From 62f3a485addac1767063ed814a6554b55b7e8f99 Mon Sep 17 00:00:00 2001
From: Bryan Van Deusen <bryan.vandeusen@fabledsword.com>
Date: Mon, 21 Sep 2026 20:47:16 -0400
Subject: [PATCH] fix(usage): one seam attaches the surfaced-vs-opened chip,
 and the Knowledge browse uses it (#4230)
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`usage_for_notes` is named for notes and works on every note row, yet the chip
reached snippets and rules only. Notes had it nowhere. Lessons had it collected
and shown nowhere a person could reach, because #4196 taught `/api/lessons` to
attach it and `KnowledgeView` — the only lesson list in the UI — browses through
`/api/knowledge`, so `listLessons` still has no consumer.

The cause was not a missing line. SEVEN call sites carried their own copy of the
same few lines: two REST lists, two REST details, two MCP lists, one MCP detail.
Each read perfectly well alone, so "which doors attach usage?" had no answer
anywhere in the code — the same asymmetry test_system_tagging_door_parity.py
records for System tagging (#4249), where whichever door nobody exercised for a
kind is the one that never grew the feature.

`attach_usage(rows, key="id")` is now that answer, and all seven go through it.
A detail payload is a one-row list, so the single-record doors share the seam
rather than keeping a second shape beside it. Deliberately NO try/except: the
fail-open already lives in `usage_for_notes`, which reports through
`_report_failure("readout")` and returns the zero-filled map. Wrapping it again
would swallow the REPORT as well as the error, and a silently-swallowed readout
failure is exactly #2663 — every counter reading zero in production for weeks
while the writes landed fine.

`/api/knowledge` now attaches usage, which closes both holes at once: it is how
notes, lessons and processes are all browsed. `KnowledgeView` renders the badge
on the card footer, looking the advice up per row because the feed is mixed.

The advice moves to utils/deadWeight.ts. Canon #3460 says each caller owns its
own const, and that held while each caller showed ONE kind; a mixed feed would
need five of its own and the next surface another five. The canon's actual
invariant — advice is kind-specific and never baked into the badge — is kept:
it is still a prop. The three existing callers now read the same table, so the
sentence has one home rather than four. Recorded against #3460 so the next
reader is not left re-litigating it.

`_row_id` rejects bools explicitly: `int(True)` is 1, so a row carrying a flag
under the key would be credited with note #1's counts, and a wrong chip is worse
than no chip because it reads as a measurement. A row with no usable id is
skipped rather than failing the page.

Tests pin the PROPERTY, not one route: no door calls the aggregate directly
(AST, so a comment naming it is not a false positive), and every door that shows
usage reaches the seam. Plus the N+1 guard — one aggregate per page, asserted on
await_count, because the per-row version reads more naturally and is invisible
in review.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy
---
 .../src/components/rules/RuleListPane.vue     |  13 +-
 frontend/src/utils/deadWeight.ts              |  68 ++++++
 frontend/src/views/KnowledgeView.vue          |  17 ++
 frontend/src/views/LessonDetailView.vue       |   3 +-
 frontend/src/views/SnippetListView.vue        |  11 +-
 src/scribe/mcp/tools/lessons.py               |  10 +-
 src/scribe/mcp/tools/snippets.py              |   6 +-
 src/scribe/routes/knowledge.py                |  16 +-
 src/scribe/routes/lessons.py                  |  10 +-
 src/scribe/routes/snippets.py                 |  10 +-
 src/scribe/services/note_usage.py             |  62 ++++++
 tests/test_usage_attach_seam.py               | 205 ++++++++++++++++++
 12 files changed, 385 insertions(+), 46 deletions(-)
 create mode 100644 frontend/src/utils/deadWeight.ts
 create mode 100644 tests/test_usage_attach_seam.py

diff --git a/frontend/src/components/rules/RuleListPane.vue b/frontend/src/components/rules/RuleListPane.vue
index 87a8311..82f63f6 100644
--- a/frontend/src/components/rules/RuleListPane.vue
+++ b/frontend/src/components/rules/RuleListPane.vue
@@ -1,16 +1,7 @@
 <script setup lang="ts">
 import type { RuleHeader } from "@/api/rulebooks";
 import UsageBadge from "@/components/UsageBadge.vue";
-
-/** The dead-weight nudge for a RULE — two remedies, not one, which is the
- *  whole reason this advice is per-kind. A snippet nobody opens should
- *  probably go. A rule nobody opens may be perfectly good and simply firing on
- *  the wrong thing, so "delete it" would be the wrong nudge half the time and
- *  the operator has to be the one who picks. */
-const RULE_DEAD_WEIGHT =
-  "Kept arriving without being read. Either its trigger fires on the wrong " +
-  "work — reword “when to apply” so it says when — or it is not wanted here. " +
-  "Until one or the other, it takes a slot in every write it matches.";
+import { DEAD_WEIGHT_ADVICE } from "@/utils/deadWeight";
 
 defineProps<{ topicId: number; rules: RuleHeader[] }>();
 const emit = defineEmits<{
@@ -54,7 +45,7 @@ const emit = defineEmits<{
               ? 'Asserts a fact nobody has confirmed yet'
               : `Check last passed ${r.last_verified}`"
           >{{ r.last_verified === "never" ? "unverified" : `checked ${r.last_verified}` }}</span>
-          <UsageBadge :usage="r.usage" :dead-weight-advice="RULE_DEAD_WEIGHT" />
+          <UsageBadge :usage="r.usage" :dead-weight-advice="DEAD_WEIGHT_ADVICE.rule" />
         </div>
         <div class="statement">{{ r.statement }}</div>
         <div v-if="r.when_to_apply || r.updated_at" class="meta">
diff --git a/frontend/src/utils/deadWeight.ts b/frontend/src/utils/deadWeight.ts
new file mode 100644
index 0000000..9036d2d
--- /dev/null
+++ b/frontend/src/utils/deadWeight.ts
@@ -0,0 +1,68 @@
+/**
+ * What to suggest when a record looks like dead weight — one sentence per kind.
+ *
+ * `UsageBadge` deliberately takes the advice as a prop rather than deriving it,
+ * because the COUNTS read identically for every kind and the REMEDY does not: a
+ * snippet nobody opens should probably go, while a rule or a lesson in the same
+ * position more often has a trigger that fires on the wrong work. Telling an
+ * operator to delete one of those would be the wrong nudge about half the time.
+ *
+ * The table lives here because the copy was about to exist in five places.
+ * Three already had their own: `SnippetListView`, `RuleListPane`, and
+ * `LessonDetailView` with the sentence inline in its template. The unified
+ * Knowledge browse renders notes, tasks, processes, snippets and lessons in one
+ * mixed feed (#4230), so it needs all of them at once — and a per-view constant
+ * is how three surfaces end up giving three different answers to the same
+ * question.
+ *
+ * Every sentence names the COST of leaving it, not just the fact. "Never
+ * opened" is an observation; "takes a slot in every future menu" is why the
+ * reader should care, and it is the half that makes the chip actionable.
+ */
+
+/** The record kinds the Knowledge feed can show, plus the ones only their own
+ *  views show. Keyed by `note_type`, with `rule` alongside — rules live in a
+ *  separate table but answer the same question (see `RecordUsage`). */
+export type DeadWeightKind =
+  | "note"
+  | "task"
+  | "process"
+  | "snippet"
+  | "lesson"
+  | "rule";
+
+export const DEAD_WEIGHT_ADVICE: Record<DeadWeightKind, string> = {
+  snippet:
+    "Offered repeatedly without ever being opened — consider rewriting its " +
+    "“when to reach for it” so it says when, or deleting it. It takes a slot " +
+    "in every future auto-inject menu.",
+  rule:
+    "Kept arriving without being read. Either its trigger fires on the wrong " +
+    "work — reword “when to apply” so it says when — or it is not wanted here. " +
+    "Until one or the other, it takes a slot in every write it matches.",
+  lesson:
+    "Repeatedly offered and never opened usually means the trigger fires on " +
+    "the wrong situation — re-key `when_to_apply` rather than deleting the " +
+    "claim.",
+  note:
+    "Surfaced again and again and never opened. Usually the title is the " +
+    "problem: recall matches on it first, so a note titled for its author " +
+    "rather than for the situation keeps winning slots it cannot pay for.",
+  process:
+    "Offered without ever being run. Either the steps no longer match how " +
+    "the work is actually done, or it is being matched on the wrong trigger " +
+    "— check which before retiring it.",
+  task:
+    "Surfaced repeatedly and never opened. On a task this more often says " +
+    "the work has gone stale than that the record is wrong — decide whether " +
+    "it is still wanted before re-titling it.",
+};
+
+/** The advice for a kind, falling back to the note wording.
+ *
+ * The fallback is deliberate rather than an empty string: a kind added to the
+ * feed later should still get a usable sentence, and "the title is doing the
+ * matching" is the reading that holds for any record recall can choose. */
+export function deadWeightAdvice(kind: string | null | undefined): string {
+  return DEAD_WEIGHT_ADVICE[kind as DeadWeightKind] ?? DEAD_WEIGHT_ADVICE.note;
+}
diff --git a/frontend/src/views/KnowledgeView.vue b/frontend/src/views/KnowledgeView.vue
index 4c93381..ea84fd5 100644
--- a/frontend/src/views/KnowledgeView.vue
+++ b/frontend/src/views/KnowledgeView.vue
@@ -3,7 +3,10 @@ import { ref, computed, watch, onMounted, onUnmounted, nextTick } from "vue";
 import { useRouter } from "vue-router";
 import { apiGet } from "@/api/client";
 import type { TaskKind, TaskStatus, TaskPriority } from "@/types/note";
+import type { RecordUsage } from "@/types/usage";
+import { deadWeightAdvice } from "@/utils/deadWeight";
 import KindBadge from "@/components/KindBadge.vue";
+import UsageBadge from "@/components/UsageBadge.vue";
 import NoteSweepPane from "@/components/NoteSweepPane.vue";
 import StatusBadge from "@/components/StatusBadge.vue";
 import PriorityBadge from "@/components/PriorityBadge.vue";
@@ -44,6 +47,10 @@ interface KnowledgeItem {
   project_id: number | null;
   created_at: string;
   updated_at: string;
+  /** Surfaced-vs-opened counts, zero-filled by the route for EVERY row, so
+   *  "never surfaced" is a value here rather than a missing field (#4230).
+   *  `UsageBadge` renders nothing at all below one surfacing. */
+  usage?: RecordUsage | null;
   // Set only when another user owns this record — their suggestion, not one of
   // yours. Absent means it's yours.
   shared?: boolean;
@@ -659,6 +666,16 @@ onUnmounted(() => {
                 class="shared-tag"
                 :title="`Shared by ${item.owner ?? 'another user'} — their record, not yours`"
               >by {{ item.owner ?? "another user" }}</span>
+              <!-- Surfaced-vs-opened (#4230). This is the only list in the UI
+                   that browses notes and lessons, so it is the only place
+                   those two kinds can show the counter at all. The advice is
+                   looked up per row because this feed is mixed — the remedy
+                   for a snippet nobody opens is not the remedy for a lesson. -->
+              <UsageBadge
+                :usage="item.usage"
+                :noun="item.note_type"
+                :dead-weight-advice="deadWeightAdvice(item.note_type)"
+              />
               <span class="k-card-date">{{ formatDate(item.updated_at) }}</span>
             </div>
           </div>
diff --git a/frontend/src/views/LessonDetailView.vue b/frontend/src/views/LessonDetailView.vue
index 81c5617..f6f8238 100644
--- a/frontend/src/views/LessonDetailView.vue
+++ b/frontend/src/views/LessonDetailView.vue
@@ -25,6 +25,7 @@ import { deleteLesson, getLesson, type Lesson } from "@/api/lessons";
 import ConfirmDialog from "@/components/ConfirmDialog.vue";
 import TagPill from "@/components/TagPill.vue";
 import UsageBadge from "@/components/UsageBadge.vue";
+import { DEAD_WEIGHT_ADVICE } from "@/utils/deadWeight";
 import { useToastStore } from "@/stores/toast";
 import { renderMarkdown } from "@/utils/markdown";
 
@@ -110,7 +111,7 @@ onMounted(load);
         <UsageBadge
           :usage="lesson.usage"
           noun="lesson"
-          dead-weight-advice="Repeatedly offered and never opened usually means the trigger fires on the wrong situation — re-key `when_to_apply` rather than deleting the claim."
+          :dead-weight-advice="DEAD_WEIGHT_ADVICE.lesson"
         />
       </div>
 
diff --git a/frontend/src/views/SnippetListView.vue b/frontend/src/views/SnippetListView.vue
index 6d86289..599448c 100644
--- a/frontend/src/views/SnippetListView.vue
+++ b/frontend/src/views/SnippetListView.vue
@@ -10,6 +10,7 @@ import {
 } from "@/api/snippets";
 import { useToastStore } from "@/stores/toast";
 import UsageBadge from "@/components/UsageBadge.vue";
+import { DEAD_WEIGHT_ADVICE } from "@/utils/deadWeight";
 
 const router = useRouter();
 const toast = useToastStore();
@@ -237,14 +238,6 @@ function driftTitle(s: SnippetListItem): string {
   const what = reasons[v.status] ?? "";
   return v.detail ? `${when}: ${what}. ${v.detail}` : `${when}: ${what}.`;
 }
-
-/** The dead-weight nudge for a SNIPPET, passed to the shared badge. Kept here
- *  rather than inside the component because the remedy is kind-specific — a
- *  rule in the same position gets different advice (milestone 333 step 5). */
-const SNIPPET_DEAD_WEIGHT =
-  "Offered repeatedly without ever being opened — consider rewriting its " +
-  "“when to reach for it” so it says when, or deleting it. It takes a slot " +
-  "in every future auto-inject menu.";
 </script>
 
 <template>
@@ -431,7 +424,7 @@ const SNIPPET_DEAD_WEIGHT =
           <span v-if="driftBadge(s)" class="drift-tag" :title="driftTitle(s)">
             {{ driftBadge(s) }}
           </span>
-          <UsageBadge :usage="s.usage" :dead-weight-advice="SNIPPET_DEAD_WEIGHT" />
+          <UsageBadge :usage="s.usage" :dead-weight-advice="DEAD_WEIGHT_ADVICE.snippet" />
           <span v-if="s.shared" class="shared-tag" :title="`Shared by ${s.owner ?? 'another user'} — a suggestion, not your own record`">
             by {{ s.owner ?? "another user" }}
           </span>
diff --git a/src/scribe/mcp/tools/lessons.py b/src/scribe/mcp/tools/lessons.py
index 0e507ac..24d46c9 100644
--- a/src/scribe/mcp/tools/lessons.py
+++ b/src/scribe/mcp/tools/lessons.py
@@ -17,7 +17,7 @@ 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.mcp.tools import systems as systems_tools
-from scribe.services.note_usage import empty_usage, record_pulled, usage_for_notes
+from scribe.services.note_usage import attach_usage, record_pulled
 
 
 # The payload shape lives in the service (`lesson_to_dict`), shared with the
@@ -65,7 +65,7 @@ async def list_lessons(
     # per lesson (#4196). An agent listing lessons can see which of its own
     # triggers are firing and which are not, which is the reading that leads to
     # `update_lesson` rather than to a second lesson about the same failure.
-    usage = await usage_for_notes([int(it["id"]) for it in labelled])
+    await attach_usage(labelled)
     rows = [
         {
             "id": it["id"], "title": it["title"], "tags": it.get("tags", []),
@@ -73,7 +73,7 @@ async def list_lessons(
             # Projected by `_note_to_item` straight off the `data` mirror —
             # absent when the row carries none, rather than an empty string.
             "when_to_apply": it.get("when_to_apply", ""),
-            "usage": usage.get(int(it["id"]), empty_usage()),
+            "usage": it["usage"],
             **({"shared": True, "owner": it.get("owner")} if it.get("shared") else {}),
         }
         for it in labelled
@@ -212,9 +212,7 @@ async def get_lesson(lesson_id: int, project_id: int = 0) -> dict:
     # Read BEFORE the pull is recorded, so the number an agent is shown is the
     # one that was true when it asked — otherwise every first read of a lesson
     # reports a pull that is its own.
-    out["usage"] = (await usage_for_notes([int(note.id)])).get(
-        int(note.id), empty_usage()
-    )
+    await attach_usage([out])
     record_pulled(
         user_id=uid, note_id=int(note.id),
         source="mcp_get_lesson", project_id=project_id,
diff --git a/src/scribe/mcp/tools/snippets.py b/src/scribe/mcp/tools/snippets.py
index 5824d6b..f9a0b47 100644
--- a/src/scribe/mcp/tools/snippets.py
+++ b/src/scribe/mcp/tools/snippets.py
@@ -15,7 +15,7 @@ from scribe.mcp.tools import systems as systems_tools
 from scribe.services import access as access_svc
 from scribe.services import dedup as dedup_svc
 from scribe.services import snippets as snippets_svc
-from scribe.services.note_usage import empty_usage, record_pulled, usage_for_notes
+from scribe.services.note_usage import attach_usage, record_pulled
 from scribe.services import systems as systems_svc
 
 
@@ -90,9 +90,7 @@ async def list_snippets(
         repo=repo, path=path, symbol=symbol, verification=verification,
     )
     labeled = await access_svc.label_shared_items(uid, items)
-    usage = await usage_for_notes([int(it["id"]) for it in labeled])
-    for it in labeled:
-        it["usage"] = usage.get(int(it["id"]), empty_usage())
+    await attach_usage(labeled)
     return {"snippets": labeled, "total": total}
 
 
diff --git a/src/scribe/routes/knowledge.py b/src/scribe/routes/knowledge.py
index b30ce93..c7c79d3 100644
--- a/src/scribe/routes/knowledge.py
+++ b/src/scribe/routes/knowledge.py
@@ -7,6 +7,7 @@ from scribe.auth import get_current_user_id, login_required
 from scribe.routes.utils import parse_pagination
 from scribe.services.access import label_shared_items
 from scribe.services.knowledge import FACET_TYPES
+from scribe.services.note_usage import attach_usage
 
 logger = logging.getLogger(__name__)
 
@@ -62,10 +63,23 @@ async def list_knowledge():
         offset=offset,
     )
 
+    items = await label_shared_items(uid, items)
+    # The surfaced-vs-opened counts, on the list a person actually browses
+    # (#4230). `usage_for_notes` always worked on every note row, but only the
+    # snippet and rule lists ever attached it — and this is the ONLY lesson and
+    # note list in the UI, so those two kinds had the counter collected and
+    # shown nowhere. Attaching here rather than teaching `/api/lessons` a
+    # second time is what closes both holes at once: `/knowledge` is how notes,
+    # lessons and processes are all browsed.
+    #
+    # Mixed kinds is not a problem for this: usage keys on the note row, which
+    # every facet of this feed is.
+    await attach_usage(items)
+
     return jsonify({
         # Mark rows another user owns: this feed can be mixed-ownership, and an
         # unmarked card reads as one the viewer wrote.
-        "items": await label_shared_items(uid, items),
+        "items": items,
         "total": total,
         "page": page,
         "per_page": limit,
diff --git a/src/scribe/routes/lessons.py b/src/scribe/routes/lessons.py
index e7e19d6..9137796 100644
--- a/src/scribe/routes/lessons.py
+++ b/src/scribe/routes/lessons.py
@@ -36,7 +36,7 @@ from scribe.services.access import (
     describe_provenance,
     label_shared_items,
 )
-from scribe.services.note_usage import empty_usage, record_pulled, usage_for_notes
+from scribe.services.note_usage import attach_usage, record_pulled
 
 logger = logging.getLogger(__name__)
 
@@ -83,9 +83,7 @@ async def list_lessons_route():
     # surfaced AND repeatedly opened, and the far commoner reading of the same
     # row is that the trigger fires on the wrong situation, which `update_lesson`
     # exists to fix.
-    usage = await usage_for_notes([int(it["id"]) for it in items])
-    for it in items:
-        it["usage"] = usage.get(int(it["id"]), empty_usage())
+    await attach_usage(items)
     return jsonify({"lessons": items, "total": total})
 
 
@@ -194,9 +192,7 @@ async def get_lesson_route(lesson_id: int):
         uid, out["learned_from"]
     )
     out.update(await describe_provenance(uid, note))
-    out["usage"] = (await usage_for_notes([lesson_id])).get(
-        lesson_id, empty_usage()
-    )
+    await attach_usage([out])
     # 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).
diff --git a/src/scribe/routes/snippets.py b/src/scribe/routes/snippets.py
index ff28699..585d803 100644
--- a/src/scribe/routes/snippets.py
+++ b/src/scribe/routes/snippets.py
@@ -19,7 +19,7 @@ from scribe.routes.utils import not_found, parse_pagination
 from scribe.services import dedup as dedup_svc
 from scribe.services import snippets as snippets_svc
 from scribe.services import systems as systems_svc
-from scribe.services.note_usage import empty_usage, record_pulled, usage_for_notes
+from scribe.services.note_usage import attach_usage, record_pulled
 from scribe.services.access import (
     can_write_note,
     describe_provenance,
@@ -75,9 +75,7 @@ async def list_snippets_route():
     # One aggregate for the whole page — a per-row lookup here would be N+1 by
     # construction. Every row gets the key, zero-filled, so the UI renders
     # "never pulled" rather than having to treat a missing field as a state.
-    usage = await usage_for_notes([int(it["id"]) for it in items])
-    for it in items:
-        it["usage"] = usage.get(int(it["id"]), empty_usage())
+    await attach_usage(items)
     return jsonify({"snippets": items, "total": total})
 
 
@@ -168,9 +166,7 @@ async def get_snippet_route(snippet_id: int):
         for s in await systems_svc.list_record_systems(note.user_id, snippet_id)
     ]
     data.update(await describe_provenance(uid, note))
-    data["usage"] = (await usage_for_notes([snippet_id])).get(
-        snippet_id, empty_usage()
-    )
+    await attach_usage([data])
     # Opening the detail view IS a pull — the operator chose to look. Tagged
     # apart from the MCP sources so "the agent reused it" and "a human read it"
     # stay distinguishable; they mean different things for pruning (#2085).
diff --git a/src/scribe/services/note_usage.py b/src/scribe/services/note_usage.py
index 5b0400e..cdada2b 100644
--- a/src/scribe/services/note_usage.py
+++ b/src/scribe/services/note_usage.py
@@ -30,6 +30,7 @@ from __future__ import annotations
 
 import asyncio
 import logging
+from collections.abc import Sequence
 
 from sqlalchemy import case, func, select
 
@@ -271,3 +272,64 @@ async def usage_for_notes(note_ids: list[int]) -> dict[int, dict]:
             if latest and (slot["last_pulled_at"] or "") < latest:
                 slot["last_pulled_at"] = latest
     return out
+
+
+def _row_id(row: dict, key: str) -> int | None:
+    """The note id on a payload row, or None when there is not one to read.
+
+    Skipping is deliberate: an id this cannot parse is not a reason to fail a
+    whole list, and GUESSING one would credit another record's counts to this
+    row — a wrong chip is worse than no chip, because it reads as a
+    measurement. `bool` is excluded explicitly because `int(True)` is 1, which
+    would quietly attach note #1's usage to a row carrying a flag.
+    """
+    raw = row.get(key)
+    if raw is None or isinstance(raw, bool):
+        return None
+    try:
+        return int(raw)
+    except (TypeError, ValueError):
+        return None
+
+
+async def attach_usage(rows: Sequence[dict], *, key: str = "id") -> None:
+    """Add `usage` to every row of a payload a door is about to return (#4230).
+
+    The one seam both doors and every record kind share. Before this, four call
+    sites carried their own copy of the same lines — two list routes and two
+    detail routes — and `/api/knowledge`, which is the list a person ACTUALLY
+    browses notes and lessons in, was about to become a fifth. That is how the
+    chip came to reach two record kinds out of four while a service named
+    `usage_for_notes` worked on all of them: each door read fine on its own,
+    and nobody was comparing them.
+
+    ONE AGGREGATE FOR THE WHOLE PAGE. `usage_for_notes` is a single GROUP BY
+    over the id set; calling it per row would be N+1 by construction, which is
+    the one shape a list route must not have.
+
+    EVERY ROW GETS THE KEY, zero-filled, so a record predating the table reads
+    as "never surfaced, never pulled" rather than making the UI treat a missing
+    field as a state. `UsageBadge` then renders nothing at all below one
+    surfacing, because "0/0" would look like a verdict where there is only an
+    absence of evidence.
+
+    NO try/except HERE, deliberately — it is not an oversight. The fail-open
+    already lives one layer down: `usage_for_notes` catches its own failure,
+    reports it through `_report_failure("readout")` and returns the zero-filled
+    map, so a broken readout degrades without breaking the list it decorates.
+    Wrapping it again would swallow the REPORT along with the error, and a
+    silently-swallowed readout failure is exactly #2663 — every counter reading
+    zero in production for weeks while the writes landed fine.
+
+    Mutates in place and returns None, matching how the call sites already used
+    it: these rows are the payload, not a copy of it.
+
+    A detail payload is just a one-row list — `await attach_usage([data])` —
+    so the single-record doors share this seam rather than keeping a second
+    shape that could drift from it.
+    """
+    pairs = [(row, _row_id(row, key)) for row in rows]
+    usage = await usage_for_notes([nid for _, nid in pairs if nid is not None])
+    for row, nid in pairs:
+        if nid is not None:
+            row["usage"] = usage.get(nid, empty_usage())
diff --git a/tests/test_usage_attach_seam.py b/tests/test_usage_attach_seam.py
new file mode 100644
index 0000000..c5d936d
--- /dev/null
+++ b/tests/test_usage_attach_seam.py
@@ -0,0 +1,205 @@
+"""One seam attaches `usage`, and every door that shows it uses that seam (#4230).
+
+WHAT WENT WRONG. `usage_for_notes` is named for notes and works on every note
+row. Yet the surfaced-vs-opened chip reached snippets and rules only: notes had
+it nowhere, and lessons had it collected but shown nowhere a person could
+reach, because the only lesson LIST in the UI is the Knowledge browse and that
+route never attached it.
+
+The cause was not any one missing line. Seven call sites carried their own copy
+of the same few lines — two REST lists, two REST details, two MCP lists, one
+MCP detail — and each read perfectly well on its own. Nobody was comparing
+them, so "which doors attach usage?" had no answer anywhere in the code. That
+is the same failure `test_system_tagging_door_parity.py` records for System
+tagging (#4249): whichever door nobody exercised for a kind is the one that
+never grew the feature, and a human reviewer does not reliably catch it because
+each door is only ever read alone.
+
+So this file asserts the PROPERTY, not the behaviour of one route: the attach
+logic exists once, and no door re-implements it. A kind added next month either
+goes through the seam or fails here.
+"""
+from __future__ import annotations
+
+import ast
+import pathlib
+from unittest.mock import AsyncMock, patch
+
+import pytest
+
+from scribe.services.note_usage import attach_usage, empty_usage
+
+ROOT = pathlib.Path(__file__).resolve().parents[1] / "src" / "scribe"
+
+# The aggregate the seam is built around. Calling it from a door is the shape
+# this file exists to prevent — not because the call is wrong, but because
+# seven of them drift.
+AGGREGATE = "usage_for_notes"
+
+
+def _counts(surfaced: int = 5, pulled: int = 0) -> dict:
+    u = empty_usage()
+    u["surfaced_count"] = surfaced
+    u["pull_count"] = pulled
+    return u
+
+
+def _aggregate_returns(mapping: dict[int, dict]) -> AsyncMock:
+    return patch(
+        "scribe.services.note_usage.usage_for_notes",
+        AsyncMock(return_value=mapping),
+    )
+
+
+# ── the seam itself ───────────────────────────────────────────────────────
+
+
+async def test_every_row_gets_the_key_even_with_no_events() -> None:
+    """Zero-filled, never absent. The UI must not have to tell "no events"
+    from "no field" — and `UsageBadge` renders nothing below one surfacing, so
+    an un-surfaced record is quiet without the caller doing anything."""
+    rows = [{"id": 1}, {"id": 2}]
+    with _aggregate_returns({1: _counts(surfaced=3)}):
+        await attach_usage(rows)
+    assert rows[0]["usage"]["surfaced_count"] == 3
+    assert rows[1]["usage"] == empty_usage()
+
+
+async def test_one_aggregate_for_the_whole_page() -> None:
+    """The N+1 guard. A per-row lookup here would be N+1 by construction, which
+    is the one shape a list route must not have — and it is invisible in
+    review, because the per-row version reads more naturally."""
+    rows = [{"id": n} for n in range(25)]
+    mock = AsyncMock(return_value={})
+    with patch("scribe.services.note_usage.usage_for_notes", mock):
+        await attach_usage(rows)
+    assert mock.await_count == 1, "usage must be read once per page, not per row"
+    assert sorted(mock.await_args.args[0]) == list(range(25))
+
+
+async def test_a_detail_payload_is_just_a_one_row_list() -> None:
+    """The single-record doors share the seam rather than keeping a second
+    shape beside it. Two shapes for one job is how the seven copies started."""
+    data = {"id": 7, "title": "x"}
+    with _aggregate_returns({7: _counts(surfaced=9, pulled=2)}):
+        await attach_usage([data])
+    assert data["usage"]["pull_count"] == 2
+
+
+async def test_a_row_with_no_id_is_skipped_rather_than_failing_the_list() -> None:
+    """An unusable id is not a reason to 500 a page of otherwise fine rows."""
+    rows = [{"id": 1}, {"title": "no id here"}]
+    with _aggregate_returns({1: _counts()}):
+        await attach_usage(rows)
+    assert "usage" in rows[0]
+    assert "usage" not in rows[1]
+
+
+async def test_a_boolean_is_not_an_id() -> None:
+    """`int(True)` is 1, so a row carrying a flag under the key would silently
+    be credited with note #1's counts. A wrong chip is worse than no chip: it
+    reads as a measurement."""
+    rows = [{"id": True}]
+    with _aggregate_returns({1: _counts(surfaced=40)}):
+        await attach_usage(rows)
+    assert "usage" not in rows[0]
+
+
+async def test_a_string_id_still_resolves() -> None:
+    """Payload rows come from several serialisers; one of them handing back a
+    stringified id should not silently drop the chip."""
+    rows = [{"id": "12"}]
+    with _aggregate_returns({12: _counts(surfaced=4)}):
+        await attach_usage(rows)
+    assert rows[0]["usage"]["surfaced_count"] == 4
+
+
+@pytest.mark.parametrize("key", ["note_id", "record_id"])
+async def test_the_key_can_be_named(key: str) -> None:
+    rows = [{key: 3}]
+    with _aggregate_returns({3: _counts()}):
+        await attach_usage(rows, key=key)
+    assert "usage" in rows[0]
+
+
+async def test_an_empty_page_asks_nothing_and_breaks_nothing() -> None:
+    mock = AsyncMock(return_value={})
+    with patch("scribe.services.note_usage.usage_for_notes", mock):
+        await attach_usage([])
+    assert mock.await_args.args[0] == []
+
+
+# ── the property: one seam, and every door uses it ────────────────────────
+
+
+def _calls(tree: ast.Module) -> set[str]:
+    out = set()
+    for node in ast.walk(tree):
+        if isinstance(node, ast.Call):
+            fn = node.func
+            name = fn.attr if isinstance(fn, ast.Attribute) else getattr(fn, "id", None)
+            if name:
+                out.add(name)
+    return out
+
+
+def _door_modules() -> list[pathlib.Path]:
+    return sorted(
+        [*(ROOT / "routes").glob("*.py"), *(ROOT / "mcp" / "tools").glob("*.py")]
+    )
+
+
+def test_no_door_calls_the_aggregate_directly() -> None:
+    """THE GUARD. Seven doors each called `usage_for_notes` and zero-filled by
+    hand; the eighth would have been `/api/knowledge`, and the chip would have
+    kept reaching some kinds and not others.
+
+    Keyed on the CALL, not on the text, so a module that merely names the
+    function in a comment explaining the seam is not a false positive — and a
+    hand-kept skip list, which would itself go stale, is not needed (rule 167).
+    """
+    offenders = []
+    for path in _door_modules():
+        if AGGREGATE in _calls(ast.parse(path.read_text())):
+            offenders.append(str(path.relative_to(ROOT.parent.parent)))
+    assert not offenders, (
+        f"these doors call {AGGREGATE}() themselves instead of attach_usage(); "
+        f"that is how the chip came to reach two record kinds out of four: "
+        f"{offenders}"
+    )
+
+
+# (module, the functions that return note-bearing payloads)
+#
+# Not a list of everything that COULD attach usage — a list of the doors that
+# demonstrably show it today. A door dropping its call silently is the exact
+# regression this pins.
+DOORS = [
+    ("routes/lessons.py", "list_lessons_route or get_lesson_route"),
+    ("routes/snippets.py", "list/get snippet routes"),
+    ("routes/knowledge.py", "list_knowledge — the only note & lesson list in the UI"),
+    ("mcp/tools/lessons.py", "list_lessons / get_lesson"),
+    ("mcp/tools/snippets.py", "list_snippets"),
+]
+
+
+@pytest.mark.parametrize(("module", "why"), DOORS)
+def test_every_door_that_shows_usage_goes_through_the_seam(module: str, why: str) -> None:
+    assert "attach_usage" in _calls(ast.parse((ROOT / module).read_text())), (
+        f"{module} no longer attaches usage ({why}). If that is deliberate, "
+        f"remove it from DOORS and say why; a door that silently stops "
+        f"attaching looks exactly like a corpus nobody uses."
+    )
+
+
+def test_the_knowledge_browse_is_covered_because_it_is_the_only_note_list() -> None:
+    """Pinned on its own, with the reason, because it is the non-obvious one.
+
+    `/api/lessons` already attached usage and it did not help: no view calls
+    it. `KnowledgeView` is the only list in the UI that renders notes and
+    lessons, so `/api/knowledge` is the only route through which those two
+    kinds can show the counter at all. Deleting this line would restore the
+    original bug while every other test here still passed.
+    """
+    assert any(m == "routes/knowledge.py" for m, _ in DOORS)
+    assert "attach_usage" in _calls(ast.parse((ROOT / "routes" / "knowledge.py").read_text()))