diff --git a/alembic/versions/0091_task_kind_spike.py b/alembic/versions/0091_task_kind_spike.py
new file mode 100644
index 0000000..5b03374
--- /dev/null
+++ b/alembic/versions/0091_task_kind_spike.py
@@ -0,0 +1,66 @@
+"""task_kind gains 'spike' — the investigation, not the change
+(milestone 312 step 5)
+
+Revision ID: 0091
+Revises: 0090
+Create Date: 2026-08-27
+
+A spike is a task shape the others cannot hold. `work` ships a change;
+`issue` fixes something broken. A spike is time-boxed and its output is
+KNOWLEDGE — it succeeds by producing an answer, and nothing ships at the
+end of it. "Find out whether the runner can be given a bash shell" is not
+work, and filing it as work makes a finished investigation look like an
+abandoned change.
+
+It is the record a failed check asks for. Milestone 312 gave rules a
+`verify_with`; when one of those fails, the rule is wrong and the next move
+is often to go and find out what replaced it. `notes.arose_from_id` already
+exists (0065), so that constraint -> spike link needs no further schema.
+
+Rule 36: `task_kind` is gated by a CHECK whitelist, so the value and the
+widened constraint land in the SAME migration — DROP then ADD, exactly as
+0065 did when it introduced 'issue'. Adding the value and constraining it
+later leaves a window where the database accepts anything.
+
+'plan' stays in the list though it is retired (plans are milestones since
+0066): historical plan-tasks still carry it, and dropping it from the
+whitelist would make old rows unwritable.
+"""
+from alembic import op
+
+revision = "0091"
+down_revision = "0090"
+branch_labels = None
+depends_on = None
+
+# One tuple so the upgrade and the downgrade cannot disagree about what the
+# list was on either side of this migration.
+_KINDS_AFTER = ("work", "plan", "issue", "spike")
+_KINDS_BEFORE = ("work", "plan", "issue")
+
+
+# Restated rather than imported from 0088, which has the same helper. A
+# migration is a snapshot: it must keep working when the code around it has
+# moved on, so it never imports from live modules or from its siblings. Six
+# duplicated lines are the price of that, and the cheap half of the bargain.
+def _in_list(values: tuple[str, ...]) -> str:
+    return "task_kind IN (" + ", ".join(f"'{v}'" for v in values) + ")"
+
+
+def upgrade() -> None:
+    op.drop_constraint("notes_task_kind_check", "notes", type_="check")
+    op.create_check_constraint(
+        "notes_task_kind_check", "notes", _in_list(_KINDS_AFTER),
+    )
+
+
+def downgrade() -> None:
+    # Any row already filed as a spike would violate the narrowed constraint,
+    # so they are demoted to 'work' first. Lossy and deliberately so: the
+    # alternative is a downgrade that fails on real data, which is worse than
+    # a downgrade that says what it did.
+    op.execute("UPDATE notes SET task_kind = 'work' WHERE task_kind = 'spike'")
+    op.drop_constraint("notes_task_kind_check", "notes", type_="check")
+    op.create_check_constraint(
+        "notes_task_kind_check", "notes", _in_list(_KINDS_BEFORE),
+    )
diff --git a/frontend/src/types/note.ts b/frontend/src/types/note.ts
index 64105e4..c11a9d3 100644
--- a/frontend/src/types/note.ts
+++ b/frontend/src/types/note.ts
@@ -2,7 +2,16 @@ import type { System } from "@/api/systems";
 
 export type TaskStatus = "todo" | "in_progress" | "done" | "cancelled";
 export type TaskPriority = "none" | "low" | "medium" | "high";
-export type TaskKind = "work" | "plan" | "issue";
+/**
+ * What KIND of work a task is, not how it is going.
+ *   work  — ships a change (default)
+ *   issue — corrective; something was broken
+ *   spike — time-boxed, output is knowledge; it succeeds by producing an
+ *           answer and nothing ships at the end of it
+ *   plan  — retired (plans are milestones); kept so historical plan-tasks
+ *           still render their kind
+ */
+export type TaskKind = "work" | "plan" | "issue" | "spike";
 export type NoteType = "note" | "process" | "snippet";
 
 export interface Note {
diff --git a/frontend/src/views/TaskEditorView.vue b/frontend/src/views/TaskEditorView.vue
index f19115f..4212ca5 100644
--- a/frontend/src/views/TaskEditorView.vue
+++ b/frontend/src/views/TaskEditorView.vue
@@ -578,6 +578,7 @@ useEditorGuards(dirty, save);
             <select v-model="kind" @change="markDirty" class="sb-select">
               <option value="work">Work</option>
               <option value="issue">Issue</option>
+              <option value="spike">Spike</option>
               <!-- 'plan' is retired (plans are milestones via start_planning);
                    offered only so legacy plan-tasks display their kind. -->
               <option v-if="kind === 'plan'" value="plan">Plan (legacy)</option>
diff --git a/src/scribe/mcp/tools/systems.py b/src/scribe/mcp/tools/systems.py
index 79ca91c..df0050e 100644
--- a/src/scribe/mcp/tools/systems.py
+++ b/src/scribe/mcp/tools/systems.py
@@ -336,7 +336,8 @@ async def list_system_records(
     slice, search(system_id=...) filters semantic search to this association.
 
     Args:
-        kind: filter by task_kind — 'issue', 'work', or 'plan'. Omit for all.
+        kind: filter by task_kind — 'issue', 'work', 'spike' (or the retired
+            'plan'). Omit for all.
         open_only: limit to tasks not done/cancelled (e.g. open issues only).
     """
     uid = current_user_id()
diff --git a/src/scribe/mcp/tools/tasks.py b/src/scribe/mcp/tools/tasks.py
index 1c30055..1c81988 100644
--- a/src/scribe/mcp/tools/tasks.py
+++ b/src/scribe/mcp/tools/tasks.py
@@ -46,7 +46,8 @@ async def list_tasks(
             whenever a project is in scope so you list that project's tasks, not
             every project's. 0 = no filter (all projects — use only for a
             deliberate cross-project view).
-        kind: Filter by task kind — 'work', 'plan', or 'issue'. Omit (empty) for all kinds.
+        kind: Filter by task kind — 'work', 'issue', 'spike' (or the retired
+            'plan'). Omit (empty) for all kinds.
 
     Results are ordered by last-updated descending.
     """
@@ -138,14 +139,24 @@ async def create_task(
         milestone_id: Place within a project milestone (0 = no milestone).
         parent_id: Make this a sub-task of another task (0 = top-level).
         tags: List of plain-string tags without # prefix.
-        kind: 'work' (default) or 'issue'. An issue is corrective work — a
-            problem you fixed or are fixing; record symptom → root cause → fix
-            in the body. (Plans are milestones now — call start_planning to begin
-            a plan; 'plan' is not a valid kind here.)
+        kind: 'work' (default), 'issue', or 'spike'.
+            An ISSUE is corrective work — a problem you fixed or are fixing;
+            record symptom → root cause → fix in the body.
+            A SPIKE is time-boxed and its output is KNOWLEDGE rather than a
+            change: "find out whether the runner can be given a bash shell",
+            "work out why the index is not used". It succeeds by producing an
+            answer, so nothing ships at the end of it — which is why filing
+            one as `work` makes a finished investigation look like an
+            abandoned change. Reach for it when the honest deliverable is a
+            finding, and say in the body what would close the box: a time, or
+            the question being answered well enough to act on.
+            (Plans are milestones now — call start_planning to begin a plan;
+            'plan' is not a valid kind here.)
         system_ids: Ids of the project's Systems (reusable subsystem/area
             objects; see list_systems / create_system) to associate this task with.
-        arose_from_id: For an issue, the id of the task/feature it arose from
-            (provenance). 0 = none.
+        arose_from_id: For an issue, the id of the task/feature it arose from;
+            for a spike, the record that raised the question — including a
+            standing rule whose check just failed. 0 = none.
         force: Bypass the near-duplicate gate. By default, if a title- or
             meaning-similar task already exists in the same project, creation is
             BLOCKED and the existing task's id is returned so you update it
diff --git a/src/scribe/models/note.py b/src/scribe/models/note.py
index f09b16e..762883b 100644
--- a/src/scribe/models/note.py
+++ b/src/scribe/models/note.py
@@ -61,10 +61,16 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
     # 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.)
     note_type: Mapped[str] = mapped_column(Text, default="note", server_default="note")
-    # Task sub-kind — 'work' (default), 'plan', or 'issue' (corrective work).
+    # Task sub-kind — what KIND of work this is, not how it is going:
+    #   work   (default) — ships a change
+    #   issue  — corrective; something was broken (0065)
+    #   spike  — time-boxed, and its output is KNOWLEDGE rather than a change;
+    #            it succeeds by producing an answer, and nothing ships (0091)
+    #   plan   — retired since 0066 (plans are milestones), kept in the CHECK
+    #            so historical plan-tasks stay writable
     # Only meaningful when the note is a task (status is not None); ordinary
     # notes keep the 'work' default and ignore it. Orthogonal to note_type
-    # (which is the note/entity axis).
+    # (which is the note/entity axis). CHECK notes_task_kind_check (rule 36).
     task_kind: Mapped[str] = mapped_column(Text, default="work", server_default="work")
     # Queryable structured fields for typed records — currently snippets, whose
     # name/language/signature/locations live here so they can be INDEXED. The
diff --git a/tests/test_integration_task_kind_spike.py b/tests/test_integration_task_kind_spike.py
new file mode 100644
index 0000000..a2e528b
--- /dev/null
+++ b/tests/test_integration_task_kind_spike.py
@@ -0,0 +1,71 @@
+"""Real-Postgres test that the CHECK actually accepts 'spike' (0091).
+
+Rule 36 exists because the value and the constraint can drift apart: the
+code starts writing a new kind while the database still refuses it, and
+nothing catches it until a write fails in front of someone. A mock cannot
+show that — it has no CHECK — so the constraint gets its own real-DB test,
+the same way migration 0090's nullability did.
+
+The negative half matters as much as the positive one. A test that only
+proves 'spike' is accepted would also pass against a table with NO
+constraint at all, which is the other way this goes wrong.
+"""
+import pytest
+import pytest_asyncio
+from sqlalchemy.exc import IntegrityError
+
+from scribe.models import async_session
+from scribe.models.note import Note
+from tests.helpers import ensure_user
+
+pytestmark = [pytest.mark.integration, pytest.mark.usefixtures("_dispose_engine")]
+
+
+@pytest_asyncio.fixture
+async def owner_id():
+    async with async_session() as s:
+        owner = await ensure_user(s, "spike_owner")
+        uid = owner.id
+        await s.commit()
+    return uid
+
+
+async def _write(uid: int, kind: str) -> int:
+    async with async_session() as s:
+        note = Note(
+            user_id=uid, title=f"kind {kind}", body="", is_task=True,
+            status="todo", task_kind=kind,
+        )
+        s.add(note)
+        await s.commit()
+        return note.id
+
+
+async def test_a_spike_can_be_written(owner_id):
+    note_id = await _write(owner_id, "spike")
+    async with async_session() as s:
+        assert (await s.get(Note, note_id)).task_kind == "spike"
+
+
+async def test_the_older_kinds_still_write(owner_id):
+    """0091 widens the whitelist; it must not narrow it by accident.
+
+    'plan' is retired — plans are milestones since 0066 — but historical
+    plan-tasks still carry it, and a row that cannot be rewritten is a row
+    that cannot be edited, restored, or migrated.
+    """
+    for kind in ("work", "issue", "plan"):
+        note_id = await _write(owner_id, kind)
+        async with async_session() as s:
+            assert (await s.get(Note, note_id)).task_kind == kind
+
+
+async def test_an_unknown_kind_is_still_refused(owner_id):
+    """The half that proves a constraint is there at all.
+
+    Without this, every assertion above would pass just as happily against a
+    table whose CHECK had been dropped and never re-added — which is exactly
+    the failure rule 36 is written against.
+    """
+    with pytest.raises(IntegrityError):
+        await _write(owner_id, "investigation")
diff --git a/tests/test_mcp_tool_tasks_kind.py b/tests/test_mcp_tool_tasks_kind.py
index 4b123f4..832f013 100644
--- a/tests/test_mcp_tool_tasks_kind.py
+++ b/tests/test_mcp_tool_tasks_kind.py
@@ -33,3 +33,17 @@ async def test_list_tasks_kind_empty_means_no_filter():
         from scribe.mcp.tools.tasks import list_tasks
         await list_tasks()
     assert mock.call_args.kwargs["task_kind"] is None
+
+
+@pytest.mark.asyncio
+async def test_create_task_passes_spike():
+    """The kind a failed rule-check asks for (milestone 312).
+
+    Time-boxed, and its output is knowledge rather than a change — filing one
+    as `work` makes a finished investigation look like an abandoned change.
+    """
+    mock = AsyncMock(return_value=fake_note(task_kind="spike"))
+    with patch("scribe.mcp.tools.tasks.notes_svc.create_note", mock):
+        from scribe.mcp.tools.tasks import create_task
+        await create_task(title="Can the runner be given a bash shell?", kind="spike")
+    assert mock.call_args.kwargs["task_kind"] == "spike"