MCP Protocol Versions and Compatibility
One of the easiest ways to create confusion in MCP projects is to mix up the protocol version with the version of a language SDK.
Protocol version
The MCP specification is versioned independently. The current major specification referenced in this course is 2026-07-28.
SDK version
Python, TypeScript, Go, C#, and other SDKs have their own package versions. Installing version 2 of a Python package does not mean you are using “MCP protocol v2.” An SDK can support more than one protocol revision and negotiate with the server.
Breaking changes
The 2026-07-28 release changed important assumptions: the core became stateless, header-based routing was added, cache semantics improved, authorization behavior hardened, and extensions became formal. It also established deprecation rules so features are not removed without a transition period.
Compatibility strategy
- Pin SDK versions in production.
- Read SDK migration notes before major upgrades.
- Test against the actual servers your application uses.
- Record negotiated protocol versions in logs.
- Avoid coding directly against deprecated behavior unless you need legacy compatibility.
- Keep integration tests for tool schemas and transport behavior.
Legacy transports and features
For example, OpenAI's Agents SDK currently labels HTTP with SSE as a legacy/deprecated MCP transport and recommends Streamable HTTP or stdio for new work. The 2026 MCP release also marked some older core features for deprecation while retaining compatibility during the policy window.