[BUG] WebSearch tool fails with "adaptive thinking is not supported on this model" (400)

What's Wrong?

Every call to the WebSearch tool in this session fails with a 400 error:

API Error: 400 adaptive thinking is not supported on this model

Per the official troubleshooting doc (https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting#a-400-error-says-adaptive-thinking-is-not-supported), this specific error string means:

This happens because the model supports only extended thinking ... Use thinking: {type: "enabled", budget_tokens: N} instead.

This is the mirror-image bug of #50100 (which was "thinking.type.enabled" is not supported for this model — client sending legacy enabled to an adaptive-only model). Here, the client appears to be sending thinking: {type: "adaptive"} on a request that's actually being served by an extended-thinking-only model, or by a provider/model combination that doesn't support adaptive thinking.

What Should Happen?

Claude Code should pick the correct thinking.type (adaptive vs. extended/enabled+budget_tokens) for whichever model is actually handling the WebSearch tool call, so the tool works regardless of which chat model (/model) is selected.

Steps to Reproduce

1. In an active Claude Code session, trigger the WebSearch tool (e.g. ask Claude to search the web for something).
2. Observe: API Error: 400 adaptive thinking is not supported on this model.
3. Switching the session's chat model (/model opus, /model sonnet) does not resolve it — error persists identically regardless of selected model, suggesting the WebSearch tool call itself is misconfigured or routed to a fixed backend model independent of the user's model selection.

Additional Information

• Related/possibly-relevant prior reports: #50100 (opposite error, thinking.type.enabled rejected — Bedrock + [1m] context suffix), #43950 (WebSearch 400s with a different message, went stale/closed without fix)
• Root-cause diagnosis based on official docs: https://platform.claude.com/docs/en/build-with-claude/thinking-troubleshooting

Claude Code Version

Current (session-reported model: Claude Sonnet 5)

Platform

Anthropic API (claude.ai / Claude Code session, non-Bedrock)

Operating System

macOS (Darwin 25.5.0)