ucsandman/claude-code-capability-primer/claude-code-effort-models

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.

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.

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.

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.

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.

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.

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.

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