# Claude Code Effort Models

> Use when deciding how to work on a task - choosing effort level, model, or fast mode to balance reasoning depth, speed, and token cost.

- Skill: `ucsandman/claude-code-effort-models` (Agent Skill)
- Install (CLI): `npx skillmds@latest add ucsandman/claude-code-effort-models`
- Raw SKILL.md: https://api.skillmd.com/api/skills/ucsandman/claude-code-effort-models/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: AI & ML
- Author: ucsandman (https://skillmd.com/u/ucsandman)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/ucsandman/claude-code-effort-models

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# Claude Code Effort, Models, and Fast Mode

Quick reference for tuning how Claude Code works: model selection, effort levels, and speed mode. Read this mid-session when deciding how to approach a task.

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## Models via /model

Switch with `/model` (opens picker) or `/model <name>` to set directly. Persists to next session.

**Available models** (aliases resolve to the latest version for your provider; on the Anthropic API `opus`→Opus 4.8, `sonnet`→Sonnet 5):
- `opus` → Claude Opus 4.8 (strongest Opus-tier reasoning)
- `sonnet` → Claude Sonnet 5 (daily coding, best speed/intelligence balance; **native 1M-token context** on the Anthropic API — no `[1m]` suffix needed) — requires Claude Code v2.1.197+
- `haiku` → Claude Haiku 4.5 (fast, simple tasks)
- `fable` → Claude Fable 5 (most capable; for tasks larger than one sitting — long autonomous sessions, root-cause debugging, architecture) — requires v2.1.170+
- `best` → Fable 5 where your org has access, otherwise the latest Opus
- `opusplan` → opus during planning, auto-switches to sonnet for execution
- `opus[1m]` / `sonnet[1m]` → 1M token context window (no effect once `sonnet` already resolves to Sonnet 5's native 1M window)

**Model IDs (full names):**
- `claude-fable-5`
- `claude-opus-4-8`
- `claude-sonnet-5`
- `claude-haiku-4-5-20251001` (alias `claude-haiku-4-5`)

**Default model** (via `default` / picker): Opus 4.8 on Max, Team Premium, Enterprise pay-as-you-go, and Anthropic API; **Sonnet 5 on Pro, Team Standard, and Enterprise subscription seats**. Fable 5 is never a default — select it explicitly.

**When to pick:**
- **Fable**: your hardest, longest-running work — hand it the outcome and let it plan; it investigates and verifies with less prompting. Note: requests flagged by its cybersecurity/biology safety classifiers auto-fall back to Opus.
- **Opus**: hard reasoning, architecture decisions, complex refactors, code review, debugging. Higher token cost.
- **Sonnet**: most coding work, features, edits, tests. Good speed/capability tradeoff; 1M context suits large codebases.
- **Haiku**: searches, simple formatting, routine tasks. Cheapest.
- **opusplan**: complex feature design where planning matters but execution is straightforward.

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## Effort Levels via /effort

Controls adaptive reasoning depth per message. Raise it for complex problems; lower it for routine tasks. Persistent across sessions unless overridden by env var.

**Available levels (varies by model):**
- `low` — minimal thinking, fastest, cheapest. Use: latency-sensitive, low-complexity tasks.
- `medium` — lighter reasoning, cost-conscious work that trades some intelligence.
- `high` — default on Fable 5, Sonnet 5, Opus 4.8, Opus 4.6, Sonnet 4.6. Balances tokens and capability.
- `xhigh` — deeper reasoning, higher token spend. Default on Opus 4.7. Use: tricky architecture, intricate bugs.
- `max` — deepest reasoning, unbounded tokens, session-only. Can overthink; test first.

Fable 5, Sonnet 5, Opus 4.8, and Opus 4.7 support `low`–`max`. Opus 4.6 and Sonnet 4.6 support `low/medium/high/max` only (no `xhigh` — it falls back to `high`). Setting an unsupported level falls back to the highest supported at or below it.

**Special:** `/effort ultracode` (Opus only, session-only) sends `xhigh` to model AND orchestrates dynamic workflows for substantive tasks. Reserved for ambitious multi-phase work.

**Usage:**
- `/effort` — open slider picker
- `/effort high` — set directly
- `/effort auto` — reset to model default
- Env: `CLAUDE_CODE_EFFORT_LEVEL=xhigh`
- Skill frontmatter: `effort: xhigh`

**Token tradeoff:** low < medium < high < xhigh < max. Each step costs more tokens but enables deeper reasoning for complex tasks.

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## Fast Mode via /fast

Opus only. Same model quality, ~2.5x faster output, higher cost per token. Toggle with `/fast` or `"fastMode": true` in settings.json.

**Pricing (per MTok):**
- Opus 4.8 fast: $10 input / $50 output (vs $5/$25 standard)
- Opus 4.7 fast: $30 input / $150 output (vs $5/$25 standard)
- Opus 4.6 fast: $30 input / $150 output (deprecated; migrate to 4.8 or 4.7)

**How it works:**
- NOT a different model. Same Opus, different API config prioritizing latency.
- Auto-switches you to Opus if on Sonnet/Haiku.
- Persists to next session by default (admins can set per-session reset).
- ↯ icon shows it's active.
- Shares rate limit pool across Opus 4.8/4.7/4.6; auto-falls back to standard speed if rate-limited.

**When to use:**
- Rapid iteration, live debugging, tight deadlines. Cost matters less than speed.
- NOT good for long autonomous tasks, batch work, CI/CD.

**Cost gotcha:** enabling fast mode mid-conversation re-caches full history at fast-mode price. Enable at session start for best cost.

**Requirements:**
- Anthropic API or Claude subscription (Pro/Max/Team/Enterprise) with usage credits enabled.
- NOT available on Bedrock, Vertex, Foundry, or AWS Platform.
- Team/Enterprise admins must explicitly enable it; disabled by default org-wide.

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## Token and Cache Angle

**Model choice drives cost** (per MTok input/output, Anthropic API):
- Haiku 4.5 — $1/$5. Cheapest; searches/format work.
- Sonnet 5 — $3/$15 standard (**introductory $2/$10 through Aug 31, 2026**). Most coding; 1M context at standard pricing (no premium past 200K).
- Opus 4.8 — $5/$25. Hard reasoning; justifiable when depth pays off.
- Fable 5 — $10/$50. Reserve for the hardest, longest tasks.

**Effort and fast mode interact:**
- Higher effort = more tokens on same model.
- Fast mode = same tokens, higher per-token price, faster latency.
- Combine lower effort + fast mode for max speed on straightforward tasks.
- Don't raise both for cost-sensitive work.

**Prompt caching:** a stable warm prefix re-reads at ~10% of input price. Keep the prefix stable (don't rewrite early messages mid-session). Switching `/model` mid-session invalidates the cache for the next turn.

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## Quick Decision Tree

**Task is routine (format, search, simple edit)?** → `haiku`, `low` effort, standard mode.

**Task is typical coding (features, tests, refactors)?** → `sonnet`, `high` effort (or `medium` to cut cost), standard mode.

**Task is hard (architecture, complex bug, design)?** → `opus`, `xhigh` effort, standard mode.

**You need output in seconds, not minutes?** → Use fast mode on Opus (higher cost, lower latency). Not a model change.

**You need the deepest reasoning on an ambitious task?** → `opus`, `max` effort, standard mode. Session-only, unbounded tokens.

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## Environment Variables

- `ANTHROPIC_MODEL=<name>` — set model for this session only.
- `CLAUDE_CODE_EFFORT_LEVEL=<level>` — effort level; overrides session choice.
- `CLAUDE_CODE_DISABLE_FAST_MODE=1` — disable fast mode entirely.
- `ANTHROPIC_DEFAULT_OPUS_MODEL` / `ANTHROPIC_DEFAULT_SONNET_MODEL` / `ANTHROPIC_DEFAULT_HAIKU_MODEL` / `ANTHROPIC_DEFAULT_FABLE_MODEL` — pin specific model versions for each alias (useful on Bedrock, Vertex, Foundry).
- `CLAUDE_CODE_DISABLE_1M_CONTEXT=1` — remove 1M variants from the picker; treats Sonnet 5 sessions as 200K.

**Fallback models:** set `fallbackModel` (settings, array) or `--fallback-model sonnet,haiku` to try backups in order when the primary is overloaded/unavailable. Chain capped at 3; lasts the current turn only.

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## See Also

- Model config docs: https://code.claude.com/docs/en/model-config.md
- Fast mode docs: https://code.claude.com/docs/en/fast-mode
- Commands reference: https://code.claude.com/docs/en/commands.md