Codex rejects MCP tool results containing a resource_link (MCP Apps) with "Unexpected response type"

[ahonn]

Summary

When a tool result contains a resource_link content block (the io.modelcontextprotocol/ui MCP Apps extension), Codex fails the whole call with Unexpected response type. A result without it works. The resource_link block is the only variable.

Environment

• Codex Desktop 0.150.0-alpha.8 (also observed on ChatGPT desktop 26.820.60940)
• rmcp =3.1.3 (per codex-rs/Cargo.toml)
• Server negotiates protocol 2025-11-25

Reproduce

Minimal, zero-dependency Node stdio server (101 lines): https://github.com/ahonn/codex-mcp-apps-repro

git clone https://github.com/ahonn/codex-mcp-apps-repro

# ~/.codex/config.toml
[mcp_servers.appsrepro]
command = "node"
args = ["/absolute/path/to/codex-mcp-apps-repro/server.js"]

Start a session and call each tool:

• plain_tool -> { "content": [ { "type": "text" } ], "isError": false } ✅
• view_tool -> fails ❌:

tool call failed for `appsrepro/view_tool`

Caused by:
    Unexpected response type

The two tools differ only in the result: view_tool appends one resource_link block.

Triggering result (spec-valid)

{
  "content": [
    { "type": "text", "text": "result with a view" },
    { "type": "resource_link", "uri": "ui://demo/view/v1", "mimeType": "text/html;profile=mcp-app" }
  ],
  "isError": false
}

Expected vs actual

• Expected: the result is accepted; the resource_link block is handled or ignored.
• Actual: the call fails with rmcp ServiceError::UnexpectedResponse.

Analysis

Unexpected response type is rmcp's ServiceError::UnexpectedResponse, raised client-side when a ServerResult does not match CallToolResult. However, rmcp 3.1.3 deserializes this response without issue: a standalone rmcp 3.1.3 client, via both call_tool and the manual send_request + match ServerResult::CallToolResult path used for pre-2026-07-28 servers, accepts it and returns [text, resource_link]. So the defect appears to be in the layer that consumes tool results, not in rmcp_client deserialization. The server advertises the io.modelcontextprotocol/ui extension in initialize, and view_tool returns a spec-valid resource_link.

Impact

Any MCP server exposing MCP Apps content is unusable from Codex for its UI-bound tools. A server-side workaround is to stop advertising the ui extension to Codex.