diff --git a/packages/core/src/__tests__/workflow-compiler.test.ts b/packages/core/src/__tests__/workflow-compiler.test.ts
new file mode 100644
index 0000000000..83fef4360f
--- /dev/null
+++ b/packages/core/src/__tests__/workflow-compiler.test.ts
@@ -0,0 +1,162 @@
+import { describe, it, expect } from "vitest";
+
+import {
+  compileWorkflowToSteps,
+  validateLinearity,
+  WorkflowCompileError,
+} from "../workflow-compiler.js";
+import { serializeWorkflowIr, parseWorkflowIr } from "../workflow-ir.js";
+import type { WorkflowIr } from "../workflow-ir-types.js";
+
+/** Linear graph: start → (user nodes) → execute → review → merge → (user nodes) → end. */
+function graph(
+  preMerge: WorkflowIr["nodes"],
+  postMerge: WorkflowIr["nodes"] = [],
+  { withSeams = true }: { withSeams?: boolean } = {},
+): WorkflowIr {
+  const seamNodes: WorkflowIr["nodes"] = withSeams
+    ? [
+        { id: "execute", kind: "prompt", config: { seam: "execute" } },
+        { id: "review", kind: "prompt", config: { seam: "review" } },
+        { id: "merge", kind: "prompt", config: { seam: "merge" } },
+      ]
+    : [];
+
+  const ordered = [
+    { id: "start", kind: "start" as const },
+    ...preMerge,
+    ...seamNodes,
+    ...postMerge,
+    { id: "end", kind: "end" as const },
+  ];
+
+  const edges: WorkflowIr["edges"] = [];
+  for (let i = 0; i < ordered.length - 1; i += 1) {
+    edges.push({ from: ordered[i].id, to: ordered[i + 1].id, condition: "success" });
+  }
+  // Canonical seam failure edges to end.
+  if (withSeams) {
+    for (const seam of ["execute", "review", "merge"]) {
+      edges.push({ from: seam, to: "end", condition: "failure" });
+    }
+  }
+
+  return { version: "v1", name: "test", nodes: ordered, edges };
+}
+
+describe("compileWorkflowToSteps (U2)", () => {
+  it("compiles a linear pre-merge gate + prompt in authored order", () => {
+    const ir = graph([
+      { id: "lint", kind: "gate", config: { name: "Lint", scriptName: "lint" } },
+      { id: "spec", kind: "prompt", config: { name: "Spec check", prompt: "Check the spec" } },
+    ]);
+    const steps = compileWorkflowToSteps(ir);
+    expect(steps).toHaveLength(2);
+    expect(steps[0].name).toBe("Lint");
+    expect(steps[0].phase).toBe("pre-merge");
+    expect(steps[0].mode).toBe("script");
+    expect(steps[0].gateMode).toBe("gate");
+    expect(steps[1].name).toBe("Spec check");
+    expect(steps[1].mode).toBe("prompt");
+    expect(steps[1].gateMode).toBe("advisory");
+  });
+
+  it("partitions nodes after the merge seam into post-merge", () => {
+    const ir = graph(
+      [{ id: "pre", kind: "prompt", config: { prompt: "before" } }],
+      [{ id: "post", kind: "script", config: { scriptName: "notify" } }],
+    );
+    const steps = compileWorkflowToSteps(ir);
+    expect(steps.map((s) => s.phase)).toEqual(["pre-merge", "post-merge"]);
+    expect(steps[1].mode).toBe("script");
+    expect(steps[1].scriptName).toBe("notify");
+  });
+
+  it("does not emit the execute/review/merge seams as steps", () => {
+    const ir = graph([{ id: "only", kind: "prompt", config: { prompt: "x" } }]);
+    const steps = compileWorkflowToSteps(ir);
+    expect(steps).toHaveLength(1);
+    expect(steps.every((s) => s.name !== "execute" && s.name !== "review" && s.name !== "merge")).toBe(true);
+  });
+
+  it("treats a gate node as gateMode=gate regardless of mode", () => {
+    const ir = graph([{ id: "g", kind: "gate", config: { prompt: "block?" } }]);
+    const steps = compileWorkflowToSteps(ir);
+    expect(steps[0].gateMode).toBe("gate");
+    expect(steps[0].mode).toBe("prompt");
+  });
+
+  it("carries prompt-node model overrides into the step", () => {
+    const ir = graph([
+      {
+        id: "p",
+        kind: "prompt",
+        config: { prompt: "x", modelProvider: "anthropic", modelId: "claude-sonnet-4-5" },
+      },
+    ]);
+    const [step] = compileWorkflowToSteps(ir);
+    expect(step.modelProvider).toBe("anthropic");
+    expect(step.modelId).toBe("claude-sonnet-4-5");
+  });
+
+  it("rejects a graph with branching (fan-out beyond success/failure)", () => {
+    const ir: WorkflowIr = {
+      version: "v1",
+      name: "branchy",
+      nodes: [
+        { id: "start", kind: "start" },
+        { id: "a", kind: "prompt", config: { prompt: "a" } },
+        { id: "b", kind: "prompt", config: { prompt: "b" } },
+        { id: "end", kind: "end" },
+      ],
+      edges: [
+        { from: "start", to: "a", condition: "success" },
+        { from: "a", to: "b", condition: "success" },
+        { from: "a", to: "end", condition: "success" }, // illegal second success branch
+        { from: "b", to: "end", condition: "success" },
+      ],
+    };
+    expect(() => compileWorkflowToSteps(ir)).toThrow(WorkflowCompileError);
+    expect(() => compileWorkflowToSteps(ir)).toThrow(/interpreter \(deferred\)/i);
+  });
+
+  it("rejects a graph missing the start/end nodes via parse", () => {
+    const ir = { version: "v1", name: "x", nodes: [{ id: "p", kind: "prompt" }], edges: [] } as WorkflowIr;
+    expect(() => compileWorkflowToSteps(ir)).toThrow();
+  });
+
+  it("rejects a disconnected node not on the main path", () => {
+    const ir: WorkflowIr = {
+      version: "v1",
+      name: "orphan",
+      nodes: [
+        { id: "start", kind: "start" },
+        { id: "a", kind: "prompt", config: { prompt: "a" } },
+        { id: "orphan", kind: "prompt", config: { prompt: "o" } },
+        { id: "end", kind: "end" },
+      ],
+      edges: [
+        { from: "start", to: "a", condition: "success" },
+        { from: "a", to: "end", condition: "success" },
+        { from: "orphan", to: "end", condition: "success" },
+      ],
+    };
+    const err = validateLinearity(ir);
+    expect(err).toBeInstanceOf(WorkflowCompileError);
+  });
+
+  it("returns an empty step set for a graph with only start/seams/end", () => {
+    const ir = graph([]);
+    expect(compileWorkflowToSteps(ir)).toEqual([]);
+  });
+
+  it("is deterministic across a serialize/parse round-trip", () => {
+    const ir = graph([
+      { id: "lint", kind: "gate", config: { name: "Lint", scriptName: "lint" } },
+      { id: "spec", kind: "prompt", config: { name: "Spec", prompt: "x" } },
+    ]);
+    const first = compileWorkflowToSteps(ir);
+    const second = compileWorkflowToSteps(parseWorkflowIr(serializeWorkflowIr(ir)));
+    expect(second).toEqual(first);
+  });
+});
diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index ccc1ee812e..d94b28b0fb 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -60,6 +60,11 @@ export type {
   WorkflowDefinitionUpdate,
   WorkflowNodeLayout,
 } from "./workflow-definition-types.js";
+export {
+  compileWorkflowToSteps,
+  validateLinearity,
+  WorkflowCompileError,
+} from "./workflow-compiler.js";
 
 // ── Engine wiring (set by @fusion/engine at module load) ────────────
 export {
diff --git a/packages/core/src/workflow-compiler.ts b/packages/core/src/workflow-compiler.ts
new file mode 100644
index 0000000000..6df0f4fd4e
--- /dev/null
+++ b/packages/core/src/workflow-compiler.ts
@@ -0,0 +1,201 @@
+import type { WorkflowIr, WorkflowIrNode, WorkflowIrEdge } from "./workflow-ir-types.js";
+import { parseWorkflowIr } from "./workflow-ir.js";
+import type { WorkflowStepInput, WorkflowStepGateMode } from "./types.js";
+
+/**
+ * Raised when a WorkflowIr graph cannot be compiled onto the executable
+ * WorkflowStep engine — typically because it branches beyond the canonical
+ * seam success/failure chain and therefore requires the (deferred) graph
+ * interpreter rather than the linear pre/post-merge step runner.
+ */
+export class WorkflowCompileError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "WorkflowCompileError";
+  }
+}
+
+/** Seam anchor kinds, encoded on IR nodes as `config.seam`. These map to the
+ *  fixed execute → review → merge pipeline and are not emitted as steps. */
+const SEAM_NAMES = new Set(["execute", "review", "merge"]);
+
+function seamOf(node: WorkflowIrNode): string | undefined {
+  const seam = node.config?.seam;
+  return typeof seam === "string" && SEAM_NAMES.has(seam) ? seam : undefined;
+}
+
+function configString(node: WorkflowIrNode, key: string): string | undefined {
+  const value = node.config?.[key];
+  return typeof value === "string" && value.trim() ? value : undefined;
+}
+
+function buildOutgoing(ir: WorkflowIr): Map<string, WorkflowIrEdge[]> {
+  const outgoing = new Map<string, WorkflowIrEdge[]>();
+  for (const edge of ir.edges) {
+    const list = outgoing.get(edge.from);
+    if (list) list.push(edge);
+    else outgoing.set(edge.from, [edge]);
+  }
+  return outgoing;
+}
+
+function mainEdge(edges: WorkflowIrEdge[]): WorkflowIrEdge | undefined {
+  return edges.find((edge) => edge.condition !== "failure");
+}
+
+/**
+ * Validate that a workflow graph reduces to a linear pre-merge → seams →
+ * post-merge chain the WorkflowStep engine can run. Returns a
+ * WorkflowCompileError describing the first problem, or null when compilable.
+ *
+ * Allowed shape: a single path from start to end. Seam nodes may carry an extra
+ * `failure` edge to the end node; every other non-terminal node has exactly one
+ * outgoing edge. Anything else (true branching) requires the deferred interpreter.
+ */
+export function validateLinearity(ir: WorkflowIr): WorkflowCompileError | null {
+  const nodesById = new Map(ir.nodes.map((node) => [node.id, node]));
+  for (const edge of ir.edges) {
+    if (!nodesById.has(edge.from)) return new WorkflowCompileError(`edge references unknown node '${edge.from}'`);
+    if (!nodesById.has(edge.to)) return new WorkflowCompileError(`edge references unknown node '${edge.to}'`);
+  }
+
+  const endNode = ir.nodes.find((node) => node.kind === "end");
+  const startNode = ir.nodes.find((node) => node.kind === "start");
+  if (!startNode || !endNode) {
+    return new WorkflowCompileError("workflow must contain exactly one start and one end node");
+  }
+
+  const outgoing = buildOutgoing(ir);
+
+  for (const node of ir.nodes) {
+    const outs = outgoing.get(node.id) ?? [];
+    if (node.kind === "end") {
+      if (outs.length > 0) return new WorkflowCompileError("end node must have no outgoing edges");
+      continue;
+    }
+
+    const seam = seamOf(node);
+    if (seam) {
+      const failureEdges = outs.filter((edge) => edge.condition === "failure");
+      const mainEdges = outs.filter((edge) => edge.condition !== "failure");
+      if (mainEdges.length !== 1) {
+        return new WorkflowCompileError(`seam '${node.id}' must have exactly one success path`);
+      }
+      if (failureEdges.length > 1) {
+        return new WorkflowCompileError(`seam '${node.id}' has multiple failure edges`);
+      }
+      if (failureEdges[0] && failureEdges[0].to !== endNode.id) {
+        return new WorkflowCompileError(`seam '${node.id}' failure edge must target the end node`);
+      }
+      continue;
+    }
+
+    // start or a user node (prompt/script/gate): exactly one outgoing edge.
+    if (outs.length === 0) {
+      return new WorkflowCompileError(`node '${node.id}' has no outgoing edge`);
+    }
+    if (outs.length > 1) {
+      return new WorkflowCompileError(
+        `node '${node.id}' branches into ${outs.length} edges — graphs with branches require the workflow interpreter (deferred)`,
+      );
+    }
+  }
+
+  // Reachability: the single main path must reach end and cover every node.
+  const visited = new Set<string>();
+  let cursor: string | undefined = startNode.id;
+  while (cursor && !visited.has(cursor)) {
+    visited.add(cursor);
+    if (cursor === endNode.id) break;
+    cursor = mainEdge(outgoing.get(cursor) ?? [])?.to;
+  }
+  if (!visited.has(endNode.id)) {
+    return new WorkflowCompileError("workflow main path does not reach the end node");
+  }
+  const unreached = ir.nodes.filter((node) => !visited.has(node.id));
+  if (unreached.length > 0) {
+    return new WorkflowCompileError(
+      `node '${unreached[0].id}' is not on the main path — disconnected nodes require the workflow interpreter (deferred)`,
+    );
+  }
+
+  return null;
+}
+
+function defaultGateMode(node: WorkflowIrNode, mode: "prompt" | "script"): WorkflowStepGateMode {
+  if (node.kind === "gate") return "gate";
+  const explicit = node.config?.gateMode;
+  if (explicit === "gate" || explicit === "advisory") return explicit;
+  return mode === "script" ? "gate" : "advisory";
+}
+
+function nodeToStepInput(node: WorkflowIrNode, phase: "pre-merge" | "post-merge"): WorkflowStepInput {
+  const scriptName = configString(node, "scriptName");
+  const mode: "prompt" | "script" = node.kind === "script" || (node.kind === "gate" && scriptName) ? "script" : "prompt";
+  const gateMode = defaultGateMode(node, mode);
+
+  const input: WorkflowStepInput = {
+    name: configString(node, "name") ?? node.id,
+    description: configString(node, "description") ?? "",
+    mode,
+    phase,
+    gateMode,
+  };
+
+  if (mode === "script") {
+    input.scriptName = scriptName;
+  } else {
+    input.prompt = configString(node, "prompt") ?? "";
+    input.toolMode = node.config?.toolMode === "coding" ? "coding" : "readonly";
+    const provider = configString(node, "modelProvider");
+    const modelId = configString(node, "modelId");
+    if (provider && modelId) {
+      input.modelProvider = provider;
+      input.modelId = modelId;
+    }
+  }
+
+  return input;
+}
+
+/**
+ * Compile a workflow graph into an ordered list of WorkflowStep inputs ready to
+ * persist and run on the existing engine. User prompt/script/gate nodes become
+ * steps; execute/review seams are skipped; the merge seam is the pre-/post-merge
+ * boundary. Throws WorkflowCompileError for non-linear graphs.
+ *
+ * The returned array order is the execution order (it maps directly onto a
+ * task's `enabledWorkflowSteps`).
+ */
+export function compileWorkflowToSteps(ir: WorkflowIr): WorkflowStepInput[] {
+  const parsed = parseWorkflowIr(ir);
+  const error = validateLinearity(parsed);
+  if (error) throw error;
+
+  const nodesById = new Map(parsed.nodes.map((node) => [node.id, node]));
+  const outgoing = buildOutgoing(parsed);
+  const startNode = parsed.nodes.find((node) => node.kind === "start")!;
+
+  const steps: WorkflowStepInput[] = [];
+  let phase: "pre-merge" | "post-merge" = "pre-merge";
+  const visited = new Set<string>();
+  let cursor: string | undefined = startNode.id;
+
+  while (cursor && !visited.has(cursor)) {
+    visited.add(cursor);
+    const node = nodesById.get(cursor);
+    if (!node) break;
+
+    const seam = seamOf(node);
+    if (seam === "merge") {
+      phase = "post-merge";
+    } else if (!seam && node.kind !== "start" && node.kind !== "end") {
+      steps.push(nodeToStepInput(node, phase));
+    }
+
+    if (node.kind === "end") break;
+    cursor = mainEdge(outgoing.get(cursor) ?? [])?.to;
+  }
+
+  return steps;
+}