fix: fall through to prose slice parser when checkbox parser yields empty under ## Slices (#1744)

When the ## Slices section exists but contains H3 prose headers instead of
checkboxes, parseRoadmapSlices returned an empty array because the prose
fallback was only invoked when the ## Slices heading was entirely absent.
Now, when the checkbox parser finds zero slices, it falls through to
parseProseSliceHeaders as a second-chance fallback.

Also adds a missing_slice_dir diagnostic in doctor.ts when resolveSlicePath
returns null, with auto-fix via mkdir.

Fixes #1711

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/gsd/doctor-types.ts

@@ -57,6 +57,7 @@ export type DoctorIssueCode =
   // GSD state structural checks
   | "circular_slice_dependency"
   | "orphaned_slice_directory"
+  | "missing_slice_dir"
   | "duplicate_task_id"
   | "task_file_not_in_plan"
   | "stale_replan_file"

src/resources/extensions/gsd/doctor.ts

@@ -706,7 +706,26 @@ export async function runGSDDoctor(basePath: string, options?: { fix?: boolean;
       }
 
       const slicePath = resolveSlicePath(basePath, milestoneId, slice.id);
-      if (!slicePath) continue;
+      if (!slicePath) {
+        const expectedPath = relSlicePath(basePath, milestoneId, slice.id);
+        issues.push({
+          severity: slice.done ? "warning" : "error",
+          code: "missing_slice_dir",
+          scope: "slice",
+          unitId,
+          message: slice.done
+            ? `Missing slice directory for ${unitId} (slice is complete — cosmetic only)`
+            : `Missing slice directory for ${unitId}`,
+          file: expectedPath,
+          fixable: true,
+        });
+        if (fix) {
+          const absoluteSliceDir = join(milestonePath, "slices", slice.id);
+          mkdirSync(absoluteSliceDir, { recursive: true });
+          fixesApplied.push(`created ${absoluteSliceDir}`);
+        }
+        continue;
+      }
 
       const tasksDir = resolveTasksDir(basePath, milestoneId, slice.id);
       if (!tasksDir) {

src/resources/extensions/gsd/roadmap-slices.ts

@@ -184,6 +184,14 @@ export function parseRoadmapSlices(content: string): RoadmapSliceEntry[] {
   }
 
   if (currentSlice) slices.push(currentSlice);
+
+  // When the ## Slices section exists but the checkbox parser found nothing
+  // (e.g. the LLM used H3 prose headers instead of checkboxes), fall through
+  // to the prose-header parser as a second-chance fallback.
+  if (slices.length === 0) {
+    return parseProseSliceHeaders(content);
+  }
+
   return slices;
 }
 

src/resources/extensions/gsd/tests/roadmap-slices.test.ts

@@ -198,3 +198,58 @@ Not done.
   assert.equal(slices[0]?.title, "Done Slice");
   assert.equal(slices[1]?.done, false);
 });
+
+// ── Regression tests for #1711 ─────────────────────────────────────────────
+
+test("parseRoadmapSlices: H3 prose headers under ## Slices section triggers prose fallback (#1711)", () => {
+  const proseUnderSlices = `# M010: My Milestone
+
+**Vision:** Ship it.
+
+## Slices
+
+### S01 — Setup Environment
+Set up the dev environment and tooling.
+
+### S02 — Build Core
+Implement the core logic.
+**Depends on:** S01
+
+### S03 — Polish UI
+Final polish and theming.
+**Depends on:** S01, S02
+`;
+  const slices = parseRoadmapSlices(proseUnderSlices);
+  assert.equal(slices.length, 3, "should find 3 slices from H3 prose headers under ## Slices");
+  assert.equal(slices[0]?.id, "S01");
+  assert.equal(slices[0]?.title, "Setup Environment");
+  assert.equal(slices[1]?.id, "S02");
+  assert.deepEqual(slices[1]?.depends, ["S01"]);
+  assert.equal(slices[2]?.id, "S03");
+  assert.deepEqual(slices[2]?.depends, ["S01", "S02"]);
+});
+
+test("parseRoadmapSlices: ## Slices with valid checkboxes does NOT invoke prose fallback", () => {
+  const slices = parseRoadmapSlices(content);
+  assert.equal(slices.length, 3);
+  assert.equal(slices[0]?.id, "S01");
+  assert.equal(slices[0]?.done, true);
+});
+
+test("parseRoadmapSlices: ## Slices with only non-matching lines returns prose fallback results", () => {
+  const weirdContent = `# M020: Odd
+
+## Slices
+Some introductory text that is not a checkbox or a slice header.
+
+### S01: First Thing
+Do the first thing.
+
+### S02: Second Thing
+Do the second thing.
+`;
+  const slices = parseRoadmapSlices(weirdContent);
+  assert.equal(slices.length, 2, "should fall through to prose parser");
+  assert.equal(slices[0]?.id, "S01");
+  assert.equal(slices[1]?.id, "S02");
+});