Sub-agent MCP tool calls fail with structuredContent: expected record, received array when server returns JSON arrays

[mihaiLucian]

Describe the bug

When a sub-agent launched via the task tool calls an MCP tool that returns a JSON array, the call fails with a Zod validation error. The exact same tool with the exact same parameters works fine when called from the primary/orchestrator agent.

The error is:

MCP server 'my-mcp-server': [{"expected":"record","code":"invalid_type","path":["structuredContent"],"message":"Invalid input: expected record, received array"}]

The root cause is in sdk/index.js where the CallToolResult schema defines structuredContent: z.record(z.string(), z.unknown()).optional(). z.record() only accepts plain objects, so when an MCP server returns a JSON array, the Zod validation rejects it. This validation only runs in the sub-agent code path, not the primary agent path.

Context	Response type	Result
Primary agent	JSON array	Works
Primary agent	String	Works
Primary agent	JSON object	Works
Sub-agent (via task)	String	Works
Sub-agent (via task)	JSON object	Works
Sub-agent (via task)	JSON array	Fails

Affected version

Reproduced on 1.0.36 and 1.0.39 (latest as of 2026-04-29). Using @github/copilot-sdk 0.3.0.

Steps to reproduce the behavior

1. Register a stdio MCP server with a tool that returns a JSON array:

   [
     { "id": "abc-123", "status": "Succeeded", "startTime": "2026-04-27T10:00:00Z" },
     { "id": "def-456", "status": "Running", "startTime": "2026-04-28T01:00:00Z" }
   ]

2. Configure a custom sub-agent with access to that tool:

   name: my-agent
   tools:
     - my-mcp-server/list_items

3. Create a session with sub-agents enabled and have the orchestrator delegate to my-agent via the task tool

4. The sub-agent calls my-mcp-server/list_items and gets the error above

A tool on the same server that returns a string (e.g., "No items found") succeeds in the same sub-agent session, confirming the issue is specifically with array responses.

Expected behavior

MCP tool calls returning JSON arrays should work the same way in sub-agents as they do in the primary agent. A few possible fixes:

1. Accept arrays in structuredContent: z.union([z.record(z.string(), z.unknown()), z.array(z.unknown())])
2. Normalize array responses before validation by wrapping [...] into { items: [...] }
3. Apply the same lenient parsing that the primary agent uses to sub-agent MCP responses as well

Additional context

• OS: Windows 11 (x86_64)
• Shell: PowerShell 7
• Terminal: Windows Terminal
• No workaround available from the consumer side. The onPostToolUse session hook fires after the Zod validation, so there is no way to intercept the response before rejection.
• Related: MCP stdio transport for sub-agents (task tool) closes after ~4 seconds while agent is still running #2892 (MCP stdio transport for sub-agents closes after ~4 seconds, different symptom but same area)

[SteveSandersonMS]

@mihaiLucian Thanks for filing this.

On investigation this doesn't appear to be correct. If I'm misunderstanding, sorry - please correct me!

1. This implies we should accept JSON arrays as MCP tool responses. But that would violate the MCP spec. https://raw.githubusercontent.com/modelcontextprotocol/modelcontextprotocol/refs/heads/main/schema/2025-11-25/schema.json sets the default outputSchema to "type": "object". Even if a custom schema is declared for the MCP tool, the custom schema is itself required to specify "type": "object". With the MCP server package on NPM, it's not even possible to return a nonconforming response like this as the server validates its own output and won't send it.
2. The issue says that main agents and subagents differ in their handling of this response format. I wasn't able to repro that - in my repro the main and subagents all reject these response shapes in the same way. For clarification here's my repro code and results: https://gist.github.com/SteveSandersonMS/15d02ddd2ba0275c6905c25bdfc17bb9

If you can still repro a main/subagent difference, can you let me know how to repro that, preferably supplying code that does it?

[moyin1004]

mcp has a related sep json-schema.
but it is a draft. @SteveSandersonMS