GPT-6 Astra vs Claude Fable 5.1: Complete Comparison

Name: GPT-6 Astra vs Claude Fable 5.1
Keywords: Frontier flagship comparison

GPT-6 Astra and Claude Fable 5.1 are the September 2026 flagships from OpenAI and Anthropic.

Featured · Updated today · Last verified: September 2026 · Score 96

Choose GPT-6 Astra when

OpenAI-stack teams that need the hardest coding, computer-use, and document agents.

Choose Claude Fable 5.1 when

Claude-stack long-horizon coding and research when Opus 5 at higher effort still falls short.

Short verdict

Astra is OpenAI's new hardest model. Fable 5.1 is Anthropic's generally available long-horizon flagship with cheaper caches.

Key differences

Astra documents 1.05M context, max effort, and Critical cyber. Fable 5.1 documents 1M context, $0.25 cache reads, and multi-cloud general availability.

Best for

Pick Astra if the stack is already OpenAI Responses/Codex. Pick Fable 5.1 if Claude Code and cheaper cached agent loops are the constraint.

Reasoning fit

Pin effort on both. Astra max and Fable high will inflate tokens.

Coding workflow fit

Score merged PRs and tool errors, not isolated HumanEval-style rows.

Multimodal fit

Image-in only. Use Omni or MAI image models for generation.

Enterprise fit

Confirm workspace enablement (Astra) vs account-team Mythos if you need permissive cyber.

Who should not choose this?

Do not choose Astra expecting a free tier.
Do not choose Fable 5.1 when Opus 5 already clears your evals.
Do not treat this as Astra vs Mythos 5.1.

Cost considerations

Same $10/$50 sticker; Astra long-context multipliers vs Fable cache-read cut.

Limitations

Verified September 6, 2026 against vendor docs.

Final recommendation

Pilot both on one repository and one research task. Keep GPT-5.6 Sol and Claude Opus 5 as cheaper fallbacks.