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Title: Send SEP-2243 Mcp-Method/Mcp-Name headers from the StreamableHTTP client by vidigoat · Pull Request #2730 · modelcontextprotocol/python-sdk · GitHub

Open Graph Title: Send SEP-2243 Mcp-Method/Mcp-Name headers from the StreamableHTTP client by vidigoat · Pull Request #2730 · modelcontextprotocol/python-sdk

X Title: Send SEP-2243 Mcp-Method/Mcp-Name headers from the StreamableHTTP client by vidigoat · Pull Request #2730 · modelcontextprotocol/python-sdk

Description: 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??= (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.

Open Graph Description: 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 w...

X Description: 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 w...

Opengraph URL: https://github.com/modelcontextprotocol/python-sdk/pull/2730

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b72f110 Send SEP-2243 Mcp-Method/Mcp-Name headers from the StreamableHTTP client vidigoat May 30, 2026 https://github.com/modelcontextprotocol/python-sdk/pull/2730/commits/b72f1103b63eca6004a88836d2c05566185cea96
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