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

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.:

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

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