diff --git a/src/resources/extensions/gsd/state.ts b/src/resources/extensions/gsd/state.ts
index 738d13a88..c9f85b54e 100644
--- a/src/resources/extensions/gsd/state.ts
+++ b/src/resources/extensions/gsd/state.ts
@@ -550,6 +550,30 @@ async function _deriveStateImpl(basePath: string): Promise<GSDState> {
     };
   }
 
+  // ── Zero-slice roadmap guard (#1785) ─────────────────────────────────
+  // A stub roadmap (placeholder text, no slice definitions) has a truthy
+  // roadmap object but an empty slices array. Without this check the
+  // slice-finding loop below finds nothing and returns phase: "blocked".
+  // An empty slices array means the roadmap still needs slice definitions,
+  // so the correct phase is pre-planning.
+  if (activeRoadmap.slices.length === 0) {
+    return {
+      activeMilestone,
+      activeSlice: null,
+      activeTask: null,
+      phase: 'pre-planning',
+      recentDecisions: [],
+      blockers: [],
+      nextAction: `Milestone ${activeMilestone.id} has a roadmap but no slices defined. Add slices to the roadmap.`,
+      registry,
+      requirements,
+      progress: {
+        milestones: milestoneProgress,
+        slices: { done: 0, total: 0 },
+      },
+    };
+  }
+
   // Check if active milestone needs validation or completion (all slices done)
   if (isMilestoneComplete(activeRoadmap)) {
     const validationFile = resolveMilestoneFile(basePath, activeMilestone.id, "VALIDATION");
diff --git a/src/resources/extensions/gsd/tests/derive-state.test.ts b/src/resources/extensions/gsd/tests/derive-state.test.ts
index 550cb567f..c228107a4 100644
--- a/src/resources/extensions/gsd/tests/derive-state.test.ts
+++ b/src/resources/extensions/gsd/tests/derive-state.test.ts
@@ -957,6 +957,28 @@ slice: S01
     }
   }
 
+  // ─── Test: zero-slice roadmap → pre-planning, not blocked (#1785) ────
+  console.log('\n=== zero-slice roadmap → pre-planning, not blocked (#1785) ===');
+  {
+    const base = createFixtureBase();
+    try {
+      // Write a stub roadmap with zero slices (placeholder text, no slice definitions)
+      writeRoadmap(base, 'M001', `# M001: Stub Milestone\n\n**Vision:** Placeholder.\n\n## Slices\n\n_No slices defined yet._\n`);
+
+      const state = await deriveState(base);
+
+      assertEq(state.phase, 'pre-planning', 'phase is pre-planning when roadmap has zero slices');
+      assertTrue(state.activeMilestone !== null, 'activeMilestone is set');
+      assertEq(state.activeMilestone?.id, 'M001', 'activeMilestone is M001');
+      assertEq(state.activeSlice, null, 'activeSlice is null');
+      assertEq(state.activeTask, null, 'activeTask is null');
+      assertEq(state.blockers.length, 0, 'no blockers reported');
+      assertTrue(state.nextAction.includes('M001'), 'nextAction references M001');
+    } finally {
+      cleanup(base);
+    }
+  }
+
   report();
 }