DeepSeek-compatible Anthropic endpoints reject requests that combine an explicit disabled-thinking block with an effort value. The desktop UI can create that shape when users disable thinking and keep medium effort selected, so the request builder now omits effort whenever it will send explicit disabled thinking.
Constraint: DeepSeek-compatible Anthropic endpoints reject disabled thinking with effort
Rejected: Only patch Desktop provider settings | hidden title and side request paths still share the request builder
Confidence: high
Scope-risk: narrow
Directive: Keep explicit disabled-thinking providers from sending effort unless the upstream contract changes
Tested: UI repro on pre-fix DeepSeek provider path reproduced the 400
Tested: desktop/scripts/e2e-deepseek-thinking-ui-agent-browser.sh
Tested: bun test src/utils/__tests__/thinking.test.ts src/server/__tests__/provider-presets.test.ts src/server/__tests__/title-service.test.ts
Tested: git diff --check
Not-tested: Real DeepSeek API key live call
