AE-KHB97FZZ
A hallucinated or unregistered tool name aborts the whole agent run with ModelBehaviorError instead of letting the model recover
HALLUCINATIONseverity: MEDIUMcause: UNKNOWNoutcome: RESOLVED UNVERIFIEDconfidence: LOW
When the model returned a function tool call whose name was not registered on the current agent (for example a hallucinated tool name or a tool removed between turns), the SDK raised ModelBehaviorError and ended the run. The reporter still saw the error on 0.17.3; a maintainer PR added an opt-in mode that returns the error to the model instead.
- Framework / agent
- OpenAI Agents SDK (Python) · tool-calling agent (OpenAI Agents SDK)
- Remediation attempts
- TESTED
- Recurrence
- not documented
- Source languages
- en
- Updated
- 2026-09-29
Sources
- GITHUB ISSUE Make unknown tool calls recoverable instead of aborting with ModelBehaviorError — github.com/openai/openai-agents-python, retrieved 2026-09-29
- GITHUB PULL REQUEST fix: #3459 add opt-in recovery for missing function tools — github.com/openai/openai-agents-python, retrieved 2026-09-29
Symptoms
- The run aborts with ModelBehaviorError when the model calls a tool that is not registered on the agent
- Long-running agent sessions cannot recover from the aborted run
The full record — root-cause evidence, every remediation attempt with its status and verification, failed attempts, patch references, verbatim quotes and recurrence — is a paid lookup (0.018 USDC via x402). Agents:
GET /api/v1/cases/AE-KHB97FZZ. PricingSimilarity to your system is not implied. A remediation that worked in the documented context may not work in yours.