HTTP 200. Broken tool call. Now you can test it with aimock.

CopilotKit has added model-misbehavior faults to aimock, its open-source LLM API mock server, enabling developers to deliberately test how AI agents handle malformed tool calls and other bad model outputs. The update addresses production reliability issues in which invalid JSON, schema violations, unknown tools, duplicate IDs, truncated calls, empty outputs, refusals, or filtering responses can derail agent runs, contaminate conversation history, and cause repeated failures even when requests return successfully. Developers can configure deterministic, targeted failures by provider format, test ID, fixture, or request header, then verify that applications avoid executing tools with invalid arguments, retry appropriately, preserve usable history, and recover without leaving users stuck. aimock records fault-selection and response details in a request journal to help distinguish successful recovery from tests where a fault did not actually occur. Support spans several API formats, including OpenAI, Anthropic, Bedrock, Gemini, Cohere, and Ollama, though availability varies by failure type and streaming mode, and the tool is intended to test application behavior rather than reproduce the real-world frequency of provider failures.

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