Fix GeminiModel to handle empty tools in Vertex AI mode

[pshiko]

Description

GeminiModel fails with 400 INVALID_ARGUMENT when no tools are provided in Vertex AI mode (GOOGLE_GENAI_USE_VERTEXAI=True). This issue does not occur with the standard Gemini API.

Root Cause:
The current implementation always sends a tools field with an empty function_declarations array. Vertex AI strictly enforces the proto oneof definition and rejects empty arrays, while the standard API accepts them.

Solution:
Modified _format_request_config to only include tools when tools are provided:

tools=self._format_request_tools(tool_specs) if tool_specs else None,

Changes:

• src/strands/models/gemini.py: Updated _format_request_config to conditionally set tools
• tests/strands/models/test_gemini.py: Updated unit tests to remove empty tools arrays from expected configs

Note:
This is a validation difference between the standard Gemini API and Vertex AI. While it may be a Gemini API inconsistency, the SDK must support both deployment modes. Omitting the tools field when no tools are provided works correctly for both backends.

Related Issues

#1039

Documentation PR

Type of Change

Bug fix

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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[Copilot]

Pull Request Overview

Fix GeminiModel to omit empty tools in Vertex AI mode to avoid INVALID_ARGUMENT errors caused by sending an empty function_declarations array.

• Conditionally set tools in _format_request_config only when tool_specs are provided.
• Updated tests to remove empty tools arrays from expected request configs.
• Ensures compatibility with both standard Gemini API and Vertex AI by aligning with Vertex AI’s oneof validation rules.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/strands/models/gemini.py	Applies conditional inclusion of tools in GenerateContentConfig to prevent sending empty tools.
tests/strands/models/test_gemini.py	Updates expected configs across tests to no longer include empty tools arrays.

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