fix(@langchain/google-genai): support custom agent names in createAgent

[maslo55555]

Description

Fixes an issue where using createAgent with a custom name parameter (e.g., name: 'my_agent') causes Google GenAI to throw an error: Unknown / unsupported author: <custom_agent_name>.

Problem

When creating a ReactAgent with a custom name, the agent sets this name on AIMessage objects. However, Google GenAI's getMessageAuthor function was returning the custom name instead of the message type, causing convertAuthorToRole to fail since Google GenAI only supports standard roles: model, user, function, and system.

Solution

Modified getMessageAuthor in libs/providers/langchain-google-genai/src/utils/common.ts to always return the message type, ignoring custom names which are metadata for tracking in multi-agent scenarios but not supported by Google GenAI's API.

Changes

• Updated getMessageAuthor to return message type instead of message.name ?? type
• Added comprehensive tests to verify AIMessages with custom names work correctly
• Added documentation explaining why custom names are ignored

Testing

Added unit tests in chat_models.test.ts:

• Test that getMessageAuthor returns message type for AIMessages with custom names
• Test that convertBaseMessagesToContent handles messages with custom agent names without throwing errors

Example

After this fix, the following code works correctly:

import { createAgent, tool } from 'langchain';

const agent = createAgent({
  model: 'google-genai:gemini-2.0-flash',
  tools: [myTool],
  name: 'my_agent', // This now works!
});

[changeset-bot]

🦋 Changeset detected

Latest commit: 22b67e2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 6 packages

Name	Type
@langchain/google-genai	Patch
@langchain/google-common	Patch
@langchain/google-gauth	Patch
@langchain/google-webauth	Patch
@langchain/google-vertexai	Patch
@langchain/google-vertexai-web	Patch

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