diff --git a/packages/cli-workflow/__tests__/serve.test.ts b/packages/cli-workflow/__tests__/serve.test.ts
new file mode 100644
index 0000000..bba5047
--- /dev/null
+++ b/packages/cli-workflow/__tests__/serve.test.ts
@@ -0,0 +1,98 @@
+import { describe, expect, test } from "bun:test";
+
+import { createApp } from "../src/commands/serve/app.js";
+
+function buildApp(storageRoot: string) {
+  const app = createApp(storageRoot);
+  return {
+    fetch: (path: string, init?: RequestInit) =>
+      app.fetch(new Request(`http://localhost${path}`, init)),
+  };
+}
+
+describe("serve /healthz", () => {
+  test("returns ok", async () => {
+    const { fetch } = buildApp("/tmp/uncaged-serve-test-nonexistent");
+    const res = await fetch("/healthz");
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { ok: boolean };
+    expect(body.ok).toBe(true);
+  });
+});
+
+describe("serve /api/workflows", () => {
+  test("returns empty list for missing storage", async () => {
+    const { fetch } = buildApp("/tmp/uncaged-serve-test-nonexistent");
+    const res = await fetch("/api/workflows");
+    // Registry file won't exist, should return error
+    expect(res.status).toBe(200);
+  });
+});
+
+describe("serve /api/threads", () => {
+  test("returns empty list for missing storage", async () => {
+    const { fetch } = buildApp("/tmp/uncaged-serve-test-nonexistent");
+    const res = await fetch("/api/threads");
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { threads: unknown[] };
+    expect(body.threads).toEqual([]);
+  });
+
+  test("returns 404 for missing thread", async () => {
+    const { fetch } = buildApp("/tmp/uncaged-serve-test-nonexistent");
+    const res = await fetch("/api/threads/nonexistent-id");
+    expect(res.status).toBe(404);
+  });
+});
+
+describe("serve /api/threads/running", () => {
+  test("returns empty list for missing storage", async () => {
+    const { fetch } = buildApp("/tmp/uncaged-serve-test-nonexistent");
+    const res = await fetch("/api/threads/running");
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { threads: unknown[] };
+    expect(body.threads).toEqual([]);
+  });
+});
+
+describe("serve /api/cas", () => {
+  test("returns empty list for missing storage", async () => {
+    const { fetch } = buildApp("/tmp/uncaged-serve-test-nonexistent");
+    const res = await fetch("/api/cas");
+    expect(res.status).toBe(200);
+    const body = (await res.json()) as { hashes: unknown[] };
+    expect(body.hashes).toEqual([]);
+  });
+
+  test("returns 404 for missing hash", async () => {
+    const { fetch } = buildApp("/tmp/uncaged-serve-test-nonexistent");
+    const res = await fetch("/api/cas/nonexistent-hash");
+    expect(res.status).toBe(404);
+  });
+});
+
+describe("serve CAS round-trip", () => {
+  const tmpDir = `/tmp/uncaged-serve-cas-test-${Date.now()}`;
+
+  test("put then get", async () => {
+    const { fetch } = buildApp(tmpDir);
+
+    const putRes = await fetch("/api/cas", {
+      method: "POST",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ content: "hello world" }),
+    });
+    expect(putRes.status).toBe(201);
+    const putBody = (await putRes.json()) as { hash: string };
+    expect(typeof putBody.hash).toBe("string");
+
+    const getRes = await fetch(`/api/cas/${putBody.hash}`);
+    expect(getRes.status).toBe(200);
+    const getBody = (await getRes.json()) as { content: string };
+    expect(getBody.content).toBe("hello world");
+
+    // cleanup
+    const delRes = await fetch(`/api/cas/${putBody.hash}`, { method: "DELETE" });
+    expect(delRes.status).toBe(200);
+  });
+});
diff --git a/packages/cli-workflow/package.json b/packages/cli-workflow/package.json
index a51aadc..c156c29 100644
--- a/packages/cli-workflow/package.json
+++ b/packages/cli-workflow/package.json
@@ -7,6 +7,7 @@
   },
   "dependencies": {
     "@uncaged/workflow": "workspace:*",
+    "hono": "^4.12.18",
     "yaml": "^2.8.4"
   },
   "scripts": {
diff --git a/packages/cli-workflow/src/cli-dispatch.ts b/packages/cli-workflow/src/cli-dispatch.ts
index 84778d7..5b58fa0 100644
--- a/packages/cli-workflow/src/cli-dispatch.ts
+++ b/packages/cli-workflow/src/cli-dispatch.ts
@@ -4,6 +4,7 @@ import { getCommandRegistry } from "./cli-registry.js";
 import { formatCliUsage as formatCliUsageWithGroups } from "./cli-usage.js";
 import { createCasDispatcher } from "./commands/cas/index.js";
 import { createInitDispatcher } from "./commands/init/index.js";
+import { dispatchServe } from "./commands/serve/index.js";
 import { createThreadDispatcher, dispatchLive, dispatchRun } from "./commands/thread/index.js";
 import { createWorkflowDispatcher } from "./commands/workflow/index.js";
 import { formatSkillIndex, formatSkillTopic, getSkillTopics } from "./skill.js";
@@ -71,6 +72,7 @@ const COMMAND_TABLE: Record<string, DispatchFn> = {
   skill: dispatchSkill,
   run: dispatchRun,
   live: dispatchLive,
+  serve: dispatchServe,
 };
 
 export async function runCli(storageRoot: string, argv: string[]): Promise<number> {
diff --git a/packages/cli-workflow/src/cli-usage.ts b/packages/cli-workflow/src/cli-usage.ts
index 0834800..81c6d35 100644
--- a/packages/cli-workflow/src/cli-usage.ts
+++ b/packages/cli-workflow/src/cli-usage.ts
@@ -57,6 +57,17 @@ export function formatCliUsage(
   );
   lines.push("");
 
+  lines.push("Server:");
+  lines.push(
+    ...formatUsageCommandLines([
+      {
+        prefix: "serve [--port N] [--host ADDR]",
+        description: "Start HTTP API server (default: 127.0.0.1:7860)",
+      },
+    ]),
+  );
+  lines.push("");
+
   lines.push("Reference:");
   const skillTopicNames = skillTopics.map((t) => t.name).join(", ");
   lines.push(
diff --git a/packages/cli-workflow/src/commands/serve/app.ts b/packages/cli-workflow/src/commands/serve/app.ts
new file mode 100644
index 0000000..76cadfc
--- /dev/null
+++ b/packages/cli-workflow/src/commands/serve/app.ts
@@ -0,0 +1,20 @@
+import { Hono } from "hono";
+import { cors } from "hono/cors";
+
+import { createCasRoutes } from "./routes-cas.js";
+import { createThreadRoutes } from "./routes-thread.js";
+import { createWorkflowRoutes } from "./routes-workflow.js";
+
+export function createApp(storageRoot: string): Hono {
+  const app = new Hono();
+
+  app.use("*", cors());
+
+  app.get("/healthz", (c) => c.json({ ok: true }));
+
+  app.route("/api/workflows", createWorkflowRoutes(storageRoot));
+  app.route("/api/threads", createThreadRoutes(storageRoot));
+  app.route("/api/cas", createCasRoutes(storageRoot));
+
+  return app;
+}
diff --git a/packages/cli-workflow/src/commands/serve/index.ts b/packages/cli-workflow/src/commands/serve/index.ts
new file mode 100644
index 0000000..ce37a75
--- /dev/null
+++ b/packages/cli-workflow/src/commands/serve/index.ts
@@ -0,0 +1,3 @@
+export { createApp } from "./app.js";
+export { dispatchServe, startServer } from "./serve.js";
+export type { ServeOptions } from "./types.js";
diff --git a/packages/cli-workflow/src/commands/serve/routes-cas.ts b/packages/cli-workflow/src/commands/serve/routes-cas.ts
new file mode 100644
index 0000000..7aeb89a
--- /dev/null
+++ b/packages/cli-workflow/src/commands/serve/routes-cas.ts
@@ -0,0 +1,56 @@
+import { createCasStore, garbageCollectCas, getGlobalCasDir } from "@uncaged/workflow";
+import { Hono } from "hono";
+
+export function createCasRoutes(storageRoot: string): Hono {
+  const app = new Hono();
+
+  app.get("/", async (c) => {
+    const casDir = getGlobalCasDir(storageRoot);
+    const cas = createCasStore(casDir);
+    const hashes = await cas.list();
+    return c.json({ hashes });
+  });
+
+  app.get("/:hash", async (c) => {
+    const casDir = getGlobalCasDir(storageRoot);
+    const cas = createCasStore(casDir);
+    const content = await cas.get(c.req.param("hash"));
+    if (content === null) {
+      return c.json({ error: "not found" }, 404);
+    }
+    return c.json({ hash: c.req.param("hash"), content });
+  });
+
+  app.post("/", async (c) => {
+    const body = await c.req.json<{ content: string }>();
+    if (typeof body.content !== "string") {
+      return c.json({ error: "content field required" }, 400);
+    }
+    const casDir = getGlobalCasDir(storageRoot);
+    const cas = createCasStore(casDir);
+    const hash = await cas.put(body.content);
+    return c.json({ hash }, 201);
+  });
+
+  app.delete("/:hash", async (c) => {
+    const casDir = getGlobalCasDir(storageRoot);
+    const cas = createCasStore(casDir);
+    const hash = c.req.param("hash");
+    const content = await cas.get(hash);
+    if (content === null) {
+      return c.json({ error: "not found" }, 404);
+    }
+    await cas.delete(hash);
+    return c.json({ ok: true });
+  });
+
+  app.post("/gc", async (c) => {
+    const result = await garbageCollectCas(storageRoot);
+    if (!result.ok) {
+      return c.json({ error: result.error }, 500);
+    }
+    return c.json(result.value);
+  });
+
+  return app;
+}
diff --git a/packages/cli-workflow/src/commands/serve/routes-thread.ts b/packages/cli-workflow/src/commands/serve/routes-thread.ts
new file mode 100644
index 0000000..573d1a9
--- /dev/null
+++ b/packages/cli-workflow/src/commands/serve/routes-thread.ts
@@ -0,0 +1,46 @@
+import { Hono } from "hono";
+
+import { readTextFileIfExists } from "../../fs-utils.js";
+import {
+  listHistoricalThreads,
+  listRunningThreads,
+  resolveThreadDataPath,
+} from "../../thread-scan.js";
+
+export function createThreadRoutes(storageRoot: string): Hono {
+  const app = new Hono();
+
+  app.get("/", async (c) => {
+    const nameFilter = c.req.query("workflow") ?? null;
+    const rows = await listHistoricalThreads(storageRoot, nameFilter);
+    return c.json({ threads: rows });
+  });
+
+  app.get("/running", async (c) => {
+    const rows = await listRunningThreads(storageRoot);
+    return c.json({ threads: rows });
+  });
+
+  app.get("/:threadId", async (c) => {
+    const threadId = c.req.param("threadId");
+    const dataPath = await resolveThreadDataPath(storageRoot, threadId);
+    if (dataPath === null) {
+      return c.json({ error: `thread not found: ${threadId}` }, 404);
+    }
+    const text = await readTextFileIfExists(dataPath);
+    if (text === null) {
+      return c.json({ error: `thread data missing: ${threadId}` }, 404);
+    }
+    const lines = text.trim().split("\n");
+    const records = lines.map((line) => {
+      try {
+        return JSON.parse(line) as unknown;
+      } catch {
+        return { raw: line };
+      }
+    });
+    return c.json({ threadId, records });
+  });
+
+  return app;
+}
diff --git a/packages/cli-workflow/src/commands/serve/routes-workflow.ts b/packages/cli-workflow/src/commands/serve/routes-workflow.ts
new file mode 100644
index 0000000..ea728c3
--- /dev/null
+++ b/packages/cli-workflow/src/commands/serve/routes-workflow.ts
@@ -0,0 +1,55 @@
+import {
+  getRegisteredWorkflow,
+  listRegisteredWorkflowNames,
+  readWorkflowRegistry,
+} from "@uncaged/workflow";
+import { Hono } from "hono";
+
+export function createWorkflowRoutes(storageRoot: string): Hono {
+  const app = new Hono();
+
+  app.get("/", async (c) => {
+    const reg = await readWorkflowRegistry(storageRoot);
+    if (!reg.ok) {
+      return c.json({ error: reg.error.message }, 500);
+    }
+    const names = listRegisteredWorkflowNames(reg.value);
+    const workflows = names.map((name) => {
+      const entry = reg.value.workflows[name];
+      return {
+        name,
+        hash: entry?.hash ?? null,
+        timestamp: entry?.timestamp ?? null,
+      };
+    });
+    return c.json({ workflows });
+  });
+
+  app.get("/:name", async (c) => {
+    const reg = await readWorkflowRegistry(storageRoot);
+    if (!reg.ok) {
+      return c.json({ error: reg.error.message }, 500);
+    }
+    const name = c.req.param("name");
+    const entry = getRegisteredWorkflow(reg.value, name);
+    if (entry === null) {
+      return c.json({ error: `workflow not found: ${name}` }, 404);
+    }
+    return c.json({ name, ...entry });
+  });
+
+  app.get("/:name/history", async (c) => {
+    const reg = await readWorkflowRegistry(storageRoot);
+    if (!reg.ok) {
+      return c.json({ error: reg.error.message }, 500);
+    }
+    const name = c.req.param("name");
+    const entry = getRegisteredWorkflow(reg.value, name);
+    if (entry === null) {
+      return c.json({ error: `workflow not found: ${name}` }, 404);
+    }
+    return c.json({ name, history: entry.history });
+  });
+
+  return app;
+}
diff --git a/packages/cli-workflow/src/commands/serve/serve.ts b/packages/cli-workflow/src/commands/serve/serve.ts
new file mode 100644
index 0000000..8609efb
--- /dev/null
+++ b/packages/cli-workflow/src/commands/serve/serve.ts
@@ -0,0 +1,69 @@
+import { err, ok, type Result } from "@uncaged/workflow";
+import { serve } from "bun";
+
+import { printCliLine } from "../../cli-output.js";
+import { createApp } from "./app.js";
+import type { ServeOptions } from "./types.js";
+
+export function startServer(storageRoot: string, options: ServeOptions): void {
+  const app = createApp(storageRoot);
+
+  const server = serve({
+    fetch: app.fetch,
+    port: options.port,
+    hostname: options.hostname,
+  });
+
+  printCliLine(`uncaged-workflow API server listening on http://${server.hostname}:${server.port}`);
+}
+
+function parsePortValue(value: string | undefined): Result<number, string> {
+  if (value === undefined) {
+    return err("--port requires a value");
+  }
+  const parsed = Number.parseInt(value, 10);
+  if (!Number.isFinite(parsed) || parsed < 0 || parsed > 65535) {
+    return err(`invalid port: ${value}`);
+  }
+  return ok(parsed);
+}
+
+function parseServeArgv(argv: string[]): Result<ServeOptions, string> {
+  let port = 7860;
+  let hostname = "127.0.0.1";
+
+  for (let i = 0; i < argv.length; i++) {
+    const arg = argv[i];
+    if (arg === "--port" || arg === "-p") {
+      const portResult = parsePortValue(argv[i + 1]);
+      if (!portResult.ok) {
+        return portResult;
+      }
+      port = portResult.value;
+      i++;
+    } else if (arg === "--host") {
+      const next = argv[i + 1];
+      if (next === undefined) {
+        return err("--host requires a value");
+      }
+      hostname = next;
+      i++;
+    }
+  }
+
+  return ok({ port, hostname });
+}
+
+export async function dispatchServe(storageRoot: string, argv: string[]): Promise<number> {
+  const parsed = parseServeArgv(argv);
+  if (!parsed.ok) {
+    printCliLine(`error: ${parsed.error}`);
+    return 1;
+  }
+
+  startServer(storageRoot, parsed.value);
+
+  // Keep process alive
+  await new Promise(() => {});
+  return 0;
+}
diff --git a/packages/cli-workflow/src/commands/serve/types.ts b/packages/cli-workflow/src/commands/serve/types.ts
new file mode 100644
index 0000000..ad8ef10
--- /dev/null
+++ b/packages/cli-workflow/src/commands/serve/types.ts
@@ -0,0 +1,4 @@
+export type ServeOptions = {
+  port: number;
+  hostname: string;
+};