diff --git a/packages/core/src/__tests__/default-workflow-hooks.test.ts b/packages/core/src/__tests__/default-workflow-hooks.test.ts
new file mode 100644
index 0000000000..c650a89a78
--- /dev/null
+++ b/packages/core/src/__tests__/default-workflow-hooks.test.ts
@@ -0,0 +1,87 @@
+// @vitest-environment node
+//
+// U4: the default-workflow side effects are resolved THROUGH the trait registry
+// (the DI seam, KTD-2/U2). This pins:
+//   - registerDefaultWorkflowHooks() wires the impls so resolution finds them
+//     (no missing-hook-impl warning on the happy path);
+//   - a missing registration degrades to a no-op + audit warning (not a crash);
+//   - applyDefaultWorkflowMoveEffects mutates the task per the legacy contract.
+
+import { describe, it, expect, beforeEach } from "vitest";
+import {
+  __resetTraitRegistryForTests,
+  getTraitRegistry,
+} from "../trait-registry.js";
+import { registerBuiltinTraits } from "../builtin-traits.js";
+import {
+  __resetDefaultWorkflowHooksForTests,
+  applyDefaultWorkflowMoveEffects,
+  registerDefaultWorkflowHooks,
+  type DefaultWorkflowMoveContext,
+} from "../default-workflow-hooks.js";
+import type { Task } from "../types.js";
+
+function makeCtx(overrides: Partial<DefaultWorkflowMoveContext> = {}): DefaultWorkflowMoveContext {
+  const task = {
+    id: "FN-1",
+    column: "in-progress",
+    columnMovedAt: new Date().toISOString(),
+    steps: [],
+    dependencies: [],
+  } as unknown as Task;
+  return {
+    task,
+    fromColumn: "todo",
+    toColumn: "in-progress",
+    moveSource: "user",
+    bypassGuards: false,
+    movedAt: new Date().toISOString(),
+    settings: undefined,
+    options: {},
+    resetSteps: () => {},
+    ...overrides,
+  };
+}
+
+describe("default-workflow-hooks registry wiring", () => {
+  beforeEach(() => {
+    __resetTraitRegistryForTests();
+    __resetDefaultWorkflowHooksForTests();
+    registerBuiltinTraits();
+  });
+
+  it("resolves all default-workflow hooks without a missing-impl warning once registered", () => {
+    registerDefaultWorkflowHooks();
+    const ctx = makeCtx({ fromColumn: "todo", toColumn: "in-progress" });
+    const { warnings } = applyDefaultWorkflowMoveEffects(ctx);
+    expect(warnings).toHaveLength(0);
+    // timing.onEnter stamped cumulativeActiveMs on entry to in-progress.
+    expect(ctx.task.cumulativeActiveMs).toBe(0);
+  });
+
+  it("degrades to a no-op + audit warning when a hook impl is not registered", () => {
+    // Built-in DEFINITIONS are registered (so the trait declares the hook) but
+    // we deliberately do NOT call registerDefaultWorkflowHooks() — no impls.
+    const registry = getTraitRegistry();
+    // sanity: the trait declares the hook descriptor
+    expect(registry.getTrait("timing")?.hooks?.onEnter).toBe(true);
+    const ctx = makeCtx({ fromColumn: "todo", toColumn: "in-progress" });
+    const { warnings } = applyDefaultWorkflowMoveEffects(ctx);
+    // Every declared hook with no impl yields a degraded-no-op warning.
+    expect(warnings.length).toBeGreaterThan(0);
+    expect(warnings.every((w) => w.kind === "missing-hook-impl")).toBe(true);
+    // No crash; task unmutated by the (no-op) hooks.
+    expect(ctx.task.cumulativeActiveMs).toBeUndefined();
+  });
+
+  it("applies userPaused only for user-source reopen to todo", () => {
+    registerDefaultWorkflowHooks();
+    const userCtx = makeCtx({ fromColumn: "in-progress", toColumn: "todo", moveSource: "user" });
+    applyDefaultWorkflowMoveEffects(userCtx);
+    expect(userCtx.task.userPaused).toBe(true);
+
+    const engineCtx = makeCtx({ fromColumn: "in-progress", toColumn: "todo", moveSource: "engine" });
+    applyDefaultWorkflowMoveEffects(engineCtx);
+    expect(engineCtx.task.userPaused).toBeUndefined();
+  });
+});
diff --git a/packages/core/src/__tests__/move-task-characterization.test.ts b/packages/core/src/__tests__/move-task-characterization.test.ts
new file mode 100644
index 0000000000..66c4f4901a
--- /dev/null
+++ b/packages/core/src/__tests__/move-task-characterization.test.ts
@@ -0,0 +1,205 @@
+// @vitest-environment node
+//
+// CHARACTERIZATION SUITE (U4 Execution Note — written FIRST, before any change
+// to `moveTaskInternal`).
+//
+// This suite pins the CURRENT behavior of `moveTaskInternal` for every (from,
+// to) pair in VALID_TRANSITIONS' domain and both moveSource values, plus the
+// key column side effects:
+//   - merge-blocker on in-review → done (user source)
+//   - userPaused set only for user-source in-progress → todo
+//   - reopen field/step resets on in-review/done → todo|triage
+//   - autoMerge stamping on → in-review
+//   - timing fields (cumulativeActiveMs / executionStartedAt) on in-progress
+//
+// It runs GREEN against the unmodified store first, then runs forever against
+// BOTH flag states (workflowColumns OFF and ON) — see the `flagStates` loop.
+// Any divergence between the two flag states is a U4 parity FAILURE.
+
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { VALID_TRANSITIONS } from "../types.js";
+import type { Column, Task } from "../types.js";
+import { createTaskStoreTestHarness } from "./store-test-helpers.js";
+
+const ALL_COLUMNS: Column[] = ["triage", "todo", "in-progress", "in-review", "done", "archived"];
+
+// Flag states the characterization runs against. OFF is the legacy path; ON is
+// the workflow-resolved path. The default workflow MUST reproduce identical
+// outcomes for both, so the same expectations apply.
+const flagStates: Array<{ label: string; workflowColumns: boolean }> = [
+  { label: "flag OFF (legacy path)", workflowColumns: false },
+  { label: "flag ON (workflow-resolved default workflow)", workflowColumns: true },
+];
+
+for (const flag of flagStates) {
+  describe(`moveTaskInternal characterization — ${flag.label}`, () => {
+    const harness = createTaskStoreTestHarness();
+    let store: ReturnType<typeof harness.store>;
+
+    beforeEach(async () => {
+      await harness.beforeEach();
+      store = harness.store();
+      if (flag.workflowColumns) {
+        await store.updateGlobalSettings({ experimentalFeatures: { workflowColumns: true } });
+      }
+    });
+
+    afterEach(async () => {
+      await harness.afterEach();
+    });
+
+    /**
+     * Drive a freshly-created task (starts in `triage`) into `column` using only
+     * legal, side-effect-tolerant moves. Returns the task.
+     */
+    async function seedInColumn(column: Column): Promise<Task> {
+      const task = await store.createTask({ description: `seed-${column}` });
+      switch (column) {
+        case "triage":
+          return task;
+        case "todo":
+          return store.moveTask(task.id, "todo", { moveSource: "user" });
+        case "in-progress":
+          await store.moveTask(task.id, "todo", { moveSource: "user" });
+          return store.moveTask(task.id, "in-progress", { moveSource: "user" });
+        case "in-review":
+          await store.moveTask(task.id, "todo", { moveSource: "user" });
+          await store.moveTask(task.id, "in-progress", { moveSource: "user" });
+          return store.moveTask(task.id, "in-review", { moveSource: "user", allowDirectInReviewMove: true });
+        case "done":
+          await store.moveTask(task.id, "todo", { moveSource: "user" });
+          await store.moveTask(task.id, "in-progress", { moveSource: "user" });
+          await store.moveTask(task.id, "in-review", { moveSource: "user", allowDirectInReviewMove: true });
+          return store.moveTask(task.id, "done", { moveSource: "engine", skipMergeBlocker: true });
+        case "archived":
+          await store.moveTask(task.id, "todo", { moveSource: "user" });
+          await store.moveTask(task.id, "in-progress", { moveSource: "user" });
+          await store.moveTask(task.id, "in-review", { moveSource: "user", allowDirectInReviewMove: true });
+          await store.moveTask(task.id, "done", { moveSource: "engine", skipMergeBlocker: true });
+          return store.moveTask(task.id, "archived", { moveSource: "user" });
+        default:
+          throw new Error(`unhandled column ${column}`);
+      }
+    }
+
+    describe("transition allow/reject matrix (every from×to×moveSource)", () => {
+      for (const from of ALL_COLUMNS) {
+        for (const to of ALL_COLUMNS) {
+          for (const moveSource of ["user", "engine"] as const) {
+            const allowed = from === to || VALID_TRANSITIONS[from].includes(to);
+            const label = `${from} → ${to} [${moveSource}] should ${allowed ? "ALLOW" : "REJECT"}`;
+            it(label, async () => {
+              const task = await seedInColumn(from);
+              // Same-column move is a no-op success in legacy behavior.
+              if (from === to) {
+                const result = await store.moveTask(task.id, to, { moveSource });
+                expect(result.column).toBe(to);
+                return;
+              }
+              if (allowed) {
+                // in-review → done with merge-blocker only blocks for user source
+                // and only when a blocker exists; our seeded task has no blocker.
+                // Bare in-review targets bypass the handoff invariant via
+                // allowDirectInReviewMove, matching production drag behavior.
+                const opts =
+                  to === "in-review"
+                    ? { moveSource, allowDirectInReviewMove: true }
+                    : { moveSource };
+                const result = await store.moveTask(task.id, to, opts);
+                expect(result.column).toBe(to);
+              } else {
+                await expect(
+                  store.moveTask(task.id, to, { moveSource }),
+                ).rejects.toThrow(/Invalid transition/);
+              }
+            });
+          }
+        }
+      }
+    });
+
+    describe("merge-blocker side effect (in-review → done)", () => {
+      it("blocks a user move to done when a merge blocker exists", async () => {
+        const task = await seedInColumn("in-review");
+        // Incomplete steps create a merge blocker (getTaskMergeBlocker).
+        await store.updateTask(task.id, {
+          steps: [{ name: "x", status: "pending" }] as Task["steps"],
+        });
+        await expect(
+          store.moveTask(task.id, "done", { moveSource: "user" }),
+        ).rejects.toThrow(/Cannot move .* to done/);
+      });
+
+      it("skipMergeBlocker bypasses the blocker", async () => {
+        const task = await seedInColumn("in-review");
+        await store.updateTask(task.id, {
+          steps: [{ name: "x", status: "pending" }] as Task["steps"],
+        });
+        const result = await store.moveTask(task.id, "done", {
+          moveSource: "engine",
+          skipMergeBlocker: true,
+        });
+        expect(result.column).toBe("done");
+      });
+    });
+
+    describe("userPaused side effect (in-progress → todo)", () => {
+      it("sets userPaused for a user-source move", async () => {
+        const task = await seedInColumn("in-progress");
+        const result = await store.moveTask(task.id, "todo", { moveSource: "user" });
+        expect(result.userPaused).toBe(true);
+      });
+
+      it("does NOT set userPaused for an engine-source move", async () => {
+        const task = await seedInColumn("in-progress");
+        const result = await store.moveTask(task.id, "todo", { moveSource: "engine" });
+        expect(result.userPaused).toBeUndefined();
+      });
+    });
+
+    describe("reopen resets (in-review → todo)", () => {
+      it("clears branch/summary/baseCommitSha on reopen to todo", async () => {
+        const task = await seedInColumn("in-review");
+        await store.updateTask(task.id, {
+          branch: "fusion/fn-x",
+          summary: "did stuff",
+          baseCommitSha: "abc123",
+        });
+        const result = await store.moveTask(task.id, "todo", { moveSource: "user" });
+        expect(result.branch).toBeUndefined();
+        expect(result.summary).toBeUndefined();
+        expect(result.baseCommitSha).toBeUndefined();
+      });
+    });
+
+    describe("autoMerge stamping (→ in-review)", () => {
+      it("stamps autoMerge from settings when undefined", async () => {
+        await store.updateSettings({ autoMerge: true });
+        const task = await seedInColumn("in-progress");
+        const result = await store.moveTask(task.id, "in-review", {
+          moveSource: "user",
+          allowDirectInReviewMove: true,
+        });
+        expect(result.autoMerge).toBe(true);
+      });
+    });
+
+    describe("timing fields (→ in-progress)", () => {
+      it("sets executionStartedAt and initializes cumulativeActiveMs on entry", async () => {
+        const task = await seedInColumn("todo");
+        const result = await store.moveTask(task.id, "in-progress", { moveSource: "user" });
+        expect(result.executionStartedAt).toBeTruthy();
+        expect(result.cumulativeActiveMs).toBe(0);
+      });
+
+      it("accumulates cumulativeActiveMs on exit from in-progress", async () => {
+        const task = await seedInColumn("in-progress");
+        const result = await store.moveTask(task.id, "in-review", {
+          moveSource: "user",
+          allowDirectInReviewMove: true,
+        });
+        expect(result.cumulativeActiveMs).toBeGreaterThanOrEqual(0);
+      });
+    });
+  });
+}
diff --git a/packages/core/src/__tests__/transition-parity.test.ts b/packages/core/src/__tests__/transition-parity.test.ts
new file mode 100644
index 0000000000..3587fe59fa
--- /dev/null
+++ b/packages/core/src/__tests__/transition-parity.test.ts
@@ -0,0 +1,253 @@
+// @vitest-environment node
+//
+// TRANSITION-PARITY SUITE (U4).
+//
+// Proves the flag-ON workflow-resolved transition path reproduces the legacy
+// VALID_TRANSITIONS contract for the default workflow, and exercises the U4
+// plan scenarios:
+//   - VALID_TRANSITIONS parity (allowed AND rejected sets identical)
+//   - FN-5147 terminal-until-merged (both paths)
+//   - hard-cancel user vs engine (userPaused + abort-on-exit bypass)
+//   - handoff bypass + exactly-once enqueue across a simulated crash
+//   - crash-mid-transition marker recovery (SQLite authoritative)
+//   - unknown-column rejection
+//   - guard rejection typed (flag-ON) vs legacy string (flag-OFF)
+//   - bypassGuards capacity pass-through (documenting; U6 fills enforcement)
+
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+import { VALID_TRANSITIONS } from "../types.js";
+import type { Column, Task } from "../types.js";
+import { TransitionRejectionError } from "../store.js";
+import { resolveAllowedColumns, workflowHasColumn } from "../workflow-transitions.js";
+import { BUILTIN_CODING_WORKFLOW_IR } from "../builtin-coding-workflow-ir.js";
+import { readTransitionPending } from "../transition-pending.js";
+import { createTaskStoreTestHarness } from "./store-test-helpers.js";
+
+const ALL_COLUMNS: Column[] = ["triage", "todo", "in-progress", "in-review", "done", "archived"];
+
+describe("transition-parity — default workflow column adjacency == VALID_TRANSITIONS", () => {
+  it("reproduces VALID_TRANSITIONS exactly for every column (allowed + rejected)", () => {
+    for (const from of ALL_COLUMNS) {
+      const legacy = new Set(VALID_TRANSITIONS[from]);
+      const resolved = new Set(resolveAllowedColumns(BUILTIN_CODING_WORKFLOW_IR, from));
+      // Allowed sets identical.
+      expect([...resolved].sort()).toEqual([...legacy].sort());
+      // Rejected sets identical (complement over all columns).
+      for (const to of ALL_COLUMNS) {
+        if (from === to) continue;
+        expect(resolved.has(to)).toBe(legacy.has(to));
+      }
+    }
+  });
+
+  it("recognizes exactly the six default columns", () => {
+    for (const c of ALL_COLUMNS) {
+      expect(workflowHasColumn(BUILTIN_CODING_WORKFLOW_IR, c)).toBe(true);
+    }
+    expect(workflowHasColumn(BUILTIN_CODING_WORKFLOW_IR, "made-up")).toBe(false);
+  });
+});
+
+describe("transition-parity — store flag-ON scenarios", () => {
+  const harness = createTaskStoreTestHarness();
+  let store: ReturnType<typeof harness.store>;
+
+  beforeEach(async () => {
+    await harness.beforeEach();
+    store = harness.store();
+    await store.updateGlobalSettings({ experimentalFeatures: { workflowColumns: true } });
+  });
+  afterEach(async () => {
+    await harness.afterEach();
+  });
+
+  async function seedInColumn(column: Column): Promise<Task> {
+    const task = await store.createTask({ description: `seed-${column}` });
+    const u = { moveSource: "user" as const };
+    if (column === "triage") return task;
+    await store.moveTask(task.id, "todo", u);
+    if (column === "todo") return store.getTask(task.id) as Promise<Task>;
+    await store.moveTask(task.id, "in-progress", u);
+    if (column === "in-progress") return store.getTask(task.id) as Promise<Task>;
+    await store.moveTask(task.id, "in-review", { ...u, allowDirectInReviewMove: true });
+    if (column === "in-review") return store.getTask(task.id) as Promise<Task>;
+    await store.moveTask(task.id, "done", { moveSource: "engine", skipMergeBlocker: true });
+    if (column === "done") return store.getTask(task.id) as Promise<Task>;
+    await store.moveTask(task.id, "archived", u);
+    return store.getTask(task.id) as Promise<Task>;
+  }
+
+  it("FN-5147: user move in-review → done blocked by merge-blocker with typed rejection", async () => {
+    const task = await seedInColumn("in-review");
+    await store.updateTask(task.id, { steps: [{ name: "x", status: "pending" }] as Task["steps"] });
+    let caught: unknown;
+    try {
+      await store.moveTask(task.id, "done", { moveSource: "user" });
+    } catch (e) {
+      caught = e;
+    }
+    expect(caught).toBeInstanceOf(TransitionRejectionError);
+    expect((caught as TransitionRejectionError).rejection.code).toBe("merge-blocked");
+    expect((caught as TransitionRejectionError).rejection.retryable).toBe(true);
+  });
+
+  it("FN-5147: engine-sourced move bypasses the merge-blocker guard", async () => {
+    const task = await seedInColumn("in-review");
+    await store.updateTask(task.id, { steps: [{ name: "x", status: "pending" }] as Task["steps"] });
+    const moved = await store.moveTask(task.id, "done", { moveSource: "engine" });
+    expect(moved.column).toBe("done");
+  });
+
+  it("hard-cancel: user in-progress → todo sets userPaused; engine does not", async () => {
+    const userTask = await seedInColumn("in-progress");
+    const u = await store.moveTask(userTask.id, "todo", { moveSource: "user" });
+    expect(u.userPaused).toBe(true);
+
+    const engineTask = await seedInColumn("in-progress");
+    const e = await store.moveTask(engineTask.id, "todo", { moveSource: "engine" });
+    expect(e.userPaused).toBeUndefined();
+  });
+
+  it("unknown column rejects with typed unknown-column code, card untouched", async () => {
+    const task = await seedInColumn("todo");
+    let caught: unknown;
+    try {
+      await store.moveTask(task.id, "made-up" as Column, { moveSource: "user" });
+    } catch (e) {
+      caught = e;
+    }
+    expect(caught).toBeInstanceOf(TransitionRejectionError);
+    expect((caught as TransitionRejectionError).rejection.code).toBe("unknown-column");
+    const after = await store.getTask(task.id);
+    expect(after?.column).toBe("todo");
+  });
+
+  it("guard/adjacency rejection is typed (not a bare Error string)", async () => {
+    const task = await seedInColumn("archived");
+    // archived → todo is not a legal default-workflow transition.
+    let caught: unknown;
+    try {
+      await store.moveTask(task.id, "todo", { moveSource: "user" });
+    } catch (e) {
+      caught = e;
+    }
+    expect(caught).toBeInstanceOf(TransitionRejectionError);
+    expect((caught as TransitionRejectionError).rejection.code).toBe("guard-rejected");
+  });
+
+  it("handoffToReview maps skipMergeBlocker onto bypassGuards and enqueues exactly once", async () => {
+    const task = await seedInColumn("in-progress");
+    await store.handoffToReview(task.id, {
+      ownerAgentId: "agent-1",
+      evidence: { runId: "run-1", agentId: "agent-1", reason: "complete" },
+    } as Parameters<typeof store.handoffToReview>[1]);
+    const after = await store.getTask(task.id);
+    expect(after?.column).toBe("in-review");
+    // Idempotent re-handoff (same-column path) must not double-enqueue.
+    await store.handoffToReview(task.id, {
+      ownerAgentId: "agent-1",
+      evidence: { runId: "run-2", agentId: "agent-1", reason: "complete" },
+    } as Parameters<typeof store.handoffToReview>[1]);
+    const queueCount = (store as unknown as { db: { prepare: (s: string) => { get: (...a: unknown[]) => unknown } } }).db
+      .prepare("SELECT COUNT(*) AS n FROM mergeQueue WHERE taskId = ?")
+      .get(task.id) as { n: number };
+    expect(queueCount.n).toBe(1);
+  });
+
+  it("transitionPending marker is written in-txn and cleared post-commit (happy path)", async () => {
+    const task = await seedInColumn("todo");
+    await store.moveTask(task.id, "in-progress", { moveSource: "user" });
+    const db = (store as unknown as { db: Parameters<typeof readTransitionPending>[0] }).db;
+    // Happy path: marker cleared after the post-commit hook runner.
+    expect(readTransitionPending(db, task.id)).toBeNull();
+  });
+
+  it("crash-mid-transition: a persisted marker is recoverable from SQLite with hooksRemaining intact", async () => {
+    const task = await seedInColumn("todo");
+    await store.moveTask(task.id, "in-progress", { moveSource: "user" });
+    // Simulate a crash AFTER commit but BEFORE the marker clear by re-writing a
+    // marker directly (the in-txn write path is the same helper). Recovery reads
+    // it back from SQLite (authoritative), not from task.json.
+    const db = (store as unknown as { db: Parameters<typeof readTransitionPending>[0] }).db;
+    (db as unknown as { prepare: (s: string) => { run: (...a: unknown[]) => unknown } })
+      .prepare("UPDATE tasks SET transitionPending = ? WHERE id = ?")
+      .run(
+        JSON.stringify({ toColumn: "in-progress", hooksRemaining: ["default-workflow:postCommit"], startedAt: Date.now() }),
+        task.id,
+      );
+    const pending = readTransitionPending(db, task.id);
+    expect(pending).not.toBeNull();
+    expect(pending?.toColumn).toBe("in-progress");
+    expect(pending?.hooksRemaining).toContain("default-workflow:postCommit");
+  });
+
+  it("worktree ordering: allocateWorktree runs (and is applied) for a flag-ON move into in-progress", async () => {
+    const task = await seedInColumn("todo");
+    let allocatorCalled = false;
+    const moved = await store.moveTask(task.id, "in-progress", {
+      moveSource: "user",
+      allocateWorktree: () => {
+        allocatorCalled = true;
+        return "/tmp/wt/seed-todo";
+      },
+    });
+    expect(allocatorCalled).toBe(true);
+    expect(moved.worktree).toBe("/tmp/wt/seed-todo");
+    // Worktree allocation is NOT a hook — it is a substrate capability invoked
+    // synchronously before the move commits; the committed row carries it.
+    const after = await store.getTask(task.id);
+    expect(after?.worktree).toBe("/tmp/wt/seed-todo");
+  });
+
+  it("bypassGuards capacity pass-through (U4 documenting test): engine move into in-progress is NOT blocked by capacity (U6 fills enforcement)", async () => {
+    // U4 intentionally leaves the per-(workflow,column) capacity check as a
+    // pass-through slot; capacity enforcement lands in U6. This test pins the
+    // U4 contract: no WIP-constrained scenario is enforced yet, and an engine
+    // move (bypassGuards) into a wip-flagged column commits. It must be UPDATED
+    // by U6 (capacity is NEVER bypassable, KTD-10) — not silently left green.
+    const t1 = await seedInColumn("todo");
+    const t2 = await seedInColumn("todo");
+    const m1 = await store.moveTask(t1.id, "in-progress", { moveSource: "engine" });
+    const m2 = await store.moveTask(t2.id, "in-progress", { moveSource: "engine" });
+    expect(m1.column).toBe("in-progress");
+    expect(m2.column).toBe("in-progress");
+  });
+});
+
+describe("transition-parity — flag-OFF keeps legacy thrown strings (no behavior change)", () => {
+  const harness = createTaskStoreTestHarness();
+  let store: ReturnType<typeof harness.store>;
+  beforeEach(async () => {
+    await harness.beforeEach();
+    store = harness.store();
+  });
+  afterEach(async () => {
+    await harness.afterEach();
+  });
+
+  it("rejects an illegal move with a bare Error containing the legacy message (not TransitionRejectionError)", async () => {
+    const task = await store.createTask({ description: "legacy reject" });
+    await store.moveTask(task.id, "todo", { moveSource: "user" });
+    await store.moveTask(task.id, "in-progress", { moveSource: "user" });
+    await store.moveTask(task.id, "in-review", { moveSource: "user", allowDirectInReviewMove: true });
+    await store.moveTask(task.id, "done", { moveSource: "engine", skipMergeBlocker: true });
+    await store.moveTask(task.id, "archived", { moveSource: "user" });
+    let caught: unknown;
+    try {
+      await store.moveTask(task.id, "todo", { moveSource: "user" });
+    } catch (e) {
+      caught = e;
+    }
+    expect(caught).toBeInstanceOf(Error);
+    expect(caught).not.toBeInstanceOf(TransitionRejectionError);
+    expect((caught as Error).message).toMatch(/Invalid transition/);
+  });
+
+  it("flag-OFF does NOT write a transitionPending marker", async () => {
+    const task = await store.createTask({ description: "no marker" });
+    await store.moveTask(task.id, "todo", { moveSource: "user" });
+    await store.moveTask(task.id, "in-progress", { moveSource: "user" });
+    const db = (store as unknown as { db: Parameters<typeof readTransitionPending>[0] }).db;
+    expect(readTransitionPending(db, task.id)).toBeNull();
+  });
+});
diff --git a/packages/core/src/default-workflow-hooks.ts b/packages/core/src/default-workflow-hooks.ts
new file mode 100644
index 0000000000..b257c98799
--- /dev/null
+++ b/packages/core/src/default-workflow-hooks.ts
@@ -0,0 +1,313 @@
+/**
+ * Default-workflow trait hook implementations (U4).
+ *
+ * The legacy per-column side effects of `moveTaskInternal` — timing /
+ * `cumulativeActiveMs` accounting, reopen field/step resets, autoMerge stamping
+ * + merge-queue enqueue, and abort-on-exit (hard-cancel incl. `userPaused` only
+ * for user-source moves) — become the default workflow's trait hook
+ * implementations, registered through U2's DI seam (`registerTraitHookImpl`).
+ *
+ * IMPORTANT (per U4): this is the FLAG-ON path. The legacy inline code in
+ * `store.ts` is NOT deleted — it IS the flag-off path. The implementations here
+ * are a deliberate parallel of that inline logic so the two paths can be parity-
+ * checked against each other; "moved, not duplicated" applies to the flag-ON
+ * path only.
+ *
+ * Hook classes (KTD-2):
+ *   - guard  (sync, in-lock): merge-blocker, human-review. Implemented as the
+ *     `evaluateDefaultWorkflowGuards` reader; pure DB-free reads off the task.
+ *   - onEnter / onExit (mutating, applied in-lock to the in-memory task before
+ *     the commit for field effects; queue effects run in-txn): timing,
+ *     reset-on-entry, abort-on-exit, merge.
+ *
+ * Worktree allocation is explicitly NOT a hook (it stays a substrate capability
+ * invoked before the move; see store.ts) — there is no `allocateWorktree` hook
+ * here by design.
+ *
+ * The hooks are registered into the shared trait registry on `init` via
+ * `registerDefaultWorkflowHooks()` (idempotent). They are resolved through
+ * `getTraitRegistry().resolveTraitHook(...)` so a missing registration degrades
+ * to a no-op + audit warning rather than crashing.
+ */
+
+import { getTraitRegistry } from "./trait-registry.js";
+import type { TraitAuditWarning } from "./trait-registry.js";
+import { getTaskMergeBlocker } from "./task-merge.js";
+import type { Settings, Task } from "./types.js";
+
+// ── Guard evaluation (sync, in-lock) ─────────────────────────────────────────
+
+/** A guard verdict: undefined = allow; a string reason = reject. */
+export type GuardVerdict = string | undefined;
+
+/**
+ * Evaluate the default workflow's sync guards for a move. Reproduces the legacy
+ * `getTaskMergeBlocker` gate on `in-review → done`. (The default workflow does
+ * not carry the human-review trait — see the Trait Vocabulary note — so there
+ * is no human-review guard on this workflow.)
+ *
+ * `bypassGuards` (engine-sourced moves, KTD-9) skips guards entirely — the
+ * caller is responsible for honoring that; this function still computes the
+ * verdict so callers can choose. The store only consults it when not bypassing.
+ */
+export function evaluateMergeBlockerGuard(
+  task: Pick<Task, "column" | "paused" | "status" | "error" | "steps" | "workflowStepResults">,
+  fromColumn: string,
+  toColumn: string,
+): GuardVerdict {
+  if (fromColumn === "in-review" && toColumn === "done") {
+    return getTaskMergeBlocker(task);
+  }
+  return undefined;
+}
+
+// ── Move-effect context ───────────────────────────────────────────────────────
+
+/** Side-effect callbacks the store provides so the hooks stay engine-free and
+ *  DB-handle-free; the store wires these to its in-txn / post-commit machinery. */
+export interface DefaultWorkflowMoveContext {
+  task: Task;
+  fromColumn: string;
+  toColumn: string;
+  moveSource: "user" | "engine";
+  /** True when guards + abort-on-exit are bypassed (engine/recovery, KTD-9). */
+  bypassGuards: boolean;
+  movedAt: string;
+  /** Settings snapshot for autoMerge stamping (only read when entering review). */
+  settings: Pick<Settings, "autoMerge"> | undefined;
+  /** Move options that influence reopen/timing semantics. */
+  options: {
+    preserveStatus?: boolean;
+    preserveResumeState?: boolean;
+    preserveProgress?: boolean;
+    preserveWorktree?: boolean;
+  };
+  /** Reset all steps to pending + currentStep 0 (store owns the impl). */
+  resetSteps: () => void;
+}
+
+// ── Field-mutation effects (applied in-lock, before commit) ───────────────────
+//
+// These mirror the inline flag-off mutations in store.ts exactly. They run as
+// the resolved onEnter/onExit hook bodies for the default workflow's traits.
+
+/** `timing` trait (in-progress): accumulate active ms on exit, stamp timing on
+ *  entry. */
+export function applyTimingEffects(ctx: DefaultWorkflowMoveContext): void {
+  const { task, fromColumn, toColumn } = ctx;
+  if (fromColumn === "in-progress" && toColumn !== "in-progress") {
+    const segmentStartMs = Date.parse(task.executionStartedAt ?? task.columnMovedAt ?? ctx.movedAt);
+    const segmentEndMs = Date.parse(task.columnMovedAt ?? ctx.movedAt);
+    const segmentDeltaMs =
+      Number.isFinite(segmentStartMs) && Number.isFinite(segmentEndMs)
+        ? Math.max(0, segmentEndMs - segmentStartMs)
+        : 0;
+    task.cumulativeActiveMs = Math.max(0, task.cumulativeActiveMs ?? 0) + segmentDeltaMs;
+  }
+  if (toColumn === "in-progress") {
+    task.cumulativeActiveMs ??= 0;
+    if (!task.firstExecutionAt) task.firstExecutionAt = task.columnMovedAt;
+    if (!task.executionStartedAt) task.executionStartedAt = task.columnMovedAt;
+    task.userPaused = undefined;
+  }
+}
+
+/** Stamp `executionCompletedAt` on entry to a completion column. */
+export function applyCompletionTimingEffects(ctx: DefaultWorkflowMoveContext): void {
+  const { task, toColumn } = ctx;
+  if (toColumn === "done" && !task.executionCompletedAt) {
+    task.executionCompletedAt = task.columnMovedAt;
+  }
+}
+
+/** `reset-on-entry` trait (todo/triage reopen) + `abort-on-exit` userPaused
+ *  semantics. Reproduces the legacy reopen block. */
+export function applyResetOnEntryEffects(ctx: DefaultWorkflowMoveContext): void {
+  const { task, fromColumn, toColumn, moveSource, options } = ctx;
+  const isReopenToTodoOrTriage =
+    (fromColumn === "in-progress" || fromColumn === "done" || fromColumn === "in-review") &&
+    (toColumn === "todo" || toColumn === "triage");
+  if (!isReopenToTodoOrTriage) return;
+
+  if (!options.preserveStatus) {
+    task.status = undefined;
+    task.error = undefined;
+    task.pausedReason = undefined;
+  }
+  task.blockedBy = undefined;
+  task.overlapBlockedBy = undefined;
+  task.paused = undefined;
+  task.pausedByAgentId = undefined;
+  // abort-on-exit userPaused: only for user-source moves to todo (KTD-9).
+  if (moveSource === "user" && toColumn === "todo") {
+    task.userPaused = true;
+  } else {
+    task.userPaused = undefined;
+  }
+
+  const hasNonPendingStepProgress = task.steps.some((step) => step.status !== "pending");
+  const preserveStepProgress =
+    options.preserveResumeState || (options.preserveProgress === true && hasNonPendingStepProgress);
+
+  if (!options.preserveWorktree) {
+    task.worktree = undefined;
+  }
+  if (!options.preserveResumeState) {
+    task.executionStartedAt = undefined;
+    task.executionCompletedAt = undefined;
+  } else {
+    task.executionCompletedAt = undefined;
+  }
+  if (!preserveStepProgress) {
+    ctx.resetSteps();
+    // Prompt-checkbox reset is a filesystem effect; the store performs it
+    // post-hook (it owns the task dir). Not modeled here.
+  }
+}
+
+/** `merge` trait onEnter (in-review): autoMerge stamping + scheduler-state
+ *  clearing. The queue enqueue itself is in-txn and store-owned (handoff path);
+ *  the field effects mirror the legacy in-review block. */
+export function applyInReviewEnterEffects(ctx: DefaultWorkflowMoveContext): void {
+  const { task, toColumn, settings } = ctx;
+  if (toColumn !== "in-review") return;
+  if (task.autoMerge === undefined && settings) {
+    task.autoMerge = settings.autoMerge;
+  }
+  task.recoveryRetryCount = undefined;
+  task.nextRecoveryAt = undefined;
+  if (task.status === "queued") {
+    task.status = undefined;
+  }
+  task.blockedBy = undefined;
+  task.overlapBlockedBy = undefined;
+}
+
+/** Reopen-from-review/done field clears (branch/summary/workflowStepResults). */
+export function applyReopenFieldClears(ctx: DefaultWorkflowMoveContext): void {
+  const { task, fromColumn, toColumn } = ctx;
+  if (
+    (fromColumn === "in-review" && (toColumn === "todo" || toColumn === "in-progress" || toColumn === "triage")) ||
+    (fromColumn === "done" && (toColumn === "todo" || toColumn === "triage"))
+  ) {
+    task.workflowStepResults = undefined;
+  }
+  if (fromColumn === "in-review" && (toColumn === "todo" || toColumn === "triage")) {
+    task.branch = undefined;
+    task.executionStartBranch = undefined;
+    task.baseCommitSha = undefined;
+    task.summary = undefined;
+    task.recoveryRetryCount = undefined;
+    task.nextRecoveryAt = undefined;
+  }
+}
+
+/**
+ * Apply ALL default-workflow field-mutation move effects (the parallel of the
+ * legacy inline block) in the legacy order. Pure in-memory mutation of
+ * `ctx.task`; queue/filesystem/post-commit effects remain store-owned.
+ *
+ * This is the entry point the flag-ON store path calls. It resolves each
+ * trait's hook through the registry first (so a missing registration degrades to
+ * a no-op + audit warning, satisfying the "invokes through the registry"
+ * contract and the degraded-hook path); resolution warnings are collected and
+ * returned for the store to forward to audit.
+ */
+export function applyDefaultWorkflowMoveEffects(
+  ctx: DefaultWorkflowMoveContext,
+): { warnings: TraitAuditWarning[] } {
+  const registry = getTraitRegistry();
+  const warnings: TraitAuditWarning[] = [];
+
+  // Resolve the hooks through the registry. The resolved impls are the closures
+  // registered by registerDefaultWorkflowHooks(); resolution surfaces a warning
+  // (and a no-op) if a registration is missing.
+  const toRun: Array<{ traitId: string; hookKind: "onEnter" | "onExit" }> = [
+    { traitId: "timing", hookKind: "onExit" },
+    { traitId: "timing", hookKind: "onEnter" },
+    { traitId: "reset-on-entry", hookKind: "onEnter" },
+    { traitId: "abort-on-exit", hookKind: "onExit" },
+    { traitId: "merge", hookKind: "onEnter" },
+  ];
+  for (const { traitId, hookKind } of toRun) {
+    const { impl, warning } = registry.resolveTraitHook(traitId, hookKind);
+    if (warning) warnings.push(warning);
+    if (impl) impl(ctx);
+  }
+
+  return { warnings };
+}
+
+// ── Registration into the trait registry (DI seam) ───────────────────────────
+
+let registered = false;
+
+/**
+ * Register the default-workflow hook implementations into the shared trait
+ * registry. Idempotent. Called at store init (the store is the engine-adjacent
+ * owner of the move lifecycle). Each registration is a thin adapter that runs
+ * the corresponding field-effect function over the move context.
+ *
+ * The legacy effects map onto traits as:
+ *   timing.onExit / timing.onEnter   → applyTimingEffects + completion stamp
+ *   reset-on-entry.onEnter           → applyResetOnEntryEffects + reopen clears
+ *   abort-on-exit.onExit             → (userPaused handled in reset-on-entry;
+ *                                       session abort is an engine effect U6/U7)
+ *   merge.onEnter                    → applyInReviewEnterEffects
+ */
+export function registerDefaultWorkflowHooks(): void {
+  if (registered) return;
+  const registry = getTraitRegistry();
+
+  const cast = (fn: (ctx: DefaultWorkflowMoveContext) => void) =>
+    ((...args: unknown[]) => fn(args[0] as DefaultWorkflowMoveContext)) as (
+      ...args: unknown[]
+    ) => unknown;
+
+  registry.registerTraitHookImpl(
+    "timing",
+    "onExit",
+    cast((ctx) => {
+      applyTimingEffects(ctx);
+    }),
+  );
+  registry.registerTraitHookImpl(
+    "timing",
+    "onEnter",
+    cast((ctx) => {
+      applyCompletionTimingEffects(ctx);
+    }),
+  );
+  registry.registerTraitHookImpl(
+    "reset-on-entry",
+    "onEnter",
+    cast((ctx) => {
+      applyResetOnEntryEffects(ctx);
+      applyReopenFieldClears(ctx);
+    }),
+  );
+  registry.registerTraitHookImpl(
+    "abort-on-exit",
+    "onExit",
+    cast(() => {
+      // userPaused is set in applyResetOnEntryEffects (the legacy ordering keeps
+      // it with the reopen block). Session-abort wiring is an engine effect that
+      // lands with U6/U7; here it is intentionally a no-op so the resolved hook
+      // exists (not a missing-impl warning) while carrying no field mutation.
+    }),
+  );
+  registry.registerTraitHookImpl(
+    "merge",
+    "onEnter",
+    cast((ctx) => {
+      applyInReviewEnterEffects(ctx);
+    }),
+  );
+
+  registered = true;
+}
+
+/** Test-only: allow re-registration after a registry reset. */
+export function __resetDefaultWorkflowHooksForTests(): void {
+  registered = false;
+}
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index da4b6ccec2..d02c509c2b 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -124,6 +124,14 @@ export type {
   TransitionPendingDbHandle,
   ReconcileHooksResult,
 } from "./transition-pending.js";
+// ── U4: workflow-resolved transition adjacency + flag accessor ───────────────
+export {
+  resolveColumnAdjacency,
+  resolveAllowedColumns,
+  workflowHasColumn,
+} from "./workflow-transitions.js";
+export type { ColumnAdjacency } from "./workflow-transitions.js";
+export { isWorkflowColumnsEnabled } from "./workflow-columns-settings.js";
 export {
   readTransitionPending,
   writeTransitionPending,
@@ -259,6 +267,7 @@ export {
   MergeQueueLeaseOwnershipError,
   InvalidMergeQueueLeaseDurationError,
   HandoffInvariantViolationError,
+  TransitionRejectionError,
 } from "./store.js";
 export {
   STOPWORDS,
diff --git a/packages/core/src/store.ts b/packages/core/src/store.ts
index 2322ed9883..061ebc4d12 100644
--- a/packages/core/src/store.ts
+++ b/packages/core/src/store.ts
@@ -8,6 +8,25 @@ import { createActivityLogSnapshot, createRunAuditSnapshot, createTaskMetadataSn
 import { VALID_TRANSITIONS, DEFAULT_SETTINGS, isGlobalOnlySettingsKey, WORKFLOW_STEP_TEMPLATES, validateDocumentKey } from "./types.js";
 import { DEFAULT_PROJECT_SETTINGS } from "./settings-schema.js";
 import { parseWorkflowIr, serializeWorkflowIr } from "./workflow-ir.js";
+import { isWorkflowColumnsEnabled } from "./workflow-columns-settings.js";
+import { resolveAllowedColumns, workflowHasColumn } from "./workflow-transitions.js";
+import {
+  type DefaultWorkflowMoveContext,
+  applyDefaultWorkflowMoveEffects,
+  evaluateMergeBlockerGuard,
+  registerDefaultWorkflowHooks,
+} from "./default-workflow-hooks.js";
+import {
+  type TransitionRejection,
+  makeTransitionRejection,
+  makeTransitionPending,
+} from "./transition-types.js";
+import { writeTransitionPending, clearTransitionPending } from "./transition-pending.js";
+import { BUILTIN_CODING_WORKFLOW_IR } from "./builtin-coding-workflow-ir.js";
+import type { WorkflowIr } from "./workflow-ir-types.js";
+// Side-effect import: registers the 14 built-in trait DEFINITIONS into the
+// shared trait registry on load (the flag-ON path resolves traits by id).
+import "./builtin-traits.js";
 import type {
   WorkflowDefinition,
   WorkflowDefinitionInput,
@@ -1047,6 +1066,28 @@ export class HandoffInvariantViolationError extends Error {
   }
 }
 
+/**
+ * Thrown by the flag-ON (`workflowColumns`) `moveTaskInternal` path when a move
+ * is rejected, carrying the typed {@link TransitionRejection} (KTD-3/R13). The
+ * existing callers of `moveTask` catch thrown `Error`s (e.g. the dashboard move
+ * route inspects `err.message`), so the rejection rides on an `Error` subclass
+ * — `.message` reproduces the legacy human-readable string so flag-ON callers
+ * that only read the message keep working, while `.rejection` exposes the
+ * machine-stable code/messageKey/retryable for surfaces that want it.
+ *
+ * The FLAG-OFF path still throws the bare legacy `Error` strings unchanged
+ * (zero behavior change while the flag is off — proven by the characterization
+ * suite).
+ */
+export class TransitionRejectionError extends Error {
+  readonly rejection: TransitionRejection;
+  constructor(rejection: TransitionRejection, message: string) {
+    super(message);
+    this.name = "TransitionRejectionError";
+    this.rejection = rejection;
+  }
+}
+
 interface MoveTaskOptions {
   preserveResumeState?: boolean;
   preserveProgress?: boolean;
@@ -1056,6 +1097,15 @@ interface MoveTaskOptions {
   moveSource?: "user" | "engine";
   skipMergeBlocker?: boolean;
   allowDirectInReviewMove?: boolean;
+  /**
+   * KTD-9: engine/recovery moves bypass trait guards and abort-on-exit effects
+   * (the generalization of `skipMergeBlocker`). It NEVER bypasses capacity
+   * (KTD-10). Engine-internal only: HTTP move endpoints hardcode it off and must
+   * never forward a caller-supplied value (mirrors the hardcoded
+   * `moveSource: "user"` posture). When unset, the flag-ON path derives it from
+   * `moveSource === "engine"` plus `skipMergeBlocker`.
+   */
+  bypassGuards?: boolean;
 }
 
 interface MoveTaskInternalOptions {
@@ -1395,7 +1445,14 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
 
   async init(): Promise<void> {
     await mkdir(this.tasksDir, { recursive: true });
-    
+
+    // U4: register the default-workflow trait hook implementations into the
+    // shared trait registry (the flag-ON moveTaskInternal path resolves the
+    // legacy per-column effects through these). Idempotent; built-in trait
+    // DEFINITIONS self-register on import of ./builtin-traits.js (pulled in
+    // transitively via default-workflow-hooks / trait-registry).
+    registerDefaultWorkflowHooks();
+
     // Initialize SQLite database
     if (!this._db) {
       // Startup corruption guard: before opening, detect a malformed fusion.db
@@ -5533,6 +5590,9 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
         {
           ...opts.moveOptions,
           skipMergeBlocker: true,
+          // KTD-9: handoff is an engine/recovery-class move; its skipMergeBlocker
+          // maps onto bypassGuards under the flag (identical behavior both paths).
+          bypassGuards: true,
         },
         {
           fromHandoff: true,
@@ -5560,6 +5620,27 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     const task = currentTask ?? await this.readTaskForMove(id);
     const moveSource = options?.moveSource ?? "engine";
 
+    // ── U4: flag-gated workflow-resolved transition path (KTD-8) ─────────────
+    // Flag OFF (default): the legacy `VALID_TRANSITIONS` / inline-side-effect
+    // path below runs byte-identical (proven by the characterization suite).
+    // Flag ON: validate against the task's resolved workflow column graph, run
+    // sync trait guards (unless bypassed), and route the legacy per-column side
+    // effects through the default-workflow trait hooks.
+    // `experimentalFeatures` is a global-scoped setting, so the project-only
+    // `getSettingsSync()` row would miss it — read merged settings (global +
+    // project) via getSettingsFast(). This is an async read taken before the
+    // lock-sensitive transaction; it does not touch the task lock.
+    const useWorkflow = isWorkflowColumnsEnabled(await this.getSettingsFast());
+    // bypassGuards (KTD-9): engine-sourced moves + the existing skipMergeBlocker
+    // call sites map onto it. Capacity (KTD-10) is NEVER bypassed by this — the
+    // capacity check is not a guard (U6 fills the enforcement; U4 leaves a
+    // pass-through slot). An explicit option value wins; otherwise derive it.
+    const bypassGuards =
+      options?.bypassGuards ?? (moveSource === "engine" || options?.skipMergeBlocker === true);
+    const workflowIr: WorkflowIr | undefined = useWorkflow
+      ? this.resolveTaskWorkflowIrSync(id)
+      : undefined;
+
     if (task.column === toColumn) {
       if (internal.fromHandoff && toColumn === "in-review") {
         this.db.transactionImmediate(() => {
@@ -5616,19 +5697,70 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       return task;
     }
 
-    const validTargets = VALID_TRANSITIONS[task.column];
-    if (!validTargets.includes(toColumn)) {
-      throw new Error(
-        `Invalid transition: '${task.column}' → '${toColumn}'. ` +
-          `Valid targets: ${validTargets.join(", ") || "none"}`,
-      );
-    }
-
     const fromColumn = task.column;
-    if (fromColumn === "in-review" && toColumn === "done" && !options?.skipMergeBlocker) {
-      const mergeBlocker = getTaskMergeBlocker(task);
-      if (mergeBlocker) {
-        throw new Error(`Cannot move ${id} to done: ${mergeBlocker}`);
+
+    if (useWorkflow && workflowIr) {
+      // ── Flag-ON validation + sync guards (typed rejections, KTD-3/R13) ─────
+      // 1. Target column must exist in the task's workflow → unknown-column.
+      if (!workflowHasColumn(workflowIr, toColumn)) {
+        throw new TransitionRejectionError(
+          makeTransitionRejection(
+            "unknown-column",
+            "transition.rejected.unknownColumn",
+            false,
+            `Column '${toColumn}' is not defined in this task's workflow`,
+          ),
+          `Invalid transition: '${fromColumn}' → '${toColumn}'. Unknown column for this workflow.`,
+        );
+      }
+      // 2. Column-graph adjacency. For the default workflow this reproduces
+      //    VALID_TRANSITIONS verbatim (resolveAllowedColumns); the
+      //    transition-parity suite machine-checks the equivalence.
+      const allowed = resolveAllowedColumns(workflowIr, fromColumn);
+      if (!allowed.includes(toColumn)) {
+        throw new TransitionRejectionError(
+          makeTransitionRejection(
+            "guard-rejected",
+            "transition.rejected.invalidTransition",
+            false,
+            `Valid targets: ${allowed.join(", ") || "none"}`,
+          ),
+          `Invalid transition: '${fromColumn}' → '${toColumn}'. ` +
+            `Valid targets: ${allowed.join(", ") || "none"}`,
+        );
+      }
+      // 3. Sync trait guards (in-lock). Skipped entirely when bypassGuards
+      //    (engine/recovery moves, KTD-9). The default workflow's merge-blocker
+      //    trait reads the same getTaskMergeBlocker.
+      if (!bypassGuards) {
+        const guardReason = evaluateMergeBlockerGuard(task, fromColumn, toColumn);
+        if (guardReason) {
+          throw new TransitionRejectionError(
+            makeTransitionRejection(
+              "merge-blocked",
+              "transition.rejected.mergeBlocked",
+              true,
+              guardReason,
+            ),
+            `Cannot move ${id} to done: ${guardReason}`,
+          );
+        }
+      }
+    } else {
+      // ── Flag-OFF legacy path (unchanged) ───────────────────────────────────
+      const validTargets = VALID_TRANSITIONS[task.column];
+      if (!validTargets.includes(toColumn)) {
+        throw new Error(
+          `Invalid transition: '${task.column}' → '${toColumn}'. ` +
+            `Valid targets: ${validTargets.join(", ") || "none"}`,
+        );
+      }
+
+      if (fromColumn === "in-review" && toColumn === "done" && !options?.skipMergeBlocker) {
+        const mergeBlocker = getTaskMergeBlocker(task);
+        if (mergeBlocker) {
+          throw new Error(`Cannot move ${id} to done: ${mergeBlocker}`);
+        }
       }
     }
 
@@ -5642,106 +5774,153 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     task.columnMovedAt = movedAt;
     task.updatedAt = movedAt;
 
-    if (fromColumn === "in-progress" && toColumn !== "in-progress") {
-      const segmentStartMs = Date.parse(task.executionStartedAt ?? task.columnMovedAt);
-      const segmentEndMs = Date.parse(task.columnMovedAt);
-      const segmentDeltaMs =
-        Number.isFinite(segmentStartMs) && Number.isFinite(segmentEndMs)
-          ? Math.max(0, segmentEndMs - segmentStartMs)
-          : 0;
-      task.cumulativeActiveMs = Math.max(0, task.cumulativeActiveMs ?? 0) + segmentDeltaMs;
-    }
-
-    if (toColumn === "in-progress") {
-      task.cumulativeActiveMs ??= 0;
-      if (!task.firstExecutionAt) {
-        task.firstExecutionAt = task.columnMovedAt;
-      }
-      if (!task.executionStartedAt) {
-        task.executionStartedAt = task.columnMovedAt;
-      }
-      task.userPaused = undefined;
-    }
-    if (toColumn === "done" && !task.executionCompletedAt) {
-      task.executionCompletedAt = task.columnMovedAt;
-    }
-
-    if (toColumn === "done") {
-      this.clearDoneTransientFields(task);
-    }
-
-    const isReopenToTodoOrTriage =
-      (fromColumn === "in-progress" || fromColumn === "done" || fromColumn === "in-review")
-      && (toColumn === "todo" || toColumn === "triage");
-
-    if (isReopenToTodoOrTriage) {
-      if (!options?.preserveStatus) {
-        task.status = undefined;
-        task.error = undefined;
-        task.pausedReason = undefined;
-      }
-      task.blockedBy = undefined;
-      task.overlapBlockedBy = undefined;
-      task.paused = undefined;
-      task.pausedByAgentId = undefined;
-      if (moveSource === "user" && toColumn === "todo") {
-        task.userPaused = true;
-      } else {
-        task.userPaused = undefined;
-      }
-
+    if (useWorkflow) {
+      // ── Flag-ON: route the legacy per-column side effects through the
+      //    default-workflow trait hooks (timing, reset-on-entry, abort-on-exit,
+      //    merge.onEnter). "Moved, not duplicated" applies to this path; the
+      //    flag-off branch below keeps the legacy inline code verbatim. ───────
+      const ctx: DefaultWorkflowMoveContext = {
+        task,
+        fromColumn,
+        toColumn,
+        moveSource,
+        bypassGuards,
+        movedAt,
+        settings: settingsForInReview,
+        options: {
+          preserveStatus: options?.preserveStatus,
+          preserveResumeState: options?.preserveResumeState,
+          preserveProgress: options?.preserveProgress,
+          preserveWorktree: options?.preserveWorktree,
+        },
+        resetSteps: () => this.resetAllStepsToPending(task),
+      };
+      const isReopenToTodoOrTriage =
+        (fromColumn === "in-progress" || fromColumn === "done" || fromColumn === "in-review") &&
+        (toColumn === "todo" || toColumn === "triage");
       const hasNonPendingStepProgress = task.steps.some((step) => step.status !== "pending");
       const preserveStepProgress =
-        options?.preserveResumeState || (options?.preserveProgress === true && hasNonPendingStepProgress);
-
-      if (!options?.preserveWorktree) {
-        task.worktree = undefined;
+        options?.preserveResumeState ||
+        (options?.preserveProgress === true && hasNonPendingStepProgress);
+      const { warnings } = applyDefaultWorkflowMoveEffects(ctx);
+      for (const warning of warnings) {
+        storeLog.warn("Default-workflow trait hook degraded to no-op", {
+          phase: "moveTaskInternal:workflow-hooks",
+          taskId: id,
+          ...warning,
+        });
       }
-
-      if (!options?.preserveResumeState) {
-        task.executionStartedAt = undefined;
-        task.executionCompletedAt = undefined;
-      } else {
-        task.executionCompletedAt = undefined;
+      // Store-owned effects the hooks intentionally do NOT perform (filesystem /
+      // store-private): clearing done transient fields + prompt-checkbox reset.
+      if (toColumn === "done") {
+        this.clearDoneTransientFields(task);
       }
-
-      if (!preserveStepProgress) {
-        this.resetAllStepsToPending(task);
+      if (isReopenToTodoOrTriage && !preserveStepProgress) {
         await this.resetPromptCheckboxes(dir);
       }
-    }
-
-    if (toColumn === "in-review") {
-      if (task.autoMerge === undefined && settingsForInReview) {
-        task.autoMerge = settingsForInReview.autoMerge;
+    } else {
+      // ── Flag-OFF legacy inline side effects (UNCHANGED — the flag-off path) ──
+      if (fromColumn === "in-progress" && toColumn !== "in-progress") {
+        const segmentStartMs = Date.parse(task.executionStartedAt ?? task.columnMovedAt);
+        const segmentEndMs = Date.parse(task.columnMovedAt);
+        const segmentDeltaMs =
+          Number.isFinite(segmentStartMs) && Number.isFinite(segmentEndMs)
+            ? Math.max(0, segmentEndMs - segmentStartMs)
+            : 0;
+        task.cumulativeActiveMs = Math.max(0, task.cumulativeActiveMs ?? 0) + segmentDeltaMs;
       }
-      task.recoveryRetryCount = undefined;
-      task.nextRecoveryAt = undefined;
-      // Clear scheduler-side dispatch state: `queued`, `blockedBy`, and
-      // `overlapBlockedBy` are stamped while the task waits in `todo`. If
-      // they survive the transition into `in-review` they permanently block
-      // the merge gate (see getTaskMergeBlocker's BLOCKING_TASK_STATUSES).
-      if (task.status === "queued") {
-        task.status = undefined;
+
+      if (toColumn === "in-progress") {
+        task.cumulativeActiveMs ??= 0;
+        if (!task.firstExecutionAt) {
+          task.firstExecutionAt = task.columnMovedAt;
+        }
+        if (!task.executionStartedAt) {
+          task.executionStartedAt = task.columnMovedAt;
+        }
+        task.userPaused = undefined;
+      }
+      if (toColumn === "done" && !task.executionCompletedAt) {
+        task.executionCompletedAt = task.columnMovedAt;
       }
-      task.blockedBy = undefined;
-      task.overlapBlockedBy = undefined;
-    }
 
-    if (
-      (fromColumn === "in-review" && (toColumn === "todo" || toColumn === "in-progress" || toColumn === "triage"))
-      || (fromColumn === "done" && (toColumn === "todo" || toColumn === "triage"))
-    ) {
-      task.workflowStepResults = undefined;
-    }
+      if (toColumn === "done") {
+        this.clearDoneTransientFields(task);
+      }
 
-    if (fromColumn === "in-review" && (toColumn === "todo" || toColumn === "triage")) {
-      task.branch = undefined;
-      task.executionStartBranch = undefined;
-      task.baseCommitSha = undefined;
-      task.summary = undefined;
-      task.recoveryRetryCount = undefined;
-      task.nextRecoveryAt = undefined;
+      const isReopenToTodoOrTriage =
+        (fromColumn === "in-progress" || fromColumn === "done" || fromColumn === "in-review")
+        && (toColumn === "todo" || toColumn === "triage");
+
+      if (isReopenToTodoOrTriage) {
+        if (!options?.preserveStatus) {
+          task.status = undefined;
+          task.error = undefined;
+          task.pausedReason = undefined;
+        }
+        task.blockedBy = undefined;
+        task.overlapBlockedBy = undefined;
+        task.paused = undefined;
+        task.pausedByAgentId = undefined;
+        if (moveSource === "user" && toColumn === "todo") {
+          task.userPaused = true;
+        } else {
+          task.userPaused = undefined;
+        }
+
+        const hasNonPendingStepProgress = task.steps.some((step) => step.status !== "pending");
+        const preserveStepProgress =
+          options?.preserveResumeState || (options?.preserveProgress === true && hasNonPendingStepProgress);
+
+        if (!options?.preserveWorktree) {
+          task.worktree = undefined;
+        }
+
+        if (!options?.preserveResumeState) {
+          task.executionStartedAt = undefined;
+          task.executionCompletedAt = undefined;
+        } else {
+          task.executionCompletedAt = undefined;
+        }
+
+        if (!preserveStepProgress) {
+          this.resetAllStepsToPending(task);
+          await this.resetPromptCheckboxes(dir);
+        }
+      }
+
+      if (toColumn === "in-review") {
+        if (task.autoMerge === undefined && settingsForInReview) {
+          task.autoMerge = settingsForInReview.autoMerge;
+        }
+        task.recoveryRetryCount = undefined;
+        task.nextRecoveryAt = undefined;
+        // Clear scheduler-side dispatch state: `queued`, `blockedBy`, and
+        // `overlapBlockedBy` are stamped while the task waits in `todo`. If
+        // they survive the transition into `in-review` they permanently block
+        // the merge gate (see getTaskMergeBlocker's BLOCKING_TASK_STATUSES).
+        if (task.status === "queued") {
+          task.status = undefined;
+        }
+        task.blockedBy = undefined;
+        task.overlapBlockedBy = undefined;
+      }
+
+      if (
+        (fromColumn === "in-review" && (toColumn === "todo" || toColumn === "in-progress" || toColumn === "triage"))
+        || (fromColumn === "done" && (toColumn === "todo" || toColumn === "triage"))
+      ) {
+        task.workflowStepResults = undefined;
+      }
+
+      if (fromColumn === "in-review" && (toColumn === "todo" || toColumn === "triage")) {
+        task.branch = undefined;
+        task.executionStartBranch = undefined;
+        task.baseCommitSha = undefined;
+        task.summary = undefined;
+        task.recoveryRetryCount = undefined;
+        task.nextRecoveryAt = undefined;
+      }
     }
 
     if (toColumn === "in-progress" && !task.worktree && options?.allocateWorktree) {
@@ -5785,6 +5964,22 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       });
       this.dequeueMergeQueueOnColumnExit(id, fromColumn, toColumn, movedAt);
 
+      // U4 (flag-ON): write the crash-safe transitionPending marker in the SAME
+      // transaction as the column change (KTD-2). It records the post-commit
+      // hooks that still owe idempotent execution so a crash mid-transition is
+      // recoverable from SQLite (the authoritative store, ADR-0001). The store
+      // clears it immediately after the post-commit hook runner completes
+      // (below). For the default workflow the field effects already applied
+      // in-lock; the marker guards the post-commit completion so recovery never
+      // double-runs (idempotent) and never strands the card.
+      if (useWorkflow) {
+        writeTransitionPending(
+          this.db,
+          id,
+          makeTransitionPending(toColumn, ["default-workflow:postCommit"], Date.parse(movedAt) || Date.now()),
+        );
+      }
+
       if (toColumn === "in-review" && !internal.fromHandoff && options?.allowDirectInReviewMove !== true) {
         this.insertRunAuditEventRow({
           taskId: id,
@@ -5859,6 +6054,23 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
 
     if (this.isWatching) this.taskCache.set(id, { ...task });
 
+    // U4 (flag-ON): post-commit hook completion. The default-workflow field
+    // effects already ran in-lock and committed; the post-commit phase here is
+    // the fire-and-forget hook runner per KTD-2. It is idempotent and clears the
+    // transitionPending marker once done. A crash before this point leaves the
+    // marker for the recovery sweep to re-run (re-running is a no-op for the
+    // default workflow's already-committed field effects). We clear it
+    // synchronously here because the default workflow has no async post-commit
+    // hook bodies in U4 (merge enqueue is in-txn via the handoff path);
+    // plugin/async post-commit hooks land in U7/U8 and will defer the clear.
+    if (useWorkflow) {
+      try {
+        clearTransitionPending(this.db, id);
+      } catch {
+        // Clearing is best-effort; the marker recovery sweep is the backstop.
+      }
+    }
+
     if (fromColumn !== toColumn) {
       this.emit("task:moved", { task, from: fromColumn, to: toColumn, source: moveSource });
     }
@@ -11436,6 +11648,36 @@ ${stepsSection}`;
   }
 
   /** Read the workflow currently selected for a task, if any. */
+  /**
+   * Synchronously resolve the parsed WorkflowIr that governs a task's columns
+   * (U4, flag-ON path). Resolution order:
+   *   1. the task's workflow selection (side table) → that workflow's IR;
+   *   2. null/missing selection → the built-in default workflow IR (KTD-1).
+   * Built-in workflow IRs are resolved from the parsed module constant; custom
+   * workflows are read + parsed from the `workflows` row. Pure DB read, safe to
+   * call inside `withTaskLock` (no further locks taken). A parse failure or
+   * missing custom row falls back to the default workflow so a move is never
+   * stranded by a corrupt definition (degraded, not crashed).
+   */
+  private resolveTaskWorkflowIrSync(taskId: string): WorkflowIr {
+    const selection = this.getTaskWorkflowSelection(taskId);
+    const workflowId = selection?.workflowId;
+    if (!workflowId) return BUILTIN_CODING_WORKFLOW_IR;
+    if (isBuiltinWorkflowId(workflowId)) {
+      const builtin = getBuiltinWorkflow(workflowId);
+      return builtin?.ir ?? BUILTIN_CODING_WORKFLOW_IR;
+    }
+    try {
+      const row = this.db
+        .prepare("SELECT ir FROM workflows WHERE id = ?")
+        .get(workflowId) as { ir: string } | undefined;
+      if (!row) return BUILTIN_CODING_WORKFLOW_IR;
+      return parseWorkflowIr(row.ir);
+    } catch {
+      return BUILTIN_CODING_WORKFLOW_IR;
+    }
+  }
+
   getTaskWorkflowSelection(taskId: string): { workflowId: string; stepIds: string[] } | undefined {
     const row = this.db
       .prepare("SELECT workflowId, stepIds FROM task_workflow_selection WHERE taskId = ?")
diff --git a/packages/core/src/workflow-columns-settings.ts b/packages/core/src/workflow-columns-settings.ts
new file mode 100644
index 0000000000..e5a946546f
--- /dev/null
+++ b/packages/core/src/workflow-columns-settings.ts
@@ -0,0 +1,18 @@
+import { isExperimentalFeatureEnabled } from "./experimental-features.js";
+import type { Settings } from "./types.js";
+
+/**
+ * The `experimentalFeatures.workflowColumns` flag (KTD-8). OFF: the legacy
+ * enum/`VALID_TRANSITIONS` path runs untouched. ON: `moveTaskInternal` resolves
+ * each task's workflow column graph + trait guards. Default OFF until the
+ * transition-parity suite and field observations prove zero drift (U12).
+ *
+ * Mirrors `isSandboxExperimentalEnabled` / `isEvalsViewEnabled` — a thin,
+ * named accessor over the shared experimental-features map so the literal flag
+ * key lives in exactly one place.
+ */
+export function isWorkflowColumnsEnabled(
+  settings: Pick<Settings, "experimentalFeatures"> | undefined,
+): boolean {
+  return isExperimentalFeatureEnabled(settings, "workflowColumns");
+}
diff --git a/packages/core/src/workflow-transitions.ts b/packages/core/src/workflow-transitions.ts
new file mode 100644
index 0000000000..ed81c1243d
--- /dev/null
+++ b/packages/core/src/workflow-transitions.ts
@@ -0,0 +1,108 @@
+/**
+ * Workflow-resolved transition adjacency (U4, R4/R9/R13).
+ *
+ * `moveTaskInternal` (flag ON) and `board.ts` both derive "which columns can a
+ * card move to from here" from the SAME helper so the two surfaces never
+ * diverge — `resolveAllowedColumns(ir, fromColumn)`.
+ *
+ * ── Why an explicit adjacency, not pure graph-derivation ──────────────────────
+ *
+ * The plan asks: derive allowed column adjacency from node placement + edges,
+ * and for the DEFAULT workflow it MUST reproduce `VALID_TRANSITIONS` exactly.
+ * Pure graph-edge derivation CANNOT reproduce it: `VALID_TRANSITIONS` encodes
+ * backward/reopen edges (in-review → todo, done → todo, archived → done, …) and
+ * cross edges (in-progress → done) that have no counterpart in the linear
+ * execute → review → merge → end pipeline graph. The IR edges describe the
+ * forward automation walk; the column adjacency describes legal *board* moves
+ * (drags, reopens, recovery), which is a strictly larger, partly-cyclic set.
+ *
+ * So per the plan's documented fallback we attach an explicit per-column
+ * `transitions` adjacency:
+ *   - For the BUILT-IN default workflow we reproduce `VALID_TRANSITIONS` verbatim
+ *     (keyed by the legacy column ids, which are exactly the default workflow's
+ *     column ids — KTD-1). This is the parity contract the transition-parity
+ *     suite machine-checks.
+ *   - For CUSTOM workflows (no explicit adjacency authored yet — authoring lands
+ *     with the editor in U10) we derive a linear forward+back adjacency from the
+ *     declared column ORDER: each column may move to its neighbors (prev/next).
+ *     This is a safe, predictable default that keeps every column reachable and
+ *     never strands a card; richer custom adjacency is future work.
+ *
+ * The adjacency is intentionally a column→columns map computed once per IR; it
+ * is read-only and pure.
+ */
+
+import { VALID_TRANSITIONS } from "./types.js";
+import type { Column } from "./types.js";
+import type { WorkflowIr, WorkflowIrV2 } from "./workflow-ir-types.js";
+import { DEFAULT_WORKFLOW_COLUMN_IDS } from "./workflow-ir.js";
+
+/** A column→allowed-target-columns adjacency map. */
+export type ColumnAdjacency = Map<string, string[]>;
+
+/** True when the IR's columns are exactly the legacy default-workflow column ids
+ *  (same set), i.e. this is the built-in default workflow (or an equivalent). */
+function isDefaultWorkflowColumns(ir: WorkflowIrV2): boolean {
+  const ids = ir.columns.map((c) => c.id);
+  if (ids.length !== DEFAULT_WORKFLOW_COLUMN_IDS.length) return false;
+  const set = new Set(ids);
+  return DEFAULT_WORKFLOW_COLUMN_IDS.every((id) => set.has(id));
+}
+
+/** Build the verbatim `VALID_TRANSITIONS` adjacency keyed by column id. */
+function defaultWorkflowAdjacency(): ColumnAdjacency {
+  const adj: ColumnAdjacency = new Map();
+  for (const [from, targets] of Object.entries(VALID_TRANSITIONS) as [Column, Column[]][]) {
+    adj.set(from, [...targets]);
+  }
+  return adj;
+}
+
+/** Derive a neighbor (prev/next by declared order) adjacency for a custom
+ *  workflow. Each column can move to the column before and after it in the
+ *  authored order. Endpoints have a single neighbor. */
+function orderDerivedAdjacency(ir: WorkflowIrV2): ColumnAdjacency {
+  const adj: ColumnAdjacency = new Map();
+  const ids = ir.columns.map((c) => c.id);
+  for (let i = 0; i < ids.length; i++) {
+    const targets: string[] = [];
+    if (i > 0) targets.push(ids[i - 1]);
+    if (i < ids.length - 1) targets.push(ids[i + 1]);
+    adj.set(ids[i], targets);
+  }
+  return adj;
+}
+
+/**
+ * Resolve the full column adjacency for a workflow IR. The default workflow
+ * reproduces `VALID_TRANSITIONS` exactly; custom workflows use order-derived
+ * neighbor adjacency.
+ */
+export function resolveColumnAdjacency(ir: WorkflowIr): ColumnAdjacency {
+  // v1 IR is upgraded to v2 on parse, but accept either defensively.
+  const v2 = ir as WorkflowIrV2;
+  if (!Array.isArray(v2.columns)) {
+    // No columns (shouldn't happen post-parse) → empty adjacency.
+    return new Map();
+  }
+  if (isDefaultWorkflowColumns(v2)) {
+    return defaultWorkflowAdjacency();
+  }
+  return orderDerivedAdjacency(v2);
+}
+
+/**
+ * The allowed target columns for a move out of `fromColumn` under this workflow.
+ * Returns an empty array when `fromColumn` is unknown to the workflow (callers
+ * should first check column existence to distinguish "unknown column" from "no
+ * legal targets").
+ */
+export function resolveAllowedColumns(ir: WorkflowIr, fromColumn: string): string[] {
+  return resolveColumnAdjacency(ir).get(fromColumn) ?? [];
+}
+
+/** True when `toColumn` is a defined column of the workflow. */
+export function workflowHasColumn(ir: WorkflowIr, columnId: string): boolean {
+  const v2 = ir as WorkflowIrV2;
+  return Array.isArray(v2.columns) && v2.columns.some((c) => c.id === columnId);
+}
diff --git a/packages/dashboard/src/routes/__tests__/register-task-workflow-routes.move-bypassguards.test.ts b/packages/dashboard/src/routes/__tests__/register-task-workflow-routes.move-bypassguards.test.ts
new file mode 100644
index 0000000000..a0e91911a6
--- /dev/null
+++ b/packages/dashboard/src/routes/__tests__/register-task-workflow-routes.move-bypassguards.test.ts
@@ -0,0 +1,52 @@
+// @vitest-environment node
+//
+// U4 hardening: `bypassGuards` is engine-internal (KTD-9). The HTTP move
+// endpoint hardcodes its move options (mirroring the hardcoded
+// `moveSource: "user"` posture) and must NEVER forward a caller-supplied
+// `bypassGuards` (or `moveSource`) from the request body — otherwise a remote
+// caller could bypass trait guards / abort-on-exit.
+
+import { describe, it, expect, vi } from "vitest";
+import express from "express";
+import type { TaskStore } from "@fusion/core";
+import { createApiRoutes } from "../../routes.js";
+import { request as REQUEST } from "../../test-request.js";
+
+describe("task move route — bypassGuards is not forwardable", () => {
+  it("ignores a caller-supplied bypassGuards/moveSource in the request body", async () => {
+    const moveTask = vi.fn(async (_id: string, column: string, _options?: Record<string, unknown>) => ({
+      id: "FN-001",
+      column,
+      dependencies: [],
+      steps: [],
+      currentStep: 0,
+    }));
+
+    const store: TaskStore = {
+      getRootDir: vi.fn(() => process.cwd()),
+      getTask: vi.fn(async () => ({ id: "FN-001", column: "todo" })),
+      getSettings: vi.fn(async () => ({})),
+      moveTask,
+    } as unknown as TaskStore;
+
+    const app = express();
+    app.use(express.json());
+    app.use("/api", createApiRoutes(store));
+
+    const res = await REQUEST(
+      app,
+      "POST",
+      "/api/tasks/FN-001/move",
+      JSON.stringify({ column: "triage", bypassGuards: true, moveSource: "engine" }),
+      { "content-type": "application/json" },
+    );
+
+    expect(res.status).toBe(200);
+    expect(moveTask).toHaveBeenCalledTimes(1);
+    const passedOptions = moveTask.mock.calls[0][2] as Record<string, unknown> | undefined;
+    // The route constructs its own options; the injected fields must not leak.
+    expect(passedOptions?.bypassGuards).toBeUndefined();
+    // The route hardcodes moveSource: "user" — the body's "engine" is ignored.
+    expect(passedOptions?.moveSource).toBe("user");
+  });
+});