Codex 0.20.0 fails to convert several MCP tools (JSON Schema parsing): integer unsupported, union types rejected, and missing type — Obsidian MCP server

[mickahouan]

What version of Codex is running?

codex-cli 0.20.0

Which model were you using?

GPT-5

What platform is your computer?

• Windows 11 • PowerShell 7 (x64) - WSL2 (Debian 2) • bash

What steps can reproduce the bug?

Summary
On Codex 0.20.0, MCP tool discovery fails for 3 tools exposed by the cyanheads/obsidian-mcp-server. The exact same server lists 8/8 tools and works in other clients (e.g., Gemini CLI). Codex logs show schema-conversion errors during tools/list and hides those tools.

Environment

• Client: Codex CLI codex-cli 0.20.0

• Node/npm: v22.13.1 / 11.5.2

• MCP server: cyanheads/obsidian-mcp-server (Docker)

• OS / shells tested (same result)

  • Windows 11 • PowerShell 5.1
  • Windows 11 • PowerShell 7 (x64)
  • WSL2 (Ubuntu) • bash

• Config: standard ~/.codex/config.toml with a [mcp_servers.obsidian] entry invoking the Docker image (same invocation used by Gemini CLI)

Expected
All 8 tools from the Obsidian MCP server are visible and callable from Codex (including obsidian_global_search, obsidian_list_notes, obsidian_manage_frontmatter).

Actual
Codex hides 3 tools and logs conversion errors on startup:

2025-08-11T23:51:01.117396Z ERROR Failed to convert "obsidian__obsidian_global_search" MCP tool to OpenAI tool: Error("unknown variant `integer`, expected one of `boolean`, `string`, `number`, `array`, `object`", line: 0, column: 0)
2025-08-11T23:51:01.117515Z ERROR Failed to convert "obsidian__obsidian_list_notes" MCP tool to OpenAI tool: Error("invalid type: sequence, expected variant identifier", line: 0, column: 0)
2025-08-11T23:51:01.117598Z ERROR Failed to convert "obsidian__obsidian_manage_frontmatter" MCP tool to OpenAI tool: Error("missing field `type`", line: 0, column: 0)

Repro steps

1. Install Codex 0.20.0.

2. Configure ~/.codex/config.toml with an MCP server entry, e.g.:

   [mcp_servers.obsidian]
   command = "docker"
   args = ["run","--rm","-i","--env-file","PATH_TO_.env","obsidian-mcp-server-fixed"]

3. Launch codex, trigger MCP discovery (/mcp list), then check ~/.codex/log/codex-tui.log.

4. Observe the 3 conversion errors above and the missing tools.

Cross-checks

• With Gemini CLI against the exact same Docker command, all 8 tools are listed and callable.
• Reproduced identically on PowerShell 5.1, PowerShell 7 (x64), and WSL2 (Ubuntu).

Notes / Hypothesis

• Errors suggest the converter currently rejects JSON Schema inputs such as:

  • type: "integer" (accepted by JSON Schema, but seemingly unsupported here — only number works)
  • Union/nullable forms (e.g., type: ["string","null"])
  • Missing explicit type: "object" at schema roots

• Request: clarify the supported subset of JSON Schema for MCP tools, or consider relaxing the converter to accept common patterns above.

Ask

• Confirm whether integer and union-types are intentionally unsupported or if this is a regression.
• If unsupported by design, please document the required JSON-Schema subset.
• If regression, please fix so Codex loads the full toolset from community MCP servers like cyanheads/obsidian-mcp-server.

Labels suggested: bug, mcp, schema-conversion

What is the expected behavior?

No response

What do you see instead?

No response

Additional information

No response

[bolinfest]

FYI, #1975 went into https://github.com/openai/codex/releases/tag/rust-v0.21.0, which was just released, so give that a try?

[mickahouan]

Great, that solves the problem. Thanks a lot.

[aldinokemal]

FYI, #1975 went into https://github.com/openai/codex/releases/tag/rust-v0.21.0, which was just released, so give that a try?

is that mean we can use mcp Json or still use toml?