Add Vertex AI Discovery Engine Rerank Support

[Sameerlite]

Title

Add Vertex AI Discovery Engine Rerank Support

Relevant issues

Fixes LIT-1107

Pre-Submission checklist

Please complete all items before asking a LiteLLM maintainer to review your PR

• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• I have added a screenshot of my new test passing locally
• 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

🆕 New Feature

Changes

Implementation

• Added Vertex AI Discovery Engine rerank support via VertexAIRerankConfig class
• Authentication: Reuses existing Vertex AI authentication infrastructure with proper Google Cloud headers
• URL Construction: Returns Discovery Engine ranking API URL with project ID
• Request Transformation: Converts Cohere-format requests to Vertex AI Discovery Engine format
• Response Transformation: Converts Vertex AI responses back to Cohere format for consistency
• Parameter Mapping: Maps return_documents to ignoreRecordDetailsInResponse for ID-only responses

Usage Examples

SDK Usage:

from litellm import rerank

response = rerank(
    model="vertex_ai/semantic-ranker-default@latest",
    query="What is Google Gemini?",
    documents=["doc1", "doc2", "doc3"],
    top_n=2,
    return_documents=True
)

Proxy Usage:

model_list:
  - model_name: semantic-ranker-default@latest
    litellm_params:
      model: vertex_ai/semantic-ranker-default@latest
      vertex_ai_project: "your-project-id"
      vertex_ai_location: "us-central1"
      vertex_ai_credentials: "path/to/service-account.json"

Supported Models

• semantic-ranker-default@latest (1024 tokens, 25 languages)
• semantic-ranker-fast@latest (1024 tokens, 25 languages)
• semantic-ranker-default-003 (512 tokens, 25 languages)
• semantic-ranker-default-002 (512 tokens, English only)

Testing

[vercel]

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Project	Deployment	Preview	Comments	Updated (UTC)
litellm	Ready	Preview	Comment	Oct 14, 2025 0:23am

[CLAassistant]

All committers have signed the CLA.

[ishaan-jaff]

LGTM!