docs(self-surfacing): derive groups are drift not audit material — shape-accounting + reusing-code skills, static floor, plugin README (after-write hook), api-reference rows (#2902, milestone 299 step 4)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/api-reference.md

@@ -172,6 +172,8 @@ endpoint at `/mcp`, not these REST routes.
 | GET | `/api/plugin/context` | SessionStart context payload (rules + active-project) |
 | GET | `/api/plugin/retrieve` | Title-first knowledge-injection candidates |
 | GET | `/api/plugin/processes` | Stored Processes for skill-stub sync |
+| GET | `/api/plugin/prior-art` | Write-path hint for the plugin hooks (params: `path`, `code`, `repo`, `shapes`, `exclude_ids`, `exclude_sync_ids`, `exclude_derive`); returns `context`, `note_ids`, `sync_note_ids`, `stamped`, `divergence`, `derive`, `derive_keys` |
+| GET / POST | `/api/projects/<id>/coverage`, `…/coverage/refresh` | Shape-ledger accounting (`pattern_coverage` line, counts, `derive_groups`, `derive_new`, `divergence`, `recheck`) |
 | GET / PUT | `/api/plugin/marketplace-url` | Read / set the plugin marketplace URL |
 
 ## Dashboard, Export, Trash, Users

plugin/README.md

@@ -52,8 +52,18 @@ On install you'll be asked for:
   but never stop it; silent when nothing is recorded, which is most of the time.
   Two framings: a REUSE menu (similar/nearby records), and a SYNC nudge when a
   snippet records the exact file being edited — "updating the record is part of
-  the edit" — each with its own once-per-session dedup.
+  the edit" — each with its own once-per-session dedup. A third, ledger-fed
+  line names a duplicate family (no canon) or a canon recorded elsewhere for
+  the names being written (its own dedup channel, `exclude_derive`).
   Toggle in **Settings → Knowledge auto-inject**.
+- `hooks/hooks.json` → PostToolUse hook on `Bash`
+  (`hooks/scribe_after_write.sh`): code written through sed/heredocs/scripts
+  never reaches the PreToolUse hook, so this one diffs the working tree after
+  every Bash call (per-session path+blob snapshot; one `git status` when
+  nothing changed) and runs the same arms on the definitions just written,
+  through the same endpoint and the same dedup channels. `additionalContext`
+  only; silent on any failure. The extractor, the prose/data skip list and the
+  local by-name duplicate arm are shared in `hooks/scribe_defs.sh`.
 - `skills/` → the universal process-skills, surfaced by description match.
 - `hooks/scribe_sync_processes.sh` (a 2nd SessionStart hook) + the `/scribe:sync`
   command → generate `~/.claude/skills/scribe-proc-*` stubs from your Scribe

plugin/hooks/scribe_after_write.sh

@@ -187,6 +187,11 @@ while IFS= read -r rel_path; do
         printf '%s' "$body" | jq -r '((.note_ids // []) - (.sync_note_ids // []))[]?' 2>/dev/null >> "$idfile" || true
         printf '%s' "$body" | jq -r '(.sync_note_ids // [])[]?' 2>/dev/null >> "$syncfile" || true
         printf '%s' "$body" | jq -r '(.derive_keys // [])[]?' 2>/dev/null >> "$derivefile" || true
+        # Several files in one call may name the same family: keep each
+        # token once, so the next request's exclude list stays exact.
+        for f in "$idfile" "$syncfile" "$derivefile"; do
+          [ -s "$f" ] && { sort -u -o "$f" "$f" 2>/dev/null || true; }
+        done
       fi
     fi
   fi

plugin/hooks/scribe_static_context.md

@@ -66,7 +66,10 @@ for the operator's work, and as your own working memory across sessions.
   should read as a map of every shape in it. The backstop still holds:
   noticing the second copy of anything, or consolidating copies into a shared
   X, means X gets recorded before that work is finished — which is how a
-  codebase is kept from growing four `.btn-primary` definitions.
+  codebase is kept from growing four `.btn-primary` definitions. The write-path
+  hooks (before a Write/Edit, and after any Bash call that changed the tree)
+  name a known duplicate family or a canon elsewhere for what was just
+  written — act on that line at the write, not at the next audit.
 - Do **not** keep the operator's rules, plans, or project notes in local
   memory / CLAUDE.md in parallel with Scribe — Scribe holds the single copy.
 - **Compact at clean seams** — because you record as you go, a context

plugin/skills/reusing-code/SKILL.md

@@ -44,6 +44,13 @@ through recall/auto-inject; this skill is the active reflex around that.
   it before you go any further. Either it's the helper you were about to
   duplicate — reuse it and drop yours — or it isn't, and the record needs the new
   location adding. Both are cheaper now than after the duplicate settles in.
+- **A `Shape ledger at …` line is the ledger speaking, not the record.** It
+  names a duplicate family ("identical body in N other files, no canon") or a
+  canon elsewhere for a name you just wrote — for edits made through Bash
+  (sed, heredocs, scripts) as much as through Write/Edit. Derive the family or
+  reuse the canon *now*; a family that is convention rather than copies is
+  dismissed with `classify_shapes(..., status="exempt",
+  reason_code="convention-plumbing")`, never ignored.
 - **A `[records this file]` hint is a duty, not a menu.** When the hint says a
   snippet records the very file you're editing, the record's freshness is now
   YOUR edit's responsibility: if the edit changes the recorded shape,

plugin/skills/shape-accounting/SKILL.md

@@ -85,6 +85,34 @@ the dominant form, `create_snippet` it, migrate the outliers, then classify
 the rest as instances. Canon is determined from the code; consistency comes
 from the derivation, not from asking permission.
 
+## Derive groups are drift, not audit material
+
+The catalogue exists so the codebase is DRY **from inception**, not as DRY as
+the last sweep left it. Three surfaces say so without anyone running an audit
+(milestone 299):
+
+- **At the write** — the prior-art hint (the Write/Edit hook, and since
+  0.1.39 the after-write hook on Bash, so sed/heredoc/script edits count too)
+  carries a `Shape ledger at <path>` line when a name just written is a known
+  **duplicate family** ("identical body in N other files, no canon") or a
+  **canon elsewhere** ("snippet #N at <path> — reuse, don't redefine"). Act
+  on it *then*: pull the canon and build from it, or derive the family now —
+  `create_snippet` the dominant form, repoint the copies, `classify_shapes`
+  them `instance`. A family is named once per session.
+- **On arrival** — the coverage line's `standing:` block (shown even when
+  nothing is unclassified) and `derive_new` ("+N new copies since last
+  refresh: .x in <path>") name what drifted since the previous refresh. That
+  is the todo of the moment, sized to the last batch — not a backlog.
+- **A family that is convention, not copies** — component-local `load` /
+  `toggle` / `save` that happen to share a name — is dismissed, not
+  consolidated: `classify_shapes(..., status="exempt",
+  reason_code="convention-plumbing", reason=…)` (or `classify_shapes_by_rule`
+  for a whole family) removes it from the queue. Dismissal is a judgment and
+  it is recorded; silence is not.
+
+After the one-time pay-down the derive queue reads empty; anything in it
+afterwards is drift of the moment, and the hint already said so at the write.
+
 ## The divergence readout — button B where button A is canon
 
 Three questions the ledger answers mechanically (#2793):

tests/test_write_path_trigger.py

@@ -853,14 +853,18 @@ def test_hook_exits_silently_when_unconfigured():
 
 
 def test_hook_skips_prose_and_data_files():
-    """No round-trip for a markdown edit — the server would return nothing anyway."""
-    src = HOOK.read_text()
-    skip = re.search(r"case \"\$file_path\" in\n(.*?)esac", src, re.S)
-    assert skip, "expected an extension skip list"
+    """No round-trip for a markdown edit — the server would return nothing
+    anyway. The list lives in the shared library (#2901) and the hook asks it."""
+    lib = (PLUGIN / "hooks" / "scribe_defs.sh").read_text()
+    skip = re.search(r"scribe_skip_path\(\) \{\n  case \"\$1\" in\n(.*?)esac", lib, re.S)
+    assert skip, "expected an extension skip list in scribe_defs.sh"
     for ext in ("*.md", "*.json", "*.lock", "*.png"):
         assert ext in skip.group(1)
     # Config formats are deliberately NOT skipped — a workflow file is reusable.
     assert "*.yml" not in skip.group(1)
+    src = HOOK.read_text()
+    assert 'scribe_skip_path "$file_path" && exit 0' in src
+    assert '/scribe_defs.sh"' in src                   # sourced, not copied
 
 
 def test_plugin_version_bumped_with_the_hook():
@@ -1150,7 +1154,9 @@ def test_hook_names_the_shapes_being_written():
     that changes a body, not a signature — the definition enclosing the edit,
     found by walking the target file upward from the edited lines."""
     src = HOOK.read_text()
-    assert "scribe_defs()" in src                      # one extractor, two consumers
+    lib = (PLUGIN / "hooks" / "scribe_defs.sh").read_text()
+    assert "scribe_defs()" in lib                      # one extractor, shared (#2901)
+    assert "scribe_defs()" not in src                  # ...not a second copy here
     assert ".tool_input.old_string" in src             # the Edit's anchor
     assert "| tac | scribe_defs | head -1" in src      # nearest definition above
     # The ledger feed sends NAMES, never bodies, and stays on the one GET.