[v2] completable() loses completion when a Zod method is chained after it

Finding: In modelcontextprotocol/typescript-sdk, an external report says that chaining a Zod method such as .describe() after completable() silently drops the completion. The server then does not advertise the completions capability, and client.complete() fails with -32601 Method not found. The reported environments are main at 8aabbdce (zod 4.3.6, Node 24.19.0) and published 2.3.0 (zod 4.6.5), with v1 1.32.0 also reported. This is an external report with no KFA reproduction. Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/2949 Reported behavior: According to the issue author, completable(z.string(), cb).describe('Language') yields no completion, while completable(z.string().describe('Language'), cb) works. The author also reports the same loss with .min(1), .default('x'), .nullable() and .meta({}). .optional() is said to be the only method that survives, and order matters: .optional().describe('x') works but .describe('x').optional() does not. Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/2949 Cause status: The author's hypothesis is that completable() marks the schema instance it receives, that chained Zod methods return new instances without the mark, and that the mcp.ts lookup unwraps only one outer optional. A later commenter says they confirmed the behavior on current main. They point to completable.ts lines 51-58 and mcp.ts line 851. This is a non-maintainer source reading, not a maintainer-confirmed cause. Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/2949#issuecomment-5982938199 Maintainer and fix status: The supplied content shows no maintainer response. The issue is open, and no fix commit or release is shown. The commenter only suggests directions: re-attach the marker on chained results, or make the lookup see through wrappers. The author also notes that the migration guide advises .optional() after completable() and that the limitation is not documented. They offer a docs PR and mention #1474 as a related change that would remove the problem. Source: https://github.com/modelcontextprotocol/typescript-sdk/issues/2949 Practical diagnosis, source-backed and not verified by KFA: if a prompt argument uses completable() and the completions capability is missing or complete() returns -32601, check whether any Zod method other than a final .optional() follows completable(). Per the report, applying such methods to the inner schema first avoids the issue. Treat this as a reported ordering workaround rather than a confirmed fix. Limitations: Everything above is external-report evidence. KFA did not reproduce it, and the versions are as stated by the reporters. Middle comments may be missing from the supplied coverage.