feat(cli): add workflow semantic validation before execution

Implements validateWorkflow() that performs deep semantic checks on
parsed WorkflowPayload before registration or execution:

- Role reference integrity (unknown roles, orphans, reserved names)
- Graph structure (/ constraints, reachability, edge targets)
- Status-edge consistency (single/multi-exit matching)
- Mustache template variable existence
- oneOf discriminant validity ( const check)

All errors collected (not fail-fast). Integrated into:
- uwf workflow add (before CAS registration)
- uwf thread start (local workflow materialization)

Closes #506

packages/cli-workflow/src/__tests__/validate-semantic.test.ts

@@ -0,0 +1,366 @@
+import type { WorkflowPayload } from "@uncaged/workflow-protocol";
+import { describe, expect, test } from "vitest";
+import { validateWorkflow } from "../validate-semantic.js";
+
+/** Build a valid two-role workflow that passes all checks. */
+function makeWorkflow(overrides?: Partial<WorkflowPayload>): WorkflowPayload {
+  const base: WorkflowPayload = {
+    name: "test-workflow",
+    description: "A test workflow",
+    roles: {
+      writer: {
+        description: "Writes content",
+        goal: "Write content",
+        capabilities: ["writing"],
+        procedure: "Write it",
+        output: "The content",
+        frontmatter: {
+          type: "object",
+          properties: {
+            $status: { enum: ["_"] },
+            plan: { type: "string" },
+          },
+          required: ["$status", "plan"],
+        } as unknown as string,
+      },
+      reviewer: {
+        description: "Reviews content",
+        goal: "Review content",
+        capabilities: ["reviewing"],
+        procedure: "Review it",
+        output: "The review",
+        frontmatter: {
+          type: "object",
+          oneOf: [
+            {
+              properties: {
+                $status: { const: "approved" },
+                summary: { type: "string" },
+              },
+              required: ["$status", "summary"],
+            },
+            {
+              properties: {
+                $status: { const: "rejected" },
+                reason: { type: "string" },
+              },
+              required: ["$status", "reason"],
+            },
+          ],
+        } as unknown as string,
+      },
+    },
+    graph: {
+      $START: { _: { role: "writer", prompt: "Begin writing" } },
+      writer: { _: { role: "reviewer", prompt: "Review this: {{{plan}}}" } },
+      reviewer: {
+        approved: { role: "$END", prompt: "Done: {{{summary}}}" },
+        rejected: { role: "writer", prompt: "Fix: {{{reason}}}" },
+      },
+    },
+  };
+
+  if (!overrides) return base;
+  return { ...base, ...overrides };
+}
+
+describe("Suite 1: Role Reference Integrity", () => {
+  test("1.1 graph references unknown role", () => {
+    const wf = makeWorkflow();
+    wf.graph.nonexistent = { _: { role: "$END", prompt: "done" } };
+    const errors = validateWorkflow(wf);
+    expect(errors.some((e) => e.includes('unknown role "nonexistent"'))).toBe(true);
+  });
+
+  test("1.2 orphan role not in graph", () => {
+    const wf = makeWorkflow();
+    wf.roles.orphan = {
+      description: "Orphan",
+      goal: "Nothing",
+      capabilities: [],
+      procedure: "None",
+      output: "None",
+      frontmatter: {
+        type: "object",
+        properties: { $status: { enum: ["_"] } },
+        required: ["$status"],
+      } as unknown as string,
+    };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) => e.includes('role "orphan" is defined but not referenced in graph')),
+    ).toBe(true);
+  });
+
+  test("1.3 $START in roles", () => {
+    const wf = makeWorkflow();
+    (wf.roles as Record<string, unknown>).$START = {
+      description: "Bad",
+      goal: "Bad",
+      capabilities: [],
+      procedure: "Bad",
+      output: "Bad",
+      frontmatter: { type: "object", properties: {}, required: [] },
+    };
+    const errors = validateWorkflow(wf);
+    expect(errors.some((e) => e.includes('reserved name "$START"'))).toBe(true);
+  });
+
+  test("1.4 $END in roles", () => {
+    const wf = makeWorkflow();
+    (wf.roles as Record<string, unknown>).$END = {
+      description: "Bad",
+      goal: "Bad",
+      capabilities: [],
+      procedure: "Bad",
+      output: "Bad",
+      frontmatter: { type: "object", properties: {}, required: [] },
+    };
+    const errors = validateWorkflow(wf);
+    expect(errors.some((e) => e.includes('reserved name "$END"'))).toBe(true);
+  });
+
+  test("1.5 valid workflow returns no errors", () => {
+    const wf = makeWorkflow();
+    const errors = validateWorkflow(wf);
+    expect(errors).toEqual([]);
+  });
+});
+
+describe("Suite 2: Graph Structure", () => {
+  test("2.1 $START missing from graph", () => {
+    const wf = makeWorkflow();
+    delete wf.graph.$START;
+    const errors = validateWorkflow(wf);
+    expect(errors.some((e) => e.includes("$START must be defined in graph"))).toBe(true);
+  });
+
+  test("2.2 $START has multiple status keys", () => {
+    const wf = makeWorkflow();
+    wf.graph.$START = {
+      _: { role: "writer", prompt: "Begin" },
+      other: { role: "reviewer", prompt: "Also" },
+    };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) => e.includes('$START must have exactly one edge with status "_"')),
+    ).toBe(true);
+  });
+
+  test("2.3 $START edge uses non-_ status", () => {
+    const wf = makeWorkflow();
+    wf.graph.$START = { ready: { role: "writer", prompt: "Begin" } };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) => e.includes('$START must have exactly one edge with status "_"')),
+    ).toBe(true);
+  });
+
+  test("2.4 $END has outgoing edges", () => {
+    const wf = makeWorkflow();
+    wf.graph.$END = { _: { role: "writer", prompt: "Loop" } };
+    const errors = validateWorkflow(wf);
+    expect(errors.some((e) => e.includes("$END must not have outgoing edges"))).toBe(true);
+  });
+
+  test("2.5 unreachable role", () => {
+    const wf = makeWorkflow();
+    wf.roles.isolated = {
+      description: "Isolated",
+      goal: "Isolated",
+      capabilities: [],
+      procedure: "Isolated",
+      output: "Isolated",
+      frontmatter: {
+        type: "object",
+        properties: { $status: { enum: ["_"] } },
+        required: ["$status"],
+      } as unknown as string,
+    };
+    wf.graph.isolated = { _: { role: "$END", prompt: "done" } };
+    const errors = validateWorkflow(wf);
+    expect(errors.some((e) => e.includes('role "isolated" is not reachable from $START'))).toBe(
+      true,
+    );
+  });
+
+  test("2.6 edge target references invalid role", () => {
+    const wf = makeWorkflow();
+    wf.graph.writer = { _: { role: "ghost", prompt: "Go to ghost" } };
+    const errors = validateWorkflow(wf);
+    expect(errors.some((e) => e.includes('unknown target role "ghost"'))).toBe(true);
+  });
+});
+
+describe("Suite 3: Status-Edge Consistency", () => {
+  test("3.1 single-exit role with multiple graph keys", () => {
+    const wf = makeWorkflow();
+    wf.graph.writer = {
+      _: { role: "reviewer", prompt: "Review" },
+      extra: { role: "$END", prompt: "Done" },
+    };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) =>
+        e.includes('role "writer" is single-exit but has status keys other than "_"'),
+      ),
+    ).toBe(true);
+  });
+
+  test("3.2 single-exit role missing _ key", () => {
+    const wf = makeWorkflow();
+    wf.graph.writer = { done: { role: "reviewer", prompt: "Review" } };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) => e.includes('role "writer" is single-exit but graph has no "_" key')),
+    ).toBe(true);
+  });
+
+  test("3.3 multi-exit role with extra statuses", () => {
+    const wf = makeWorkflow();
+    wf.graph.reviewer = {
+      approved: { role: "$END", prompt: "Done" },
+      rejected: { role: "writer", prompt: "Fix" },
+      timeout: { role: "$END", prompt: "Timed out" },
+    };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) => e.includes('role "reviewer" graph has extra status keys: timeout')),
+    ).toBe(true);
+  });
+
+  test("3.4 multi-exit role missing a status", () => {
+    const wf = makeWorkflow();
+    wf.graph.reviewer = {
+      approved: { role: "$END", prompt: "Done" },
+    };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) => e.includes('role "reviewer" graph is missing status keys: rejected')),
+    ).toBe(true);
+  });
+
+  test("3.5 multi-exit role with _ key", () => {
+    const wf = makeWorkflow();
+    wf.graph.reviewer = { _: { role: "$END", prompt: "Done" } };
+    const errors = validateWorkflow(wf);
+    expect(errors.some((e) => e.includes('role "reviewer" is multi-exit but graph uses "_"'))).toBe(
+      true,
+    );
+  });
+});
+
+describe("Suite 4: Mustache Template Variable Existence", () => {
+  test("4.1 prompt references nonexistent variable (single-exit)", () => {
+    const wf = makeWorkflow();
+    wf.graph.writer = { _: { role: "reviewer", prompt: "Review: {{{branch}}}" } };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) =>
+        e.includes('prompt variable "branch" not found in role "writer" frontmatter'),
+      ),
+    ).toBe(true);
+  });
+
+  test("4.2 prompt references nonexistent variable (multi-exit)", () => {
+    const wf = makeWorkflow();
+    wf.graph.reviewer = {
+      approved: { role: "$END", prompt: "Done: {{{branch}}}" },
+      rejected: { role: "writer", prompt: "Fix: {{{reason}}}" },
+    };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) =>
+        e.includes('prompt variable "branch" not found in role "reviewer" variant "approved"'),
+      ),
+    ).toBe(true);
+  });
+
+  test("4.3 valid mustache variables pass", () => {
+    const wf = makeWorkflow();
+    const errors = validateWorkflow(wf);
+    expect(errors).toEqual([]);
+  });
+
+  test("4.4 $status variable is always valid", () => {
+    const wf = makeWorkflow();
+    wf.graph.writer = { _: { role: "reviewer", prompt: "Status: {{$status}}" } };
+    const errors = validateWorkflow(wf);
+    expect(errors).toEqual([]);
+  });
+});
+
+describe("Suite 5: oneOf Discriminant Validity", () => {
+  test("5.1 oneOf without $status const", () => {
+    const wf = makeWorkflow();
+    wf.roles.reviewer = {
+      ...wf.roles.reviewer,
+      frontmatter: {
+        type: "object",
+        oneOf: [
+          { properties: { summary: { type: "string" } }, required: ["summary"] },
+          { properties: { reason: { type: "string" } }, required: ["reason"] },
+        ],
+      } as unknown as string,
+    };
+    const errors = validateWorkflow(wf);
+    expect(
+      errors.some((e) => e.includes('oneOf variants must have "$status" as const discriminant')),
+    ).toBe(true);
+  });
+
+  test("5.2 oneOf with non-const $status", () => {
+    const wf = makeWorkflow();
+    wf.roles.reviewer = {
+      ...wf.roles.reviewer,
+      frontmatter: {
+        type: "object",
+        oneOf: [
+          {
+            properties: { $status: { type: "string" }, summary: { type: "string" } },
+            required: ["$status", "summary"],
+          },
+          {
+            properties: { $status: { type: "string" }, reason: { type: "string" } },
+            required: ["$status", "reason"],
+          },
+        ],
+      } as unknown as string,
+    };
+    const errors = validateWorkflow(wf);
+    expect(errors.some((e) => e.includes("oneOf variant $status must be a const value"))).toBe(
+      true,
+    );
+  });
+
+  test("5.3 valid oneOf passes", () => {
+    const wf = makeWorkflow();
+    const errors = validateWorkflow(wf);
+    expect(errors).toEqual([]);
+  });
+});
+
+describe("Suite 6: Multiple Errors Collection", () => {
+  test("6.1 multiple errors collected", () => {
+    const wf = makeWorkflow();
+    // orphan role
+    wf.roles.orphan = {
+      description: "Orphan",
+      goal: "Nothing",
+      capabilities: [],
+      procedure: "None",
+      output: "None",
+      frontmatter: {
+        type: "object",
+        properties: { $status: { enum: ["_"] } },
+        required: ["$status"],
+      } as unknown as string,
+    };
+    // unknown graph reference
+    wf.graph.nonexistent = { _: { role: "$END", prompt: "done" } };
+    // bad mustache var
+    wf.graph.writer = { _: { role: "reviewer", prompt: "{{{badvar}}}" } };
+    const errors = validateWorkflow(wf);
+    expect(errors.length).toBeGreaterThanOrEqual(3);
+  });
+});

packages/cli-workflow/src/__tests__/workflow-resolution.test.ts

@@ -20,23 +20,37 @@ async function makeUwfStore(storageRoot: string): Promise<UwfStore> {
   return { storageRoot, store, schemas };
 }
 
-async function storeWorkflow(uwf: UwfStore, name: string): Promise<CasRef> {
-  const payload: WorkflowPayload = {
+function makeMinimalPayload(name: string, description: string): WorkflowPayload {
+  return {
     name,
-    description: "Test workflow",
-    roles: {},
-    graph: {},
+    description,
+    roles: {
+      worker: {
+        description: "worker role",
+        goal: "do work",
+        capabilities: [],
+        procedure: "",
+        output: "",
+        frontmatter: { type: "0000000000000" } as unknown as CasRef,
+      },
+    },
+    graph: {
+      $START: { _: { role: "worker", prompt: "start working" } },
+      worker: { _: { role: "$END", prompt: "done" } },
+    },
   };
+}
+
+async function storeWorkflow(uwf: UwfStore, name: string): Promise<CasRef> {
+  const payload = makeMinimalPayload(name, "Test workflow");
   return await uwf.store.put(uwf.schemas.workflow, payload);
 }
 
 async function createWorkflowYaml(name: string, version: string | null = null): Promise<string> {
-  const payload: WorkflowPayload = {
+  const payload = makeMinimalPayload(
     name,
-    description: version !== null ? `Test workflow (${version})` : "Test workflow",
-    roles: {},
-    graph: {},
-  };
+    version !== null ? `Test workflow (${version})` : "Test workflow",
+  );
   const yaml = stringify(payload);
   return yaml;
 }

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

@@ -40,6 +40,7 @@ import {
   type UwfStore,
 } from "../store.js";
 import { checkWorkflowFilenameConsistency, isCasRef, parseWorkflowPayload } from "../validate.js";
+import { validateWorkflow } from "../validate-semantic.js";
 import {
   type ChainState,
   collectOrderedSteps,
@@ -169,6 +170,11 @@ async function materializeLocalWorkflow(uwf: UwfStore, filePath: string): Promis
     fail(filenameError);
   }
 
+  const semanticErrors = validateWorkflow(payload);
+  if (semanticErrors.length > 0) {
+    fail(`workflow validation failed:\n${semanticErrors.map((e) => `  - ${e}`).join("\n")}`);
+  }
+
   const materialized = await materializeWorkflowPayload(uwf, payload);
   const hash = await uwf.store.put(uwf.schemas.workflow, materialized);
   const stored = uwf.store.get(hash);

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

@@ -15,6 +15,7 @@ import {
   type UwfStore,
 } from "../store.js";
 import { checkWorkflowFilenameConsistency, parseWorkflowPayload } from "../validate.js";
+import { validateWorkflow } from "../validate-semantic.js";
 
 export type WorkflowOrigin = "local" | "global";
 
@@ -136,6 +137,11 @@ export async function cmdWorkflowAdd(
     fail(filenameError);
   }
 
+  const semanticErrors = validateWorkflow(payload);
+  if (semanticErrors.length > 0) {
+    fail(`workflow validation failed:\n${semanticErrors.map((e) => `  - ${e}`).join("\n")}`);
+  }
+
   const uwf = await createUwfStore(storageRoot);
   const materialized = await materializeWorkflowPayload(uwf, payload);
 

packages/cli-workflow/src/validate-semantic.ts

@@ -0,0 +1,278 @@
+import type { WorkflowPayload } from "@uncaged/workflow-protocol";
+
+type SchemaObj = Record<string, unknown>;
+
+const RESERVED_NAMES = new Set(["$START", "$END"]);
+
+/** Extract mustache variable names from a prompt string. */
+function extractMustacheVars(prompt: string): string[] {
+  const vars: string[] = [];
+  const re = /\{\{\{?([^}]+)\}\}\}?/g;
+  let m: RegExpExecArray | null = re.exec(prompt);
+  while (m !== null) {
+    vars.push(m[1]);
+    m = re.exec(prompt);
+  }
+  return vars;
+}
+
+/** Check if a frontmatter schema is a oneOf (multi-exit) type. */
+function isOneOfSchema(fm: unknown): fm is SchemaObj & { oneOf: SchemaObj[] } {
+  if (typeof fm !== "object" || fm === null) return false;
+  const obj = fm as SchemaObj;
+  return Array.isArray(obj.oneOf);
+}
+
+/** Get property names from a schema object. */
+function getPropertyNames(schema: SchemaObj): Set<string> {
+  const props = schema.properties;
+  if (typeof props !== "object" || props === null) return new Set();
+  return new Set(Object.keys(props as Record<string, unknown>));
+}
+
+/** Extract $status const values from oneOf variants. */
+function getOneOfStatuses(variants: SchemaObj[]): string[] {
+  const statuses: string[] = [];
+  for (const variant of variants) {
+    const props = variant.properties as Record<string, SchemaObj> | undefined;
+    if (props?.$status) {
+      const statusDef = props.$status;
+      if (typeof statusDef.const === "string") {
+        statuses.push(statusDef.const);
+      }
+    }
+  }
+  return statuses;
+}
+
+/** Check reserved names and role/graph reference integrity. */
+function checkRoleReferences(payload: WorkflowPayload, errors: string[]): void {
+  const roleNames = new Set(Object.keys(payload.roles));
+  const graphNodes = new Set(Object.keys(payload.graph));
+
+  for (const name of roleNames) {
+    if (RESERVED_NAMES.has(name)) {
+      errors.push(`reserved name "${name}" must not appear in roles`);
+    }
+  }
+
+  for (const node of graphNodes) {
+    if (!RESERVED_NAMES.has(node) && !roleNames.has(node)) {
+      errors.push(`graph references unknown role "${node}"`);
+    }
+  }
+
+  for (const name of roleNames) {
+    if (RESERVED_NAMES.has(name)) continue;
+    if (!graphNodes.has(name)) {
+      errors.push(`role "${name}" is defined but not referenced in graph`);
+    }
+  }
+}
+
+/** Check $START/$END constraints, edge targets, and reachability. */
+function checkGraphStructure(payload: WorkflowPayload, errors: string[]): void {
+  const roleNames = new Set(Object.keys(payload.roles));
+  const graphNodes = new Set(Object.keys(payload.graph));
+
+  if (!graphNodes.has("$START")) {
+    errors.push("$START must be defined in graph");
+  } else {
+    const startKeys = Object.keys(payload.graph.$START);
+    if (startKeys.length !== 1 || startKeys[0] !== "_") {
+      errors.push('$START must have exactly one edge with status "_"');
+    }
+  }
+
+  if (graphNodes.has("$END")) {
+    errors.push("$END must not have outgoing edges");
+  }
+
+  for (const [node, statusMap] of Object.entries(payload.graph)) {
+    for (const [status, target] of Object.entries(statusMap)) {
+      if (target.role !== "$END" && !roleNames.has(target.role)) {
+        errors.push(`edge ${node}→${status}: unknown target role "${target.role}"`);
+      }
+    }
+  }
+
+  checkReachability(roleNames, collectReachableRoles(payload.graph), errors);
+}
+
+/** BFS to collect all roles reachable from $START. */
+function collectReachableRoles(graph: WorkflowPayload["graph"]): Set<string> {
+  const reachable = new Set<string>();
+  const startEdges = graph.$START;
+  if (!startEdges) return reachable;
+
+  const queue: string[] = [];
+  for (const target of Object.values(startEdges)) {
+    if (target.role !== "$END" && !reachable.has(target.role)) {
+      reachable.add(target.role);
+      queue.push(target.role);
+    }
+  }
+
+  while (queue.length > 0) {
+    const current = queue.shift() as string;
+    const edges = graph[current];
+    if (!edges) continue;
+    for (const target of Object.values(edges)) {
+      if (target.role !== "$END" && !reachable.has(target.role)) {
+        reachable.add(target.role);
+        queue.push(target.role);
+      }
+    }
+  }
+
+  return reachable;
+}
+
+/** Check that all defined roles are reachable from $START. */
+function checkReachability(roleNames: Set<string>, reachable: Set<string>, errors: string[]): void {
+  for (const name of roleNames) {
+    if (RESERVED_NAMES.has(name)) continue;
+    if (!reachable.has(name)) {
+      errors.push(`role "${name}" is not reachable from $START`);
+    }
+  }
+}
+
+/** Check oneOf discriminant validity for a role. */
+function checkOneOfDiscriminant(
+  roleName: string,
+  variants: SchemaObj[],
+  statuses: string[],
+  errors: string[],
+): void {
+  if (statuses.length === variants.length) return;
+
+  let foundMissing = false;
+  for (const variant of variants) {
+    const props = variant.properties as Record<string, SchemaObj> | undefined;
+    if (!props?.$status) {
+      errors.push(`role "${roleName}": oneOf variants must have "$status" as const discriminant`);
+      foundMissing = true;
+      break;
+    }
+    if (typeof props.$status.const !== "string") {
+      errors.push(`role "${roleName}": oneOf variant $status must be a const value`);
+      foundMissing = true;
+      break;
+    }
+  }
+
+  if (!foundMissing) {
+    errors.push(`role "${roleName}": oneOf variant $status must be a const value`);
+  }
+}
+
+/** Check status-edge consistency for a multi-exit role. */
+function checkMultiExitEdges(
+  roleName: string,
+  graphKeys: Set<string>,
+  statusSet: Set<string>,
+  errors: string[],
+): void {
+  if (graphKeys.has("_")) {
+    errors.push(`role "${roleName}" is multi-exit but graph uses "_"`);
+    return;
+  }
+
+  const extraKeys = [...graphKeys].filter((k) => !statusSet.has(k));
+  const missingKeys = [...statusSet].filter((k) => !graphKeys.has(k));
+  if (extraKeys.length > 0) {
+    errors.push(`role "${roleName}" graph has extra status keys: ${extraKeys.join(", ")}`);
+  }
+  if (missingKeys.length > 0) {
+    errors.push(`role "${roleName}" graph is missing status keys: ${missingKeys.join(", ")}`);
+  }
+}
+
+/** Check mustache variables for multi-exit role. */
+function checkMultiExitMustache(
+  roleName: string,
+  graphEntry: Record<string, { role: string; prompt: string }>,
+  variants: SchemaObj[],
+  errors: string[],
+): void {
+  for (const [status, target] of Object.entries(graphEntry)) {
+    const vars = extractMustacheVars(target.prompt);
+    const variant = variants.find((v) => {
+      const props = v.properties as Record<string, SchemaObj> | undefined;
+      return props?.$status?.const === status;
+    });
+    if (!variant) continue;
+    const propNames = getPropertyNames(variant);
+    for (const v of vars) {
+      if (v === "$status") continue;
+      if (!propNames.has(v)) {
+        errors.push(`prompt variable "${v}" not found in role "${roleName}" variant "${status}"`);
+      }
+    }
+  }
+}
+
+/** Check status-edge consistency and mustache for each role. */
+function checkRoleConsistency(payload: WorkflowPayload, errors: string[]): void {
+  for (const [roleName, role] of Object.entries(payload.roles)) {
+    if (RESERVED_NAMES.has(roleName)) continue;
+    const graphEntry = payload.graph[roleName];
+    if (!graphEntry) continue;
+
+    const fm = role.frontmatter as unknown;
+    const graphKeys = new Set(Object.keys(graphEntry));
+
+    if (isOneOfSchema(fm)) {
+      const variants = fm.oneOf as SchemaObj[];
+      const statuses = getOneOfStatuses(variants);
+
+      checkOneOfDiscriminant(roleName, variants, statuses, errors);
+      checkMultiExitEdges(roleName, graphKeys, new Set(statuses), errors);
+      checkMultiExitMustache(roleName, graphEntry, variants, errors);
+    } else {
+      checkSingleExitRole(roleName, graphKeys, graphEntry, fm as SchemaObj | null, errors);
+    }
+  }
+}
+
+/** Check single-exit role status and mustache. */
+function checkSingleExitRole(
+  roleName: string,
+  graphKeys: Set<string>,
+  graphEntry: Record<string, { role: string; prompt: string }>,
+  fm: SchemaObj | null,
+  errors: string[],
+): void {
+  if (graphKeys.size > 1 || (graphKeys.size === 1 && !graphKeys.has("_"))) {
+    if (!graphKeys.has("_")) {
+      errors.push(`role "${roleName}" is single-exit but graph has no "_" key`);
+    } else {
+      errors.push(`role "${roleName}" is single-exit but has status keys other than "_"`);
+    }
+  }
+
+  const singleTarget = graphEntry._;
+  if (!singleTarget) return;
+
+  const vars = extractMustacheVars(singleTarget.prompt);
+  const propNames = fm ? getPropertyNames(fm) : new Set<string>();
+  for (const v of vars) {
+    if (v === "$status") continue;
+    if (!propNames.has(v)) {
+      errors.push(`prompt variable "${v}" not found in role "${roleName}" frontmatter`);
+    }
+  }
+}
+
+/**
+ * Validate a parsed WorkflowPayload for semantic correctness.
+ * Returns an array of error messages. Empty array = valid.
+ */
+export function validateWorkflow(payload: WorkflowPayload): string[] {
+  const errors: string[] = [];
+  checkRoleReferences(payload, errors);
+  checkGraphStructure(payload, errors);
+  checkRoleConsistency(payload, errors);
+  return errors;
+}