Model Routing Claude model choice is the biggest cost lever in Claude Code. Match the model to the work. Decision matrix | Task type | Model | Why | |---|---|---| | Architecture decision | Opus | Multi-step reasoning; hidden-cost detection | | Root-cause debugging (hard) | Opus | Hypothesis trees, multi-source evidence | | Security review | Opus | Risk sensitivity; knowledge of OWASP/CWE | | Feature implementation | Sonnet | Standard generation; good reasoning | | Code review (routine PR) | Sonnet | Fast; catches most issues | | Test writing | Sonnet | Pattern-based | | Research / docs lookup…