fix(core): compile branching merge-region in builtin workflows

FN-6035 modeled the merge lifecycle as a branching subgraph of merge/
retry/branch-group primitives instead of a single `merge` seam, but the
linear workflow compiler still tried to lower those nodes and rejected
merge-gate's fan-out — so task creation failed with "node 'merge-gate'
branches into 2 edges — graphs with branches require the workflow
interpreter (deferred)".

Treat the merge-region primitive kinds (merge-gate, merge-attempt,
manual-merge-hold, retry-backoff, recovery-router,
branch-group-member-integration, branch-group-promotion, pr-merge) as an
engine-owned terminal boundary: exempt from the single-edge linearity
rule and never lowered to a step. Linear-prefix workflows compile to
their pre-merge step list again.

Also refresh the stale builtin-workflows assertions that referenced the
removed `merge` seam node.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.changeset/fix-workflow-compiler-merge-region.md

@@ -0,0 +1,5 @@
+---
+"@fusion/core": patch
+---
+
+Fix task creation failing with "node 'merge-gate' branches into 2 edges — graphs with branches require the workflow interpreter (deferred)". The built-in coding workflow now models the merge lifecycle as a branching region of merge/retry/branch-group primitives (FN-6035), but the linear workflow compiler still tried to lower those nodes and rejected their fan-out. The compiler now treats the merge-region primitive kinds (merge-gate, merge-attempt, manual-merge-hold, retry-backoff, recovery-router, branch-group-member-integration, branch-group-promotion, pr-merge) as an engine-owned terminal boundary — exempt from the single-edge linearity rule and never lowered to a step — so linear-prefix workflows compile to their pre-merge step list again.

packages/core/src/__tests__/builtin-workflows.test.ts

@@ -138,7 +138,9 @@ describe("built-in workflows", () => {
     expect(byId.get("execute")?.column).toBe("in-progress");
     expect(byId.get("workflow-step")?.column).toBe("in-progress");
     expect(byId.get("review")?.column).toBe("in-review");
-    expect(byId.get("merge")?.column).toBe("in-review");
+    // Merge is the native primitive region (FN-6035), placed in in-review.
+    expect(byId.get("merge")).toBeUndefined();
+    expect(byId.get("merge-attempt")?.column).toBe("in-review");
     expect(ir.settings).toEqual(BUILTIN_WORKFLOW_SETTINGS);
   });
 
@@ -195,10 +197,11 @@ describe("built-in workflows", () => {
       const byId = new Map(candidate.nodes.map((node) => [node.id, node]));
       expect(byId.get("workflow-step")?.config?.name).toBe("Pre-merge workflow steps");
       expect(byId.get("review")?.config?.name).toBe("Review");
-      expect(byId.get("merge")?.config?.name).toBe("Merge boundary");
       expect(byId.get("workflow-step")?.config?.maxRetries).toBeUndefined();
       expect(byId.get("review")?.config?.maxRetries).toBeUndefined();
-      expect(byId.get("merge")?.config?.maxRetries).toBeUndefined();
+      // The merge lifecycle is no longer a single `merge` seam node (FN-6035): it
+      // is expressed as the merge-gate/merge-attempt/branch-group primitive region.
+      expect(byId.get("merge")).toBeUndefined();
     }
   });
 
@@ -320,11 +323,11 @@ describe("built-in workflows", () => {
       const coding = getBuiltinWorkflow("builtin:coding");
       const execute = coding?.ir.nodes.find((node) => node.id === "execute");
       const review = coding?.ir.nodes.find((node) => node.id === "review");
-      const merge = coding?.ir.nodes.find((node) => node.id === "merge");
 
       expect((execute?.config as { prompt?: string } | undefined)?.prompt).toContain("You are a task execution agent");
       expect((review?.config as { prompt?: string } | undefined)?.prompt).toContain("You are an independent code and plan reviewer");
-      expect((merge?.config as { prompt?: string } | undefined)?.prompt).toContain("You are a merge agent");
+      // No `merge` seam node post-FN-6035 — merge runs as native primitives.
+      expect(coding?.ir.nodes.find((node) => node.id === "merge")).toBeUndefined();
     });
 
     it("rejects editing or deleting a built-in", async () => {

packages/core/src/__tests__/workflow-compiler.test.ts

@@ -120,6 +120,40 @@ describe("compileWorkflowToSteps (U2)", () => {
     expect(() => compileWorkflowToSteps(ir)).toThrow(/interpreter \(deferred\)/i);
   });
 
+  it("compiles a workflow whose post-review merge region branches into primitives (FN-6035)", () => {
+    // Mirrors the builtin:coding shape: review → merge-gate fans out into the
+    // engine-owned merge/branch-group/retry subgraph. These primitive kinds are a
+    // terminal boundary, so the graph still compiles to its pre-merge step list
+    // instead of failing as interpreter-only.
+    const ir: WorkflowIr = {
+      version: "v1",
+      name: "merge-region",
+      nodes: [
+        { id: "start", kind: "start" },
+        { id: "spec", kind: "prompt", config: { name: "Spec", prompt: "spec" } },
+        { id: "review", kind: "prompt", config: { seam: "review" } },
+        { id: "merge-gate", kind: "merge-gate", config: { gate: "auto-merge" } },
+        { id: "merge-attempt", kind: "merge-attempt" },
+        { id: "merge-hold", kind: "manual-merge-hold" },
+        { id: "end", kind: "end" },
+      ],
+      edges: [
+        { from: "start", to: "spec", condition: "success" },
+        { from: "spec", to: "review", condition: "success" },
+        { from: "review", to: "merge-gate", condition: "success" },
+        { from: "review", to: "end", condition: "failure" },
+        { from: "merge-gate", to: "merge-attempt", condition: "outcome:auto-on" },
+        { from: "merge-gate", to: "merge-hold", condition: "outcome:auto-off" },
+        { from: "merge-attempt", to: "end", condition: "success" },
+        { from: "merge-hold", to: "merge-attempt", condition: "success" },
+      ],
+    };
+    expect(validateLinearity(parseWorkflowIr(ir))).toBeNull();
+    const steps = compileWorkflowToSteps(ir);
+    // Only the pre-merge user node lowers; the merge primitives emit no steps.
+    expect(steps.map((s) => s.name)).toEqual(["Spec"]);
+  });
+
   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();

packages/core/src/workflow-compiler.ts

@@ -20,6 +20,30 @@ export class WorkflowCompileError extends Error {
  *  not emitted as steps. */
 const SEAM_NAMES = new Set(["planning", "execute", "workflow-step", "review", "merge"]);
 
+/** Workflow-owned merge/retry/recovery policy node kinds (FN-6035). After review,
+ *  the builtin workflows express the merge lifecycle as a branching subgraph of
+ *  these primitives instead of the single legacy `merge` seam node. The linear
+ *  compiler treats the whole region as one engine-owned terminal boundary: it is
+ *  exempt from the single-outgoing-edge rule, never lowered to a WorkflowStep, and
+ *  ends the linear walk (the legacy pipeline runs the merge lifecycle natively,
+ *  the graph interpreter runs the branches). This keeps `builtin:coding` and other
+ *  linear-prefix workflows compilable to their pre-merge step list rather than
+ *  failing as "interpreter (deferred)". */
+const MERGE_REGION_KINDS = new Set([
+  "merge-gate",
+  "merge-attempt",
+  "manual-merge-hold",
+  "retry-backoff",
+  "recovery-router",
+  "branch-group-member-integration",
+  "branch-group-promotion",
+  "pr-merge",
+]);
+
+function isMergeRegion(node: WorkflowIrNode): boolean {
+  return MERGE_REGION_KINDS.has(node.kind);
+}
+
 function seamOf(node: WorkflowIrNode): string | undefined {
   const seam = node.config?.seam;
   return typeof seam === "string" && SEAM_NAMES.has(seam) ? seam : undefined;
@@ -75,6 +99,11 @@ export function validateLinearity(ir: WorkflowIr): WorkflowCompileError | null {
       continue;
     }
 
+    // Merge-region primitives are an engine-owned terminal boundary (FN-6035):
+    // they legitimately branch (e.g. merge-gate's auto-on/auto-off outcome edges)
+    // and are never lowered to steps, so they are exempt from the linearity rules.
+    if (isMergeRegion(node)) continue;
+
     const seam = seamOf(node);
     if (seam) {
       const failureEdges = outs.filter((edge) => edge.condition === "failure");
@@ -121,10 +150,18 @@ export function validateLinearity(ir: WorkflowIr): WorkflowCompileError | null {
   const seenSeams = new Set<string>();
   let nextExpectedSeamIndex = 0;
   const visited = new Set<string>();
+  // Reaching the engine-owned merge region counts as reaching the terminal
+  // lifecycle: the linear walk stops there and the branching merge subgraph
+  // (plus the end node it eventually leads to) is owned by the merge runtime.
+  let reachedTerminal = false;
   let cursor: string | undefined = startNode.id;
   while (cursor && !visited.has(cursor)) {
     visited.add(cursor);
     const node = nodesById.get(cursor);
+    if (node && isMergeRegion(node)) {
+      reachedTerminal = true;
+      break;
+    }
     const seam = node ? seamOf(node) : undefined;
     if (seam) {
       if (seenSeams.has(seam)) {
@@ -144,13 +181,21 @@ export function validateLinearity(ir: WorkflowIr): WorkflowCompileError | null {
       seenSeams.add(seam);
       nextExpectedSeamIndex += 1;
     }
-    if (cursor === endNode.id) break;
+    if (cursor === endNode.id) {
+      reachedTerminal = true;
+      break;
+    }
     cursor = mainEdge(outgoing.get(cursor) ?? [])?.to;
   }
-  if (!visited.has(endNode.id)) {
+  if (!reachedTerminal) {
     return new WorkflowCompileError("workflow main path does not reach the end node");
   }
-  const unreached = ir.nodes.filter((node) => !visited.has(node.id));
+  // Merge-region nodes and the end node may be reached only through the branching
+  // merge subgraph (not the linear walk), so they are not required to appear on the
+  // pre-merge main path. Every other node must.
+  const unreached = ir.nodes.filter(
+    (node) => !visited.has(node.id) && node.kind !== "end" && !isMergeRegion(node),
+  );
   if (unreached.length > 0) {
     return new WorkflowCompileError(
       `node '${unreached[0].id}' is not on the main path — disconnected nodes require the workflow interpreter (deferred)`,
@@ -236,6 +281,11 @@ export function compileWorkflowToSteps(ir: WorkflowIr): WorkflowStepInput[] {
     const node = nodesById.get(cursor);
     if (!node) break;
 
+    // The merge region is an engine-owned terminal boundary: it carries no
+    // lowerable user steps and ends the linear lowering walk (mirrors how the
+    // legacy `merge` seam terminated the pre-merge chain).
+    if (isMergeRegion(node)) break;
+
     const seam = seamOf(node);
     if (seam === "merge") {
       phase = "post-merge";