fix(server): negotiate initialize as legacy lifecycle

[AprilNEA]

Treat an initialize opener as selecting the legacy lifecycle, including when the client offers 2026-07-28. Negotiate only supported legacy revisions and reject initialize only when the server supports no legacy revision.

Motivation and Context

The 2026-07-28 protocol replaces the initialize handshake with the discover lifecycle. However, a client that still opens with initialize is explicitly selecting legacy lifecycle semantics.

Previously, a dual-era server could echo 2026-07-28 from initialize, even though that revision does not use the initialize lifecycle. Rejecting every such offer would also unnecessarily break clients capable of using an older revision.

This change makes a dual-era server negotiate the offer down to its preferred supported legacy revision (2025-11-25 by default). A modern-only server, with no supported legacy revision, returns Method Not Found.

The same rule is applied across direct transports, stateless HTTP handling, and stateful Streamable HTTP session classification. Discover/per-request modern requests remain sessionless.

How Has This Been Tested?

The following focused tests pass locally:

• cargo test -p rmcp --test test_protocol_version_negotiation --features client
• cargo test -p rmcp --test test_stateless_protocol_version --features 'transport-streamable-http-server reqwest'
• cargo test -p rmcp --test test_client_lifecycle_modes --features client high_level_server_accepts_discover_startup_without_initialize
• cargo test -p rmcp --test test_subscriptions_streamable_http --features 'client transport-streamable-http-client-reqwest transport-streamable-http-server reqwest' modern_http_lifecycle_stays_sessionless_for_older_application_version
• cargo clippy -p rmcp --lib --features client -- -D warnings
• cargo fmt --all --check
• git diff --check

The behavior was also exercised in MCPlex's Streamable HTTP gateway integration test:

• initialize(protocolVersion = 2026-07-28) opens a legacy session and negotiates 2025-11-25.
• Requests carrying complete modern per-request metadata remain stateless.
• The complete MCPlex test suite and clippy checks pass.

Breaking Changes

No API changes are required for users.

This corrects protocol negotiation behavior: an initialize request can no longer negotiate 2026-07-28. Servers supporting legacy revisions negotiate an appropriate legacy revision; modern-only servers reject the obsolete method.

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
• 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

The lifecycle is selected by how the client opens:

• initialize selects legacy lifecycle semantics, regardless of the offered revision.
• server/discover or a request carrying complete modern _meta selects the modern stateless lifecycle.

ServerHandler::supported_protocol_versions continues to advertise all supported revisions through discovery, while initialize negotiation considers only revisions older than 2026-07-28.

[Copilot]

Pull request overview

Adjusts MCP protocol negotiation so an initialize opener always selects the legacy lifecycle (even when the client offers the modern 2026-07-28 revision), negotiating down to a supported legacy revision when possible and rejecting initialize only when the server supports no legacy revisions. This aligns direct transports and streamable HTTP behavior with the intended dual-era semantics.

Changes:

• Update initialize protocol negotiation to consider only legacy revisions and return method not found when no legacy revision is supported.
• Ensure streamable HTTP request classification treats initialize as legacy lifecycle regardless of offered version.
• Expand/adjust tests to cover modern offers being negotiated down and modern-only servers rejecting initialize.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/rmcp/tests/test_stateless_protocol_version.rs	Updates/extends streamable HTTP tests to validate modern-offer negotiation-down and modern-only rejection behavior.
crates/rmcp/tests/test_protocol_version_negotiation.rs	Extends direct transport tests to verify modern initialize offers negotiate down to the preferred legacy revision.
crates/rmcp/src/transport/streamable_http_server/tower.rs	Applies legacy negotiation result to stateless HTTP initialize handling and treats initialize as selecting legacy lifecycle for classification.
crates/rmcp/src/service/server.rs	Changes negotiation function to return Option and rejects initialize when no legacy protocol is supported; updates handshake path accordingly.
crates/rmcp/src/service.rs	Updates docs to clarify how supported versions bound legacy initialize negotiation vs discover lifecycle selection.
crates/rmcp/src/handler/server.rs	Updates default ServerHandler::initialize to reject initialize when no legacy version can be negotiated.

Suppressed comments (1)

crates/rmcp/tests/test_stateless_protocol_version.rs:209

• Same issue as the JSON variant: this loop only skips exactly 2026-07-28, so it may fail when newer modern revisions are added. Skip >= 2026-07-28 to keep the test aligned with legacy-only initialize negotiation.

        if version == &ProtocolVersion::V_2026_07_28 {
            continue;
        }

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