diff --git a/src/resources/extensions/gsd/state.ts b/src/resources/extensions/gsd/state.ts
index e4877552e..fca596ecf 100644
--- a/src/resources/extensions/gsd/state.ts
+++ b/src/resources/extensions/gsd/state.ts
@@ -366,6 +366,10 @@ function buildCompletenessSet(basePath: string, milestones: MilestoneRow[]) {
   const completeMilestoneIds = new Set<string>();
   const parkedMilestoneIds = new Set<string>();
 
+  // DB-authoritative: a milestone is only "complete" when its DB row says so.
+  // SUMMARY-file presence is NOT a completion signal here — an orphan SUMMARY
+  // (crashed complete-milestone turn, partial merge, manual edit) must not
+  // flip derived state to complete and cascade into a false auto-merge (#4179).
   for (const m of milestones) {
     const parkedFile = resolveMilestoneFile(basePath, m.id, "PARKED");
     if (parkedFile || m.status === 'parked') {
@@ -376,11 +380,6 @@ function buildCompletenessSet(basePath: string, milestones: MilestoneRow[]) {
       completeMilestoneIds.add(m.id);
       continue;
     }
-    const summaryFile = resolveMilestoneFile(basePath, m.id, "SUMMARY");
-    if (summaryFile) {
-      completeMilestoneIds.add(m.id);
-      continue;
-    }
   }
   return { completeMilestoneIds, parkedMilestoneIds };
 }
@@ -409,18 +408,22 @@ async function buildRegistryAndFindActive(
       if (isGhostMilestone(basePath, m.id)) continue;
     }
 
-    const summaryFile = resolveMilestoneFile(basePath, m.id, "SUMMARY");
-
-    if (completeMilestoneIds.has(m.id) || (summaryFile !== null)) {
+    // DB-authoritative completeness (#4179): only trust completeMilestoneIds,
+    // which is itself derived from DB status. SUMMARY-file presence alone must
+    // not imply completion. The summary file may still be consulted below as a
+    // title source for legitimately-complete milestones whose DB row has no title.
+    if (completeMilestoneIds.has(m.id)) {
       let title = stripMilestonePrefix(m.title) || m.id;
-      if (summaryFile && !m.title) {
-        const summaryContent = await loadFile(summaryFile);
-        if (summaryContent) {
-          title = parseSummary(summaryContent).title || m.id;
+      if (!m.title) {
+        const summaryFile = resolveMilestoneFile(basePath, m.id, "SUMMARY");
+        if (summaryFile) {
+          const summaryContent = await loadFile(summaryFile);
+          if (summaryContent) {
+            title = parseSummary(summaryContent).title || m.id;
+          }
         }
       }
       registry.push({ id: m.id, title, status: 'complete' });
-      completeMilestoneIds.add(m.id);
       continue;
     }
 
@@ -461,7 +464,14 @@ async function buildRegistryAndFindActive(
         const validationContent = validationFile ? await loadFile(validationFile) : null;
         const validationTerminal = validationContent ? isValidationTerminal(validationContent) : false;
 
-        if (!validationTerminal || (validationTerminal && !summaryFile)) {
+        // DB-authoritative (#4179): completeness is already decided by
+        // completeMilestoneIds above. If we reached this branch, the DB says
+        // the milestone is NOT complete — so any SUMMARY file on disk is an
+        // orphan (crashed complete-milestone, partial merge, manual edit) and
+        // must not short-circuit this path. When validation is terminal, fall
+        // through to the default active-push below so `complete-milestone` can
+        // re-run idempotently.
+        if (!validationTerminal) {
           activeMilestone = { id: m.id, title };
           activeMilestoneSlices = slices;
           activeMilestoneFound = true;
diff --git a/src/resources/extensions/gsd/tests/derive-state-helpers.test.ts b/src/resources/extensions/gsd/tests/derive-state-helpers.test.ts
index 035e5efb2..c4f7f5ddb 100644
--- a/src/resources/extensions/gsd/tests/derive-state-helpers.test.ts
+++ b/src/resources/extensions/gsd/tests/derive-state-helpers.test.ts
@@ -307,27 +307,87 @@ describe('derive-state-helpers', () => {
     }
   });
 
-  // ─── buildCompletenessSet: SUMMARY-on-disk marks complete ───────────
-  test('buildCompletenessSet: milestone with SUMMARY on disk treated as complete', async () => {
+  // ─── buildCompletenessSet: DB status is authoritative ──────────────
+  test('buildCompletenessSet: DB status=complete marks milestone complete', async () => {
     const base = createFixtureBase();
     try {
-      // M001 has summary on disk but DB status is still 'active'
       writeFile(base, 'milestones/M001/M001-ROADMAP.md', ROADMAP_CONTENT);
       writeFile(base, 'milestones/M001/M001-SUMMARY.md', '# M001 Summary\n\nDone.');
-      // M002 is the real active milestone
       writeFile(base, 'milestones/M002/M002-CONTEXT.md', '# M002\n\nActive.');
 
       openDatabase(':memory:');
-      insertMilestone({ id: 'M001', title: 'First', status: 'active' });
+      insertMilestone({ id: 'M001', title: 'First', status: 'complete' });
       insertMilestone({ id: 'M002', title: 'Second', status: 'active' });
 
       invalidateStateCache();
       const state = await deriveStateFromDb(base);
 
-      // M001 should be complete (summary on disk), M002 should be active
       const m1 = state.registry.find(e => e.id === 'M001');
-      assert.equal(m1?.status, 'complete', 'summary-disk: M001 marked complete via disk SUMMARY');
-      assert.equal(state.activeMilestone?.id, 'M002', 'summary-disk: M002 is active');
+      assert.equal(m1?.status, 'complete', 'DB status=complete → registry entry complete');
+      assert.equal(state.activeMilestone?.id, 'M002', 'M002 is the active milestone');
+    } finally {
+      closeDatabase();
+      cleanup(base);
+    }
+  });
+
+  // ─── Regression #4179: orphan SUMMARY must NOT flip DB-active milestone ───
+  // A crashed complete-milestone turn (or stale/manual SUMMARY.md) can leave
+  // a milestone SUMMARY on disk while the DB row still reads 'active'. The
+  // read-side of state derivation must NOT treat the orphan SUMMARY as a
+  // completion signal, or the auto-loop advances and merges work that was
+  // never actually finished (same failure class as #4175, read-side twin).
+  test('buildCompletenessSet (#4179): orphan SUMMARY on disk does not mark DB-active milestone complete', async () => {
+    const base = createFixtureBase();
+    try {
+      writeFile(base, 'milestones/M001/M001-ROADMAP.md', ROADMAP_CONTENT);
+      writeFile(base, 'milestones/M001/M001-SUMMARY.md', '# M001 Orphan Summary\n\nLeft over from crashed turn.');
+
+      openDatabase(':memory:');
+      insertMilestone({ id: 'M001', title: 'First', status: 'active' });
+      // Slice still in-flight — auto should resume, not merge.
+      insertSlice({ id: 'S01', milestoneId: 'M001', title: 'First', status: 'active', risk: 'low', depends: [] });
+      insertSlice({ id: 'S02', milestoneId: 'M001', title: 'Second', status: 'pending', risk: 'low', depends: ['S01'] });
+      insertTask({ id: 'T01', sliceId: 'S01', milestoneId: 'M001', title: 'In-flight', status: 'pending' });
+
+      invalidateStateCache();
+      const state = await deriveStateFromDb(base);
+
+      const m1 = state.registry.find(e => e.id === 'M001');
+      assert.notEqual(m1?.status, 'complete', 'orphan SUMMARY must not mark milestone complete');
+      assert.equal(m1?.status, 'active', 'M001 remains active — DB is authoritative');
+      assert.equal(state.activeMilestone?.id, 'M001', 'M001 is still the active milestone');
+      assert.notEqual(state.phase, 'completing-milestone', 'must not short-circuit into completion');
+    } finally {
+      closeDatabase();
+      cleanup(base);
+    }
+  });
+
+  // Regression #4179 (companion): DB-active milestone with all slices done +
+  // validation terminal + orphan SUMMARY must still flow through completing-milestone
+  // (re-runs complete-milestone), not be reported as already-complete.
+  test('buildRegistryAndFindActive (#4179): orphan SUMMARY with validation-terminal falls through to completing-milestone', async () => {
+    const base = createFixtureBase();
+    try {
+      writeFile(base, 'milestones/M001/M001-ROADMAP.md', ROADMAP_CONTENT);
+      writeFile(base, 'milestones/M001/slices/S01/S01-PLAN.md', PLAN_CONTENT);
+      writeFile(base, 'milestones/M001/slices/S02/S02-PLAN.md', PLAN_CONTENT);
+      writeFile(base, 'milestones/M001/M001-VALIDATION.md', '---\nverdict: passed\n---\n# Validation\nAll good.');
+      writeFile(base, 'milestones/M001/M001-SUMMARY.md', '# M001 Orphan Summary\n\nLeft over.');
+
+      openDatabase(':memory:');
+      insertMilestone({ id: 'M001', title: 'First', status: 'active' });
+      insertSlice({ id: 'S01', milestoneId: 'M001', title: 'First', status: 'complete', risk: 'low', depends: [] });
+      insertSlice({ id: 'S02', milestoneId: 'M001', title: 'Second', status: 'complete', risk: 'low', depends: ['S01'] });
+
+      invalidateStateCache();
+      const state = await deriveStateFromDb(base);
+
+      const m1 = state.registry.find(e => e.id === 'M001');
+      assert.equal(m1?.status, 'active', 'M001 stays active despite orphan SUMMARY + validation-terminal');
+      assert.equal(state.activeMilestone?.id, 'M001', 'M001 is still the active milestone');
+      assert.equal(state.phase, 'completing-milestone', 'phase flows through completing-milestone (re-run)');
     } finally {
       closeDatabase();
       cleanup(base);