fix(langchain): serialize ToolMessage/FunctionMessage with correct role and tool_call_id

[milanagm]

Problem

extractChatMessageContent in the LangChain CallbackHandler serializes ToolMessage and FunctionMessage with role: message.name. name is optional on ToolMessage, and idiomatic usage (new ToolMessage({ content, tool_call_id })) never sets it, so the serialized message ends up with no role at all. When name is set, the role becomes the tool's function name (e.g. "get_debt"), not a valid ChatML role. On top of that, tool_call_id was dropped entirely: it's a top-level field on ToolMessage, not part of additional_kwargs.

This breaks "Open in Playground" for any generation whose history contains tool results. The web playground converter (playgroundConverter.ts) only maps a message to a ToolResult when it finds a tool_call_id, and otherwise falls back to role || ChatMessageRole.Assistant, so tool results render as plain assistant messages and re-running the completion loses the tool linkage.

Fixes #930

Linear: https://linear.app/clickhouse/issue/LFE-15880/langchain-toolmessage-serialized-without-role-and-tool-call-id-breaks

Changes

• ToolMessage → { role: "tool", name, content, additional_kwargs, tool_call_id }
• FunctionMessage (same pattern one branch above) → { role: "function", name, content, additional_kwargs }
• name is kept as its own field rather than dropped, matching what LangChain itself sends to the OpenAI Chat Completions API and what the Python SDK's _convert_message_to_dict already does for ToolMessage/FunctionMessage.
• Extended the exported LlmMessage type with optional name and tool_call_id fields instead of attaching them with as any.

Verification

• pnpm lint
• pnpm typecheck
• pnpm test
• pnpm test:integration
• pnpm test:e2e (not run: requires a reachable Langfuse server and API credentials not available in this environment)
• pnpm format:check

Also verified end-to-end against a local Langfuse instance using the exact history from the issue: before the fix, the ToolMessage history opens in the Playground as a plain Assistant message with no tool linkage; after the fix, it opens as a Tool message linked to the originating tool call (call_1).

Release info

Bump level

• Major
• Minor
• Patch

Libraries affected

• All of them
• @langfuse/core
• @langfuse/client
• @langfuse/tracing
• @langfuse/otel
• @langfuse/openai
• @langfuse/langchain

Changelog notes

• Fixed @langfuse/langchain serializing ToolMessage/FunctionMessage without a valid role and dropping tool_call_id, which broke "Open in Playground" for traces containing tool results.

Confidence Score: 5/5

The PR appears safe to merge with no actionable correctness, security, or repository-rule issues identified.

Summary

• Serializes tool messages with role: "tool", optional name, and top-level tool_call_id.
• Serializes function messages with role: "function" and preserves name separately.
• Extends LlmMessage with typed optional metadata fields.
• Adds integration coverage for named and unnamed tool messages and function messages.

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