Claude Opus 5 introduces three API‑level changes that affect existing integrations. First, thinking is now enabled by default. Any request that previously relied on max_tokens to cover only the response must now account for the thinking tokens as well. Review your max_tokens settings and increase them if you see truncated outputs. Second, setting thinking disabled together with effort xhigh or max returns a 400 error. To keep thinking off, either cap effort at high or remove the thinking field entirely. Update your code to check the effort value before sending a disabled thinking flag. Third, Anthropic advises deleting verification prompts such as “include a final verification step”. The model already performs internal verification, and extra prompts cause over‑verification and wasted tokens. Remove those instructions from your prompts and rely on the model’s built‑in checks.
Developers using long‑context workloads should note that the minimum cacheable prompt size dropped from 1024 to 512 tokens. Existing prompts that were too short to cache will now create cache entries automatically, potentially reducing latency and cost without code changes. However, the synchronous Messages API still caps output at 128k tokens; for larger outputs you must switch to the Message Batches API with the output‑300k‑2026‑03‑24 beta header.
For coding agents, Opus 5 delivers strong scores on SWE‑bench Verified and FrontierBench, but safety classifiers now flag about 5% of API calls. If you see unexpected refusals, inspect the safety logs and consider adjusting prompts to avoid triggering classifiers. In agentic workflows, the model benefits more from tool use than from raising thinking effort alone. Equip your agents with simple tools for cropping images, running code, or checking work to achieve better performance without increasing token consumption.
Update your client libraries to send the effort parameter explicitly, verify max_tokens includes thinking budget, strip verification prompts, adjust caching logic for the new 512‑token minimum, and use the Batches API when you need over 128k output. These steps will keep your integration stable and cost‑effective with Opus 5.
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