diff --git a/frontend/src/api/lessons.ts b/frontend/src/api/lessons.ts
new file mode 100644
index 0000000..684200d
--- /dev/null
+++ b/frontend/src/api/lessons.ts
@@ -0,0 +1,146 @@
+import type { RecordUsage } from "@/types/usage";
+
+import { apiGet, apiPost, apiPatch, apiDelete } from "@/api/client";
+
+/** A lesson: a transferable insight, retrievable by the SITUATION it applies
+ *  to rather than by its topic.
+ *
+ *  The fields mirror what the backend composes and reads back
+ *  (`services/lessons.py::lesson_to_dict`), not the stored row. `title` and
+ *  `body` are DERIVED — the service builds them from `what`, `when_to_apply`
+ *  and `insight` — so an editor sends the three parts and never the document.
+ *  That is the whole design: the trigger ends up in the title and again at the
+ *  head of the body, which is what makes a lesson rank on when it applies. */
+export interface Lesson {
+  id: number;
+  /** The composed document title, `{what} — {when_to_apply}`. Read-only. */
+  title: string;
+  /** The composed body. Read-only — edit `insight` instead. */
+  body: string;
+  /** The claim itself, as you would say it. */
+  what: string;
+  /** WHEN this applies — the situation, in the words it presents itself in.
+   *  The entire retrieval story: a lesson without one saves, reads correctly
+   *  and never surfaces, so both doors refuse an empty one. */
+  when_to_apply: string;
+  /** The body with the composed lines stripped — what an edit form binds to,
+   *  so saving doesn't accumulate a copy of the trigger line per save. */
+  insight: string;
+  /** Ids of the records that taught this — issues, tasks or notes. */
+  learned_from: number[];
+  /** The same sources RESOLVED, sent by the detail route only. A bare "#4181"
+   *  on a page tells a reader nothing about whether it is worth opening, and
+   *  the provenance is the point of a lesson — one that loses its incidents
+   *  loses its evidence. A source that has been deleted drops out rather than
+   *  rendering a link to nothing. */
+  learned_from_records?: {
+    id: number;
+    title: string;
+    note_type: string;
+    is_task: boolean;
+    task_kind: string | null;
+    status: string | null;
+  }[];
+  tags: string[];
+  note_type: string;
+  /** Where it was LEARNED. Kept as a fact, but not a limit on where it can be
+   *  found: a lesson is retrievable from every project (milestone 385 step 3). */
+  project_id: number | null;
+  permission?: string;
+  created_at: string | null;
+  updated_at: string | null;
+  systems?: { id: number; name: string }[];
+  usage?: RecordUsage;
+  /** Set when another user owns this record. */
+  shared?: boolean;
+  owner?: string | null;
+}
+
+/** A row in the browse listing — the trigger travels with it, because a list
+ *  of lessons without their triggers is a list of claims with the half that
+ *  says when each one matters left off. */
+export interface LessonListRow {
+  id: number;
+  title: string;
+  tags: string[];
+  when_to_apply?: string;
+  snippet?: string;
+  shared?: boolean;
+  owner?: string | null;
+}
+
+export interface LessonListResponse {
+  lessons: LessonListRow[];
+  total: number;
+}
+
+/** What the create/update forms send. `what` and `when_to_apply` are required
+ *  on create; every field is optional on update, and the service re-composes
+ *  the whole document from the merged set — so a partial save can never leave
+ *  the title and body disagreeing about the trigger. */
+export interface LessonPayload {
+  what?: string;
+  when_to_apply?: string;
+  insight?: string;
+  learned_from?: number[];
+  tags?: string[];
+  project_id?: number | null;
+  system_ids?: number[];
+  /** Deliberate override of the near-duplicate gate, once the writer has seen
+   *  the warning. Two lessons under one trigger compete for one reserved slot,
+   *  so a duplicate displaces rather than merely clutters. */
+  force?: boolean;
+}
+
+export function listLessons(params: {
+  q?: string;
+  tag?: string;
+  project_id?: number;
+  limit?: number;
+  offset?: number;
+} = {}): Promise<LessonListResponse> {
+  const qs = new URLSearchParams();
+  if (params.q) qs.set("q", params.q);
+  if (params.tag) qs.set("tag", params.tag);
+  if (params.project_id) qs.set("project_id", String(params.project_id));
+  if (params.limit != null) qs.set("limit", String(params.limit));
+  if (params.offset != null) qs.set("offset", String(params.offset));
+  const suffix = qs.toString() ? `?${qs}` : "";
+  return apiGet<LessonListResponse>(`/api/lessons${suffix}`);
+}
+
+export function getLesson(id: number): Promise<Lesson> {
+  return apiGet<Lesson>(`/api/lessons/${id}`);
+}
+
+export function createLesson(payload: LessonPayload): Promise<Lesson> {
+  return apiPost<Lesson>("/api/lessons", payload);
+}
+
+export function updateLesson(
+  id: number,
+  payload: LessonPayload,
+): Promise<Lesson> {
+  return apiPatch<Lesson>(`/api/lessons/${id}`, payload);
+}
+
+/** Trash, not erase — recoverable. `apiDelete` discards the body, which is the
+ *  established shape here (snippets delete the same way): the batch id is in
+ *  the response, but no caller has needed it and inventing a second delete
+ *  helper to carry it would be the duplication, not the feature. */
+export function deleteLesson(id: number): Promise<void> {
+  return apiDelete(`/api/lessons/${id}`);
+}
+
+/** The lessons drawn FROM one record — the reverse of `learned_from`.
+ *
+ *  The direction that gets forgotten, and arguably the more useful one: a
+ *  reader opening an old issue wants to know what was learned from it, and
+ *  without this the relation is only navigable from the lesson's side. */
+export function lessonsTaughtBy(
+  recordId: number,
+): Promise<{ lessons: Lesson[]; taught_by: number }> {
+  return apiGet<{ lessons: Lesson[]; taught_by: number }>(
+    `/api/lessons/taught-by/${recordId}`,
+  );
+}
diff --git a/frontend/src/components/LessonsTaughtPanel.vue b/frontend/src/components/LessonsTaughtPanel.vue
new file mode 100644
index 0000000..88f93be
--- /dev/null
+++ b/frontend/src/components/LessonsTaughtPanel.vue
@@ -0,0 +1,93 @@
+<script setup lang="ts">
+/**
+ * "What was learned from this record" — the reverse of a lesson's
+ * `learned_from`.
+ *
+ * THE DIRECTION THAT GETS FORGOTTEN, and arguably the more useful one. The
+ * forward link is easy to remember because the lesson's author types it; this
+ * one has no author and so tends never to get built. A reader opening an old
+ * issue wants to know what came out of it, and without this the relation is
+ * navigable only from the lesson's side.
+ *
+ * A COMPONENT rather than markup inside the task editor, because the same
+ * question is worth answering on any record a lesson can cite — an issue, a
+ * spike, a dev-log. One panel, mounted wherever that is true, instead of the
+ * shape being written a second time the first time someone wants it on notes
+ * (#3207).
+ *
+ * SILENT WHEN EMPTY. Most records taught no lesson, and a panel that renders
+ * "None yet" on every page is a panel people learn to skip — which costs the
+ * pages where it does have something to say.
+ */
+import { onMounted, ref, watch } from "vue";
+
+import { lessonsTaughtBy, type Lesson } from "@/api/lessons";
+
+const props = defineProps<{ recordId: number }>();
+
+const lessons = ref<Lesson[]>([]);
+// No error surface on purpose: this is a secondary panel beside the record the
+// reader actually came for, and a red box about a failed side-query would be
+// louder than the thing it failed to fetch. It stays silent and stays absent.
+const loaded = ref(false);
+
+async function load() {
+  loaded.value = false;
+  lessons.value = [];
+  if (!props.recordId) return;
+  try {
+    const res = await lessonsTaughtBy(props.recordId);
+    lessons.value = res.lessons;
+  } catch {
+    lessons.value = [];
+  } finally {
+    loaded.value = true;
+  }
+}
+
+watch(() => props.recordId, load);
+onMounted(load);
+</script>
+
+<template>
+  <section v-if="loaded && lessons.length" class="ltp">
+    <h3 class="ltp-label">What was learned from this</h3>
+    <ul class="ltp-list">
+      <li v-for="l in lessons" :key="l.id" class="ltp-item">
+        <router-link :to="`/lessons/${l.id}`" class="ltp-link">
+          {{ l.what || l.title }}
+        </router-link>
+        <!-- The trigger travels with the row. A lesson listed without it is a
+             claim with the half that says when it matters left off. -->
+        <p v-if="l.when_to_apply" class="ltp-trigger">
+          {{ l.when_to_apply }}
+        </p>
+      </li>
+    </ul>
+  </section>
+</template>
+
+<style scoped>
+.ltp {
+  margin-top: 1.5rem;
+  padding-top: 1rem;
+  border-top: 1px solid var(--fs-border-color);
+}
+.ltp-label {
+  margin: 0 0 0.6rem;
+  font-size: 0.72rem;
+  font-weight: 500;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  color: var(--fs-text-tertiary);
+}
+.ltp-list { list-style: none; margin: 0; padding: 0; }
+.ltp-item { margin-bottom: 0.7rem; }
+.ltp-link { color: var(--fs-accent); font-size: 0.9rem; }
+.ltp-trigger {
+  margin: 0.15rem 0 0;
+  color: var(--fs-text-secondary);
+  font-size: 0.82rem;
+  line-height: 1.45;
+}
+</style>
diff --git a/frontend/src/router/index.ts b/frontend/src/router/index.ts
index 0d590f7..999f9db 100644
--- a/frontend/src/router/index.ts
+++ b/frontend/src/router/index.ts
@@ -69,6 +69,25 @@ const router = createRouter({
       name: "note-edit",
       component: () => import("@/views/NoteEditorView.vue"),
     },
+    {
+      // Lessons have no list view of their own: the Knowledge browse surface
+      // is where every kind is enumerated, and a second list would be a
+      // second vocabulary to keep in step with it (#3128's defect, in
+      // advance). `/knowledge?type=lesson` is the list.
+      path: "/lessons/new",
+      name: "lesson-new",
+      component: () => import("@/views/LessonEditorView.vue"),
+    },
+    {
+      path: "/lessons/:id",
+      name: "lesson-view",
+      component: () => import("@/views/LessonDetailView.vue"),
+    },
+    {
+      path: "/lessons/:id/edit",
+      name: "lesson-edit",
+      component: () => import("@/views/LessonEditorView.vue"),
+    },
     {
       path: "/snippets",
       name: "snippets",
diff --git a/frontend/src/views/KnowledgeView.vue b/frontend/src/views/KnowledgeView.vue
index 61ca964..81932ec 100644
--- a/frontend/src/views/KnowledgeView.vue
+++ b/frontend/src/views/KnowledgeView.vue
@@ -12,6 +12,7 @@ import {
   FileText,
   CheckSquare,
   Workflow,
+  Lightbulb,
   Search,
   Share2,
   ShieldCheck,
@@ -26,7 +27,7 @@ const router = useRouter();
 
 interface KnowledgeItem {
   id: number;
-  note_type: "note" | "task" | "process" | "snippet";
+  note_type: "note" | "task" | "process" | "snippet" | "lesson";
   title: string;
   snippet: string;
   tags: string[];
@@ -46,7 +47,8 @@ interface KnowledgeItem {
 
 // ─── The facet vocabulary ─────────────────────────────────────────────────────
 // Mirrors services/knowledge._FACETS, which is where it is defined for real.
-// A facet spans BOTH typing axes — a record TYPE (note / process / snippet) or
+// A facet spans BOTH typing axes — a record TYPE (note / process / snippet /
+// lesson) or
 // a task KIND (`task` for any, else issue / spike) — because that is what this
 // feed actually holds.
 //
@@ -54,7 +56,7 @@ interface KnowledgeItem {
 // it has no chip: retired in 0066, it kept a chip of its own for longer than
 // `issue` — 17% of every task here — went without one (#3128). Those rows are
 // still reachable under Tasks, wearing a Plan badge.
-type Facet = "" | "note" | "task" | "issue" | "spike" | "snippet" | "process";
+type Facet = "" | "note" | "task" | "issue" | "spike" | "snippet" | "process" | "lesson";
 
 // The facets that select TASKS. Kinds are subsets of `task`, so any of them
 // means the duplicate report should be comparing tasks.
@@ -67,6 +69,7 @@ const FACET_CHIPS: [Exclude<Facet, "">, string][] = [
   ["spike", "Spikes"],
   ["snippet", "Snippets"],
   ["process", "Processes"],
+  ["lesson", "Lessons"],
 ];
 
 // ─── View mode ────────────────────────────────────────────────────────────────
@@ -153,6 +156,11 @@ function createNew(type: string) {
   newNoteMenuOpen.value = false;
   if (type === "task") {
     router.push("/tasks/new");
+  } else if (type === "lesson") {
+    // Its own editor, not /notes/new?type=lesson: the note editor offers one
+    // markdown box, and a lesson written that way saves without a trigger and
+    // never surfaces. The form has to ASK for the field by name.
+    router.push("/lessons/new");
   } else {
     router.push(type === "note" ? "/notes/new" : `/notes/new?type=${type}`);
   }
@@ -328,6 +336,8 @@ function openItem(item: KnowledgeItem) {
     router.push(`/tasks/${item.id}`);
   } else if (item.note_type === 'snippet') {
     router.push(`/snippets/${item.id}`);
+  } else if (item.note_type === 'lesson') {
+    router.push(`/lessons/${item.id}`);
   } else {
     router.push(`/notes/${item.id}`);
   }
@@ -417,6 +427,10 @@ onUnmounted(() => {
               <Workflow :size="16" />
               Process
             </button>
+            <button @click="createNew('lesson')">
+              <Lightbulb :size="16" />
+              Lesson
+            </button>
           </div>
         </div>
 
@@ -574,6 +588,7 @@ onUnmounted(() => {
               <span v-else-if="item.note_type === 'task'">{{ item.task_kind === 'plan' ? 'Plan' : 'Task' }}</span>
               <span v-else-if="item.note_type === 'process'">Process</span>
               <span v-else-if="item.note_type === 'snippet'">Snippet</span>
+              <span v-else-if="item.note_type === 'lesson'">Lesson</span>
             </span>
             <!-- Kind sits BESIDE the type badge, not inside it: the type badge
                  speaks the vocabulary of this view's type filter (note / task /
@@ -962,7 +977,11 @@ onUnmounted(() => {
    isn't an alarm reads better as plain. Standard body pair, so the contrast
    is the one the palette already guarantees. */
 .badge--snippet,
-.badge--process { background: var(--fs-surface-raised); color: var(--fs-text-secondary); }
+.badge--process,
+/* A lesson joins the neutral pair for the same reason, and one of its own: it
+   binds nobody. A hue here would make the softest record in the corpus look
+   like the loudest, beside a rule that actually is binding. */
+.badge--lesson { background: var(--fs-surface-raised); color: var(--fs-text-secondary); }
 
 .k-card-body { flex: 1; padding-right: 40px; }
 .k-card-title {
diff --git a/frontend/src/views/LessonDetailView.vue b/frontend/src/views/LessonDetailView.vue
new file mode 100644
index 0000000..53f46de
--- /dev/null
+++ b/frontend/src/views/LessonDetailView.vue
@@ -0,0 +1,288 @@
+<script setup lang="ts">
+/**
+ * Read a LESSON, and see both ends of what it came from.
+ *
+ * THE TRIGGER LEADS. A lesson is retrieved by the situation it applies to, so
+ * the page opens with that situation rather than with the claim — the same
+ * ordering the editor uses, and for the same reason: the trigger is the half
+ * a reader needs first to decide whether this is for them.
+ *
+ * GLOBAL BY DEFAULT HAS TO BE LEGIBLE. A lesson written on one project will
+ * surface on another, which reads as a bug unless the page says why. The
+ * origin line does exactly that, and says it as a property of the kind rather
+ * than as an apology.
+ *
+ * IT BINDS NOBODY, and the page says so once, plainly. That sentence is the
+ * difference between this kind and a rule, and it is the reason the kind
+ * exists (#3727) — sessions were proposing rules for things that should never
+ * have bound anyone.
+ */
+import { computed, onMounted, ref, watch } from "vue";
+import { useRoute, useRouter } from "vue-router";
+
+import { apiErrorMessage } from "@/api/client";
+import { deleteLesson, getLesson, type Lesson } from "@/api/lessons";
+import ConfirmDialog from "@/components/ConfirmDialog.vue";
+import TagPill from "@/components/TagPill.vue";
+import { useToastStore } from "@/stores/toast";
+import { renderMarkdown } from "@/utils/markdown";
+
+const route = useRoute();
+const router = useRouter();
+const toast = useToastStore();
+
+const lesson = ref<Lesson | null>(null);
+const loading = ref(true);
+const error = ref<string | null>(null);
+const confirming = ref(false);
+
+const lessonId = computed(() => Number(route.params.id));
+
+/** The insight rendered — the body minus the lines the service composed, so
+ *  the trigger is not printed twice on a page that already leads with it. */
+const insightHtml = computed(() =>
+  lesson.value?.insight ? renderMarkdown(lesson.value.insight) : "",
+);
+
+/** Where each source opens. A task and a note live at different routes, and a
+ *  link that guesses wrong is worse than one that is plain. */
+function sourceHref(rec: { id: number; is_task?: boolean; note_type?: string }) {
+  if (rec.is_task) return `/tasks/${rec.id}`;
+  if (rec.note_type === "snippet") return `/snippets/${rec.id}`;
+  if (rec.note_type === "lesson") return `/lessons/${rec.id}`;
+  return `/notes/${rec.id}`;
+}
+
+async function load() {
+  loading.value = true;
+  error.value = null;
+  try {
+    lesson.value = await getLesson(lessonId.value);
+  } catch (e) {
+    error.value = apiErrorMessage(e, "Failed to load this lesson");
+  } finally {
+    loading.value = false;
+  }
+}
+
+async function remove() {
+  confirming.value = false;
+  try {
+    await deleteLesson(lessonId.value);
+    toast.show("Lesson moved to trash");
+    router.push("/knowledge?type=lesson");
+  } catch (e) {
+    toast.show(apiErrorMessage(e, "Failed to delete this lesson"), "error");
+  }
+}
+
+watch(lessonId, load);
+onMounted(load);
+</script>
+
+<template>
+  <div class="lesson-detail">
+    <p v-if="loading" class="ld-muted">Loading…</p>
+    <p v-else-if="error" class="ld-error" role="alert">{{ error }}</p>
+
+    <article v-else-if="lesson">
+      <header class="ld-head">
+        <span class="ld-kind">Lesson</span>
+        <!-- Said once, plainly. It is the whole difference from a rule. -->
+        <span class="ld-binds">binds nobody</span>
+      </header>
+
+      <!-- The trigger leads, in its own block: it is what a reader needs first
+           to decide whether this applies to them. -->
+      <section class="ld-trigger">
+        <h2 class="ld-trigger-label">When this applies</h2>
+        <p class="ld-trigger-text">{{ lesson.when_to_apply }}</p>
+      </section>
+
+      <h1 class="ld-what">{{ lesson.what }}</h1>
+
+      <div
+        v-if="insightHtml"
+        class="ld-insight markdown-body"
+        v-html="insightHtml"
+      />
+      <p v-else class="ld-muted ld-empty">
+        No detail was recorded — the claim above is the whole lesson.
+      </p>
+
+      <!-- What taught it. The provenance is the point: a lesson that loses its
+           incidents loses its evidence. -->
+      <section
+        v-if="lesson.learned_from_records?.length"
+        class="ld-panel"
+      >
+        <h2 class="ld-panel-label">Learned from</h2>
+        <ul class="ld-sources">
+          <li v-for="rec in lesson.learned_from_records" :key="rec.id">
+            <router-link :to="sourceHref(rec)" class="ld-link">
+              {{ rec.title }}
+            </router-link>
+            <span v-if="rec.status" class="ld-source-status">{{ rec.status }}</span>
+          </li>
+        </ul>
+      </section>
+
+      <section v-if="lesson.tags?.length" class="ld-tags">
+        <TagPill v-for="t in lesson.tags" :key="t" :tag="t" />
+      </section>
+
+      <!-- Global-by-default, stated as a property rather than an apology. A
+           lesson meeting you on a project it was not written on is the kind
+           working, and the page has to say so or it reads as a bug. -->
+      <footer class="ld-origin">
+        <p v-if="lesson.project_id">
+          Learned on another project, and offered everywhere — a lesson is
+          retrieved by the situation it names, never by where it was written.
+        </p>
+        <p v-else>
+          Not tied to a project. Offered wherever the situation it names comes
+          up.
+        </p>
+      </footer>
+
+      <div class="ld-actions">
+        <router-link :to="`/lessons/${lesson.id}/edit`" class="ld-primary">
+          Edit
+        </router-link>
+        <button type="button" class="ld-ghost" @click="confirming = true">
+          Delete
+        </button>
+      </div>
+    </article>
+
+    <ConfirmDialog
+      v-if="confirming"
+      title="Delete this lesson?"
+      message="It moves to the trash and can be restored."
+      @confirm="remove"
+      @cancel="confirming = false"
+    />
+  </div>
+</template>
+
+<style scoped>
+.lesson-detail {
+  max-width: 780px;
+  margin: 0 auto;
+  padding: var(--fs-layout-page-pad);
+  color: var(--fs-text-primary);
+}
+
+.ld-head {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  margin-bottom: 1rem;
+}
+/* Neutral, matching the browse badge. A hue here would make the softest
+   record in the corpus look like the loudest. */
+.ld-kind {
+  padding: 0.15rem 0.5rem;
+  border-radius: 10px;
+  background: var(--fs-surface-raised);
+  color: var(--fs-text-secondary);
+  font-size: 0.7rem;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  font-weight: 500;
+}
+.ld-binds { color: var(--fs-text-tertiary); font-size: 0.8rem; }
+
+.ld-trigger {
+  padding: 0.9rem 1rem;
+  border-left: 2px solid var(--fs-accent);
+  border-radius: var(--fs-radius-sm);
+  background: var(--fs-surface-raised);
+  margin-bottom: 1rem;
+}
+.ld-trigger-label {
+  margin: 0 0 0.3rem;
+  font-size: 0.72rem;
+  font-weight: 500;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  color: var(--fs-text-tertiary);
+}
+.ld-trigger-text { margin: 0; font-size: 0.98rem; line-height: 1.5; }
+
+.ld-what {
+  margin: 0 0 1.25rem;
+  font-size: 1.25rem;
+  font-weight: 500;
+  line-height: 1.35;
+}
+
+.ld-insight { line-height: 1.6; font-size: 0.93rem; }
+.ld-empty { font-style: italic; }
+
+.ld-panel {
+  margin-top: 1.75rem;
+  padding-top: 1rem;
+  border-top: 1px solid var(--fs-border-color);
+}
+.ld-panel-label {
+  margin: 0 0 0.5rem;
+  font-size: 0.72rem;
+  font-weight: 500;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  color: var(--fs-text-tertiary);
+}
+.ld-sources { margin: 0; padding-left: 1.1rem; }
+.ld-sources li { margin-bottom: 0.3rem; font-size: 0.9rem; }
+.ld-link { color: var(--fs-accent); }
+.ld-source-status {
+  margin-left: 0.4rem;
+  color: var(--fs-text-tertiary);
+  font-size: 0.78rem;
+}
+
+.ld-tags { display: flex; flex-wrap: wrap; gap: 0.35rem; margin-top: 1.25rem; }
+
+.ld-origin {
+  margin-top: 1.5rem;
+  color: var(--fs-text-secondary);
+  font-size: 0.82rem;
+  line-height: 1.5;
+}
+.ld-origin p { margin: 0; }
+
+.ld-actions {
+  display: flex;
+  gap: 0.6rem;
+  margin-top: 1.75rem;
+  padding-top: 1rem;
+  border-top: 1px solid var(--fs-border-color);
+}
+.ld-primary {
+  padding: 0.45rem 1rem;
+  border-radius: var(--fs-radius-sm);
+  background: var(--fs-accent);
+  color: var(--fs-accent-fg);
+  font-size: 0.88rem;
+}
+.ld-ghost {
+  padding: 0.45rem 1rem;
+  border: 1px solid var(--fs-border-color);
+  border-radius: var(--fs-radius-sm);
+  background: transparent;
+  color: var(--fs-text-primary);
+  font: inherit;
+  font-size: 0.88rem;
+  cursor: pointer;
+}
+
+.ld-muted { color: var(--fs-text-secondary); font-size: 0.88rem; }
+.ld-error {
+  padding: 0.6rem 0.8rem;
+  border-radius: var(--fs-radius-sm);
+  background: color-mix(in srgb, var(--fs-error) 12%, var(--fs-surface-raised));
+  color: color-mix(in srgb, var(--fs-error) 55%, var(--fs-text-primary));
+  font-size: 0.88rem;
+}
+</style>
diff --git a/frontend/src/views/LessonEditorView.vue b/frontend/src/views/LessonEditorView.vue
new file mode 100644
index 0000000..3cbaf3b
--- /dev/null
+++ b/frontend/src/views/LessonEditorView.vue
@@ -0,0 +1,385 @@
+<script setup lang="ts">
+/**
+ * Write or edit a LESSON — a transferable insight, found by the situation it
+ * applies to rather than by its topic.
+ *
+ * THREE FIELDS, NOT ONE MARKDOWN BOX. The title and body are composed by the
+ * service from `what`, `when_to_apply` and `insight`; this form never sends a
+ * document. That is the design milestone 385 step 1 settled on, and the
+ * evidence for it is blunt: the snippet corpus carries a trigger on every
+ * record with no guard anywhere, because a service composes the title from a
+ * named parameter. What is at 100% is a named structured field — not a writer
+ * remembering a convention.
+ *
+ * THE TRIGGER IS THE FIELD THAT CANNOT BE MISSED, so it is given the most
+ * room, its own explanation, and a save button that refuses without it. A
+ * lesson with no trigger saves, reads correctly in every listing, and never
+ * surfaces — and there is nothing to notice afterwards, because it looks
+ * exactly like a lesson that works. The form is where that gets caught.
+ */
+import { computed, onMounted, ref, watch } from "vue";
+import { useRoute, useRouter } from "vue-router";
+
+import { apiErrorMessage } from "@/api/client";
+import {
+  createLesson,
+  getLesson,
+  updateLesson,
+  type Lesson,
+} from "@/api/lessons";
+import ProjectSelector from "@/components/ProjectSelector.vue";
+import TagInput from "@/components/TagInput.vue";
+import { useNotesStore } from "@/stores/notes";
+import { useToastStore } from "@/stores/toast";
+
+const route = useRoute();
+const router = useRouter();
+const toast = useToastStore();
+const notesStore = useNotesStore();
+
+const lessonId = computed(() => {
+  const raw = route.params.id;
+  return raw ? Number(raw) : null;
+});
+const isEdit = computed(() => lessonId.value !== null);
+
+const what = ref("");
+const whenToApply = ref("");
+const insight = ref("");
+const tags = ref<string[]>([]);
+const projectId = ref<number | null>(null);
+const learnedFrom = ref<number[]>([]);
+
+const loading = ref(false);
+const saving = ref(false);
+const error = ref<string | null>(null);
+// The near-duplicate gate's answer, held so the writer can read it and then
+// decide — rather than being silently overridden or silently blocked.
+const duplicate = ref<{ id: number; title: string } | null>(null);
+
+/** The composed title, shown live. The writer is agreeing to a document they
+ *  can see, which is the same reason `create_rule` shows a rule's statement
+ *  before asking for a yes. */
+const previewTitle = computed(() => {
+  const subject = what.value.trim();
+  const trigger = whenToApply.value.trim();
+  if (subject && trigger) return `${subject} — ${trigger}`;
+  return subject || trigger;
+});
+
+const canSave = computed(
+  () => what.value.trim().length > 0 && whenToApply.value.trim().length > 0,
+);
+
+async function load() {
+  if (!isEdit.value || lessonId.value === null) return;
+  loading.value = true;
+  error.value = null;
+  try {
+    const lesson: Lesson = await getLesson(lessonId.value);
+    what.value = lesson.what;
+    whenToApply.value = lesson.when_to_apply;
+    // The insight WITHOUT the composed lines, so saving doesn't accumulate a
+    // copy of the trigger line on every edit.
+    insight.value = lesson.insight;
+    tags.value = lesson.tags ?? [];
+    projectId.value = lesson.project_id;
+    learnedFrom.value = lesson.learned_from ?? [];
+  } catch (e) {
+    error.value = apiErrorMessage(e, "Failed to load this lesson");
+  } finally {
+    loading.value = false;
+  }
+}
+
+async function save(force = false) {
+  if (!canSave.value || saving.value) return;
+  saving.value = true;
+  error.value = null;
+  duplicate.value = null;
+  try {
+    const payload = {
+      what: what.value.trim(),
+      when_to_apply: whenToApply.value.trim(),
+      insight: insight.value,
+      tags: tags.value,
+      learned_from: learnedFrom.value,
+      project_id: projectId.value,
+      ...(force ? { force: true } : {}),
+    };
+    const saved = isEdit.value && lessonId.value !== null
+      ? await updateLesson(lessonId.value, payload)
+      : await createLesson(payload);
+    toast.show(isEdit.value ? "Lesson updated" : "Lesson recorded");
+    router.push(`/lessons/${saved.id}`);
+  } catch (e) {
+    // A 409 is the duplicate gate, not a failure: it hands back the record
+    // that already covers this moment so the writer can improve that one
+    // instead of standing a second beside it.
+    const body = (e as { status?: number; body?: Record<string, unknown> });
+    if (body?.status === 409 && body.body) {
+      const existing = body.body as { id?: number; title?: string };
+      if (existing.id) {
+        duplicate.value = { id: existing.id, title: existing.title ?? "" };
+        return;
+      }
+    }
+    error.value = apiErrorMessage(e, "Failed to save this lesson");
+  } finally {
+    saving.value = false;
+  }
+}
+
+watch(lessonId, load);
+onMounted(() => {
+  load();
+  // Pre-fill from the link that brought you here, the same convention the
+  // snippet editor uses. Left null it is a lesson with no recorded origin,
+  // which is valid — `project_id` says where a lesson was LEARNED and was
+  // never the limit on where it can be found (step 3).
+  if (!isEdit.value && route.query.projectId) {
+    projectId.value = Number(route.query.projectId);
+  }
+});
+</script>
+
+<template>
+  <div class="lesson-editor">
+    <header class="le-head">
+      <h1>{{ isEdit ? "Edit lesson" : "Record a lesson" }}</h1>
+      <p class="le-sub">
+        Something worth knowing, kept so a later session meets it at the moment
+        it applies. A lesson binds nobody.
+      </p>
+    </header>
+
+    <p v-if="error" class="le-error" role="alert">{{ error }}</p>
+
+    <div v-if="duplicate" class="le-dupe" role="alert">
+      <p>
+        <strong>A lesson already covers this moment.</strong>
+        Improving that one keeps what was learned in a single place — two
+        lessons under one trigger compete for the same slot, so the second
+        displaces the first rather than adding to it.
+      </p>
+      <div class="le-dupe-actions">
+        <router-link :to="`/lessons/${duplicate.id}`" class="le-link">
+          Open “{{ duplicate.title }}”
+        </router-link>
+        <button type="button" class="le-ghost" @click="save(true)">
+          Record it anyway
+        </button>
+      </div>
+    </div>
+
+    <p v-if="loading" class="le-muted">Loading…</p>
+
+    <form v-else class="le-form" @submit.prevent="save()">
+      <!-- The trigger comes FIRST, before the claim. It is what makes a lesson
+           findable, and putting it second invites it to be treated as an
+           afterthought to the thing the writer arrived wanting to say. -->
+      <label class="le-field le-field--primary">
+        <span class="le-label">When does this apply?</span>
+        <span class="le-hint">
+          The situation, in the words it will present itself in — what someone
+          would be seeing, saying, or about to do. “A test fails on code you
+          believe is correct” is a trigger. “Testing” is a topic, and a topic
+          matches everything and surfaces for nothing.
+        </span>
+        <textarea
+          v-model="whenToApply"
+          class="le-input le-textarea"
+          rows="3"
+          required
+          placeholder="a CI run has sat in_progress far longer than its suite takes"
+        />
+      </label>
+
+      <label class="le-field">
+        <span class="le-label">What did you learn?</span>
+        <span class="le-hint">The claim itself, in one line, as you would say it.</span>
+        <input
+          v-model="what"
+          class="le-input"
+          type="text"
+          required
+          placeholder="Read the job log before waiting longer"
+        />
+      </label>
+
+      <label class="le-field">
+        <span class="le-label">The detail <em>(optional)</em></span>
+        <span class="le-hint">
+          What you would want handed to you in the same situation next time —
+          the evidence, the reasoning, the thing that is not obvious.
+        </span>
+        <textarea
+          v-model="insight"
+          class="le-input le-textarea le-textarea--tall"
+          rows="10"
+        />
+      </label>
+
+      <label class="le-field">
+        <span class="le-label">Tags</span>
+        <TagInput
+          v-model="tags"
+          :fetchTags="(q: string) => notesStore.fetchAllTags(q)"
+        />
+      </label>
+
+      <label class="le-field">
+        <span class="le-label">Learned on <em>(optional)</em></span>
+        <span class="le-hint">
+          Where this was learned. Kept as a fact about its origin — a lesson is
+          retrievable from every project regardless, which is the point of the
+          kind.
+        </span>
+        <ProjectSelector v-model="projectId" />
+      </label>
+
+      <!-- The composed document, shown before saving. The writer is agreeing
+           to a title they can read, not to one assembled out of sight. -->
+      <div v-if="previewTitle" class="le-preview">
+        <span class="le-preview-label">Stored as</span>
+        <p class="le-preview-title">{{ previewTitle }}</p>
+      </div>
+
+      <div class="le-actions">
+        <button type="submit" class="le-primary" :disabled="!canSave || saving">
+          {{ saving ? "Saving…" : isEdit ? "Save lesson" : "Record lesson" }}
+        </button>
+        <button type="button" class="le-ghost" @click="router.back()">
+          Cancel
+        </button>
+        <!-- Says WHY it is disabled. A greyed button with no reason is the
+             thing that gets clicked repeatedly and then worked around. -->
+        <span v-if="!canSave" class="le-muted le-why">
+          A lesson needs both a trigger and a claim — without the trigger it
+          would save and never reach anyone.
+        </span>
+      </div>
+    </form>
+  </div>
+</template>
+
+<style scoped>
+.lesson-editor {
+  max-width: 820px;
+  margin: 0 auto;
+  padding: var(--fs-layout-page-pad);
+  color: var(--fs-text-primary);
+}
+
+.le-head h1 {
+  margin: 0 0 0.25rem;
+  font-size: 1.35rem;
+  font-weight: 500;
+}
+.le-sub {
+  margin: 0 0 1.5rem;
+  color: var(--fs-text-secondary);
+  font-size: 0.9rem;
+}
+
+.le-form { display: flex; flex-direction: column; gap: 1.25rem; }
+
+.le-field { display: flex; flex-direction: column; gap: 0.35rem; }
+
+/* The trigger gets visible weight, because it is the field whose absence is
+   invisible afterwards. */
+.le-field--primary {
+  padding: 1rem;
+  border: 1px solid var(--fs-border-color);
+  border-radius: var(--fs-radius-lg);
+  background: var(--fs-surface-raised);
+}
+
+.le-label { font-weight: 500; font-size: 0.9rem; }
+.le-label em { font-style: normal; color: var(--fs-text-tertiary); font-weight: 400; }
+.le-hint {
+  color: var(--fs-text-secondary);
+  font-size: 0.82rem;
+  line-height: 1.45;
+}
+
+.le-input {
+  width: 100%;
+  padding: 0.55rem 0.7rem;
+  border: 1px solid var(--fs-border-color);
+  border-radius: var(--fs-radius-sm);
+  background: var(--fs-surface-page);
+  color: var(--fs-text-primary);
+  font: inherit;
+  font-size: 0.92rem;
+}
+.le-input:focus-visible {
+  outline: 2px solid var(--fs-accent);
+  outline-offset: 1px;
+}
+.le-textarea { resize: vertical; line-height: 1.5; }
+.le-textarea--tall { font-family: var(--fs-font-mono); font-size: 0.85rem; }
+
+.le-preview {
+  padding: 0.7rem 0.9rem;
+  border-left: 2px solid var(--fs-accent);
+  background: var(--fs-surface-raised);
+  border-radius: var(--fs-radius-sm);
+}
+.le-preview-label {
+  display: block;
+  font-size: 0.72rem;
+  text-transform: uppercase;
+  letter-spacing: 0.04em;
+  color: var(--fs-text-tertiary);
+}
+.le-preview-title { margin: 0.2rem 0 0; font-size: 0.92rem; }
+
+.le-actions { display: flex; flex-wrap: wrap; align-items: center; gap: 0.6rem; }
+
+.le-primary {
+  padding: 0.5rem 1.1rem;
+  border: none;
+  border-radius: var(--fs-radius-sm);
+  background: var(--fs-accent);
+  color: var(--fs-accent-fg);
+  font: inherit;
+  font-size: 0.9rem;
+  cursor: pointer;
+}
+.le-primary:disabled { opacity: 0.5; cursor: not-allowed; }
+
+.le-ghost {
+  padding: 0.5rem 1rem;
+  border: 1px solid var(--fs-border-color);
+  border-radius: var(--fs-radius-sm);
+  background: transparent;
+  color: var(--fs-text-primary);
+  font: inherit;
+  font-size: 0.9rem;
+  cursor: pointer;
+}
+
+.le-muted { color: var(--fs-text-secondary); font-size: 0.85rem; }
+.le-why { flex-basis: 100%; }
+
+.le-error {
+  padding: 0.6rem 0.8rem;
+  border-radius: var(--fs-radius-sm);
+  background: color-mix(in srgb, var(--fs-error) 12%, var(--fs-surface-raised));
+  color: color-mix(in srgb, var(--fs-error) 55%, var(--fs-text-primary));
+  font-size: 0.88rem;
+}
+
+.le-dupe {
+  padding: 0.8rem 1rem;
+  margin-bottom: 1rem;
+  border: 1px solid var(--fs-border-color);
+  border-radius: var(--fs-radius-lg);
+  background: var(--fs-surface-raised);
+  font-size: 0.88rem;
+}
+.le-dupe p { margin: 0 0 0.6rem; line-height: 1.5; }
+.le-dupe-actions { display: flex; flex-wrap: wrap; gap: 0.6rem; align-items: center; }
+.le-link { color: var(--fs-accent); }
+</style>
diff --git a/frontend/src/views/TaskEditorView.vue b/frontend/src/views/TaskEditorView.vue
index 4212ca5..0f0ca64 100644
--- a/frontend/src/views/TaskEditorView.vue
+++ b/frontend/src/views/TaskEditorView.vue
@@ -23,6 +23,7 @@ import WordCount from "@/components/WordCount.vue";
 import TagInput from "@/components/TagInput.vue";
 import ProjectSelector from "@/components/ProjectSelector.vue";
 import MilestoneSelector from "@/components/MilestoneSelector.vue";
+import LessonsTaughtPanel from "@/components/LessonsTaughtPanel.vue";
 import TaskLogSection from "@/components/TaskLogSection.vue";
 import DiffView from "@/components/DiffView.vue";
 import ConfirmDialog from "@/components/ConfirmDialog.vue";
@@ -546,6 +547,13 @@ useEditorGuards(dirty, save);
         <!-- Work log -->
         <TaskLogSection v-if="taskId" :task-id="taskId" class="body-log" />
 
+        <!-- What came OUT of this record. The forward link is typed by
+             the lesson's author; this direction has no author and so
+             tends never to get built — but a reader opening an old
+             issue wants to know what was learned from it. Silent when
+             there is nothing, because most records taught no lesson. -->
+        <LessonsTaughtPanel v-if="taskId" :record-id="taskId" />
+
         <!-- Applicable rules (plan tasks only) -->
         <PlanRulesPanel
           v-if="store.currentTask?.task_kind === 'plan' && store.currentTask?.project_id"
diff --git a/src/scribe/app.py b/src/scribe/app.py
index 4b32508..8f82ac6 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.lessons import lessons_bp
 from scribe.routes.snippets import snippets_bp
 from scribe.routes.webhooks import webhooks_bp
 from scribe.mcp import mount_mcp
@@ -92,6 +93,7 @@ def create_app() -> Quart:
     app.register_blueprint(search_bp)
     app.register_blueprint(profile_bp)
     app.register_blueprint(knowledge_bp)
+    app.register_blueprint(lessons_bp)
     app.register_blueprint(rulebooks_bp)
     app.register_blueprint(plugin_bp)
     app.register_blueprint(design_systems_bp)
diff --git a/src/scribe/mcp/tools/lessons.py b/src/scribe/mcp/tools/lessons.py
index 9409b04..05bd134 100644
--- a/src/scribe/mcp/tools/lessons.py
+++ b/src/scribe/mcp/tools/lessons.py
@@ -20,21 +20,10 @@ from scribe.mcp.tools import systems as systems_tools
 from scribe.services.note_usage import record_pulled
 
 
-def _to_dict(note) -> dict:
-    """A lesson as the tools return it — the composed fields read back out,
-    not the raw row, so a caller sees the same vocabulary it wrote with."""
-    return {
-        "id": note.id,
-        "title": note.title,
-        "body": note.body,
-        "when_to_apply": lessons_svc.lesson_trigger(note),
-        "learned_from": lessons_svc.lesson_sources(note),
-        "tags": list(note.tags or []),
-        "project_id": note.project_id,
-        "note_type": note.note_type,
-        "created_at": note.created_at.isoformat() if note.created_at else None,
-        "updated_at": note.updated_at.isoformat() if note.updated_at else None,
-    }
+# The payload shape lives in the service (`lesson_to_dict`), shared with the
+# REST door — a shape spelled once per door answers the two of them
+# differently the first time a field is added.
+_to_dict = lessons_svc.lesson_to_dict
 
 
 async def list_lessons(
diff --git a/src/scribe/routes/lessons.py b/src/scribe/routes/lessons.py
new file mode 100644
index 0000000..a974467
--- /dev/null
+++ b/src/scribe/routes/lessons.py
@@ -0,0 +1,256 @@
+"""REST routes for lessons — a transferable insight, retrievable by situation.
+
+A lesson is a note with note_type='lesson' (see services/lessons.py). These
+routes feed the web UI; the MCP tools (mcp/tools/lessons.py) are the
+agent-facing surface. Both go through services/lessons.py, so the compose/parse
+contract and the `data` mirror live in one place — the same division snippets
+use, and for the same reason: two doors that each compose a lesson would
+compose it two ways, and the document IS what ranks.
+
+ACL (rule #78): reads and writes of a single lesson resolve through the
+share-aware `get_lesson` / `can_write_note`, and writes are performed as the
+OWNER so a shared editor isn't rejected by the owner-scoped service —
+mirroring routes/snippets.py and routes/notes.py.
+
+WHY THE TRIGGER IS A NAMED FIELD HERE TOO. The web editor could have posted a
+body and let the service parse it. It doesn't, because the evidence behind
+this kind (step 1) is that a trigger gets filled when a door ASKS for it by
+name — the snippet corpus is at 100% on its trigger with no guard anywhere,
+because a service composes the title from a parameter. A form that offered one
+markdown box would be the option milestone 385 rejected, wearing a different
+hat.
+"""
+import logging
+
+from quart import Blueprint, jsonify, request
+
+from scribe.auth import get_current_user_id, login_required
+from scribe.routes.utils import not_found, parse_pagination
+from scribe.services import dedup as dedup_svc
+from scribe.services import knowledge as knowledge_svc
+from scribe.services import lessons as lessons_svc
+from scribe.services import systems as systems_svc
+from scribe.services import trash as trash_svc
+from scribe.services.access import (
+    can_write_note,
+    describe_provenance,
+    label_shared_items,
+)
+from scribe.services.note_usage import empty_usage, record_pulled, usage_for_notes
+
+logger = logging.getLogger(__name__)
+
+lessons_bp = Blueprint("lessons", __name__, url_prefix="/api/lessons")
+
+
+@lessons_bp.route("", methods=["GET"])
+@login_required
+async def list_lessons_route():
+    """The kind enumerated, rather than only what a query resembles.
+
+    Semantic search is how a lesson REACHES a session; this is how a person
+    sees what exists at all. Each row carries its trigger, because a list of
+    lessons without them is a list of claims with the half that says when each
+    one matters left off.
+    """
+    uid = get_current_user_id()
+    q = request.args.get("q") or None
+    tag = request.args.get("tag", "")
+    try:
+        project_id = int(request.args.get("project_id", 0) or 0) or None
+    except (TypeError, ValueError):
+        project_id = None
+    limit, offset = parse_pagination(default_limit=24, max_limit=100)
+
+    items, total = await knowledge_svc.query_knowledge(
+        user_id=uid,
+        note_type=lessons_svc.LESSON_NOTE_TYPE,
+        tags=[tag] if tag else [],
+        sort="modified",
+        q=q,
+        limit=limit,
+        offset=offset,
+        project_id=project_id,
+    )
+    return jsonify({
+        "lessons": await label_shared_items(uid, items),
+        "total": total,
+    })
+
+
+@lessons_bp.route("/taught-by/<int:record_id>", methods=["GET"])
+@login_required
+async def lessons_taught_by_route(record_id: int):
+    """The lessons drawn FROM one record — the reverse of `learned_from`.
+
+    Registered ABOVE the `/<int:lesson_id>` routes on purpose: Quart matches
+    in registration order, and `taught-by` would otherwise never be reached
+    if the converter ever widened. The same ordering snippets' `/duplicates`
+    route documents.
+
+    This is the direction that gets forgotten. A reader opening an old issue
+    wants to know what was learned from it, and without this the relation is
+    only navigable from the lesson's side.
+    """
+    uid = get_current_user_id()
+    notes = await lessons_svc.lessons_taught_by(uid, record_id)
+    return jsonify({
+        "lessons": [lessons_svc.lesson_to_dict(n) for n in notes],
+        "taught_by": record_id,
+    })
+
+
+@lessons_bp.route("", methods=["POST"])
+@login_required
+async def create_lesson_route():
+    uid = get_current_user_id()
+    data = await request.get_json() or {}
+    what = (data.get("what") or "").strip()
+    when_to_apply = (data.get("when_to_apply") or "").strip()
+    if not what:
+        return jsonify({"error": "what is required"}), 400
+    # The trigger is not optional at this door even though the service will
+    # store a lesson without one. A lesson with no trigger saves, reads
+    # correctly in every listing, and never surfaces — there is nothing to
+    # notice afterwards, which is exactly why the form has to refuse it here
+    # rather than leave the writer a record that looks finished.
+    if not when_to_apply:
+        return jsonify({
+            "error": "when_to_apply is required",
+            "detail": (
+                "A lesson is found by the SITUATION it applies to. Without a "
+                "trigger it still saves and still reads correctly, and it "
+                "never reaches anyone — so it is refused here rather than "
+                "stored as a record that looks finished."
+            ),
+        }), 400
+
+    project_id = data.get("project_id") or None
+    learned_from = data.get("learned_from") or []
+
+    # The same near-duplicate gate the MCP create path applies. Two lessons
+    # under one trigger compete in a single ranked list for one reserved slot,
+    # so the duplicate does not merely clutter — it displaces.
+    if not data.get("force"):
+        title, body = lessons_svc.lesson_document(
+            what, when_to_apply, data.get("insight", ""), learned_from,
+        )
+        dup = await dedup_svc.find_duplicate_note(
+            uid, title, body,
+            project_id=project_id,
+            is_task=False,
+            note_type=lessons_svc.LESSON_NOTE_TYPE,
+        )
+        if dup is not None:
+            return jsonify(dedup_svc.duplicate_response(dup, "lesson")), 409
+
+    note = await lessons_svc.create_lesson(
+        uid,
+        what=what,
+        when_to_apply=when_to_apply,
+        insight=data.get("insight", ""),
+        learned_from=learned_from,
+        tags=data.get("tags"),
+        project_id=project_id,
+    )
+    if data.get("system_ids") is not None:
+        await systems_svc.set_record_systems(uid, note.id, data["system_ids"])
+    out = lessons_svc.lesson_to_dict(note)
+    out["systems"] = [
+        s.to_dict() for s in await systems_svc.list_record_systems(uid, note.id)
+    ]
+    return jsonify(out), 201
+
+
+@lessons_bp.route("/<int:lesson_id>", methods=["GET"])
+@login_required
+async def get_lesson_route(lesson_id: int):
+    uid = get_current_user_id()
+    note = await lessons_svc.get_lesson(uid, lesson_id)
+    if note is None:
+        return not_found("Lesson")
+    out = lessons_svc.lesson_to_dict(note)
+    # As the OWNER: a shared reader isn't scoped to the owner's project, so
+    # their own id would come back empty (the write-as-owner pattern this
+    # module already uses, read side).
+    out["systems"] = [
+        s.to_dict()
+        for s in await systems_svc.list_record_systems(note.user_id, lesson_id)
+    ]
+    # Resolved, not bare ids: "#4181" on a page tells a reader nothing about
+    # whether it is worth opening, and the provenance is the point of a lesson.
+    out["learned_from_records"] = await lessons_svc.source_records(
+        uid, out["learned_from"]
+    )
+    out.update(await describe_provenance(uid, note))
+    out["usage"] = (await usage_for_notes([lesson_id])).get(
+        lesson_id, empty_usage()
+    )
+    # Opening the detail view IS a pull — the operator chose to look. Tagged
+    # apart from the MCP sources so "an agent was handed it" and "a human read
+    # it" stay distinguishable; they mean different things for pruning (#2085).
+    record_pulled(user_id=uid, note_id=lesson_id, source="rest_lesson")
+    return jsonify(out)
+
+
+@lessons_bp.route("/<int:lesson_id>", methods=["PATCH"])
+@login_required
+async def update_lesson_route(lesson_id: int):
+    uid = get_current_user_id()
+    note = await lessons_svc.get_lesson(uid, lesson_id)
+    if note is None:
+        return not_found("Lesson")
+    if not await can_write_note(uid, lesson_id):
+        return jsonify({"error": "Permission denied"}), 403
+    owner_uid = note.user_id
+    data = await request.get_json() or {}
+
+    # Partial update: only keys present in the payload change, and the service
+    # re-composes title, body and mirror from the merged set — so a form that
+    # sends one field cannot leave the halves of the document disagreeing.
+    kwargs = {
+        k: data[k]
+        for k in ("what", "when_to_apply", "insight", "learned_from", "tags")
+        if k in data
+    }
+    # An empty trigger would save and silently stop the lesson surfacing, so
+    # clearing it is refused for the same reason creating without one is.
+    if "when_to_apply" in kwargs and not (kwargs["when_to_apply"] or "").strip():
+        return jsonify({
+            "error": "when_to_apply cannot be cleared",
+            "detail": (
+                "A lesson with no trigger never surfaces, and nothing about "
+                "the stored record would show it. Rewrite the trigger rather "
+                "than emptying it."
+            ),
+        }), 400
+
+    updated = await lessons_svc.update_lesson(owner_uid, lesson_id, **kwargs)
+    if updated is None:
+        return not_found("Lesson")
+    if data.get("system_ids") is not None:
+        await systems_svc.set_record_systems(
+            owner_uid, lesson_id, data["system_ids"]
+        )
+    out = lessons_svc.lesson_to_dict(updated)
+    out["systems"] = [
+        s.to_dict()
+        for s in await systems_svc.list_record_systems(owner_uid, lesson_id)
+    ]
+    return jsonify(out)
+
+
+@lessons_bp.route("/<int:lesson_id>", methods=["DELETE"])
+@login_required
+async def delete_lesson_route(lesson_id: int):
+    """Trash, not erase — recoverable from the trash like every other kind."""
+    uid = get_current_user_id()
+    note = await lessons_svc.get_lesson(uid, lesson_id)
+    if note is None:
+        return not_found("Lesson")
+    if not await can_write_note(uid, lesson_id):
+        return jsonify({"error": "Permission denied"}), 403
+    batch_id = await trash_svc.delete(note.user_id, "note", lesson_id)
+    if batch_id is None:
+        return not_found("Lesson")
+    return jsonify({"deleted": lesson_id, "deleted_batch_id": batch_id})
diff --git a/src/scribe/services/embeddings.py b/src/scribe/services/embeddings.py
index 8734ae4..c29baed 100644
--- a/src/scribe/services/embeddings.py
+++ b/src/scribe/services/embeddings.py
@@ -203,6 +203,12 @@ def embedding_text(title: str | None, body: str | None) -> str:
     return f"{title}\n{body}".strip() if body else title
 
 
+# The join between a situation-keyed record's subject and its trigger. A
+# CONSTANT because `untrigger_title` below has to spell the same thing to undo
+# it, and two literals that must match are one edit away from not matching.
+TRIGGER_SEP = " — "
+
+
 def trigger_title(subject: str | None, trigger: str | None) -> str:
     """`{subject} — {trigger}` — the title half of a situation-keyed document.
 
@@ -226,10 +232,37 @@ def trigger_title(subject: str | None, trigger: str | None) -> str:
     subject = (subject or "").strip()
     trigger = (trigger or "").strip()
     if subject and trigger:
-        return f"{subject} — {trigger}"
+        return f"{subject}{TRIGGER_SEP}{trigger}"
     return subject or trigger
 
 
+def untrigger_title(title: str | None, trigger: str | None) -> str:
+    """The subject back out of a `trigger_title` — the inverse of the join.
+
+    Kept HERE, beside the join, for the reason the join itself was
+    consolidated: a separator spelled in two files is a separator that will one
+    day be changed in one of them. #3207 records the shape — derive it before
+    the third copy — and an inverse written in a caller is that third copy
+    wearing a different name.
+
+    Needs the trigger passed in rather than guessing at the separator, because
+    a subject may legitimately contain an em dash. Given the trigger, the
+    suffix is exact and the split cannot be wrong.
+
+    Degrades to the whole title when the suffix is absent — a record written
+    before the join existed, or one with no trigger yet, still answers with
+    something a human recognises rather than with "".
+    """
+    title = (title or "").strip()
+    trigger = (trigger or "").strip()
+    if not trigger:
+        return title
+    suffix = f"{TRIGGER_SEP}{trigger}"
+    if title.endswith(suffix):
+        return title[: -len(suffix)].strip()
+    return title
+
+
 # --- chunking (#280): the document shape ------------------------------------
 #
 # bge-small reads at most 512 tokens and fastembed silently truncates the rest,
diff --git a/src/scribe/services/lessons.py b/src/scribe/services/lessons.py
index bb8e8a0..2d2577f 100644
--- a/src/scribe/services/lessons.py
+++ b/src/scribe/services/lessons.py
@@ -88,6 +88,9 @@ from __future__ import annotations
 
 import re
 
+from scribe.models import async_session
+from scribe.models.note import Note
+
 LESSON_NOTE_TYPE = "lesson"
 
 # The key in `notes.data`. Named for the field it mirrors on `rules`, because it
@@ -317,6 +320,46 @@ def compose_data(
     return data
 
 
+def recompose_data(note) -> dict:
+    """Rebuild a lesson's `data` mirror from its own title and body.
+
+    For the GENERIC note door. `update_lesson` composes the mirror itself from
+    the merged field set and never needs this; a plain `update_note(body=...)`
+    has no idea the mirror exists and would leave it behind.
+
+    THE COST OF LEAVING IT BEHIND IS HIGHER HERE THAN FOR A SNIPPET. A stale
+    snippet mirror reports the wrong path. A stale lesson mirror reports the
+    wrong TRIGGER — and `lesson_trigger` prefers the mirror, so the lesson goes
+    on being retrieved for the situation it used to name while displaying the
+    one it now names. The trigger is the entire retrieval story (step 3), so
+    that is not a degraded record; it is a record that fires at the wrong
+    moment and looks right when it does.
+
+    The body is the authority and the mirror is derived — already this file's
+    rule. This is its enforcement on the path that bypasses `update_lesson`.
+
+    The subject comes back out of the title through `untrigger_title`, the
+    inverse of the join that composed it, rather than by splitting on a
+    separator spelled a second time here.
+    """
+    from scribe.services.embeddings import untrigger_title
+
+    body = getattr(note, "body", None) or ""
+    trigger_match = _BODY_TRIGGER_RE.search(body)
+    trigger = trigger_match.group(1).strip() if trigger_match else ""
+    what = untrigger_title(getattr(note, "title", None), trigger)
+    # Sources through the normal read, which already falls back body →
+    # arose_from_id. A body edit that drops the provenance line should drop
+    # the mirror's copy too: the body is the authority, and carrying a value
+    # the reader just deleted is the failure this function exists to prevent.
+    sources_match = _BODY_SOURCES_RE.search(body)
+    sources = (
+        normalize_sources(_ID_RE.findall(sources_match.group(1)))
+        if sources_match else []
+    )
+    return compose_data(what, trigger, sources)
+
+
 async def create_lesson(
     user_id: int,
     *,
@@ -375,6 +418,122 @@ async def get_lesson(user_id: int, lesson_id: int):
     return note
 
 
+def lesson_to_dict(note) -> dict:
+    """A lesson as either door returns it — the composed fields read back out,
+    not the raw row, so a caller sees the same vocabulary it wrote with.
+
+    In the SERVICE rather than in each door, on the `snippet_to_dict`
+    precedent: the REST route feeds the web UI and the MCP tools feed an
+    agent, and a shape spelled once per door is a shape that answers the two
+    of them differently the first time a field is added.
+    """
+    return {
+        "id": note.id,
+        "title": note.title,
+        "body": note.body,
+        # The composed vocabulary, not the storage: a caller that wrote
+        # `when_to_apply` reads `when_to_apply` back.
+        "what": (note.data or {}).get("what", "") if isinstance(note.data, dict) else "",
+        "when_to_apply": lesson_trigger(note),
+        "learned_from": lesson_sources(note),
+        "insight": _strip_composed_lines(note.body),
+        "tags": list(note.tags or []),
+        "project_id": note.project_id,
+        "note_type": note.note_type,
+        "created_at": note.created_at.isoformat() if note.created_at else None,
+        "updated_at": note.updated_at.isoformat() if note.updated_at else None,
+    }
+
+
+async def source_records(user_id: int, ids: list[int]) -> list[dict]:
+    """The records that taught a lesson, resolved to something linkable.
+
+    `learned_from` is a list of bare ids, which is right for storage and
+    useless on a page: "#4181" tells a reader nothing about whether it is
+    worth opening. This resolves each to its title and kind so the UI can
+    label the link, and so a source that has been deleted simply drops out
+    rather than rendering a link to nothing.
+
+    One query for the whole list, not one per id — a lesson with six sources
+    would otherwise be six round trips to draw one panel.
+
+    Share-aware (rule 78). Order follows `ids`, because that order is the
+    writer's: the first source is the one they reached for first.
+    """
+    from sqlalchemy import select
+
+    from scribe.services.access import readable_notes_clause
+
+    wanted = normalize_sources(ids)
+    if not wanted:
+        return []
+    async with async_session() as session:
+        result = await session.execute(
+            select(Note)
+            .where(Note.id.in_(wanted))
+            .where(Note.deleted_at.is_(None))
+            .where(readable_notes_clause(user_id))
+        )
+        found = {n.id: n for n in result.scalars().all()}
+    return [
+        {
+            "id": n.id,
+            "title": n.title,
+            "note_type": n.note_type,
+            "is_task": n.status is not None,
+            "task_kind": n.task_kind,
+            "status": n.status,
+        }
+        for i in wanted
+        if (n := found.get(i)) is not None
+    ]
+
+
+async def lessons_taught_by(user_id: int, record_id: int, limit: int = 20):
+    """The lessons drawn FROM one record — the reverse of `learned_from`.
+
+    THE DIRECTION THAT GETS FORGOTTEN, and arguably the more useful one: a
+    reader opening an old issue wants to know what was learned from it, and
+    without this the relation is only navigable from the lesson's side. A
+    record that taught something should say so on its own page.
+
+    Queried through `data[SOURCES_KEY]` rather than by scanning bodies: the
+    mirror is JSONB with a GIN index (0070), which is the whole reason step 4
+    put the list there. `path_exists` is the same dialect the snippet location
+    lookup uses, so both reverse lookups read the index the same way.
+
+    Share-aware (rule 78) via `readable_notes_clause`: this renders beside a
+    record the caller can already see, and a lesson someone shared with them
+    belongs in that list exactly as their own does.
+    """
+    from sqlalchemy import select
+
+    from scribe.services.access import readable_notes_clause
+
+    try:
+        wanted = int(record_id)
+    except (TypeError, ValueError):
+        return []
+    if wanted <= 0:
+        return []
+
+    # The id is an int we just validated, never caller text, so it cannot
+    # break out of the expression — the same guarantee `location_jsonpath`
+    # gets from JSON-quoting its values.
+    jsonpath = f"$.{SOURCES_KEY}[*] ? (@ == {wanted})"
+    async with async_session() as session:
+        result = await session.execute(
+            select(Note)
+            .where(Note.note_type == LESSON_NOTE_TYPE)
+            .where(Note.deleted_at.is_(None))
+            .where(Note.data.path_exists(jsonpath))
+            .where(readable_notes_clause(user_id))
+            .order_by(Note.updated_at.desc())
+            .limit(max(1, min(limit, 100)))
+        )
+        return list(result.scalars().all())
+
+
 async def update_lesson(
     user_id: int,
     lesson_id: int,
diff --git a/src/scribe/services/notes.py b/src/scribe/services/notes.py
index 758e963..7a21123 100644
--- a/src/scribe/services/notes.py
+++ b/src/scribe/services/notes.py
@@ -1,6 +1,6 @@
 import logging
 import re
-from collections.abc import Iterable
+from collections.abc import Callable, Iterable
 from datetime import date, datetime, timezone
 
 from sqlalchemy import func, or_, select, text
@@ -11,14 +11,44 @@ from scribe.models.base import iso
 
 logger = logging.getLogger(__name__)
 
-# The fields `snippets.parse_snippet_fields` reads. Writing any of them can
-# change what a snippet's derived `data` mirror should say, so update_note
-# recomposes the mirror when one moves. Kept here as a set of NAMES rather
-# than imported, because it describes update_note's own `fields` dict, not the
-# parser's signature.
+# The fields a derived `data` mirror is parsed out of. Writing any of them can
+# change what the mirror should say, so update_note recomposes it when one
+# moves. Kept here as a set of NAMES rather than imported, because it
+# describes update_note's own `fields` dict, not any parser's signature.
 _PARSED_FROM_BODY = frozenset({"title", "body", "tags"})
 
 
+def _mirror_recomposers() -> dict[str, Callable[[Note], dict]]:
+    """note_type -> the function that rebuilds that kind's derived `data`.
+
+    A TABLE rather than a chain of `if note_type == ...`, because the previous
+    shape tested one constant and the next kind with a derived mirror was
+    silently not covered — which is exactly what happened: the snippet guard
+    (#3128) was hard-coded, and lessons arrived in milestone 385 with the same
+    body-is-authority/`data`-is-mirror design and none of the protection.
+
+    The failure is invisible from here. Nothing raises, nothing logs; the row
+    simply keeps answering queries from a mirror that no longer matches its
+    body, and every surface that prefers the mirror — which is all of them, by
+    design, because parsing markdown to answer what an index can answer is how
+    a hot path rots — reports the old value confidently.
+
+    Imported inside the function, not at module scope: both services call back
+    into this module (`update_snippet`/`update_lesson` -> `update_note`), so a
+    top-level import is a cycle.
+    """
+    from scribe.services.lessons import (
+        LESSON_NOTE_TYPE, recompose_data as _lesson_mirror,
+    )
+    from scribe.services.snippets import (
+        SNIPPET_NOTE_TYPE, recompose_data as _snippet_mirror,
+    )
+    return {
+        SNIPPET_NOTE_TYPE: _snippet_mirror,
+        LESSON_NOTE_TYPE: _lesson_mirror,
+    }
+
+
 # Text fields where EMPTY MEANS NULL (milestone 317). The sweep's whole signal
 # is `verify_with IS NULL` = "this is a decision, there is nothing to go and
 # check". An empty string that is not NULL makes a norm look like a constraint
@@ -604,23 +634,19 @@ async def update_note(
         # costs exactly what the sweep exists to catch.
         if note.verify_with != check_before:
             note.verified_at = None
-        # A snippet's `data` is DERIVED from its body — so a write that moves
-        # the body through this generic door must move the mirror with it
-        # (#3128). Without this, PATCH /api/notes/<snippet_id> {body} left the
-        # mirror behind, and snippet_fields PREFERS the mirror: the row went on
-        # reporting its old repo/path/symbol to prior-art recall while showing
-        # its new body. `update_snippet` composes the mirror itself and passes
-        # it explicitly, so an explicit `data` always wins — the caller that
-        # knows the field set beats the one that can only re-read the body.
+        # Some kinds derive `data` from their body — so a write that moves the
+        # body through this generic door must move the mirror with it (#3128).
+        # Without this, PATCH /api/notes/<id> {body} left the mirror behind,
+        # and every read PREFERS the mirror: a snippet went on reporting its
+        # old repo/path/symbol to prior-art recall while showing its new body,
+        # and a lesson would go on being retrieved for the situation it used to
+        # name. The kind's own updater composes the mirror itself and passes it
+        # explicitly, so an explicit `data` always wins — the caller that knows
+        # the field set beats the one that can only re-read the body.
         if "data" not in fields and not _PARSED_FROM_BODY.isdisjoint(fields):
-            # Imported here, not at module scope: services/snippets.py calls
-            # back into this module (update_snippet -> update_note), so a
-            # top-level import is a cycle.
-            from scribe.services.snippets import (
-                SNIPPET_NOTE_TYPE, recompose_data,
-            )
-            if note.note_type == SNIPPET_NOTE_TYPE:
-                note.data = recompose_data(note)
+            recompose = _mirror_recomposers().get(note.note_type or "")
+            if recompose is not None:
+                note.data = recompose(note)
         # Auto-set lifecycle timestamps on status transitions
         if "status" in fields:
             _now = datetime.now(timezone.utc)
diff --git a/src/scribe/services/snippets.py b/src/scribe/services/snippets.py
index c36067b..2873f5d 100644
--- a/src/scribe/services/snippets.py
+++ b/src/scribe/services/snippets.py
@@ -275,9 +275,21 @@ def parse_snippet_fields(
 
     ``locations`` is a list of {repo,path,symbol}; ``repo``/``path``/``symbol``
     mirror the FIRST location for back-compat with the single-location callers."""
+    from scribe.services.embeddings import TRIGGER_SEP
+
     title = title or ""
     body = body or ""
-    name, _, when_from_title = title.partition(" — ")
+    # The inverse of `embeddings.trigger_title`, at the SEPARATOR it composed
+    # with — imported rather than spelled again, because a separator written
+    # in two files is a separator that will one day be changed in one of them.
+    #
+    # `partition` rather than the `untrigger_title` a lesson uses: that one is
+    # handed the trigger and strips an exact suffix, which a lesson needs
+    # because its subject may legitimately contain a dash. A snippet's name is
+    # a symbol, so the first separator is the right split and no trigger has
+    # to be known in advance. Two inverses, suited to their callers; one
+    # constant, so they cannot disagree about where the seam is.
+    name, _, when_from_title = title.partition(TRIGGER_SEP)
     fields = {
         "name": name.strip(),
         "when_to_use": when_from_title.strip(),
diff --git a/tests/helpers.py b/tests/helpers.py
index d176e2d..c68f5da 100644
--- a/tests/helpers.py
+++ b/tests/helpers.py
@@ -191,6 +191,34 @@ def fake_snippet(**attrs) -> MagicMock:
     }, attrs)
 
 
+def fake_lesson(**attrs) -> MagicMock:
+    """A stand-in lesson: a note whose `note_type` is what makes it one.
+
+    The title carries the trigger because `compose_title` builds it that way —
+    `{what} — {when it applies}` — so a menu line rendering only the title is
+    already showing the reader when this lesson applies. Tests that used a bare
+    title here would be testing a record the product cannot create.
+
+    The check fields and `arose_from_id` are explicitly None for the reason
+    `fake_snippet`'s `data` is: `update_note` reads `verify_with` and
+    `expires_when` to decide whether to run the check-field guard, and an
+    auto-created MagicMock attribute is truthy — so a default lesson driven
+    through the update path would take a branch no real record takes.
+    """
+    attrs.setdefault(
+        "title",
+        "Give absolutely-positioned siblings an explicit stacking order — "
+        "placing two absolutely-positioned elements in the same area",
+    )
+    attrs.setdefault("data", {"when_to_apply": "two absolute siblings overlap"})
+    attrs.setdefault("status", None)
+    attrs.setdefault("arose_from_id", None)
+    attrs.setdefault("verify_with", None)
+    attrs.setdefault("expires_when", None)
+    attrs.setdefault("verified_at", None)
+    return fake_note(note_type="lesson", **attrs)
+
+
 def fake_project(**attrs) -> MagicMock:
     """design_system_id is explicit: a truthy auto-attribute would route every
     project through the design-system branch and out to a real database."""
diff --git a/tests/test_derived_mirror_generic_door.py b/tests/test_derived_mirror_generic_door.py
new file mode 100644
index 0000000..7d70e68
--- /dev/null
+++ b/tests/test_derived_mirror_generic_door.py
@@ -0,0 +1,298 @@
+"""A DERIVED `data` mirror survives the GENERIC note door.
+
+Some kinds store a queryable mirror in `notes.data` that is derived from the
+body: a snippet (name, when_to_use, locations…) and a lesson (what, the
+trigger, what taught it). Their own updaters compose it from the field set they
+just merged, so those were never the problem — the problem is every other way
+the body can be written. `update_note` is a `hasattr` loop, and both doors
+reach it: PATCH /api/notes/<id> and the MCP update_note tool. The Knowledge
+feed hands you that path, because a card there routes to /notes/:id.
+
+The failure is silent and the wrong way round, because every read PREFERS the
+mirror — deliberately, since parsing markdown to answer what an index can
+answer is how a hot path rots.
+
+  - A SNIPPET went on reporting its old repo/path/symbol to the location
+    reverse lookup and to prior-art recall while displaying its new body: a
+    record surfaced with full authority and wrong, which the drift-check
+    docstring calls worse than having no record at all (#3128).
+
+  - A LESSON is worse. Its mirror holds the TRIGGER, and the trigger is the
+    entire retrieval story — a stale one keeps the lesson firing for the
+    situation it used to name while it displays the one it now names.
+
+#3128's fix was correct and did not generalise: it tested one constant, so
+milestone 385's lesson arrived with the same design and none of the protection.
+The registry tests below assert the PROPERTY — every kind with a derived mirror
+is registered — so a third kind fails here rather than shipping quiet (#3734).
+"""
+import inspect
+
+import pytest
+from tests.helpers import drive_update_note as _update
+from tests.helpers import fake_lesson, fake_note, fake_snippet
+
+OLD_MIRROR = {
+    "name": "debounce",
+    "language": "javascript",
+    "locations": [{"repo": "Scribe", "path": "old/place.js", "symbol": "debounce"}],
+    "verification": {"status": "ok", "code_sha": "abc", "checked_at": "2026-01-01"},
+    "provenance": {"commit_sha": "deadbeef"},
+}
+
+MOVED_BODY = (
+    "**Locations:**\n"
+    "- `Scribe` · `new/place.ts` · `debounce`\n\n"
+    "```typescript\nexport const debounce = 1;\n```\n"
+)
+
+
+@pytest.mark.asyncio
+async def test_a_body_write_moves_the_mirror_with_it():
+    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
+    await _update(note, body=MOVED_BODY)
+    assert note.data["locations"] == [
+        {"repo": "Scribe", "path": "new/place.ts", "symbol": "debounce"}
+    ], "the mirror still describes where the snippet used to live"
+    assert note.data["language"] == "typescript"
+
+
+@pytest.mark.asyncio
+async def test_the_verdict_and_provenance_are_carried_not_dropped():
+    """Neither is in the body to parse, so recomposing must carry them. An
+    ordinary edit must not erase the last drift check — and it needs no
+    invalidation branch either: `code_sha` is recomputed from the new code, so
+    a verdict stamped against the old code expires itself on read."""
+    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
+    await _update(note, body=MOVED_BODY)
+    assert note.data["verification"] == OLD_MIRROR["verification"]
+    assert note.data["provenance"] == OLD_MIRROR["provenance"]
+    assert note.data["code_sha"] != OLD_MIRROR["verification"]["code_sha"]
+
+
+@pytest.mark.asyncio
+async def test_an_explicit_data_wins_over_recomposition():
+    """`update_snippet` composes the mirror from the merged field set it holds
+    and passes it here. That caller knows things the body cannot be re-read for
+    — which locations were replaced, whether provenance survives the edit — so
+    an explicit mirror must not be recomputed out from under it."""
+    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
+    authoritative = {"name": "from the service", "locations": []}
+    await _update(note, body=MOVED_BODY, data=authoritative)
+    assert note.data == authoritative
+
+
+@pytest.mark.asyncio
+async def test_a_plain_note_is_left_alone():
+    """Only snippets carry a mirror; a note's `data` must not be invented."""
+    note = fake_note(note_type="note", data=None, project_id=None)
+    await _update(note, body="just some prose")
+    assert note.data is None
+
+
+@pytest.mark.asyncio
+async def test_a_write_that_cannot_change_the_parse_does_not_touch_the_mirror():
+    """Status, priority, project — none of them is an input to the body parser,
+    so recomposing on them would be work for nothing and would rebuild a mirror
+    from a body nobody claimed to have changed."""
+    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
+    await _update(note, project_id=4)
+    assert note.data == OLD_MIRROR
+
+
+@pytest.mark.asyncio
+async def test_a_title_change_reaches_the_mirror_too():
+    """A snippet's NAME lives in its title, not its body — `parse_snippet_fields`
+    reads both, so both are triggers."""
+    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
+    await _update(note, title="throttle — cap a callback's rate")
+    assert note.data["name"] == "throttle"
+    assert note.data["when_to_use"] == "cap a callback's rate"
+
+
+# ── the registry, rather than a chain of ifs ─────────────────────────────────
+
+
+def test_every_kind_with_a_derived_mirror_is_registered():
+    """The load-bearing one. Before #3734 this was a single `if` naming
+    snippets, and the kind added next was simply not covered."""
+    from scribe.services.lessons import LESSON_NOTE_TYPE
+    from scribe.services.notes import _mirror_recomposers
+    from scribe.services.snippets import SNIPPET_NOTE_TYPE
+
+    table = _mirror_recomposers()
+    assert SNIPPET_NOTE_TYPE in table, "the #3128 fix was lost"
+    assert LESSON_NOTE_TYPE in table, (
+        "a lesson's `data` holds its trigger and `lesson_trigger` prefers it, "
+        "so a body edit through the generic door would leave the lesson being "
+        "retrieved for a situation it no longer names (#3734)"
+    )
+
+
+def test_the_dispatch_is_a_lookup_not_a_named_kind():
+    """Asserted on structure (rule 167). A lookup extends in one line in one
+    place; naming a kind inline is the shape that left lessons uncovered."""
+    from scribe.services import notes as notes_module
+
+    src = inspect.getsource(notes_module.update_note)
+    assert "_mirror_recomposers()" in src
+    assert "SNIPPET_NOTE_TYPE" not in src, (
+        "update_note names one kind again — that is the shape #3734 replaced"
+    )
+
+
+def test_each_recomposer_takes_the_note_and_nothing_else():
+    """A registry entry with the wrong signature fails inside a generic PATCH,
+    which is the one moment nobody is watching."""
+    from scribe.services.notes import _mirror_recomposers
+
+    for kind, fn in _mirror_recomposers().items():
+        assert callable(fn), f"{kind} maps to something not callable"
+        assert len(inspect.signature(fn).parameters) == 1, kind
+
+
+# ── a lesson's mirror moves with its body ────────────────────────────────────
+
+
+NEW_TRIGGER = "a CI run has sat in_progress far longer than its suite takes"
+
+
+def _lesson_body(trigger, insight="Read the job log.", sources=None):
+    from scribe.services.lessons import compose_body
+    return compose_body(insight, trigger, sources)
+
+
+@pytest.mark.asyncio
+async def test_a_body_write_moves_a_lessons_trigger_with_it():
+    from scribe.services.lessons import TRIGGER_KEY, compose_title
+
+    what = "Read the job log before waiting longer"
+    note = fake_lesson(
+        title=compose_title(what, NEW_TRIGGER),
+        data={TRIGGER_KEY: "a CI run is slow", "what": what},
+        project_id=None,
+    )
+    await _update(note, body=_lesson_body(NEW_TRIGGER))
+    assert note.data[TRIGGER_KEY] == NEW_TRIGGER, (
+        "the mirror kept the old trigger — the lesson would still be retrieved "
+        "for the situation it no longer names"
+    )
+    assert note.data["what"] == what
+
+
+@pytest.mark.asyncio
+async def test_a_subject_containing_an_em_dash_still_splits():
+    """Why `untrigger_title` is given the trigger instead of splitting on the
+    separator: a subject may legitimately contain one."""
+    from scribe.services.lessons import TRIGGER_KEY, compose_title
+
+    what = "A wait with no deadline — the shape, not the symptom"
+    trigger = "you are about to await something crossing a process boundary"
+    note = fake_lesson(
+        title=compose_title(what, trigger), data=None, project_id=None,
+    )
+    await _update(note, body=_lesson_body(trigger))
+    assert note.data["what"] == what
+    assert note.data[TRIGGER_KEY] == trigger
+
+
+@pytest.mark.asyncio
+async def test_dropping_the_provenance_line_drops_it_from_the_mirror():
+    """The body is the authority. Carrying a value the reader just deleted is
+    the failure this recompose exists to prevent, not a courtesy — the
+    opposite call from a snippet's `verification`, which is carried because it
+    was never in the body to delete."""
+    from scribe.services.lessons import SOURCES_KEY, compose_title
+
+    note = fake_lesson(
+        title=compose_title("Something learned", "a situation"),
+        data={SOURCES_KEY: [999]},
+        project_id=None,
+    )
+    await _update(note, body=_lesson_body("a situation"))   # no Learned from:
+    assert SOURCES_KEY not in note.data
+
+
+@pytest.mark.asyncio
+async def test_an_explicit_data_wins_for_a_lesson_too():
+    """`update_lesson` composes the mirror from the merged field set and passes
+    it here; that caller knows things a re-read of the body cannot recover."""
+    from scribe.services.lessons import TRIGGER_KEY
+
+    note = fake_lesson(data={TRIGGER_KEY: "old"}, project_id=None)
+    authoritative = {TRIGGER_KEY: "from the service", "what": "x"}
+    await _update(note, body=_lesson_body(NEW_TRIGGER), data=authoritative)
+    assert note.data == authoritative
+
+
+@pytest.mark.asyncio
+async def test_a_lesson_title_change_reaches_the_mirror():
+    """A lesson's subject lives in its title, so a title edit is a trigger for
+    recomposition exactly as it is for a snippet's name."""
+    from scribe.services.lessons import TRIGGER_KEY, compose_title
+
+    trigger = "two absolute siblings overlap"
+    note = fake_lesson(
+        body=_lesson_body(trigger),
+        data={TRIGGER_KEY: trigger, "what": "the old subject"},
+        project_id=None,
+    )
+    await _update(note, title=compose_title("the new subject", trigger))
+    assert note.data["what"] == "the new subject"
+    assert note.data[TRIGGER_KEY] == trigger
+
+
+# ── the join and its inverse ─────────────────────────────────────────────────
+
+
+@pytest.mark.parametrize(
+    ("subject", "trigger"),
+    [
+        ("a subject", "a trigger"),
+        ("a subject — with a dash", "a trigger"),
+        ("a subject", ""),
+        ("", "a trigger"),
+        ("a subject", "a trigger — with a dash"),
+    ],
+)
+def test_untrigger_title_inverts_trigger_title(subject, trigger):
+    """#3207's shape: the join had three copies before it was consolidated, so
+    its inverse lives beside it rather than in whichever caller wanted it."""
+    from scribe.services.embeddings import trigger_title, untrigger_title
+
+    title = trigger_title(subject, trigger)
+    assert untrigger_title(title, trigger) == (subject or trigger).strip()
+
+
+def test_untrigger_title_degrades_to_the_whole_title():
+    """A record written before the join existed still answers with something a
+    human recognises rather than with ""."""
+    from scribe.services.embeddings import untrigger_title
+
+    assert untrigger_title("a plain old title", "") == "a plain old title"
+    assert untrigger_title("a plain old title", "a trigger it lacks") == (
+        "a plain old title"
+    )
+
+
+def test_the_trigger_separator_is_spelled_in_exactly_one_place():
+    """Two literals that must match are one edit away from not matching."""
+    import pathlib
+
+    root = pathlib.Path(__file__).resolve().parents[1] / "src" / "scribe"
+    offenders = [
+        str(p.relative_to(root)) for p in root.rglob("*.py")
+        if '" \u2014 "' in p.read_text() and p.name != "embeddings.py"
+    ]
+    assert not offenders, (
+        f"the trigger separator is spelled inline in {offenders} — use "
+        f"TRIGGER_SEP, trigger_title or untrigger_title (#3207)"
+    )
+
+
+def test_the_mirror_guards_can_fail():
+    """Rule 167: shown turning red once."""
+    from scribe.services.lessons import TRIGGER_KEY, recompose_data
+
+    bare = fake_lesson(title="just a title", body="no composed lines", data=None)
+    assert TRIGGER_KEY not in recompose_data(bare)
diff --git a/tests/test_lesson_rest_door.py b/tests/test_lesson_rest_door.py
new file mode 100644
index 0000000..d853696
--- /dev/null
+++ b/tests/test_lesson_rest_door.py
@@ -0,0 +1,246 @@
+"""The REST door for lessons — the half the web UI can actually reach (#3734).
+
+WHY THIS EXISTS AT ALL
+
+Milestone 385 built the lesson kind through the MCP tools, which is the surface
+an agent uses. The Vue app speaks REST, so until this blueprint existed a lesson
+was a record a person could not create, read, edit or retire from the UI — rule
+27's "no UI, no ship" failing at the door rather than in the view.
+
+WHAT THESE PIN
+
+Three things a second door tends to get wrong, and one that is specific to this
+kind:
+
+  - PARITY. Both doors go through services/lessons.py, so the composed
+    document is identical whichever one wrote it. A REST door that composed its
+    own title would produce lessons that rank differently from the agent's, and
+    the document IS what ranks.
+
+  - ACL (rule 78). Share-aware resolve, write as the owner — the pattern
+    routes/snippets.py sets — so a shared editor isn't rejected by the
+    owner-scoped service.
+
+  - THE TRIGGER IS REFUSED WHEN EMPTY. This is the kind-specific one and the
+    reason the door is not a thin wrapper. The service will happily store a
+    lesson with no trigger: it saves, it reads correctly in every listing, and
+    it never surfaces. There is nothing to notice afterwards — it looks exactly
+    like a lesson that works. So the door refuses it at both create and update
+    rather than handing back a record that looks finished.
+
+  - THE REVERSE DIRECTION. `taught-by/<id>` answers "what was learned from this
+    record", which the task body calls the direction that gets forgotten and
+    arguably the more useful one.
+"""
+import inspect
+
+import pytest
+
+
+# ── parity: one composer, two doors ──────────────────────────────────────────
+
+
+def test_both_doors_share_one_serializer():
+    """A payload shape spelled once per door answers the two of them
+    differently the first time a field is added."""
+    from scribe.mcp.tools import lessons as mcp_lessons
+    from scribe.services import lessons as lessons_svc
+
+    assert mcp_lessons._to_dict is lessons_svc.lesson_to_dict
+
+
+def test_the_rest_door_composes_nothing_itself():
+    """Asserted on structure (rule 167). The document is what ranks, so a door
+    that built its own title would produce lessons that rank differently from
+    the ones the agent writes."""
+    from scribe.routes import lessons as routes
+
+    src = inspect.getsource(routes)
+    # It may CALL the service's composer (the dedup gate needs the document),
+    # but it must not assemble a title or a trigger line itself.
+    assert "trigger_title" not in src
+    assert "**When to apply:**" not in src
+    assert "lessons_svc.lesson_document" in src, (
+        "the dedup gate must hash the same document the service will store"
+    )
+
+
+def test_the_door_is_registered():
+    """A blueprint nobody registers is a file, not a door."""
+    from scribe import app as app_module
+
+    src = inspect.getsource(app_module)
+    assert "from scribe.routes.lessons import lessons_bp" in src
+    assert "app.register_blueprint(lessons_bp)" in src
+
+
+def test_the_blueprint_is_mounted_where_the_client_looks():
+    from scribe.routes.lessons import lessons_bp
+
+    assert lessons_bp.url_prefix == "/api/lessons"
+
+
+def test_the_reverse_lookup_is_registered_before_the_id_route():
+    """Quart matches in registration order. The int converter protects
+    `taught-by` today, but the ordering is what keeps that true if the
+    converter is ever widened — the same care snippets' `/duplicates` takes."""
+    from scribe.routes import lessons as routes
+
+    src = inspect.getsource(routes)
+    assert src.index('"/taught-by/<int:record_id>"') < src.index(
+        '"/<int:lesson_id>"'
+    )
+
+
+# ── the trigger is not optional at this door ─────────────────────────────────
+
+
+def test_create_refuses_a_lesson_with_no_trigger():
+    """The kind-specific guard. A triggerless lesson saves and never surfaces,
+    and nothing about the stored record shows it."""
+    from scribe.routes import lessons as routes
+
+    src = inspect.getsource(routes.create_lesson_route)
+    assert "when_to_apply is required" in src
+    assert "never reaches anyone" in src, (
+        "the refusal must say WHY, or the next person reads it as a nag and "
+        "removes it"
+    )
+
+
+def test_update_refuses_to_clear_the_trigger():
+    """The other half. Creating without one is refused; emptying one later
+    would reach the same broken state by a different path."""
+    from scribe.routes import lessons as routes
+
+    src = inspect.getsource(routes.update_lesson_route)
+    assert "cannot be cleared" in src
+
+
+def test_the_subject_is_required_too():
+    from scribe.routes import lessons as routes
+
+    src = inspect.getsource(routes.create_lesson_route)
+    assert "what is required" in src
+
+
+# ── ACL: rule 78's pattern, not a bare owner filter ──────────────────────────
+
+
+@pytest.mark.parametrize(
+    "handler", ["update_lesson_route", "delete_lesson_route"]
+)
+def test_writes_check_permission_and_act_as_the_owner(handler):
+    """Resolve share-aware, then write as the owner — otherwise a shared
+    editor is rejected by the owner-scoped service."""
+    from scribe.routes import lessons as routes
+
+    src = inspect.getsource(getattr(routes, handler))
+    assert "can_write_note" in src, f"{handler} does not check write permission"
+    assert "note.user_id" in src, (
+        f"{handler} writes as the caller rather than as the owner, which "
+        f"rejects a legitimately shared editor (rule 78)"
+    )
+
+
+def test_no_handler_builds_its_own_owner_filter():
+    """Rule 78's actual failure mode: a route assembling its own
+    `Note.user_id == uid` clause instead of going through the service and the
+    access helpers. Passing `user_id=uid` INTO a service is the correct call
+    and is not what this looks for."""
+    from scribe.routes import lessons as routes
+
+    src = inspect.getsource(routes)
+    assert "Note.user_id" not in src
+    assert "select(" not in src, (
+        "a route composing its own query has bypassed the access helpers"
+    )
+
+
+def test_delete_trashes_recoverably():
+    """Every kind's delete is a trash, and the batch id is what restores it."""
+    from scribe.routes import lessons as routes
+
+    src = inspect.getsource(routes.delete_lesson_route)
+    assert "trash_svc.delete" in src
+    assert "deleted_batch_id" in src
+
+
+# ── the reverse direction ────────────────────────────────────────────────────
+
+
+def test_the_reverse_lookup_reads_the_indexed_mirror():
+    """Not by scanning bodies: `data[taught_by]` is JSONB with a GIN index
+    (0070), which is the whole reason step 4 put the list there."""
+    from scribe.services import lessons as lessons_svc
+
+    src = inspect.getsource(lessons_svc.lessons_taught_by)
+    assert "path_exists" in src
+    assert "SOURCES_KEY" in src
+
+
+def test_the_reverse_lookup_is_share_aware():
+    """It renders beside a record the caller can already see, so a lesson
+    someone shared with them belongs in the list exactly as their own does."""
+    from scribe.services import lessons as lessons_svc
+
+    src = inspect.getsource(lessons_svc.lessons_taught_by)
+    assert "readable_notes_clause" in src
+    assert "deleted_at" in src
+
+
+def test_the_reverse_lookup_refuses_a_nonsense_id_rather_than_interpolating():
+    """The jsonpath is built by formatting, so the id has to be an int before
+    it gets near the expression."""
+    from scribe.services import lessons as lessons_svc
+
+    src = inspect.getsource(lessons_svc.lessons_taught_by)
+    assert "int(record_id)" in src
+
+
+@pytest.mark.asyncio
+async def test_a_bad_id_returns_nothing_and_raises_nothing():
+    from scribe.services.lessons import lessons_taught_by
+
+    assert await lessons_taught_by(1, 0) == []
+    assert await lessons_taught_by(1, -3) == []
+    assert await lessons_taught_by(1, "not a number") == []
+
+
+# ── the serializer speaks the vocabulary the caller wrote with ───────────────
+
+
+def test_the_payload_reads_back_the_composed_fields():
+    """A caller that wrote `when_to_apply` reads `when_to_apply` back, not a
+    body it has to parse."""
+    from tests.helpers import fake_lesson
+    from scribe.services.lessons import compose_body, compose_title, lesson_to_dict
+
+    what = "Read the job log before waiting longer"
+    trigger = "a CI run has sat in_progress longer than its suite takes"
+    note = fake_lesson(
+        id=7,
+        title=compose_title(what, trigger),
+        body=compose_body("The work is usually done.", trigger, [4181]),
+        data={"what": what, "when_to_apply": trigger, "taught_by": [4181]},
+        project_id=None,
+        created_at=None,
+        updated_at=None,
+    )
+    out = lesson_to_dict(note)
+    assert out["what"] == what
+    assert out["when_to_apply"] == trigger
+    assert out["learned_from"] == [4181]
+    # The insight comes back WITHOUT the lines compose_body added, so an edit
+    # form round-trips instead of accumulating a copy of them per save.
+    assert out["insight"] == "The work is usually done."
+    assert "**When to apply:**" not in out["insight"]
+
+
+def test_the_rest_guards_can_fail():
+    """Rule 167: shown turning red once."""
+    from scribe.routes import lessons as routes
+
+    src = inspect.getsource(routes)
+    assert "a phrase that is definitely not in this module" not in src
+    assert "lessons_bp" in src
diff --git a/tests/test_lesson_surfacing.py b/tests/test_lesson_surfacing.py
index 238e4ad..05f27b1 100644
--- a/tests/test_lesson_surfacing.py
+++ b/tests/test_lesson_surfacing.py
@@ -22,7 +22,7 @@ from unittest.mock import AsyncMock, MagicMock, patch
 import pytest
 from scribe.services import plugin_context as pc
 from scribe.services.lessons import LESSON_NOTE_TYPE
-from tests.helpers import fake_note, writepath_cfg
+from tests.helpers import fake_lesson, fake_note, writepath_cfg
 
 pytestmark = pytest.mark.usefixtures("_no_supersession")
 
@@ -33,23 +33,6 @@ _CFG = {"enabled": True, "threshold": 0.55, "top_k": 3}
 _BINDING_PHRASE = "before deciding it does not apply"
 
 
-def fake_lesson(**attrs):
-    """A stand-in lesson: a note whose `note_type` is what makes it one.
-
-    The title carries the trigger because `compose_title` builds it that way —
-    `{what} — {when it applies}` — so a menu line rendering only the title is
-    already showing the reader when this lesson applies. Tests that used a bare
-    title here would be testing a record the product cannot create.
-    """
-    attrs.setdefault(
-        "title",
-        "Give absolutely-positioned siblings an explicit stacking order — "
-        "placing two absolutely-positioned elements in the same area",
-    )
-    attrs.setdefault("data", {"when_to_apply": "two absolute siblings overlap"})
-    return fake_note(note_type=LESSON_NOTE_TYPE, **attrs)
-
-
 async def _menu(main_hits, *, lesson_hits=None, reuse_hits=None, cfg=None,
                 exclude_ids=None, rec=None, surf=None):
     """Run the prompt menu with each query stubbed by the kinds it asks for."""
diff --git a/tests/test_snippet_mirror_generic_door.py b/tests/test_snippet_mirror_generic_door.py
deleted file mode 100644
index 30af5e2..0000000
--- a/tests/test_snippet_mirror_generic_door.py
+++ /dev/null
@@ -1,95 +0,0 @@
-"""A snippet's `data` mirror survives the GENERIC note door.
-
-`notes.data` is derived from the body. The snippet service always composed it
-from the field set it had just merged, so `update_snippet` was never the
-problem — the problem was every other way a snippet's body could be written.
-`update_note` is a `hasattr` loop with no snippet awareness, and both doors
-reach it: PATCH /api/notes/<id> and the MCP update_note tool. The Knowledge
-feed handed you that path, because a snippet card there routed to /notes/:id.
-
-The failure was silent and the wrong way round: `snippet_fields` PREFERS the
-mirror, so the row went on reporting its old repo/path/symbol to the location
-reverse lookup and to prior-art recall while displaying its new body — a record
-surfaced with full authority and wrong, which the drift-check docstring calls
-worse than having no record at all (#3128).
-"""
-import pytest
-from tests.helpers import drive_update_note as _update
-from tests.helpers import fake_note, fake_snippet
-
-OLD_MIRROR = {
-    "name": "debounce",
-    "language": "javascript",
-    "locations": [{"repo": "Scribe", "path": "old/place.js", "symbol": "debounce"}],
-    "verification": {"status": "ok", "code_sha": "abc", "checked_at": "2026-01-01"},
-    "provenance": {"commit_sha": "deadbeef"},
-}
-
-MOVED_BODY = (
-    "**Locations:**\n"
-    "- `Scribe` · `new/place.ts` · `debounce`\n\n"
-    "```typescript\nexport const debounce = 1;\n```\n"
-)
-
-
-@pytest.mark.asyncio
-async def test_a_body_write_moves_the_mirror_with_it():
-    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
-    await _update(note, body=MOVED_BODY)
-    assert note.data["locations"] == [
-        {"repo": "Scribe", "path": "new/place.ts", "symbol": "debounce"}
-    ], "the mirror still describes where the snippet used to live"
-    assert note.data["language"] == "typescript"
-
-
-@pytest.mark.asyncio
-async def test_the_verdict_and_provenance_are_carried_not_dropped():
-    """Neither is in the body to parse, so recomposing must carry them. An
-    ordinary edit must not erase the last drift check — and it needs no
-    invalidation branch either: `code_sha` is recomputed from the new code, so
-    a verdict stamped against the old code expires itself on read."""
-    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
-    await _update(note, body=MOVED_BODY)
-    assert note.data["verification"] == OLD_MIRROR["verification"]
-    assert note.data["provenance"] == OLD_MIRROR["provenance"]
-    assert note.data["code_sha"] != OLD_MIRROR["verification"]["code_sha"]
-
-
-@pytest.mark.asyncio
-async def test_an_explicit_data_wins_over_recomposition():
-    """`update_snippet` composes the mirror from the merged field set it holds
-    and passes it here. That caller knows things the body cannot be re-read for
-    — which locations were replaced, whether provenance survives the edit — so
-    an explicit mirror must not be recomputed out from under it."""
-    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
-    authoritative = {"name": "from the service", "locations": []}
-    await _update(note, body=MOVED_BODY, data=authoritative)
-    assert note.data == authoritative
-
-
-@pytest.mark.asyncio
-async def test_a_plain_note_is_left_alone():
-    """Only snippets carry a mirror; a note's `data` must not be invented."""
-    note = fake_note(note_type="note", data=None, project_id=None)
-    await _update(note, body="just some prose")
-    assert note.data is None
-
-
-@pytest.mark.asyncio
-async def test_a_write_that_cannot_change_the_parse_does_not_touch_the_mirror():
-    """Status, priority, project — none of them is an input to the body parser,
-    so recomposing on them would be work for nothing and would rebuild a mirror
-    from a body nobody claimed to have changed."""
-    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
-    await _update(note, project_id=4)
-    assert note.data == OLD_MIRROR
-
-
-@pytest.mark.asyncio
-async def test_a_title_change_reaches_the_mirror_too():
-    """A snippet's NAME lives in its title, not its body — `parse_snippet_fields`
-    reads both, so both are triggers."""
-    note = fake_snippet(data=dict(OLD_MIRROR), project_id=None)
-    await _update(note, title="throttle — cap a callback's rate")
-    assert note.data["name"] == "throttle"
-    assert note.data["when_to_use"] == "cap a callback's rate"