diff --git a/packages/core/src/__tests__/task-fields.test.ts b/packages/core/src/__tests__/task-fields.test.ts
new file mode 100644
index 0000000000..a35e0c855d
--- /dev/null
+++ b/packages/core/src/__tests__/task-fields.test.ts
@@ -0,0 +1,455 @@
+import { describe, it, expect, beforeEach, afterEach } from "vitest";
+
+import {
+  validateCustomFieldPatch,
+  applyFieldDefaults,
+  reconcileFieldsOnWorkflowChange,
+} from "../task-fields.js";
+import type { WorkflowFieldDefinition, WorkflowIr } from "../workflow-ir-types.js";
+import { createTaskStoreTestHarness } from "./store-test-helpers.js";
+
+/**
+ * U11 / KTD-13 — custom task fields: validation authority, defaults,
+ * reconciliation, and the store-level write authority.
+ *
+ * The pure functions in task-fields.ts are the single validation core; the
+ * store delegates to them for updateTask/updateTaskCustomFields and for
+ * workflow-switch / definition-edit reconciliation. These tests cover both.
+ */
+
+// ── Field-definition fixtures ────────────────────────────────────────────────
+
+const F = (over: Partial<WorkflowFieldDefinition> & { id: string; type: WorkflowFieldDefinition["type"] }): WorkflowFieldDefinition => ({
+  name: over.id,
+  ...over,
+});
+
+const enumOpts = [
+  { value: "high", label: "High" },
+  { value: "low", label: "Low" },
+];
+
+const ALL_TYPES: WorkflowFieldDefinition[] = [
+  F({ id: "s", type: "string" }),
+  F({ id: "tx", type: "text" }),
+  F({ id: "n", type: "number" }),
+  F({ id: "b", type: "boolean" }),
+  F({ id: "e", type: "enum", options: enumOpts }),
+  F({ id: "m", type: "multi-enum", options: enumOpts }),
+  F({ id: "d", type: "date" }),
+  F({ id: "u", type: "url" }),
+];
+
+// ── Pure validation: every type ──────────────────────────────────────────────
+
+describe("validateCustomFieldPatch — per-type validate/reject", () => {
+  it("string/text accept strings, reject non-strings", () => {
+    expect(validateCustomFieldPatch(ALL_TYPES, { s: "hi", tx: "yo" }).ok).toBe(true);
+    const r = validateCustomFieldPatch(ALL_TYPES, { s: 5 });
+    expect(r.ok).toBe(false);
+    if (!r.ok) expect(r.rejection.code).toBe("type-mismatch");
+  });
+
+  it("number accepts finite numbers, rejects NaN/Infinity/non-number", () => {
+    expect(validateCustomFieldPatch(ALL_TYPES, { n: 3 }).ok).toBe(true);
+    expect(validateCustomFieldPatch(ALL_TYPES, { n: 0 }).ok).toBe(true);
+    expect(validateCustomFieldPatch(ALL_TYPES, { n: Number.NaN }).ok).toBe(false);
+    expect(validateCustomFieldPatch(ALL_TYPES, { n: Number.POSITIVE_INFINITY }).ok).toBe(false);
+    expect(validateCustomFieldPatch(ALL_TYPES, { n: "3" }).ok).toBe(false);
+  });
+
+  it("boolean accepts booleans only", () => {
+    expect(validateCustomFieldPatch(ALL_TYPES, { b: true }).ok).toBe(true);
+    expect(validateCustomFieldPatch(ALL_TYPES, { b: "true" }).ok).toBe(false);
+  });
+
+  it("date accepts parseable ISO strings, rejects garbage", () => {
+    expect(validateCustomFieldPatch(ALL_TYPES, { d: "2026-06-04" }).ok).toBe(true);
+    expect(validateCustomFieldPatch(ALL_TYPES, { d: "2026-06-04T12:00:00Z" }).ok).toBe(true);
+    expect(validateCustomFieldPatch(ALL_TYPES, { d: "not-a-date" }).ok).toBe(false);
+    expect(validateCustomFieldPatch(ALL_TYPES, { d: 20260604 }).ok).toBe(false);
+  });
+
+  it("url accepts URL-parseable strings, rejects bad", () => {
+    expect(validateCustomFieldPatch(ALL_TYPES, { u: "https://example.com/x" }).ok).toBe(true);
+    const r = validateCustomFieldPatch(ALL_TYPES, { u: "not a url" });
+    expect(r.ok).toBe(false);
+    if (!r.ok) expect(r.rejection.code).toBe("type-mismatch");
+  });
+});
+
+describe("validateCustomFieldPatch — enum membership", () => {
+  it("accepts a declared option, rejects a non-member with enum-violation", () => {
+    expect(validateCustomFieldPatch(ALL_TYPES, { e: "high" }).ok).toBe(true);
+    const r = validateCustomFieldPatch(ALL_TYPES, { e: "medium" });
+    expect(r.ok).toBe(false);
+    if (!r.ok) {
+      expect(r.rejection.code).toBe("enum-violation");
+      expect(r.rejection.fieldId).toBe("e");
+    }
+  });
+  it("rejects a non-string enum value with type-mismatch", () => {
+    const r = validateCustomFieldPatch(ALL_TYPES, { e: 1 });
+    expect(r.ok).toBe(false);
+    if (!r.ok) expect(r.rejection.code).toBe("type-mismatch");
+  });
+});
+
+describe("validateCustomFieldPatch — multi-enum subsets + dupes", () => {
+  it("accepts a subset of options", () => {
+    const r = validateCustomFieldPatch(ALL_TYPES, { m: ["high"] });
+    expect(r.ok).toBe(true);
+    if (r.ok) expect(r.normalized.m).toEqual(["high"]);
+  });
+  it("accepts the empty array", () => {
+    expect(validateCustomFieldPatch(ALL_TYPES, { m: [] }).ok).toBe(true);
+  });
+  it("rejects a non-member with enum-violation", () => {
+    const r = validateCustomFieldPatch(ALL_TYPES, { m: ["high", "medium"] });
+    expect(r.ok).toBe(false);
+    if (!r.ok) expect(r.rejection.code).toBe("enum-violation");
+  });
+  it("rejects duplicate members", () => {
+    const r = validateCustomFieldPatch(ALL_TYPES, { m: ["high", "high"] });
+    expect(r.ok).toBe(false);
+    if (!r.ok) expect(r.rejection.code).toBe("enum-violation");
+  });
+  it("rejects a non-array", () => {
+    const r = validateCustomFieldPatch(ALL_TYPES, { m: "high" });
+    expect(r.ok).toBe(false);
+    if (!r.ok) expect(r.rejection.code).toBe("type-mismatch");
+  });
+});
+
+describe("validateCustomFieldPatch — unknown field & no-fields", () => {
+  it("rejects a patch key naming no declared field", () => {
+    const r = validateCustomFieldPatch(ALL_TYPES, { nope: 1 });
+    expect(r.ok).toBe(false);
+    if (!r.ok) {
+      expect(r.rejection.code).toBe("unknown-field");
+      expect(r.rejection.fieldId).toBe("nope");
+    }
+  });
+  it("rejects any non-empty patch when no fields are defined (no-fields-defined)", () => {
+    const r = validateCustomFieldPatch(undefined, { anything: 1 });
+    expect(r.ok).toBe(false);
+    if (!r.ok) expect(r.rejection.code).toBe("no-fields-defined");
+    const r2 = validateCustomFieldPatch([], { x: 1 });
+    expect(r2.ok).toBe(false);
+    if (!r2.ok) expect(r2.rejection.code).toBe("no-fields-defined");
+  });
+  it("accepts an EMPTY patch even with no fields defined", () => {
+    expect(validateCustomFieldPatch(undefined, {}).ok).toBe(true);
+    expect(validateCustomFieldPatch([], {}).ok).toBe(true);
+  });
+  it("treats null/undefined patch values as delete sentinels (normalized to null)", () => {
+    const r = validateCustomFieldPatch(ALL_TYPES, { s: null, n: undefined });
+    expect(r.ok).toBe(true);
+    if (r.ok) expect(r.normalized).toEqual({ s: null, n: null });
+  });
+});
+
+// ── Defaults ──────────────────────────────────────────────────────────────
+
+describe("applyFieldDefaults", () => {
+  const fields: WorkflowFieldDefinition[] = [
+    F({ id: "req", type: "string", required: true, default: "x" }),
+    F({ id: "reqNoDefault", type: "string", required: true }),
+    F({ id: "optDefault", type: "number", default: 7 }),
+  ];
+  it("fills required field defaults absent from current", () => {
+    expect(applyFieldDefaults(fields, {})).toEqual({ req: "x" });
+  });
+  it("does not override an existing value", () => {
+    expect(applyFieldDefaults(fields, { req: "kept" })).toEqual({ req: "kept" });
+  });
+  it("ignores non-required defaults and required-without-default", () => {
+    const out = applyFieldDefaults(fields, {});
+    expect(out).not.toHaveProperty("optDefault");
+    expect(out).not.toHaveProperty("reqNoDefault");
+  });
+});
+
+// ── Reconciliation ──────────────────────────────────────────────────────────
+
+describe("reconcileFieldsOnWorkflowChange", () => {
+  it("keeps same-id type-compatible values, orphans removed ids", () => {
+    const oldF = [F({ id: "a", type: "string" }), F({ id: "gone", type: "number" })];
+    const newF = [F({ id: "a", type: "string" })];
+    const { kept, orphaned } = reconcileFieldsOnWorkflowChange(oldF, newF, { a: "v", gone: 1 });
+    expect(kept).toEqual({ a: "v" });
+    expect(orphaned).toEqual({ gone: 1 });
+  });
+
+  it("orphans a value when the new type is incompatible", () => {
+    const oldF = [F({ id: "a", type: "string" })];
+    const newF = [F({ id: "a", type: "number" })];
+    const { kept, orphaned } = reconcileFieldsOnWorkflowChange(oldF, newF, { a: "still-a-string" });
+    expect(kept).toEqual({});
+    expect(orphaned).toEqual({ a: "still-a-string" });
+  });
+
+  it("keeps an enum value still in the new options, orphans one no longer present", () => {
+    const oldF = [F({ id: "e", type: "enum", options: enumOpts })];
+    const newF = [F({ id: "e", type: "enum", options: [{ value: "high", label: "H" }] })];
+    expect(reconcileFieldsOnWorkflowChange(oldF, newF, { e: "high" }).kept).toEqual({ e: "high" });
+    expect(reconcileFieldsOnWorkflowChange(oldF, newF, { e: "low" }).orphaned).toEqual({ e: "low" });
+  });
+});
+
+// ── Store authority integration ──────────────────────────────────────────────
+
+describe("store: updateTaskCustomFields + updateTask integration (U11)", () => {
+  const harness = createTaskStoreTestHarness();
+  let store: ReturnType<typeof harness.store>;
+
+  const irWith = (fields: WorkflowFieldDefinition[], name = "wf"): WorkflowIr =>
+    ({
+      version: "v2",
+      name,
+      columns: [
+        { id: "todo", name: "todo", traits: [] },
+        { id: "in-progress", name: "in-progress", traits: [] },
+        { id: "done", name: "done", traits: [] },
+      ],
+      nodes: [
+        { id: "start", kind: "start", column: "todo" },
+        { id: "end", kind: "end", column: "todo" },
+      ],
+      edges: [{ from: "start", to: "end" }],
+      fields,
+    }) as unknown as WorkflowIr;
+
+  beforeEach(async () => {
+    await harness.beforeEach();
+    store = harness.store();
+  });
+  afterEach(async () => {
+    await harness.afterEach();
+  });
+
+  async function taskWithFields(fields: WorkflowFieldDefinition[]) {
+    const def = await (store as any).createWorkflowDefinition({ name: "WF", ir: irWith(fields) });
+    const t = await store.createTask({ description: "field task" });
+    await (store as any).selectTaskWorkflow(t.id, def.id);
+    return { task: t, workflowId: def.id as string };
+  }
+
+  it("happy path: validates, merges, persists, returns ok", async () => {
+    const { task } = await taskWithFields([
+      F({ id: "sev", type: "enum", options: enumOpts }),
+      F({ id: "pts", type: "number" }),
+    ]);
+    const r = await (store as any).updateTaskCustomFields(task.id, { sev: "high", pts: 5 });
+    expect(r.ok).toBe(true);
+    const got = await store.getTask(task.id);
+    expect(got?.customFields).toEqual({ sev: "high", pts: 5 });
+  });
+
+  it("reject path: returns a typed rejection, does not mutate", async () => {
+    const { task } = await taskWithFields([F({ id: "pts", type: "number" })]);
+    const r = await (store as any).updateTaskCustomFields(task.id, { pts: "not-a-number" });
+    expect(r.ok).toBe(false);
+    expect(r.rejection.code).toBe("type-mismatch");
+    expect(r.rejection.fieldId).toBe("pts");
+    const got = await store.getTask(task.id);
+    expect(got?.customFields).toEqual({});
+  });
+
+  it("unknown-field rejection on an undeclared key", async () => {
+    const { task } = await taskWithFields([F({ id: "pts", type: "number" })]);
+    const r = await (store as any).updateTaskCustomFields(task.id, { nope: 1 });
+    expect(r.ok).toBe(false);
+    expect(r.rejection.code).toBe("unknown-field");
+  });
+
+  it("default workflow (zero fields) rejects cleanly with no-fields-defined", async () => {
+    const t = await store.createTask({ description: "default wf" });
+    const r = await (store as any).updateTaskCustomFields(t.id, { anything: 1 });
+    expect(r.ok).toBe(false);
+    expect(r.rejection.code).toBe("no-fields-defined");
+  });
+
+  it("emits task:updated on a successful write", async () => {
+    const { task } = await taskWithFields([F({ id: "pts", type: "number" })]);
+    let emitted = 0;
+    (store as any).on("task:updated", () => {
+      emitted += 1;
+    });
+    const r = await (store as any).updateTaskCustomFields(task.id, { pts: 1 });
+    expect(r.ok).toBe(true);
+    expect(emitted).toBeGreaterThanOrEqual(1);
+  });
+
+  it("null patch value deletes the stored value", async () => {
+    const { task } = await taskWithFields([F({ id: "pts", type: "number" }), F({ id: "x", type: "number" })]);
+    await (store as any).updateTaskCustomFields(task.id, { pts: 1, x: 2 });
+    await (store as any).updateTaskCustomFields(task.id, { pts: null });
+    const got = await store.getTask(task.id);
+    expect(got?.customFields).toEqual({ x: 2 });
+  });
+
+  it("updateTask with an invalid customFields patch throws CustomFieldRejectionError", async () => {
+    const { task } = await taskWithFields([F({ id: "pts", type: "number" })]);
+    await expect(store.updateTask(task.id, { customFields: { pts: "bad" } })).rejects.toThrow(/pts/);
+  });
+
+  it("applies required+default fields at workflow selection", async () => {
+    const def = await (store as any).createWorkflowDefinition({
+      name: "Defaults",
+      ir: irWith([F({ id: "tier", type: "string", required: true, default: "bronze" })]),
+    });
+    const t = await store.createTask({ description: "defaults" });
+    await (store as any).selectTaskWorkflow(t.id, def.id);
+    const got = await store.getTask(t.id);
+    expect(got?.customFields).toEqual({ tier: "bronze" });
+  });
+});
+
+describe("store: workflow switch reconciliation (U11)", () => {
+  const harness = createTaskStoreTestHarness();
+  let store: ReturnType<typeof harness.store>;
+
+  const irWith = (fields: WorkflowFieldDefinition[], name: string): WorkflowIr =>
+    ({
+      version: "v2",
+      name,
+      columns: [
+        { id: "todo", name: "todo", traits: [] },
+        { id: "in-progress", name: "in-progress", traits: [] },
+        { id: "done", name: "done", traits: [] },
+      ],
+      nodes: [
+        { id: "start", kind: "start", column: "todo" },
+        { id: "end", kind: "end", column: "todo" },
+      ],
+      edges: [{ from: "start", to: "end" }],
+      fields,
+    }) as unknown as WorkflowIr;
+
+  beforeEach(async () => {
+    await harness.beforeEach();
+    store = harness.store();
+  });
+  afterEach(async () => {
+    await harness.afterEach();
+  });
+
+  it("keeps same-id compatible values and orphans the rest (orphan-not-delete)", async () => {
+    const wfA = await (store as any).createWorkflowDefinition({
+      name: "A",
+      ir: irWith([F({ id: "shared", type: "string" }), F({ id: "onlyA", type: "number" })], "A"),
+    });
+    const wfB = await (store as any).createWorkflowDefinition({
+      name: "B",
+      ir: irWith([F({ id: "shared", type: "string" }), F({ id: "onlyB", type: "boolean" })], "B"),
+    });
+    const t = await store.createTask({ description: "switch" });
+    await (store as any).selectTaskWorkflow(t.id, wfA.id);
+    await (store as any).updateTaskCustomFields(t.id, { shared: "v", onlyA: 3 });
+
+    await (store as any).selectTaskWorkflow(t.id, wfB.id);
+    const got = await store.getTask(t.id);
+    // shared kept; onlyA orphaned but RETAINED in storage (never destroyed).
+    expect(got?.customFields).toEqual({ shared: "v", onlyA: 3 });
+  });
+});
+
+describe("store: updateWorkflowDefinition field-type change coercion (U11)", () => {
+  const harness = createTaskStoreTestHarness();
+  let store: ReturnType<typeof harness.store>;
+
+  const irWith = (fields: WorkflowFieldDefinition[], name = "WF"): WorkflowIr =>
+    ({
+      version: "v2",
+      name,
+      columns: [
+        { id: "todo", name: "todo", traits: [] },
+        { id: "in-progress", name: "in-progress", traits: [] },
+        { id: "done", name: "done", traits: [] },
+      ],
+      nodes: [
+        { id: "start", kind: "start", column: "todo" },
+        { id: "end", kind: "end", column: "todo" },
+      ],
+      edges: [{ from: "start", to: "end" }],
+      fields,
+    }) as unknown as WorkflowIr;
+
+  beforeEach(async () => {
+    await harness.beforeEach();
+    store = harness.store();
+  });
+  afterEach(async () => {
+    await harness.afterEach();
+  });
+
+  async function fieldedTaskAndWf(fields: WorkflowFieldDefinition[]) {
+    const def = await (store as any).createWorkflowDefinition({ name: "WF", ir: irWith(fields) });
+    const t = await store.createTask({ description: "edit" });
+    await (store as any).selectTaskWorkflow(t.id, def.id);
+    return { workflowId: def.id as string, taskId: t.id as string };
+  }
+
+  it("rejects an incompatible type change with occupants and no coerce", async () => {
+    const { workflowId, taskId } = await fieldedTaskAndWf([F({ id: "x", type: "string" })]);
+    await (store as any).updateTaskCustomFields(taskId, { x: "hello" });
+    await expect(
+      store.updateWorkflowDefinition(workflowId, { ir: irWith([F({ id: "x", type: "number" })]) }),
+    ).rejects.toThrow(/IncompatibleFieldChange|incompatibl/i);
+  });
+
+  it("coerce:keep-orphaned retains the now-incompatible value", async () => {
+    const { workflowId, taskId } = await fieldedTaskAndWf([F({ id: "x", type: "string" })]);
+    await (store as any).updateTaskCustomFields(taskId, { x: "hello" });
+    await store.updateWorkflowDefinition(workflowId, {
+      ir: irWith([F({ id: "x", type: "number" })]),
+      coerce: "keep-orphaned",
+    });
+    const got = await store.getTask(taskId);
+    expect(got?.customFields).toEqual({ x: "hello" });
+  });
+
+  it("coerce:drop discards the now-incompatible value", async () => {
+    const { workflowId, taskId } = await fieldedTaskAndWf([F({ id: "x", type: "string" })]);
+    await (store as any).updateTaskCustomFields(taskId, { x: "hello" });
+    await store.updateWorkflowDefinition(workflowId, {
+      ir: irWith([F({ id: "x", type: "number" })]),
+      coerce: "drop",
+    });
+    const got = await store.getTask(taskId);
+    expect(got?.customFields).toEqual({});
+  });
+
+  it("removing a field outright orphans (never blocks, value retained)", async () => {
+    const { workflowId, taskId } = await fieldedTaskAndWf([
+      F({ id: "x", type: "string" }),
+      F({ id: "y", type: "string" }),
+    ]);
+    await (store as any).updateTaskCustomFields(taskId, { x: "a", y: "b" });
+    await store.updateWorkflowDefinition(workflowId, { ir: irWith([F({ id: "x", type: "string" })]) });
+    const got = await store.getTask(taskId);
+    // y orphaned but retained.
+    expect(got?.customFields).toEqual({ x: "a", y: "b" });
+  });
+});
+
+// ── JSON round-trip stability ────────────────────────────────────────────────
+
+describe("custom-field values JSON round-trip", () => {
+  it("normalized values survive a JSON round-trip unchanged", () => {
+    const r = validateCustomFieldPatch(ALL_TYPES, {
+      s: "x",
+      n: 1.5,
+      b: false,
+      e: "low",
+      m: ["high", "low"],
+      d: "2026-06-04",
+      u: "https://x.test/",
+    });
+    expect(r.ok).toBe(true);
+    if (r.ok) {
+      expect(JSON.parse(JSON.stringify(r.normalized))).toEqual(r.normalized);
+    }
+  });
+});
diff --git a/packages/core/src/__tests__/workflow-step-instances.test.ts b/packages/core/src/__tests__/workflow-step-instances.test.ts
index 1d301698b9..8968f58452 100644
--- a/packages/core/src/__tests__/workflow-step-instances.test.ts
+++ b/packages/core/src/__tests__/workflow-step-instances.test.ts
@@ -1,6 +1,7 @@
 import { describe, it, expect, beforeEach, afterEach } from "vitest";
 
 import type { WorkflowRunStepInstance } from "../types.js";
+import type { WorkflowIr } from "../workflow-ir-types.js";
 import { createTaskStoreTestHarness } from "./store-test-helpers.js";
 
 /**
@@ -174,28 +175,87 @@ describe("workflow_run_step_instances CRUD (U4, KTD-6)", () => {
   });
 });
 
-describe("tasks.customFields raw JSON round-trip (U4 groundwork for KTD-13)", () => {
+describe("tasks.customFields JSON round-trip under a fielded workflow (U11/KTD-13)", () => {
+  // U11 behavior change vs. U4: customFields is no longer an opaque whole-object
+  // round-trip — every write is now validated against the task's workflow field
+  // schema through the single store authority (task-fields.ts). The default
+  // workflow declares no fields, so the original U4 tests (which wrote arbitrary
+  // keys onto a default-workflow task) would now be rejected with
+  // `no-fields-defined`. They are reworked here to attach a workflow that
+  // declares the fields under test, and `updateTask` is now a MERGE-with-delete
+  // patch (not whole-object replacement). The zero-fields rejection path is
+  // covered in task-fields.test.ts.
   const harness = createTaskStoreTestHarness();
   let store: ReturnType<typeof harness.store>;
 
+  // A v2 workflow declaring the fields exercised below.
+  const fieldedIr = (): WorkflowIr =>
+    ({
+      version: "v2",
+      name: "fielded",
+      columns: [
+        { id: "todo", name: "todo", traits: [] },
+        { id: "in-progress", name: "in-progress", traits: [] },
+        { id: "done", name: "done", traits: [] },
+      ],
+      nodes: [
+        { id: "start", kind: "start", column: "todo" },
+        { id: "end", kind: "end", column: "todo" },
+      ],
+      edges: [{ from: "start", to: "end" }],
+      fields: [
+        {
+          id: "severity",
+          name: "Severity",
+          type: "enum",
+          options: [
+            { value: "high", label: "High" },
+            { value: "low", label: "Low" },
+          ],
+        },
+        { id: "points", name: "Points", type: "number" },
+        { id: "flagged", name: "Flagged", type: "boolean" },
+        {
+          id: "tags",
+          name: "Tags",
+          type: "multi-enum",
+          options: [
+            { value: "a", label: "A" },
+            { value: "b", label: "B" },
+          ],
+        },
+        { id: "keep", name: "Keep", type: "string" },
+        { id: "a", name: "A", type: "number" },
+        { id: "b", name: "B", type: "number" },
+      ],
+    }) as unknown as WorkflowIr;
+
+  let workflowId: string;
+
   beforeEach(async () => {
     await harness.beforeEach();
     store = harness.store();
+    const def = await (store as any).createWorkflowDefinition({ name: "Fielded", ir: fieldedIr() });
+    workflowId = def.id;
   });
   afterEach(async () => {
     await harness.afterEach();
   });
 
+  async function fieldedTask(description: string) {
+    const t = await store.createTask({ description });
+    await (store as any).selectTaskWorkflow(t.id, workflowId);
+    return t;
+  }
+
   it("a freshly created task has no customFields (legacy-shape default)", async () => {
     const t = await store.createTask({ description: "no fields" });
     const got = await store.getTask(t.id);
-    // Stored default is '{}' which parses to an empty object; the row→Task map
-    // surfaces that as an empty object, distinguishable from later writes.
     expect(got?.customFields).toEqual({});
   });
 
-  it("round-trips a customFields object through updateTask → getTask", async () => {
-    const t = await store.createTask({ description: "fielded" });
+  it("round-trips a validated customFields object through updateTask → getTask", async () => {
+    const t = await fieldedTask("fielded");
     await store.updateTask(t.id, {
       customFields: { severity: "high", points: 3, flagged: true, tags: ["a", "b"] },
     });
@@ -203,17 +263,25 @@ describe("tasks.customFields raw JSON round-trip (U4 groundwork for KTD-13)", ()
     expect(got?.customFields).toEqual({ severity: "high", points: 3, flagged: true, tags: ["a", "b"] });
   });
 
-  it("updateTask treats customFields as a whole-object opaque patch (replaces, not merges)", async () => {
-    const t = await store.createTask({ description: "replace" });
+  it("updateTask MERGES the customFields patch (U11 change from U4's whole-object replace)", async () => {
+    const t = await fieldedTask("merge");
     await store.updateTask(t.id, { customFields: { a: 1, b: 2 } });
     await store.updateTask(t.id, { customFields: { a: 9 } });
     const got = await store.getTask(t.id);
-    // Whole-object replacement: `b` is gone. (Merge/validation is a later unit.)
-    expect(got?.customFields).toEqual({ a: 9 });
+    // U11 merge semantics: `b` survives, `a` is overwritten. (U4 replaced wholesale.)
+    expect(got?.customFields).toEqual({ a: 9, b: 2 });
+  });
+
+  it("null in the patch deletes that field's value", async () => {
+    const t = await fieldedTask("delete");
+    await store.updateTask(t.id, { customFields: { a: 1, b: 2 } });
+    await store.updateTask(t.id, { customFields: { a: null } });
+    const got = await store.getTask(t.id);
+    expect(got?.customFields).toEqual({ b: 2 });
   });
 
   it("leaves customFields untouched when an unrelated field is updated", async () => {
-    const t = await store.createTask({ description: "untouched" });
+    const t = await fieldedTask("untouched");
     await store.updateTask(t.id, { customFields: { keep: "me" } });
     await store.updateTask(t.id, { summary: "an unrelated change" });
     const got = await store.getTask(t.id);
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 67272ebed3..da2bad2e74 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -151,9 +151,11 @@ export type { ColumnCapacity } from "./workflow-capacity.js";
 export {
   OccupiedColumnsError,
   InvalidRehomeTargetError,
+  IncompatibleFieldChangeError,
   resolveEntryColumnId,
   resolveSwitchReconciliation,
   computeRemovedOccupiedColumns,
+  computeIncompatibleFieldChanges,
   assertRehomeTargetValid,
   setReconciliationAbort,
   runReconciliationAbort,
@@ -162,9 +164,24 @@ export {
 export type {
   SwitchReconciliation,
   ColumnOccupancy,
+  IncompatibleFieldChange,
   ReconciliationAbort,
   ReconciliationAbortContext,
 } from "./workflow-reconciliation.js";
+export {
+  validateCustomFieldPatch,
+  applyFieldDefaults,
+  reconcileFieldsOnWorkflowChange,
+  makeCustomFieldRejection,
+  CustomFieldRejectionError,
+  CUSTOM_FIELD_REJECTION_CODES,
+} from "./task-fields.js";
+export type {
+  CustomFieldRejection,
+  CustomFieldRejectionCode,
+  CustomFieldPatchResult,
+  FieldReconciliation,
+} from "./task-fields.js";
 export {
   readTransitionPending,
   writeTransitionPending,
diff --git a/packages/core/src/store.ts b/packages/core/src/store.ts
index f4ddcf46e7..6716e8f0ed 100644
--- a/packages/core/src/store.ts
+++ b/packages/core/src/store.ts
@@ -21,6 +21,8 @@ import {
   OccupiedColumnsError,
   assertRehomeTargetValid,
   computeRemovedOccupiedColumns,
+  computeIncompatibleFieldChanges,
+  IncompatibleFieldChangeError,
   resolveEntryColumnId,
   resolveSwitchReconciliation,
   runReconciliationAbort,
@@ -43,7 +45,14 @@ import {
   reconcileHooksRemaining,
 } from "./transition-pending.js";
 import { BUILTIN_CODING_WORKFLOW_IR } from "./builtin-coding-workflow-ir.js";
-import type { WorkflowIr, WorkflowIrColumn } from "./workflow-ir-types.js";
+import type { WorkflowIr, WorkflowIrColumn, WorkflowFieldDefinition } from "./workflow-ir-types.js";
+import {
+  validateCustomFieldPatch,
+  applyFieldDefaults,
+  reconcileFieldsOnWorkflowChange,
+  CustomFieldRejectionError,
+  type CustomFieldRejection,
+} from "./task-fields.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";
@@ -6974,6 +6983,58 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
     return this.withTaskLock(id, () => this.updateTaskUnlocked(id, updates, runContext));
   }
 
+  /**
+   * Merge a validated/normalized custom-field patch into the existing values.
+   * `null` in the patch deletes that field's value (the delete sentinel from
+   * {@link validateCustomFieldPatch}); any other value overwrites. Returns a new
+   * object (never mutates the input) so the caller assigns it onto the task.
+   */
+  private mergeCustomFieldPatch(
+    current: Record<string, unknown> | undefined,
+    patch: Record<string, unknown>,
+  ): Record<string, unknown> {
+    const next: Record<string, unknown> = { ...(current ?? {}) };
+    for (const [key, value] of Object.entries(patch)) {
+      if (value === null) {
+        delete next[key];
+      } else {
+        next[key] = value;
+      }
+    }
+    return next;
+  }
+
+  /**
+   * Single write authority for custom task fields (U11 / KTD-13).
+   *
+   * Resolves the task's workflow field definitions, validates `patch` against
+   * them via {@link validateCustomFieldPatch}, merges the normalized result into
+   * `Task.customFields` (delete-on-null), persists through the standard update
+   * path, and emits `task:updated` like every other task mutation. A workflow
+   * with no fields (e.g. the default) rejects any non-empty patch with
+   * `no-fields-defined`. Returns a typed result rather than throwing so callers
+   * (agent tools, HTTP routes) can surface the field path/code directly.
+   */
+  async updateTaskCustomFields(
+    taskId: string,
+    patch: Record<string, unknown>,
+    runContext?: RunMutationContext,
+  ): Promise<{ ok: true; task: Task } | { ok: false; rejection: CustomFieldRejection }> {
+    return this.withTaskLock(taskId, async () => {
+      const defs = this.resolveTaskCustomFieldDefsSync(taskId);
+      const result = validateCustomFieldPatch(defs, patch);
+      if (!result.ok) {
+        return { ok: false as const, rejection: result.rejection };
+      }
+      // Pass the validated PATCH through (with null delete-sentinels) — the
+      // merge-with-delete happens once, inside updateTaskUnlocked, against the
+      // freshly-read task. Pre-merging here would lose the delete semantics on
+      // the second merge.
+      const task = await this.updateTaskUnlocked(taskId, { customFields: result.normalized }, runContext);
+      return { ok: true as const, task };
+    });
+  }
+
   /**
    * The body of {@link updateTask} WITHOUT acquiring the per-task lock. Callers
    * that already hold `withTaskLock(id)` — e.g. workflow-selection mutations
@@ -7082,10 +7143,19 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
         }
       }
       if (updates.steps !== undefined) task.steps = updates.steps;
-      // U4/KTD-13 groundwork: round-trip customFields as an opaque whole-object
-      // patch. The typed validation/write authority (updateTaskCustomFields)
-      // lands in a later unit; for now updateTask just persists what it is given.
-      if (updates.customFields !== undefined) task.customFields = updates.customFields;
+      // U11/KTD-13: customFields writes are validated against the task's workflow
+      // field schema through the single authority (task-fields.ts). The patch is
+      // merged into the existing values (delete-on-null), mirroring
+      // updateTaskCustomFields. Backward-compat note: U4 round-tripped the object
+      // opaquely; the field system now enforces type/enum/unknown-id rules, so a
+      // write against a workflow with no fields (the default) is rejected with a
+      // typed CustomFieldRejectionError rather than silently persisted.
+      if (updates.customFields !== undefined) {
+        const defs = this.resolveTaskCustomFieldDefsSync(id);
+        const result = validateCustomFieldPatch(defs, updates.customFields);
+        if (!result.ok) throw new CustomFieldRejectionError(result.rejection);
+        task.customFields = this.mergeCustomFieldPatch(task.customFields, result.normalized);
+      }
       if (updates.currentStep !== undefined) task.currentStep = updates.currentStep;
       if (updates.status === null) {
         task.status = undefined;
@@ -12293,6 +12363,58 @@ ${stepsSection}`;
         pendingRehome = { rehomeTo: updates.rehomeTo, occupantTaskIds };
       }
     }
+
+    // U11/KTD-13: when the IR changes custom field types incompatibly for tasks
+    // that already hold values, block with a typed IncompatibleFieldChangeError
+    // unless `coerce` is supplied. Removed/added fields never block (removal
+    // orphans). Flag-independent: fields are orthogonal to the columns flag.
+    // Reconciliation runs per occupant task AFTER the IR save commits.
+    let pendingFieldReconcile:
+      | { oldFields: WorkflowFieldDefinition[]; newFields: WorkflowFieldDefinition[]; occupantTaskIds: string[]; coerce?: "drop" | "keep-orphaned" }
+      | undefined;
+    if (updates.ir !== undefined) {
+      const existingForFields = await this.getWorkflowDefinition(id);
+      if (!existingForFields) throw new Error(`Workflow '${id}' not found`);
+      const nextIrForFields = parseWorkflowIr(updates.ir);
+      const oldFields: WorkflowFieldDefinition[] =
+        existingForFields.ir.version === "v2" ? (existingForFields.ir.fields ?? []) : [];
+      const newFields: WorkflowFieldDefinition[] =
+        nextIrForFields.version === "v2" ? (nextIrForFields.fields ?? []) : [];
+      const fieldsChanged =
+        JSON.stringify(oldFields) !== JSON.stringify(newFields);
+      if (fieldsChanged) {
+        const occupantTaskIds = this.listWorkflowOccupantTaskIds(id, false);
+        const occupantsByField = new Map<string, number>();
+        const occupantsWithFields: string[] = [];
+        for (const taskId of occupantTaskIds) {
+          const row = this.db.prepare("SELECT customFields FROM tasks WHERE id = ?").get(taskId) as
+            | { customFields: string | null }
+            | undefined;
+          const values = row?.customFields
+            ? (fromJson<Record<string, unknown>>(row.customFields) ?? {})
+            : {};
+          if (Object.keys(values).length === 0) continue;
+          occupantsWithFields.push(taskId);
+          for (const key of Object.keys(values)) {
+            occupantsByField.set(key, (occupantsByField.get(key) ?? 0) + 1);
+          }
+        }
+        const incompatible = computeIncompatibleFieldChanges(
+          existingForFields.ir,
+          nextIrForFields,
+          occupantsByField,
+        );
+        if (incompatible.length > 0 && updates.coerce === undefined) {
+          throw new IncompatibleFieldChangeError(id, incompatible);
+        }
+        pendingFieldReconcile = {
+          oldFields,
+          newFields,
+          occupantTaskIds: occupantsWithFields,
+          coerce: updates.coerce,
+        };
+      }
+    }
     const saved = await this.withConfigLock(async () => {
       const existing = await this.getWorkflowDefinition(id);
       if (!existing) throw new Error(`Workflow '${id}' not found`);
@@ -12341,6 +12463,23 @@ ${stepsSection}`;
         });
       }
     }
+
+    // U11/KTD-13: now that the new field schema is committed, reconcile each
+    // occupant task's stored values against it (orphan-not-delete by default;
+    // coerce:"drop" discards orphans). Each runs under its own task lock.
+    if (pendingFieldReconcile) {
+      const dropOrphans = pendingFieldReconcile.coerce === "drop";
+      for (const taskId of pendingFieldReconcile.occupantTaskIds) {
+        await this.withTaskLock(taskId, () =>
+          this.reconcileTaskCustomFieldsForSchema(
+            taskId,
+            pendingFieldReconcile!.oldFields,
+            pendingFieldReconcile!.newFields,
+            dropOrphans,
+          ),
+        );
+      }
+    }
     return saved;
   }
 
@@ -12898,6 +13037,16 @@ ${stepsSection}`;
     return list;
   }
 
+  /**
+   * Resolve the custom-field definitions (KTD-13) governing a task, via its
+   * workflow selection. v1 IR and the default workflow declare none → `[]`.
+   * Pure DB read, safe inside transactions.
+   */
+  private resolveTaskCustomFieldDefsSync(taskId: string): WorkflowFieldDefinition[] {
+    const ir = this.resolveTaskWorkflowIrSync(taskId);
+    return ir.version === "v2" ? (ir.fields ?? []) : [];
+  }
+
   private resolveTaskWorkflowIrSync(taskId: string): WorkflowIr {
     const selection = this.getTaskWorkflowSelection(taskId);
     const workflowId = selection?.workflowId;
@@ -13130,6 +13279,12 @@ ${stepsSection}`;
       // prior selection's rows, so a mid-flight failure never leaves the task
       // referencing already-deleted step ids.
       const priorSelection = this.getTaskWorkflowSelection(taskId);
+      // U11/KTD-13: capture the OLD field schema (from the prior selection's IR)
+      // before the selection row flips, so we can reconcile existing field values
+      // against the NEW workflow's schema below.
+      const oldFieldDefs = this.resolveTaskCustomFieldDefsSync(taskId);
+      const newFieldDefs: WorkflowFieldDefinition[] =
+        def.ir.version === "v2" ? (def.ir.fields ?? []) : [];
       const ids = await this.materializeWorkflowSteps(workflowId, inputs);
       try {
         await this.updateTaskUnlocked(taskId, { enabledWorkflowSteps: ids });
@@ -13155,10 +13310,56 @@ ${stepsSection}`;
         }
         this.workflowStepsCache = null;
       }
+
+      // U11/KTD-13: reconcile custom field values against the NEW workflow's
+      // schema. Same-id, type-compatible values are kept; incompatible/removed
+      // ids are orphaned — but RETAINED in storage (orphan-not-delete) so a later
+      // switch back, or the orphaned-fields disclosure, can still surface them.
+      // Then fill defaults for the new workflow's required+default fields that
+      // are absent. The merged object is written DIRECTLY (bypassing the
+      // validating patch path) because orphaned ids are by definition unknown to
+      // the new schema and would otherwise be rejected.
+      await this.reconcileTaskCustomFieldsForSchema(taskId, oldFieldDefs, newFieldDefs);
+
       return ids;
     });
   }
 
+  /**
+   * U11/KTD-13: reconcile a task's stored custom field values when its governing
+   * field schema changes (workflow switch or definition edit). Values are
+   * partitioned by {@link reconcileFieldsOnWorkflowChange}; orphans are retained
+   * (never destroyed). Required+default fields absent from the result are filled.
+   * Writes the merged values directly onto task.json — orphaned ids are unknown
+   * to the new schema, so this deliberately bypasses the validating patch path.
+   * Assumes the caller already holds the per-task lock.
+   */
+  private async reconcileTaskCustomFieldsForSchema(
+    taskId: string,
+    oldFieldDefs: WorkflowFieldDefinition[],
+    newFieldDefs: WorkflowFieldDefinition[],
+    dropOrphans = false,
+  ): Promise<void> {
+    const dir = this.taskDir(taskId);
+    const task = await this.readTaskJson(dir);
+    const current = task.customFields ?? {};
+    const { kept, orphaned } = reconcileFieldsOnWorkflowChange(oldFieldDefs, newFieldDefs, current);
+    // Default (keep-orphaned): storage keeps everything (kept ∪ orphaned).
+    // coerce:"drop" discards the orphaned values entirely.
+    const base = dropOrphans ? { ...kept } : { ...kept, ...orphaned };
+    const reconciled = applyFieldDefaults(newFieldDefs, base);
+    // Skip the write when nothing changed (no defaults added, same keys/values).
+    const unchanged =
+      Object.keys(reconciled).length === Object.keys(current).length &&
+      Object.entries(reconciled).every(([k, v]) => current[k] === v);
+    if (unchanged) return;
+    task.customFields = reconciled;
+    task.updatedAt = new Date().toISOString();
+    await this.atomicWriteTaskJson(dir, task);
+    if (this.isWatching) this.taskCache.set(taskId, { ...task });
+    this.emitTaskLifecycleEventSafely("task:updated", [task]);
+  }
+
   /**
    * U5 (R20) workflow switch: select a workflow for a task and, when the
    * `workflowColumns` flag is ON, reconcile the card's board column against the
diff --git a/packages/core/src/task-fields.ts b/packages/core/src/task-fields.ts
new file mode 100644
index 0000000000..ce4c0cd019
--- /dev/null
+++ b/packages/core/src/task-fields.ts
@@ -0,0 +1,362 @@
+/**
+ * Custom task field validation & reconciliation authority (U11 / KTD-13).
+ *
+ * Workflows declare typed custom task fields ({@link WorkflowFieldDefinition});
+ * task values live in `tasks.customFields` (a JSON object keyed by field id).
+ * This module is the single, side-effect-free validation core that the store
+ * write authority (`updateTaskCustomFields` / `updateTask`) delegates to. It
+ * mirrors the `TransitionRejection` style: a flat, JSON-safe typed rejection
+ * with a machine-stable `code`, the offending `fieldId`, and a non-localized
+ * `detail` string for audit/logs.
+ *
+ * Three operations:
+ *  - {@link validateCustomFieldPatch} — validate a `Record<string, unknown>`
+ *    patch against a field schema, normalizing accepted values. `null`/`undefined`
+ *    in the patch is a delete sentinel for that field (always accepted).
+ *  - {@link applyFieldDefaults} — fill `default` for required fields absent from
+ *    the current values (task create / workflow selection).
+ *  - {@link reconcileFieldsOnWorkflowChange} — partition existing values into
+ *    `kept` (same id, type-compatible) and `orphaned` (everything else) when a
+ *    workflow's fields change or the task switches workflows. Orphans are
+ *    RETAINED in storage — this only computes the partition so the UI can render
+ *    the orphaned-fields disclosure.
+ */
+
+import type {
+  WorkflowFieldDefinition,
+  WorkflowFieldType,
+} from "./workflow-ir-types.js";
+
+// ---------------------------------------------------------------------------
+// Typed rejection (TransitionRejection-style: flat, JSON-safe, no class)
+// ---------------------------------------------------------------------------
+
+/**
+ * Reason codes for a rejected custom-field write. Stable string literals — they
+ * cross the agent-tool / HTTP boundary and are matched by surfaces for copy, so
+ * they must not change without migrating consumers.
+ */
+export type CustomFieldRejectionCode =
+  | "no-fields-defined"
+  | "unknown-field"
+  | "type-mismatch"
+  | "enum-violation";
+
+/** The full, immutable set of custom-field rejection codes. */
+export const CUSTOM_FIELD_REJECTION_CODES: readonly CustomFieldRejectionCode[] = [
+  "no-fields-defined",
+  "unknown-field",
+  "type-mismatch",
+  "enum-violation",
+] as const;
+
+/**
+ * A typed custom-field rejection. Flat and JSON-safe by construction — mirrors
+ * {@link import("./transition-types.js").TransitionRejection}.
+ *
+ * - `code` — machine-stable {@link CustomFieldRejectionCode}.
+ * - `fieldId` — the offending field id (the patch key that failed).
+ * - `detail` — non-localized diagnostic context for audit/logs.
+ */
+export interface CustomFieldRejection {
+  code: CustomFieldRejectionCode;
+  fieldId: string;
+  detail: string;
+}
+
+/** Result of validating a custom-field patch. Discriminated on `ok`. */
+export type CustomFieldPatchResult =
+  | { ok: true; normalized: Record<string, unknown> }
+  | { ok: false; rejection: CustomFieldRejection };
+
+/** Construct a {@link CustomFieldRejection}. */
+export function makeCustomFieldRejection(
+  code: CustomFieldRejectionCode,
+  fieldId: string,
+  detail: string,
+): CustomFieldRejection {
+  return { code, fieldId, detail };
+}
+
+/**
+ * Thrown by the throw-based write paths (`updateTask` with a `customFields`
+ * patch) when validation rejects. `updateTaskCustomFields` returns the typed
+ * rejection instead; this wrapper exists for the legacy throw contract so a bad
+ * `updateTask` write fails loudly rather than silently round-tripping an invalid
+ * value (the U4 opaque behavior). Carries the structured rejection so HTTP/agent
+ * surfaces can recover the field path and code.
+ */
+export class CustomFieldRejectionError extends Error {
+  readonly rejection: CustomFieldRejection;
+  constructor(rejection: CustomFieldRejection) {
+    super(`custom field '${rejection.fieldId}' rejected (${rejection.code}): ${rejection.detail}`);
+    this.name = "CustomFieldRejectionError";
+    this.rejection = rejection;
+  }
+}
+
+// ---------------------------------------------------------------------------
+// Per-type value validation
+// ---------------------------------------------------------------------------
+
+/** True iff `value` is a non-empty option-value member of `field.options`. */
+function isEnumMember(field: WorkflowFieldDefinition, value: string): boolean {
+  return (field.options ?? []).some((o) => o.value === value);
+}
+
+/**
+ * Validate (and normalize) a single non-null value against a field's type.
+ * Returns the normalized value on success, or a rejection. The caller has
+ * already resolved the field definition.
+ */
+function validateValue(
+  field: WorkflowFieldDefinition,
+  value: unknown,
+): { ok: true; value: unknown } | { ok: false; rejection: CustomFieldRejection } {
+  const reject = (
+    code: CustomFieldRejectionCode,
+    detail: string,
+  ): { ok: false; rejection: CustomFieldRejection } => ({
+    ok: false,
+    rejection: makeCustomFieldRejection(code, field.id, detail),
+  });
+
+  switch (field.type) {
+    case "string":
+    case "text": {
+      if (typeof value !== "string") {
+        return reject("type-mismatch", `field '${field.id}' expects a string, got ${typeof value}`);
+      }
+      return { ok: true, value };
+    }
+    case "number": {
+      if (typeof value !== "number" || !Number.isFinite(value)) {
+        return reject(
+          "type-mismatch",
+          `field '${field.id}' expects a finite number, got ${typeof value === "number" ? String(value) : typeof value}`,
+        );
+      }
+      return { ok: true, value };
+    }
+    case "boolean": {
+      if (typeof value !== "boolean") {
+        return reject("type-mismatch", `field '${field.id}' expects a boolean, got ${typeof value}`);
+      }
+      return { ok: true, value };
+    }
+    case "enum": {
+      if (typeof value !== "string") {
+        return reject("type-mismatch", `field '${field.id}' (enum) expects a string option value, got ${typeof value}`);
+      }
+      if (!isEnumMember(field, value)) {
+        return reject("enum-violation", `field '${field.id}' value '${value}' is not a declared option`);
+      }
+      return { ok: true, value };
+    }
+    case "multi-enum": {
+      if (!Array.isArray(value)) {
+        return reject("type-mismatch", `field '${field.id}' (multi-enum) expects an array, got ${typeof value}`);
+      }
+      const seen = new Set<string>();
+      for (const item of value) {
+        if (typeof item !== "string") {
+          return reject("type-mismatch", `field '${field.id}' (multi-enum) members must be strings`);
+        }
+        if (!isEnumMember(field, item)) {
+          return reject("enum-violation", `field '${field.id}' member '${item}' is not a declared option`);
+        }
+        if (seen.has(item)) {
+          return reject("enum-violation", `field '${field.id}' has duplicate member '${item}'`);
+        }
+        seen.add(item);
+      }
+      return { ok: true, value: [...value] as string[] };
+    }
+    case "date": {
+      if (typeof value !== "string") {
+        return reject("type-mismatch", `field '${field.id}' (date) expects an ISO date string, got ${typeof value}`);
+      }
+      const ms = Date.parse(value);
+      if (Number.isNaN(ms)) {
+        return reject("type-mismatch", `field '${field.id}' value '${value}' is not a parseable date`);
+      }
+      return { ok: true, value };
+    }
+    case "url": {
+      if (typeof value !== "string") {
+        return reject("type-mismatch", `field '${field.id}' (url) expects a string, got ${typeof value}`);
+      }
+      try {
+        // eslint-disable-next-line no-new
+        new URL(value);
+      } catch {
+        return reject("type-mismatch", `field '${field.id}' value '${value}' is not a valid URL`);
+      }
+      return { ok: true, value };
+    }
+    default: {
+      // Exhaustiveness guard — an unknown type cannot validate.
+      const _exhaustive: never = field.type;
+      return reject("type-mismatch", `field '${field.id}' has unsupported type '${String(_exhaustive)}'`);
+    }
+  }
+}
+
+// ---------------------------------------------------------------------------
+// Patch validation authority
+// ---------------------------------------------------------------------------
+
+/**
+ * Validate a custom-field `patch` against a workflow's field `fields`.
+ *
+ * - A `null`/`undefined` patch value is a DELETE sentinel: the field's stored
+ *   value should be removed. It is always accepted (even for required fields —
+ *   required is not a write-time gate this round, KTD-13) and surfaces in
+ *   `normalized` as `null` so the caller can apply the delete uniformly.
+ * - A non-null value is validated/normalized per the field's type.
+ * - A patch key that names no declared field → `unknown-field`.
+ * - When `fields` is undefined/empty and the patch carries any key → the whole
+ *   patch is rejected `no-fields-defined` (the default workflow declares no
+ *   fields; nothing can be written). An empty patch against no fields is `ok`.
+ *
+ * Validation is fail-fast: the first offending key produces the rejection.
+ */
+export function validateCustomFieldPatch(
+  fields: WorkflowFieldDefinition[] | undefined,
+  patch: Record<string, unknown>,
+): CustomFieldPatchResult {
+  const keys = Object.keys(patch);
+  const byId = new Map<string, WorkflowFieldDefinition>((fields ?? []).map((f) => [f.id, f]));
+
+  if (byId.size === 0) {
+    if (keys.length === 0) return { ok: true, normalized: {} };
+    return {
+      ok: false,
+      rejection: makeCustomFieldRejection(
+        "no-fields-defined",
+        keys[0]!,
+        "the resolved workflow declares no custom fields; no values may be written",
+      ),
+    };
+  }
+
+  const normalized: Record<string, unknown> = {};
+  for (const key of keys) {
+    const value = patch[key];
+    const field = byId.get(key);
+    if (!field) {
+      return {
+        ok: false,
+        rejection: makeCustomFieldRejection(
+          "unknown-field",
+          key,
+          `field '${key}' is not declared by the task's workflow`,
+        ),
+      };
+    }
+    // null/undefined = delete this field's value.
+    if (value === null || value === undefined) {
+      normalized[key] = null;
+      continue;
+    }
+    const res = validateValue(field, value);
+    if (!res.ok) return res;
+    normalized[key] = res.value;
+  }
+  return { ok: true, normalized };
+}
+
+// ---------------------------------------------------------------------------
+// Defaults at create / workflow selection
+// ---------------------------------------------------------------------------
+
+/**
+ * Fill `default` values for REQUIRED fields that are absent from `current`.
+ * Returns a NEW merged object (does not mutate `current`); existing values win.
+ * Non-required fields and fields without a declared `default` are left absent.
+ *
+ * Used at task create / workflow selection so a workflow with required+default
+ * fields lands sensible initial values. Defaults are taken on trust from the
+ * (already-validated-at-save) field schema.
+ */
+export function applyFieldDefaults(
+  fields: WorkflowFieldDefinition[] | undefined,
+  current: Record<string, unknown> | undefined,
+): Record<string, unknown> {
+  const out: Record<string, unknown> = { ...(current ?? {}) };
+  for (const field of fields ?? []) {
+    if (!field.required) continue;
+    if (field.default === undefined) continue;
+    if (Object.prototype.hasOwnProperty.call(out, field.id) && out[field.id] !== undefined) {
+      continue;
+    }
+    out[field.id] = field.default;
+  }
+  return out;
+}
+
+// ---------------------------------------------------------------------------
+// Reconciliation on workflow edit / switch
+// ---------------------------------------------------------------------------
+
+/** Two field types are "enum-kind" siblings (enum / multi-enum). */
+function isEnumKind(type: WorkflowFieldType): boolean {
+  return type === "enum" || type === "multi-enum";
+}
+
+/**
+ * A stored value for `field` is type-compatible with a new field definition iff
+ * the new value re-validates cleanly. For enum-kind fields, compatibility also
+ * requires the value still be a member of the new options (handled by
+ * re-validation). This is the same gate {@link validateValue} applies on write,
+ * so "kept" values are guaranteed re-writable under the new schema.
+ */
+function valueCompatible(newField: WorkflowFieldDefinition, value: unknown): boolean {
+  if (value === null || value === undefined) return true;
+  return validateValue(newField, value).ok;
+}
+
+/** Partition of existing values produced by {@link reconcileFieldsOnWorkflowChange}. */
+export interface FieldReconciliation {
+  /** Values whose id survives in the new schema AND remain type-compatible. */
+  kept: Record<string, unknown>;
+  /**
+   * Values that no longer fit: id removed from the new schema, or the type
+   * changed incompatibly (including an enum value no longer in the new options).
+   * RETAINED in storage — listed here only so the UI can render them under the
+   * orphaned-fields disclosure.
+   */
+  orphaned: Record<string, unknown>;
+}
+
+/**
+ * Reconcile stored `values` when a workflow's field schema changes (edit) or a
+ * task switches workflows. Same-id values are KEPT when the new field is
+ * type-compatible (same type, or both enum-kind with the value still a member —
+ * enforced by re-validation); everything else is ORPHANED.
+ *
+ * Storage keeps EVERYTHING — this function only computes the partition. Callers
+ * persist `{...kept, ...orphaned}` (i.e. the original values, unchanged) and use
+ * `orphaned` purely for UI disclosure. `oldFields` is accepted for symmetry and
+ * future heuristics; the decision is driven entirely by `newFields` + the value.
+ */
+export function reconcileFieldsOnWorkflowChange(
+  oldFields: WorkflowFieldDefinition[] | undefined,
+  newFields: WorkflowFieldDefinition[] | undefined,
+  values: Record<string, unknown> | undefined,
+): FieldReconciliation {
+  void oldFields; // reserved for future migration heuristics; intentionally unused
+  const newById = new Map<string, WorkflowFieldDefinition>((newFields ?? []).map((f) => [f.id, f]));
+  const kept: Record<string, unknown> = {};
+  const orphaned: Record<string, unknown> = {};
+
+  for (const [id, value] of Object.entries(values ?? {})) {
+    const newField = newById.get(id);
+    if (newField && valueCompatible(newField, value)) {
+      kept[id] = value;
+    } else {
+      orphaned[id] = value;
+    }
+  }
+  return { kept, orphaned };
+}
diff --git a/packages/core/src/workflow-definition-types.ts b/packages/core/src/workflow-definition-types.ts
index 026f544684..60aee809e4 100644
--- a/packages/core/src/workflow-definition-types.ts
+++ b/packages/core/src/workflow-definition-types.ts
@@ -48,4 +48,14 @@ export interface WorkflowDefinitionUpdate {
    * the `workflowColumns` flag is ON.
    */
   rehomeTo?: string;
+  /**
+   * U11/KTD-13: when an IR update changes a custom field's type incompatibly for
+   * tasks that already hold a value under that field, the update is blocked with
+   * a typed {@link import("./workflow-reconciliation.js").IncompatibleFieldChangeError}
+   * unless `coerce` is supplied. `"drop"` discards the now-incompatible stored
+   * values; `"keep-orphaned"` retains them as orphans (rendered under the
+   * orphaned-fields disclosure). Removing a field outright always orphans (never
+   * blocks). Mirrors the `rehomeTo` conflict-resolution posture for columns.
+   */
+  coerce?: "drop" | "keep-orphaned";
 }
diff --git a/packages/core/src/workflow-reconciliation.ts b/packages/core/src/workflow-reconciliation.ts
index 382d3645fa..e417c79a9e 100644
--- a/packages/core/src/workflow-reconciliation.ts
+++ b/packages/core/src/workflow-reconciliation.ts
@@ -32,7 +32,12 @@
  * is independently testable and reused identically across switch/edit/delete.
  */
 
-import type { WorkflowIr, WorkflowIrV2, WorkflowIrColumn } from "./workflow-ir-types.js";
+import type {
+  WorkflowIr,
+  WorkflowIrV2,
+  WorkflowIrColumn,
+  WorkflowFieldDefinition,
+} from "./workflow-ir-types.js";
 import { resolveColumnFlags } from "./trait-registry.js";
 import { workflowHasColumn } from "./workflow-transitions.js";
 
@@ -181,6 +186,90 @@ export function assertRehomeTargetValid(nextIr: WorkflowIr, rehomeTo: string): v
   }
 }
 
+// ── Custom-field schema-evolution reconciliation (U11/KTD-13) ────────────────
+
+/** A field whose type changed incompatibly while tasks hold values under it. */
+export interface IncompatibleFieldChange {
+  fieldId: string;
+  fromType: string;
+  toType: string;
+  /** Number of tasks (under this workflow) currently holding a value for it. */
+  occupantCount: number;
+}
+
+/**
+ * Thrown by the workflow update path when an IR edit changes one or more custom
+ * fields' types incompatibly for tasks that already hold a value, and no
+ * `coerce` option was supplied. Mirrors {@link OccupiedColumnsError}: a typed,
+ * conflict-signaling error the surface maps to a 409 prompting for a coercion
+ * choice (`drop` | `keep-orphaned`).
+ */
+export class IncompatibleFieldChangeError extends Error {
+  readonly workflowId: string;
+  readonly changes: IncompatibleFieldChange[];
+  constructor(workflowId: string, changes: IncompatibleFieldChange[]) {
+    const summary = changes
+      .map((c) => `${c.fieldId} (${c.fromType}→${c.toType}, ${c.occupantCount})`)
+      .join(", ");
+    super(
+      `Workflow '${workflowId}' edit changes field type(s) incompatibly: ${summary}. ` +
+        `Supply coerce ("drop" | "keep-orphaned") to proceed.`,
+    );
+    this.name = "IncompatibleFieldChangeError";
+    this.workflowId = workflowId;
+    this.changes = changes;
+  }
+}
+
+/** The v2 fields of an IR, or `[]` when absent (v1 or undeclared). */
+function fieldsOf(ir: WorkflowIr): WorkflowFieldDefinition[] {
+  const v2 = ir as WorkflowIrV2;
+  return Array.isArray(v2.fields) ? v2.fields : [];
+}
+
+/** Enum-kind sibling check (enum / multi-enum). */
+function sameEnumKind(a: string, b: string): boolean {
+  const enumKind = (t: string) => t === "enum" || t === "multi-enum";
+  return enumKind(a) && enumKind(b);
+}
+
+/**
+ * Compute which custom fields change type INCOMPATIBLY between `existingIr` and
+ * `nextIr` AND still have occupant tasks holding a value. A type is compatible
+ * with itself; enum↔multi-enum is treated as compatible-shape (values are
+ * re-validated against the new options at reconcile time — a value dropped by
+ * the new options orphans individually, not via a hard block). A field removed
+ * outright is NOT a conflict (removal always orphans, never blocks). Returns one
+ * entry per blocking change in the existing IR's field order.
+ *
+ * `occupantsByField` maps a field id to the count of tasks (under this workflow)
+ * currently holding a value for it.
+ */
+export function computeIncompatibleFieldChanges(
+  existingIr: WorkflowIr,
+  nextIr: WorkflowIr,
+  occupantsByField: Map<string, number>,
+): IncompatibleFieldChange[] {
+  const nextById = new Map(fieldsOf(nextIr).map((f) => [f.id, f]));
+  const changes: IncompatibleFieldChange[] = [];
+  for (const oldField of fieldsOf(existingIr)) {
+    const next = nextById.get(oldField.id);
+    if (!next) continue; // removed → orphan, not a block
+    if (next.type === oldField.type) continue; // identical type → fine
+    if (sameEnumKind(oldField.type, next.type)) continue; // enum↔multi-enum → soft
+    const occupantCount = occupantsByField.get(oldField.id) ?? 0;
+    if (occupantCount > 0) {
+      changes.push({
+        fieldId: oldField.id,
+        fromType: oldField.type,
+        toType: next.type,
+        occupantCount,
+      });
+    }
+  }
+  return changes;
+}
+
 // ── Abort-on-switch DI seam (core stays engine-free) ─────────────────────────
 //
 // A workflow switch must abort the card's in-flight processing BEFORE the move