Claude Code Makes Claude Fable 5.1 Its Default Fable Model
Claude CodeAnthropic released Claude Fable 5.1 and Claude Mythos 5.1 on September 1, 2026, and Claude Code 2.1.257 immediately made claude-fable-5-1 the default Fable model, defaulting to High effort in the CLI. The new model keeps Fable 5's list pricing but cuts cache-read costs by 75%, delivering roughly 25% lower costs on typical workloads and up to 45% on heavily agentic tasks. It also posts large benchmark gains, more than doubling Fable 5's Terminal-Bench-Science score and beating Opus 5 on Terminal-Bench 4.0. Mythos 5.1, the restricted-access counterpart, cuts cybersecurity false positives by 60% and biology false positives by 85%, while gaining the ability to discover (not exploit) software vulnerabilities.
Key Takeaways
- Claude Code now defaults to High-effort Fable 5.1, a change that shipped in 2.1.257 the same day Anthropic announced the model publicly.
- Cache reads got 75% cheaper, dropping from $1.00 to $0.25 per million tokens, which Anthropic estimates cuts typical costs by roughly 25% and agentic-workload costs by up to 45%.
- Terminal-Bench-Science 0.1 score more than doubled, jumping from 24.7% on Fable 5 to 52.6% on Fable 5.1.
- Fable 5.1 beat Opus 5 on Terminal-Bench 4.0 in third-party testing, scoring 55.8% versus 52.3%.
- Mythos 5.1's safeguards got far less trigger-happy, cutting cybersecurity false positives 60% and biology false positives 85%, while adding defensive vulnerability discovery.
- Gateway users on unconfigured deployments still get Fable 5, since the generic
fable/bestaliases won't resolve to 5.1 until an operator updates their gateway.
Sources & Mentions
5 external resources covering this update
Anthropic's new Fable release is cheaper, less restrictive
TechCrunch
Anthropic's Claude Fable 5.1 and Mythos 5.1 arrive with a 75% cost reduction for Fable cache reads
VentureBeat
Anthropic upgrades Claude with new Fable 5.1 model, details here
9to5Mac
Anthropic Launches Claude Fable 5.1 With Lower Costs and Fewer False Positives
MacRumors
Claude Fable 5.1 and Claude Mythos 5.1
Hacker News
A New Default Model for Claude Code
Claude Code 2.1.257, shipped September 1, 2026, made claude-fable-5-1 the default Fable model across the CLI, replacing Claude Fable 5. In Claude Code specifically, the model defaults to High effort; Claude Cowork and claude.ai default to Medium effort instead. The change landed alongside Anthropic's broader launch of Claude Fable 5.1 and Claude Mythos 5.1, described by Anthropic as its most capable pair of models yet for coding, long-running agentic work, and research tasks.
Fable 5.1 and Mythos 5.1 are the same underlying model shipped with different levels of safeguards. Fable 5.1 is generally available through Claude.ai, the Claude API, AWS, Google Cloud, and Microsoft Azure. Mythos 5.1 remains limited to vetted organizations working in cybersecurity and life sciences through Anthropic's trusted-access programs.
Pricing and Performance
Fable 5.1 keeps the same headline API pricing as Fable 5, $10 per million input tokens and $50 per million output tokens, but cuts the cost of a cache read from $1.00 to $0.25 per million tokens, a 75% reduction. Anthropic estimates that change alone lowers costs by roughly 25% for typical workloads and by as much as 45% for highly agentic, cache-heavy tasks, which describes a large share of what Claude Code does during long coding sessions.
The model also shows substantial benchmark gains over its predecessor. On Terminal-Bench-Science 0.1, Fable 5.1 scored 52.6% against Fable 5's 24.7%, more than double. On Terminal-Bench 4.0 it reached 55.8%, ahead of both Fable 5 (42.0%) and, according to third-party testing, Opus 5 (52.3%). It also improved on Humanity's Last Exam (60.9% vs. 57.8%) and CursorBench 3.2.0 (73.4% vs. 70.5%). Anthropic says the model shows particular strength at identifying the root cause of software issues and carrying complex, multi-step coding tasks through to completion.
Safer Defaults for Mythos 5.1
Mythos 5.1's safeguards were retuned to cut false positives sharply: 60% fewer false triggers in cybersecurity contexts and 85% fewer for benign elementary biology and medical questions. In exchange, Mythos 5.1 gained the ability to discover software vulnerabilities for defensive purposes, though not to develop exploits for them, and gained enhanced biology research capabilities through a Life Sciences Verification Program partnership with the U.S. government. Access to Mythos 5.1's expanded capabilities is currently limited to U.S. organizations, with international expansion planned. Anthropic reports the model held steady on prompt-injection robustness and malicious-request refusal benchmarks compared to Mythos 5, Sonnet 5, and Opus 5.
Rollout Notes
Because gateway deployments not yet configured for Fable 5.1 would reject requests naming it directly, Claude Code 2.1.257 keeps the generic fable and best aliases resolving to Fable 5 for Claude apps gateway sessions until an operator updates their configuration; developers who want Fable 5.1 specifically can select it by name in /model.