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Mirror x-mcp-header tool arguments into Mcp-Param-* request headers (SEP-2243) (#2990)

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Marcelo Trylesinski 2026-06-26 14:36:56 +02:00 • committed by GitHub
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docs/migration.md
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@ -399,6 +399,10 @@ For an in-process `Client(server)` (where `server` is a `Server` or `MCPServer`
For protocol 2026-07-28, a `tools/call` request may return an `InputRequiredResult` asking the client to supply additional input and retry. By default `call_tool` (on `ClientSession`, `Client`, and `ClientSessionGroup`) still returns `CallToolResult` and raises `RuntimeError` if the server requests input. Pass `allow_input_required=True` to receive the `InputRequiredResult` instead, then retry with `input_responses=` / `request_state=`.
### `call_tool` mirrors `x-mcp-header` arguments into `Mcp-Param-*` headers (SEP-2243)
For protocol 2026-07-28 over Streamable HTTP, a tool's input-schema property may carry an `x-mcp-header` annotation. When a tool the client has listed is called, each annotated argument is mirrored into an `Mcp-Param-<name>` request header (string verbatim, integer as decimal, boolean as `true`/`false`, base64-sentinel-wrapped when not header-safe; `null`/absent arguments are omitted). The argument is also left in the request body. `list_tools` caches a tool's annotations, so list a tool before calling it to enable mirroring; a tool the client never listed emits no `Mcp-Param-*` headers. Other transports ignore the annotation.
### `McpError` renamed to `MCPError`
The `McpError` exception class has been renamed to `MCPError` for consistent naming with the MCP acronym style used throughout the SDK.