feat(quiz): diff coverage map in the sidebar

src/cognit/mcp/assets/quiz_mcp.js

@@ -384,6 +384,52 @@ function renderSidebar() {
         ` Q${i + 1} · ${TYPE_LABEL[q.type].split(" ")[0].toLowerCase()}`,
       ]))),
   ]));
+  renderCoverageBlock();  // async: appends "Diff coverage" once the file list is fetched
+}
+
+// ── diff coverage map ───────────────────────────────────────────
+// Sidebar list of the PR's changed files, marked covered when some question is
+// anchored to them. Display only (the "ask host to cover this" steer is Track B).
+let changedFiles = null;  // string[] — fetched once per page (null = not yet fetched)
+
+async function ensureChangedFiles() {
+  if (changedFiles !== null) return changedFiles;
+  try {
+    const r = await fetch("/changed-files");
+    changedFiles = r.ok ? ((await r.json()).files || []) : [];
+  } catch (e) {
+    console.warn("changed-files fetch failed", e);
+    changedFiles = [];
+  }
+  return changedFiles;
+}
+
+// Mirror of the server's do_file_diff path matching (exact, repo-relative suffix,
+// or basename) so a covered marker lines up with what /diff would actually serve.
+function fileMatchesAnchor(file, anchorPath) {
+  if (!anchorPath) return false;
+  if (file === anchorPath || file.endsWith("/" + anchorPath) || anchorPath.endsWith("/" + file)) {
+    return true;
+  }
+  return file.split("/").pop() === anchorPath.split("/").pop();
+}
+
+async function renderCoverageBlock() {
+  const files = await ensureChangedFiles();
+  if (!files.length || !quiz) return;  // no diff info, or quiz cleared while awaiting
+  sidebarRoot.querySelector(".side-block--coverage")?.remove();  // idempotent re-render
+  const anchorPaths = quiz.questions.map((q) => q.anchor && q.anchor.path).filter(Boolean);
+  const rows = files.map((f) => ({ path: f, covered: anchorPaths.some((ap) => fileMatchesAnchor(f, ap)) }));
+  const coveredCount = rows.filter((r) => r.covered).length;
+  sidebarRoot.appendChild(el("div", { class: "side-block side-block--coverage" }, [
+    el("div", { class: "side-title", text: "Diff coverage" }),
+    el("div", { class: "progress-text", text: `${coveredCount} of ${rows.length} files probed` }),
+    el("ul", { class: "sidelist coverage" },
+      rows.map((r) => el("li", { class: r.covered ? "covered" : "uncovered", title: r.path }, [
+        el("span", { class: `ic ${r.covered ? "ok" : "empty"}`, text: r.covered ? "✓" : "○" }),
+        el("span", { class: "coverage__file", text: ` ${r.path.split("/").pop()}` }),
+      ]))),
+  ]));
 }
 
 function updateSidebarProgress() {

src/cognit/mcp/assets/styles.css

@@ -869,6 +869,14 @@ textarea.open:focus { border-color: var(--blue); outline: none; box-shadow: 0 0
 .diff-line.anchor-hit { box-shadow: inset 3px 0 0 var(--blue); }
 .codepanel__note { padding: 10px 12px; font-size: 12px; color: var(--fg-mute); background: var(--bg-canvas); border-radius: 0 0 6px 6px; }
 
+/* diff coverage map (sidebar) ────────────────────────────────────── */
+.sidelist .ic.empty { background: transparent; color: var(--fg-faint); box-shadow: inset 0 0 0 1.5px var(--border); }
+.sidelist.coverage .coverage__file {
+  font-family: var(--mono); font-size: 12px;
+  overflow: hidden; text-overflow: ellipsis; white-space: nowrap;
+}
+.sidelist.coverage li.uncovered .coverage__file { color: var(--fg-mute); }
+
 @media (prefers-reduced-motion: reduce) {
   .codepanel__icon { transition: none; }
 }

src/cognit/mcp/server.py

@@ -168,6 +168,7 @@ def _start_web(
         post_comment=lambda body: gh_post_comment(pr_url, body),
         grade=lambda: grade_state(state, llm=llm),
         diff_section=lambda path: do_file_diff(path, diffs.sections()),
+        changed_files=lambda: list(diffs.sections()),
         pr_url=pr_url,
     )
     url = f"http://127.0.0.1:{port}"

src/cognit/mcp/web.py

@@ -39,6 +39,7 @@ def build_web_app(
     post_comment: Callable[[str], str],
     grade: Callable[[], Results] | None = None,
     diff_section: Callable[[str], str] | None = None,
+    changed_files: Callable[[], list[str]] | None = None,
     pr_url: str = "",
 ) -> FastAPI:
     """Browser projection over `state`.
@@ -47,8 +48,10 @@ def build_web_app(
     button. It runs the same handler-owned grading the agent's `grade` tool uses and
     must store the result in `state`; we return the Results to the page. `diff_section`
     (when provided) returns the unified-diff section for one changed file — GET /diff
-    serves it so the browser can show a question's anchored hunk inline. `pr_url` feeds
-    the page chrome (the "on GitHub" links).
+    serves it so the browser can show a question's anchored hunk inline. `changed_files`
+    (when provided) lists the PR's changed-file paths — GET /changed-files serves it so
+    the browser can render the diff coverage map. `pr_url` feeds the page chrome (the
+    "on GitHub" links).
     """
     app = FastAPI()
     app.mount("/static", StaticFiles(directory=str(_ASSETS_DIR)), name="static")
@@ -82,6 +85,12 @@ def get_diff(path: str = "") -> PlainTextResponse:
             return PlainTextResponse("diff not available", status_code=503)
         return PlainTextResponse(diff_section(path))
 
+    @app.get("/changed-files")
+    def get_changed_files() -> JSONResponse:
+        if changed_files is None:
+            return JSONResponse({"error": "diff not available"}, status_code=503)
+        return JSONResponse({"files": changed_files()})
+
     @app.post("/grade")
     async def do_grade() -> JSONResponse:
         if grade is None:

tests/mcp/test_web.py

@@ -209,6 +209,38 @@ def test_diff_endpoint_503_when_unavailable(client) -> None:
     assert c.get("/diff", params={"path": "x.py"}).status_code == 503
 
 
+def test_changed_files_endpoint_lists_files(tmp_path: Path) -> None:
+    state = QuizState(pr_number=7, snapshot_path=tmp_path / "s.json")
+    state.set_quiz(
+        Quiz(
+            pr_number=7,
+            questions=[
+                MCQQuestion(id="q1", prompt="p", options=["A", "B"], answer="A", explanation="x")
+            ],
+        )
+    )
+    app = build_web_app(
+        state,
+        post_comment=lambda b: "http://c/1",
+        changed_files=lambda: ["src/a.py", "src/b.py"],
+    )
+    port = _free_port()
+    server, t = _serve(app, port)
+    try:
+        r = httpx.get(f"http://127.0.0.1:{port}/changed-files")
+        assert r.status_code == 200
+        assert r.json()["files"] == ["src/a.py", "src/b.py"]
+    finally:
+        server.should_exit = True
+        t.join(timeout=2)
+
+
+def test_changed_files_503_when_unavailable(client) -> None:
+    # the default fixture app wires no `changed_files`
+    c, _state, _ = client
+    assert c.get("/changed-files").status_code == 503
+
+
 def test_publish_before_grading_409(tmp_path: Path) -> None:
     state = QuizState(pr_number=7, snapshot_path=tmp_path / "s.json")
     state.set_quiz(