ModelSpec is an analysis of alternatives for AI models: it decides which models fit a job from your requirements, sourced benchmarks and real cost, and shows its work.

Claude Fable 5 vs Claude Opus 5.5 for General assistant (best available), as of 2026-09-23

Find the active assistant whose open-ended responses people rate most highly.

Worked example as of 2026-09-23, from the public data image.

What the evidence says

The evidence separates them: Claude Opus 5.5 leads for General assistant (best available), with a 64% chance of being the best model in the full lineup for this job, as of 2026-09-23.

Claude Fable 5 has a 25% chance of matching or beating Claude Opus 5.5 on this job's objective.

probability bands: a model joins the leader when P(its score >= the leader's) >= 0.25 under the score posteriors; a weighted capability with an 80% interval wider than 2.8 is not enough evidence

Decision status: partial. P(best) and score intervals retain the full ranked lineup for this job, including models beyond this pair and models with thin evidence. Models with unknown objective values remain unranked and are outside this probability. P(best) is not a head-to-head probability. Separation from this challenger need not mean a unique best model across that lineup.

Scores and score intervals use the objective's dimensionless normalised units. Cost per task is in USD for the selected offering and the applied spec's task size. When task size is omitted, the engine uses its documented default. Unavailable means the engine returned no value.

Most preferred responses, cost no object.

Requirements

• model.class = text-generator: A conversational assistant needs a text-generating model.
• model.lifecycle = active: An active model is still supported by its lab or provider.

Preferences

• chat_preference: Rank only by the chat-preference capability estimate.

Evidence for Claude Fable 5

Missing facts for Claude Fable 5

• model.hardware_fit_indeterminate: unknown
• model.languages: unknown
• model.parameters_total: unknown
• origin.base_lineage: not_disclosed
• origin.weights_hosting: not_disclosed
• offering.api_model_id: unknown
• offering.data.zero_retention: requires_contract
• offering.private_deployment: unknown
• offering.router_slug: unknown
• offering.speed.throughput: unknown
• offering.speed.time_to_first_token: unknown

Evidence for Claude Opus 5.5

Missing facts for Claude Opus 5.5

• model.hardware_fit_indeterminate: unknown
• model.languages: unknown
• model.parameters_total: unknown
• origin.base_lineage: not_disclosed
• origin.weights_hosting: not_disclosed
• offering.api_model_id: unknown
• offering.data.zero_retention: requires_contract
• offering.private_deployment: unknown
• offering.router_slug: unknown
• offering.speed.throughput: unknown
• offering.speed.time_to_first_token: unknown

What this template teaches

One Must selects the class, one Prefer states what best means.

Decision dec_7f2a6357d164309eef5edbee on public snapshot snap_05a963d2477ac2fb.

Read the full dated job answer.

The live board uses current data. This link applies the template to its full lineup.

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