metadataFilter on File Search tool returns zero grounding/empty responses (confirmed with raw @google/genai SDK)

Gemini API
1

Bug: metadataFilter on File Search breaks retrieval entirely

Environment

  • SDK: @google/genai (raw SDK, no AI SDK middleware)
  • Model: gemini-2.5-pro (also tested with gemini-3-flash-preview)
  • Runtime: Bun / Node.js

Setup

  • Single fileSearchStore with 20 PDF documents
  • Each document has customMetadata: [{ key: “partySigla”, stringValue: “” }]
  • Store and metadata confirmed working via ai.fileSearchStores.documents.list()

Reproduction

  • Without metadataFilter → works (2000+ chars, grounding metadata present):
const response = await ai.models.generateContent({
  model: "gemini-2.5-pro",
  contents: "¿Qué propone el PLN sobre educación?",
  config: {
    tools: [{
      fileSearch: {
        fileSearchStoreNames: [storeName],
      },
    }],
  },
});
// ✅ 2303 chars, groundingMetadata with chunks from other parties
// ❌ BUT: PLN document is never retrieved despite being in the store

With metadataFilter → completely broken (empty response, no grounding):

const response = await ai.models.generateContent({
  model: "gemini-2.5-pro",
  contents: "¿Qué propone el PLN sobre educación?",
  config: {
    tools: [{
      fileSearch: {
        fileSearchStoreNames: [storeName],
        metadataFilter: 'partySigla = "PLN"',
      },
    }],
  },
});
// ❌ 188 chars: "I couldn't find information about PLN"
// ❌ groundingMetadata: undefined (zero grounding chunks)

Observed behavior

  1. metadataFilter kills retrieval entirely — zero grounding chunks, model says it can’t find anything
  2. Even without the filter, certain documents are never retrieved despite being confirmed present in the store with correct metadata
  3. Tested on a freshly created store (deleted and re-uploaded all 20 documents) — same result

Expected behavior

metadataFilter should filter the retrieval to matching documents, not return zero results
All indexed documents should be retrievable by the file search tool