McpToolResponse.isError is dropped before MCP tool result reaches the client

[codylittle]

McpToolResponse accepts isError: true, but the Node.js worker conversion path appears to drop that field before the result is sent to the Azure Functions MCP extension. As a result, MCP clients receive a
successful tools/call result instead of a tool-error result.

Environment

• @azure/functions: 4.16.0
• Azure Functions MCP extension: observed with extension bundle preview using MCP extension 1.5.0
• Runtime: Node.js Azure Functions app using app.mcpTool(...)

Reproduction

Return an MCP tool response with isError: true:

return new McpToolResponse({
  content: [
    new McpTextContent("Invalid file")
  ],
  isError: true,
});

The public type supports this:

export interface McpToolResponseInit {
  content: McpContentBlock[];
  structuredContent?: unknown;
  isError?: boolean;
}

But toMcpToolResult() serializes only type, content, and optional structuredContent:

const out: McpToolResult = { type, content: contentStr };

if (response.structuredContent !== undefined && response.structuredContent !== null) {
  out.structuredContent =
    typeof response.structuredContent === "string"
      ? response.structuredContent
      : JSON.stringify(response.structuredContent);
}

return out;

Actual Behavior

A handler-level error response reaches the MCP client like this:

{
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Invalid file"
      }
    ]
  },
  "id": 5,
  "jsonrpc": "2.0"
}

There is no isError field.

Expected Behavior

The client-visible MCP result should include:

{
  "result": {
    "content": [
      {
        "type": "text",
        "text": "Invalid file"
      }
    ],
    "isError": true
  },
  "id": 5,
  "jsonrpc": "2.0"
}

Per the MCP schema, missing isError defaults to false, so clients currently treat handler-level tool errors as successful tool results.

Spec reference: https://modelcontextprotocol.io/specification/2025-06-18/schema#tools-call

Notes

I'm unsure if this would best be solved in the node library, or in the extension handler same way _meta is (in preview builds)

[lilyjma]

thanks for reporting this Cody Little (@codylittle)! We talked about this in our status update today. Swapnil Nagar (@swapnil-nagar) will look into this and address your question

[swapnil-nagar]

PR Created(#446) for the work, we will work on the fixing this in next release. Expect a new version Early next week.