[theamodhshetty]

Motivation and Context

The reqwest Streamable HTTP client rejects valid Application/JSON and Text/Event-Stream responses, while admitting different types such as application/jsonx and text/event-streaming. On a JSON 503 response, the case mismatch also loses the original JSON-RPC error.

Fixes #1389. This is separate from #1382, which changes server request validation.

Use one private matcher for the complete response media-type essence across GET admission, POST JSON/SSE dispatch and JSON error classification. Compare type/subtype case-insensitively, preserve parameter tolerance, and reject suffixes and comma-joined essences. Parameter bytes are left opaque. The existing accepted media types and body-free 429 policy are unchanged. No API or dependency changes.

How Has This Been Tested?

On macOS arm64 with Rust 1.96.1:

• 22 real HTTP cases against axum listeners: 12 fail on unchanged upstream 2a401c4, with 10 passing controls. All pass with this fix.
• Two additional byte-level cases preserve opaque parameter bytes and reject a non-ASCII essence.
• All 411 rmcp library tests and 61 tests across eight related HTTP integration targets pass.
• Strict Clippy for the library and tests passes with the relevant HTTP and stdio/child-process features enabled. Nightly workspace formatting and diff checks pass.
• Full cross-platform and feature-matrix CI were not run locally.

The wire tests exercise GET, POST JSON/SSE, parameters, absent headers, case variants, suffix/list rejection, JSON 503 preservation and 429 status handling. No live external service or credentials are used.

Breaking Changes

Previously misclassified response types such as application/jsonx now return the existing unexpected-content-type error on success responses. They are not JSON/SSE media types. Valid response types remain accepted, with added case-insensitive support.

Types of changes

• Bug fix
• New feature
• Breaking API change
• Documentation update

Additional context

Submitted as a draft under the one-open-ready-PR limit while #1387 is in review. Issue #1389 records the proposal; maintainer agreement on the scope is still pending. This patch is scoped to the reqwest backend; Unix-socket response handling is unchanged. Developed with Codex assistance. Local checks and hosted CI are reported separately.

[github-actions]

@DaleSeo (assignment failed) is on triage rotation this week and is the reviewer for this one.

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