diff --git a/packages/workflow-execute/__tests__/json-cas-engine.test.ts b/packages/workflow-execute/__tests__/json-cas-engine.test.ts
new file mode 100644
index 0000000..e904aa7
--- /dev/null
+++ b/packages/workflow-execute/__tests__/json-cas-engine.test.ts
@@ -0,0 +1,869 @@
+import { describe, expect, test } from "bun:test";
+import { createMemoryStore, type Store, walk } from "@uncaged/json-cas";
+import {
+  registerWorkflowSchemas,
+  type WorkflowSchemaHashes,
+  type ContentPayload,
+  type ThreadEndPayload,
+  type ThreadStartPayload,
+  type ThreadStepPayload,
+} from "@uncaged/json-cas-workflow";
+import { registerWorkflow, type WorkflowInput } from "@uncaged/workflow-json-def";
+
+import {
+  buildJsonCasThreadContext,
+  buildJsonCasThreadSnapshot,
+  readContentText,
+} from "../src/engine/json-cas-context.js";
+import { executeJsonCasThread } from "../src/engine/json-cas-engine.js";
+import type {
+  JsonCasAgentFn,
+  JsonCasEngineIo,
+  JsonCasEngineOptions,
+} from "../src/engine/json-cas-types.js";
+
+// ── Test fixtures ─────────────────────────────────────────────────────
+
+const START = "__start__";
+const END = "__end__";
+
+const SIMPLE_WORKFLOW: WorkflowInput = {
+  name: "test-simple",
+  description: "A simple two-role workflow for testing",
+  roles: {
+    planner: {
+      description: "Plans the work",
+      systemPrompt: "You are a planner.",
+      extractPrompt: "Extract planner output.",
+      schema: {
+        type: "object",
+        required: ["plan"],
+        properties: { plan: { type: "string" } },
+      },
+    },
+    coder: {
+      description: "Implements the plan",
+      systemPrompt: "You are a coder.",
+      extractPrompt: "Extract coder output.",
+      schema: {
+        type: "object",
+        required: ["code"],
+        properties: { code: { type: "string" } },
+      },
+    },
+  },
+  moderator: [
+    { from: START, to: "planner", when: null },
+    { from: "planner", to: "coder", when: null },
+    { from: "coder", to: END, when: null },
+  ],
+};
+
+const SINGLE_ROLE_WORKFLOW: WorkflowInput = {
+  name: "test-single",
+  description: "A single-role workflow",
+  roles: {
+    worker: {
+      description: "Does all the work",
+      systemPrompt: "You are a worker.",
+      extractPrompt: "Extract worker output.",
+      schema: {
+        type: "object",
+        required: ["result"],
+        properties: { result: { type: "string" } },
+      },
+    },
+  },
+  moderator: [
+    { from: START, to: "worker", when: null },
+    { from: "worker", to: END, when: null },
+  ],
+};
+
+const CONDITIONAL_WORKFLOW: WorkflowInput = {
+  name: "test-conditional",
+  description: "A workflow with JSONata conditions",
+  roles: {
+    checker: {
+      description: "Checks the input",
+      systemPrompt: "You are a checker.",
+      extractPrompt: "Extract checker output.",
+      schema: {
+        type: "object",
+        required: ["status"],
+        properties: { status: { type: "string" } },
+      },
+    },
+    fixer: {
+      description: "Fixes issues",
+      systemPrompt: "You are a fixer.",
+      extractPrompt: "Extract fixer output.",
+      schema: {
+        type: "object",
+        required: ["fix"],
+        properties: { fix: { type: "string" } },
+      },
+    },
+  },
+  moderator: [
+    { from: START, to: "checker", when: null },
+    { from: "checker", to: END, when: "steps[-1].meta.status = 'ok'" },
+    { from: "checker", to: "fixer", when: null },
+    { from: "fixer", to: "checker", when: null },
+  ],
+};
+
+function noLogger(): (tag: string, content: string) => void {
+  return () => {};
+}
+
+async function setupStore(): Promise<{
+  store: Store;
+  typeHashes: WorkflowSchemaHashes;
+}> {
+  const store = createMemoryStore();
+  const typeHashes = await registerWorkflowSchemas(store);
+  return { store, typeHashes };
+}
+
+async function setupWorkflow(
+  store: Store,
+  typeHashes: WorkflowSchemaHashes,
+  workflowDef: WorkflowInput,
+) {
+  const workflowHash = await registerWorkflow(store, typeHashes, workflowDef);
+  return { workflowHash };
+}
+
+function makeOptions(overrides: Partial<JsonCasEngineOptions> = {}): JsonCasEngineOptions {
+  return {
+    depth: 0,
+    parentThread: null,
+    signal: new AbortController().signal,
+    agents: {},
+    ...overrides,
+  };
+}
+
+function makeIo(
+  store: Store,
+  typeHashes: WorkflowSchemaHashes,
+  threadId: string,
+): JsonCasEngineIo {
+  return { threadId, store, typeHashes };
+}
+
+/**
+ * A mock agent that returns a canned text and meta for each role.
+ */
+function createMockAgent(
+  responses: Record<string, { text: string; meta: Record<string, unknown> }>,
+): JsonCasAgentFn {
+  return async (role, _systemPrompt, _snapshot) => {
+    const resp = responses[role];
+    if (resp === undefined) {
+      throw new Error(`mock agent: no response configured for role "${role}"`);
+    }
+    return resp;
+  };
+}
+
+// ── Tests ─────────────────────────────────────────────────────────────
+
+describe("executeJsonCasThread", () => {
+  describe("thread lifecycle", () => {
+    test("simple two-role workflow creates start, two steps, and end nodes", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SIMPLE_WORKFLOW);
+
+      const agentFn = createMockAgent({
+        planner: { text: "I will plan", meta: { plan: "phase-1" } },
+        coder: { text: "I wrote code", meta: { code: "done" } },
+      });
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "Build a widget",
+        moderatorRules: SIMPLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD01"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      expect(result.returnCode).toBe(0);
+      expect(result.summary).toContain("END");
+      expect(result.rootHash).toBeTruthy();
+
+      const endNode = store.get(result.rootHash);
+      expect(endNode).not.toBeNull();
+      const endPayload = endNode!.payload as ThreadEndPayload;
+      expect(endPayload.returnCode).toBe(0);
+      expect(endPayload.start).toBeTruthy();
+      expect(endPayload.lastStep).toBeTruthy();
+    });
+
+    test("single-role workflow creates correct chain", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SINGLE_ROLE_WORKFLOW);
+
+      const agentFn = createMockAgent({
+        worker: { text: "work done", meta: { result: "success" } },
+      });
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "Do the thing",
+        moderatorRules: SINGLE_ROLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD02"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      expect(result.returnCode).toBe(0);
+
+      const endNode = store.get(result.rootHash);
+      expect(endNode).not.toBeNull();
+      const endPayload = endNode!.payload as ThreadEndPayload;
+
+      const lastStepNode = store.get(endPayload.lastStep);
+      expect(lastStepNode).not.toBeNull();
+      const lastStepPayload = lastStepNode!.payload as ThreadStepPayload;
+      expect(lastStepPayload.role).toBe("worker");
+      expect(lastStepPayload.previous).toBeNull();
+
+      const startNode = store.get(endPayload.start);
+      expect(startNode).not.toBeNull();
+      const startPayload = startNode!.payload as ThreadStartPayload;
+      expect(startPayload.input).toBe("Do the thing");
+      expect(startPayload.depth).toBe(0);
+    });
+  });
+
+  describe("CAS node structure", () => {
+    test("thread-start contains workflow ref, input, depth, agents", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SINGLE_ROLE_WORKFLOW);
+
+      const agentFn = createMockAgent({
+        worker: { text: "ok", meta: { result: "ok" } },
+      });
+
+      const agentHash = await store.put(typeHashes.agent, {
+        package: "test-agent",
+        version: "1.0.0",
+        config: {},
+      });
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "Test input",
+        moderatorRules: SINGLE_ROLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD03"),
+        options: makeOptions({ agents: { worker: agentHash }, depth: 2 }),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+      const startPayload = store.get(endPayload.start)!.payload as ThreadStartPayload;
+
+      expect(startPayload.workflow).toBe(workflowHash);
+      expect(startPayload.input).toBe("Test input");
+      expect(startPayload.depth).toBe(2);
+      expect(startPayload.parentThread).toBeNull();
+      expect(startPayload.agents).toEqual({ worker: agentHash });
+    });
+
+    test("thread-start records parentThread when provided", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SINGLE_ROLE_WORKFLOW);
+
+      const agentFn = createMockAgent({
+        worker: { text: "nested", meta: { result: "nested" } },
+      });
+
+      const fakeParent = "FAKEPARENT0001";
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "nested task",
+        moderatorRules: SINGLE_ROLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD04"),
+        options: makeOptions({ parentThread: fakeParent, depth: 1 }),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+      const startPayload = store.get(endPayload.start)!.payload as ThreadStartPayload;
+      expect(startPayload.parentThread).toBe(fakeParent);
+      expect(startPayload.depth).toBe(1);
+    });
+
+    test("each thread-step has content, react, start, and previous refs", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SIMPLE_WORKFLOW);
+
+      const agentFn = createMockAgent({
+        planner: { text: "plan text", meta: { plan: "p1" } },
+        coder: { text: "code text", meta: { code: "c1" } },
+      });
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "go",
+        moderatorRules: SIMPLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD05"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+      const startHash = endPayload.start;
+
+      const step2 = store.get(endPayload.lastStep)!.payload as ThreadStepPayload;
+      expect(step2.role).toBe("coder");
+      expect(step2.start).toBe(startHash);
+      expect(step2.previous).not.toBeNull();
+
+      const contentNode2 = store.get(step2.content);
+      expect(contentNode2).not.toBeNull();
+      expect((contentNode2!.payload as ContentPayload).text).toBe("code text");
+
+      const reactNode2 = store.get(step2.react);
+      expect(reactNode2).not.toBeNull();
+
+      const step1 = store.get(step2.previous!)!.payload as ThreadStepPayload;
+      expect(step1.role).toBe("planner");
+      expect(step1.start).toBe(startHash);
+      expect(step1.previous).toBeNull();
+
+      const contentNode1 = store.get(step1.content);
+      expect(contentNode1).not.toBeNull();
+      expect((contentNode1!.payload as ContentPayload).text).toBe("plan text");
+    });
+
+    test("thread-end references start and last step", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SIMPLE_WORKFLOW);
+
+      const agentFn = createMockAgent({
+        planner: { text: "plan", meta: { plan: "x" } },
+        coder: { text: "code", meta: { code: "x" } },
+      });
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "test",
+        moderatorRules: SIMPLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD06"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+      expect(endPayload.returnCode).toBe(0);
+      expect(endPayload.summary).toBeTruthy();
+
+      const startNode = store.get(endPayload.start);
+      expect(startNode).not.toBeNull();
+      expect((startNode!.payload as ThreadStartPayload).workflow).toBe(workflowHash);
+
+      const lastStepNode = store.get(endPayload.lastStep);
+      expect(lastStepNode).not.toBeNull();
+      expect((lastStepNode!.payload as ThreadStepPayload).role).toBe("coder");
+    });
+
+    test("content nodes store the agent text verbatim", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SINGLE_ROLE_WORKFLOW);
+
+      const longText = "This is a longer text with\nnewlines\nand special chars: <>&\"'";
+
+      const agentFn = createMockAgent({
+        worker: { text: longText, meta: { result: "done" } },
+      });
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "process this",
+        moderatorRules: SINGLE_ROLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD07"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+      const stepPayload = store.get(endPayload.lastStep)!.payload as ThreadStepPayload;
+      const contentPayload = store.get(stepPayload.content)!.payload as ContentPayload;
+      expect(contentPayload.text).toBe(longText);
+    });
+
+    test("meta is stored in thread-step payload", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SIMPLE_WORKFLOW);
+
+      const complexMeta = {
+        plan: "phase-1",
+        phases: [{ hash: "abc", title: "first" }],
+        nested: { deep: true },
+      };
+
+      const agentFn = createMockAgent({
+        planner: { text: "plan", meta: complexMeta },
+        coder: { text: "code", meta: { code: "done" } },
+      });
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "go",
+        moderatorRules: SIMPLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD08"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+      const step2 = store.get(endPayload.lastStep)!.payload as ThreadStepPayload;
+      const step1 = store.get(step2.previous!)!.payload as ThreadStepPayload;
+
+      expect(step1.meta).toEqual(complexMeta);
+      expect(step2.meta).toEqual({ code: "done" });
+    });
+  });
+
+  describe("moderator routing", () => {
+    test("conditional moderator routes based on agent meta", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, CONDITIONAL_WORKFLOW);
+
+      let checkerCallCount = 0;
+      const agentFn: JsonCasAgentFn = async (role, _sp, _snap) => {
+        if (role === "checker") {
+          checkerCallCount++;
+          if (checkerCallCount === 1) {
+            return { text: "found issue", meta: { status: "bad" } };
+          }
+          return { text: "all good now", meta: { status: "ok" } };
+        }
+        return { text: "fixed it", meta: { fix: "patched" } };
+      };
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "check and fix",
+        moderatorRules: CONDITIONAL_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD09"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      expect(result.returnCode).toBe(0);
+      expect(checkerCallCount).toBe(2);
+
+      const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+      const lastStep = store.get(endPayload.lastStep)!.payload as ThreadStepPayload;
+      expect(lastStep.role).toBe("checker");
+
+      const step2 = store.get(lastStep.previous!)!.payload as ThreadStepPayload;
+      expect(step2.role).toBe("fixer");
+
+      const step1 = store.get(step2.previous!)!.payload as ThreadStepPayload;
+      expect(step1.role).toBe("checker");
+      expect(step1.previous).toBeNull();
+    });
+
+    test("immediate END from moderator still produces a valid thread", async () => {
+      const { store, typeHashes } = await setupStore();
+
+      const immediateEnd: WorkflowInput = {
+        name: "test-immediate-end",
+        description: "Ends immediately",
+        roles: {
+          worker: {
+            description: "Never called",
+            systemPrompt: "N/A",
+            extractPrompt: "N/A",
+            schema: { type: "object" },
+          },
+        },
+        moderator: [{ from: START, to: END, when: null }],
+      };
+
+      const { workflowHash } = await setupWorkflow(store, typeHashes, immediateEnd);
+
+      const agentFn: JsonCasAgentFn = async () => {
+        throw new Error("should not be called");
+      };
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "skip",
+        moderatorRules: immediateEnd.moderator,
+        io: makeIo(store, typeHashes, "THREAD10"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      expect(result.returnCode).toBe(0);
+
+      const endNode = store.get(result.rootHash);
+      expect(endNode).not.toBeNull();
+      const endPayload = endNode!.payload as ThreadEndPayload;
+      expect(endPayload.start).toBeTruthy();
+      expect(endPayload.lastStep).toBeTruthy();
+    });
+  });
+
+  describe("abort handling", () => {
+    test("aborted signal produces returnCode 130", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SINGLE_ROLE_WORKFLOW);
+
+      const ac = new AbortController();
+      ac.abort();
+
+      const agentFn: JsonCasAgentFn = async () => {
+        throw new Error("should not be called");
+      };
+
+      const result = await executeJsonCasThread({
+        workflowHash,
+        input: "will abort",
+        moderatorRules: SINGLE_ROLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD11"),
+        options: makeOptions({ signal: ac.signal }),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      expect(result.returnCode).toBe(130);
+      expect(result.summary).toContain("abort");
+
+      const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+      expect(endPayload.returnCode).toBe(130);
+    });
+  });
+
+  describe("agent receives correct context", () => {
+    test("agent receives role name, system prompt, and accumulated steps", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, SIMPLE_WORKFLOW);
+
+      const { loadWorkflow } = await import("@uncaged/workflow-json-def");
+      const hydrated = loadWorkflow(store, typeHashes, workflowHash);
+
+      const receivedCalls: Array<{
+        role: string;
+        systemPrompt: string;
+        stepCount: number;
+        input: string;
+      }> = [];
+
+      const agentFn: JsonCasAgentFn = async (role, systemPrompt, snapshot) => {
+        receivedCalls.push({
+          role,
+          systemPrompt,
+          stepCount: snapshot.steps.length,
+          input: snapshot.start.input,
+        });
+        return { text: `output for ${role}`, meta: {} };
+      };
+
+      await executeJsonCasThread({
+        workflowHash,
+        input: "my prompt",
+        moderatorRules: SIMPLE_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD12"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: hydrated,
+      });
+
+      expect(receivedCalls.length).toBe(2);
+
+      expect(receivedCalls[0]!.role).toBe("planner");
+      expect(receivedCalls[0]!.systemPrompt).toBe("You are a planner.");
+      expect(receivedCalls[0]!.stepCount).toBe(0);
+      expect(receivedCalls[0]!.input).toBe("my prompt");
+
+      expect(receivedCalls[1]!.role).toBe("coder");
+      expect(receivedCalls[1]!.systemPrompt).toBe("You are a coder.");
+      expect(receivedCalls[1]!.stepCount).toBe(1);
+    });
+
+    test("snapshot accumulates step meta from previous rounds", async () => {
+      const { store, typeHashes } = await setupStore();
+      const { workflowHash } = await setupWorkflow(store, typeHashes, CONDITIONAL_WORKFLOW);
+
+      let round = 0;
+      const snapshots: Array<{ role: string; steps: readonly { role: string; meta: Record<string, unknown> }[] }> = [];
+
+      const agentFn: JsonCasAgentFn = async (role, _sp, snapshot) => {
+        snapshots.push({ role, steps: [...snapshot.steps] });
+        round++;
+        if (role === "checker") {
+          return round === 1
+            ? { text: "bad", meta: { status: "bad" } }
+            : { text: "ok", meta: { status: "ok" } };
+        }
+        return { text: "fixed", meta: { fix: "yes" } };
+      };
+
+      await executeJsonCasThread({
+        workflowHash,
+        input: "go",
+        moderatorRules: CONDITIONAL_WORKFLOW.moderator,
+        io: makeIo(store, typeHashes, "THREAD13"),
+        options: makeOptions(),
+        agentFn,
+        logger: noLogger(),
+        workflow: null,
+      });
+
+      expect(snapshots.length).toBe(3);
+
+      expect(snapshots[0]!.steps.length).toBe(0);
+
+      expect(snapshots[1]!.steps.length).toBe(1);
+      expect(snapshots[1]!.steps[0]!.role).toBe("checker");
+      expect(snapshots[1]!.steps[0]!.meta).toEqual({ status: "bad" });
+
+      expect(snapshots[2]!.steps.length).toBe(2);
+      expect(snapshots[2]!.steps[0]!.role).toBe("checker");
+      expect(snapshots[2]!.steps[1]!.role).toBe("fixer");
+    });
+  });
+});
+
+describe("buildJsonCasThreadSnapshot", () => {
+  test("builds snapshot from start + step chain", async () => {
+    const { store, typeHashes } = await setupStore();
+    const { workflowHash } = await setupWorkflow(store, typeHashes, SIMPLE_WORKFLOW);
+
+    const agentFn = createMockAgent({
+      planner: { text: "plan text", meta: { plan: "alpha" } },
+      coder: { text: "code text", meta: { code: "beta" } },
+    });
+
+    const result = await executeJsonCasThread({
+      workflowHash,
+      input: "build it",
+      moderatorRules: SIMPLE_WORKFLOW.moderator,
+      io: makeIo(store, typeHashes, "THREAD_SNAP"),
+      options: makeOptions(),
+      agentFn,
+      logger: noLogger(),
+      workflow: null,
+    });
+
+    const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+    const startHash = endPayload.start;
+    const lastStepHash = endPayload.lastStep;
+
+    const snapshot = buildJsonCasThreadSnapshot(
+      store, typeHashes, startHash, lastStepHash, "THREAD_SNAP",
+    );
+
+    expect(snapshot.threadId).toBe("THREAD_SNAP");
+    expect(snapshot.start.input).toBe("build it");
+    expect(snapshot.start.workflowHash).toBe(workflowHash);
+    expect(snapshot.steps.length).toBe(2);
+    expect(snapshot.steps[0]!.role).toBe("planner");
+    expect(snapshot.steps[0]!.meta).toEqual({ plan: "alpha" });
+    expect(snapshot.steps[1]!.role).toBe("coder");
+    expect(snapshot.steps[1]!.meta).toEqual({ code: "beta" });
+  });
+
+  test("builds snapshot with null headStepHash (start only)", async () => {
+    const { store, typeHashes } = await setupStore();
+    const { workflowHash } = await setupWorkflow(store, typeHashes, SINGLE_ROLE_WORKFLOW);
+
+    const startHash = await store.put(typeHashes.threadStart, {
+      workflow: workflowHash,
+      input: "just started",
+      depth: 0,
+      parentThread: null,
+      agents: {},
+    });
+
+    const snapshot = buildJsonCasThreadSnapshot(
+      store, typeHashes, startHash, null, "THREAD_SNAP2",
+    );
+
+    expect(snapshot.threadId).toBe("THREAD_SNAP2");
+    expect(snapshot.start.input).toBe("just started");
+    expect(snapshot.steps.length).toBe(0);
+  });
+});
+
+describe("buildJsonCasThreadContext", () => {
+  test("builds a protocol-compatible ThreadContext", async () => {
+    const { store, typeHashes } = await setupStore();
+    const { workflowHash } = await setupWorkflow(store, typeHashes, SIMPLE_WORKFLOW);
+
+    const agentFn = createMockAgent({
+      planner: { text: "plan text", meta: { plan: "ctx-test" } },
+      coder: { text: "code text", meta: { code: "ctx-done" } },
+    });
+
+    const result = await executeJsonCasThread({
+      workflowHash,
+      input: "context test",
+      moderatorRules: SIMPLE_WORKFLOW.moderator,
+      io: makeIo(store, typeHashes, "THREAD_CTX"),
+      options: makeOptions({ depth: 3 }),
+      agentFn,
+      logger: noLogger(),
+      workflow: null,
+    });
+
+    const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+    const ctx = buildJsonCasThreadContext(
+      store, typeHashes, endPayload.start, endPayload.lastStep,
+    );
+
+    expect(ctx.threadId).toBe("");
+    expect(ctx.depth).toBe(3);
+    expect(ctx.bundleHash).toBe(workflowHash);
+    expect(ctx.start.role).toBe("__start__");
+    expect(ctx.start.content).toBe("context test");
+    expect(ctx.steps.length).toBe(2);
+    expect(ctx.steps[0]!.role).toBe("planner");
+    expect(ctx.steps[0]!.meta).toEqual({ plan: "ctx-test" });
+    expect(ctx.steps[1]!.role).toBe("coder");
+    expect(ctx.steps[1]!.meta).toEqual({ code: "ctx-done" });
+  });
+
+  test("context from start-only thread has empty steps", async () => {
+    const { store, typeHashes } = await setupStore();
+    const { workflowHash } = await setupWorkflow(store, typeHashes, SINGLE_ROLE_WORKFLOW);
+
+    const startHash = await store.put(typeHashes.threadStart, {
+      workflow: workflowHash,
+      input: "start only",
+      depth: 0,
+      parentThread: null,
+      agents: {},
+    });
+
+    const ctx = buildJsonCasThreadContext(store, typeHashes, startHash, null);
+
+    expect(ctx.start.content).toBe("start only");
+    expect(ctx.steps.length).toBe(0);
+  });
+});
+
+describe("readContentText", () => {
+  test("reads text from a content node", async () => {
+    const { store, typeHashes } = await setupStore();
+    const hash = await store.put(typeHashes.content, { text: "hello world" });
+
+    const text = readContentText(store, hash);
+    expect(text).toBe("hello world");
+  });
+
+  test("returns null for missing hash", async () => {
+    const { store } = await setupStore();
+    const text = readContentText(store, "NONEXISTENT0001");
+    expect(text).toBeNull();
+  });
+});
+
+describe("CAS graph integrity", () => {
+  test("all nodes are reachable via walk from thread-end", async () => {
+    const { store, typeHashes } = await setupStore();
+    const { workflowHash } = await setupWorkflow(store, typeHashes, SIMPLE_WORKFLOW);
+
+    const agentFn = createMockAgent({
+      planner: { text: "plan", meta: { plan: "x" } },
+      coder: { text: "code", meta: { code: "y" } },
+    });
+
+    const result = await executeJsonCasThread({
+      workflowHash,
+      input: "walk test",
+      moderatorRules: SIMPLE_WORKFLOW.moderator,
+      io: makeIo(store, typeHashes, "THREAD_WALK"),
+      options: makeOptions(),
+      agentFn,
+      logger: noLogger(),
+      workflow: null,
+    });
+
+    const visited = new Set<string>();
+    walk(store, result.rootHash, (hash) => {
+      visited.add(hash);
+    });
+
+    expect(visited.has(result.rootHash)).toBe(true);
+
+    const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+    expect(visited.has(endPayload.start)).toBe(true);
+    expect(visited.has(endPayload.lastStep)).toBe(true);
+
+    const step2 = store.get(endPayload.lastStep)!.payload as ThreadStepPayload;
+    expect(visited.has(step2.content)).toBe(true);
+    expect(visited.has(step2.react)).toBe(true);
+    expect(visited.has(step2.start)).toBe(true);
+
+    if (step2.previous !== null) {
+      expect(visited.has(step2.previous)).toBe(true);
+      const step1 = store.get(step2.previous)!.payload as ThreadStepPayload;
+      expect(visited.has(step1.content)).toBe(true);
+      expect(visited.has(step1.react)).toBe(true);
+    }
+  });
+
+  test("react session nodes have placeholder structure", async () => {
+    const { store, typeHashes } = await setupStore();
+    const { workflowHash } = await setupWorkflow(store, typeHashes, SINGLE_ROLE_WORKFLOW);
+
+    const agentFn = createMockAgent({
+      worker: { text: "w", meta: { result: "r" } },
+    });
+
+    const result = await executeJsonCasThread({
+      workflowHash,
+      input: "react check",
+      moderatorRules: SINGLE_ROLE_WORKFLOW.moderator,
+      io: makeIo(store, typeHashes, "THREAD_REACT"),
+      options: makeOptions(),
+      agentFn,
+      logger: noLogger(),
+      workflow: null,
+    });
+
+    const endPayload = store.get(result.rootHash)!.payload as ThreadEndPayload;
+    const stepPayload = store.get(endPayload.lastStep)!.payload as ThreadStepPayload;
+    const reactNode = store.get(stepPayload.react);
+
+    expect(reactNode).not.toBeNull();
+    const reactPayload = reactNode!.payload as Record<string, unknown>;
+    expect(reactPayload.turns).toEqual([]);
+    expect(reactPayload.totalTokens).toBe(0);
+    expect(reactPayload.durationMs).toBe(0);
+    expect(reactPayload.role).toBe("worker");
+    expect(typeof reactPayload.agent).toBe("string");
+  });
+});
diff --git a/packages/workflow-execute/package.json b/packages/workflow-execute/package.json
index e9e18d1..e54787e 100644
--- a/packages/workflow-execute/package.json
+++ b/packages/workflow-execute/package.json
@@ -24,6 +24,9 @@
     "@uncaged/workflow-cas": "workspace:^",
     "@uncaged/workflow-reactor": "workspace:^",
     "@uncaged/workflow-register": "workspace:^",
+    "@uncaged/json-cas": "file:../../../json-cas/packages/json-cas",
+    "@uncaged/json-cas-workflow": "file:../../../json-cas/packages/json-cas-workflow",
+    "@uncaged/workflow-json-def": "workspace:^",
     "yaml": "^2.7.1"
   },
   "peerDependencies": {
diff --git a/packages/workflow-execute/src/engine/index.ts b/packages/workflow-execute/src/engine/index.ts
index 4a64b4d..9d3c1ab 100644
--- a/packages/workflow-execute/src/engine/index.ts
+++ b/packages/workflow-execute/src/engine/index.ts
@@ -7,6 +7,18 @@ export {
   walkStateFramesNewestFirst,
 } from "./fork-thread.js";
 export { garbageCollectCas } from "./gc.js";
+export { buildJsonCasThreadContext, buildJsonCasThreadSnapshot, readContentText } from "./json-cas-context.js";
+export { executeJsonCasThread } from "./json-cas-engine.js";
+export type {
+  AgentBindings,
+  JsonCasAgentFn,
+  JsonCasEngineIo,
+  JsonCasEngineOptions,
+  JsonCasStartSnapshot,
+  JsonCasStepSnapshot,
+  JsonCasThreadPauseGate,
+  JsonCasThreadSnapshot,
+} from "./json-cas-types.js";
 export { createThreadPauseGate } from "./thread-pause-gate.js";
 export type { ThreadHistoryEntry, ThreadIndex, ThreadIndexEntry } from "./threads-index.js";
 export {
diff --git a/packages/workflow-execute/src/engine/json-cas-context.ts b/packages/workflow-execute/src/engine/json-cas-context.ts
new file mode 100644
index 0000000..156c3b7
--- /dev/null
+++ b/packages/workflow-execute/src/engine/json-cas-context.ts
@@ -0,0 +1,130 @@
+import type { Hash, Store } from "@uncaged/json-cas";
+import type {
+  ContentPayload,
+  ThreadStartPayload,
+  ThreadStepPayload,
+  WorkflowSchemaHashes,
+} from "@uncaged/json-cas-workflow";
+import type { ThreadContext } from "@uncaged/workflow-protocol";
+import { START } from "@uncaged/workflow-protocol";
+
+import type { JsonCasStepSnapshot, JsonCasThreadSnapshot } from "./json-cas-types.js";
+
+// ── Snapshot builder (lightweight, for agent & moderator) ─────────────
+
+/**
+ * Walk the thread-step chain backwards via `previous` refs, then reverse
+ * to get chronological order. Returns a {@link JsonCasThreadSnapshot}.
+ */
+export function buildJsonCasThreadSnapshot(
+  store: Store,
+  _typeHashes: WorkflowSchemaHashes,
+  startHash: Hash,
+  headStepHash: Hash | null,
+  threadId: string,
+): JsonCasThreadSnapshot {
+  const startNode = store.get(startHash);
+  if (startNode === null) {
+    throw new Error(`buildJsonCasThreadSnapshot: missing thread-start node at ${startHash}`);
+  }
+  const startPayload = startNode.payload as ThreadStartPayload;
+
+  const steps: JsonCasStepSnapshot[] = [];
+
+  let cursor: Hash | null = headStepHash;
+  while (cursor !== null) {
+    const stepNode = store.get(cursor);
+    if (stepNode === null) {
+      throw new Error(`buildJsonCasThreadSnapshot: missing thread-step node at ${cursor}`);
+    }
+    const stepPayload = stepNode.payload as ThreadStepPayload;
+    steps.push({
+      role: stepPayload.role,
+      meta: stepPayload.meta,
+      contentHash: stepPayload.content,
+    });
+    cursor = stepPayload.previous;
+  }
+
+  steps.reverse();
+
+  return {
+    threadId,
+    start: {
+      input: startPayload.input,
+      depth: startPayload.depth,
+      workflowHash: startPayload.workflow,
+    },
+    steps,
+  };
+}
+
+// ── ThreadContext builder (protocol-compatible) ───────────────────────
+
+/**
+ * Build a full {@link ThreadContext} from a json-cas thread chain.
+ * Reads the thread-start node, walks thread-step backwards, and resolves
+ * content text from each step's content node.
+ *
+ * `bundleHash` is set from the workflow ref in the thread-start payload.
+ * `threadId` is set to `""` — callers should overwrite when known.
+ */
+export function buildJsonCasThreadContext(
+  store: Store,
+  _typeHashes: WorkflowSchemaHashes,
+  startHash: Hash,
+  headStepHash: Hash | null,
+): ThreadContext {
+  const startNode = store.get(startHash);
+  if (startNode === null) {
+    throw new Error(`buildJsonCasThreadContext: missing thread-start node at ${startHash}`);
+  }
+  const startPayload = startNode.payload as ThreadStartPayload;
+
+  const rawSteps: ThreadStepPayload[] = [];
+  let cursor: Hash | null = headStepHash;
+  while (cursor !== null) {
+    const stepNode = store.get(cursor);
+    if (stepNode === null) {
+      throw new Error(`buildJsonCasThreadContext: missing thread-step node at ${cursor}`);
+    }
+    const payload = stepNode.payload as ThreadStepPayload;
+    rawSteps.push(payload);
+    cursor = payload.previous;
+  }
+  rawSteps.reverse();
+
+  const steps = rawSteps.map((sp) => ({
+    role: sp.role,
+    meta: sp.meta,
+    contentHash: sp.content,
+    refs: [] as string[],
+    timestamp: 0,
+  }));
+
+  return {
+    threadId: "",
+    depth: startPayload.depth,
+    bundleHash: startPayload.workflow,
+    start: {
+      role: START,
+      content: startPayload.input,
+      meta: {},
+      timestamp: 0,
+      parentState: startPayload.parentThread,
+    },
+    steps,
+  };
+}
+
+/**
+ * Read the text payload from a content node.
+ */
+export function readContentText(store: Store, contentHash: Hash): string | null {
+  const node = store.get(contentHash);
+  if (node === null) {
+    return null;
+  }
+  const payload = node.payload as ContentPayload;
+  return payload.text;
+}
diff --git a/packages/workflow-execute/src/engine/json-cas-engine.ts b/packages/workflow-execute/src/engine/json-cas-engine.ts
new file mode 100644
index 0000000..c898a36
--- /dev/null
+++ b/packages/workflow-execute/src/engine/json-cas-engine.ts
@@ -0,0 +1,317 @@
+import type { Hash, Store } from "@uncaged/json-cas";
+import type {
+  ContentPayload,
+  ThreadEndPayload,
+  ThreadStartPayload,
+  ThreadStepPayload,
+  WorkflowSchemaHashes,
+} from "@uncaged/json-cas-workflow";
+import type { HydratedWorkflow } from "@uncaged/workflow-json-def";
+import type { ModeratorRule, WorkflowResult } from "@uncaged/workflow-protocol";
+import { END, evaluateModerator, START } from "@uncaged/workflow-protocol";
+import type { LogFn } from "@uncaged/workflow-util";
+
+import type {
+  AgentBindings,
+  JsonCasAgentFn,
+  JsonCasEngineIo,
+  JsonCasEngineOptions,
+  JsonCasStepSnapshot,
+  JsonCasThreadSnapshot,
+} from "./json-cas-types.js";
+
+// ── Helpers: CAS node writers ─────────────────────────────────────────
+
+async function writeContent(
+  store: Store,
+  typeHashes: WorkflowSchemaHashes,
+  text: string,
+): Promise<Hash> {
+  const payload: ContentPayload = { text };
+  return store.put(typeHashes.content, payload);
+}
+
+async function writePlaceholderReactSession(
+  store: Store,
+  typeHashes: WorkflowSchemaHashes,
+  role: string,
+  agentHash: Hash,
+): Promise<Hash> {
+  return store.put(typeHashes.reactSession, {
+    agent: agentHash,
+    role,
+    turns: [],
+    totalTokens: 0,
+    durationMs: 0,
+  });
+}
+
+async function writeThreadStart(
+  store: Store,
+  typeHashes: WorkflowSchemaHashes,
+  params: {
+    workflowHash: Hash;
+    input: string;
+    depth: number;
+    parentThread: Hash | null;
+    agents: AgentBindings;
+  },
+): Promise<Hash> {
+  const payload: ThreadStartPayload = {
+    workflow: params.workflowHash,
+    input: params.input,
+    depth: params.depth,
+    parentThread: params.parentThread,
+    agents: params.agents,
+  };
+  return store.put(typeHashes.threadStart, payload);
+}
+
+async function writeThreadStep(
+  store: Store,
+  typeHashes: WorkflowSchemaHashes,
+  params: {
+    role: string;
+    meta: Record<string, unknown>;
+    contentHash: Hash;
+    reactHash: Hash;
+    startHash: Hash;
+    previousHash: Hash | null;
+  },
+): Promise<Hash> {
+  const payload: ThreadStepPayload = {
+    role: params.role,
+    meta: params.meta,
+    content: params.contentHash,
+    react: params.reactHash,
+    start: params.startHash,
+    previous: params.previousHash,
+  };
+  return store.put(typeHashes.threadStep, payload);
+}
+
+async function writeThreadEnd(
+  store: Store,
+  typeHashes: WorkflowSchemaHashes,
+  params: {
+    returnCode: number;
+    summary: string;
+    startHash: Hash;
+    lastStepHash: Hash;
+  },
+): Promise<Hash> {
+  const payload: ThreadEndPayload = {
+    returnCode: params.returnCode,
+    summary: params.summary,
+    start: params.startHash,
+    lastStep: params.lastStepHash,
+  };
+  return store.put(typeHashes.threadEnd, payload);
+}
+
+// ── Placeholder agent ─────────────────────────────────────────────────
+
+async function ensurePlaceholderAgent(
+  store: Store,
+  typeHashes: WorkflowSchemaHashes,
+): Promise<Hash> {
+  return store.put(typeHashes.agent, {
+    package: "placeholder",
+    version: "0.0.0",
+    config: {},
+  });
+}
+
+// ── JSONata moderator adapter ─────────────────────────────────────────
+
+function snapshotToModeratorContext(
+  snapshot: JsonCasThreadSnapshot,
+): Parameters<typeof evaluateModerator>[1] {
+  return {
+    threadId: snapshot.threadId,
+    depth: snapshot.start.depth,
+    bundleHash: snapshot.start.workflowHash,
+    start: {
+      role: START,
+      content: snapshot.start.input,
+      meta: {},
+      timestamp: 0,
+      parentState: null,
+    },
+    steps: snapshot.steps.map((s) => ({
+      role: s.role,
+      meta: s.meta,
+      contentHash: s.contentHash,
+      refs: [],
+      timestamp: 0,
+    })),
+  };
+}
+
+// ── Main engine ───────────────────────────────────────────────────────
+
+/**
+ * Execute a workflow thread using json-cas as the storage layer.
+ *
+ * Drives the moderator→agent loop:
+ *   1. Writes a thread-start node.
+ *   2. On each round: evaluates the moderator, invokes the agent, writes
+ *      content + thread-step nodes (react is a placeholder for now).
+ *   3. On END: writes a thread-end node and returns the result.
+ *
+ * The `agentFn` callback is invoked for each role step. It receives the
+ * role name, system prompt, and current thread snapshot, and returns the
+ * agent's text output plus structured meta.
+ */
+export async function executeJsonCasThread(params: {
+  workflowHash: Hash;
+  input: string;
+  moderatorRules: readonly ModeratorRule[];
+  io: JsonCasEngineIo;
+  options: JsonCasEngineOptions;
+  agentFn: JsonCasAgentFn;
+  logger: LogFn;
+  /** Hydrated workflow for role system prompts. Null disables prompt forwarding. */
+  workflow: HydratedWorkflow | null;
+}): Promise<WorkflowResult> {
+  const { io, options, agentFn, logger, moderatorRules, workflow } = params;
+  const { store, typeHashes, threadId } = io;
+
+  const placeholderAgentHash = await ensurePlaceholderAgent(store, typeHashes);
+
+  const startHash = await writeThreadStart(store, typeHashes, {
+    workflowHash: params.workflowHash,
+    input: params.input,
+    depth: options.depth,
+    parentThread: options.parentThread,
+    agents: options.agents,
+  });
+
+  logger("X3RK7QWN", `json-cas thread ${threadId} started`);
+
+  let previousStepHash: Hash | null = null;
+  let headStepHash: Hash | null = null;
+  const stepSnapshots: JsonCasStepSnapshot[] = [];
+
+  while (true) {
+    if (options.signal.aborted) {
+      return abortThread(store, typeHashes, startHash, headStepHash, logger, threadId);
+    }
+
+    const snapshot: JsonCasThreadSnapshot = {
+      threadId,
+      start: {
+        input: params.input,
+        depth: options.depth,
+        workflowHash: params.workflowHash,
+      },
+      steps: stepSnapshots,
+    };
+
+    const modCtx = snapshotToModeratorContext(snapshot);
+    const nextRole = await evaluateModerator(moderatorRules, modCtx);
+
+    if (nextRole === END) {
+      logger("Y5TN8RVK", `json-cas thread ${threadId} moderator returned END`);
+
+      if (headStepHash === null) {
+        const dummyContentHash = await writeContent(store, typeHashes, "no-op");
+        const dummyReactHash = await writePlaceholderReactSession(
+          store,
+          typeHashes,
+          END,
+          placeholderAgentHash,
+        );
+        headStepHash = await writeThreadStep(store, typeHashes, {
+          role: END,
+          meta: {},
+          contentHash: dummyContentHash,
+          reactHash: dummyReactHash,
+          startHash,
+          previousHash: null,
+        });
+      }
+
+      const endHash = await writeThreadEnd(store, typeHashes, {
+        returnCode: 0,
+        summary: "completed: moderator returned END",
+        startHash,
+        lastStepHash: headStepHash,
+      });
+
+      return { returnCode: 0, summary: "completed: moderator returned END", rootHash: endHash };
+    }
+
+    const roleSystemPrompt =
+      workflow !== null && workflow.roles[nextRole] !== undefined
+        ? workflow.roles[nextRole].systemPrompt
+        : "";
+
+    const agentResult = await agentFn(nextRole, roleSystemPrompt, snapshot);
+
+    const contentHash = await writeContent(store, typeHashes, agentResult.text);
+
+    const agentHash = options.agents[nextRole] ?? placeholderAgentHash;
+    const reactHash = await writePlaceholderReactSession(store, typeHashes, nextRole, agentHash);
+
+    const stepHash = await writeThreadStep(store, typeHashes, {
+      role: nextRole,
+      meta: agentResult.meta,
+      contentHash,
+      reactHash,
+      startHash,
+      previousHash: previousStepHash,
+    });
+
+    previousStepHash = stepHash;
+    headStepHash = stepHash;
+    stepSnapshots.push({
+      role: nextRole,
+      meta: agentResult.meta,
+      contentHash,
+    });
+
+    logger("Z7WP4NHK", `json-cas thread ${threadId} wrote role ${nextRole}`);
+  }
+}
+
+async function abortThread(
+  store: Store,
+  typeHashes: WorkflowSchemaHashes,
+  startHash: Hash,
+  headStepHash: Hash | null,
+  logger: LogFn,
+  threadId: string,
+): Promise<WorkflowResult> {
+  logger("A8QK3VNR", `json-cas thread ${threadId} aborted`);
+
+  const placeholderAgentHash = await ensurePlaceholderAgent(store, typeHashes);
+
+  let lastStep = headStepHash;
+  if (lastStep === null) {
+    const dummyContentHash = await writeContent(store, typeHashes, "thread aborted");
+    const dummyReactHash = await writePlaceholderReactSession(
+      store,
+      typeHashes,
+      END,
+      placeholderAgentHash,
+    );
+    lastStep = await writeThreadStep(store, typeHashes, {
+      role: END,
+      meta: {},
+      contentHash: dummyContentHash,
+      reactHash: dummyReactHash,
+      startHash,
+      previousHash: null,
+    });
+  }
+
+  const endHash = await writeThreadEnd(store, typeHashes, {
+    returnCode: 130,
+    summary: "thread aborted",
+    startHash,
+    lastStepHash: lastStep,
+  });
+
+  return { returnCode: 130, summary: "thread aborted", rootHash: endHash };
+}
diff --git a/packages/workflow-execute/src/engine/json-cas-types.ts b/packages/workflow-execute/src/engine/json-cas-types.ts
new file mode 100644
index 0000000..20706af
--- /dev/null
+++ b/packages/workflow-execute/src/engine/json-cas-types.ts
@@ -0,0 +1,71 @@
+import type { Hash, Store } from "@uncaged/json-cas";
+import type { WorkflowSchemaHashes } from "@uncaged/json-cas-workflow";
+
+import type { Result } from "@uncaged/workflow-util";
+
+// ── Engine IO ─────────────────────────────────────────────────────────
+
+export type JsonCasEngineIo = {
+  threadId: string;
+  store: Store;
+  typeHashes: WorkflowSchemaHashes;
+};
+
+// ── Agent binding ─────────────────────────────────────────────────────
+
+/**
+ * Maps each role name to a CAS hash referencing an agent node.
+ * Phase 4 uses a simple role→hash mapping; full agent resolution comes later.
+ */
+export type AgentBindings = Record<string, Hash>;
+
+// ── Engine options ────────────────────────────────────────────────────
+
+export type JsonCasEngineOptions = {
+  depth: number;
+  parentThread: Hash | null;
+  signal: AbortSignal;
+  agents: AgentBindings;
+};
+
+// ── Agent function (mock-friendly) ────────────────────────────────────
+
+/**
+ * Invoked for each role step. Returns the agent's raw text output and
+ * structured meta. The engine stores the text in a content node and the
+ * meta inside the thread-step node.
+ */
+export type JsonCasAgentFn = (
+  role: string,
+  systemPrompt: string,
+  context: JsonCasThreadSnapshot,
+) => Promise<{ text: string; meta: Record<string, unknown> }>;
+
+// ── Thread snapshot (read-only view for agents & moderator) ───────────
+
+export type JsonCasStartSnapshot = {
+  input: string;
+  depth: number;
+  workflowHash: Hash;
+};
+
+export type JsonCasStepSnapshot = {
+  role: string;
+  meta: Record<string, unknown>;
+  contentHash: Hash;
+};
+
+export type JsonCasThreadSnapshot = {
+  threadId: string;
+  start: JsonCasStartSnapshot;
+  steps: readonly JsonCasStepSnapshot[];
+};
+
+// ── Thread pause gate (re-use from existing types) ────────────────────
+
+export type JsonCasThreadPauseGate = {
+  awaitAfterYield: () => Promise<void>;
+  pause: () => Result<void, string>;
+  resume: () => Result<void, string>;
+  isPaused: () => boolean;
+};
diff --git a/packages/workflow-execute/src/index.ts b/packages/workflow-execute/src/index.ts
index 92b477f..dcdaae5 100644
--- a/packages/workflow-execute/src/index.ts
+++ b/packages/workflow-execute/src/index.ts
@@ -4,6 +4,18 @@ export {
   walkStateFramesNewestFirst,
 } from "./engine/fork-thread.js";
 export { garbageCollectCas } from "./engine/gc.js";
+export { buildJsonCasThreadContext, buildJsonCasThreadSnapshot, readContentText } from "./engine/json-cas-context.js";
+export { executeJsonCasThread } from "./engine/json-cas-engine.js";
+export type {
+  AgentBindings,
+  JsonCasAgentFn,
+  JsonCasEngineIo,
+  JsonCasEngineOptions,
+  JsonCasStartSnapshot,
+  JsonCasStepSnapshot,
+  JsonCasThreadPauseGate,
+  JsonCasThreadSnapshot,
+} from "./engine/json-cas-types.js";
 export type {
   ThreadHistoryEntry,
   ThreadIndex,
diff --git a/packages/workflow-execute/tsconfig.json b/packages/workflow-execute/tsconfig.json
index 2b204f7..0b8dc57 100644
--- a/packages/workflow-execute/tsconfig.json
+++ b/packages/workflow-execute/tsconfig.json
@@ -11,6 +11,7 @@
     { "path": "../workflow-util" },
     { "path": "../workflow-cas" },
     { "path": "../workflow-reactor" },
-    { "path": "../workflow-register" }
+    { "path": "../workflow-register" },
+    { "path": "../workflow-json-def" }
   ]
 }