feat(mcp): expose server instructions from InitializeResult on MCPClient

[ShotaroKataoka]

Description

Store InitializeResult.instructions on MCPClient.server_instructions so Host applications can access MCP server instructions and inject them into the LLM's system prompt.

The MCP specification defines instructions in InitializeResult for servers to provide LLMs with contextual knowledge about tool interdependencies and operational constraints. Currently MCPClient discards the InitializeResult, making instructions inaccessible to Host applications.

Changes:

• mcp_client.py: Store init_result.instructions after session.initialize() (3 lines added)
• conftest.py: Fix mock to return proper InitializeResult with instructions field
• test_mcp_client.py: Add 2 tests (default empty string, populated from server)

The SDK's responsibility is only to make the data accessible — how/whether to inject into the system prompt remains the Host's decision.

Related Issues

Closes #1813

Documentation PR

N/A — Public attribute server_instructions is self-documenting. Docs update can follow if needed.

Type of Change

New feature

Testing

• 2106 passed, 2 failed (pre-existing failures unrelated to this PR — region-dependent default model prefix apac vs us)
• 128 MCP tests all pass
• 2 new tests added

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

[github-actions]

Assessment: Comment

This is a well-scoped, minimal change that exposes MCP server instructions to Host applications - aligning with the MCP specification. The implementation is clean and tests are included.

Review Summary

• API Design: The existing review comment covers the key improvement - using str | None instead of str with empty string default to distinguish between "no instructions" vs "empty instructions"
• Code Organization: Public attribute should be positioned before private attributes
• Testing: Good coverage with tests for both the default case and populated instructions

Small, focused feature addition. Once the requested changes are addressed, this looks good to merge.

[github-actions]

Assessment: Approve ✅

The requested changes have been addressed:

1. ✅ Type annotation: Changed to str | None = None - correctly distinguishes "server didn't provide instructions" (None) from "server provided empty instructions" ("")
2. ✅ Attribute position: Moved above _consumers
3. ✅ Tests updated: Now correctly asserts is None for the default case

Clean implementation that aligns with the MCP specification. Ready to merge.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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