diff --git a/gitnexus/src/mcp/local/local-backend.ts b/gitnexus/src/mcp/local/local-backend.ts
index e99f81aa4..534989792 100644
--- a/gitnexus/src/mcp/local/local-backend.ts
+++ b/gitnexus/src/mcp/local/local-backend.ts
@@ -35,7 +35,10 @@ import {
   type ExactEmbeddingRow,
 } from '../../core/embeddings/exact-search.js';
 import { EMBEDDING_TABLE_NAME, EMBEDDING_INDEX_NAME } from '../../core/lbug/schema.js';
-import { getExactScanLimit } from '../../core/platform/capabilities.js';
+import {
+  getExactScanLimit,
+  isVectorExtensionSupportedByPlatform,
+} from '../../core/platform/capabilities.js';
 import { PhaseTimer } from '../../core/search/phase-timer.js';
 import { checkStaleness, checkCwdMatch } from '../../core/git-staleness.js';
 // AI context generation is CLI-only (gitnexus analyze)
@@ -223,6 +226,14 @@ export class LocalBackend {
    */
   private warnedSiblingDrift: Set<string> = new Set();
 
+  /**
+   * One-shot stderr warning for the VECTOR-extension fallback. Without this
+   * guard the diagnostic would fire on every `semanticSearch()` call on
+   * platforms where the extension is unsupported (e.g. Windows), making MCP
+   * stderr noisy per DoD §2.8.
+   */
+  private warnedVectorUnsupported = false;
+
   /**
    * Cross-repo group tools (CLI). Shares logic with MCP `group_*` handlers.
    */
@@ -1071,9 +1082,10 @@ export class LocalBackend {
         string,
         { distance: number; chunkIndex: number; startLine: number; endLine: number }
       >();
-      try {
-        bestChunks = await collectBestChunks(limit, async (fetchLimit) => {
-          const vectorQuery = `
+      if (isVectorExtensionSupportedByPlatform()) {
+        try {
+          bestChunks = await collectBestChunks(limit, async (fetchLimit) => {
+            const vectorQuery = `
             CALL QUERY_VECTOR_INDEX('${EMBEDDING_TABLE_NAME}', '${EMBEDDING_INDEX_NAME}',
               CAST(${queryVecStr} AS FLOAT[${dims}]), ${fetchLimit})
             YIELD node AS emb, distance
@@ -1084,17 +1096,28 @@ export class LocalBackend {
             ORDER BY distance
           `;
 
-          const embResults = await executeQuery(repo.id, vectorQuery);
-          return embResults.map((row) => ({
-            nodeId: row.nodeId ?? row[0],
-            chunkIndex: row.chunkIndex ?? row[1] ?? 0,
-            startLine: row.startLine ?? row[2] ?? 0,
-            endLine: row.endLine ?? row[3] ?? 0,
-            distance: row.distance ?? row[4],
-          }));
-        });
-      } catch {
-        bestChunks = new Map();
+            const embResults = await executeQuery(repo.id, vectorQuery);
+            return embResults.map((row) => ({
+              nodeId: row.nodeId ?? row[0],
+              chunkIndex: row.chunkIndex ?? row[1] ?? 0,
+              startLine: row.startLine ?? row[2] ?? 0,
+              endLine: row.endLine ?? row[3] ?? 0,
+              distance: row.distance ?? row[4],
+            }));
+          });
+        } catch {
+          bestChunks = new Map();
+        }
+      } else if (!this.warnedVectorUnsupported) {
+        // Rare diagnostic: surface why we fell back to the exact scan path so
+        // operators can see at a glance that the VECTOR extension is missing on
+        // this runtime (e.g. Windows builds without the optional native
+        // dependency). Emitted once per `LocalBackend` instance lifetime to
+        // avoid noisy stderr on hot semantic-search paths (DoD §2.8).
+        this.warnedVectorUnsupported = true;
+        console.error(
+          'GitNexus [query:vector]: VECTOR index unavailable for this runtime; using exact scan fallback',
+        );
       }
 
       if (bestChunks.size === 0) {
diff --git a/gitnexus/test/unit/calltool-dispatch.test.ts b/gitnexus/test/unit/calltool-dispatch.test.ts
index d57bd9051..2299f90a3 100644
--- a/gitnexus/test/unit/calltool-dispatch.test.ts
+++ b/gitnexus/test/unit/calltool-dispatch.test.ts
@@ -13,7 +13,7 @@ import { describe, it, expect, vi, beforeEach } from 'vitest';
 // local-backend.ts imports from core/lbug/pool-adapter.js; the mcp/core/lbug-adapter.js
 // re-exports from the same module, so we mock the canonical source.
 // vi.hoisted runs before vi.mock hoisting, making the fns available to both factories.
-const { lbugMocks } = vi.hoisted(() => ({
+const { lbugMocks, platformMocks } = vi.hoisted(() => ({
   lbugMocks: {
     initLbug: vi.fn().mockResolvedValue(undefined),
     executeQuery: vi.fn().mockResolvedValue([]),
@@ -21,6 +21,9 @@ const { lbugMocks } = vi.hoisted(() => ({
     closeLbug: vi.fn().mockResolvedValue(undefined),
     isLbugReady: vi.fn().mockReturnValue(true),
   },
+  platformMocks: {
+    isVectorExtensionSupportedByPlatform: vi.fn().mockReturnValue(true),
+  },
 }));
 
 vi.mock('../../src/core/lbug/pool-adapter.js', async (importOriginal) => {
@@ -48,6 +51,14 @@ vi.mock('../../src/core/git-staleness.js', () => ({
   checkCwdMatch: vi.fn().mockResolvedValue({ match: 'none' }),
 }));
 
+vi.mock('../../src/core/platform/capabilities.js', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('../../src/core/platform/capabilities.js')>();
+  return {
+    ...actual,
+    isVectorExtensionSupportedByPlatform: platformMocks.isVectorExtensionSupportedByPlatform,
+  };
+});
+
 // Also mock the search modules to avoid loading onnxruntime
 vi.mock('../../src/core/search/bm25-index.js', () => ({
   searchFTSFromLbug: vi.fn().mockResolvedValue([]),
@@ -157,6 +168,7 @@ describe('LocalBackend.callTool', () => {
 
   beforeEach(async () => {
     vi.clearAllMocks();
+    platformMocks.isVectorExtensionSupportedByPlatform.mockReturnValue(true);
     backend = new LocalBackend();
     setupSingleRepo();
     await backend.init();
@@ -181,6 +193,38 @@ describe('LocalBackend.callTool', () => {
     expect(result).toHaveProperty('definitions');
   });
 
+  it('skips vector index query when VECTOR is unsupported by the platform', async () => {
+    const consoleError = vi.spyOn(console, 'error').mockImplementation(() => undefined);
+    platformMocks.isVectorExtensionSupportedByPlatform.mockReturnValue(false);
+    (executeQuery as any).mockImplementation(async (_repoId: string, cypher: string) => {
+      if (cypher.includes('COUNT(*) AS cnt')) return [{ cnt: 1 }];
+      if (cypher.includes('MATCH (e:CodeEmbedding)')) return [];
+      return [];
+    });
+    (executeParameterized as any).mockResolvedValue([]);
+
+    try {
+      await backend.callTool('query', { query: 'auth' });
+
+      const queries = (executeQuery as any).mock.calls.map(
+        ([, cypher]: [string, string]) => cypher,
+      );
+      expect(queries.some((cypher: string) => cypher.includes('QUERY_VECTOR_INDEX'))).toBe(false);
+      expect(
+        queries.some(
+          (cypher: string) =>
+            cypher.includes('RETURN e.nodeId AS nodeId') &&
+            cypher.includes('e.embedding AS embedding'),
+        ),
+      ).toBe(true);
+      expect(consoleError).toHaveBeenCalledWith(
+        expect.stringContaining('GitNexus [query:vector]: VECTOR index unavailable'),
+      );
+    } finally {
+      consoleError.mockRestore();
+    }
+  });
+
   it('query tool returns error for empty query', async () => {
     const result = await backend.callTool('query', { query: '' });
     expect(result.error).toContain('query parameter is required');