Send SEP-2243 Mcp-Method/Mcp-Name headers from the StreamableHTTP client

[vidigoat]

Summary

Fixes #2715. Per SEP-2243 (accepted — see the draft changelog), the Streamable HTTP transport requires Mcp-Method and Mcp-Name headers on POST requests so servers/intermediaries can route without parsing the JSON-RPC body. The Python client did not send them.

Changes

• _set_mcp_request_headers() sets, for outgoing POST messages:
  • Mcp-Method: the JSON-RPC method — for requests and notifications
  • Mcp-Name: params.name (tools/prompts) or, when absent, params.uri (resources)
  • responses/errors (no method) get neither header
• _encode_mcp_header_value() applies the SEP-2243 value-encoding rules to Mcp-Name: safe printable-ASCII values pass through; non-ASCII, control characters, or significant leading/trailing whitespace are wrapped as =?base64?<b64>?= (matching the spec's examples).

The encoding is not just spec-compliance — without it, a tool name/URI containing non-ASCII raises UnicodeEncodeError and breaks the request, and a value containing CR/LF is a header-injection vector. Verified both: café now sends as =?base64?Y2Fmw6k=?= and is accepted by httpx; "a\r\nEvil: 1" is base64-wrapped.

The broader x-mcp-header custom-header part of SEP-2243 is intentionally out of scope here (separate feature); this PR covers the Mcp-Method/Mcp-Name requirement from #2715.

Tests

• test_set_mcp_request_headers — request with name, with uri, no params, non-string name, notification, response, error, and a non-ASCII name (end-to-end encoding).
• test_encode_mcp_header_value — passthrough + each base64 trigger (non-ASCII, whitespace, control char, sentinel collision), using SEP-2243's own example values.

Full tests/shared/test_streamable_http.py passes (76 tests); ruff and pyright clean.

uv run --frozen pytest tests/shared/test_streamable_http.py

Authored with the assistance of an AI coding assistant; reviewed and verified by me.

[vidigoat]

Updated to also apply SEP-2243's value-encoding rules to Mcp-Name. Without it, a tool name or URI containing non-ASCII characters raises UnicodeEncodeError and breaks the request, and CR/LF in a value is a header-injection vector. Now safe printable-ASCII values pass through unchanged and everything else is wrapped as =?base64?<b64>?=, matching the spec's examples. Added test_encode_mcp_header_value covering passthrough plus each base64 trigger (non-ASCII, leading/trailing whitespace, control char, sentinel collision).