T8 - serve 2026-07-28 requests next to legacy requests

Target branch: 2026-07-28-protocol-support

Requires:

• https://github.com/softwaremill/chimp/issues/239
• https://github.com/softwaremill/chimp/issues/242

Server accepts 2026-07-28 clients and 2025-xx clients on the same endpoint. Each request is served by the rules of its own protocol revision. Legacy clients see no change in behaviour.

Era selection

• A request is a 2026-07-28 request when it carries _meta["io.modelcontextprotocol/protocolVersion"], when its MCP-Protocol-Version header names a stateless version, or when it uses a method that exists only in 2026-07-28, every other request is legacy
• On stdio there are no headers so _meta and the method decide
• The server never uses an earlier request to interpret the current one

Modern request validation

• A request without _meta.protocolVersion or _meta.clientCapabilities fails with -32602.
• A version in the header that differs from the version in `_meta`` fails with -32020.
• A version chimp does not support fails with -32022, with supported and requested in the error data.
• Any unknown method fail with -32601. notifications/initialized is not accepted.
• Over HTTP, validation errors return 400 and unknown methods return 404, as a plain JSON error and never as an SSE stream, every error response echoes the request id

server/discover

• The server answers with supported version (stateless version only), capabilities, instructions, cache hints and server info

Modern results

• Every result carries resultType: complete and _meta.serverInfo
• The cacheable results (tools/list, prompts/list, resources/list, resources/templates/list, resources/read, server/discover) carry TTL and cache scope, with 0 and private as fixed defaults
• An unknown resource fails with -32602 and data.uri, legacy requests keep -32002.
• notifications/message is sent only when the request carried _meta.logLevel, and only at or above that level

Request facts for handlers

• A tool, prompt or resource handler can read the request's protocol version, client capabilities, client info and log level via server context

Coverage

• The rules above hold for the sync and streaming servers, over HTTP and stdio, on ZIO, Ox and Pekko
• The conformance server offers the fixture tools test_missing_capability and test_logging_tool

Spec:

Basic - Statelessness
Basic - _meta
Basic - Error Codes
Versioning - Protocol Version Negotiation
Versioning - Backward Compatibility
Streamable HTTP - Protocol Version Header
Streamable HTTP - Standard Request Headers
Streamable HTTP - Server Validation
Streamable HTTP - Earlier Streamable HTTP Revisions
stdio - Request Metadata
server/discover
Caching - Cacheable Results
Resources - Error Handling
Logging - Per-request log level

Conformance scenarios that must pass after this task:

• Server: caching, sep-2164-resource-not-found, dns-rebinding-protection, completion-complete, tools-list,
  tools-call-simple-text, tools-call-image, tools-call-audio, tools-call-embedded-resource,
  tools-call-mixed-content, tools-call-error, tools-call-with-progress, server-sse-multiple-streams,
  resources-list, resources-read-text, resources-read-binary, resources-templates-read, prompts-list,
  prompts-get-simple, prompts-get-with-args, prompts-get-embedded-resource, prompts-get-with-image,
  input-required-result-unsupported-methods. They leave the 2026-07-28 baseline.

Docs: yes.

• New page docs/server/protocol.md (added to the toctree): supported versions, how the server selects the wire per
  request, what a handler can read from the request context, the 400 and 404 mapping, no sessions
  and no GET or DELETE, the resource error codes per era

Start by reviewing the target branch and the prerequisite issues 239 and 242, then use the listed conformance scenarios as the behavioral checklist across HTTP, stdio, sync, and streaming servers. Add docs/server/protocol.md to the toctree and document version selection, request context, HTTP errors, sessions, methods, and era-specific resource errors. Done means the scenarios pass and the 2026-07-28 baseline remains intact.