diff --git a/packages/core/src/__tests__/workflow-ir-settings.test.ts b/packages/core/src/__tests__/workflow-ir-settings.test.ts
new file mode 100644
index 0000000000..a3ce1c3692
--- /dev/null
+++ b/packages/core/src/__tests__/workflow-ir-settings.test.ts
@@ -0,0 +1,229 @@
+import { describe, expect, it } from "vitest";
+import {
+  parseWorkflowIr,
+  serializeWorkflowIr,
+  downgradeIrToV1IfPure,
+  WorkflowIrError,
+} from "../workflow-ir.js";
+import { BUILTIN_CODING_WORKFLOW_IR } from "../builtin-coding-workflow-ir.js";
+import { BUILTIN_WORKFLOW_SETTINGS } from "../builtin-workflow-settings.js";
+import { DEFAULT_PROJECT_SETTINGS } from "../types.js";
+import type {
+  WorkflowIrV2,
+  WorkflowIrNode,
+  WorkflowSettingDefinition,
+} from "../workflow-ir-types.js";
+
+const startEnd: WorkflowIrNode[] = [
+  { id: "start", kind: "start" },
+  { id: "end", kind: "end" },
+];
+
+function withSettings(settings: WorkflowSettingDefinition[]): WorkflowIrV2 {
+  return {
+    version: "v2",
+    name: "test",
+    columns: [],
+    nodes: startEnd,
+    edges: [{ from: "start", to: "end" }],
+    settings,
+  };
+}
+
+describe("parseWorkflowIr — workflow settings declarations (U1)", () => {
+  it("parses and round-trips a valid declaration of each type", () => {
+    const settings: WorkflowSettingDefinition[] = [
+      { id: "s-string", name: "S", type: "string", default: "x" },
+      { id: "s-text", name: "T", type: "text", default: "long" },
+      { id: "s-number", name: "N", type: "number", default: 42 },
+      { id: "s-boolean", name: "B", type: "boolean", default: true },
+      {
+        id: "s-enum",
+        name: "E",
+        type: "enum",
+        default: "a",
+        options: [
+          { value: "a", label: "A" },
+          { value: "b", label: "B" },
+        ],
+      },
+      {
+        id: "s-multi",
+        name: "M",
+        type: "multi-enum",
+        default: ["a"],
+        options: [
+          { value: "a", label: "A" },
+          { value: "b", label: "B" },
+        ],
+        render: { widget: "chips" },
+      },
+    ];
+    const parsed = parseWorkflowIr(withSettings(settings)) as WorkflowIrV2;
+    expect(parsed.settings).toEqual(settings);
+    const reparsed = parseWorkflowIr(serializeWorkflowIr(parsed));
+    expect(reparsed).toEqual(parsed);
+  });
+
+  it("allows a declaration with no default and a description", () => {
+    const parsed = parseWorkflowIr(
+      withSettings([
+        { id: "lane", name: "Lane", type: "string", description: "a model lane" },
+      ]),
+    ) as WorkflowIrV2;
+    expect(parsed.settings?.[0].default).toBeUndefined();
+  });
+
+  it("rejects duplicate setting ids", () => {
+    expect(() =>
+      parseWorkflowIr(
+        withSettings([
+          { id: "dup", name: "A", type: "string" },
+          { id: "dup", name: "B", type: "string" },
+        ]),
+      ),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects an empty id", () => {
+    expect(() =>
+      parseWorkflowIr(withSettings([{ id: "", name: "A", type: "string" }])),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects an unknown type", () => {
+    expect(() =>
+      parseWorkflowIr(
+        withSettings([{ id: "x", name: "A", type: "date" as never }]),
+      ),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects an enum without options", () => {
+    expect(() =>
+      parseWorkflowIr(withSettings([{ id: "x", name: "A", type: "enum" }])),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects options on a non-enum type", () => {
+    expect(() =>
+      parseWorkflowIr(
+        withSettings([
+          { id: "x", name: "A", type: "number", options: [{ value: "a", label: "A" }] },
+        ]),
+      ),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects duplicate option values", () => {
+    expect(() =>
+      parseWorkflowIr(
+        withSettings([
+          {
+            id: "x",
+            name: "A",
+            type: "enum",
+            options: [
+              { value: "a", label: "A" },
+              { value: "a", label: "A2" },
+            ],
+          },
+        ]),
+      ),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects a disallowed render widget", () => {
+    expect(() =>
+      parseWorkflowIr(
+        withSettings([
+          { id: "x", name: "A", type: "string", render: { widget: "slider" as never } },
+        ]),
+      ),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects a default violating its own type (number with string)", () => {
+    expect(() =>
+      parseWorkflowIr(
+        withSettings([{ id: "x", name: "A", type: "number", default: "x" }]),
+      ),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects a default violating boolean type", () => {
+    expect(() =>
+      parseWorkflowIr(
+        withSettings([{ id: "x", name: "A", type: "boolean", default: "true" }]),
+      ),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects an enum default not among options", () => {
+    expect(() =>
+      parseWorkflowIr(
+        withSettings([
+          {
+            id: "x",
+            name: "A",
+            type: "enum",
+            default: "c",
+            options: [
+              { value: "a", label: "A" },
+              { value: "b", label: "B" },
+            ],
+          },
+        ]),
+      ),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("rejects a multi-enum default containing an unknown option", () => {
+    expect(() =>
+      parseWorkflowIr(
+        withSettings([
+          {
+            id: "x",
+            name: "A",
+            type: "multi-enum",
+            default: ["a", "c"],
+            options: [
+              { value: "a", label: "A" },
+              { value: "b", label: "B" },
+            ],
+          },
+        ]),
+      ),
+    ).toThrow(WorkflowIrError);
+  });
+
+  it("does not downgrade an IR with settings present to v1", () => {
+    const parsed = parseWorkflowIr(
+      withSettings([{ id: "x", name: "A", type: "string", default: "v" }]),
+    );
+    const down = downgradeIrToV1IfPure(parsed);
+    expect(down.version).toBe("v2");
+  });
+});
+
+describe("built-in workflow settings parity anchor (U1, R4)", () => {
+  it("the built-in coding workflow declares the full moved-key catalog", () => {
+    const builtin = BUILTIN_CODING_WORKFLOW_IR as WorkflowIrV2;
+    const declaredIds = new Set((builtin.settings ?? []).map((s) => s.id));
+    for (const setting of BUILTIN_WORKFLOW_SETTINGS) {
+      expect(declaredIds.has(setting.id)).toBe(true);
+    }
+    expect(builtin.settings).toEqual(BUILTIN_WORKFLOW_SETTINGS);
+  });
+
+  it("each declaration default strictly equals the legacy DEFAULT_PROJECT_SETTINGS literal", () => {
+    const legacy = DEFAULT_PROJECT_SETTINGS as Record<string, unknown>;
+    for (const setting of BUILTIN_WORKFLOW_SETTINGS) {
+      // Catalog keys must exist as a known project-settings key.
+      expect(Object.prototype.hasOwnProperty.call(legacy, setting.id)).toBe(true);
+      // A declared default must byte-equal the legacy literal; an omitted
+      // default corresponds to a legacy `undefined` literal.
+      expect(setting.default).toStrictEqual(legacy[setting.id]);
+    }
+  });
+});
diff --git a/packages/core/src/builtin-coding-workflow-ir.ts b/packages/core/src/builtin-coding-workflow-ir.ts
index 381d78156e..3ae675ed8d 100644
--- a/packages/core/src/builtin-coding-workflow-ir.ts
+++ b/packages/core/src/builtin-coding-workflow-ir.ts
@@ -1,5 +1,6 @@
 import type { WorkflowIr } from "./workflow-ir-types.js";
 import { parseWorkflowIr } from "./workflow-ir.js";
+import { BUILTIN_WORKFLOW_SETTINGS } from "./builtin-workflow-settings.js";
 
 /**
  * The built-in default workflow as a v2 IR. Its six columns have ids that are
@@ -59,6 +60,9 @@ const RAW_BUILTIN_CODING_WORKFLOW_IR: WorkflowIr = {
     { from: "review", to: "end", condition: "failure" },
     { from: "merge", to: "end", condition: "failure" },
   ],
+  // Workflow-settings (U1, R4): declare the full moved-key catalog with defaults
+  // byte-equal to today's DEFAULT_PROJECT_SETTINGS literals. Inert until U3.
+  settings: BUILTIN_WORKFLOW_SETTINGS,
 };
 
 export const BUILTIN_CODING_WORKFLOW_IR = parseWorkflowIr(RAW_BUILTIN_CODING_WORKFLOW_IR);
diff --git a/packages/core/src/builtin-stepwise-coding-workflow-ir.ts b/packages/core/src/builtin-stepwise-coding-workflow-ir.ts
index 150e47f96e..179ae134f5 100644
--- a/packages/core/src/builtin-stepwise-coding-workflow-ir.ts
+++ b/packages/core/src/builtin-stepwise-coding-workflow-ir.ts
@@ -1,5 +1,6 @@
 import type { WorkflowIr } from "./workflow-ir-types.js";
 import { parseWorkflowIr } from "./workflow-ir.js";
+import { BUILTIN_WORKFLOW_SETTINGS } from "./builtin-workflow-settings.js";
 
 /**
  * The built-in **stepwise** coding workflow (KTD-9) — the demonstration of step
@@ -144,6 +145,8 @@ const RAW_BUILTIN_STEPWISE_CODING_WORKFLOW_IR: WorkflowIr = {
     { from: "merge", to: "end", condition: "success" },
     { from: "merge", to: "end", condition: "failure" },
   ],
+  // Workflow-settings (U1, R4): same moved-key catalog as the default builtin.
+  settings: BUILTIN_WORKFLOW_SETTINGS,
 };
 
 export const BUILTIN_STEPWISE_CODING_WORKFLOW_IR = parseWorkflowIr(
diff --git a/packages/core/src/builtin-workflow-settings.ts b/packages/core/src/builtin-workflow-settings.ts
new file mode 100644
index 0000000000..7f3cce9676
--- /dev/null
+++ b/packages/core/src/builtin-workflow-settings.ts
@@ -0,0 +1,266 @@
+import type { WorkflowSettingDefinition } from "./workflow-ir-types.js";
+
+/**
+ * The moved-key catalog declared as workflow settings (U1, R4).
+ *
+ * Single source of truth, imported by both built-in workflow IR files
+ * (`builtin-coding-workflow-ir.ts`, `builtin-stepwise-coding-workflow-ir.ts`) so
+ * the catalog has exactly one definition.
+ *
+ * Each `default` here MUST be byte-equal to the corresponding literal in
+ * `DEFAULT_PROJECT_SETTINGS` (`settings-schema.ts`) — this is the parity anchor
+ * for the U4 hard-move migration. The U1 test
+ * (`workflow-ir-settings.test.ts`) asserts strict equality against the legacy
+ * literals. Keys with `undefined` legacy defaults (the per-phase model lanes)
+ * omit `default` entirely, which round-trips to the same effective value.
+ *
+ * NOTE: these declarations are inert in U1 — nothing reads them until the
+ * effective-settings resolver and engine integration land (U3). Adding them does
+ * not change any built-in workflow's behavior.
+ *
+ * Keys deliberately NOT in this catalog (per KTD-4 / the catalog-shrink rule):
+ *   - `completionDocumentationMode` — read outside per-task scope (triage), stays
+ *     in project settings.
+ *   - merge-cluster keys + `maxConcurrent` — owned by the columns/traits track.
+ */
+export const BUILTIN_WORKFLOW_SETTINGS: WorkflowSettingDefinition[] = [
+  // ── Step execution ─────────────────────────────────────────────────────
+  {
+    id: "workflowStepTimeoutMs",
+    name: "Step timeout (ms)",
+    type: "number",
+    default: 360_000,
+    description: "Maximum time a single workflow step may run before it is timed out.",
+  },
+  {
+    id: "workflowStepScopeEnforcement",
+    name: "Step scope enforcement",
+    type: "enum",
+    default: "block",
+    options: [
+      { value: "block", label: "Block" },
+      { value: "warn", label: "Warn" },
+      { value: "off", label: "Off" },
+    ],
+    description: "How to handle a step that writes outside its declared file scope.",
+  },
+  {
+    id: "planOnlyScopeLeakEnforcement",
+    name: "Plan-only scope leak enforcement",
+    type: "enum",
+    default: "warn",
+    options: [
+      { value: "off", label: "Off" },
+      { value: "warn", label: "Warn" },
+      { value: "block", label: "Block" },
+    ],
+    description: "How to handle code changes during a plan-only step.",
+  },
+  {
+    id: "workflowRevisionForkOnScopeMismatch",
+    name: "Fork workflow revision on scope mismatch",
+    type: "boolean",
+    default: true,
+    description: "Fork a new workflow revision when a step's actual scope diverges from its plan.",
+  },
+  {
+    id: "strictScopeEnforcement",
+    name: "Strict scope enforcement",
+    type: "boolean",
+    default: false,
+    description: "Enforce declared step scope strictly, rejecting any out-of-scope change.",
+  },
+  {
+    id: "runStepsInNewSessions",
+    name: "Run steps in new sessions",
+    type: "boolean",
+    default: false,
+    description: "Run each workflow step in its own agent session instead of a shared one.",
+  },
+  {
+    id: "maxParallelSteps",
+    name: "Max parallel steps",
+    type: "number",
+    default: 2,
+    description: "Maximum number of steps to run in parallel when running steps in new sessions.",
+  },
+  {
+    id: "buildRetryCount",
+    name: "Build retry count",
+    type: "number",
+    default: 0,
+    description: "Number of times to retry a failing build before giving up.",
+  },
+  {
+    id: "buildTimeoutMs",
+    name: "Build timeout (ms)",
+    type: "number",
+    default: 300_000,
+    description: "Maximum time a build command may run before it is timed out.",
+  },
+  {
+    id: "verificationFixRetries",
+    name: "Verification fix retries",
+    type: "number",
+    default: 3,
+    description: "Number of automatic fix attempts after a failed verification.",
+  },
+  {
+    id: "maxPostReviewFixes",
+    name: "Max post-review fixes",
+    type: "number",
+    default: 1,
+    description: "Maximum number of automatic fix passes after review feedback.",
+  },
+
+  // ── Review / approval ──────────────────────────────────────────────────
+  {
+    id: "requirePrApproval",
+    name: "Require PR approval",
+    type: "boolean",
+    default: false,
+    description: "Require explicit approval before a pull request can be merged.",
+  },
+  {
+    id: "requirePlanApproval",
+    name: "Require plan approval",
+    type: "boolean",
+    default: false,
+    description: "Require explicit approval of the plan before execution begins.",
+  },
+  {
+    id: "reviewHandoffPolicy",
+    name: "Review handoff policy",
+    type: "enum",
+    default: "disabled",
+    options: [
+      { value: "disabled", label: "Disabled" },
+      { value: "comment-triggered", label: "Comment-triggered" },
+      { value: "always", label: "Always" },
+    ],
+    description: "When to hand off a task to a human reviewer.",
+  },
+  {
+    id: "maxReviewerContextRetries",
+    name: "Max reviewer context retries",
+    type: "number",
+    default: 2,
+    description: "Maximum reviewer retries due to insufficient context before falling back.",
+  },
+  {
+    id: "maxReviewerFallbackRetries",
+    name: "Max reviewer fallback retries",
+    type: "number",
+    default: 2,
+    description: "Maximum reviewer retries on the fallback model before failing.",
+  },
+  {
+    id: "reflectionEnabled",
+    name: "Reflection enabled",
+    type: "boolean",
+    default: false,
+    description: "Enable periodic reflection passes over completed work.",
+  },
+  {
+    id: "reflectionIntervalMs",
+    name: "Reflection interval (ms)",
+    type: "number",
+    default: 3_600_000,
+    description: "How often to run a reflection pass when reflection is enabled.",
+  },
+  {
+    id: "reflectionAfterTask",
+    name: "Reflect after each task",
+    type: "boolean",
+    default: true,
+    description: "Run a reflection pass after each task completes.",
+  },
+
+  // ── Per-phase model lanes ──────────────────────────────────────────────
+  // Legacy defaults are all `undefined`; `default` is omitted so resolution
+  // falls through to the global lane / project default (KTD-7).
+  {
+    id: "executionProvider",
+    name: "Execution provider",
+    type: "string",
+    description: "Provider for the execution phase. Empty falls through to the global lane.",
+  },
+  {
+    id: "executionModelId",
+    name: "Execution model",
+    type: "string",
+    description: "Model id for the execution phase. Empty falls through to the global lane.",
+  },
+  {
+    id: "planningProvider",
+    name: "Planning provider",
+    type: "string",
+    description: "Provider for the planning phase. Empty falls through to the global lane.",
+  },
+  {
+    id: "planningModelId",
+    name: "Planning model",
+    type: "string",
+    description: "Model id for the planning phase. Empty falls through to the global lane.",
+  },
+  {
+    id: "planningFallbackProvider",
+    name: "Planning fallback provider",
+    type: "string",
+    description: "Fallback provider for the planning phase.",
+  },
+  {
+    id: "planningFallbackModelId",
+    name: "Planning fallback model",
+    type: "string",
+    description: "Fallback model id for the planning phase.",
+  },
+  {
+    id: "validatorProvider",
+    name: "Validator provider",
+    type: "string",
+    description: "Provider for the validation phase. Empty falls through to the global lane.",
+  },
+  {
+    id: "validatorModelId",
+    name: "Validator model",
+    type: "string",
+    description: "Model id for the validation phase. Empty falls through to the global lane.",
+  },
+  {
+    id: "validatorFallbackProvider",
+    name: "Validator fallback provider",
+    type: "string",
+    description: "Fallback provider for the validation phase.",
+  },
+  {
+    id: "validatorFallbackModelId",
+    name: "Validator fallback model",
+    type: "string",
+    description: "Fallback model id for the validation phase.",
+  },
+  {
+    id: "titleSummarizerProvider",
+    name: "Title summarizer provider",
+    type: "string",
+    description: "Provider for summarizing task titles.",
+  },
+  {
+    id: "titleSummarizerModelId",
+    name: "Title summarizer model",
+    type: "string",
+    description: "Model id for summarizing task titles.",
+  },
+  {
+    id: "titleSummarizerFallbackProvider",
+    name: "Title summarizer fallback provider",
+    type: "string",
+    description: "Fallback provider for summarizing task titles.",
+  },
+  {
+    id: "titleSummarizerFallbackModelId",
+    name: "Title summarizer fallback model",
+    type: "string",
+    description: "Fallback model id for summarizing task titles.",
+  },
+];
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 999eb211cb..4d99fe1da2 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -50,6 +50,8 @@ export {
   serializeWorkflowIr,
   WorkflowIrError,
   DEFAULT_WORKFLOW_COLUMN_IDS,
+  WORKFLOW_SETTING_TYPES,
+  SETTING_RENDER_WIDGETS,
 } from "./workflow-ir.js";
 export type {
   WorkflowIr,
@@ -70,9 +72,15 @@ export type {
   WorkflowFieldType,
   WorkflowFieldOption,
   WorkflowFieldRender,
+  // Workflow-settings (U1): typed setting declaration IR types.
+  WorkflowSettingDefinition,
+  WorkflowSettingType,
+  WorkflowSettingOption,
+  WorkflowSettingRender,
 } from "./workflow-ir-types.js";
 export { BUILTIN_CODING_WORKFLOW_IR } from "./builtin-coding-workflow-ir.js";
 export { BUILTIN_STEPWISE_CODING_WORKFLOW_IR } from "./builtin-stepwise-coding-workflow-ir.js";
+export { BUILTIN_WORKFLOW_SETTINGS } from "./builtin-workflow-settings.js";
 
 // ── Trait model (U2) ─────────────────────────────────────────────────
 export type {
diff --git a/packages/core/src/workflow-ir-types.ts b/packages/core/src/workflow-ir-types.ts
index b96e538fb5..91b78e0743 100644
--- a/packages/core/src/workflow-ir-types.ts
+++ b/packages/core/src/workflow-ir-types.ts
@@ -97,6 +97,48 @@ export interface WorkflowFieldDefinition {
   render?: WorkflowFieldRender;
 }
 
+/** Workflow-settings (U1): the supported setting value types. A whitelist
+ *  mirroring the scalar/enum subset of `WorkflowFieldType` — settings carry
+ *  workflow-scoped policy (step timeouts, review gates, model lanes), so the
+ *  date/url field types do not apply. */
+export type WorkflowSettingType =
+  | "string"
+  | "text"
+  | "number"
+  | "boolean"
+  | "enum"
+  | "multi-enum";
+
+/** A single enum/multi-enum option for a workflow setting (mirrors
+ *  `WorkflowFieldOption`). */
+export interface WorkflowSettingOption {
+  value: string;
+  label: string;
+  color?: string;
+}
+
+/** Rendering instructions for a workflow setting (U1, KTD-1). Settings get their
+ *  OWN render-hint type: a widget only — NO `card`/`detail` placement, which is
+ *  task-card-specific. The widget whitelist mirrors the field render widgets. */
+export interface WorkflowSettingRender {
+  widget?: "select" | "radio" | "chips" | "input" | "textarea" | "toggle";
+}
+
+/** Workflow-settings (U1, R1, KTD-1): a workflow-declared typed setting. Clones
+ *  the shape of `WorkflowFieldDefinition` (one level up) — declarations describe
+ *  the schema; the per-`(workflowId, projectId)` value table (U2) carries data.
+ *  `default` is consumed by the engine's effective-settings resolver (U3), so it
+ *  is validated against its own type/options at parse time. */
+export interface WorkflowSettingDefinition {
+  id: string;
+  name: string;
+  type: WorkflowSettingType;
+  default?: unknown;
+  options?: WorkflowSettingOption[];
+  description?: string;
+  render?: WorkflowSettingRender;
+}
+
 /** A single trait configuration applied to a column. The `trait` is an opaque
  *  registry id (resolved by the trait registry shipped in U2); `config` carries
  *  trait-specific options validated by that trait's schema. */
@@ -145,6 +187,9 @@ export interface WorkflowIrV2 {
   edges: WorkflowIrEdge[];
   artifacts?: WorkflowIrArtifact[];
   fields?: WorkflowFieldDefinition[];
+  /** Workflow-settings (U1, R1): typed setting declarations. Additive; absent on
+   *  legacy graphs. Values persist per-`(workflowId, projectId)` (U2), not here. */
+  settings?: WorkflowSettingDefinition[];
 }
 
 /** Either IR version. v1 graphs upgrade to v2 on parse (see parseWorkflowIr). */
diff --git a/packages/core/src/workflow-ir.ts b/packages/core/src/workflow-ir.ts
index 6a3cc5eff8..265f22c3e7 100644
--- a/packages/core/src/workflow-ir.ts
+++ b/packages/core/src/workflow-ir.ts
@@ -10,6 +10,8 @@ import type {
   WorkflowForeachConfig,
   WorkflowFieldDefinition,
   WorkflowFieldType,
+  WorkflowSettingDefinition,
+  WorkflowSettingType,
 } from "./workflow-ir-types.js";
 
 export class WorkflowIrError extends Error {
@@ -64,6 +66,27 @@ const FIELD_RENDER_WIDGETS: ReadonlySet<string> = new Set([
   "toggle",
 ]);
 
+/** Workflow-settings (U1) value-type whitelist (mirrors WORKFLOW_FIELD_TYPES). */
+export const WORKFLOW_SETTING_TYPES: ReadonlySet<WorkflowSettingType> = new Set([
+  "string",
+  "text",
+  "number",
+  "boolean",
+  "enum",
+  "multi-enum",
+]);
+
+/** Workflow-settings render-widget whitelist (mirrors FIELD_RENDER_WIDGETS;
+ *  no placement — settings have no card/detail placement). */
+export const SETTING_RENDER_WIDGETS: ReadonlySet<string> = new Set([
+  "select",
+  "radio",
+  "chips",
+  "input",
+  "textarea",
+  "toggle",
+]);
+
 /** Hard cap on a foreach `maxReworkCycles` (KTD-5: default 3, clamp >10 to 10,
  *  reject <1). */
 const MAX_REWORK_CYCLES_CAP = 10;
@@ -726,6 +749,139 @@ function validateFields(fields: WorkflowFieldDefinition[] | undefined): void {
   }
 }
 
+/** Validate that a setting's `default` conforms to its own type/options (U1).
+ *  Unlike `validateFields`, settings validate defaults because the engine's
+ *  effective-settings resolver (U3) consumes the default directly — a malformed
+ *  default would feed garbage into execution. */
+function validateSettingDefault(setting: WorkflowSettingDefinition): void {
+  const value = setting.default;
+  if (value === undefined) return;
+  const id = setting.id;
+  switch (setting.type) {
+    case "string":
+    case "text":
+      if (typeof value !== "string") {
+        throw new WorkflowIrError(
+          `Workflow setting '${id}' default must be a string for type '${setting.type}'`,
+        );
+      }
+      break;
+    case "number":
+      if (typeof value !== "number" || !Number.isFinite(value)) {
+        throw new WorkflowIrError(
+          `Workflow setting '${id}' default must be a finite number`,
+        );
+      }
+      break;
+    case "boolean":
+      if (typeof value !== "boolean") {
+        throw new WorkflowIrError(`Workflow setting '${id}' default must be a boolean`);
+      }
+      break;
+    case "enum": {
+      const allowed = new Set((setting.options ?? []).map((o) => o.value));
+      if (typeof value !== "string" || !allowed.has(value)) {
+        throw new WorkflowIrError(
+          `Workflow setting '${id}' default '${String(value)}' is not one of its enum options`,
+        );
+      }
+      break;
+    }
+    case "multi-enum": {
+      const allowed = new Set((setting.options ?? []).map((o) => o.value));
+      if (!Array.isArray(value)) {
+        throw new WorkflowIrError(
+          `Workflow setting '${id}' default must be an array for type 'multi-enum'`,
+        );
+      }
+      for (const entry of value) {
+        if (typeof entry !== "string" || !allowed.has(entry)) {
+          throw new WorkflowIrError(
+            `Workflow setting '${id}' default '${String(entry)}' is not one of its enum options`,
+          );
+        }
+      }
+      break;
+    }
+  }
+}
+
+/** Validate `settings` declarations (U1, R1). Mirrors `validateFields`: non-empty
+ *  unique ids, type whitelist, options iff enum-kind, unique option values, render
+ *  widget whitelist — plus default validation (settings need it; see
+ *  `validateSettingDefault`). */
+function validateSettings(settings: WorkflowSettingDefinition[] | undefined): void {
+  if (settings === undefined) return;
+  if (!Array.isArray(settings)) {
+    throw new WorkflowIrError("Workflow IR settings must be an array");
+  }
+  const seen = new Set<string>();
+  for (const setting of settings) {
+    if (!setting || typeof setting.id !== "string" || setting.id === "") {
+      throw new WorkflowIrError("Workflow setting must have a non-empty id");
+    }
+    if (seen.has(setting.id)) {
+      throw new WorkflowIrError(`Workflow IR has duplicate setting id '${setting.id}'`);
+    }
+    seen.add(setting.id);
+    if (typeof setting.name !== "string" || setting.name === "") {
+      throw new WorkflowIrError(
+        `Workflow setting '${setting.id}' must have a non-empty name`,
+      );
+    }
+    if (!WORKFLOW_SETTING_TYPES.has(setting.type)) {
+      throw new WorkflowIrError(
+        `Workflow setting '${setting.id}' has unknown type '${String(setting.type)}'`,
+      );
+    }
+    const isEnum = setting.type === "enum" || setting.type === "multi-enum";
+    if (isEnum) {
+      if (!Array.isArray(setting.options) || setting.options.length === 0) {
+        throw new WorkflowIrError(
+          `Workflow setting '${setting.id}' of type '${setting.type}' must declare non-empty options`,
+        );
+      }
+      const optSeen = new Set<string>();
+      for (const opt of setting.options) {
+        if (!opt || typeof opt.value !== "string" || opt.value === "") {
+          throw new WorkflowIrError(
+            `Workflow setting '${setting.id}' option must have a non-empty value`,
+          );
+        }
+        if (typeof opt.label !== "string" || opt.label === "") {
+          throw new WorkflowIrError(
+            `Workflow setting '${setting.id}' option '${opt.value}' must have a non-empty label`,
+          );
+        }
+        if (optSeen.has(opt.value)) {
+          throw new WorkflowIrError(
+            `Workflow setting '${setting.id}' has duplicate option value '${opt.value}'`,
+          );
+        }
+        optSeen.add(opt.value);
+      }
+    } else if (setting.options !== undefined) {
+      throw new WorkflowIrError(
+        `Workflow setting '${setting.id}' of type '${setting.type}' must not declare options`,
+      );
+    }
+    if (setting.description !== undefined && typeof setting.description !== "string") {
+      throw new WorkflowIrError(
+        `Workflow setting '${setting.id}' description must be a string`,
+      );
+    }
+    if (setting.render !== undefined) {
+      const r = setting.render;
+      if (r.widget !== undefined && !SETTING_RENDER_WIDGETS.has(r.widget)) {
+        throw new WorkflowIrError(
+          `Workflow setting '${setting.id}' render.widget '${String(r.widget)}' is not allowed`,
+        );
+      }
+    }
+    validateSettingDefault(setting);
+  }
+}
+
 function validateColumns(ir: WorkflowIrV2): void {
   if (!Array.isArray(ir.columns)) {
     throw new WorkflowIrError("Workflow IR v2 columns must be an array");
@@ -781,6 +937,7 @@ function validateV2(ir: WorkflowIrV2): void {
   validateParseStepsNodes(ir);
   validateCodeNodes(ir.nodes);
   validateFields(ir.fields);
+  validateSettings(ir.settings);
 
   // Rework edges are legal only intra-template; any rework edge at the top level
   // is rejected (template rework edges are validated inside validateForeach and
@@ -869,8 +1026,13 @@ export function downgradeIrToV1IfPure(ir: WorkflowIr): WorkflowIr {
     if (!V1_NODE_KINDS.has(node.kind)) return ir;
   }
 
-  // Step-inversion declarations (artifacts/fields) are v2-only features.
-  if ((ir.artifacts && ir.artifacts.length > 0) || (ir.fields && ir.fields.length > 0)) {
+  // Step-inversion declarations (artifacts/fields) and workflow settings (U1)
+  // are v2-only features.
+  if (
+    (ir.artifacts && ir.artifacts.length > 0) ||
+    (ir.fields && ir.fields.length > 0) ||
+    (ir.settings && ir.settings.length > 0)
+  ) {
     return ir;
   }