MCP Python SDK v2.2.0

What it claims

v2.2.0 changes several defaults in the MCP Python SDK (pip install -U mcp) that can break existing 2.x servers and clients on upgrade. The headline shifts are around HTTP redirect handling, idle session expiry, and OAuth issuer validation — plus two new deprecation warnings that foreshadow required arguments in 3.0.

Behaviour changes to check before upgrading

• Redirects are origin-scoped (#3397). Client(...), streamable_http_client and sse_client follow a redirect only if it stays on the same scheme/host/port (or upgrades http→https on the same host). Anything else fails with MCPError (SSE fails with httpx2.HTTPStatusError); the session stays usable. If the redirect target is really your server, set it as the endpoint URL directly. follow_redirects on a passed-in httpx2.AsyncClient is no longer used for MCP requests.
• Idle stateful Streamable HTTP sessions now expire (#3395). A stateful session with nothing in flight for 30 minutes is closed; the client's next request gets a 404 and must re-initialize. Clients that keep the GET stream open (the SDK's own Client does) are unaffected, as are stateless servers and 2026-07-28 connections. A server also caps at 10,000 concurrent sessions — beyond that, new sessions get a 503.
• Stricter OAuth issuer checks on the legacy path (#3398). Authorization-server metadata whose issuer isn't the server's own origin is rejected with OAuthFlowError: Authorization server metadata issuer mismatch. A 403 that isn't an insufficient_scope challenge is returned rather than retried; a 5xx/429 fetching protected-resource metadata now stops the flow instead of falling back to legacy endpoints.

Turning the new defaults off

mcp.run(transport="streamable-http", session_idle_timeout=None, max_sessions=None)

Same arguments exist on streamable_http_app() and run_streamable_http_async().

Deprecations aimed at 3.0

• ClientCredentialsOAuthProvider / PrivateKeyJWTOAuthProvider without issuer= warn now; 3.0 will require it.
• AuthSettings with resource_server_url set but validate_token_resource unset warns; 3.0 defaults it to True.
• Both still work in 2.x — this bites mainly if your tests promote warnings to errors.

Also new: AuthSettings.validate_token_resource (accept only tokens your TokenVerifier reports as issued for this server, #3447). Fix: $refs in a tool's outputSchema now resolve within that schema only; unresolvable ones surface as RuntimeError: Invalid schema for tool ... (#3394).

Limits

The tasks extension (SEP-2663), DPoP (SEP-1932), and the jwt-bearer grant are not yet implemented; the repo's ROADMAP.md tracks them.