feat(agent-kit): render per-variant output instructions for discriminated oneOf

buildOutputFormatInstruction now detects discriminated union schemas
(oneOf with shared const/ property) and renders separate YAML
example blocks per variant, so agents see exactly which fields belong
to which outcome instead of a flat merge.

Non-discriminated oneOf/anyOf schemas fall back to the existing flat
merge behavior.

Refs #502

.workflows/solve-issue.yaml

@@ -22,16 +22,17 @@ roles:
       After producing the test spec:
       1. Store it via `uwf cas put-text "<markdown content>"` and capture the returned hash
       2. Put the hash in frontmatter.plan (required when status=ready)
-    output: "Output a brief summary of the test spec. Frontmatter must include: status (ready or insufficient_info) and plan (CAS hash of the test spec, required when status=ready)."
+    output: "Output a brief summary of the test spec. Set $status to ready (with plan hash and repoPath) or insufficient_info."
     frontmatter:
-      type: object
-      properties:
-        status:
-          type: string
-          enum: [ready, insufficient_info]
-        plan:
-          type: string
-      required: [status]
+      oneOf:
+        - properties:
+            $status: { const: "ready" }
+            plan: { type: string }
+            repoPath: { type: string }
+          required: [$status, plan, repoPath]
+        - properties:
+            $status: { const: "insufficient_info" }
+          required: [$status]
   developer:
     description: "TDD implementation per test spec"
     goal: "You are a developer agent. You implement code changes following TDD — write tests first, then implementation."
@@ -58,14 +59,13 @@ roles:
       8. Implement the code to make tests pass
       9. Ensure `bun run build` passes with no errors
       10. Run `bun test` to verify all tests pass
-    output: "List all files changed and provide a summary. Frontmatter must include: status (done or failed)."
+    output: "List all files changed and provide a summary. Include branch name and worktree path in frontmatter."
     frontmatter:
       type: object
       properties:
-        status:
-          type: string
-          enum: [done, failed]
-      required: [status]
+        branch: { type: string }
+        worktree: { type: string }
+      required: [branch, worktree]
   reviewer:
     description: "Code standards compliance check"
     goal: "You are a code reviewer. You verify code standards compliance — NOT functionality (that's the tester's job)."
@@ -95,14 +95,18 @@ roles:
 
       Only review standards compliance. Do NOT test functionality.
       If rejecting, you MUST explain the specific reason in your output.
-    output: "Explain your decision with specific file/line references. Frontmatter must include: status (approved or rejected)."
+    output: "Explain your decision with specific file/line references. Set $status to approved (with branch/worktree) or rejected (with comments)."
     frontmatter:
-      type: object
-      properties:
-        status:
-          type: string
-          enum: [approved, rejected]
-      required: [status]
+      oneOf:
+        - properties:
+            $status: { const: "approved" }
+            branch: { type: string }
+            worktree: { type: string }
+          required: [$status, branch, worktree]
+        - properties:
+            $status: { const: "rejected" }
+            comments: { type: string }
+          required: [$status, comments]
   tester:
     description: "Functional correctness verification"
     goal: "You are a tester agent. You verify that the implementation correctly satisfies every scenario in the test spec."
@@ -118,14 +122,22 @@ roles:
          - passed: all scenarios verified, tests pass
          - fix_code: tests fail or implementation doesn't match spec → send back to developer
          - fix_spec: the spec itself is wrong or incomplete → send back to planner
-    output: "Report test results per scenario. Frontmatter must include: status (passed, fix_code, or fix_spec)."
+    output: "Report test results per scenario. Set $status to passed (with branch/worktree), fix_code (with report), or fix_spec (with report)."
     frontmatter:
-      type: object
-      properties:
-        status:
-          type: string
-          enum: [passed, fix_code, fix_spec]
-      required: [status]
+      oneOf:
+        - properties:
+            $status: { const: "passed" }
+            branch: { type: string }
+            worktree: { type: string }
+          required: [$status, branch, worktree]
+        - properties:
+            $status: { const: "fix_code" }
+            report: { type: string }
+          required: [$status, report]
+        - properties:
+            $status: { const: "fix_spec" }
+            report: { type: string }
+          required: [$status, report]
   committer:
     description: "Commits and creates PR"
     goal: "You are a committer agent. You create a clean commit and push a PR linking the original issue."
@@ -146,30 +158,32 @@ roles:
       5. After PR creation, clean up the worktree:
          - `cd ~/repos/workflow`
          - `git worktree remove ~/repos/workflow-worktrees/fix/<issue-number>-<slug>`
-    output: "Include PR URL on success or error log on failure. Frontmatter must include: status (committed or hook_failed)."
+    output: "Include PR URL on success or error log on failure. Set $status to committed (with prUrl) or hook_failed (with error)."
     frontmatter:
-      type: object
-      properties:
-        status:
-          type: string
-          enum: [committed, hook_failed]
-      required: [status]
+      oneOf:
+        - properties:
+            $status: { const: "committed" }
+            prUrl: { type: string }
+          required: [$status, prUrl]
+        - properties:
+            $status: { const: "hook_failed" }
+            error: { type: string }
+          required: [$status, error]
 graph:
   $START:
     _: { role: "planner", prompt: "Analyze the issue and produce an implementation plan." }
   planner:
     insufficient_info: { role: "$END", prompt: "Insufficient information to proceed; end the workflow." }
-    ready: { role: "developer", prompt: "Implement the plan from the planner." }
+    ready: { role: "developer", prompt: "Implement the TDD test spec (CAS hash: {{{plan}}}) in repo {{{repoPath}}}." }
   developer:
-    failed: { role: "$END", prompt: "Development failed; end the workflow." }
-    done: { role: "reviewer", prompt: "Send the implementation to the reviewer." }
+    _: { role: "reviewer", prompt: "Review branch {{{branch}}} at {{{worktree}}} for code standards compliance." }
   reviewer:
-    rejected: { role: "developer", prompt: "Reviewer rejected the implementation; fix the issues." }
-    approved: { role: "tester", prompt: "Review passed; run tests on the implementation." }
+    rejected: { role: "developer", prompt: "Reviewer rejected: {{{comments}}}. Fix the issues." }
+    approved: { role: "tester", prompt: "Review passed. Run tests on branch {{{branch}}} at {{{worktree}}}." }
   tester:
-    fix_code: { role: "developer", prompt: "Tests found code issues; return to developer." }
-    fix_spec: { role: "planner", prompt: "Tests found spec issues; return to planner." }
-    passed: { role: "committer", prompt: "Tests passed; commit and push the changes." }
+    fix_code: { role: "developer", prompt: "Tests found code issues: {{{report}}}. Fix and re-submit." }
+    fix_spec: { role: "planner", prompt: "Tests found spec issues: {{{report}}}. Revise the test spec." }
+    passed: { role: "committer", prompt: "All tests passed. Commit and push branch {{{branch}}} from {{{worktree}}}." }
   committer:
-    hook_failed: { role: "developer", prompt: "Push hook failed; return to developer to fix." }
-    committed: { role: "$END", prompt: "Commit succeeded; complete the workflow." }
+    hook_failed: { role: "developer", prompt: "Push hook failed: {{{error}}}. Fix and re-submit." }
+    committed: { role: "$END", prompt: "PR created: {{{prUrl}}}. Workflow complete." }

examples/analyze-topic.yaml

@@ -22,7 +22,7 @@ roles:
     frontmatter:
       type: object
       properties:
-        status:
+        $status:
           enum: ["_"]
         thesis:
           type: string
@@ -32,7 +32,7 @@ roles:
             type: string
         caveats:
           type: string
-      required: [status, thesis, keyPoints]
+      required: [$status, thesis, keyPoints]
 graph:
   $START:
     _: { role: "analyst", prompt: "Analyze the topic in the task and produce a structured summary with key points." }

examples/debate.yaml

@@ -21,11 +21,11 @@ roles:
     frontmatter:
       type: object
       properties:
-        status:
+        $status:
           enum: ["continue", "conceded"]
         argument:
           type: string
-      required: [status, argument]
+      required: [$status, argument]
   for:
     description: "Argues for the proposition"
     goal: |
@@ -46,11 +46,11 @@ roles:
     frontmatter:
       type: object
       properties:
-        status:
+        $status:
           enum: ["continue", "conceded"]
         argument:
           type: string
-      required: [status, argument]
+      required: [$status, argument]
 graph:
   $START:
     _: { role: "against", prompt: "Present your opening argument against the proposition." }

examples/solve-issue.yaml

@@ -27,13 +27,13 @@ roles:
     frontmatter:
       type: object
       properties:
-        status:
+        $status:
           enum: ["_"]
         repoPath:
           type: string
         plan:
           type: string
-      required: [status, repoPath, plan]
+      required: [$status, repoPath, plan]
   developer:
     description: "Implements code changes"
     goal: "You are a developer agent. You implement code changes according to plans."
@@ -52,7 +52,7 @@ roles:
     frontmatter:
       type: object
       properties:
-        status:
+        $status:
           enum: ["_"]
         filesChanged:
           type: array
@@ -60,7 +60,7 @@ roles:
             type: string
         summary:
           type: string
-      required: [status, filesChanged, summary]
+      required: [$status, filesChanged, summary]
   reviewer:
     description: "Reviews code changes"
     goal: "You are a code reviewer. You review implementations for correctness and quality."
@@ -75,11 +75,11 @@ roles:
     frontmatter:
       type: object
       properties:
-        status:
+        $status:
           enum: ["approved", "rejected"]
         comments:
           type: string
-      required: [status, comments]
+      required: [$status, comments]
 graph:
   $START:
     _: { role: "planner", prompt: "Analyze the issue described in the task and produce a detailed implementation plan." }

packages/cli-workflow/src/__tests__/solve-issue-tea-worktree.test.ts

@@ -81,17 +81,18 @@ describe("solve-issue workflow: tea pr create worktree fix", () => {
     expect(workflow.roles.committer?.frontmatter).toBeDefined();
   });
 
-  test("committer frontmatter schema should require status field", async () => {
+  test("committer frontmatter schema should be oneOf with $status discriminant", async () => {
     const yamlContent = await readFile(workflowPath, "utf-8");
     // Parse as any to access the raw YAML structure (frontmatter is inline JSON Schema in YAML)
     // eslint-disable-next-line @typescript-eslint/no-explicit-any
     const workflow = parse(yamlContent) as any;
-
     const frontmatter = workflow.roles.committer?.frontmatter;
     expect(frontmatter).toBeDefined();
-    expect(frontmatter?.type).toBe("object");
-    expect(frontmatter?.properties?.status).toBeDefined();
-    expect(frontmatter?.properties?.status?.enum).toContain("committed");
-    expect(frontmatter?.required).toContain("status");
+    expect(frontmatter?.oneOf).toBeDefined();
+    const committedVariant = frontmatter.oneOf.find(
+      (v: any) => v.properties?.["$status"]?.const === "committed",
+    );
+    expect(committedVariant).toBeDefined();
+    expect(committedVariant.required).toContain("$status");
   });
 });

packages/cli-workflow/src/commands/thread.ts

@@ -669,14 +669,16 @@ function formatThreadReadMarkdown(options: {
   return parts.join("\n\n---\n\n");
 }
 
-type EvaluateLastOutput = Record<string, unknown> & { status: string };
+type EvaluateLastOutput = Record<string, unknown>;
+
+const STATUS_KEY = "$status";
 
 function resolveEvaluateArgs(
   uwf: UwfStore,
   chain: ChainState,
 ): { lastRole: string; lastOutput: EvaluateLastOutput } {
   if (chain.headIsStart) {
-    return { lastRole: START_ROLE, lastOutput: { status: "_" } };
+    return { lastRole: START_ROLE, lastOutput: { [STATUS_KEY]: "_" } };
   }
 
   const lastStep = chain.stepsNewestFirst[0];
@@ -689,11 +691,10 @@ function resolveEvaluateArgs(
     typeof raw === "object" && raw !== null && !Array.isArray(raw)
       ? (raw as Record<string, unknown>)
       : {};
-  const status = typeof base.status === "string" ? base.status : "_";
 
   return {
     lastRole: lastStep.role,
-    lastOutput: { ...base, status },
+    lastOutput: base,
   };
 }
 

packages/workflow-agent-kit/__tests__/build-output-format-instruction.test.ts

@@ -87,7 +87,7 @@ describe("buildOutputFormatInstruction", () => {
     expect(result).toContain("beta: <number>");
   });
 
-  test("lists union of fields from a oneOf schema", () => {
+  test("lists union of fields from a oneOf schema (no discriminant — flat merge)", () => {
     const schema = {
       oneOf: [
         {
@@ -101,12 +101,71 @@ describe("buildOutputFormatInstruction", () => {
       ],
     };
     const result = buildOutputFormatInstruction(schema);
+    // No discriminant detected → falls back to flat merge
     expect(result).toContain("`foo`");
     expect(result).toContain("`bar`");
     expect(result).toContain("foo: <string>");
     expect(result).toContain("bar: true  # true | false");
   });
 
+  test("renders per-variant instructions for discriminated oneOf", () => {
+    const schema = {
+      oneOf: [
+        {
+          type: "object",
+          properties: {
+            $status: { const: "ready" },
+            plan: { type: "string" },
+          },
+          required: ["$status", "plan"],
+        },
+        {
+          type: "object",
+          properties: {
+            $status: { const: "insufficient_info" },
+          },
+          required: ["$status"],
+        },
+      ],
+    };
+    const result = buildOutputFormatInstruction(schema);
+    expect(result).toContain("Choose ONE of the following variants");
+    expect(result).toContain("When `$status: ready`");
+    expect(result).toContain("When `$status: insufficient_info`");
+    expect(result).toContain("plan: <string>");
+    // The insufficient_info variant should NOT mention plan
+    const insufficientBlock = result.split("When `$status: insufficient_info`")[1];
+    expect(insufficientBlock).not.toContain("plan:");
+  });
+
+  test("renders per-variant for single-enum discriminant", () => {
+    const schema = {
+      oneOf: [
+        {
+          type: "object",
+          properties: {
+            $status: { type: "string", enum: ["approved"] },
+            branch: { type: "string" },
+          },
+          required: ["$status"],
+        },
+        {
+          type: "object",
+          properties: {
+            $status: { type: "string", enum: ["rejected"] },
+            comments: { type: "string" },
+          },
+          required: ["$status"],
+        },
+      ],
+    };
+    const result = buildOutputFormatInstruction(schema);
+    expect(result).toContain("When `$status: approved`");
+    expect(result).toContain("When `$status: rejected`");
+    expect(result).toContain("branch: <string>");
+    expect(result).toContain("comments: <string>");
+  });
+
   test("falls back gracefully for a non-object schema with no properties", () => {
     const result = buildOutputFormatInstruction({ type: "string" });
     expect(result).toContain("schema fields will be extracted automatically");

packages/workflow-agent-kit/src/build-output-format-instruction.ts

@@ -166,14 +166,109 @@ function buildFieldList(properties: SchemaProperty[]): string {
     .join("\n");
 }
 
+/**
+ * Detect the discriminant property name from a oneOf schema.
+ * Returns the property name if all variants share a const/single-enum string property, else null.
+ */
+function detectDiscriminant(variants: JSONSchema[]): string | null {
+  // Find property names that appear in ALL variants with const or single-enum
+  const candidateNames = new Set<string>();
+
+  for (const variant of variants) {
+    const props = variant.properties as Record<string, JSONSchema> | null | undefined;
+    if (typeof props !== "object" || props === null) return null;
+
+    for (const [name, propSchema] of Object.entries(props)) {
+      const isConst =
+        propSchema.const !== undefined ||
+        (Array.isArray(propSchema.enum) && propSchema.enum.length === 1);
+      if (isConst) candidateNames.add(name);
+    }
+  }
+
+  // Check which candidate appears in ALL variants
+  for (const name of candidateNames) {
+    const allHaveIt = variants.every((v) => {
+      const props = v.properties as Record<string, JSONSchema> | null | undefined;
+      if (typeof props !== "object" || props === null) return false;
+      const propSchema = props[name];
+      if (!propSchema) return false;
+      return (
+        propSchema.const !== undefined ||
+        (Array.isArray(propSchema.enum) && propSchema.enum.length === 1)
+      );
+    });
+    if (allHaveIt) return name;
+  }
+
+  return null;
+}
+
+function getConstValue(propSchema: JSONSchema): string {
+  if (propSchema.const !== undefined) return String(propSchema.const);
+  if (Array.isArray(propSchema.enum) && propSchema.enum.length === 1)
+    return String(propSchema.enum[0]);
+  return "<unknown>";
+}
+
+function buildVariantBlock(variant: JSONSchema, discriminant: string): string {
+  const props = extractSchemaProperties(variant);
+  const value = getConstValue(
+    ((variant.properties as Record<string, JSONSchema>) ?? {})[discriminant] ?? {},
+  );
+  const yamlExample = buildYamlExampleBlock(props);
+  const fieldList = buildFieldList(props);
+
+  return `### When \`${discriminant}: ${value}\`
+
+\`\`\`
+${yamlExample}
+\`\`\`
+
+Fields:
+${fieldList}`;
+}
+
 /**
  * Build a concise output format instruction block for an agent role.
  *
- * The instruction describes the expected frontmatter markdown format and lists
- * the meta fields derived from the JSON Schema.  It is prepended to the agent's
- * system prompt so the deliverable format is the first thing the agent sees.
+ * For discriminated union schemas (oneOf with a shared const/$status field),
+ * renders per-variant instructions so the agent knows exactly which fields
+ * belong to which outcome.
+ *
+ * For flat object schemas, renders a single YAML example block.
  */
 export function buildOutputFormatInstruction(schema: JSONSchema): string {
+  // Check for discriminated union (oneOf with shared discriminant)
+  const unionKey = Array.isArray(schema.oneOf)
+    ? "oneOf"
+    : Array.isArray(schema.anyOf)
+      ? "anyOf"
+      : null;
+
+  if (unionKey !== null) {
+    const variants = schema[unionKey] as JSONSchema[];
+    const discriminant = detectDiscriminant(variants);
+
+    if (discriminant !== null && variants.length > 1) {
+      const variantBlocks = variants.map((v) => buildVariantBlock(v, discriminant)).join("\n\n");
+
+      return `## Deliverable Format
+
+Your response MUST begin with a YAML frontmatter block followed by your markdown work.
+
+Choose ONE of the following variants based on your outcome:
+
+${variantBlocks}
+
+The frontmatter is the **primary deliverable** — the engine reads it directly.
+Output ONLY the fields listed for your chosen variant. Do not add extra fields that are not specified in the schema.
+
+Focus exclusively on YOUR role's deliverable. Do not perform actions outside your role's scope.`;
+    }
+  }
+
+  // Flat object schema fallback
   const properties = extractSchemaProperties(schema);
   const yamlExample = buildYamlExampleBlock(properties);
   const fieldList = buildFieldList(properties);

packages/workflow-moderator/__tests__/evaluate.test.ts

@@ -21,7 +21,7 @@ const solveIssueGraph: WorkflowPayload["graph"] = {
 
 describe("evaluate", () => {
   test("$START → first role (unit status _)", () => {
-    const result = evaluate(solveIssueGraph, "$START", { status: "_" });
+    const result = evaluate(solveIssueGraph, "$START", { $status: "_" });
     expect(result).toEqual({
       ok: true,
       value: { role: "planner", prompt: "Start planning from the issue in the task." },
@@ -30,7 +30,7 @@ describe("evaluate", () => {
 
   test("status-based routing (reviewer rejected → developer)", () => {
     const result = evaluate(solveIssueGraph, "reviewer", {
-      status: "rejected",
+      $status: "rejected",
       comments: "missing tests",
     });
     expect(result).toEqual({
@@ -40,7 +40,7 @@ describe("evaluate", () => {
   });
 
   test("status-based routing (reviewer approved → $END)", () => {
-    const result = evaluate(solveIssueGraph, "reviewer", { status: "approved" });
+    const result = evaluate(solveIssueGraph, "reviewer", { $status: "approved" });
     expect(result).toEqual({
       ok: true,
       value: { role: "$END", prompt: "Done." },
@@ -48,7 +48,7 @@ describe("evaluate", () => {
   });
 
   test("missing role in graph → error", () => {
-    const result = evaluate(solveIssueGraph, "unknown-role", { status: "_" });
+    const result = evaluate(solveIssueGraph, "unknown-role", { $status: "_" });
     expect(result.ok).toBe(false);
     if (!result.ok) {
       expect(result.error.message).toBe('no transitions defined for role "unknown-role"');
@@ -56,7 +56,7 @@ describe("evaluate", () => {
   });
 
   test("missing status in graph → error", () => {
-    const result = evaluate(solveIssueGraph, "reviewer", { status: "pending" });
+    const result = evaluate(solveIssueGraph, "reviewer", { $status: "pending" });
     expect(result.ok).toBe(false);
     if (!result.ok) {
       expect(result.error.message).toBe('no transition for role "reviewer" with status "pending"');
@@ -65,7 +65,7 @@ describe("evaluate", () => {
 
   test("mustache template rendering with simple fields", () => {
     const result = evaluate(solveIssueGraph, "planner", {
-      status: "_",
+      $status: "_",
       plan: "Add auth middleware",
     });
     expect(result).toEqual({
@@ -76,7 +76,7 @@ describe("evaluate", () => {
 
   test("mustache does not HTML-escape prompt content", () => {
     const result = evaluate(solveIssueGraph, "reviewer", {
-      status: "rejected",
+      $status: "rejected",
       comments: 'use <T> & "Result<T, E>" types',
     });
     expect(result).toEqual({
@@ -92,7 +92,7 @@ describe("evaluate", () => {
       },
     };
     const result = evaluate(graph, "reviewer", {
-      status: "_",
+      $status: "_",
       comments: "<script>alert(1)</script>",
     });
     expect(result).toEqual({
@@ -101,6 +101,16 @@ describe("evaluate", () => {
     });
   });
 
+  test("missing $status defaults to _ (unit routing)", () => {
+    const result = evaluate(solveIssueGraph, "planner", {
+      plan: "Add auth middleware",
+    });
+    expect(result).toEqual({
+      ok: true,
+      value: { role: "developer", prompt: "Implement the plan: Add auth middleware" },
+    });
+  });
+
   test("mustache template with nested object paths", () => {
     const graph: Record<string, Record<string, Target>> = {
       reviewer: {
@@ -111,7 +121,7 @@ describe("evaluate", () => {
       },
     };
     const result = evaluate(graph, "reviewer", {
-      status: "_",
+      $status: "_",
       review: { comments: "refactor the handler" },
     });
     expect(result).toEqual({

packages/workflow-moderator/src/evaluate.ts

@@ -9,14 +9,21 @@ mustache.escape = (text: string) => text;
 const START_ROLE = "$START";
 const UNIT_STATUS = "_";
 
-type LastOutput = Record<string, unknown> & { status: string };
+type LastOutput = Record<string, unknown>;
+
+const STATUS_KEY = "$status";
 
 export function evaluate(
   graph: Record<string, Record<string, Target>>,
   lastRole: string,
   lastOutput: LastOutput,
 ): Result<EvaluateResult, Error> {
-  const status = lastRole === START_ROLE ? UNIT_STATUS : lastOutput.status;
+  const status =
+    lastRole === START_ROLE
+      ? UNIT_STATUS
+      : typeof lastOutput[STATUS_KEY] === "string"
+        ? (lastOutput[STATUS_KEY] as string)
+        : UNIT_STATUS;
 
   const roleTargets = graph[lastRole];
   if (roleTargets === undefined) {