Output validation error: Invalid structured content for tool

What it means

When a tool declares an outputSchema, the server SDK checks structuredContent against it before the result leaves the server. Our tool declared { sum: number } and returned { total: 'five' }. The TypeScript SDK v2 turned the result into an error result:

Output validation error: Invalid structured content for tool bad_output: sum: Invalid input: expected number, received undefined

with isError: true. This is a bug in the server, not in the client or the model: the tool ran, but its answer was thrown away.

The handler returns a different shape

Compare the field names and types in the message with what the handler builds. Typical slips: a renamed field (total vs sum), a number serialized as a string, or a nested object where the schema says flat. Fix whichever side is wrong, and add a test that calls the tool, for example with the Inspector CLI.

The handler returns no structuredContent

If a tool declares outputSchema, every successful result must include structuredContent. A tool that returned only content: [{ type: 'text', … }] got this instead:

Output validation error: Tool text_only has an output schema but no structured content was provided

Either return both (text for clients that only show text, structured data for programs), or remove outputSchema if the tool does not produce structured data.

Related

• Input validation error
• Your first server in TypeScript: a tool with both content and structuredContent.