diff --git a/src/resources/extensions/gsd/migrate/writer.ts b/src/resources/extensions/gsd/migrate/writer.ts
index 27b627ba9..4fa12eaf9 100644
--- a/src/resources/extensions/gsd/migrate/writer.ts
+++ b/src/resources/extensions/gsd/migrate/writer.ts
@@ -483,6 +483,45 @@ export async function writeGSDDirectory(
       counts.research++;
     }
 
+    // For fully-completed milestones (all slices done), write a pass-through
+    // validation file so deriveState() doesn't enter validating-milestone
+    // phase for historical milestones that predate the validation gate (#819).
+    const allSlicesDone = milestone.slices.length > 0 && milestone.slices.every(s => s.done);
+    if (allSlicesDone) {
+      const validationPath = join(mDir, `${milestone.id}-VALIDATION.md`);
+      const validationContent = [
+        `---`,
+        `verdict: pass`,
+        `migrated: true`,
+        `---`,
+        ``,
+        `# ${milestone.id} Validation`,
+        ``,
+        `Migrated milestone — all slices were completed in the original project.`,
+        ``,
+      ].join('\n');
+      await saveFile(validationPath, validationContent);
+      paths.push(validationPath);
+      counts.other++;
+
+      // Also write a milestone summary if one doesn't exist
+      const summaryPath = join(mDir, `${milestone.id}-SUMMARY.md`);
+      const summaryContent = [
+        `---`,
+        `status: done`,
+        `migrated: true`,
+        `---`,
+        ``,
+        `# ${milestone.id}: ${milestone.title}`,
+        ``,
+        `Migrated from .planning — ${milestone.slices.length} slices completed.`,
+        ``,
+      ].join('\n');
+      await saveFile(summaryPath, summaryContent);
+      paths.push(summaryPath);
+      counts.other++;
+    }
+
     // Slices
     for (const slice of milestone.slices) {
       const sDir = join(mDir, 'slices', slice.id);
diff --git a/src/resources/extensions/gsd/tests/migrate-writer-integration.test.ts b/src/resources/extensions/gsd/tests/migrate-writer-integration.test.ts
index f86dae777..fca6a533b 100644
--- a/src/resources/extensions/gsd/tests/migrate-writer-integration.test.ts
+++ b/src/resources/extensions/gsd/tests/migrate-writer-integration.test.ts
@@ -11,6 +11,7 @@ import { writeGSDDirectory } from '../migrate/writer.ts';
 import { generatePreview } from '../migrate/preview.ts';
 import { parseRoadmap, parsePlan, parseSummary } from '../files.ts';
 import { deriveState } from '../state.ts';
+import { invalidateAllCaches } from '../cache.ts';
 import type {
   GSDProject,
   GSDMilestone,
@@ -207,6 +208,7 @@ async function main(): Promise<void> {
 
       // (e) deriveState
       console.log('  --- deriveState ---');
+      invalidateAllCaches();
       const state = await deriveState(base);
       assertEq(state.phase, 'executing', 'incomplete: deriveState phase is executing');
       assertTrue(state.activeMilestone !== null, 'incomplete: deriveState has activeMilestone');
@@ -262,14 +264,18 @@ async function main(): Promise<void> {
       assertTrue(!existsSync(join(m, 'M001-RESEARCH.md')), 'complete: M001-RESEARCH.md NOT written (null)');
       // No REQUIREMENTS.md since empty requirements
       assertTrue(!existsSync(join(base, '.gsd', 'REQUIREMENTS.md')), 'complete: REQUIREMENTS.md NOT written (empty)');
+      // Completed milestone should have VALIDATION and SUMMARY from migration (#819)
+      assertTrue(existsSync(join(m, 'M001-VALIDATION.md')), 'complete: M001-VALIDATION.md written for completed milestone');
+      assertTrue(existsSync(join(m, 'M001-SUMMARY.md')), 'complete: M001-SUMMARY.md written for completed milestone');
 
-      // deriveState: all slices done, all tasks done — needs validation then milestone summary
-      // Without VALIDATION file, it should be 'validating-milestone'
+      // deriveState: all slices done, all tasks done — migration now writes
+      // VALIDATION.md and SUMMARY.md for completed milestones (#819),
+      // so the milestone should be fully complete.
+      invalidateAllCaches();
       const state = await deriveState(base);
-      // All slices are done in roadmap. No VALIDATION or SUMMARY exists.
-      // deriveState should return 'validating-milestone' since validation gate precedes completion.
-      assertEq(state.phase, 'validating-milestone', 'complete: deriveState phase is validating-milestone');
-      assertTrue(state.activeMilestone !== null, 'complete: deriveState has activeMilestone');
+      assertEq(state.phase, 'complete', 'complete: deriveState phase is complete (validation + summary written by migration)');
+      // When all milestones are complete, activeMilestone points to the last entry (for display)
+      assertTrue(state.activeMilestone !== null, 'complete: deriveState has activeMilestone (last entry)');
       assertEq(state.activeMilestone!.id, 'M001', 'complete: deriveState activeMilestone is M001');
 
       // generatePreview for complete project