fix(vertex_ai/gemini): fix Gemini MIME detection for extensionless GCS URIs (addresses Greptile feedback on #27278)

[krrish-berri-2]

Relevant issues

Addresses Greptile feedback on PR #27278 (Fix Gemini MIME detection for extensionless GCS URIs).

Pre-Submission checklist

• I have Added testing in the tests/test_litellm/ directory
• 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
✅ Test

Changes

This PR implements the changes from PR #27278 with all Greptile-identified issues resolved:

Issues Fixed (from Greptile score 3/5)

1. Event-loop blocking — async_transform_request_body was calling _transform_request_body synchronously, which in turn calls _get_gcs_object_content_type (up to 5s sync httpx.get). Fixed by wrapping with asyncify() to offload to a thread pool worker.

2. Unmocked test making real HTTP calls — All tests now mock _get_gcs_object_content_type so no real outbound requests are made to storage.googleapis.com.

Core Feature (from PR #27278)

• GCS metadata lookup: _get_gcs_object_content_type resolves MIME type from GCS Storage JSON API for extensionless gs:// URIs
• litellm_params propagation: vertex_project/vertex_credentials are threaded from request body through _transform_messages → _gemini_convert_messages_with_history → _process_gemini_media → _get_gcs_object_content_type
• Security: Anonymous-only requests when no Vertex credentials are supplied (prevents server's default Google credentials from being used as an oracle for private GCS objects on the Google AI Studio path)
• MIME normalization: _normalize_and_validate_gemini_mime_type normalizes aliases (e.g., image/jpg → image/jpeg) and validates against Gemini's accepted file types
• Explicit MIME fields: mime_type/content_type accepted as aliases for format in image_url and file content blocks

Tests Added (12 new tests, all mock-only)

• GCS URI with extension — explicit MIME success path
• GCS URI without extension — metadata lookup success path
• GCS URI without extension — clear error when metadata unavailable
• Metadata MIME alias normalization (image/jpg → image/jpeg)
• Unsupported MIME type rejection
• Credential propagation to auth header
• No-credential path skips auth (security assertion)
• Event-loop non-blocking verification via concurrent sleep timing
• Dotted bucket name length validation
• File block with mime_type field
• Real extensionless file_id resolution via mocked metadata

Affected files

• litellm/llms/vertex_ai/gemini/transformation.py
• litellm/llms/gemini/chat/transformation.py
• litellm/llms/vertex_ai/gemini/vertex_and_google_ai_studio_gemini.py
• tests/test_litellm/llms/vertex_ai/test_vertex.py

  

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[codecov]

Codecov Report

❌ Patch coverage is 78.21782% with 22 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
litellm/llms/vertex_ai/gemini/transformation.py	77.77%	22 Missing ⚠️

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[github-actions]

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