fix(gemini): add missing role="user" to function response content blocks

[Chesars]

Relevant issues

Fixes #22003
Fixes #20690
Ref #17949

Pre-Submission checklist

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• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• My PR passes all unit tests on make test-unit
• My PR's scope is as isolated as possible, it only solves 1 specific problem

Type

🐛 Bug Fix

Changes

Gemini API only accepts two roles: "user" and "model". Function responses
must be sent with role: "user" per the
Gemini API spec.

_gemini_convert_messages_with_history() was appending function response
ContentType blocks without a role field (2 call sites), causing Gemini to
reject multi-turn tool-calling conversations with:

• "Please use a valid role: user, model." ([Bug]: gemini-3 multi-turn function calling error #22003)
• INVALID_ARGUMENT 400 ([Bug]: Gemini function_response missing role="user" causes INVALID_ARGUMENT #20690)

The fix adds role="user" to both ContentType(parts=tool_call_responses)
call sites in transformation.py.

Verified against the live Gemini API with a multi-turn function calling
conversation.

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Project	Deployment	Actions	Updated (UTC)
litellm	Ready	Preview, Comment	Feb 28, 2026 4:14pm

[greptile-apps]

Greptile Summary

This PR fixes a bug where _gemini_convert_messages_with_history() was constructing ContentType blocks for tool/function responses without a role field. The Gemini API requires every content block to have role: "user" or role: "model", and function responses must use role: "user" per the Gemini API spec. The missing role caused 400 INVALID_ARGUMENT errors during multi-turn tool-calling conversations.

• Adds role="user" to both ContentType(parts=tool_call_responses) call sites in transformation.py (lines 503 and 513)
• Adds two mock-only unit tests covering single-turn and multi-turn function calling scenarios
• All 5 ContentType(...) usages in the function now consistently include an explicit role
• Fixes [Bug]: gemini-3 multi-turn function calling error #22003 and [Bug]: Gemini function_response missing role="user" causes INVALID_ARGUMENT #20690

Confidence Score: 5/5

• This PR is safe to merge — it's a minimal, well-scoped bug fix with clear evidence of correctness.
• The change is a two-line fix adding a required field (role="user") to two call sites, matching the Gemini API spec and the existing pattern used by all other ContentType usages in the same function. The fix is well-tested with two mock-only unit tests, verified against the live API per the PR description, and no regressions are likely since the field was simply missing before.
• No files require special attention

Important Files Changed

Filename	Overview
litellm/llms/vertex_ai/gemini/transformation.py	Adds role="user" to both ContentType(parts=tool_call_responses) call sites (lines 503 and 513), fixing Gemini API 400 errors on multi-turn tool-calling conversations. The fix is minimal, correct, and consistent with the Gemini API spec.
tests/test_litellm/llms/vertex_ai/gemini/test_vertex_ai_gemini_transformation.py	Adds two well-structured mock-only tests covering single-turn and multi-turn function calling scenarios, asserting that all function response content blocks have role="user". Minor: missing trailing newline.

Sequence Diagram

sequenceDiagram
    participant User as User Message
    participant LiteLLM as _gemini_convert_messages_with_history()
    participant Gemini as Gemini API

    User->>LiteLLM: messages (OpenAI format with tool results)
    Note over LiteLLM: Convert user content → ContentType(role="user")
    Note over LiteLLM: Convert assistant content → ContentType(role="model")
    Note over LiteLLM: Convert tool responses → ContentType(role="user") ← FIX
    LiteLLM->>Gemini: contents[] with all roles set
    Gemini-->>LiteLLM: Valid response (no 400 error)

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[greptile-apps]

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[shin-bot-litellm]

Review

1. Does this PR fix the issue it describes?
Yes. Fixes #22003 and #20690 — Gemini was rejecting multi-turn tool-calling conversations with "Please use a valid role: user, model" errors.

The root cause: ContentType(parts=tool_call_responses) was created without a role field. Per Gemini API spec, function responses must have role="user".

The fix adds role="user" to both callsites in transformation.py. Verified against live Gemini API.

2. Has this issue already been solved elsewhere?
No. Searched for other Gemini function response handling — this is the correct place to fix it. The two issues (#22003, #20690) had been open without resolution.

✅ LGTM — minimal, correct fix with test coverage and live API verification.

[jquinter]

@Chesars can you rebase with main? most of tests failed

[Chesars]

@Chesars can you rebase with main? most of tests failed

Rebased 👍

[jquinter]

CI Failure: PLR0915 lint errors

These 3 PLR0915 "too many statements" errors are pre-existing on main and unrelated to this PR.

Fix PR: #22328

[Chesars]

Thanks @jquinter