diff --git a/packages/feature-studio/src/server/draft-writer.ts b/packages/feature-studio/src/server/draft-writer.ts
new file mode 100644
index 0000000..a4100d3
--- /dev/null
+++ b/packages/feature-studio/src/server/draft-writer.ts
@@ -0,0 +1,31 @@
+import type { AppConfig } from "@stc/contracts";
+import { createStudioStore, StudioError, safeAuthor, type Identity } from "./store";
+
+/** Programmatic draft API for other packages (WP-D's studio generator). Writes only to `draft/<authorId>`, never publishes. */
+export interface DraftWriter {
+  writeDraftFiles(
+    courseId: string,
+    authorId: string,
+    files: { path: string; content: string }[],
+    message: string,
+  ): Promise<{ branch: string; commit: string }>;
+}
+
+export interface DraftWriterOptions {
+  /** Commit author for `authorId`. Without it (or when it returns null) commits use `<authorId> <authorId@users.stcourses.invalid>`. */
+  identity?: (authorId: string) => Promise<Identity | null> | Identity | null;
+}
+
+export function createDraftWriter(ctx: { config: Pick<AppConfig, "dataDir"> }, opts: DraftWriterOptions = {}): DraftWriter {
+  const store = createStudioStore({ dataDir: ctx.config.dataDir });
+  return {
+    async writeDraftFiles(courseId, authorId, files, message) {
+      if (!store.isManaged(courseId)) throw new StudioError(`course ${courseId} is not managed by the studio`, 404);
+      const id = (await opts.identity?.(authorId)) ?? { name: authorId, email: `${safeAuthor(authorId)}@users.stcourses.invalid` };
+      const r = await store.saveFiles(courseId, { ...id, id: authorId }, files.map((f) => ({ path: f.path, content: f.content })), message);
+      if (!r.ok) throw new StudioError("draft write conflict", 409); // unreachable: no base hashes are given
+      const st = await store.status(courseId, authorId);
+      return { branch: st.branch, commit: r.commit ?? st.draftHead };
+    },
+  };
+}
diff --git a/packages/feature-studio/src/server/paths.ts b/packages/feature-studio/src/server/paths.ts
new file mode 100644
index 0000000..f95b9f0
--- /dev/null
+++ b/packages/feature-studio/src/server/paths.ts
@@ -0,0 +1,17 @@
+import { join, resolve } from "node:path";
+import type { AppConfig } from "@stc/contracts";
+
+/** Where learners read studio courses from: `<dataDir>/worktrees/<id>/published` (depth 2; drafts sit at depth 3 and are never discovered). */
+export function studioWorktreesDir(dataDir: string): string {
+  return join(dataDir, "worktrees");
+}
+
+/**
+ * The course dirs the CourseRepository must scan so studio courses load from their published worktree.
+ * Phase 0's server builds the repository from `config.courseDirs` only; see docs/contract-requests/studio.md.
+ */
+export function studioCourseDirs(config: Pick<AppConfig, "dataDir" | "courseDirs">): string[] {
+  const wt = resolve(studioWorktreesDir(config.dataDir));
+  const dirs = config.courseDirs.map((d) => resolve(d));
+  return dirs.includes(wt) ? dirs : [...dirs, wt];
+}
diff --git a/packages/feature-studio/src/server/watch.ts b/packages/feature-studio/src/server/watch.ts
new file mode 100644
index 0000000..29d484d
--- /dev/null
+++ b/packages/feature-studio/src/server/watch.ts
@@ -0,0 +1,52 @@
+import type { CourseRepository } from "@stc/course-format";
+import type { StudioStore } from "./store";
+
+export interface PublishedWatcher {
+  /** Compares every managed course's published HEAD with the last one seen; reloads the repository for changed ones. */
+  check(): Promise<string[]>;
+  /** Records a head the studio itself just produced (publish, sync) after reloading. */
+  note(courseId: string, head: string): void;
+  start(): void;
+  stop(): void;
+}
+
+/**
+ * Spec 12: "The platform watches the published worktree and reloads when it changes, after an external commit or a pull."
+ * Polling `git rev-parse HEAD` is portable (fs.watch on .git internals is not) and costs one git spawn per course per tick.
+ */
+export function createPublishedWatcher(store: StudioStore, courses: CourseRepository, opts: { intervalMs?: number } = {}): PublishedWatcher {
+  const seen = new Map<string, string>();
+  let timer: ReturnType<typeof setInterval> | null = null;
+  let running: Promise<string[]> | null = null;
+
+  const run = async (): Promise<string[]> => {
+    const changed: string[] = [];
+    for (const id of await store.listManaged()) {
+      let head: string;
+      try { head = await store.publishedHead(id); } catch { continue; }
+      const prev = seen.get(id);
+      seen.set(id, head);
+      if (prev === undefined ? courses.get(id) === null : prev !== head) changed.push(id);
+    }
+    for (const id of changed) await courses.reload(id);
+    return changed;
+  };
+
+  const w: PublishedWatcher = {
+    check() {
+      running ??= run().finally(() => { running = null; });
+      return running;
+    },
+    note(courseId, head) { seen.set(courseId, head); },
+    start() {
+      if (timer) return;
+      timer = setInterval(() => { w.check().catch((e) => console.error("[studio] watcher", e)); }, opts.intervalMs ?? 3000);
+      (timer as { unref?: () => void }).unref?.();
+    },
+    stop() {
+      if (timer) clearInterval(timer);
+      timer = null;
+    },
+  };
+  return w;
+}
diff --git a/packages/feature-studio/test/draft-writer.test.ts b/packages/feature-studio/test/draft-writer.test.ts
new file mode 100644
index 0000000..3833051
--- /dev/null
+++ b/packages/feature-studio/test/draft-writer.test.ts
@@ -0,0 +1,29 @@
+import { expect, test } from "bun:test";
+import { existsSync, readFileSync } from "node:fs";
+import { join } from "node:path";
+import { createDraftWriter } from "../src/server/draft-writer";
+import { ada, extGit, newStore } from "./helpers";
+
+test("writeDraftFiles commits on draft/<author>, never on the published branch", async () => {
+  const { dataDir, store } = await newStore();
+  await store.createCourse({ id: "c1", title: "C" }, ada);
+  const writer = createDraftWriter({ config: { dataDir } }, { identity: async (id) => (id === ada.id ? ada : null) });
+  const r = await writer.writeDraftFiles("c1", ada.id, [{ path: "lessons/generated.md", content: "# Generated\n" }], "AI draft: module 2");
+  expect(r.branch).toBe("draft/u-ada");
+  const draft = store.paths("c1").draft(ada.id);
+  expect(await extGit(draft, "rev-parse", "HEAD")).toBe(r.commit);
+  expect(await extGit(draft, "log", "-1", "--format=%an|%s")).toBe("Ada Lovelace|AI draft: module 2");
+  expect(readFileSync(join(draft, "lessons/generated.md"), "utf8")).toBe("# Generated\n");
+  expect(existsSync(join(store.paths("c1").published, "lessons/generated.md"))).toBe(false);
+});
+
+test("unknown identity falls back to the author id; unmanaged course and bad paths throw", async () => {
+  const { dataDir, store } = await newStore();
+  await store.createCourse({ id: "c1", title: "C" }, ada);
+  const writer = createDraftWriter({ config: { dataDir } });
+  const r = await writer.writeDraftFiles("c1", "u-x", [{ path: "a.md", content: "a" }], "m");
+  expect(await extGit(store.paths("c1").draft("u-x"), "log", "-1", "--format=%an <%ae>")).toBe("u-x <u-x@users.stcourses.invalid>");
+  expect(r.branch).toBe("draft/u-x");
+  await expect(writer.writeDraftFiles("pure-snake", "u-x", [{ path: "a.md", content: "a" }], "m")).rejects.toThrow(/not managed/);
+  await expect(writer.writeDraftFiles("c1", "u-x", [{ path: "../a.md", content: "a" }], "m")).rejects.toThrow(/path/);
+});
diff --git a/packages/feature-studio/test/watch.test.ts b/packages/feature-studio/test/watch.test.ts
new file mode 100644
index 0000000..05de072
--- /dev/null
+++ b/packages/feature-studio/test/watch.test.ts
@@ -0,0 +1,44 @@
+import { expect, test } from "bun:test";
+import { writeFileSync } from "node:fs";
+import { join } from "node:path";
+import { createCourseRepository } from "@stc/course-format";
+import { createPublishedWatcher } from "../src/server/watch";
+import { studioCourseDirs } from "../src/server/paths";
+import { ada, extGit, newStore } from "./helpers";
+
+test("an external commit in the published worktree reloads the CourseRepository", async () => {
+  const { dataDir, store } = await newStore();
+  const courses = await createCourseRepository(studioCourseDirs({ dataDir, courseDirs: [] }));
+  let reloads = 0;
+  const realReload = courses.reload.bind(courses);
+  courses.reload = async (id?: string) => { reloads++; await realReload(id); };
+  const w = createPublishedWatcher(store, courses, { intervalMs: 50 });
+
+  await store.createCourse({ id: "c1", title: "C" }, ada);
+  expect(await w.check()).toEqual(["c1"]); // new course, not yet loaded
+  expect(courses.get("c1")?.status).toBe("ok");
+  expect(await w.check()).toEqual([]);
+
+  const pub = store.paths("c1").published;
+  writeFileSync(join(pub, "lessons/welcome.md"), "# External\n");
+  await extGit(pub, "commit", "-qam", "external");
+  expect(await w.check()).toEqual(["c1"]);
+  expect(await courses.readLessonMarkdown("c1", "welcome")).toBe("# External\n");
+  expect(reloads).toBe(2);
+
+  // the timer path
+  writeFileSync(join(pub, "lessons/welcome.md"), "# External 2\n");
+  await extGit(pub, "commit", "-qam", "external 2");
+  w.start();
+  for (let i = 0; i < 40 && (await courses.readLessonMarkdown("c1", "welcome")) !== "# External 2\n"; i++) await Bun.sleep(25);
+  w.stop();
+  expect(await courses.readLessonMarkdown("c1", "welcome")).toBe("# External 2\n");
+});
+
+test("drafts are not discovered as courses", async () => {
+  const { dataDir, store } = await newStore();
+  await store.createCourse({ id: "c1", title: "C" }, ada);
+  await store.ensureDraft("c1", ada.id);
+  const courses = await createCourseRepository(studioCourseDirs({ dataDir, courseDirs: [] }));
+  expect(courses.list().map((c) => [c.id, c.root])).toEqual([["c1", store.paths("c1").published]]);
+});