Explain the artifacts that can appear in Claude Opus 5's output when thinking is disabled?

With thinking disabled, at effort high or below, Claude Opus 5 can occasionally write what should have been a structured tool call directly into its user-facing text output instead of emitting a proper tool_use block - the turn completes normally, but the intended tool call never actually runs.

This is particularly disruptive in agentic loops, since the leaked, non-functional text describing the intended tool call stays in the conversation history going forward, meaning later turns are also affected by that malformed artifact rather than the issue being contained to a single turn.

This behavior shows up most commonly on tool-heavy workloads, such as search-driven tasks, where the model is frequently deciding whether and how to invoke a tool as part of its normal flow.

Anthropic's primary recommended mitigation is to avoid disabling thinking in the first place and instead manage token cost through a lower effort level, since for most tasks, thinking enabled at low effort tends to outperform thinking disabled at a similar overall cost - addressing the root cause rather than working around the artifact after the fact.