Model Context Protocol specification 2026-07-28, Versioning and Compatibility

Publisher

Model Context Protocol project

URL

https://modelcontextprotocol.io/specification/2026-07-28/basic/versioning

Published

2026-07-28

Updated

unknown

Accessed

2026-09-05

HTTP status

200

License

MIT license; permitted under the specification license

Source notes citing this source

• Because legacy clients have no fall-forward mechanism, a server that must keep existing initialize-based integrations working while adopting 2026-07-28 has to be dual-era rather than modern-only, and it may serve both eras concurrently on one endpoint. inferred in context
• In MCP revision 2026-07-28, clients SHOULD cache the modern-or-legacy era determination for the lifetime of the server process on stdio or of the origin on HTTP, and MAY persist it across restarts of the same server configuration, re-probing if the cached assumption later fails. in context
• Per the MCP 2026-07-28 compatibility matrix, a legacy client against a modern-only server fails: on stdio the server rejects initialize with an implementation-defined JSON-RPC error, and on HTTP the request lacks the required headers and is rejected with 400 Bad Request; legacy clients have no fall-forward mechanism. in context
• The Model Context Protocol specification revisions published as of the access date are 2024-11-05 2025-03-26 2025-06-18 2025-11-25 and 2026-07-28 and the latest alias resolves to 2026-07-28. in context

Cite this source record

APA

WarmTransfer. (2026, July 28). Model Context Protocol specification 2026-07-28, Versioning and Compatibility. WarmTransfer. https://warmtransfer.net/knowledge/sources/mcp-spec-2026-07-28-versioning

BibTeX

@misc{warmtransfer-mcp-spec-2026-07-28-versioning,
  title  = {Model Context Protocol specification 2026-07-28, Versioning and Compatibility},
  author = {{WarmTransfer}},
  year   = {2026},
  url    = {https://warmtransfer.net/knowledge/sources/mcp-spec-2026-07-28-versioning},
  note   = {Model Context Protocol project, accessed 2026-09-05}
}