src/scribe/models/note.py

@@ -76,8 +76,27 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
     recurrence_next_spawn_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True
     )
-    # Note type — 'note' (default) or 'process' (a stored process). Task-ness is
-    # tracked by `status`, not here. (person/place/list entity types removed 2026-07.)
+    # WHAT KIND of record this is, on the note/entity axis. Task-ness is tracked
+    # by `status`, not here (person/place/list entity types removed 2026-07):
+    #   note    (default) — authored prose, findable by what it is ABOUT
+    #   process — a stored procedure, synced to a client as a skill
+    #   snippet — a reusable shape, with its structured fields mirrored in
+    #             `data` and its trigger composed into the title (0070)
+    #   lesson  — a transferable insight, findable by WHEN IT APPLIES rather
+    #             than by topic (milestone 385). Same mirror discipline as a
+    #             snippet: the trigger lives in `data.when_to_apply` and is
+    #             composed into the title and the head of the body, which is
+    #             what puts it in the embedded document. A lesson is never a
+    #             task — see `status` — because a lesson that acquired one
+    #             would start appearing in open-work listings.
+    #
+    # DELIBERATELY UNGATED. Unlike `task_kind` there is no CHECK on this column:
+    # migration 0036 added it as plain Text with a server default and nothing
+    # has constrained it since, so rule 36 has no whitelist to expand when a
+    # kind is added. The vocabulary that actually decides what a reader can
+    # reach is `services.knowledge._FACETS`; an unrecognised value there
+    # resolves to a filter matching nothing, which is the intended answer to a
+    # typo.
     note_type: Mapped[str] = mapped_column(Text, default="note", server_default="note")
     # Task sub-kind — what KIND of work this is, not how it is going:
     #   work   (default) — ships a change

src/scribe/services/embeddings.py

@@ -203,6 +203,33 @@ def embedding_text(title: str | None, body: str | None) -> str:
     return f"{title}\n{body}".strip() if body else title
 
 
+def trigger_title(subject: str | None, trigger: str | None) -> str:
+    """`{subject} — {trigger}` — the title half of a situation-keyed document.
+
+    ONE definition, because this join had three. `rule_document` built it for
+    rules, `snippets.compose_title` for snippets, and milestone 385 needed a
+    fourth for lessons — the shape #3207 records, where a fix or an improvement
+    then has to be found in N places by someone who does not know N.
+
+    WHY THE JOIN MATTERS AT ALL, measured in note #2485: the snippet was the
+    only sharp record in the corpus — a 0.153 top-to-second gap against
+    0.010–0.023 for everything else — and the cause was this title plus the
+    same trigger repeated in the body, so purpose appears twice in a short
+    document and dominates the vector. Every kind that must be findable by WHEN
+    IT APPLIES rather than what it is about is built on this line.
+
+    Either side alone is returned as-is: a record with no trigger yet degrades
+    to its subject and still embeds, just less sharply — which is an argument
+    for backfilling triggers, not for padding the title with whatever text is
+    to hand.
+    """
+    subject = (subject or "").strip()
+    trigger = (trigger or "").strip()
+    if subject and trigger:
+        return f"{subject} — {trigger}"
+    return subject or trigger
+
+
 # --- chunking (#280): the document shape ------------------------------------
 #
 # bge-small reads at most 512 tokens and fastembed silently truncates the rest,
@@ -785,7 +812,7 @@ def rule_document(
     if not trigger:
         return name or None, body or None
     return (
-        f"{name} — {trigger}" if name else trigger,
+        trigger_title(name, trigger),
         f"When to apply: {trigger}\n\n{body}" if body else f"When to apply: {trigger}",
     )
 

src/scribe/services/knowledge.py

@@ -292,6 +292,13 @@ _FACETS: dict[str, tuple[bool, str | None]] = {
     "note": (False, "note"),
     "process": (False, "process"),
     "snippet": (False, "snippet"),
+    # A transferable insight, keyed by the situation it applies to rather than
+    # by its topic (milestone 385). It reaches the browse surface through this
+    # one entry: the door's validation, the counts and both dialects of the
+    # type filter are all generated from this table, which is the property
+    # #3161 asked for so that adding a kind is a single edit rather than four
+    # that must agree.
+    "lesson": (False, "lesson"),
 }
 
 # The non-task record types, for the counts query. Derived so it cannot drift

src/scribe/services/lessons.py

@@ -0,0 +1,138 @@
+"""Lesson service — a transferable insight, retrievable by situation.
+
+A *lesson* is a Note with ``note_type='lesson'``: a better way to think about a
+problem, or a solution that transfers, recorded so a later session meets it at
+the moment it applies — and **without binding the reader**.
+
+WHY THE KIND EXISTS (milestone 385, from note #3727)
+
+Rules were the only surface that is global AND situation-keyed, so an agent
+holding a transferable insight had one door, and that door binds. The observed
+symptom was sessions offering rule proposals for things that should not be
+rules.
+
+The gap is a document-shape fact, not a threshold:
+
+  - a note is embedded as ``title\\nbody`` and is findable by **what it is
+    about**;
+  - a rule is embedded as ``{title} — {trigger}`` with ``When to apply:``
+    repeated at the head of the body, so the trigger appears twice in a short
+    document and dominates the vector — findable by **when it applies**.
+
+No tuning reaches across that: the field the query would match on is simply not
+in a note's document. So a lesson carries a trigger and is embedded like a rule,
+while staying a note in every other respect.
+
+WHERE THE TRIGGER LIVES (decision #4157, milestone 385 step 1)
+
+In ``notes.data`` under ``when_to_apply``, written through a named parameter and
+mirrored into the title and the head of the body — the shape snippets already
+use for ``when_to_use``. Not a column on ``notes``.
+
+That decision was measured rather than assumed. The whole snippet corpus —
+164 of 164 — carries a ``when_to_use`` with **no guard anywhere**, which refutes
+the premise that an unenforced field gets skipped. What it does NOT show is that
+an agent types a title convention correctly: ``compose_title`` builds the title
+from the parameter, so what is at 100% is a named structured field. A column
+would have bought enforceability at the price of deciding, for every note kind
+at once, a question nothing had measured.
+
+The mirror is what makes the vector sharp, and it is why nothing re-embeds:
+``chunk_document`` is untouched, so ``CHUNKER_VERSION`` does not move. The
+trigger reaches the document by being in the text, exactly as a snippet's is.
+
+WHAT A LESSON INHERITS, AND THE CELLS LEFT EMPTY ON PURPOSE (#3163)
+
+A new kind inherits the note machinery wholesale, and #3163 asks which parts it
+should NOT get — so that an empty cell is a decision rather than an oversight.
+
+Inherited, all deliberately:
+
+  - **versions** — a lesson is reworded as understanding improves, and what it
+    used to say is worth as much as any note's history.
+  - **supersession** — the event this most needs. A lesson replaced by a better
+    lesson is precisely what ``note_supersessions`` models, and the demotion
+    penalty already exists.
+  - **trash**, **the share ACL**, **tags**, **project and System tagging**,
+    **chunked embeddings**, **the near-duplicate gate**.
+
+NOT inherited, and each for a stated reason:
+
+  - **status / task_kind / milestone_id** — a lesson is not work. ``is_task`` is
+    ``status is not None``, so a lesson that acquired a status would become a
+    task and appear in open-work listings. This is the one cell where filling it
+    in by accident silently changes what the record IS.
+  - **recurrence** — task-only, and a lesson does not recur.
+  - **verify_with / expires_when** — available, because they are generic note
+    fields, but not part of a lesson's contract and not asked for on create. The
+    milestone-312 distinction is why: those mark a record that asserts a FACT
+    about someone else's software and can go false unwatched. A lesson is closer
+    to a norm — "a better way to think about this" has no truth value that rots
+    on its own. A lesson that does assert such a fact can still carry them.
+
+WHY THERE IS NO MIGRATION
+
+``note_type`` carries **no CHECK constraint** — only ``task_kind`` does
+(``notes_task_kind_check``, migrations 0056 / 0065). Migration 0036 added
+``note_type`` as plain ``Text`` with a server default and nothing has gated it
+since. So rule 36 has nothing to expand here, and the failure it guards against
+— a value the database refuses on an instance predating its migration — cannot
+arise for this column.
+
+The real vocabulary is ``services.knowledge._FACETS``, which is where a kind
+becomes reachable on the browse surface and validated at the door. That is one
+table feeding both dialects of the type filter, so adding a kind there is a
+single edit — a property #3161 recommended and that landed before this.
+"""
+from __future__ import annotations
+
+import re
+
+LESSON_NOTE_TYPE = "lesson"
+
+# The key in `notes.data`. Named for the field it mirrors on `rules`, because it
+# answers the same question and a reader who knows one should not have to learn
+# a second word for it.
+TRIGGER_KEY = "when_to_apply"
+
+# The body's trigger line, and the pattern that reads it back. The body is the
+# readable form and the thing that gets embedded; `data` is the queryable
+# mirror. Reads prefer the mirror and fall back to this, which is the discipline
+# `snippet_fields` follows and the reason a row written before the mirror
+# existed is still readable.
+_BODY_TRIGGER_RE = re.compile(r"^\*\*When to apply:\*\*\s*(.+?)\s*$", re.M)
+
+
+def lesson_trigger(note) -> str:
+    """When this lesson applies, or "" — the mirror first, then the body.
+
+    Prefers `data` for the same reason every snippet read does: it is indexed,
+    and parsing a body to answer a question the database can answer is how a
+    hot path ends up regexing markdown. The fallback is not dead code — it is
+    what makes a lesson readable if the mirror is ever absent, and an absent
+    mirror must degrade to the right answer rather than to silence.
+    """
+    data = getattr(note, "data", None) or {}
+    from_mirror = (data.get(TRIGGER_KEY) or "").strip() if isinstance(data, dict) else ""
+    if from_mirror:
+        return from_mirror
+    match = _BODY_TRIGGER_RE.search(getattr(note, "body", None) or "")
+    return match.group(1).strip() if match else ""
+
+
+def compose_title(what: str, when_to_apply: str = "") -> str:
+    """`{what} — {when it applies}`, the half of the document that ranks.
+
+    Built HERE rather than asked of the caller, and that distinction is the
+    whole evidence base for this design: the snippet corpus is at 100% on its
+    trigger because a service composes the title from a named parameter, not
+    because agents type separators reliably. A caller made to spell the
+    convention is the option milestone 385 step 1 rejected.
+
+    The join is `embeddings.trigger_title` — shared with rules and snippets, so
+    the three kinds that rank on a trigger cannot drift apart in how they say
+    so.
+    """
+    from scribe.services.embeddings import trigger_title
+
+    return trigger_title(what, when_to_apply)

src/scribe/services/snippets.py

@@ -55,10 +55,15 @@ UNSET: object = object()
 # --- serialize: structured fields -> note (title/body/tags) ------------------
 
 def compose_title(name: str, when_to_use: str = "") -> str:
-    """`name — when to use` (or just `name` when no usage note is given)."""
-    name = (name or "").strip()
-    when = (when_to_use or "").strip()
-    return f"{name} — {when}" if when else name
+    """`name — when to use` (or just `name` when no usage note is given).
+
+    The join itself lives in `embeddings.trigger_title`, which rules and
+    lessons build their titles from too. Kept as a named function here because
+    it is this module's public vocabulary and callers say `compose_title`.
+    """
+    from scribe.services.embeddings import trigger_title
+
+    return trigger_title(name, when_to_use)
 
 
 def compose_tags(language: str = "", tags: list[str] | None = None) -> list[str]:

tests/test_integration_lesson_kind.py

@@ -0,0 +1,112 @@
+"""A lesson is a row the database actually accepts (milestone 385 step 2).
+
+WHY THIS IS THE REAL GUARD, AND WHY IT IS HERE
+
+The step asked for "a guard that the CHECK actually accepts `lesson` and
+rejects a typo", citing #3128 — a `spike` kind that could not be written on an
+instance predating its migration. That failure mode belongs to `task_kind`,
+which IS gated (`notes_task_kind_check`, migrations 0056 and 0065).
+`note_type` is not gated at all: migration 0036 added it as plain Text with a
+server default and nothing has constrained it since.
+
+So there is no whitelist to expand and no rejection to assert. Writing the
+guard as "the constraint admits lesson" would have pinned a constraint that
+does not exist; writing it as "no constraint exists" would pin today's schema
+rather than the behaviour that matters, and would go red on a change that is
+perfectly fine.
+
+Asserting the WRITE covers both worlds. It passes today, it keeps passing if a
+CHECK is added that admits `lesson`, and it goes red the day one is added that
+does not — which is the only outcome anyone needs to be told about.
+
+The second test is the cell #3163 asks to be left empty on purpose: a lesson
+must not be a task. `is_task` is a read-only property over `status`, so this
+cannot be asserted by setting a flag — it has to be observed on a real row.
+"""
+import pytest
+import pytest_asyncio
+from sqlalchemy import select
+
+from scribe.models import async_session
+from scribe.models.note import Note
+from scribe.services import lessons as lessons_svc
+from scribe.services import notes as notes_svc
+from tests.helpers import ensure_user
+
+pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine")]
+
+OWNER_USERNAME = "lesson_kind_owner"
+
+TRIGGER = "the operator pasted a stack trace and said it is still broken"
+SUBJECT = "Change one thing, then look"
+
+
+@pytest_asyncio.fixture
+async def owner_id():
+    async with async_session() as s:
+        owner = await ensure_user(s, OWNER_USERNAME)
+        await s.commit()
+        uid = owner.id
+        # Cleaned at SETUP rather than teardown: create_note fires a detached
+        # embedding refresh that opens its own connection and writes the row,
+        # and a teardown delete would race it. A fresh loop has already
+        # cancelled whatever the previous test left in flight.
+        for note in (await s.execute(
+            select(Note).where(
+                Note.user_id == uid,
+                Note.note_type == lessons_svc.LESSON_NOTE_TYPE,
+            )
+        )).scalars().all():
+            await s.delete(note)
+        await s.commit()
+    return uid
+
+
+async def test_a_lesson_is_a_row_the_database_accepts(owner_id):
+    """The whole point. Goes red if `note_type` is ever gated without this
+    value, and stays correct if it is gated with it."""
+    lesson = await notes_svc.create_note(
+        owner_id,
+        title=lessons_svc.compose_title(SUBJECT, TRIGGER),
+        body=f"**When to apply:** {TRIGGER}\n\nOne change at a time.",
+        note_type=lessons_svc.LESSON_NOTE_TYPE,
+        data={lessons_svc.TRIGGER_KEY: TRIGGER},
+    )
+
+    async with async_session() as s:
+        stored = (await s.execute(
+            select(Note).where(Note.id == lesson.id)
+        )).scalars().one()
+
+    assert stored.note_type == "lesson"
+    # The trigger survives the round trip on both halves — the indexed mirror
+    # and the readable body — because the vector is built from the text and
+    # the queries are built from the mirror.
+    assert lessons_svc.lesson_trigger(stored) == TRIGGER
+    assert stored.title == f"{SUBJECT} — {TRIGGER}"
+    assert "**When to apply:**" in (stored.body or "")
+
+
+async def test_a_lesson_is_not_a_task(owner_id):
+    """#3163's cell left empty on purpose.
+
+    `is_task` is `status is not None` and is read-only, so this is only
+    observable on a stored row. A lesson that arrived with a status would join
+    the open-work listings — the one way an unfilled field changes what the
+    record IS rather than what it says.
+    """
+    lesson = await notes_svc.create_note(
+        owner_id,
+        title=lessons_svc.compose_title(SUBJECT, TRIGGER),
+        body="One change at a time.",
+        note_type=lessons_svc.LESSON_NOTE_TYPE,
+    )
+
+    async with async_session() as s:
+        stored = (await s.execute(
+            select(Note).where(Note.id == lesson.id)
+        )).scalars().one()
+
+    assert stored.status is None
+    assert stored.is_task is False
+    assert stored.milestone_id is None

tests/test_lesson_kind.py

@@ -0,0 +1,114 @@
+"""The `lesson` kind — its trigger, its title, and its place in the vocabulary.
+
+WHY THIS EXISTS (milestone 385 step 2, #3729)
+
+A lesson is a note that must be findable by WHEN IT APPLIES rather than by what
+it is about. That is a document-shape fact: the trigger has to reach the
+embedded text, and decision #4157 put it in `notes.data` with a mirror in the
+title and the head of the body — the shape snippets already use.
+
+THE ONE THAT MATTERS MOST
+
+`test_one_join_builds_every_trigger_title`. Three kinds now rank on a
+`{subject} — {trigger}` title: rules, snippets and lessons. That join had three
+implementations before this step and would have had four; #3207 records what
+that costs. The guard is behavioural rather than `assert a is b`, because the
+three are reached through different public names and a test that compared
+identities would pass on a re-implementation that merely re-exported.
+
+WHAT IS DELIBERATELY NOT TESTED HERE
+
+That the database accepts `note_type='lesson'`. `note_type` carries no CHECK —
+only `task_kind` does — so there is nothing to assert against in a unit test,
+and asserting the absence would pin the schema's current shape rather than the
+behaviour that matters. The real guard is in the integration lane, where a
+lesson is written and read back: it holds whether or not a constraint exists,
+and goes red the day one is added without this value.
+"""
+from types import SimpleNamespace
+
+from scribe.services import knowledge as knowledge_svc
+from scribe.services import lessons as lessons_svc
+from scribe.services import snippets as snippets_svc
+from scribe.services.embeddings import trigger_title
+
+
+def _note(data=None, body=""):
+    return SimpleNamespace(data=data, body=body)
+
+
+def test_the_trigger_is_read_from_the_indexed_mirror():
+    note = _note(data={"when_to_apply": "a stack trace, and it is still broken"})
+    assert lessons_svc.lesson_trigger(note) == "a stack trace, and it is still broken"
+
+
+def test_the_body_answers_when_the_mirror_is_absent():
+    """Not dead code. A row written before the mirror existed is still readable,
+    and an absent mirror has to degrade to the right answer rather than to
+    silence — the discipline `snippet_fields` follows for the same reason."""
+    note = _note(body="**When to apply:** about to reach for a second hypothesis\n\nbody")
+    assert lessons_svc.lesson_trigger(note) == "about to reach for a second hypothesis"
+
+
+def test_the_mirror_wins_when_both_are_present():
+    note = _note(
+        data={"when_to_apply": "from the mirror"},
+        body="**When to apply:** from the body",
+    )
+    assert lessons_svc.lesson_trigger(note) == "from the mirror"
+
+
+def test_no_trigger_reads_as_empty_rather_than_raising():
+    """A lesson with no trigger is unreachable, not broken. Whatever refuses to
+    write one belongs on the write path; a reader's job is to say so plainly."""
+    assert lessons_svc.lesson_trigger(_note()) == ""
+    assert lessons_svc.lesson_trigger(_note(data={}, body="no trigger line here")) == ""
+
+
+def test_one_join_builds_every_trigger_title():
+    """THE GUARD. Rules, snippets and lessons rank on the same title shape.
+
+    Behavioural on purpose — see the module docstring. Each of the three is
+    called through the name its own callers use, so a fourth hand-rolled copy
+    fails here even though it would look correct in isolation.
+    """
+    subject, trigger = "Pace hard debugging", "a stack trace and it is still broken"
+    expected = f"{subject} — {trigger}"
+
+    assert trigger_title(subject, trigger) == expected
+    assert lessons_svc.compose_title(subject, trigger) == expected
+    assert snippets_svc.compose_title(subject, trigger) == expected
+
+
+def test_a_subject_with_no_trigger_degrades_to_the_subject():
+    """It still embeds, just less sharply — an argument for backfilling
+    triggers, not for padding the title with whatever text is to hand."""
+    assert trigger_title("debounce", "") == "debounce"
+    assert lessons_svc.compose_title("  debounce  ") == "debounce"
+    assert trigger_title("", "when it applies") == "when it applies"
+
+
+def test_the_kind_is_in_the_browse_vocabulary():
+    """A kind the browse surface does not know is a record nobody can filter to.
+
+    Asserted through the public table rather than a literal, because the door's
+    validation, the counts and both dialects of the type filter are all
+    generated from it (#3161) — so this is the one place that decides.
+    """
+    assert lessons_svc.LESSON_NOTE_TYPE in knowledge_svc.FACET_TYPES
+    assert lessons_svc.LESSON_NOTE_TYPE in knowledge_svc.NON_TASK_FACETS
+
+
+def test_a_lesson_is_not_a_task_on_either_arm_of_the_filter():
+    """The cell left empty on purpose (#3163).
+
+    `is_task` IS `status is not None`, so a lesson that acquired a status would
+    become a task and start appearing in open-work listings — the one way
+    filling a field in by accident changes what the record IS.
+    """
+    assert knowledge_svc.facet_is_task(lessons_svc.LESSON_NOTE_TYPE) is False
+    lesson = SimpleNamespace(note_type="lesson", is_task=False, task_kind="work")
+    assert knowledge_svc.matches_facet(lesson, "lesson") is True
+    assert knowledge_svc.matches_facet(lesson, "task") is False
+    # And it does not answer to another kind's facet.
+    assert knowledge_svc.matches_facet(lesson, "note") is False

tests/test_services_knowledge_counts.py

@@ -71,7 +71,10 @@ async def test_total_counts_snippets_and_counts_no_task_twice():
 async def test_absent_facets_report_zero_rather_than_missing():
     counts, _ = await _counts([("note", 1)], [])
     assert counts["note"] == 1
-    for key in ("process", "snippet", "task", "work", "issue", "spike", "plan"):
+    # `lesson` is here from milestone 385 step 2: a kind added to the facet
+    # table and not to the counts would show an empty chip beside a feed that
+    # has rows in it, which is defect 3b in #3161 repeating itself.
+    for key in ("process", "snippet", "lesson", "task", "work", "issue", "spike", "plan"):
         assert counts[key] == 0, key
     assert counts["total"] == 1