docs(performance): explain adaptive reasoning effort controls

rules/common/performance.md

@@ -32,16 +32,21 @@ Lower context sensitivity tasks:
 
 ## Extended Thinking + Plan Mode
 
-Extended thinking is enabled by default, reserving up to 31,999 tokens for internal reasoning.
+In Claude Code, adaptive reasoning lets the model choose how much to think for each task.
+For Fable models, Sonnet 5 and later, and Opus 4.7 and later, use effort to control reasoning
+rather than a fixed token budget. Available levels depend on the model.
 
-Control extended thinking via:
-- **Toggle**: Option+T (macOS) / Alt+T (Windows/Linux)
-- **Config**: Set `alwaysThinkingEnabled` in `~/.claude/settings.json`
-- **Budget cap**: `export MAX_THINKING_TOKENS=10000` (bash) or `$env:MAX_THINKING_TOKENS = "10000"` (PowerShell)
+Control thinking via:
+- **Effort**: Use `/effort` in a session, `--effort <level>` at launch, or `CLAUDE_CODE_EFFORT_LEVEL`. Save a per-model level in `modelSettings` in `~/.claude/settings.json`; the older user-level `effortLevel` key does not apply to Opus 5.5 or newer models.
+- **Toggle**: Option+T (macOS) / Alt+T (Windows/Linux), or `alwaysThinkingEnabled` in settings. These cannot disable thinking on Opus 5.5, Sonnet 5.5, or Fable models.
+- **Fixed budget**: A positive `MAX_THINKING_TOKENS` does not cap adaptive reasoning. On Opus 4.6 or Sonnet 4.6, set `CLAUDE_CODE_DISABLE_ADAPTIVE_THINKING=1` to use a fixed budget with `MAX_THINKING_TOKENS`.
 - **Verbose mode**: Ctrl+O to see thinking output
 
+See [Claude Code model configuration](https://code.claude.com/docs/en/model-config#adjust-effort-level)
+and [environment variables](https://code.claude.com/docs/en/env-vars) for supported controls.
+
 For complex tasks requiring deep reasoning:
-1. Ensure extended thinking is enabled (on by default)
+1. Choose a higher supported effort level, such as `/effort high`
 2. Enable **Plan Mode** for structured approach
 3. Use multiple critique rounds for thorough analysis
 4. Use split role sub-agents for diverse perspectives