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)