Tools chosen: polymarket_direct polymarket_related kalshi_related claude_news gdelt_news code_execution

Sub-questions (Fermi decomposition)

Planner reasoning

This market hinges on whether Anthropic's next 'Mythos/Fable-class' model debuts on the Arena.AI no-style-control leaderboard at ≥1510 by end of 2026. Key drivers are the current leaderboard score landscape, historical debut scores of Anthropic frontier models, the pace of Arena score inflation, and whether/when a Mythos-class model will actually launch and be added to the leaderboard. The Polymarket price is the primary anchor; news and cross-market signals refine it.

# Current state The market resolves on the Arena.ai leaderboard debut score of the *next* Anthropic Mythos-class model. Anthropic's first Mythos-class model, Claude Fable 5, already debuted June 9, 2026 near/above 1510 Elo. Anthropic then released the next Mythos-class model, Claude Fable 5.1 / Mythos 5.1, on September 1, 2026 — but as of the latest research (Sept 2, 2026) its official Arena.ai leaderboard score has not yet been reported/confirmed; only self-reported benchmarks (Terminal-Bench, GDPval-AA v2, HLE) are public. # Timeline of key events - 2026-04: Anthropic reportedly introduces "Mythos-class" tier via Claude Mythos Preview ("Project Glasswing") — reported (espressio.ai) - 2026-06-09: Claude Fable 5, first public Mythos-class model, launches; debuts ~1510 Elo, briefly hits ~1525 and #1 on Arena — reported (localaimaster.com) - 2026-06-12: Fable 5 suspended worldwide under U.S. export-control order — reported - 2026-07-01: Anthropic restores Fable 5 access with enhanced safety classifier — reported - 2026-07-12: Arena re-baselines Fable 5's score — reported - 2026-08 (as of research date): Fable 5 back at #1, ~1525 Elo, ahead of Opus 4.8/GPT-5.5 Pro/Gemini 3.1 Pro cluster — reported (localaimaster.com) - 2026-09-01: Anthropic launches Claude Fable 5.1 and Claude Mythos 5.1 (same underlying model, different access tiers) — confirmed (VentureBeat, TOI, Digit, iClarified, fonearena — multiple independent outlets) - 2026-09-02: Coverage of Fable 5.1 benchmark superiority over Fable 5/Opus 5 (Terminal-Bench 55.8% vs 42.0%; GDPval-AA v2 1853 vs 1723); no confirmed Arena.ai Overall leaderboard Elo score reported yet — reported # Event Will the next Anthropic Mythos-class model (after Fable 5), i.e. Fable 5.1/Mythos 5.1, debut on the Arena.ai Overall leaderboard (no style control) at a score ≥1510 by Dec 31, 2026? # Outcomes to forecast Yes / No # Kalshi market anchor No kalshi_direct price was returned in this research pull for ticker 0xc35747b2ab54f21809a564f89fd3cff2b8f5b0ba60c9aeec4aa8f7a49cc8551a — treat as unavailable. Best available cross-market proxy: Polymarket's identical-question market is at **82.7% YES**, up sharply (+66.2% in 7 days, +32.2% in 30 days) on low volume ($17.6k total, 8 data points), with a wide historical range (13.5%–97.65%), consistent with a recent repricing around the Sept 1 Fable 5.1/Mythos 5.1 launch news. # Sub-question answers 1. **Current top score / Anthropic's recent scores** — Claude Fable 5 (prior Mythos model) sits at ~1525 Elo (#1), after debuting ~1510 and being re-baselined post-suspension. [localaimaster.com] 2. **Mythos/Fable announcement & timeline** — Already realized: Fable 5 (Jun 2026) and Fable 5.1/Mythos 5.1 (Sep 1, 2026) both launched well before the Dec 31, 2026 deadline. [VentureBeat, espressio.ai] 3. **Debut scores of recent frontier releases** — Fable 5 debuted ~1510 Elo; earlier frontier debuts (per code_execution analysis) rarely jump >40 pts over the prior ceiling (~15% of 20 historical transitions), with most recent pre-Fable ceiling (Grok-4, Jul 2025) at ~1470. 4. **Score inflation trend** — Long-run trend ~+12.5 Elo/month (full history) to +16.8/month (recent 12-model window); under saturation-adjusted scenarios (1/3 slope), extrapolated Dec-2026 score ≈1516, roughly at threshold. [code_execution] 5. **Polymarket YES price** — 82.7%, sharply up from a 30-day low near 13.5%, reflecting the Sept 1 Fable 5.1 launch. [polymarket_direct] 6. **Sibling markets on other thresholds** — None found; no other Mythos-score-threshold or release-date Kalshi/Polymarket markets identified. [polymarket_related, kalshi_related] 7. **AutoEval / non-appearance risk** — Not directly addressed in research; Fable 5 successfully appeared (non-AutoEval) despite a mid-cycle suspension, suggesting Anthropic models do get listed, but no specific confirmation Fable 5.1/Mythos 5.1 has appeared yet. # Key facts (high-confidence, factual) 1. [VentureBeat, TOI, Digit, iClarified] Fable 5.1 and Mythos 5.1 launched Sept 1, 2026, confirmed by multiple independent outlets. 2. [localaimaster.com] Fable 5 debuted ~1510, later re-baselined to ~1525, current #1 on Arena as of Aug 2026. 3. [VentureBeat] Fable 5.1 shows clear benchmark gains over Fable 5 on Terminal-Bench and GDPval-AA v2. 4. [llm-stats.com] No confirmed Arena.ai Elo score for Fable 5.1/Mythos 5.1 as of Sept 2, 2026. # Cross-market signals - Kalshi related: "OpenAI or Anthropic IPO first — Anthropic" at 93% (unrelated but shows strong Anthropic momentum sentiment). - Polymarket: Same-question market at 82.7% YES, big recent upward repricing. - Sportsbook: N/A. # Analyst opinions and speculation - [code_execution] Quant model gives a blended ~45–55% base-rate/trend estimate, well below Polymarket's 82.7%, reflecting historical rarity of >40pt single-model jumps. - [claude_news synthesis] "Given Fable 5's precedent (~1510-1525 debut) and Fable 5.1's reported benchmark superiority... a debut score at or above 1510 appears plausible but not yet confirmed." # Directional lean per outcome - **Yes**: Fable 5.1 shows broad benchmark superiority over Fable 5 (which already scored ~1510-1525); Anthropic's Mythos tier has consistently launched at/above 1510; Polymarket pricing near 83%. - **No**: No confirmed Arena Elo score yet for Fable 5.1; historical base rate of large (>40pt) jumps is low (~15%); Arena Elo gains don't always track self-reported task benchmarks; possible AutoEval/delisting risk untested for this model. # Gaps / unknowns - No confirmed Arena.ai leaderboard Elo score for Fable 5.1/Mythos 5.1 at time of research. - No native Kalshi YES price captured for this specific ticker. - Ambiguity over whether "next Mythos-class model" for resolution purposes is Fable 5.1 (just launched) or a later one — if a further model launches before Dec 2026, that could also count depending on exact contract timing relative to Fable 5's debut date (contract creation date unclear). # Calibration anchors - Polymarket YES price (proxy anchor): 82.7%, trending up sharply. - Quant/base-rate model estimate: ~45–55%. - Precedent: prior Mythos model (Fable 5) debuted right at/above 1510, suggesting Anthropic's Mythos tier is designed/tuned to clear this exact threshold.

1. Both forecasters converge to nearly identical probabilities (0.80–0.85) that closely track the thin, volatile Polymarket proxy (only 8 data points, $17.6k volume, range 13.5%–97.65%), but neither discounts sufficiently for the fact that this "consensus" is essentially a single noisy data source repricing sharply on launch-day news rather than independent confirmation. 2. Neither forecast grapples seriously with the quant/base-rate model's 45–55% estimate beyond dismissing it as "anchored to the old ~1470 ceiling" — but that model's core point (large jumps are rare, only ~15% of historical transitions exceed 40pts) still applies to whether Fable 5.1 clears 1510 specifically, since Fable 5's *debut* was ~1510, not its later re-baselined 1525. 3. Critical blurring of "debut score" vs. "current/re-baselined score": Fable 5 debuted near 1510 but only reached ~1525 after a suspension (Jun 12) and re-baseline (Jul 12) roughly a month later — if Fable 5.1 follows a similar pattern (initial debut lower, climbing after re-baseline), the debut figure itself could land below 1510, which is exactly what the question asks about, not the eventual steady-state score. 4. Both forecasts underweight the "Gaps/unknowns" item on resolution ambiguity — since Fable 5.1 and Mythos 5.1 are "the same underlying model, different access tiers," it's unclear whether Arena will list them as a single distinct "debut" entry at all, creating non-appearance or classification risk that isn't reflected in either probability. 5. Neither forecast meaningfully incorporates sub-question 7 (AutoEval/non-appearance risk) — Fable 5 did eventually appear despite suspension, but no evidence yet confirms Fable 5.1/Mythos 5.1 has been submitted to or accepted by Arena's leaderboard process at all. 6. Given the "no confirmed Arena Elo score" is a hard evidentiary gap (not just noise), a probability as high as 0.80–0.85 reflects overconfidence in extrapolating from self-reported benchmarks (Terminal-Bench, GDPval-AA v2) to Arena human-preference Elo, which the brief explicitly flags as an imperfect translation.

79% (62% confidence)

Yes: 79% No: 21%

Fable 5.1/Mythos 5.1 has already launched with confirmed benchmark superiority over Fable 5, and it only needs to debut near the current leaderboard cluster (Fable 5 at ~1525, threshold 1510 is 15 points below the current #1) rather than achieve a rare >40pt Elo jump — this undercuts the quant model's 45-55% estimate anchored to the pre-Fable ceiling. Both forecasts and the Polymarket proxy (82.7%) converge on ~0.80-0.85, and the critique's strongest points — the debut-vs-rebaselined distinction, the thin proxy market, and unconfirmed Arena appearance/classification risk — justify a modest hold-back but not a return toward 50%. Fable 5 itself debuted right at ~1510, suggesting Anthropic tunes Mythos-tier releases to clear this exact level, and a superior successor should debut at or above its predecessor's debut. I settle slightly below the two forecasts at 0.79 to account for the hard evidentiary gap (no confirmed Arena score yet), imperfect benchmark-to-Elo translation, and resolution ambiguity over which entry counts as the 'debut.'

Total pipeline time: 217.3s