The Short Answer
GPT-5.5 wins on price; Claude Fable 5 wins on autonomy and context. GPT-5.5 costs $5/$30 against Fable 5's $10/$50, and its $0.50 cached-input rate makes long agent sessions dramatically cheaper than the sticker prices suggest. Fable 5 counters with a 1M-token context window (2.5x GPT-5.5's 400K) and the strongest long-horizon agentic performance we have seen.
Spec-for-Spec
| Claude Fable 5 | GPT-5.5 | |
|---|---|---|
| Input $/1M | $10.00 | $5.00 |
| Output $/1M | $50.00 | $30.00 |
| Cached input $/1M | $1.00 (cache read) | $0.50 |
| Context window | 1M tokens | 400K tokens |
| Reasoning mode | Adaptive thinking | Configurable effort |
| Batch discount | 50% | 50% |
Real Workload: A Coding Agent
Take an autonomous coding agent averaging 40 turns per task, with a 30K-token context re-read each turn and 2K output tokens per turn. Per task that is roughly 1.2M input and 80K output tokens. On list prices: Fable 5 ≈ $16.00, GPT-5.5 ≈ $8.40. With realistic cache hit rates (~85% of input cached), GPT-5.5 drops to roughly $3.30 per task while Fable 5 with Anthropic's prompt caching lands near $5.60. But if Fable 5 completes the task in one attempt where GPT-5.5 needs a retry or a human nudge, the ranking flips.
Where Each One Wins
Pick Claude Fable 5 for: overnight autonomous coding runs, complex multi-repo refactors, deep research over huge document sets (the 1M context means fewer lossy compactions), and any task where a failed run costs more than the tokens.
Pick GPT-5.5 for: cache-friendly agent loops, products already on OpenAI tooling, latency-sensitive interactive use, and workloads where you can A/B test and confirm quality parity at 40% lower list price.
Do Not Forget the Tier Below
Both vendors sell a near-flagship at a steep discount: Claude Opus 4.8 ($5/$25) and GPT-5.4 ($2.50/$15). For most production workloads these are the rational defaults — reserve the true flagships for the tasks that demonstrably need them.
Model both against your own traffic in the cost calculator, or line them up against Gemini 3.1 Pro and Grok 4.3 in the comparison tool.