feat(client): store InitializeResult on ClientSession, decompose on Client

[maxisbey]

ClientSession now stores the full InitializeResult via an initialize_result property. The high-level Client exposes it as three decomposed, non-nullable properties.

Closes #1018. Supersedes #1565 and #2211.

Motivation and Context

Previously, ClientSession destructured only result.capabilities and discarded server_info, instructions, and protocol_version — users had to manually capture the return value of initialize(), which Client.__aenter__ throws away.

ClientSession.initialize_result

Stores the whole InitializeResult, returns InitializeResult | None. The | None is honest here — users who manage ClientSession directly can observe the pre-init state.

The name matches the type and the method that populates it (initialize() → initialize_result). Go's SDK calls this InitializeResult(); FastMCP calls it initialize_result. The earlier candidate name server_params collided with the established server_params = StdioServerParameters(...) idiom used throughout README, examples, tests, and ClientSessionGroup's ServerParameters type alias.

Client.server_capabilities / server_info / server_instructions

Inside async with Client(...) as client, initialization is guaranteed — __aenter__ calls initialize() before returning. So Client captures the result and exposes it as three non-nullable properties:

• server_capabilities: ServerCapabilities — same name as v1, now non-nullable instead of | None
• server_info: Implementation — the server's name/version
• server_instructions: str | None — None means the server didn't provide any (spec marks instructions optional)

Each raises RuntimeError if accessed outside the context manager, matching the existing session property.

The negotiated protocol_version is not exposed on Client — it's transport plumbing. Power users can reach it via client.session.initialize_result.protocol_version.

How Has This Been Tested?

• test_initialize_result asserts None before init and all four fields after
• test_client_is_initialized asserts all three Client properties
• test_client_server_properties_before_enter covers the RuntimeError paths
• ./scripts/test — 100% coverage, strict-no-cover clean
• pyright — 0 errors

Breaking Changes

• ClientSession.get_server_capabilities() removed → use session.initialize_result.capabilities
• Client.server_capabilities return type narrowed from ServerCapabilities | None to ServerCapabilities (raises RuntimeError outside context instead of returning None)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

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