Fable 5 Effort Levels Explained: low to xhigh, and What They Cost You

Last updated: June 11, 2026

Fable 5 took away most of the dials developers used to tune Claude. Thinking is always on, budget_tokens returns a 400, and the sampling parameters are gone entirely. What is left is one control that now does almost all the work: the effort parameter. The same dial drives Opus 4.8 and 4.7, and it surfaces in Claude Code as /effort. This guide covers what each level changes, which models accept which levels, and how to reason about cost - all verified against Anthropic's documentation on June 11, 2026.

What Effort Actually Controls

Effort is set via output_config: {"effort": "..."} in the Messages API. It is GA on supported models with no beta header. Per Anthropic's effort documentation, the parameter affects all tokens in the response, not just thinking:

• Text responses and explanations
• Tool calls and function arguments
• Extended thinking (when enabled)

That last point is the key difference from the old budget_tokens approach, which only capped thinking. At lower effort, Claude makes fewer tool calls, combines operations into single calls, skips preamble, and confirms tersely. At higher effort, it makes more tool calls, explains its plan before acting, and writes more detailed summaries and code comments.

One important framing from the docs: effort is "a behavioral signal, not a strict token budget." At low, Claude will still think on genuinely hard problems - just less. If you need a hard ceiling, that is what max_tokens is for.

The Five Levels and Where They Run

The API accepts exactly five values. These are the complete set - the docs state this explicitly, which matters because Claude Code's menu shows a sixth option (more on ultracode below).

Effort is supported on Fable 5, Mythos 5, Opus 4.8, Opus 4.7, Opus 4.6, Sonnet 4.6, Opus 4.5, and Mythos Preview. Models not on that list (Sonnet 4.5, Haiku 4.5) do not support the parameter at all.

Two subtleties worth internalizing:

• xhigh is the exclusive club. Only Fable 5, Mythos 5, Opus 4.8, and Opus 4.7 have it. On Opus 4.6 and Sonnet 4.6 the ladder jumps from high straight to max.
• The scale is calibrated per model. The Claude Code model configuration docs state that "the same level name does not represent the same underlying value across models." Do not port effort settings between models without re-testing.

Defaults Differ by Model and Surface

The API default is high everywhere - omitting the parameter and setting "high" behave identically. Claude Code defaults differ per model:

Claude Code also has fallback behavior: set a level the active model does not support and it falls back to the highest supported level at or below it. Set xhigh and switch to Opus 4.6, and you silently run at high. And when you first run Fable 5, Opus 4.8, or Opus 4.7, Claude Code applies that model's default effort even if you had set a different level for another model.

One more wrinkle: low through xhigh persist across Claude Code sessions, but max applies to the current session only (unless forced through the CLAUDE_CODE_EFFORT_LEVEL environment variable). Anthropic clearly does not want anyone accidentally living at max.

How Effort Interacts with Thinking

On Fable 5, thinking cannot be turned off - effort is the only depth control. On Opus 4.8 and 4.7, adaptive thinking is the only mode, and it is off unless you send thinking: {"type": "adaptive"}. The adaptive thinking docs describe how each effort level steers it:

• max: Claude always thinks, with no constraints on depth
• xhigh: always thinks deeply, with extended exploration
• high: almost always thinks
• medium: moderate thinking; may skip it for very simple queries
• low: minimizes thinking; skips it where speed matters most

If you are coming from budget_tokens code, this mapping is your migration path - there is no 1:1 token conversion. The full migration checklist covers the other breaking changes that land alongside it.

What Each Level Costs You: the Math

Here is the part pricing pages will not spell out for you: effort does not change the per-token rate. Fable 5 bills $10 per million input tokens and $50 per million output at every effort level; Opus 4.8 and 4.7 bill $5/$25, per the pricing page. The only documented price multipliers are caching, batch, fast mode, and data residency - effort is not on the list.

What effort changes is volume, almost entirely on the output side, because thinking tokens bill as output tokens even when you never see them (the default display on Fable 5 and Opus 4.8/4.7 is "omitted", which hides thinking text but bills it identically).

A worked illustration. Suppose an agentic task on Opus 4.8 sends 20,000 input tokens per turn. These output volumes are hypothetical, but the shape matches how the docs describe the levels:

Same model, same prices, roughly 7x spread per turn purely from behavior. On Fable 5 double every number. Multiply by hundreds of turns in a long agent run and the effort setting is a bigger budget lever than the model choice in many cases - which is why effort deserves a row in any cost-per-task analysis you run.

Three practical cost notes, all from the docs:

1. Measure, do not guess. usage.output_tokens_details.thinking_tokens reports how many billed output tokens went to reasoning. Run the same eval at two effort levels and compare.
2. Give high effort room. At xhigh or max, Anthropic recommends starting max_tokens at 64,000 - it is a hard ceiling on thinking plus response text. Seeing stop_reason: "max_tokens" means raise the ceiling or lower the effort.
3. The tokenizer compounds it. Opus 4.7 and later (including Fable 5) use a new tokenizer that can produce up to 35% more tokens for the same text, so old cost baselines do not transfer.

Counterintuitively, higher effort is sometimes the cheaper total: a run that finishes in one pass at xhigh can undercut three failed retries at medium. The Fable 5 vs Opus 4.8 decision guide works through that completion-rate math for model choice; the same logic applies at the effort dial.

Picking a Level per Task Type

Synthesizing Anthropic's per-model recommendations:

On Fable 5: start at high (the default) for most work and reserve xhigh for the most capability-sensitive workloads. Anthropic's docs note that lower effort settings on Fable 5 "still perform well and often exceed xhigh performance on prior models" - a vendor claim, but a useful prior: do not assume Fable 5 needs the dial maxed. Step down to medium or low if tasks complete correctly but take longer than necessary.

On Opus 4.8 and 4.7: start with xhigh for coding and agentic work, treat high as the floor for anything intelligence-sensitive, and drop to medium only after evals confirm quality holds. The docs are blunt about max: on most workloads it "adds significant cost for relatively small quality gains" and it "can lead to overthinking" on structured-output tasks. Reserve it for genuinely frontier problems.

Everywhere: low is the documented home for subagents, classification, quick lookups, and high-volume latency-sensitive paths. If you orchestrate subagents, agent teams, or workflows, running workers at low while the orchestrator sits at high or xhigh is the cleanest cost win available.

Setting It: Claude Code and the API

In Claude Code, the CLAUDE_CODE_EFFORT_LEVEL environment variable beats everything, then your configured level, then the model default. The mechanisms:

• /effort opens an interactive slider; /effort xhigh sets directly; /effort auto resets to the model default
• Arrow keys adjust effort inside the /model picker
• --effort <level> at launch, for a single session
• effortLevel in settings (accepts low through xhigh; max and ultracode are session-only)
• effort in skill or subagent frontmatter, overriding the session level while that skill or subagent runs

About ultracode: it appears in the /effort menu but is not an API effort level. It sends xhigh to the model and additionally grants Claude Code standing permission to orchestrate dynamic workflows. Session-only, and deliberately excluded from the effortLevel setting and --effort flag. Related but different: typing ultrathink in a prompt requests deeper reasoning for that one turn via an in-context instruction - the effort level sent to the API is unchanged.

On the API, the syntax is one field:

response = client.messages.create(
    model="claude-fable-5",
    max_tokens=64000,
    output_config={"effort": "xhigh"},
    messages=[{"role": "user", "content": "..."}],
)

On Opus 4.8 or 4.7, add thinking={"type": "adaptive"} if you want thinking; on Fable 5, omit the thinking parameter entirely (it is always on, and an explicit disabled returns a 400). If raw speed is the constraint rather than depth, see whether fast mode is worth it - fast mode changes price-per-token, effort changes token volume.

FAQ

Is xhigh available on every Claude model?

No. xhigh exists only on Fable 5, Mythos 5, Opus 4.8, and Opus 4.7. Opus 4.6 and Sonnet 4.6 support low, medium, high, and max but skip xhigh. In Claude Code, setting xhigh on an unsupported model silently falls back to high.

Does higher effort cost more per token?

No. Per-token rates are fixed per model ($10/$50 for Fable 5, $5/$25 for Opus 4.8 and 4.7) and effort is not a pricing multiplier. Higher effort costs more because the model generates more tokens - more thinking (billed as output even when hidden), more tool calls, longer explanations. Check usage.output_tokens_details.thinking_tokens to see where the spend goes.

What is ultracode, and is it an API effort level?

It is a Claude Code setting, not an API level. The API accepts exactly low, medium, high, xhigh, and max. Ultracode sends xhigh to the model and adds standing permission for Claude Code to launch multi-agent dynamic workflows. It applies to the current session only.

Should I run Fable 5 at xhigh by default?

Anthropic says no - start at high, the default. Fable 5's lower effort levels are documented as often exceeding the xhigh performance of prior models, and blanket xhigh gets expensive fast. Move up only when a capability-sensitive task measurably benefits.

Sources

• Effort parameter - Anthropic docs - levels, availability, defaults, tool-use behavior (accessed June 11, 2026)
• Adaptive thinking - Anthropic docs - effort-to-thinking mapping, billing of thinking tokens, thinking_tokens usage field (accessed June 11, 2026)
• Model configuration - Claude Code docs - /effort command, per-model level tables, defaults, ultracode, ultrathink, fallback behavior (accessed June 11, 2026)
• Pricing - Anthropic docs - per-model token rates, tokenizer note, pricing multipliers (accessed June 11, 2026)
• Claude Code What's New, Week 22 - Opus 4.8 default rollout and /effort xhigh guidance (accessed June 11, 2026)