Streamable HTTP: drain SSE frames until the initialize response, igno…

[min-mwei]

Streamable HTTP: drain SSE frames until the initialize response, ignoring early notifications to prevent handshake timeouts

Motivation and Context

The streamable HTTP transport can deliver server-sent notifications (e.g., logging or connection-established events) before the JSON‑RPC initialize response. The previous implementation read only the f
irst SSE frame and treated it as the response, which could stall initialization and cause later requests (like tools/list) to time out. This change makes the handshake resilient by waiting for the actual response.

How Has This Been Tested?

• rmcp crate builds and unit tests pass with default features (cargo test -p rmcp).
• The change is localized to SSE handling; manual verification is recommended against an MCP server that emits pre-response notifications to confirm the handshake completes and subsequent requests succeed.

Breaking Changes

None. Behavior is more permissive and robust during initialization; other transports are unaffected.

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 (https://modelcontextprotocol.io)
• 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

Additional context

• Implementation detail: StreamableHttpPostResponse::expect_initialized now drains SSE frames until a ServerJsonRpcMessage::Response is received, skipping empty payloads and logging intermediate no
  tifications with tracing::debug.
• Scope: crates/rmcp/src/transport/streamable_http_client.rs only; no changes to public API or dependencies.

Protocol Compliance

• MCP allows servers to send notifications at any time; clients must tolerate and ignore them until the corresponding response arrives. This change adheres to that model for the streamable HTTP transport.
• JSON-RPC semantics are preserved: initialization completes only upon receiving a Response message; notifications and requests received before that are not treated as completion.
• SSE specifics: event data is parsed as JSON per message; the client continues draining frames until the expected response is found.