MCP: schema validation errors return a 3 kB raw Zod union dump in `error.message` while `error.issues` already holds the clean answer

[Densrt]

When an input fails a union-typed schema, the MCP tool returns an error.message that is the raw stringified ZodError: one entry per union branch, all identical, none naming the expected shape. The same response already carries a correctly flattened error.issues — so the useful message is computed, just not used.

This matters more for MCP than for a human CLI user: an agent reads message first, and 3 kB of repeated text is pure context burn for zero information. The CLI's own hand-written errors are excellent by comparison, which is what makes the contrast jarring:

delete-styles input.deletes is not supported. Expected one of: deletions.
update-design-token-styles input.token is not supported. Expected one of: designTokenId, updates. Did you mean designTokenId?

One turn to fix, no ambiguity. The Zod path gives the opposite.

Environment

webstudio@0.282.1, Node v22.23.1, Linux.

Reproduction

Pass a plain string where update-styles expects a StyleValue object — a very natural first attempt, since CSS values are strings everywhere else in the product:

$ webstudio update-styles '{"updates":[{"instanceId":"<id>","property":"color","value":"red"}],"dryRun":true}'

Result:

• error.message — 3068 characters, containing "Invalid input: expected object, received string" 15 times (once per branch of the StyleValue union), with "path": [] on every one.
• error.issues — already correct and minimal:

[{"code": "invalid_type", "path": ["value"],
  "message": "Invalid input: expected object, received string",
  "constraint": "type:object"}]

Nothing in either field says what a StyleValue actually looks like, so the caller still has to go read meta.get_more_tools to learn it is {"type":"keyword","value":"red"}.

Suggestion

1. Set error.message from the already-computed error.issues (dedup identical branch errors, keep the real path) instead of the raw Zod dump.
2. For union mismatches, name the expected shape and give one valid example — the tool's mcpExamples already contains it:

   update-styles updates[0].value must be a StyleValue object, received a string.
   Example: {"type":"keyword","value":"red"}
   

That would bring schema errors in line with the quality of the hand-written ones.