Calling a tool with structured output that returns an array of objects throws an error #872

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1. olaservo

   added

   bugSomething isn't workingSomething isn't working

   cliIssues and PRs specific to cli modeIssues and PRs specific to cli mode

   p1-high

   on Oct 26, 2025
2. johnlanni

   added 2 commits that reference this issue on Nov 5, 2025

   feat: skip outputSchema generation for array root responses

   ...

   dc78185

   feat: skip outputSchema generation for array root responses (#26)

   ...

   d1c4cf0
3. olaservo

   removed

   bugSomething isn't workingSomething isn't working

   p1-high

   on Dec 4, 2025
4. olaservo commented on Dec 4, 2025

   

   olaservo

   on Dec 4, 2025

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   I was reviewing older issues and noticed this mentions returning an array, but structuredContent can only be an object per the MCP schema:

   "structuredContent": {
       "additionalProperties": {},
       "description": "An optional JSON object that represents the structured result of the tool call.",
       "type": "object"
   }

   This is a known limitation. (btw, [SEP-834](modelcontextprotocol/modelcontextprotocol#834) proposes loosening this restriction to allow any JSON (including arrays) validated by outputSchema.)

   Workaround: Wrap the array in an object:

   // Instead of:
   [{ "id": 1 }, { "id": 2 }]
   
   // Return:
   { "sessions": [{ "id": 1 }, { "id": 2 }] }

   With a corresponding outputSchema:

   {
     "type": "object",
     "properties": {
       "sessions": {
         "type": "array",
         "items": { ... }
       }
     }
   }
5. olaservo

   closed this as not plannedon Dec 4, 2025