async function copyCommit() {
 const restoreFileInput = ref<HTMLInputElement | null>(null);
 
 // Migrate stored "admin" → "config"; unknown tabs fall back to "general"
-const VALID_TABS = new Set(["general", "account", "profile", "notifications", "integrations", "data", "apikeys", "config", "users", "logs", "groups", "areas"]);
+const VALID_TABS = new Set(["general", "account", "profile", "notifications", "integrations", "data", "apikeys", "config", "users", "logs", "groups", "areas", "platforms"]);
 const _stored = localStorage.getItem("settings_tab") ?? "general";
 const activeTab = ref(VALID_TABS.has(_stored) ? (_stored === "admin" ? "config" : _stored) : "general");
 
@@ -489,6 +559,7 @@ function _loadTabContent(tab: string) {
     else if (tab === "logs") loadLogsPanel();
     else if (tab === "groups") loadGroupsPanel();
     else if (tab === "areas") canonStore.fetchCatalog(true);
+    else if (tab === "platforms") platformsStore.fetchCatalog(true);
     else if (tab === "config" && !versionInfo.value) loadVersionPanel();
   }
   if (tab === "apikeys") { fetchApiKeys(); }
@@ -1611,7 +1682,7 @@ async function deleteUser(userId: number) {
       <div v-if="authStore.isAdmin" class="sidebar-group">
         <div class="sidebar-group-label">Admin</div>
         <button
-          v-for="tab in ['config', 'areas', 'users', 'groups', 'logs']"
+          v-for="tab in ['config', 'areas', 'platforms', 'users', 'groups', 'logs']"
           :key="tab"
           :class="['sidebar-item', { active: activeTab === tab }]"
           @click="activeTab = tab"
@@ -3179,6 +3250,99 @@ async function deleteUser(userId: number) {
       </section>
     </div>
 
+    <!-- ── Platforms ── -->
+    <div v-if="authStore.isAdmin" v-show="activeTab === 'platforms'" class="settings-grid">
+      <section class="settings-section full-width">
+        <h2>Platforms</h2>
+        <p class="field-hint">
+          What a project can be built on or ship as. A project's platforms decide which family
+          ideas reach it. Markers are repo-relative file patterns, one per line: a bare name
+          (<code>go.mod</code>) matches that file anywhere in a bound repo, and a pattern with a
+          slash (<code>.github/workflows/*</code>) matches the whole path. A platform with no
+          markers is never detected — projects declare it themselves.
+        </p>
+
+        <ul class="area-admin-list">
+          <li v-for="entry in platformsStore.catalog" :key="entry.id" class="area-admin-row">
+            <template v-if="editingPlatformId === entry.id">
+              <form class="area-admin-form" @submit.prevent="savePlatform">
+                <input v-model="editPlatformName" class="fs-input" aria-label="Platform name" />
+                <textarea
+                  v-model="editPlatformDescription"
+                  class="fs-input"
+                  rows="2"
+                  aria-label="Platform description"
+                ></textarea>
+                <textarea
+                  v-model="editPlatformMarkers"
+                  class="fs-input area-admin-markers"
+                  rows="3"
+                  aria-label="Detection markers, one per line"
+                ></textarea>
+                <div class="area-admin-actions">
+                  <button type="submit" class="btn-primary btn-compact" :disabled="!editPlatformName.trim() || savingPlatform">
+                    {{ savingPlatform ? "Saving…" : "Save" }}
+                  </button>
+                  <button type="button" class="btn-ghost btn-compact" @click="editingPlatformId = null">Cancel</button>
+                </div>
+              </form>
+            </template>
+            <template v-else>
+              <div class="area-admin-body">
+                <div class="area-admin-name-row">
+                  <span class="area-admin-name">{{ entry.name }}</span>
+                  <code class="area-admin-slug" title="The match key, and the name tools and backups use.">{{ entry.slug }}</code>
+                </div>
+                <p v-if="entry.description" class="area-admin-desc">{{ entry.description }}</p>
+                <p class="area-admin-desc">
+                  <template v-if="entry.markers.length">
+                    Detected by
+                    <code v-for="m in entry.markers" :key="m" class="area-admin-slug">{{ m }}</code>
+                  </template>
+                  <template v-else>Declare-only — never detected.</template>
+                </p>
+              </div>
+              <button
+                class="btn-ghost btn-compact"
+                @click="startEditPlatform(entry.id, entry.name, entry.description, entry.markers)"
+              >Edit</button>
+            </template>
+          </li>
+        </ul>
+        <p v-if="!platformsStore.catalog.length && !platformsStore.loading" class="settings-empty">
+          No platforms yet.
+        </p>
+
+        <form class="area-admin-form area-admin-create" @submit.prevent="createPlatform">
+          <input
+            v-model="newPlatformName"
+            class="fs-input"
+            placeholder="New platform name (e.g. Flutter app)"
+            aria-label="New platform name"
+          />
+          <textarea
+            v-model="newPlatformDescription"
+            class="fs-input"
+            rows="2"
+            placeholder="What makes a project this platform? One sentence."
+            aria-label="New platform description"
+          ></textarea>
+          <textarea
+            v-model="newPlatformMarkers"
+            class="fs-input area-admin-markers"
+            rows="2"
+            placeholder="Markers, one per line (e.g. pubspec.yaml) — leave empty for declare-only"
+            aria-label="New platform markers, one per line"
+          ></textarea>
+          <div class="area-admin-actions">
+            <button type="submit" class="btn-primary btn-compact" :disabled="!newPlatformName.trim() || creatingPlatform">
+              {{ creatingPlatform ? "Adding…" : "Add platform" }}
+            </button>
+          </div>
+        </form>
+      </section>
+    </div>
+
     <div v-if="authStore.isAdmin" v-show="activeTab === 'users'" class="settings-grid">
 
       <section class="settings-section full-width">
@@ -4386,6 +4550,8 @@ async function deleteUser(userId: number) {
 .area-admin-form { display: flex; flex-direction: column; gap: 0.5rem; flex: 1; }
 .area-admin-create { margin-top: var(--fs-space-4); }
 .area-admin-actions { display: flex; gap: 0.4rem; }
+.area-admin-markers { font-family: var(--fs-font-mono); font-size: 0.82rem; }
+.area-admin-desc .area-admin-slug { margin-right: 0.25rem; }
 
 /* The retrieval tuning trail (#4102). Reads as a record, not a control panel:
    the operator is reviewing what was done, and the reason is the part worth
diff --git a/src/scribe/app.py b/src/scribe/app.py
index 0ebfa7da..a0d4c031 100644
--- a/src/scribe/app.py
+++ b/src/scribe/app.py
@@ -32,6 +32,7 @@ from scribe.routes.trash import trash_bp
 from scribe.routes.dashboard import dashboard_bp
 from scribe.routes.systems import systems_bp
 from scribe.routes.canonical_systems import canonical_systems_bp
+from scribe.routes.platforms import platforms_bp
 from scribe.routes.lessons import lessons_bp
 from scribe.routes.snippets import snippets_bp
 from scribe.routes.webhooks import webhooks_bp
@@ -101,6 +102,7 @@ def create_app() -> Quart:
     app.register_blueprint(dashboard_bp)
     app.register_blueprint(systems_bp)
     app.register_blueprint(canonical_systems_bp)
+    app.register_blueprint(platforms_bp)
     app.register_blueprint(snippets_bp)
     app.register_blueprint(webhooks_bp)
 
diff --git a/src/scribe/mcp/server.py b/src/scribe/mcp/server.py
index 7e4f628c..18520615 100644
--- a/src/scribe/mcp/server.py
+++ b/src/scribe/mcp/server.py
@@ -160,6 +160,9 @@ _READ_ONLY_TOOLS = frozenset({
     # retrieval_telemetry's reason, and needed by a read key so that a line
     # naming a moment can be understood by whoever was shown it.
     "list_moments",
+    # The platform catalog and a project's answers (milestone 463). A pure
+    # read; set_project_platforms is the write.
+    "list_platforms",
     # The pass over the corpus and its queue (milestone 458 step 7): which
     # rules are unjudged and which proposals wait. Reads of the caller's own
     # rules, as list_rules is.
@@ -183,6 +186,7 @@ _WRITE_TOOLS = frozenset({
     # projects, Systems, repos
     "create_project", "update_project", "delete_project", "decide_project_inception",
     "create_system", "update_system", "delete_system", "map_system_to_canonical",
+    "set_project_platforms",
     "bind_repo", "unbind_repo",
     # snippets, processes, the shape ledger
     "create_snippet", "update_snippet", "delete_snippet", "verify_snippet",
diff --git a/src/scribe/mcp/tools/__init__.py b/src/scribe/mcp/tools/__init__.py
index 78bb9671..2bc5704b 100644
--- a/src/scribe/mcp/tools/__init__.py
+++ b/src/scribe/mcp/tools/__init__.py
@@ -6,7 +6,7 @@ from `mcp.server.build_mcp_server`.
 """
 from scribe.mcp.tools import (
     design_systems, lessons, milestones, notes, processes, projects, recent, repos,
-    moments, retrieval_review, retrieval_tuning,
+    moments, platforms, retrieval_review, retrieval_tuning,
     wide_net,
     rulebooks, search, shapes, snippets, systems, tags, tasks, trash,
 )
@@ -24,6 +24,7 @@ def register_all(mcp) -> None:
     projects.register(mcp)
     milestones.register(mcp)
     systems.register(mcp)
+    platforms.register(mcp)
     design_systems.register(mcp)
     tags.register(mcp)
     recent.register(mcp)
diff --git a/src/scribe/mcp/tools/platforms.py b/src/scribe/mcp/tools/platforms.py
new file mode 100644
index 00000000..ddc23fae
--- /dev/null
+++ b/src/scribe/mcp/tools/platforms.py
@@ -0,0 +1,78 @@
+"""Platform MCP tools — what a project is built on or ships as (milestone 463).
+
+Thin wrappers over services/platforms.py. The catalog is global; a project's
+platforms decide which family ideas reach it.
+"""
+from __future__ import annotations
+
+from scribe.mcp._context import current_user_id
+from scribe.services import platforms as platforms_svc
+
+
+async def list_platforms(project_id: int = 0) -> dict:
+    """The GLOBAL platform catalog — runtimes, delivery channels and
+    toolchains a project can be built on or ship as (Android app, container
+    image, Go, …) — and, with a project_id, that project's answer for each.
+
+    A project's platforms decide which family ideas reach it: an idea is for
+    some platforms, and every project that is one of them answers it. Pass
+    slugs from here to set_project_platforms and decide_project_inception.
+
+    Each platform's `markers` are the file patterns that let the coverage
+    refresh DETECT it in a bound repo. A project's `state` per platform is
+    `declared` (a person said so), `detected` (a marker said so) or `rejected`
+    (a person said no — kept so detection cannot add it back). Only declared
+    and detected are membership.
+
+    Args:
+        project_id: also return this project's answers. 0 = catalog only.
+    """
+    catalog = await platforms_svc.list_platforms()
+    out: dict = {"platforms": [p.to_dict() for p in catalog]}
+    if project_id:
+        rows = await platforms_svc.project_platforms(current_user_id(), project_id)
+        if rows is None:
+            raise ValueError(f"project {project_id} not found")
+        out["project_platforms"] = rows
+    return out
+
+
+async def set_project_platforms(
+    project_id: int,
+    declared: list[str] | None = None,
+    rejected: list[str] | None = None,
+    withdrawn: list[str] | None = None,
+) -> dict:
+    """Say which platforms a project is — or is not. Only the platforms named
+    change; everything else is left exactly as it is.
+
+    Use it when the operator corrects what detection found, or when a project
+    starts or stops shipping something (it gains an Android client; it drops
+    its container image). At project creation, decide_project_inception's
+    `platforms` is the place instead — it records the answer with the rest of
+    what the project inherits.
+
+    Args:
+        project_id: the project.
+        declared: slugs the project IS (list_platforms).
+        rejected: slugs it is NOT — recorded as a "no", so the coverage
+            refresh never detects it back.
+        withdrawn: slugs whose answer to drop entirely, so detection may
+            decide again on the next refresh.
+    """
+    updates: dict[str, str | None] = {}
+    for slug in withdrawn or []:
+        updates[slug] = None
+    for slug in rejected or []:
+        updates[slug] = "rejected"
+    for slug in declared or []:
+        updates[slug] = "declared"
+    if not updates:
+        raise ValueError("name at least one platform to declare, reject or withdraw")
+    rows = await platforms_svc.set_project_platforms(current_user_id(), project_id, updates)
+    return {"project_id": project_id, "project_platforms": rows}
+
+
+def register(mcp) -> None:
+    for fn in (list_platforms, set_project_platforms):
+        mcp.tool(name=fn.__name__)(fn)
diff --git a/src/scribe/mcp/tools/projects.py b/src/scribe/mcp/tools/projects.py
index 56491c21..087feecf 100644
--- a/src/scribe/mcp/tools/projects.py
+++ b/src/scribe/mcp/tools/projects.py
@@ -23,6 +23,7 @@ from scribe.services import design_systems as design_systems_svc
 from scribe.services import inception as inception_svc
 from scribe.services import milestones as milestones_svc
 from scribe.services import notes as notes_svc
+from scribe.services import platforms as platforms_svc
 from scribe.services import projects as projects_svc
 from scribe.services import rulebooks as rulebooks_svc
 from scribe.services import systems as systems_svc
@@ -125,9 +126,15 @@ async def enter_project(project_id: int) -> dict:
     create it with create_system rather than leaving the area unmodelled. Read
     a subsystem's accumulated records with list_system_records. Each is id and name; get_system has the charter.
 
+    `platforms` (milestone 463) is what the project is built on or ships as
+    — Android app, container image, Go, … — each with how it is known
+    (`declared` by a person, `detected` from a bound repo). They decide which
+    family ideas reach this project. An empty list on a project that plainly
+    ships something is worth correcting with set_project_platforms.
+
     `inception` (milestone 297) appears ONLY when the project is yours and
     nobody has decided what it inherits: it carries the current defaults
-    (design system, Systems), what to ask the
+    (design system, Systems, platforms), what to ask the
     operator — once — and the decide_project_inception call that answers it;
     it repeats on every enter until a decision is recorded.
 
@@ -188,6 +195,9 @@ async def enter_project(project_id: int) -> dict:
     # three days of the feature landing (#2546's audit). Untagged writes now
     # also ask with the vocabulary listed; this copy lets the first write tag.
     systems = await systems_svc.list_systems(uid, project_id)
+    platforms = platforms_svc.members(
+        await platforms_svc.project_platforms(uid, project_id) or []
+    )
 
     # The arrival-moment half of the bootstrap ask (#2683): session start is
     # when the agent has just read the project map and is not yet deep in a
@@ -258,6 +268,7 @@ async def enter_project(project_id: int) -> dict:
         },
         "pattern_coverage": coverage_svc.coverage_line(coverage) if coverage else None,
         "systems": [{"id": s.id, "name": s.name} for s in systems],
+        "platforms": platforms,
         "design_system": design_system,
         "milestone_summary": milestone_summary,
         **rulebooks_svc.rules_payload(
@@ -302,12 +313,15 @@ async def get_project(project_id: int) -> dict:
     rules (project_rules), and applicable_rules: the
     global rules tagged to an area this project works in. Every other global
     rule applies too and arrives by retrieval when the work matches it.
+    `platforms` is every platform the project has an answer for, with its
+    state — rejected ones included, unlike enter_project's brief list.
     """
     uid = current_user_id()
     project = await projects_svc.get_project(uid, project_id)
     if project is None:
         raise ValueError(f"project {project_id} not found")
     data = project.to_dict()
+    data["platforms"] = await platforms_svc.project_platforms(uid, project_id) or []
     rows = await milestones_svc.get_project_milestone_summary(uid, project_id)
     data["milestone_summary"], _ = milestones_svc.brief_milestone_summary(rows)
     applicable = await rulebooks_svc.get_applicable_rules(
@@ -317,17 +331,21 @@ async def get_project(project_id: int) -> dict:
     return data
 
 
-def _inception_choices(design_system_id, seed_systems) -> dict | None:
+def _inception_choices(design_system_id, seed_systems, platforms=None) -> dict | None:
     """The tool args → an inception choices object, or None when no inception
     arg was given at all (a bare create stays undecided and enter_project
     asks). design_system_id: 0 = not stated, -1 = explicitly none, n = that
-    system."""
-    if not design_system_id and seed_systems is None:
+    system. platforms: None = not stated (memberships untouched), a list =
+    the whole answer."""
+    if not design_system_id and seed_systems is None and platforms is None:
         return None
-    return {
+    choices = {
         "design_system_id": None if design_system_id in (0, -1) else design_system_id,
         "seed_systems": bool(seed_systems),
     }
+    if platforms is not None:
+        choices["platforms"] = list(platforms)
+    return choices
 
 
 async def create_project(
@@ -338,11 +356,12 @@ async def create_project(
     color: str = "",
     design_system_id: int = 0,
     seed_systems: bool | None = None,
+    platforms: list[str] | None = None,
 ) -> dict:
     """Create a new project in Scribe — and decide what it inherits.
 
     A project's inheritance is a decision, not a default (milestone 297):
-    before calling, ask the operator the two inception questions and pass
+    before calling, ask the operator the three inception questions and pass
     the answers; a project created without either is UNDECIDED and
     enter_project will ask until decide_project_inception records it.
     Defaults if nobody decides: no design system, no Systems. Rules are not
@@ -359,6 +378,9 @@ async def create_project(
             (list_design_systems); -1 = explicitly none; 0 = not stated.
         seed_systems: true mints the standard starter Systems (CI & Release,
             Auth & Access, …) so records can be tagged from day one.
+        platforms: slugs of what the project is built on or ships as
+            (list_platforms) — they decide which family ideas reach it. The
+            list is the whole answer; omit it to leave the question open.
     """
     uid = current_user_id()
     project = await projects_svc.create_project(
@@ -370,7 +392,7 @@ async def create_project(
         color=color or None,
     )
     data = project.to_dict()
-    choices = _inception_choices(design_system_id, seed_systems)
+    choices = _inception_choices(design_system_id, seed_systems, platforms)
     if choices is not None:
         decided = await inception_svc.decide(uid, project.id, choices=choices, via="mcp")
         data["inception"] = decided["inception"]
@@ -388,6 +410,7 @@ async def decide_project_inception(
     project_id: int,
     design_system_id: int = 0,
     seed_systems: bool | None = None,
+    platforms: list[str] | None = None,
 ) -> dict:
     """Record what a project inherits — answer enter_project's `inception` ask,
     or re-decide later (milestone 297).
@@ -400,10 +423,10 @@ async def decide_project_inception(
 
     Args: as create_project's inception args. Passing nothing records a
     decision to take nothing (no design system, no seed) — a valid answer,
-    stated.
+    stated — and leaves the project's platforms as they are.
     """
     uid = current_user_id()
-    choices = _inception_choices(design_system_id, seed_systems) or {}
+    choices = _inception_choices(design_system_id, seed_systems, platforms) or {}
     decided = await inception_svc.decide(uid, project_id, choices=choices, via="mcp")
     return {"project_id": project_id, **decided}
 
diff --git a/src/scribe/routes/platforms.py b/src/scribe/routes/platforms.py
new file mode 100644
index 00000000..0c040aad
--- /dev/null
+++ b/src/scribe/routes/platforms.py
@@ -0,0 +1,96 @@
+"""Platform routes — the GLOBAL platform catalog, and which platforms a
+project is (milestone 463 step 2).
+
+Two shapes, as with the canonical areas:
+
+- `/api/platforms` — the catalog. Readable by any signed-in user; writable
+  only by an admin, since a vocabulary anyone extends stops being shared.
+- `/api/projects/<id>/platforms` — a project's answers, authorised by the
+  PROJECT (read to see them, write to change them). The service enforces
+  both; these are thin wrappers.
+"""
+import logging
+
+from quart import Blueprint, jsonify, request
+
+from scribe.auth import admin_required, get_current_user_id, login_required
+from scribe.routes.utils import not_found
+from scribe.services import platforms as platforms_svc
+
+logger = logging.getLogger(__name__)
+
+platforms_bp = Blueprint("platforms", __name__, url_prefix="/api")
+
+_EDITABLE = ("name", "description", "order_index", "markers")
+
+
+@platforms_bp.route("/platforms", methods=["GET"])
+@login_required
+async def list_platforms_route():
+    entries = await platforms_svc.list_platforms()
+    return jsonify({"platforms": [e.to_dict() for e in entries]})
+
+
+@platforms_bp.route("/platforms", methods=["POST"])
+@admin_required
+async def create_platform_route():
+    uid = get_current_user_id()
+    data = await request.get_json() or {}
+    if not (data.get("name") or "").strip():
+        return jsonify({"error": "name is required"}), 400
+    try:
+        entry = await platforms_svc.create_platform(
+            uid, data["name"], description=data.get("description"),
+            markers=data.get("markers"),
+        )
+    except ValueError as exc:
+        return jsonify({"error": str(exc)}), 400
+    if entry is None:
+        return jsonify({"error": "Permission denied"}), 403
+    # Duplicate-gated on the slug: the entry that already is this platform
+    # comes back as a 409 rather than a second spelling of it.
+    if isinstance(entry, dict):
+        return jsonify(entry), 409
+    return jsonify(entry.to_dict()), 201
+
+
+@platforms_bp.route("/platforms/<int:platform_id>", methods=["PATCH"])
+@admin_required
+async def update_platform_route(platform_id: int):
+    uid = get_current_user_id()
+    data = await request.get_json() or {}
+    fields = {k: v for k, v in data.items() if k in _EDITABLE}
+    try:
+        entry = await platforms_svc.update_platform(uid, platform_id, **fields)
+    except ValueError as exc:
+        return jsonify({"error": str(exc)}), 400
+    if entry is None:
+        return not_found("Platform")
+    return jsonify(entry.to_dict())
+
+
+@platforms_bp.route("/projects/<int:project_id>/platforms", methods=["GET"])
+@login_required
+async def project_platforms_route(project_id: int):
+    """Every platform the project has an answer for — rejected included, so
+    a screen can show "no" as well as "yes"."""
+    rows = await platforms_svc.project_platforms(get_current_user_id(), project_id)
+    if rows is None:
+        return not_found("Project")
+    return jsonify({"project_platforms": rows})
+
+
+@platforms_bp.route("/projects/<int:project_id>/platforms", methods=["PUT"])
+@login_required
+async def set_project_platforms_route(project_id: int):
+    """Body: {"platforms": {<slug>: "declared" | "rejected" | null}}. Only
+    the slugs named change; null withdraws the answer. Applies whole or not
+    at all."""
+    data = await request.get_json() or {}
+    try:
+        rows = await platforms_svc.set_project_platforms(
+            get_current_user_id(), project_id, data.get("platforms"),
+        )
+    except ValueError as exc:
+        return jsonify({"error": str(exc)}), 400
+    return jsonify({"project_platforms": rows})
diff --git a/src/scribe/services/access.py b/src/scribe/services/access.py
index 2a433741..31f8cc87 100644
--- a/src/scribe/services/access.py
+++ b/src/scribe/services/access.py
@@ -103,6 +103,15 @@ async def can_admin_project(user_id: int, project_id: int) -> bool:
     return perm in ("admin", "owner")
 
 
+async def is_instance_admin(user_id: int) -> bool:
+    """Whether the user administers the INSTANCE (users.role == "admin") —
+    the gate on the global catalogs (canonical areas, platforms), which belong
+    to no user and no project, so no share can grant a write to them."""
+    async with async_session() as session:
+        role = await session.scalar(select(User.role).where(User.id == user_id))
+    return role == "admin"
+
+
 # ---------------------------------------------------------------------------
 # Note / task permissions
 # ---------------------------------------------------------------------------
diff --git a/src/scribe/services/canonical_systems.py b/src/scribe/services/canonical_systems.py
index d1c56dd7..d6c43136 100644
--- a/src/scribe/services/canonical_systems.py
+++ b/src/scribe/services/canonical_systems.py
@@ -30,7 +30,6 @@ from sqlalchemy import select
 from scribe.models import async_session
 from scribe.models.canonical_system import CanonicalSystem
 from scribe.models.system import System
-from scribe.models.user import User
 from scribe.services import access
 
 logger = logging.getLogger(__name__)
@@ -60,9 +59,7 @@ def _tokens(slug: str) -> frozenset[str]:
 
 
 async def _is_admin(user_id: int) -> bool:
-    async with async_session() as session:
-        role = await session.scalar(select(User.role).where(User.id == user_id))
-    return role == "admin"
+    return await access.is_instance_admin(user_id)
 
 
 async def list_canonical_systems() -> list[CanonicalSystem]:
diff --git a/src/scribe/services/coverage.py b/src/scribe/services/coverage.py
index b8f6bf99..7f8e3b28 100644
--- a/src/scribe/services/coverage.py
+++ b/src/scribe/services/coverage.py
@@ -660,6 +660,11 @@ class ArchiveScan(NamedTuple):
 
     definitions: list[ArchiveShape]
     references: dict[str, dict[str, int]]   # path → class token → count
+    # EVERY file's repo-relative path, scannable or not (milestone 463). The
+    # platform markers are files this scan otherwise skips — go.mod,
+    # AndroidManifest.xml, a Dockerfile — so detection reads the names here
+    # rather than re-walking the tarball.
+    paths: tuple[str, ...] = ()
 
 
 def definitions_from_archive(blob: bytes) -> list[ArchiveShape]:
@@ -678,11 +683,14 @@ def scan_archive(blob: bytes) -> ArchiveScan:
     """
     shapes: list[ArchiveShape] = []
     references: dict[str, dict[str, int]] = {}
+    paths: list[str] = []
     with tarfile.open(fileobj=io.BytesIO(blob), mode="r:gz") as tar:
         for member in tar:
             if not member.isfile() or "/" not in member.name:
                 continue
             path = member.name.split("/", 1)[1]
+            if path:
+                paths.append(path)
             if not path or not scannable(path) or member.size > _MAX_FILE_BYTES:
                 continue
             handle = tar.extractfile(member)
@@ -708,7 +716,7 @@ def scan_archive(blob: bytes) -> ArchiveScan:
             )
             if refs:
                 references[path] = refs
-    return ArchiveScan(shapes, references)
+    return ArchiveScan(shapes, references, tuple(paths))
 
 
 # --- matching shapes against recorded locations ------------------------------
@@ -806,6 +814,8 @@ async def compute_coverage(
         return None
 
     served: list[tuple[str, str]] = []
+    # Every file path across the project's repos, for platform detection.
+    tree_paths: list[str] = []
     recorded = await _recorded_locations(user_id, project_id)
     # The proposer's canon catalog, read once per refresh and shared across
     # the project's repos (#2792).
@@ -822,6 +832,7 @@ async def compute_coverage(
         ref = binding.ref or await forge.default_branch(api_repo)
         scan = scan_archive(await forge.archive(api_repo, ref))
         definitions = scan.definitions
+        tree_paths.extend(scan.paths)
         # The head commit is provenance sugar on the ledger rows; failing to
         # learn it must not fail the sync — the ref names the point well
         # enough and the row timestamps carry the when.
@@ -857,6 +868,16 @@ async def compute_coverage(
     if not served:
         return None
 
+    # Platform detection (milestone 463) rides the same walk: the paths are in
+    # hand once. It only ever ADDS membership the project has no answer for,
+    # and it must not be able to fail the refresh it rides on.
+    try:
+        from scribe.services import platforms as platforms_svc
+
+        await platforms_svc.detect_for_project(project_id, tree_paths)
+    except Exception:
+        logger.warning("platform detection failed for project %s", project_id, exc_info=True)
+
     await shape_ledger.mark_canonicals(project_id, recorded)
     try:
         await shape_ledger.apply_derive_groups(project_id)
diff --git a/src/scribe/services/inception.py b/src/scribe/services/inception.py
index 52f838e2..4e9d48bd 100644
--- a/src/scribe/services/inception.py
+++ b/src/scribe/services/inception.py
@@ -8,7 +8,8 @@ A project's inheritance is a decision, not a default. The record lives on
       "via": "mcp" | "ui" | "legacy",
       "choices": {
         "design_system_id": <id> | null,
-        "seed_systems": bool
+        "seed_systems": bool,
+        "platforms": [<slug>, ...] | null
       }
     }
 
@@ -29,6 +30,15 @@ applies the effects (each idempotent), and writes the record LAST, so a
 half-applied decision is re-runnable rather than recorded as done.
 ``current_defaults`` is what the enter_project ask shows: what binds today
 if nobody decides.
+
+``platforms`` (milestone 463) is which platforms the project IS — the answer
+that decides which family ideas reach it. A list of catalog SLUGS, not ids:
+the record is JSON, and a slug survives a backup restore onto an install whose
+ids differ, where an id inside JSON would come back naming another platform.
+NULL means the question was not answered here, and the project's memberships
+are left exactly as they are; a list is the full answer — every platform in it
+is declared, and any platform detection had added that is NOT in it is
+recorded as a "no", so the next refresh cannot put it back.
 """
 from __future__ import annotations
 
@@ -38,7 +48,7 @@ from scribe.models import async_session
 from scribe.models.project import Project
 
 INCEPTION_VIAS = ("mcp", "ui", "legacy")
-CHOICE_KEYS = ("design_system_id", "seed_systems")
+CHOICE_KEYS = ("design_system_id", "seed_systems", "platforms")
 
 
 def validate_inception(choices) -> str | None:
@@ -46,8 +56,9 @@ def validate_inception(choices) -> str | None:
     None. Pure and checked BEFORE any effect is applied: a decision either
     applies whole or errors whole (the StrictArgs lesson, #2709).
 
-    Accepts two keys, each optional: ``design_system_id`` an int or None,
-    ``seed_systems`` a bool. Unknown keys are an error — a typo, or a choice
+    Accepts three keys, each optional: ``design_system_id`` an int or None,
+    ``seed_systems`` a bool, ``platforms`` a list of slugs or None. Unknown
+    keys are an error — a typo, or a choice
     the product no longer offers, must not become a silently ignored one."""
     if not isinstance(choices, dict):
         return "choices must be an object"
@@ -60,16 +71,26 @@ def validate_inception(choices) -> str | None:
     seed = choices.get("seed_systems", False)
     if not isinstance(seed, bool):
         return "seed_systems must be true or false"
+    platforms = choices.get("platforms")
+    if platforms is not None and (
+        not isinstance(platforms, list)
+        or not all(isinstance(p, str) and p.strip() for p in platforms)
+    ):
+        return "platforms must be a list of platform slugs (list_platforms), or null"
     return None
 
 
 def normalize_choices(choices: dict | None) -> dict:
-    """Both keys, always present, in canonical form — what gets stored
+    """Every key, always present, in canonical form — what gets stored
     and what the UI/agent reads back. Call after validate_inception."""
     choices = choices or {}
+    platforms = choices.get("platforms")
     return {
         "design_system_id": choices.get("design_system_id"),
         "seed_systems": bool(choices.get("seed_systems", False)),
+        "platforms": (
+            sorted({p.strip() for p in platforms}) if platforms is not None else None
+        ),
     }
 
 
@@ -81,11 +102,15 @@ def is_decided(project) -> bool:
 async def current_defaults(user_id: int, project_id: int) -> dict:
     """What the project inherits if nobody decides — the ask's payload.
 
-    {design_system_id, design_systems: [{id,title}], systems: <count>}.
+    {design_system_id, design_systems: [{id,title}], systems: <count>,
+    platforms: [{slug,name}], project_platforms: [{slug,name,state}]}.
     Instance-agnostic: an install with no design systems shows an empty list,
-    and the ask says so rather than inventing a default.
+    and the ask says so rather than inventing a default. `project_platforms`
+    is what detection has already found (and anything already answered), so
+    the form can start from it.
     """
     from scribe.services import design_systems as design_systems_svc
+    from scribe.services import platforms as platforms_svc
     from scribe.services import projects as projects_svc
     from scribe.services import systems as systems_svc
 
@@ -94,10 +119,13 @@ async def current_defaults(user_id: int, project_id: int) -> dict:
         raise ValueError(f"project {project_id} not found")
     designs = await design_systems_svc.list_design_systems(user_id)
     systems = await systems_svc.list_systems(user_id, project_id, include_archived=True)
+    catalog = await platforms_svc.list_platforms()
     return {
         "design_system_id": project.design_system_id,
         "design_systems": [{"id": d.id, "title": d.title} for d in designs],
         "systems": len(systems),
+        "platforms": [{"slug": p.slug, "name": p.name} for p in catalog],
+        "project_platforms": await platforms_svc.project_platforms(user_id, project_id) or [],
     }
 
 
@@ -106,9 +134,15 @@ async def _check_targets(user_id: int, choices: dict) -> None:
     effect lands — a decision applies whole or errors whole."""
     from scribe.services import access
 
+    from scribe.services import platforms as platforms_svc
+
     ds = choices["design_system_id"]
     if ds is not None and not await access.can_read_design_system(user_id, ds):
         raise ValueError(f"design system {ds} not found (or not readable)")
+    if choices["platforms"]:
+        _, unknown = await platforms_svc.resolve_slugs(choices["platforms"])
+        if unknown:
+            raise ValueError(f"unknown platform(s): {', '.join(unknown)} (list_platforms)")
 
 
 async def decide(
@@ -127,7 +161,8 @@ async def decide(
     replaces the design system and re-seeds nothing a project already has.
 
     Returns {"inception": <record>, "effects": {design_system_id,
-    systems_seeded}}.
+    systems_seeded, platforms}} — `platforms` is the project's answers after
+    the decision, or None when the choice was left unstated.
     """
     from scribe.services import design_systems as design_systems_svc
     from scribe.services import projects as projects_svc
@@ -152,6 +187,9 @@ async def decide(
         await systems_svc.seed_standard_systems(user_id, project_id)
         if choices["seed_systems"] else []
     )
+    platforms = None
+    if choices["platforms"] is not None:
+        platforms = await _apply_platforms(user_id, project_id, choices["platforms"])
 
     record = {
         "decided_at": datetime.now(timezone.utc).isoformat(),
@@ -169,10 +207,41 @@ async def decide(
         "effects": {
             "design_system_id": choices["design_system_id"],
             "systems_seeded": [sy.name for sy in seeded],
+            "platforms": platforms,
         },
     }
 
 
+async def _apply_platforms(user_id: int, project_id: int, slugs: list[str]) -> list[dict]:
+    """The platforms answer as membership. The list is the WHOLE answer:
+    every slug in it is declared, and every platform the project was a member
+    of (declared or detected) that is left out becomes rejected — so the next
+    refresh cannot detect back what the person just said the project isn't.
+    Platforms already rejected stay rejected; platforms never answered stay
+    unanswered."""
+    from scribe.services import platforms as platforms_svc
+
+    named = set(slugs)
+    current = await platforms_svc.project_platforms(user_id, project_id) or []
+    updates: dict[str, str | None] = {slug: "declared" for slug in named}
+    for row in current:
+        if row["slug"] not in named and row["state"] in platforms_svc.MEMBER_STATES:
+            updates[row["slug"]] = "rejected"
+    return await platforms_svc.set_project_platforms(user_id, project_id, updates)
+
+
+def _platform_line(defaults: dict) -> str:
+    """What the ask says about platforms: what detection already found, and
+    the catalog to choose from."""
+    found = [
+        p["slug"] for p in defaults.get("project_platforms", [])
+        if p["state"] in ("declared", "detected")
+    ]
+    catalog = ", ".join(p["slug"] for p in defaults.get("platforms", [])) or "none"
+    lead = f"detected so far: {', '.join(found)}; " if found else ""
+    return f"{lead}catalog: {catalog}"
+
+
 async def inception_ask(user_id: int, project_id: int) -> dict:
     """The enter_project ask for an undecided project (milestone 297) — the
     sibling of the systems-bootstrap ask (#2683): the project's OWN current
@@ -190,13 +259,16 @@ async def inception_ask(user_id: int, project_id: int) -> dict:
             "inherits. Design system — "
             f"{'#' + str(defaults['design_system_id']) if defaults['design_system_id'] else 'none'} "
             f"(available: {designs}); Systems — {defaults['systems']}. Ask the operator, "
-            "once: which design system (or none), and whether to seed "
-            "the standard starter Systems — then record the answers. This ask repeats on "
-            "every enter_project until a decision is recorded."
+            "once: which design system (or none), whether to seed "
+            "the standard starter Systems, and which platforms the project is "
+            f"built on or ships as ({_platform_line(defaults)}) — then record the "
+            "answers. This ask repeats on every enter_project until a decision "
+            "is recorded."
         ),
         "call": (
             f"decide_project_inception(project_id={project_id}, "
-            "design_system_id=<id | -1 for none>, seed_systems=<true|false>)"
+            "design_system_id=<id | -1 for none>, seed_systems=<true|false>, "
+            "platforms=[<slug>, ...])"
         ),
     }
 
diff --git a/src/scribe/services/platforms.py b/src/scribe/services/platforms.py
new file mode 100644
index 00000000..675c72a5
--- /dev/null
+++ b/src/scribe/services/platforms.py
@@ -0,0 +1,335 @@
+"""Platforms — what a project is built on or ships as, and which projects are
+which (milestone 463 step 2).
+
+Membership is what makes "is this idea in family for that project?" a lookup
+rather than a judgment: a family idea is for some platforms, and it reaches
+every project that is a member of one of them.
+
+Three ways a project becomes, or refuses to become, a member:
+
+- **declared** — a person said so: at inception, or in the project's settings.
+- **detected** — a marker file in a bound repo said so, found by the coverage
+  refresh that already walks the repo archive.
+- **rejected** — a person said NO. Kept as a row, so the next refresh does not
+  detect it straight back.
+
+The one invariant everything here protects: **detection only ever ADDS, and
+only where nobody has answered.** It never overwrites a declared or rejected
+row, and it never removes anything — not even a detected row whose marker has
+since disappeared, because membership is what adoption rows hang off and a
+platform flickering in and out with a repo's file tree would churn a ledger
+of decisions nobody re-made.
+
+The catalog itself is global, like the canonical area catalog, and for the
+same reason writes to it are admin-only: a shared vocabulary anyone can extend
+stops being shared. Reads are open to any signed-in user.
+"""
+from __future__ import annotations
+
+import fnmatch
+import logging
+from datetime import datetime, timezone
+
+from sqlalchemy import select
+
+from scribe.models import async_session
+from scribe.models.family import Platform, ProjectPlatform
+from scribe.services import access
+from scribe.services.canonical_systems import canonical_slug
+
+logger = logging.getLogger(__name__)
+
+# The states that make a project a member. `rejected` is an answer, not
+# membership.
+MEMBER_STATES = ("declared", "detected")
+# What a person may set from a door. `detected` is the refresh's to write.
+SETTABLE_STATES = ("declared", "rejected")
+
+
+# --- the catalog -------------------------------------------------------------
+
+async def list_platforms() -> list[Platform]:
+    """The whole live catalog, in display order. Global — no owner filter."""
+    async with async_session() as session:
+        result = await session.execute(
+            select(Platform)
+            .where(Platform.deleted_at.is_(None))
+            .order_by(Platform.order_index.asc(), Platform.name.asc())
+        )
+        return list(result.scalars().all())
+
+
+def _clean_markers(markers) -> list[str]:
+    """Markers as stored: stripped, non-empty strings, de-duplicated in order.
+    Anything else is refused by the caller before it gets here."""
+    seen: list[str] = []
+    for m in markers or []:
+        m = str(m).strip()
+        if m and m not in seen:
+            seen.append(m)
+    return seen
+
+
+def validate_markers(markers) -> str | None:
+    """The error a markers value would earn, or None. Pure."""
+    if markers is None:
+        return None
+    if not isinstance(markers, list) or not all(isinstance(m, str) for m in markers):
+        return "markers must be a list of glob patterns"
+    if any(m.strip().startswith("/") for m in markers):
+        return "markers are repo-relative — no leading slash"
+    return None
+
+
+async def create_platform(
+    user_id: int, name: str, *, description: str | None = None,
+    markers: list[str] | None = None,
+) -> Platform | dict | None:
+    """Add a platform to the global catalog. Admin only.
+
+    Duplicate-gated on the slug, so "Android App" cannot be added beside
+    "Android app": the existing entry comes back instead of a second spelling
+    of it. None means not permitted, or no usable name.
+    """
+    if not await access.is_instance_admin(user_id):
+        return None
+    slug = canonical_slug(name)
+    if not slug:
+        return None
+    error = validate_markers(markers)
+    if error:
+        raise ValueError(error)
+    async with async_session() as session:
+        existing = await session.scalar(
+            select(Platform).where(Platform.slug == slug, Platform.deleted_at.is_(None))
+        )
+        if existing is not None:
+            return {
+                "duplicate": True,
+                "existing_id": existing.id,
+                "message": (
+                    f"'{existing.name}' (#{existing.id}) is already this platform — "
+                    f"both names reduce to '{slug}'."
+                ),
+            }
+        highest = await session.scalar(
+            select(Platform.order_index).order_by(Platform.order_index.desc()).limit(1)
+        )
+        entry = Platform(
+            name=" ".join(name.split()),
+            slug=slug,
+            description=description,
+            markers=_clean_markers(markers),
+            order_index=(highest or 0) + 1,
+        )
+        session.add(entry)
+        await session.commit()
+        await session.refresh(entry)
+        return entry
+
+
+async def update_platform(user_id: int, platform_id: int, **fields: object) -> Platform | None:
+    """Rename, re-describe, re-order or re-mark a catalog entry. Admin only.
+
+    A rename recomputes the slug — the display name and the match key must not
+    disagree. Changing the slug of a platform projects already belong to is
+    safe: membership is by id; only a backup carries the slug.
+    """
+    if not await access.is_instance_admin(user_id):
+        return None
+    if "markers" in fields:
+        error = validate_markers(fields["markers"])
+        if error:
+            raise ValueError(error)
+    async with async_session() as session:
+        entry = await session.get(Platform, platform_id)
+        if entry is None or entry.deleted_at is not None:
+            return None
+        if fields.get("name"):
+            entry.name = " ".join(str(fields["name"]).split())
+            entry.slug = canonical_slug(entry.name)
+        if fields.get("description") is not None:
+            entry.description = fields["description"] or None
+        if fields.get("order_index") is not None:
+            entry.order_index = int(fields["order_index"])
+        if fields.get("markers") is not None:
+            entry.markers = _clean_markers(fields["markers"])
+        entry.updated_at = datetime.now(timezone.utc)
+        await session.commit()
+        await session.refresh(entry)
+        return entry
+
+
+async def resolve_slugs(slugs: list[str]) -> tuple[dict[str, int], list[str]]:
+    """{slug: id} for the slugs the live catalog knows, and the ones it does
+    not. Doors take slugs — readable, stable across installs, and the form an
+    inception record and a backup both carry."""
+    wanted = [s for s in dict.fromkeys(slugs or [])]
+    if not wanted:
+        return {}, []
+    async with async_session() as session:
+        rows = (await session.execute(
+            select(Platform.slug, Platform.id).where(
+                Platform.slug.in_(wanted), Platform.deleted_at.is_(None),
+            )
+        )).all()
+    known = {slug: pid for slug, pid in rows}
+    return known, [s for s in wanted if s not in known]
+
+
+# --- detection (pure) --------------------------------------------------------
+
+def marker_matches(marker: str, path: str) -> bool:
+    """Whether one marker matches one repo-relative path.
+
+    A marker with no slash matches a file's BASENAME anywhere in the tree
+    (`go.mod`, `AndroidManifest.xml`, `vite.config.*`). A marker with a slash
+    matches the whole repo-relative path (`.github/workflows/*`), so a
+    directory-shaped marker cannot be satisfied by a same-named file
+    somewhere else.
+    """
+    marker = marker.strip()
+    if not marker:
+        return False
+    if "/" in marker:
+        return fnmatch.fnmatchcase(path, marker)
+    return fnmatch.fnmatchcase(path.rsplit("/", 1)[-1], marker)
+
+
+def detect(catalog: list, paths: list[str]) -> list[int]:
+    """The ids of every catalog platform at least one of whose markers matches
+    at least one path. Pure, so it is testable against a fixture tree with no
+    forge and no database. A platform with no markers is never detected —
+    that is what declare-only means."""
+    hits: list[int] = []
+    for platform in catalog:
+        markers = list(getattr(platform, "markers", None) or [])
+        if markers and any(marker_matches(m, p) for m in markers for p in paths):
+            hits.append(platform.id)
+    return hits
+
+
+# --- membership ----------------------------------------------------------------
+
+async def project_platforms(user_id: int, project_id: int) -> list[dict] | None:
+    """Every platform the project has an answer for, joined to the catalog.
+    None when the caller cannot read the project.
+
+    Rejected rows are included — a settings screen has to show "no" as well
+    as "yes", or it cannot let anyone change their mind.
+    """
+    if not await access.can_read_project(user_id, project_id):
+        return None
+    async with async_session() as session:
+        rows = (await session.execute(
+            select(ProjectPlatform, Platform)
+            .join(Platform, Platform.id == ProjectPlatform.platform_id)
+            .where(ProjectPlatform.project_id == project_id, Platform.deleted_at.is_(None))
+            .order_by(Platform.order_index.asc(), Platform.name.asc())
+        )).all()
+    return [
+        {"id": p.id, "slug": p.slug, "name": p.name, "state": m.state}
+        for m, p in rows
+    ]
+
+
+def members(platforms: list[dict]) -> list[dict]:
+    """The rows of `project_platforms` that are membership — the brief form
+    enter_project and get_project carry."""
+    return [
+        {"slug": p["slug"], "name": p["name"], "state": p["state"]}
+        for p in platforms if p["state"] in MEMBER_STATES
+    ]
+
+
+def validate_updates(updates) -> str | None:
+    """The error a set of membership answers would earn, or None. Pure.
+
+    `updates` is {slug: "declared" | "rejected" | None}. None withdraws the
+    answer — the row goes, and detection may add the platform again later.
+    `detected` is not settable: it is what the refresh writes, and a person
+    claiming it would erase the difference between the two."""
+    if not isinstance(updates, dict):
+        return "platforms must be an object of {slug: state}"
+    for slug, state in updates.items():
+        if not isinstance(slug, str) or not slug:
+            return "each platform is named by its slug"
+        if state is not None and state not in SETTABLE_STATES:
+            return (
+                f"'{state}' is not a state a person sets — use one of "
+                f"{', '.join(SETTABLE_STATES)}, or null to withdraw the answer"
+            )
+    return None
+
+
+async def set_project_platforms(
+    user_id: int, project_id: int, updates: dict[str, str | None],
+) -> list[dict]:
+    """Apply a person's answers about a project's platforms. Write-gated on
+    the project (rule 78). Platforms not named are left exactly as they are.
+
+    Raises ValueError, naming the problem, on a malformed update, an unknown
+    slug, or no write access — before anything is written, so the update
+    applies whole or not at all.
+    """
+    error = validate_updates(updates)
+    if error:
+        raise ValueError(error)
+    if not await access.can_write_project(user_id, project_id):
+        raise ValueError(f"project {project_id} not found or no write access")
+    known, unknown = await resolve_slugs(list(updates))
+    if unknown:
+        raise ValueError(f"unknown platform(s): {', '.join(unknown)} (list_platforms)")
+    async with async_session() as session:
+        existing = {
+            row.platform_id: row for row in (await session.execute(
+                select(ProjectPlatform).where(ProjectPlatform.project_id == project_id)
+            )).scalars().all()
+        }
+        for slug, state in updates.items():
+            pid = known[slug]
+            row = existing.get(pid)
+            if state is None:
+                if row is not None:
+                    await session.delete(row)
+            elif row is None:
+                session.add(ProjectPlatform(project_id=project_id, platform_id=pid, state=state))
+            else:
+                row.state = state
+        await session.commit()
+    return await project_platforms(user_id, project_id) or []
+
+
+async def record_detected(project_id: int, platform_ids: list[int]) -> list[int]:
+    """Add `detected` membership for each platform the project has NO answer
+    for. Returns the ids actually added.
+
+    The refresh's writer, so it takes no user: it runs on the owner's behalf
+    inside a sync the owner's keyring authorised. It never updates or deletes
+    a row — see the module docstring for why that is the whole contract.
+    """
+    if not platform_ids:
+        return []
+    async with async_session() as session:
+        answered = set((await session.execute(
+            select(ProjectPlatform.platform_id).where(
+                ProjectPlatform.project_id == project_id,
+            )
+        )).scalars().all())
+        added = [pid for pid in dict.fromkeys(platform_ids) if pid not in answered]
+        for pid in added:
+            session.add(ProjectPlatform(project_id=project_id, platform_id=pid, state="detected"))
+        await session.commit()
+    return added
+
+
+async def detect_for_project(project_id: int, paths: list[str]) -> list[int]:
+    """Run detection over a refresh's paths and record what is new. The
+    coverage refresh's single call — it must not be able to fail that
+    refresh, so the caller wraps it."""
+    hits = detect(await list_platforms(), paths)
+    added = await record_detected(project_id, hits)
+    if added:
+        logger.info("project %s: detected platform(s) %s", project_id, added)
+    return added
+
diff --git a/tests/helpers.py b/tests/helpers.py
index 54473ab9..21ace608 100644
--- a/tests/helpers.py
+++ b/tests/helpers.py
@@ -512,3 +512,18 @@ def skill_text(name: str) -> str:
     folder = pathlib.Path(__file__).resolve().parents[1] / "plugin" / "skills" / name
     refs = sorted(p for p in folder.glob("*.md") if p.name != "SKILL.md")
     return "\n\n".join(p.read_text() for p in [folder / "SKILL.md", *refs])
+
+
+def forge_tarball(files: dict[str, bytes], top: str = "widget") -> bytes:
+    """A gzipped tar shaped like a forge archive: every file under one
+    top-level directory (repo-ref/), which the scan strips."""
+    import io
+    import tarfile
+
+    buf = io.BytesIO()
+    with tarfile.open(fileobj=buf, mode="w:gz") as tar:
+        for path, data in files.items():
+            info = tarfile.TarInfo(f"{top}/{path}")
+            info.size = len(data)
+            tar.addfile(info, io.BytesIO(data))
+    return buf.getvalue()
diff --git a/tests/test_inception.py b/tests/test_inception.py
index 8e316c5e..c85dc43a 100644
--- a/tests/test_inception.py
+++ b/tests/test_inception.py
@@ -27,7 +27,7 @@ def test_project_carries_an_inception_record_and_to_dict_shows_it():
 
 
 def test_validate_inception_pins_the_choice_vocabulary():
-    assert CHOICE_KEYS == ("design_system_id", "seed_systems")
+    assert CHOICE_KEYS == ("design_system_id", "seed_systems", "platforms")
     assert validate_inception({}) is None
     assert validate_inception({"design_system_id": 3, "seed_systems": True}) is None
     assert validate_inception({"design_system_id": None}) is None
@@ -46,10 +46,28 @@ def test_validate_inception_pins_the_choice_vocabulary():
     assert "true or false" in validate_inception({"seed_systems": "yes"})
 
 
+def test_validate_inception_takes_platforms_as_slugs_or_unstated():
+    """Slugs, not ids: an inception record outlives a restore, and a slug is
+    what both sides of a restore agree on. None is "not answered", which is
+    different from [] — "none of them"."""
+    assert validate_inception({"platforms": ["android-app", "go"]}) is None
+    assert validate_inception({"platforms": []}) is None
+    assert validate_inception({"platforms": None}) is None
+    assert "platform slugs" in validate_inception({"platforms": "android-app"})
+    assert "platform slugs" in validate_inception({"platforms": [3]})
+    assert "platform slugs" in validate_inception({"platforms": ["  "]})
+
+
 def test_normalize_choices_is_canonical_and_complete():
     out = normalize_choices({"design_system_id": 4})
-    assert out == {"design_system_id": 4, "seed_systems": False}
-    assert normalize_choices(None) == {"design_system_id": None, "seed_systems": False}
+    assert out == {"design_system_id": 4, "seed_systems": False, "platforms": None}
+    assert normalize_choices(None) == {
+        "design_system_id": None, "seed_systems": False, "platforms": None,
+    }
+    # Sorted and de-duplicated, so two equal answers record identically.
+    assert normalize_choices({"platforms": ["go", " android-app", "go"]})["platforms"] == [
+        "android-app", "go",
+    ]
 
 
 def test_standard_systems_vocabulary_reads_the_catalog_not_a_constant():
diff --git a/tests/test_integration_inception.py b/tests/test_integration_inception.py
index 8a92078e..abfd372a 100644
--- a/tests/test_integration_inception.py
+++ b/tests/test_integration_inception.py
@@ -37,14 +37,16 @@ async def seeded():
 async def test_decide_applies_every_effect_and_records_last(seeded):
     owner, pid = seeded["owner"], seeded["pid"]
     defaults = await inception_svc.current_defaults(owner, pid)
-    assert set(defaults) == {"design_system_id", "design_systems", "systems"}
+    assert set(defaults) == {
+        "design_system_id", "design_systems", "systems", "platforms", "project_platforms",
+    }
     assert defaults["systems"] == 0 and defaults["design_system_id"] is None
 
     out = await inception_svc.decide(owner, pid, via="mcp", choices={
         "design_system_id": None,
         "seed_systems": True,
     })
-    assert set(out["effects"]) == {"design_system_id", "systems_seeded"}
+    assert set(out["effects"]) == {"design_system_id", "systems_seeded", "platforms"}
     catalog = await canonical_svc.list_canonical_systems()
     assert len(out["effects"]["systems_seeded"]) == len(catalog)
     # Seeded Systems come out mapped, not needing a later reconciliation.
@@ -56,7 +58,9 @@ async def test_decide_applies_every_effect_and_records_last(seeded):
         project = await s.get(Project, pid)
     assert inception_svc.is_decided(project)
     assert project.inception["via"] == "mcp" and project.inception["decided_by"] == owner
-    assert project.inception["choices"] == {"design_system_id": None, "seed_systems": True}
+    assert project.inception["choices"] == {
+        "design_system_id": None, "seed_systems": True, "platforms": None,
+    }
     # Re-deciding with seed again mints nothing twice.
     again = await inception_svc.decide(owner, pid, via="ui", choices={"seed_systems": True})
     assert again["effects"]["systems_seeded"] == []
diff --git a/tests/test_integration_platforms.py b/tests/test_integration_platforms.py
new file mode 100644
index 00000000..2b152ef2
--- /dev/null
+++ b/tests/test_integration_platforms.py
@@ -0,0 +1,134 @@
+"""Real-Postgres checks for platform membership (milestone 463 step 2).
+
+The one invariant: detection only ever ADDS, and only where nobody has
+answered. A refresh that overwrote a "no" would put a project back into a
+family it was taken out of; one that overwrote a "yes" would erase who said
+it. Each is shown here against the real writer, beside the person-facing
+writes it must defer to.
+"""
+import pytest
+import pytest_asyncio
+from sqlalchemy import select
+
+from scribe.models import async_session
+from scribe.models.family import Platform
+from scribe.models.project import Project
+from scribe.services import inception as inception_svc
+from scribe.services import platforms as platforms_svc
+from tests.helpers import ensure_user
+
+pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine")]
+
+OWNER = "platforms_owner"
+
+
+async def _id(slug: str) -> int:
+    async with async_session() as s:
+        return await s.scalar(
+            select(Platform.id).where(Platform.slug == slug, Platform.deleted_at.is_(None))
+        )
+
+
+@pytest_asyncio.fixture
+async def seeded():
+    """An owner, an outsider and a fresh project. Each test gets its own
+    project, so answers never leak between tests through a shared row."""
+    async with async_session() as s:
+        owner = await ensure_user(s, OWNER)
+        outsider = await ensure_user(s, "platforms_outsider")
+        project = Project(user_id=owner.id, title="an app with a server")
+        s.add(project)
+        await s.flush()
+        ids = {"owner": owner.id, "outsider": outsider.id, "pid": project.id}
+        await s.commit()
+    return ids
+
+
+def _states(rows: list[dict]) -> dict[str, str]:
+    return {r["slug"]: r["state"] for r in rows}
+
+
+async def test_detection_adds_only_where_nobody_has_answered(seeded):
+    owner, pid = seeded["owner"], seeded["pid"]
+    await platforms_svc.set_project_platforms(owner, pid, {
+        "go": "declared", "rust": "rejected",
+    })
+    added = await platforms_svc.detect_for_project(pid, [
+        "server/go.mod", "core/Cargo.toml", "app/src/main/AndroidManifest.xml",
+    ])
+    # Only android was unanswered; go and rust keep the person's answers.
+    assert added == [await _id("android-app")]
+    states = _states(await platforms_svc.project_platforms(owner, pid))
+    assert states == {"go": "declared", "rust": "rejected", "android-app": "detected"}
+
+
+async def test_detection_never_removes_a_platform_whose_marker_is_gone(seeded):
+    owner, pid = seeded["owner"], seeded["pid"]
+    await platforms_svc.detect_for_project(pid, ["server/go.mod"])
+    await platforms_svc.detect_for_project(pid, ["README.md"])
+    assert _states(await platforms_svc.project_platforms(owner, pid)) == {"go": "detected"}
+
+
+async def test_a_withdrawn_answer_lets_detection_decide_again(seeded):
+    owner, pid = seeded["owner"], seeded["pid"]
+    await platforms_svc.set_project_platforms(owner, pid, {"go": "rejected"})
+    assert await platforms_svc.detect_for_project(pid, ["go.mod"]) == []
+    await platforms_svc.set_project_platforms(owner, pid, {"go": None})
+    assert await platforms_svc.detect_for_project(pid, ["go.mod"]) == [await _id("go")]
+
+
+async def test_a_bad_update_writes_nothing(seeded):
+    owner, pid = seeded["owner"], seeded["pid"]
+    with pytest.raises(ValueError, match="unknown platform"):
+        await platforms_svc.set_project_platforms(owner, pid, {
+            "go": "declared", "not-a-platform": "declared",
+        })
+    assert await platforms_svc.project_platforms(owner, pid) == []
+    with pytest.raises(ValueError, match="no write access"):
+        await platforms_svc.set_project_platforms(seeded["outsider"], pid, {"go": "declared"})
+    assert await platforms_svc.project_platforms(seeded["outsider"], pid) is None
+
+
+async def test_inception_records_slugs_and_rejects_what_it_leaves_out(seeded):
+    """The inception list is the WHOLE answer. Detection found Android and
+    Go; the person says the project is Go and Python — so Android becomes a
+    recorded "no", and the next refresh cannot bring it back."""
+    owner, pid = seeded["owner"], seeded["pid"]
+    await platforms_svc.detect_for_project(pid, ["app/AndroidManifest.xml", "go.mod"])
+    defaults = await inception_svc.current_defaults(owner, pid)
+    assert {"android-app", "go"} <= {p["slug"] for p in defaults["project_platforms"]}
+
+    out = await inception_svc.decide(owner, pid, via="ui", choices={
+        "seed_systems": False, "platforms": ["python", "go"],
+    })
+    assert _states(out["effects"]["platforms"]) == {
+        "android-app": "rejected", "go": "declared", "python": "declared",
+    }
+    async with async_session() as s:
+        project = await s.get(Project, pid)
+    assert project.inception["choices"]["platforms"] == ["go", "python"]
+    assert await platforms_svc.detect_for_project(pid, ["app/AndroidManifest.xml"]) == []
+
+
+async def test_an_unknown_inception_platform_applies_nothing(seeded):
+    owner, pid = seeded["owner"], seeded["pid"]
+    with pytest.raises(ValueError, match="unknown platform"):
+        await inception_svc.decide(owner, pid, via="mcp", choices={
+            "seed_systems": True, "platforms": ["go", "cobol-mainframe"],
+        })
+    assert await platforms_svc.project_platforms(owner, pid) == []
+
+
+async def test_the_catalog_is_admin_written_and_duplicate_gated():
+    async with async_session() as s:
+        admin = await ensure_user(s, "platforms_admin", role="admin")
+        user = await ensure_user(s, OWNER)
+        await s.commit()
+        admin_id, user_id = admin.id, user.id
+    assert await platforms_svc.create_platform(user_id, "Kotlin Multiplatform") is None
+    # "Android App" reduces to the seeded android-app: the existing one comes
+    # back rather than a second spelling of it.
+    dup = await platforms_svc.create_platform(admin_id, "Android App")
+    assert dup["duplicate"] and dup["existing_id"] == await _id("android-app")
+    with pytest.raises(ValueError, match="repo-relative"):
+        await platforms_svc.create_platform(admin_id, "Odd", markers=["/abs"])
diff --git a/tests/test_mcp_tool_projects.py b/tests/test_mcp_tool_projects.py
index 5e93f689..e40dd61e 100644
--- a/tests/test_mcp_tool_projects.py
+++ b/tests/test_mcp_tool_projects.py
@@ -25,6 +25,18 @@ def _no_systems():
         yield
 
 
+@pytest.fixture(autouse=True)
+def _no_platforms():
+    """enter_project and get_project carry the project's platforms (milestone
+    463). No database in this lane, so the lookup is stubbed to the common
+    case — a project with none yet. The populated shape is asserted in its
+    own test.
+    """
+    with patch("scribe.mcp.tools.projects.platforms_svc.project_platforms",
+               AsyncMock(return_value=[])):
+        yield
+
+
 @pytest.fixture(autouse=True)
 def _no_coverage():
     """enter_project also reads the pattern-coverage cache (#2692) — same
diff --git a/tests/test_milestone_summary_brief.py b/tests/test_milestone_summary_brief.py
index febea93a..eabdb007 100644
--- a/tests/test_milestone_summary_brief.py
+++ b/tests/test_milestone_summary_brief.py
@@ -24,6 +24,19 @@ PLAN = "A plan paragraph long enough to matter. " * 125  # ~5k chars
 GOAL = "What the project is for. " * 50                  # ~1.2k chars
 
 
+@pytest.fixture(autouse=True)
+def _no_platforms():
+    """enter_project and get_project carry the project's platforms (milestone
+    463). No database in this lane, so the lookup is stubbed to the common
+    case — a project with none yet. The populated shape is asserted in its
+    own test.
+    """
+    with patch("scribe.mcp.tools.projects.platforms_svc.project_platforms",
+               AsyncMock(return_value=[])):
+        yield
+
+
+
 def _milestone(mid: int, status: str, touched_day: int) -> dict:
     """A summary row as get_project_milestone_summary returns it."""
     touched = f"2026-08-{touched_day:02d}T00:00:00+00:00"
diff --git a/tests/test_pattern_coverage.py b/tests/test_pattern_coverage.py
index ca67f79d..19c1edef 100644
--- a/tests/test_pattern_coverage.py
+++ b/tests/test_pattern_coverage.py
@@ -8,11 +8,9 @@ deliberately reuse the definitions test_write_path_trigger stages for the
 hook; extending one detector means extending both, and this comment is the
 tripwire.
 """
-import io
 import json
 import shutil
 import subprocess
-import tarfile
 from pathlib import Path
 
 import pytest
@@ -29,7 +27,7 @@ from scribe.services.coverage import (
     shapes_from_archive,
 )
 from scribe.services.shape_ledger import location_covers
-from tests.helpers import ensure_user
+from tests.helpers import ensure_user, forge_tarball
 
 PLUGIN = Path(__file__).resolve().parents[1] / "plugin"
 
@@ -173,14 +171,7 @@ def test_scannable_gates_prose_vendored_and_sourcemaps():
 # --- unit: reading shapes out of a forge tarball -----------------------------
 
 
-def _tarball(files: dict[str, bytes], top: str = "widget") -> bytes:
-    buf = io.BytesIO()
-    with tarfile.open(fileobj=buf, mode="w:gz") as tar:
-        for path, data in files.items():
-            info = tarfile.TarInfo(f"{top}/{path}")
-            info.size = len(data)
-            tar.addfile(info, io.BytesIO(data))
-    return buf.getvalue()
+_tarball = forge_tarball  # shared with tests/test_platforms.py
 
 
 TREE = {
diff --git a/tests/test_platforms.py b/tests/test_platforms.py
new file mode 100644
index 00000000..71ecafc2
--- /dev/null
+++ b/tests/test_platforms.py
@@ -0,0 +1,145 @@
+"""Platforms without a database (milestone 463 step 2).
+
+Detection is pure — a catalog and a list of repo paths in, platform ids out —
+so it is pinned here against fixture trees. What it writes, and what it must
+never overwrite, is in tests/test_integration_platforms.py.
+"""
+from __future__ import annotations
+
+from types import SimpleNamespace
+
+from scribe.mcp.server import _READ_ONLY_TOOLS, _WRITE_TOOLS
+from scribe.mcp.tools.projects import _inception_choices
+from scribe.services.coverage import scan_archive
+from tests.helpers import forge_tarball
+from scribe.services.platforms import (
+    MEMBER_STATES, SETTABLE_STATES, detect, marker_matches, members,
+    validate_markers, validate_updates,
+)
+
+
+def _platform(pid: int, *markers: str) -> SimpleNamespace:
+    return SimpleNamespace(id=pid, markers=list(markers))
+
+
+# --- a single marker ----------------------------------------------------------
+
+def test_a_bare_marker_matches_the_basename_anywhere_in_the_tree():
+    assert marker_matches("go.mod", "go.mod")
+    assert marker_matches("go.mod", "services/api/go.mod")
+    assert marker_matches("AndroidManifest.xml", "app/src/main/AndroidManifest.xml")
+    assert marker_matches("vite.config.*", "frontend/vite.config.ts")
+    assert not marker_matches("go.mod", "go.mod.bak")
+    assert not marker_matches("go.mod", "docs/go.mod.md")
+
+
+def test_a_marker_with_a_slash_matches_the_whole_path_only():
+    """A directory-shaped marker cannot be satisfied by a same-named file
+    somewhere else — `.github/workflows/*` is about the repo root's CI, not
+    a vendored copy of some other project's."""
+    assert marker_matches(".github/workflows/*", ".github/workflows/ci.yml")
+    assert not marker_matches(".github/workflows/*", "vendor/x/.github/workflows/ci.yml")
+    assert not marker_matches(".github/workflows/*", "ci.yml")
+
+
+def test_a_blank_marker_matches_nothing():
+    assert not marker_matches("", "anything")
+    assert not marker_matches("   ", "anything")
+
+
+# --- detection over a tree -------------------------------------------------------
+
+ANDROID_AND_GO = [
+    "app/build.gradle.kts",
+    "app/src/main/AndroidManifest.xml",
+    "server/go.mod",
+    "server/main.go",
+    "README.md",
+]
+
+
+def test_detect_finds_every_platform_a_tree_carries_and_nothing_else():
+    catalog = [
+        _platform(1, "AndroidManifest.xml"),
+        _platform(2, "go.mod"),
+        _platform(3, "Cargo.toml"),
+        _platform(4, ".github/workflows/*"),
+    ]
+    assert detect(catalog, ANDROID_AND_GO) == [1, 2]
+
+
+def test_a_platform_with_no_markers_is_declare_only():
+    """An empty marker list is how a platform says "a person must tell you" —
+    it must not match everything, and it must not match nothing by error."""
+    catalog = [_platform(1), _platform(2, "go.mod")]
+    assert detect(catalog, ANDROID_AND_GO) == [2]
+    assert detect([SimpleNamespace(id=9, markers=None)], ANDROID_AND_GO) == []
+
+
+def test_detect_on_an_empty_tree_finds_nothing():
+    assert detect([_platform(1, "go.mod")], []) == []
+
+
+def test_the_archive_scan_carries_every_path_not_only_the_scannable_ones():
+    """The markers that say what a project IS are mostly files the shape scan
+    skips — a manifest, a go.mod, a gradle script. Detection reads the scan's
+    paths, so a scan that listed only code files would detect nothing."""
+    scan = scan_archive(forge_tarball({
+        "app/src/main/AndroidManifest.xml": b"<manifest/>",
+        "server/go.mod": b"module x\n",
+        "server/main.py": b"def main():\n    pass\n",
+    }))
+    assert set(scan.paths) == {
+        "app/src/main/AndroidManifest.xml", "server/go.mod", "server/main.py",
+    }
+    # The forge's wrapping directory is stripped, as it is for shapes.
+    assert not any(p.startswith("widget/") for p in scan.paths)
+
+
+# --- the answers a person may give -------------------------------------------------
+
+def test_detected_is_the_refreshs_to_write_never_a_persons():
+    assert "detected" in MEMBER_STATES
+    assert "detected" not in SETTABLE_STATES
+    assert "rejected" not in MEMBER_STATES
+
+
+def test_validate_updates():
+    assert validate_updates({"go": "declared", "rust": "rejected", "python": None}) is None
+    assert validate_updates({}) is None
+    assert "slug: state" in validate_updates(["go"])
+    assert "not a state a person sets" in validate_updates({"go": "detected"})
+    assert "not a state a person sets" in validate_updates({"go": "maybe"})
+    assert "named by its slug" in validate_updates({"": "declared"})
+
+
+def test_validate_markers():
+    assert validate_markers(None) is None
+    assert validate_markers(["go.mod", ".github/workflows/*"]) is None
+    assert "list of glob patterns" in validate_markers("go.mod")
+    assert "list of glob patterns" in validate_markers([1])
+    assert "repo-relative" in validate_markers(["/etc/passwd"])
+
+
+def test_members_drops_the_rejected_answers():
+    rows = [
+        {"id": 1, "slug": "go", "name": "Go", "state": "declared"},
+        {"id": 2, "slug": "rust", "name": "Rust", "state": "rejected"},
+        {"id": 3, "slug": "python", "name": "Python", "state": "detected"},
+    ]
+    assert [m["slug"] for m in members(rows)] == ["go", "python"]
+
+
+# --- the doors ------------------------------------------------------------------------
+
+def test_the_platform_tools_are_classified():
+    assert "list_platforms" in _READ_ONLY_TOOLS
+    assert "set_project_platforms" in _WRITE_TOOLS
+
+
+def test_inception_choices_carry_platforms_only_when_given():
+    """Left out, platforms stays UNSTATED (None after normalising) — which is
+    not the same answer as an empty list, "none of them"."""
+    assert "platforms" not in _inception_choices(0, True)
+    assert _inception_choices(0, True, ["go"])["platforms"] == ["go"]
+    assert _inception_choices(0, True, [])["platforms"] == []
diff --git a/tests/test_routes_platforms.py b/tests/test_routes_platforms.py
new file mode 100644
index 00000000..c7c9ad05
--- /dev/null
+++ b/tests/test_routes_platforms.py
@@ -0,0 +1,28 @@
+"""Structural tests for the platforms blueprint (milestone 463 step 2) —
+registration and the route-to-service contract. What the writes do is in
+tests/test_integration_platforms.py."""
+import inspect
+
+
+def test_platforms_blueprint_registered_in_app():
+    from scribe.app import create_app
+    app = create_app()
+    assert "platforms" in app.blueprints
+    rules = {(r.rule, m) for r in app.url_map.iter_rules() for m in r.methods}
+    for rule, method in (
+        ("/api/platforms", "GET"),
+        ("/api/platforms", "POST"),
+        ("/api/platforms/<int:platform_id>", "PATCH"),
+        ("/api/projects/<int:project_id>/platforms", "GET"),
+        ("/api/projects/<int:project_id>/platforms", "PUT"),
+    ):
+        assert (rule, method) in rules, f"{method} {rule} is not routed"
+
+
+def test_writes_to_the_catalog_and_to_a_project_take_the_caller():
+    """The catalog's admin gate and the project's write gate both live in the
+    service, so every write must be handed the caller to check."""
+    from scribe.services import platforms as svc
+    for name in ("create_platform", "update_platform", "set_project_platforms",
+                 "project_platforms"):
+        assert "user_id" in inspect.signature(getattr(svc, name)).parameters