diff --git a/packages/course-format/src/load.ts b/packages/course-format/src/load.ts
index b0e3c57..32065f6 100644
--- a/packages/course-format/src/load.ts
+++ b/packages/course-format/src/load.ts
@@ -1,5 +1,5 @@
 import { readdir, readFile, realpath } from "node:fs/promises";
-import { realpathSync } from "node:fs";
+import { realpathSync, statSync } from "node:fs";
 import { basename, dirname, isAbsolute, join, relative, resolve, sep } from "node:path";
 import { extractHints, extractSection } from "./markdown";
 import { parseManifest, type Lesson, type Manifest } from "./schema";
@@ -67,8 +67,27 @@ export async function discoverManifests(courseDirs: string[]): Promise<string[]>
   return found;
 }
 
+/** Never throws: an unexpected failure becomes an error diagnostic on that course only. */
 export async function loadCourse(manifestPath: string): Promise<LoadedCourse> {
   manifestPath = resolve(manifestPath);
+  const dir = dirname(manifestPath);
+  try {
+    return await loadCourseUnsafe(manifestPath);
+  } catch (e) {
+    const id = basename(dir);
+    return {
+      id,
+      manifestPath,
+      root: dir,
+      manifest: null,
+      status: "error",
+      diagnostics: [{ level: "error", courseId: id, path: manifestPath, message: `unexpected error loading course: ${(e as Error).message}` }],
+      lessonIndex: new Map(),
+    };
+  }
+}
+
+async function loadCourseUnsafe(manifestPath: string): Promise<LoadedCourse> {
   const dir = dirname(manifestPath);
   const diagnostics: Diagnostic[] = [];
   const course: LoadedCourse = {
@@ -118,10 +137,16 @@ export async function loadCourse(manifestPath: string): Promise<LoadedCourse> {
     if (!mdCache.has(abs)) mdCache.set(abs, await Bun.file(abs).text());
     return mdCache.get(abs)!;
   };
-  /** Reports an error and returns false when `rel` does not exist inside the root. */
-  const requireFile = (rel: string, what: string, lessonId?: string): boolean => {
-    if (resolveInside(root, rel)) return true;
-    err(`${what} not found: ${rel}`, { lessonId, path: rel });
+  /** Reports an error and returns false when `rel` is not a regular file (or directory, if asked) inside the root. */
+  const requireFile = (rel: string, what: string, lessonId?: string, dir = false): boolean => {
+    const abs = resolveInside(root, rel);
+    if (!abs) {
+      err(`${what} not found: ${rel}`, { lessonId, path: rel });
+      return false;
+    }
+    const st = statSync(abs);
+    if (dir ? st.isDirectory() : st.isFile()) return true;
+    err(`${what} is not a ${dir ? "directory" : "regular file"}: ${rel}`, { lessonId, path: rel });
     return false;
   };
 
@@ -131,7 +156,7 @@ export async function loadCourse(manifestPath: string): Promise<LoadedCourse> {
   for (const a of manifest.attachments ?? []) requireFile(a.file, "attachment");
 
   for (const mod of manifest.modules) {
-    if (mod.code) requireFile(mod.code.dir, `module "${mod.id}" code dir`);
+    if (mod.code) requireFile(mod.code.dir, `module "${mod.id}" code dir`, undefined, true);
     for (const l of mod.lessons) {
       const id = l.id;
       let fileOk = true;
diff --git a/packages/course-format/test/load.test.ts b/packages/course-format/test/load.test.ts
index a93f3d3..ad14892 100644
--- a/packages/course-format/test/load.test.ts
+++ b/packages/course-format/test/load.test.ts
@@ -80,6 +80,34 @@ describe("overlays and errors", () => {
     expect(errs[2]!.message).toContain("Exercise 1");
   });
 
+  test("lesson file that is a directory gives a diagnostic", async () => {
+    const d = await tmp();
+    await Bun.write(join(d, "adir/x.md"), "# X\n");
+    await Bun.write(join(d, "stcourse.yaml"), manifest("dirs", "", "      - { id: as-file, title: L, type: text, file: adir }"));
+    const c = await loadCourse(join(d, "stcourse.yaml"));
+    expect(c.status).toBe("error");
+    const e = c.diagnostics.filter((x) => x.level === "error");
+    expect(e).toHaveLength(1);
+    expect(e[0]!.lessonId).toBe("as-file");
+    expect(e[0]!.path).toBe("adir");
+  });
+
+  test("one throwing course does not break loadAll", async () => {
+    const d = await tmp();
+    await Bun.write(join(d, "bad/adir/x.md"), "# X\n");
+    await Bun.write(join(d, "bad/a.md"), "# A\n## Sec\n");
+    await Bun.write(
+      join(d, "bad/stcourse.yaml"),
+      manifest("bad", "", "      - { id: h, title: L, type: exercise, file: a.md, hints: { file: adir, match: 'E' } }"),
+    );
+    await Bun.write(join(d, "good/a.md"), "# A\n");
+    await Bun.write(join(d, "good/stcourse.yaml"), manifest("good", "", "      - { id: l, title: L, type: text, file: a.md }"));
+    const all = await loadAll([d]);
+    expect(all.map((c) => c.id).sort()).toEqual(["bad", "good"]);
+    expect(all.find((c) => c.id === "bad")!.status).toBe("error");
+    expect(all.find((c) => c.id === "good")!.status).toBe("ok");
+  });
+
   test("invalid manifest gives an error course and does not throw", async () => {
     const d = await tmp();
     await Bun.write(join(d, "stcourse.yaml"), "schema: 2\nid: x\n");