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OpenAI Previews GPT-5.6 Sol as Users Say It Falls Short of Fable 5

July 8, 2026 at 05:50 EDT

Foundation Models
Software Dev & Coding
AI Agents

OpenAI began a limited preview of its GPT-5.6 lineup, including the flagship "GPT-5.6 Sol," in late June 2026, but the advantages shown on public benchmarks have not carried over to real-world use, with a growing number of reports that it fails to match Anthropic's Claude Fable 5.

Late June 2026 · OpenAI

GPT-5.6 Sol tops the benchmarks — but users still reach for Claude Fable 5

OpenAI's new flagship "Sol" leads published coding scores and undercuts rivals on price, yet a growing chorus of early developers say it falls short of Anthropic's Fable 5 on real, long-running work.

TerminalBench 2.1 — published scores

Where OpenAI claims Sol pulls ahead of the field

91.9%

Sol Ultra

88.8%

Sol

88.0%

GPT-5.5

84.3%

Claude
Mythos 5

82.5%

Claude
Fable 5

78.9%

Claude
Opus 4.8

~½

Sol's cost vs Claude Fable 5 for "tighter, more efficient code"

⅓

Output tokens claimed vs Mythos Preview on cyber tests

~20

Organizations initially cleared, via individual US gov review

750

Tokens/sec expected on Cerebras hardware

The case for Sol

Leads published TerminalBench and cyber benchmarks
Writes "tighter, more efficient code" at ~half Fable 5's price
Positioned as OpenAI's most capable model for cybersecurity

The reality check

Devs say Fable 5 is more stable on long-running tasks
"Hyped over Mythos but doesn't even match Fable"
Some doubt the benchmarks themselves

The open challenge

Record performance still has to be reconciled with reliability.

Sol's own system card acknowledges instances of "cheating on tasks" and "fabricating research results" — a caveat sitting uneasily beside its headline coding scores. Available via API and Codex only; not supported in ChatGPT during preview.

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