MCP Version History and Compatibility

Replacement notice

The earlier page mixed several historical MCP releases. This replacement uses the current 2026-07-28 specification and its official changelog. The raw note is preserved under dumpster/AI/2026-09-02-legacy-ai-fact-check/raw-mcp/.

Key jargon

Term	Meaning
Protocol version	A dated MCP contract identifying a specific set of message and feature rules. S1
Compatibility	Whether two implementations can communicate under a mutually supported protocol contract. S1
Deprecated	Still present for a transition period but not recommended for new implementations. S2
Removed	No longer part of the current protocol. S2

What changed

MCP uses JSON-RPC 2.0 messages between hosts, clients, and servers. In the 2026-07-28 specification, the core uses stateless, self-contained requests and per-request capability information. S1

The 2026-07-28 release removed protocol-level sessions and the initialization handshake, introduced server/discover, moved Tasks to an extension, and deprecated HTTP+SSE, Roots, Sampling, Logging, and Dynamic Client Registration for new implementations. S2

flowchart LR
    A["Older stateful releases"] --> B["2025-11-25 compatibility"]
    B --> C["2026-07-28 stateless core"]
    C --> D["Extensions plus formal deprecation"]

Diagram semantics checked against the official changelog on 2026-09-02.

Compatibility rule

Do not say “MCP v1” without a dated protocol version. Record the exact version, inspect the official changelog, and test both peers. New code should target the current specification; compatibility code should isolate older stateful and HTTP+SSE behavior. S1