fix: enable FTS fallback when no embedding provider available (#17725)

When no embedding provider is available (e.g., OAuth mode without API keys),
memory_search now falls back to FTS-only mode instead of returning disabled: true.

Changes:
- embeddings.ts: return null provider with reason instead of throwing
- manager.ts: handle null provider, use FTS-only search mode
- manager-search.ts: allow searching all models when provider is undefined
- memory-tool.ts: expose search mode in results

The search results now include a 'mode' field indicating 'hybrid' or 'fts-only'.

src/agents/tools/memory-tool.ts

@@ -81,12 +81,14 @@ export function createMemorySearchTool(options: {
           status.backend === "qmd"
             ? clampResultsByInjectedChars(decorated, resolved.qmd?.limits.maxInjectedChars)
             : decorated;
+        const searchMode = (status.custom as { searchMode?: string } | undefined)?.searchMode;
         return jsonResult({
           results,
           provider: status.provider,
           model: status.model,
           fallback: status.fallback,
           citations: citationsMode,
+          mode: searchMode,
         });
       } catch (err) {
         const message = err instanceof Error ? err.message : String(err);

src/memory/embeddings.test.ts

@@ -432,3 +432,63 @@ describe("local embedding normalization", () => {
     }
   });
 });
+
+describe("FTS-only fallback when no provider available", () => {
+  afterEach(() => {
+    vi.resetAllMocks();
+    vi.unstubAllGlobals();
+  });
+
+  it("returns null provider with reason when auto mode finds no providers", async () => {
+    vi.mocked(authModule.resolveApiKeyForProvider).mockRejectedValue(
+      new Error('No API key found for provider "openai"'),
+    );
+
+    const result = await createEmbeddingProvider({
+      config: {} as never,
+      provider: "auto",
+      model: "",
+      fallback: "none",
+    });
+
+    expect(result.provider).toBeNull();
+    expect(result.requestedProvider).toBe("auto");
+    expect(result.providerUnavailableReason).toBeDefined();
+    expect(result.providerUnavailableReason).toContain("No API key");
+  });
+
+  it("returns null provider when explicit provider fails with missing API key", async () => {
+    vi.mocked(authModule.resolveApiKeyForProvider).mockRejectedValue(
+      new Error('No API key found for provider "openai"'),
+    );
+
+    const result = await createEmbeddingProvider({
+      config: {} as never,
+      provider: "openai",
+      model: "text-embedding-3-small",
+      fallback: "none",
+    });
+
+    expect(result.provider).toBeNull();
+    expect(result.requestedProvider).toBe("openai");
+    expect(result.providerUnavailableReason).toBeDefined();
+  });
+
+  it("returns null provider when both primary and fallback fail with missing API keys", async () => {
+    vi.mocked(authModule.resolveApiKeyForProvider).mockRejectedValue(
+      new Error("No API key found for provider"),
+    );
+
+    const result = await createEmbeddingProvider({
+      config: {} as never,
+      provider: "openai",
+      model: "text-embedding-3-small",
+      fallback: "gemini",
+    });
+
+    expect(result.provider).toBeNull();
+    expect(result.requestedProvider).toBe("openai");
+    expect(result.fallbackFrom).toBe("openai");
+    expect(result.providerUnavailableReason).toContain("Fallback to gemini failed");
+  });
+});

src/memory/embeddings.ts

@@ -36,10 +36,11 @@ export type EmbeddingProviderFallback = EmbeddingProviderId | "none";
 const REMOTE_EMBEDDING_PROVIDER_IDS = ["openai", "gemini", "voyage"] as const;
 
 export type EmbeddingProviderResult = {
-  provider: EmbeddingProvider;
+  provider: EmbeddingProvider | null;
   requestedProvider: EmbeddingProviderRequest;
   fallbackFrom?: EmbeddingProviderId;
   fallbackReason?: string;
+  providerUnavailableReason?: string;
   openAi?: OpenAiEmbeddingClient;
   gemini?: GeminiEmbeddingClient;
   voyage?: VoyageEmbeddingClient;
@@ -183,15 +184,19 @@ export async function createEmbeddingProvider(
           missingKeyErrors.push(message);
           continue;
         }
+        // Non-auth errors (e.g., network) are still fatal
         throw new Error(message, { cause: err });
       }
     }
 
+    // All providers failed due to missing API keys - return null provider for FTS-only mode
     const details = [...missingKeyErrors, localError].filter(Boolean) as string[];
-    if (details.length > 0) {
-      throw new Error(details.join("\n\n"));
-    }
-    throw new Error("No embeddings provider available.");
+    const reason = details.length > 0 ? details.join("\n\n") : "No embeddings provider available.";
+    return {
+      provider: null,
+      requestedProvider,
+      providerUnavailableReason: reason,
+    };
   }
 
   try {
@@ -209,13 +214,31 @@ export async function createEmbeddingProvider(
           fallbackReason: reason,
         };
       } catch (fallbackErr) {
-        // oxlint-disable-next-line preserve-caught-error
-        throw new Error(
-          `${reason}\n\nFallback to ${fallback} failed: ${formatErrorMessage(fallbackErr)}`,
-          { cause: fallbackErr },
-        );
+        // Both primary and fallback failed - check if it's auth-related
+        const fallbackReason = formatErrorMessage(fallbackErr);
+        const combinedReason = `${reason}\n\nFallback to ${fallback} failed: ${fallbackReason}`;
+        if (isMissingApiKeyError(primaryErr) && isMissingApiKeyError(fallbackErr)) {
+          // Both failed due to missing API keys - return null for FTS-only mode
+          return {
+            provider: null,
+            requestedProvider,
+            fallbackFrom: requestedProvider,
+            fallbackReason: reason,
+            providerUnavailableReason: combinedReason,
+          };
+        }
+        // Non-auth errors are still fatal
+        throw new Error(combinedReason, { cause: fallbackErr });
       }
     }
+    // No fallback configured - check if we should degrade to FTS-only
+    if (isMissingApiKeyError(primaryErr)) {
+      return {
+        provider: null,
+        requestedProvider,
+        providerUnavailableReason: reason,
+      };
+    }
     throw new Error(reason, { cause: primaryErr });
   }
 }

src/memory/manager-embedding-ops.ts

@@ -202,6 +202,10 @@ class MemoryManagerEmbeddingOps {
   }
 
   private computeProviderKey(): string {
+    // FTS-only mode: no provider, use a constant key
+    if (!this.provider) {
+      return hashText(JSON.stringify({ provider: "none", model: "fts-only" }));
+    }
     if (this.provider.id === "openai" && this.openAi) {
       const entries = Object.entries(this.openAi.headers)
         .filter(([key]) => key.toLowerCase() !== "authorization")

src/memory/manager-search.ts

@@ -136,7 +136,7 @@ export function listChunks(params: {
 export async function searchKeyword(params: {
   db: DatabaseSync;
   ftsTable: string;
-  providerModel: string;
+  providerModel: string | undefined;
   query: string;
   limit: number;
   snippetMaxChars: number;
@@ -152,16 +152,20 @@ export async function searchKeyword(params: {
     return [];
   }
 
+  // When providerModel is undefined (FTS-only mode), search all models
+  const modelClause = params.providerModel ? " AND model = ?" : "";
+  const modelParams = params.providerModel ? [params.providerModel] : [];
+
   const rows = params.db
     .prepare(
       `SELECT id, path, source, start_line, end_line, text,\n` +
         `       bm25(${params.ftsTable}) AS rank\n` +
         `  FROM ${params.ftsTable}\n` +
-        ` WHERE ${params.ftsTable} MATCH ? AND model = ?${params.sourceFilter.sql}\n` +
+        ` WHERE ${params.ftsTable} MATCH ?${modelClause}${params.sourceFilter.sql}\n` +
         ` ORDER BY rank ASC\n` +
         ` LIMIT ?`,
     )
-    .all(ftsQuery, params.providerModel, ...params.sourceFilter.params, params.limit) as Array<{
+    .all(ftsQuery, ...modelParams, ...params.sourceFilter.params, params.limit) as Array<{
     id: string;
     path: string;
     source: SearchSource;

src/memory/manager.ts

@@ -46,10 +46,11 @@ export class MemoryIndexManager implements MemorySearchManager {
   private readonly agentId: string;
   private readonly workspaceDir: string;
   private readonly settings: ResolvedMemorySearchConfig;
-  private provider: EmbeddingProvider;
+  private provider: EmbeddingProvider | null;
   private readonly requestedProvider: "openai" | "local" | "gemini" | "voyage" | "auto";
   private fallbackFrom?: "openai" | "local" | "gemini" | "voyage";
   private fallbackReason?: string;
+  private readonly providerUnavailableReason?: string;
   private openAi?: OpenAiEmbeddingClient;
   private gemini?: GeminiEmbeddingClient;
   private voyage?: VoyageEmbeddingClient;
@@ -154,6 +155,7 @@ export class MemoryIndexManager implements MemorySearchManager {
     this.requestedProvider = params.providerResult.requestedProvider;
     this.fallbackFrom = params.providerResult.fallbackFrom;
     this.fallbackReason = params.providerResult.fallbackReason;
+    this.providerUnavailableReason = params.providerResult.providerUnavailableReason;
     this.openAi = params.providerResult.openAi;
     this.gemini = params.providerResult.gemini;
     this.voyage = params.providerResult.voyage;
@@ -225,6 +227,16 @@ export class MemoryIndexManager implements MemorySearchManager {
       Math.max(1, Math.floor(maxResults * hybrid.candidateMultiplier)),
     );
 
+    // FTS-only mode: no embedding provider available
+    if (!this.provider) {
+      if (!this.fts.enabled || !this.fts.available) {
+        log.warn("memory search: no provider and FTS unavailable");
+        return [];
+      }
+      const ftsResults = await this.searchKeyword(cleaned, candidates).catch(() => []);
+      return ftsResults.filter((entry) => entry.score >= minScore).slice(0, maxResults);
+    }
+
     const keywordResults = hybrid.enabled
       ? await this.searchKeyword(cleaned, candidates).catch(() => [])
       : [];
@@ -253,6 +265,10 @@ export class MemoryIndexManager implements MemorySearchManager {
     queryVec: number[],
     limit: number,
   ): Promise<Array<MemorySearchResult & { id: string }>> {
+    // This method should never be called without a provider
+    if (!this.provider) {
+      return [];
+    }
     const results = await searchVector({
       db: this.db,
       vectorTable: VECTOR_TABLE,
@@ -279,10 +295,12 @@ export class MemoryIndexManager implements MemorySearchManager {
       return [];
     }
     const sourceFilter = this.buildSourceFilter();
+    // In FTS-only mode (no provider), search all models; otherwise filter by current provider's model
+    const providerModel = this.provider?.model;
     const results = await searchKeyword({
       db: this.db,
       ftsTable: FTS_TABLE,
-      providerModel: this.provider.model,
+      providerModel,
       query,
       limit,
       snippetMaxChars: SNIPPET_MAX_CHARS,
@@ -446,6 +464,13 @@ export class MemoryIndexManager implements MemorySearchManager {
       }
       return sources.map((source) => Object.assign({ source }, bySource.get(source)!));
     })();
+
+    // Determine search mode: "fts-only" if no provider, "hybrid" otherwise
+    const searchMode = this.provider ? "hybrid" : "fts-only";
+    const providerInfo = this.provider
+      ? { provider: this.provider.id, model: this.provider.model }
+      : { provider: "none", model: undefined };
+
     return {
       backend: "builtin",
       files: files?.c ?? 0,
@@ -453,8 +478,8 @@ export class MemoryIndexManager implements MemorySearchManager {
       dirty: this.dirty || this.sessionsDirty,
       workspaceDir: this.workspaceDir,
       dbPath: this.settings.store.path,
-      provider: this.provider.id,
-      model: this.provider.model,
+      provider: providerInfo.provider,
+      model: providerInfo.model,
       requestedProvider: this.requestedProvider,
       sources: Array.from(this.sources),
       extraPaths: this.settings.extraPaths,
@@ -497,10 +522,18 @@ export class MemoryIndexManager implements MemorySearchManager {
         lastError: this.batchFailureLastError,
         lastProvider: this.batchFailureLastProvider,
       },
+      custom: {
+        searchMode,
+        providerUnavailableReason: this.providerUnavailableReason,
+      },
     };
   }
 
   async probeVectorAvailability(): Promise<boolean> {
+    // FTS-only mode: vector search not available
+    if (!this.provider) {
+      return false;
+    }
     if (!this.vector.enabled) {
       return false;
     }
@@ -508,6 +541,13 @@ export class MemoryIndexManager implements MemorySearchManager {
   }
 
   async probeEmbeddingAvailability(): Promise<MemoryEmbeddingProbeResult> {
+    // FTS-only mode: embeddings not available but search still works
+    if (!this.provider) {
+      return {
+        ok: false,
+        error: this.providerUnavailableReason ?? "No embedding provider available (FTS-only mode)",
+      };
+    }
     try {
       await this.embedBatchWithRetry(["ping"]);
       return { ok: true };