MiniMax-M3 emits truncated tool-call JSON (error 2013) and destroys resumable session

Summary

While using MiniMax-M3 through a persistent Codex/fast-subagent session, the model attempted an unrequested tool call whose arguments were truncated. The provider returned error 2013 (invalid_prompt), and the persistent session became structurally non-resumable.

Environment

• Date: 2026-08-09
• OS: Windows
• Model: MiniMax-M3
• Reasoning effort: high
• Runtime transport: Codex session wrapped by a local MCP supervisor
• MiniMax CLI was not used for the failing run

Sanitized failure

ERROR codex_core::tools::router: failed to parse function arguments:
EOF while parsing a string at line 1 column 260

provider response:
invalid params, invalid function arguments json string
code: invalid_prompt
numeric code: 2013

The last textual output before failure said the agent was going to write a research document, even though the requested outcome was a textual research list and no file write was requested.

A subsequent attempt to add a message to the same persistent session returned:

SESSION_NOT_RESUMABLE
Session cannot resume after structural failure in the previous run

Steps to reproduce

1. Start a persistent session with MiniMax-M3 and tool calling available.
2. Ask for a structured research response containing multiple candidates and URLs.
3. Let the model decide whether to invoke a tool.
4. In the observed run, the model emitted malformed/truncated JSON for a tool call.
5. Attempt to continue the same session.

I could not reproduce the malformed call in a clean second session after explicitly forbidding all tool use; that session completed normally. The original failure and non-resumable state are persisted and reproducible from its run artifact.

Expected behavior

• Tool-call arguments should always be valid complete JSON.
• If generation of a tool call fails, the provider/runtime should ideally return a recoverable turn error or allow the persistent session to continue.
• A malformed assistant tool call should not permanently destroy otherwise valid conversation state.

Actual behavior

• The turn exits with code 1 / provider error 2013.
• No token usage is returned for the failed run.
• The persistent session becomes structurally non-resumable.

No credentials, prompts containing personal data, or private file contents are included in this report.

Start with the persistent session and tool-calling runtime paths, using the preserved run artifact and the sanitized provider error as the reproduction entry point. Follow the listed reproduction steps with MiniMax-M3 and compare behavior with tool use explicitly forbidden. Done means malformed tool-call JSON is handled without permanently making the session non-resumable, with coverage for the provider error path.