TokenMix Research Lab · 2026-06-10
Claude Fable 5 vs GPT-5.5 vs Gemini 3.1 Pro: 2026 Verdict
Last Updated: 2026-06-10 Author: TokenMix Research Lab Data verified: 2026-06-10 — Anthropic announcement and API docs (pricing, models overview, migration guide), OpenAI pricing docs, Google AI pricing docs, The Decoder, TechCrunch, Hacker News launch thread
Claude Fable 5 wins the hard benchmarks, Gemini 3.1 Pro wins the price sheet, and GPT-5.5 sits in a middle that gets uncomfortable fast. The June 9 launch of Claude Fable 5 at $10/$50 per MTok reset the flagship comparison: it posts SWE-Bench Pro 80.3% against GPT-5.5's 58.6% and Gemini 3.1 Pro's 54.2%, and FrontierCode 29.3% against GPT-5.5's 5.7% — at 2-5× the price per token (The Decoder, Anthropic).
The sticker prices say Gemini 3.1 Pro at $2/$12 is 5× cheaper than Fable 5 (Google AI pricing). The cost-per-solve math says it depends entirely on task difficulty — and the long-context billing tables say something nobody quotes: past 272K input tokens, GPT-5.5's price doubles to $10/$45 (OpenAI pricing), and a 300K-token request costs $3.90 on GPT-5.5 versus $4.00 on Fable 5. The 2× sticker gap collapses to 2.5%. This comparison runs all three models through the same price, benchmark, cost-per-solve, and risk tables, with every number tagged confirmed or vendor-reported.
Table of Contents
- Quick Verdict
- Quick Comparison: Three Flagships at a Glance
- Pricing: $10/$50 vs $5/$30 vs $2/$12
- Long-Context Billing: Where the 2x Gap Collapses
- Benchmarks: SWE-Bench Pro, FrontierCode, and Vendor Math
- Cost per Solve: The Only Number That Matters
- API Behavior: Refusals, Thinking, Multimodal, Lanes
- Use Case Matrix: Route by Task, Not by Brand
- Risk Matrix: What Each Vendor Doesn't Advertise
- Final Recommendation
- FAQ
Quick Verdict
No single winner. Fable 5 is the strongest model and the worst deal for routine work; Gemini 3.1 Pro is the price-performance floor with a missing frontier scorecard; GPT-5.5 is squeezed from both sides.
| Claim | Status | Source |
|---|---|---|
| Fable 5 leads SWE-Bench Pro (80.3%) and FrontierCode (29.3%) | Confirmed — Anthropic-published eval, single test set | The Decoder |
| Gemini 3.1 Pro is the cheapest flagship at $2/$12 (≤200K prompt) | Confirmed | Google AI pricing |
| GPT-5.5 doubles to $10/$45 past 272K input tokens | Confirmed | OpenAI pricing |
| Fable 5 bills one flat rate across its full 1M context | Confirmed | Anthropic pricing docs |
| Gemini 3.1 Pro is cheapest per solved routine task ($0.81) | Confirmed math on vendor-reported pass rates | Derived below |
| Fable 5 is cheapest per solved frontier-hard task ($6.83) | Confirmed math on vendor-reported pass rates | Derived below |
| Gemini has no published FrontierCode result | Confirmed absence | Anthropic eval table |
| Cross-vendor benchmark numbers are independently replicated | Not yet — all vendor-reported as of June 10 | — |
Quick Comparison: Three Flagships at a Glance
One table before the deep dive. Opus 4.8 stays in the tables as the reference point, because for most Claude workloads it is still the default choice.
| Spec | Claude Fable 5 | GPT-5.5 | Gemini 3.1 Pro | Claude Opus 4.8 |
|---|---|---|---|---|
| Input / output per MTok | $10.00 / $50.00 | $5.00 / $30.00 | $2.00 / $12.00 | $5.00 / $25.00 |
| Long-context surcharge | None — flat to 1M | $10.00 / $45.00 past 272K | $4.00 / $18.00 past 200K | None — flat to 1M |
| Context window | 1M | 1M | 1M+ | 1M |
| Max output | 128K | — | — | 128K |
| Cache read | $1.00 | $0.50 (≤272K), $1.00 (>272K) | $0.20-$0.40 | $0.50 |
| Batch | $5.00 / $25.00 | $2.50 / $15.00 (Batch and Flex) | Separate batch rates published | $2.50 / $12.50 |
| SWE-Bench Pro (Anthropic eval) | 80.3% | 58.6% | 54.2% | 69.2% |
| FrontierCode (Anthropic eval) | 29.3% | 5.7% | Not published | 13.4% |
| Release status | GA, June 9, 2026 | GA | Preview label still attached | GA |
Sources: Anthropic pricing, OpenAI pricing, Google AI pricing.
Pricing: $10/$50 vs $5/$30 vs $2/$12
On base rates, the order is unambiguous: Gemini 3.1 Pro costs 20% of Fable 5 on input and 24% on output. GPT-5.5 sits at exactly half of Fable 5 — the same ratio Opus 4.8 holds, which is not a coincidence Anthropic hides: every Fable 5 rate is exactly 2× Opus 4.8.
| Rate | Fable 5 | GPT-5.5 | Gemini 3.1 Pro |
|---|---|---|---|
| Base input /MTok | $10.00 | $5.00 | $2.00 |
| Base output /MTok | $50.00 | $30.00 | $12.00 |
| Cached input /MTok | $1.00 | $0.50 | $0.20-$0.40 |
| Cache write | $12.50 (5-min) / $20.00 (1-hour) | No explicit write fee — automatic prefix caching | Cache storage billed per token-hour |
| Minimum cacheable prompt | 512 tokens | Prefix-match based | Model-dependent |
| Batch input / output | $5.00 / $25.00 | $2.50 / $15.00 | Separate batch rates published |
| Premium lane | None | Priority: $12.50 / $75.00 | None |
Three structural differences matter more than the headline rates:
- Caching models differ. Anthropic charges explicit cache writes ($12.50-$20 per MTok) and then $1 reads; OpenAI caches matching prefixes automatically at $0.50 with no write fee (OpenAI prompt caching); Google bills cached tokens plus storage per token-hour (Google AI pricing). For agents with a stable system prompt hit hundreds of times a day, all three converge near 90% input savings — the cache math just arrives there differently.
- Fable 5 caches shorter prompts. The 512-token minimum (down from 1,024 on Opus 4.8) makes short system prompts cacheable for the first time on a Claude flagship.
- GPT-5.5 has a premium lane nobody should buy by accident. Priority runs $12.50/$75 — above Fable 5's base rates — for latency guarantees (breakdown).
Long-Context Billing: Where the 2x Gap Collapses
This is the table that changes routing decisions. Fable 5 and Opus 4.8 bill one flat rate across the full 1M window — per Anthropic's pricing docs, a 900k-token request bills at the same per-token rate as a 9k one. GPT-5.5 doubles input past 272K. Gemini 3.1 Pro doubles input past 200K.
Worked math, 300K input / 20K output — a routine size for repo-scale agent context:
| Model | Applicable rate | Input cost | Output cost | Total |
|---|---|---|---|---|
| Claude Fable 5 | $10 / $50 flat | $3.00 | $1.00 | $4.00 |
| GPT-5.5 | $10 / $45 (>272K tier) | $3.00 | $0.90 | $3.90 |
| Gemini 3.1 Pro | $4 / $18 (>200K tier) | $1.20 | $0.36 | $1.56 |
| Claude Opus 4.8 | $5 / $25 flat | $1.50 | $0.50 | $2.00 |
At 300K tokens, GPT-5.5 costs 97.5% of Fable 5. The "GPT-5.5 is half the price" claim is only true below 272K input.
Push deeper — 800K input / 50K output, a full-codebase audit:
| Model | Input cost | Output cost | Total | vs Fable 5 |
|---|---|---|---|---|
| Claude Fable 5 | $8.00 | $2.50 | $10.50 | — |
| GPT-5.5 | $8.00 | $2.25 | $10.25 | -2.4% |
| Gemini 3.1 Pro | $3.20 | $0.90 | $4.10 | -61% |
| Claude Opus 4.8 | $4.00 | $1.25 | $5.25 | -50% |
Conclusion: above 272K, choosing between Fable 5 and GPT-5.5 on price is pointless — choose on capability, where the published gap is wide. Gemini 3.1 Pro keeps a real cost lead at every context size, even after its own >200K doubling.
Benchmarks: SWE-Bench Pro, FrontierCode, and Vendor Math
The only same-test-set comparison available is Anthropic's launch eval, which ran all four models on SWE-Bench Pro and three on FrontierCode (The Decoder):
| Benchmark | Fable 5 | GPT-5.5 | Gemini 3.1 Pro | Opus 4.8 |
|---|---|---|---|---|
| SWE-Bench Pro | 80.3% | 58.6% | 54.2% | 69.2% |
| FrontierCode | 29.3% | 5.7% | Not published | 13.4% |
Two caveats, both real:
- These are Anthropic-run numbers. Vendor-published evals favor the vendor's framing; independent replication is pending as of June 10. The direction (Fable leads, gap widens with difficulty) is consistent with early field reports in the Hacker News launch thread, but treat magnitudes as provisional.
- Each vendor quotes its own favorite benchmark. Google's published numbers for Gemini 3.1 Pro are GPQA Diamond 94.3% and SWE-bench Verified 80.6% — a different, easier test set than SWE-Bench Pro, which is why Google can report 80.6% while Anthropic's harness scores the same model at 54.2%. Neither number is wrong; they are answers to different questions. Full Gemini context in our Gemini 3.1 Pro review.
Beyond the table: Anthropic reports a frontier physics eval where Fable 5 reached in 36 hours what GPT-5.5 needed four days for, and customer evals (Anaconda) report Fable beating Opus 4.8 at every effort level while running 25-30% faster — both vendor-curated, both directionally consistent with the benchmark deltas.
Cost per Solve: The Only Number That Matters
Per-attempt cost is what the pricing page sells. Cost per solved task — attempt cost divided by pass rate — is what you pay. Reference task: 100K input / 20K output, short-context rates.
| Model | Cost per attempt | SWE-Bench Pro pass rate | Cost per solve (routine-hard) |
|---|---|---|---|
| Gemini 3.1 Pro | $0.44 | 54.2% | $0.81 |
| Claude Opus 4.8 | $1.00 | 69.2% | $1.45 |
| GPT-5.5 | $1.10 | 58.6% | $1.88 |
| Claude Fable 5 | $2.00 | 80.3% | $2.49 |
| Model | Cost per attempt | FrontierCode pass rate | Cost per solve (frontier-hard) |
|---|---|---|---|
| Claude Fable 5 | $2.00 | 29.3% | $6.83 |
| Claude Opus 4.8 | $1.00 | 13.4% | $7.46 |
| GPT-5.5 | $1.10 | 5.7% | $19.30 |
| Gemini 3.1 Pro | $0.44 | Not published | Cannot be computed |
Read the two tables together and the routing rule writes itself:
- Routine-hard work: Gemini 3.1 Pro at $0.81 per solve is the floor — roughly a third of Fable 5's $2.49. If a task type passes your evals on Gemini, paying anyone more for it is loyalty, not engineering.
- Frontier-hard work: the order inverts. GPT-5.5's 5.7% pass rate makes it the most expensive option on the board at $19.30 per solve — 17.6× its per-attempt cost. Fable 5 becomes the cheapest frontier option despite the highest sticker price.
- The Gemini gap: no FrontierCode number means no frontier cost-per-solve math. Until Google publishes one or independent runs land, routing frontier-hard work to Gemini is a bet without a posted line.
Caveat from the field: several developers in the launch thread report Fable finishing tasks in fewer turns with smaller diffs — one claims comparable results at roughly half the tokens. If that replicates, Fable's effective per-attempt cost approaches Opus parity and these tables understate its position. Variance is high; meter your own workloads.
API Behavior: Refusals, Thinking, Multimodal, Lanes
The three platforms diverge hardest in behavior, not price. These are the differences that break integrations or disqualify models outright.
| Behavior | Claude Fable 5 | GPT-5.5 | Gemini 3.1 Pro |
|---|---|---|---|
| Thinking control | Always on; effort low→max, default high; cannot disable |
Standard sampling and reasoning controls | Standard controls |
| Refusal surface | HTTP 200 + stop_reason: "refusal" + stop_details.category |
Standard error/refusal patterns | Standard error/refusal patterns |
| Safety fallback | <5% of sessions rerouted to Opus 4.8, billed at Opus rates | None equivalent | None equivalent |
| Data retention | 30-day mandatory; no zero-data-retention option | Standard retention controls | Standard retention controls |
| Modality | Text-first | Text-first | Text, image, video, audio input |
| Delivery lanes | Standard + Batch | Standard, Batch, Flex (50% off), Priority (premium) | Standard + batch rates |
| Model variants above | Mythos 5 (same model, classifiers lifted, Glasswing partners only) | GPT-5.5-pro at $30/$180 | Gemini 3.5 Pro announced, not shipped |
Three of these deserve a sentence each:
- Fable 5's refusal model is unique and silently breaks status-code error handling. A refused request returns HTTP 200; anything keyed on status codes passes a declined response downstream. The full review covers the migration checklist.
- Fable 5's retention rule is disqualifying for some industries. Covered Model status means mandatory 30-day retention — zero-data-retention agreements do not apply. Legal, health, and regulated-finance workloads stay on Opus 4.8 or a competitor regardless of benchmarks.
- Gemini's multimodal breadth is the quiet differentiator. Video and audio input at flagship quality has no equivalent on the other two; for pipelines that touch media, the comparison ends before the price table.
Use Case Matrix: Route by Task, Not by Brand
| Workload | Best pick | Why | Runner-up |
|---|---|---|---|
| Frontier-hard agentic coding | Claude Fable 5 | $6.83/solve, cheapest where others fail | Opus 4.8 ($7.46) |
| Routine coding at volume | Gemini 3.1 Pro | $0.81/solve floor | Opus 4.8 if Claude-native stack |
| Long-context, cold input >200K | Gemini 3.1 Pro | $4/$18 beats everyone above its own breakpoint | Opus 4.8 (flat $5/$25) |
| Long-context with stable cacheable prefix | Claude (Opus or Fable) | Flat rates + $0.50-$1.00 cache reads, no breakpoint management | GPT-5.5 below 272K |
| Offline/async bulk jobs | GPT-5.5 Batch or Flex | $2.50/$15 with two lane options | Claude Batch ($5/$25 Fable, $2.50/$12.50 Opus) |
| Video/audio input pipelines | Gemini 3.1 Pro | Only flagship with native video+audio input | — |
| Regulated, zero-retention requirements | Not Fable 5 | Mandatory 30-day retention | Opus 4.8 or competitor per your DPA |
| Latency-sensitive interactive UX | None of these three | All are thinking-heavy flagships | Sonnet 4.6 tier and below |
Risk Matrix: What Each Vendor Doesn't Advertise
| Risk | Fable 5 | GPT-5.5 | Gemini 3.1 Pro |
|---|---|---|---|
| Billing surprise | Rerouted sessions bill at Opus rates mid-conversation | Input doubles past 272K; Priority lane at $12.50/$75 | Input doubles past 200K; cache storage billed per token-hour |
| Capability surprise | Classifier false positives — HN reports flag MRI code and malaria research as bio risks | FrontierCode 5.7% — frontier-hard retries get expensive fast | No FrontierCode result published at all |
| Compliance | No ZDR, 30-day retention mandatory | Standard | Standard |
| Stability | New refusal/fallback semantics, 1 day old | Mature | "Preview" label still on the flagship; 3.5 Pro announced overhead |
| Lock-in pressure | effort and fallback params are Claude-specific |
Flex/Priority lanes are OpenAI-specific | Multimodal pipelines are hard to port |
Final Recommendation
Route, don't crown. Gemini 3.1 Pro takes routine volume at $0.81 per solve and anything multimodal or past 200K cold context. Claude Fable 5 takes the frontier-hard 10-20% of the workload where its 80.3%/29.3% pass rates make it the cheapest per solve on the board. GPT-5.5 keeps Batch/Flex bulk work and stacks below 272K input — above that line its price advantage over Fable 5 is 2.5% and not worth a capability discount. And if your stack is Claude-native, Opus 4.8 at $1.45 per routine solve is still the default workhorse; the full lever-by-lever spend playbook is in our Fable 5 cost optimization guide.
FAQ
Is Claude Fable 5 better than GPT-5.5?
On Anthropic's published evals, yes by a wide margin: SWE-Bench Pro 80.3% vs 58.6%, FrontierCode 29.3% vs 5.7%. The numbers are vendor-reported and not yet independently replicated. GPT-5.5 costs half as much below 272K input; above 272K its long-context rates erase that gap almost entirely.
Is Claude Fable 5 better than Gemini 3.1 Pro?
On the same-harness eval, Fable 5 leads SWE-Bench Pro 80.3% to 54.2%. Gemini 3.1 Pro costs $2/$12 versus $10/$50 and wins routine-task economics at $0.81 per solve. Gemini has no published FrontierCode result, so frontier-hard comparison is one-sided by absence.
Which flagship is cheapest per solved task?
Depends on difficulty. Routine-hard (SWE-Bench Pro tier): Gemini 3.1 Pro at $0.81, then Opus 4.8 at $1.45, GPT-5.5 at $1.88, Fable 5 at $2.49. Frontier-hard (FrontierCode tier): Fable 5 at $6.83, Opus 4.8 at $7.46, GPT-5.5 at $19.30.
Does GPT-5.5 charge more for long context?
Yes. Past 272K input tokens, GPT-5.5 bills $10 input / $45 output per MTok instead of $5/$30, and cached input doubles to $1.00. A 300K-token request costs $3.90 — within 2.5% of Claude Fable 5's $4.00.
Which model is best for long-context work?
Cold input above 200K: Gemini 3.1 Pro at $4/$18 is cheapest, even after its own surcharge. Stable cacheable prefixes: Claude's flat rates plus $0.50-$1.00 cache reads avoid breakpoint management entirely. Fable 5 and Opus 4.8 are the only two with no long-context surcharge at all.
Which has the largest context window?
All three flagships operate at the 1M-token class: Fable 5 and Opus 4.8 at 1M with 128K max output, GPT-5.5 at 1M with rate doubling past 272K, Gemini 3.1 Pro at 1M+ with rate doubling past 200K.
Which flagship should AI agents use in 2026?
Route by difficulty tier: Gemini 3.1 Pro or Opus 4.8 for the routine 80-90% of tasks, Claude Fable 5 for the frontier-hard remainder where retries on cheaper models cost more than Fable's $2.00 per attempt. Add a stop_reason check before routing anything to Fable 5 — its refusals return HTTP 200.
Sources
- Anthropic — Claude Fable 5 and Mythos 5 announcement
- Anthropic API docs — models overview
- Anthropic API docs — pricing
- Anthropic API docs — introducing Claude Fable 5 and Claude Mythos 5
- OpenAI pricing
- Google AI — Gemini API pricing
- The Decoder — Anthropic releases Claude Fable 5 and Mythos 5
- TechCrunch — Anthropic released Claude Fable 5 days after warning AI is getting too dangerous
- Hacker News — Claude Fable 5 launch thread
- OpenRouter — Claude Fable 5 listing
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- Gemini 3.1 Pro Review 2026: 94.3% GPQA at $2/$12 — Top Value
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