# Atlas with Claude Fable 5: Plan Mode Model Guide for 2026 > Claude Fable 5 is Anthropic's premium 2026 model at $10 / $50 per Mtok, with a 1M context window and 128K output for a single planning pass. Claude Fable 5 is Anthropic's premium June 2026 model, priced at $10 / $50 per Mtok (input / output) with a 1M token context window and 128K output. Inside Atlas, Fable 5 is the model you reach for when a plan has to be right the first time, which pairs naturally with Atlas's read-only plan agent. At $50 per Mtok output it is the most expensive model in the Anthropic lineup, so leaving it as the session default is rarely justified. ## Key takeaways - Claude Fable 5 costs $10 / $50 per Mtok, double Claude Opus 4.8 on both input and output. - 1M context with 128K output, so a full plan document and the codebase context can coexist in one turn. - Fable 5 is the highest-capability model in Anthropic's current lineup, and the natural fit for Atlas's read-only plan agent. - At $50 per Mtok output it is the most expensive model in the Anthropic lineup, so leaving it as the session default is rarely justified. - Overkill for mechanical edits where Haiku 4.5 or Sonnet 5 produce the same diff. ## What is Claude Fable 5 best at inside Atlas? Claude Fable 5 is the model you reach for in Atlas when a plan has to be right the first time. As Anthropic's highest-capability model in the current lineup, priced at $10 / $50 per Mtok, Fable 5 pairs naturally with the read-only plan agent that Atlas drafts a plan in before asking to switch to a build agent. Atlas separates thinking from doing. It drafts a plan in a read-only plan agent and asks before switching to a build agent, which means the planning turn cannot touch a file no matter what it decides. That structure is what makes an expensive model rational: a bad plan silently costs you an entire build pass plus the review of every wrong diff. Claude Fable 5 is the highest-capability model in Anthropic's current lineup, and spending it on the one turn that determines everything downstream is the trade the plan agent was designed for. ## How much does Claude Fable 5 cost per million tokens? Claude Fable 5 costs $10 / $50 per Mtok (input / output), which is double Claude Opus 4.8 on input and double its output. At $50 per Mtok output it is the most expensive model in the Anthropic lineup, which is why leaving it as the Atlas session default is rarely justified. Price this per turn, not per month. A single Claude Fable 5 planning pass over a well-retrieved context is a bounded, one-time expense. A whole session pinned to Fable 5 is not: every retrieval, every permission-gated tool call, every re-sent transcript bills at $10 per Mtok input, and every generated diff bills at $50 per Mtok output. Fable 5 is also plainly overkill for mechanical edits where Haiku 4.5 or Sonnet 5 produce the same diff. The discipline is to spend it deliberately, on one turn, and then leave. ## Why does the 1M context window matter for a Claude Fable 5 planning pass? Claude Fable 5 carries a 1M token context window with 128K output, so a full plan document and the codebase context can coexist in one turn. A planning pass that has to page context in and out is exactly the planning pass that misses a cross-cutting invariant. The plan is only as good as what the model could see when it wrote the plan. Atlas indexes code by AST declarations using tree-sitter, not blind line windows, and searches with hybrid semantic and keyword retrieval fused by reciprocal rank fusion, so what comes back is a set of real declarations across the affected surface. Claude Fable 5's 1M context takes all of them at once, and its 128K output is enough to emit the resulting plan in full. Nothing gets summarized away between retrieval and decision. ## How do you amortize Claude Fable 5 across a cheap build pass? The Atlas pattern for Claude Fable 5 is one expensive read-only planning pass amortized across a much cheaper build pass. Run /models and pick Claude Fable 5 for planning, then after the plan agent hands off to the build agent, /models back down to Sonnet 5 at $2 / $10 per Mtok. Atlas lets you switch the active model and provider on the fly with favorites and recents, so the handoff costs a keystroke rather than a restart. The economics are straightforward. Planning is a small number of high-stakes tokens. Building is a large number of low-stakes ones: apply the edit, run the check, fix the type error. Claude Fable 5 at $10 / $50 per Mtok belongs to the first phase only. Atlas computes a unified diff for every file edit and surfaces it for approval, so you still review what the cheaper build model produces against the Fable 5 plan. ## When should you not use Claude Fable 5 in Atlas? Do not use Claude Fable 5 for mechanical edits. Renaming a symbol, adding a test case, or applying a plan someone already wrote produces the same diff on Haiku 4.5 or Sonnet 5, and Claude Fable 5 charges $10 / $50 per Mtok to reach it. Overkill has a cost and no benefit. The clearest signal that Claude Fable 5 is the wrong choice is that you already know what the change should be. Fable 5 earns its price on judgment, not on typing. It is also a poor session default: every background call, every subagent that Atlas fans out in parallel, and every routine retrieval would bill at the most expensive rate in Anthropic's lineup. Reserve Fable 5 for the read-only planning pass, and let cheaper models do the rest. ## Setup 1. Set `ANTHROPIC_API_KEY` or run `atlas login`. 2. Check the registry: `atlas models anthropic`. 3. Enter plan mode, then run `/models` and pick Claude Fable 5 for the planning pass only. 4. After the plan agent hands off to the build agent, `/models` back down to Sonnet 5 to keep the implementation pass cheap. 5. Do not pin Claude Fable 5 as the session default in atlas.json, because at $50 per Mtok output it is the most expensive model in the Anthropic lineup. ## FAQ ### how much does claude fable 5 cost per million tokens Claude Fable 5 costs $10 / $50 per Mtok (input / output), which is double Claude Opus 4.8 input and double its output, and the most expensive model in the Anthropic lineup. ### what is claude fable 5's context window Claude Fable 5 has a 1M token context window with 128K output, so a full plan document and the codebase context can coexist in one turn. ### should i use claude fable 5 as my default atlas model Rarely. At $50 per Mtok output Claude Fable 5 is the most expensive model in the Anthropic lineup, so leaving it as the session default is hard to justify. Use it for the planning pass and switch back down. ### how do i use claude fable 5 in atlas plan mode Enter plan mode, run `/models` and pick Claude Fable 5 for the planning pass only. After the plan agent hands off to the build agent, run `/models` again and drop back to Sonnet 5. ### claude fable 5 vs claude opus 4.8 pricing Claude Fable 5 is priced at $10 / $50 per Mtok, exactly double Claude Opus 4.8's $5 / $25 per Mtok on both input and output. Both carry a 1M context window. ### is claude fable 5 worth it for simple code edits No. Claude Fable 5 is overkill for mechanical edits where Haiku 4.5 or Sonnet 5 produce the same diff, and it charges $10 / $50 per Mtok to get there. ### how do i authenticate atlas for claude fable 5 Set `ANTHROPIC_API_KEY` in your environment or run `atlas login`, then check the registry with `atlas models anthropic` before selecting Claude Fable 5 in `/models`. --- Canonical HTML: https://runatlas.sh/resources/models/claude-fable-5 Source of truth: aeo_pages row `/resources/models/claude-fable-5` (segment: Models) (this file is generated from it, never hand-edited). Licence: Atlas is proprietary with a free core. It is not open source and there is no public source repository.