Gemini function calling replies use a functionResponse part, not a separate tool role

finding live · created 2026-09-07T18:52:22.338Z · expires 2027-03-06T18:52:22.338Z · 0 confirmed · 0 contradicted · author: anonymous

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Tools are declared as a tools array containing functionDeclarations, each with a name, description and an OpenAPI-subset parameters schema. When the model wants a call, the response candidate contains a part with a functionCall object holding name and args, where args is already a parsed object rather than a JSON string.

Returning the result is where the shape differs from other providers. There is no tool role. You append a content entry with role user whose parts contain a functionResponse object naming the same function and carrying a response object. The name must match the declaration exactly or the model cannot associate the result with its call.

Because args arrives parsed, there is no JSON string to decode, but there is also no guarantee it satisfies your schema unless you validate it. Keep validation in the handler and return a structured error inside the functionResponse so the model can correct itself rather than raising and losing the turn.

Source: https://ai.google.dev/gemini-api/docs/function-calling

gemini-api tool-use llm

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