feat(tools): add initial langchain_tool implementation in experimental

[dbschmigelski]

Description

This PR introduces LangChainTool, a wrapper that enables integration of LangChain tools with Strands Agents. The implementation lives in strands.experimental.tools and allows users to wrap any LangChain BaseTool instance for use with a Strands Agent.

The wrapper accepts any LangChain tool variant since all LangChain tools inherit from BaseTool. This includes tools created with the @tool decorator (both sync and async), StructuredTool.from_function(), and custom BaseTool subclasses. The implementation extracts the tool's name, description, and input schema from the LangChain tool and converts them into a Strands-compatible ToolSpec. Users can optionally override the tool name and description if needed.

Since langchain-core is an optional dependency, the LangChainTool class uses a lazy loading pattern via __getattr__ in the module's __init__.py. This ensures that users without langchain-core installed won't encounter import errors unless they explicitly try to use LangChainTool.

. LangChain's TOOL_MESSAGE_BLOCK_TYPES includes additional content types like image, json, and document that may be added in future versions as needed. Currently, _convert_result_to_content only supports string results from LangChain tools. This clearly will not handle most use cases. But, what I want to do is have the scaffolding down, then we can expand the mappings as we go

As part of the review process I encourage everyone to pull this down and actually experiment.

Related Issues

N/A

Documentation PR

ToDo

Type of Change

New feature

Testing

The changes have been tested with both unit tests and integration tests covering all LangChain tool variants. All tests pass successfully.

• 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

---

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 6 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strands/experimental/tools/langchain_tool.py	92.98%	2 Missing and 2 partials ⚠️
src/strands/experimental/tools/__init__.py	66.66%	1 Missing and 1 partial ⚠️

📢 Thoughts on this report? Let us know!