Your Agent Is Not the Model: The Key to Debugging AI Systems

A common confusion in AI development is conflating the model with the agent system. This article clarifies the layered architecture: the model (e.g., Sonnet, Opus) is the mathematical core, the inference service (e.g., AWS Bedrock, Anthropic's API) runs it, and the harness (e.g., Claude Desktop, Cursor) shapes inputs and outputs. Using a house-building analogy, it explains why the same model behaves differently across harnesses and provides a troubleshooting table to pinpoint issues like bad reasoning, missing context, or high cost. Precision in terminology leads to faster fixes.