# What changed in mcp

> 36 changes in mcp 1 to 2 from its official release notes, upgrade guides and advisories: 18 breaking, 5 deprecations, 1 security, 6 new APIs and options. Each one cited. Sources checked 2026-10-18. HTML: https://unstale.sh/pypi/mcp/

To see which of these apply to a repository, run `npx @unstale/cli init` in it.

## mcp 2

### Breaking changes

- After a HeaderMismatch (-32020) rejection, call_tool re-lists the tools and retries once. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- initialize omits experimental when none is configured; client code reading capabilities.experimental on a legacy connection should handle None. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- The required version is now httpx2>=2.10.0, up from >=2.5.0; the new max_sse_event_size option depends on it. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- Validation now looks up the registered tool schema by name instead of running tools/list for every tools/call; middleware filtering or rewriting tools/list no longer affects validation. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- Empty _meta is omitted from outbound requests; on the receiving side ctx.meta is None rather than an empty mapping. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- Ping and list requests without a cursor omit params; middleware sees ctx.params as None for a request without params. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- Invalid header annotations now fail during tool registration with InvalidSignature, naming the tool and the problem. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- On the legacy OAuth discovery path, metadata is rejected when its issuer differs from the server's origin. (2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0)
- MCP HTTP clients and OAuth providers follow redirects only within the endpoint's origin, with an HTTP-to-HTTPS upgrade on the same host allowed; other redirects fail. (2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0)
- Streamable HTTP servers now allow at most 10,000 sessions by default; set this parameter to `None` to disable the limit. (1.30.0, 2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0), [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)
- A 403 response without an `insufficient_scope` challenge is returned to the caller instead of being retried. (2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0)
- OAuth discovery stops instead of falling back to legacy endpoints when protected resource metadata cannot be fetched because of a 5xx or 429 response. (1.30.0, 2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0), [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)
- Stateful legacy Streamable HTTP sessions now expire after 30 minutes of inactivity by default; set this parameter to `None` to disable the timeout. (1.30.0, 2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0), [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)

### Security

- Controls whether to accept only tokens that the TokenVerifier reports as issued for this server. (1.30.0, 2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0), [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)

### Deprecations

- When `resource_server_url` is set, leaving `validate_token_resource` unset emits an MCPDeprecationWarning; set it to `True` or `False` because 3.0 defaults it to `True`. (2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0)
- Constructing this provider without `issuer=` emits an MCPDeprecationWarning; pass the authorization server's issuer URL because 3.0 will require it. (1.30.0, 2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0), [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)
- Constructing this provider without `issuer=` emits an MCPDeprecationWarning; pass the authorization server's issuer URL because 3.0 will require it. (2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0)

### New APIs and options

- Adds an option to streamable_http_client and StreamableHttpParameters for setting the maximum SSE event size; pass None or raise it for larger tool results. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- Setting subscriptions=False stops serving subscriptions/listen and advertises listChanged and subscribe as false. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- Adds a way for a low-level server to supply a tool's schema for header validation without running its tools/list handler. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- ClientCredentialsOAuthProvider and PrivateKeyJWTOAuthProvider accept an issuer URL, which is required in 3.0. (2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0)

### Performance

- On Windows, stdio_client resolves the executable off the event loop. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)

### Behavior and default changes

- Client OpenTelemetry spans now record JSON-RPC error responses; in auto mode, a server without server/discover produces one ERROR span for the probe. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- ctx: Context[AppState] now works on prompts and resource templates, not only on tools. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- Request timeouts pause while redirect_handler and callback_handler wait for an interactive OAuth login; add a limit inside callback_handler if needed. (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)
- References resolve only within the tool's output schema; an unresolvable reference raises a RuntimeError identifying the invalid tool schema. (2.2.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.2.0)

### Fixes and other changes

- A progress_callback that raises no longer fails the call on an in-process Client(server). (2.3.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v2.3.0)

## mcp 1

### Breaking changes

- Authorization server metadata is rejected when its issuer does not match the server it was fetched for; client registrations are remembered per issuer. (1.30.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)
- OAuth provider requests follow the same-origin redirect rule as MCP HTTP clients. (1.30.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)
- Redirects are followed only within the endpoint's origin, or when upgrading HTTP to HTTPS on the same host; other redirects fail with httpx.HTTPStatusError. (1.30.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)
- Redirects are followed only within the endpoint's origin, or when upgrading HTTP to HTTPS on the same host; other redirects fail with httpx.HTTPStatusError. (1.30.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)
- Tool output-schema references are resolved only within the schema document. (1.30.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)

### Deprecations

- Setting resource_server_url without validate_token_resource emits a DeprecationWarning; set it to True or False. (1.30.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)
- Using the provider without issuer= emits a DeprecationWarning; pass the authorization server's issuer URL. (1.30.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)

### New APIs and options

- Accepts issuer= to specify the authorization server's issuer URL. (1.30.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)
- Accepts issuer= to specify the authorization server's issuer URL. (1.30.0) — [release notes](https://github.com/modelcontextprotocol/python-sdk/releases/tag/v1.30.0)