### Summary
In affected versions, when a connected MCP server declared an `outputSchema` for a tool, the SDK's client validated each `call_tool` result with `jsonschema`'s default resolver, which fetches any `$ref` it cannot resolve locally by opening the URL. A server could therefore put an `http://`, `https://` or `file://` reference in a tool's output schema and make the client:
- request that URL, or read that local file, from the client's own network position on every call of the tool (the content was used only as a schema and never sent back to the server), and
- stall: the fetch ran synchronously on the event loop with no timeout, so a target that accepted the connection and then went quiet froze every session in the process for as long as it held the connection.
### Am I affected?
Yes, if your application uses the SDK as an MCP client (`ClientSession` over any transport, `ClientSessionGroup`, or the 2.x `Client`) and calls tools on servers you do not fully control. There was no setting that turned the validation off.
Affected versions: 1.10.0 through 1.29.1, and 2.0.0 through 2.1.1.
Not affected: applications that only build servers. If nothing in your code uses `ClientSession`, `ClientSessionGroup` or the 2.x `Client` class, you are not affected.
### Fix
Upgrade to `mcp` 1.30.0 (1.x) or 2.2.0 (2.x) or later. The client now resolves references only within the schema document the server sent (plus the standard JSON Schema metaschemas), so result validation performs no network or file access. A tool whose output schema points outside its own document fails at call time with `RuntimeError` (`Invalid schema for tool ...`); in-document references such as `#/$defs/...` work as before.
Until you can upgrade, call tools only on servers you trust. As a partial stopgap, skip tools whose `outputSchema` contains an `$id`, or a `$ref` / `$dynamicRef` that does not start with `#`.