Regression: Codex Desktop no longer renders draw.io MCP App Server inline (worked about a month ago)

Summary

Codex Desktop can invoke the draw.io MCP App Server, but it does not render the inline MCP App UI. The same draw.io MCP setup reportedly worked about a month ago, so this may be a regression in Codex Desktop's MCP Apps rendering path.

As a workaround, switching Codex to the draw.io Tool Server works as the primary integration path because it avoids MCP Apps inline rendering.

Environment

• Date observed: 2026-06-18
• Codex Desktop app build in local environment: 26.611.61753
• Platform: macOS arm64
• MCP server under test:
• hosted App Server: https://mcp.draw.io/mcp
• local App Server from jgraph/drawio-mcp
• local Tool Server from jgraph/drawio-mcp
• Local Codex config originally had:
• [mcp_servers.drawio] url = "https://mcp.draw.io/mcp"

Regression note

The user reports that the draw.io MCP App Server rendered correctly in Codex Desktop roughly one month ago (around mid-May 2026) using the same general setup. I cannot independently verify the earlier behavior from local logs, but the current behavior consistently fails in the desktop app while the server itself appears healthy.

Reproduction

1. Configure Codex Desktop to use the hosted draw.io MCP App Server:

[mcp_servers.drawio]
url = "https://mcp.draw.io/mcp"

1. Restart Codex Desktop.
2. In a thread, invoke the draw.io tool with a minimal Mermaid diagram, e.g. a simple flowchart.
3. Observe that the tool call succeeds, but the result in Codex Desktop is only a text payload containing the submitted diagram data and build metadata.
4. No inline diagram widget / iframe is rendered.

I also cloned https://github.com/jgraph/drawio-mcp locally and installed both server variants:

• App Server: mcp-app-server
• Tool Server: mcp-tool-server

Both local stdio processes start successfully.

What I observed

Hosted App Server (https://mcp.draw.io/mcp)

• create_diagram succeeds.
• Codex Desktop returns a text payload like:

{"mermaid":"flowchart TD\nA[Inline render test] --> B[Check whether tool returns image]","_buildId":"..."}

• No inline widget is shown.
• No browser page is opened that can be captured.

Local App Server

• Starts successfully under stdio.
• Expected to have the same failure mode, because it still depends on Codex Desktop rendering MCP Apps inline UI.

Local Tool Server

• Starts successfully under stdio.
• This is a viable workaround because it avoids the MCP Apps inline rendering path entirely.

Why this looks like a Codex Desktop issue

The draw.io upstream docs explicitly say the App Server requires an MCP Apps-compatible host for inline rendering, and that in hosts without that support the tool still works but returns text instead of rendering inline.

What appears to be happening in Codex Desktop is:

• MCP tool invocation works
• but MCP Apps UI/resource rendering does not complete

This matches another open issue very closely:

• #21019 Codex Desktop does not render MCP Apps inline UI resources even when tool result includes mcp_app_resource_uri

This new report adds a likely regression signal: the same general draw.io MCP App Server workflow reportedly worked about a month earlier.

Expected behavior

When Codex Desktop is configured with the draw.io MCP App Server, create_diagram should render the diagram inline in the chat UI, not only return the raw text payload.

Actual behavior

Tool call succeeds, but Codex Desktop only shows text output and no inline diagram UI.

Workaround

Use the draw.io Tool Server as the primary drawio MCP in Codex Desktop instead of the App Server.