translator: accept thinking_blocks signatures and fall back to skip_thought_signature_validator for Gemini function calls#2366
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… Gemini function calls
Gemini 3.x rejects multi-turn tool-calling requests with HTTP 400 when a
function call is missing a thought_signature. The OpenAI->Gemini request
translator only read signatures from assistant content parts of type
"thinking" and had no fallback when a client (correctly, per the OpenAI
schema) doesn't echo one back.
- Add ThinkingBlocks to ChatCompletionAssistantMessageParam so clients that
round-trip response thinking_blocks (LiteLLM convention) can supply a
signature on requests too.
- In assistantMsgToGeminiParts, fall back to the first non-empty
thinking_blocks signature when no content-part signature is present.
- When no signature is available at all, attach Google's documented
compatibility value ("skip_thought_signature_validator") to the first
functionCall part only, per
https://ai.google.dev/gemini-api/docs/thought-signatures.
Signed-off-by: Adam Nych <adam.nych@univio.com>
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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## main #2366 +/- ##
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Coverage 84.82% 84.83%
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missBerg
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This is an real issue, and we use gemini3 heavily, and always need workarounds for this. Can someone review this? |
mathetake
reviewed
Jul 22, 2026
| // that doesn't round-trip thinking_blocks). Gemini 3.x rejects function calls in | ||
| // multi-turn requests that are missing a thought_signature, so fall back to | ||
| // Google's documented compatibility escape. | ||
| funcCallPart.ThoughtSignature = []byte(dummyThoughtSignature) |
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this causes allocation. can you declare var whatever = []byte("skip_thought_signature_validator") as a global?
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Description
Gemini 3.x requires the thought signature (see [1]) returned with a
functionCallto be echoed back on that part in subsequent requests; otherwise Vertex rejects the request with400 INVALID_ARGUMENT("Function call is missing a thought_signature in functionCall parts"). The OpenAI → GCPVertexAI translator emits the signature in responses asmessage.thinking_blocks[].signature(#1995) but on the request side only reads it from assistantcontentparts of type"thinking"(#1726), and sends nothing when absent. Standard OpenAI-schema clients have no signature field to echo, so every tool round-trip with Gemini 3.x fails.This PR changes
assistantMsgToGeminiPartsto:thinking_blocks[].signature— the same shape the gateway itself emits, so round-tripping the response message now works. Content-part signatures keep priority.skip_thought_signature_validatoron the firstfunctionCallpart when the client echoed no signature at all (matching LiteLLM's behavior). Attached to the first part only, consistent with how Gemini attaches signatures for parallel calls.ChatCompletionAssistantMessageParamgains thethinking_blocksfield to make the request/response contract symmetric.Related Issues/PRs (if applicable)
Fixes #2365
Related PRs: #1726, #1995
Special notes for reviewers (if applicable)
gemini_helper_test.go/TestOpenAIMessagesToGeminiContentsexercised tool calls without a signature and now expect the dummy value — that's the intended new behavior, not weakened assertions.aigw runagainst real Vertex (gemini-3.5-flash,eu): the signature-less round-trip returns 200 with this patch and 400 onmain;thinking_blocksecho returns 200 and carries the real signature; streaming unaffected. Vertex accepts the dummy value (documented compatibility path, so no hard dependency on Google keeping undocumented behavior).1: https://ai.google.dev/gemini-api/docs/thought-signatures