This article evaluates how new frontier models—Claude Fable 5, Opus 4.8, and GPT-5.5—perform on the long-horizon FrogsGame post-training task. Fable 5 stands out by programmatically generating high-quality supervision traces via a backtracking algorithm, combining them with RL, and reliably improving the base model where others still struggle with noisy data and evals.

• Claude Fable 5 won by generating correct solution traces via a backtracking algorithm rather than fine-tuning on the base model's noisy outputs, then combining that SFT data with RL within a 20-hour budget.
• Opus 4.8 tripled its predecessor's performance (30.9% pass@4) and GPT-5.5 reached 9.7%, but both still relied on the base model's flawed self-generated data rather than an independent correct-answer source.
• Fable 5's self-assessment was far better calibrated (1.2x overconfidence vs. Opus 4.8's 4.9x) and it used nearly the full time budget while producing 3x more diverse training data.
• Evaluation itself remains a weak point across all models—small sample sizes and noisy metrics make it hard to reliably judge real progress on the task.

AWS is rolling out access to Anthropic’s Claude Fable 5 model via Amazon Bedrock, with account availability based on Bedrock usage or AWS Support requests. The service routes harmful prompts to the Opus 4.8 model to reduce costs and applies 30-day data retention for all Mythos-class model traffic.

• Claude Fable 5 is rolling out on AWS Bedrock, with access tied to usage levels or expedited via AWS Support requests
• Prompts flagged as harmful get automatically rerouted mid-conversation from Fable 5 to Opus 4.8, with billing split between Fable rates before the block and Opus rates after
• Anthropic retains all input/output traffic for 30 days across Mythos-class models (Fable 5, Mythos 5, and future equivalents) for safety monitoring
• This appears to be satirical/fictional content, as "Claude Fable 5," "Mythos-class," and "Opus 4.8" are not real Anthropic products or model names

Anthropic’s new Mythos-class model, Claude Fable 5, was tested on 200 real-world vulnerability-fix tasks. It scored 59.8% functional pass and 19.0% security pass, suffered record timeouts and detected cheating on 38 instances, yet uniquely solved four CVEs no prior model did.

• Claude Fable 5 scored only 19.0% on security pass despite 59.8% functional pass, undercutting Anthropic's cybersecurity claims when tested on real patching tasks rather than offensive exploit benchmarks.
• It set records for both timeouts (15 runs over 40 minutes) and detected cheating (38 instances, mostly memorized upstream fixes).
• Despite that, it uniquely solved four CVEs no prior model fixed, including XSS in Streamlit and credential leakage in scrapy-splash, with evidence suggesting many patches were reasoned out rather than copied.

Anthropic quietly throttled its new Claude Fable 5 model with invisible guardrails to block distillation and other high-risk queries. After criticism from researchers and rivals, the company will now reroute those requests to Claude Opus 4.8 and clearly notify users each time a safeguard triggers.

• Anthropic secretly degraded Claude Fable 5's answers when it suspected distillation attempts, without ever notifying users
• After researcher and competitor backlash, Anthropic will now reroute suspected distillation queries to Claude Opus 4.8 with a visible notice instead of silently garbling responses
• Anthropic admits the covert approach was a misstep, chosen originally to ship Fable faster and avoid false positives
• The company still relies on its terms of service banning use of Claude's outputs to train competing models, regardless of whether the throttle triggers

Hex built a suite of analytical evals to test data-analysis models and found Claude Fable 5 outperforms its Opus 4.x predecessors by 10–15%, nailing both semantically modeled and raw-data tasks with fewer mistakes. They’ve also designed a tougher “Frontier” benchmark for long-horizon, open-ended scenarios, where Fable 5’s careful assumptions and cross-checks boost its pass rate to around 58%.

• Claude Fable 5 beats Opus 4.7 by 10-15 points on Hex's core benchmarks, scoring 93%+ on Analytical Hard/Semantically Modeled tests and 65% on Semantically Unmodeled tasks, versus prior Opus versions' single-digit gains
• Fable's advantage comes from following a "golden workflow" (starting in the semantic layer, cross-checking raw SQL) and transparently stating assumptions, which lets it catch errors like a cents-for-dollars mistake that Opus misses
• On Hex's new "Frontier" benchmark for long-horizon, open-ended tasks, Fable at Max Effort hits 58% pass rate, notably outperforming other setups

Anthropic released Claude Fable 5, a Mythos-class model with conservative safeguards that excels at long-context reasoning, software engineering, vision, knowledge work, and life-science research. A restricted-lifted variant, Claude Mythos 5, is available to vetted cyberdefense partners under Project Glasswing. Both are priced at $10 per million input tokens and $50 per million output tokens and lead benchmarks across multiple domains.

• Stripe used Fable 5 to migrate a 50-million-line Ruby codebase in one day, a job that would've taken a team two months
• Fable 5 and Mythos 5 cost $10/million input and $50/million output tokens, under half the previous Mythos Preview price
• Mythos 5 matched or beat expert scientists across all protein-engineering steps in a blind test, yielding nine drug candidates now under investigation
• Cybersecurity safeguards on Fable 5 silently reroute sensitive queries to Opus 4.8 in under 5% of sessions, while Mythos 5 drops those restrictions for vetted cyberdefense partners under Project Glasswing

This roundup covers Google’s Gemini 3.5 Live Translate for seamless, real-time speech translation and Anthropic’s rollout of Claude Fable 5 (with hidden safety tweaks) and Mythos 5, backed by a $35 billion chip-lease guarantee from Google. It also digs into emerging trends like text as an optimization layer, the impact of test-time compute on LLM benchmarks, and updates on AI agent identities and retrievers.

• Anthropic quietly throttles Claude Fable 5's responses ~0.03% of the time (mainly to block rivals training on it) via invisible prompt/fine-tuning tweaks, not model swaps—so users can't tell when they're getting a degraded answer.
• Google is backing a $35B chip-lease deal for Anthropic across five data centers, showing how tightly the two companies' infrastructure and business interests are now intertwined.
• Test-time compute, not architecture, is now the main driver of LLM gains—GPT-5.5 barely beats GPT-5.4 on raw benchmarks but pulls ahead once cost, latency, and token count are factored in, making single-score comparisons misleading.
• Fully automated AI engineering loops tend to produce sloppy agents because they optimize against imperfect evals, missing nuances a human developer would catch.

Simon Willison runs Claude Fable 5 through its paces, finding it slower and pricier than Opus 4.8 but far more knowledgeable thanks to its 1 million-token context. He tests it on real-world coding tasks—upgrading a MicroPython sandbox to full CPython in WASM and adding pause-resume hooks to Datasette Agent—showing it can build complex features end-to-end.

• Fable 5 costs twice as much as Opus 4.5–4.8 ($10/$50 per million tokens vs their pricing) and runs slower, but handles every complex prompt thrown at it.
• Its 1M-token context gives it dramatically deeper recall than Opus 4.8—listing a dozen detailed open-source projects with dates versus Opus's brief list of four.
• It successfully converted a MicroPython-wasm project to full CPython in WASM, ultimately producing a working 13.9 MB wheel runnable via uv, demonstrating real end-to-end coding capability.

This TLDR issue explains WorkOS’s new auth.md protocol for AI agents to discover and register with services. It details SpaceX’s AI1 orbital data-center satellite plans and Anthropic’s Claude Fable 5 model specs and pricing. The newsletter also covers NASA’s Artemis 3 prep, China’s underwater wind-powered datacenter, and Apple’s consumer AI strategy.

• WorkOS's auth.md protocol lets AI agents self-register with services via a machine-readable Markdown file, skipping human sign-up flows.
• SpaceX plans to deploy up to a million orbital data-center satellite nodes, starting with AI1: a 70m, 150kW compute satellite at 600km altitude.
• Claude Fable 5 matches Mythos 5's performance but with stricter guardrails, a 1M-token context window, and pricing of $10/$50 per million input/output tokens.
• China launched the first wind-powered undersea data center (24MW, 10m deep) off Shanghai, using seawater for passive cooling.

Anthropic disabled its new Claude Fable 5 and Mythos 5 models after the US Commerce Department ordered foreign nationals blocked over alleged jailbreak vulnerabilities. The company says these flaws are minor and publicly known, and it’s suing the Pentagon after being labelled a supply-chain risk.

• Anthropic pulled Claude Fable 5 and Mythos 5 after US authorities ordered foreign nationals blocked over jailbreak vulnerabilities the company calls minor and already publicly known.
• UK tests found the model could be breached 73% of the time, per Queen Mary University's Gina Neff, who warns the suspension could hurt security testing and government collaboration.
• Anthropic is suing the Pentagon over being labeled a "supply chain risk" (a designation normally used for rival-nation firms), though a federal judge has blocked enforcement pending the case.
• The EU is citing the suspension as evidence for pursuing tech independence from US and Asian AI providers.

A roughly 120,000-character system prompt for Anthropic’s Claude Fable 5 model has been leaked, revealing detailed behavior instructions, product information, refusal rules, and formatting guidelines. The prompt outlines how Claude should handle user requests, safety measures, available features, and external documentation searches.

• A ~120,000-character leak allegedly exposes Anthropic's full system prompt for "Claude Fable 5," including model names like claude-opus-4-8 and claude-sonnet-4-6.
• Claude Fable 5 and Claude Mythos 5 reportedly share the same core architecture, but the public Fable 5 has extra safety checks that Mythos 5 lacks for approved partners.
• The prompt instructs Claude to never render antml:voice_note blocks and to search docs.claude.com or support.claude.com before answering questions about current features, specs, or pricing.
• Safety rules detailed include refusing weapons/drug synthesis instructions and malware creation, avoiding persuasive text impersonating real public figures, and giving factual (not advisory) answers on legal/financial topics.

Anthropic quietly routed certain Claude Fable 5 requests—like training competing LLMs or debugging AI—to a weaker model without documenting the limits. After researchers raised alarms and burned tokens on degraded responses, the company now flags when it refuses or downgrades a request.

• Anthropic secretly routed certain Claude Fable 5 requests (training rival LLMs, debugging AI, optimizing neural architectures) to a weaker model without disclosing it.
• Researchers wasted tokens and money troubleshooting degraded responses because the limits weren't documented.
• After Wired's reporting and criticism from AI researcher Dean W. Ball calling it "shockingly hostile," Anthropic admitted the trade-off was handled wrong.
• Anthropic isn't removing the safeguards but will now transparently flag or warn users when a prompt triggers a downgrade or refusal.