GPT-5.6 vs Claude Mythos 5

The defining difference here is access, not raw capability: both GPT-5.6 (Sol) and Claude Mythos 5 are gated to a limited set of organizations, and most readers cannot get either today. GPT-5.6 is in a limited preview open only to vetted organizations at the US government's request. Claude Mythos 5 is the same underlying model as Claude Fable 5, but offered without the dual-use safety measures to only approved organizations.

So the honest answer for nearly every buyer is this: your realistic option is Claude Fable 5, which is Mythos 5's generally available sibling, or GPT-5.6's eventual general availability. This page still compares the two restricted flagships in full, because a small number of readers do qualify and need to choose an approval path.

For those who qualify, the real question is which vendor's approval process and safety posture fits your governance. We compare eligibility, price, safety design, and compliance on the terms a buyer controls, not benchmark rankings.

GPT-5.6 (Sol) vs. Claude Mythos 5: Side-by-Side

Eligibility: can you get either one?

Most organizations cannot access GPT-5.6 or Claude Mythos 5 today, because both are restricted flagships with separate approval paths. GPT-5.6 is in a limited preview open only to a small set of vetted organizations, via API and Codex, at the US government's request. Claude Mythos 5 is offered without dual-use safety measures to only approved organizations.

These are two different gates, not one. Being cleared by OpenAI does not clear you with Anthropic, and the reverse is also true. Each vendor runs its own vetting, so plan for two separate applications if you want to weigh both.

If you do not qualify for either, do not wait on a model you cannot license. Claude Fable 5 is Mythos 5's generally available sibling with safeguards on, and GPT-5.6 general availability is planned in the coming weeks.

• GPT-5.6: vetted-organization preview at the US government's request.
• Mythos 5: approved organizations only, safeguards removed.
• Neither qualifies you for the other; expect two separate approvals.

Price: what each restricted flagship costs

GPT-5.6 Sol is cheaper per token than Claude Mythos 5, which is priced the same as Claude Fable 5. Sol costs $5 per million input tokens and $30 per million output tokens. Mythos 5 shares Fable 5's pricing at $10 input and $50 output per million tokens.

Price should not lead this decision while access is the real gate. A lower token price on a model you cannot license has no value to you today. Weigh cost only after you know which approval path you can actually clear.

If price matters and you cannot get either flagship, the cheaper GPT-5.6 tiers change the math once general availability arrives. Terra costs $2.50 input and $15 output; Luna costs $1 input and $6 output.

• GPT-5.6 Sol: $5 input / $30 output per million tokens.
• Claude Mythos 5: $10 input / $50 output, same as Fable 5.
• Cheaper GPT-5.6 tiers later: Terra $2.50/$15, Luna $1/$6.

→ Compare per-token pricing for every AI model

Safety design: layered safeguards vs safeguards removed

The two flagships take opposite approaches to safety, and that is the core governance choice. GPT-5.6 ships with a layered safeguard stack, automated red-teaming, and enhanced cyber features. Claude Mythos 5 removes the dual-use safety measures that Fable 5 keeps on, and it drops Fable 5's fallback to Claude Opus 4.8 for approved organizations.

That is why Mythos 5 is gated so tightly. Fable 5 blocks high-risk cyber and bio requests and falls back to Opus 4.8, with Anthropic reporting more than 95% of sessions never triggering that fallback. Mythos 5 hands approved organizations the same base model without that backstop.

For most governance teams, safeguards are a feature, not a limit. If your policy requires documented refusal behavior on risky prompts, a model with safeguards removed is harder to defend, which points back to Fable 5 or GPT-5.6's layered stack.

• GPT-5.6: layered safeguards, automated red-teaming, enhanced cyber features.
• Mythos 5: dual-use safety measures removed for approved organizations.
• Mythos 5 drops Fable 5's Opus 4.8 fallback backstop.

Compliance: which fits regulated data?

Both flagships inherit their vendor's compliance surface, but each carries a caveat you must confirm before sending regulated data. GPT-5.6 inherits OpenAI SOC 2, ISO 27001, and a HIPAA BAA on the API and Enterprise, once a model is in scope. Because GPT-5.6 is still a preview, confirm the specific model is named in your BAA first.

Claude Mythos 5 inherits Anthropic SOC 2, ISO 27001, and ISO 42001, with a mandatory 30-day safety retention on business accounts, so full zero-data-retention is not available. Because Mythos 5 is an approved-only model with safeguards removed, treat its terms as program-specific and confirm them in writing.

The retention rule is the key tradeoff on the Anthropic side. The 30-day data is used only for safety, then deleted, but teams that require zero retention cannot meet that on the Mythos-class model.

• GPT-5.6: OpenAI SOC 2, ISO 27001, HIPAA BAA once in scope; confirm the model is named.
• Mythos 5: Anthropic SOC 2, ISO 27001, ISO 42001; mandatory 30-day safety retention.
• Zero-data-retention requirement: not available on the Mythos-class model.

How to choose, and what most buyers should do

Choose on eligibility first, then safety posture, then price. If you can clear only one vendor's approval, that decides it. If you can clear both, pick the safety design your governance can defend: GPT-5.6's layered safeguards or Mythos 5's removed safeguards for approved work.

For nearly every buyer, the realistic move is different: use Claude Fable 5 now, or wait for GPT-5.6 general availability. Fable 5 is Mythos 5's generally available sibling with the same base model and safeguards on. It is deployable worldwide today and runs on the standard Enterprise compliance surface.

Run a short pilot on whichever you can actually access. Test refusal behavior, latency, and cost on your own tasks before you commit. A one-week trial on Fable 5 tells you more than a benchmark table on a model you cannot license.

• Can clear only one vendor: that approval decides it.
• Can clear both: pick the safety posture you can defend.
• Cannot clear either: use Claude Fable 5 now, or wait for GPT-5.6 GA.

The Verdict

Neither GPT-5.6 nor Claude Mythos 5 is a general-purpose buy, because both are restricted flagships with separate approval paths. For nearly every business, the realistic option is Claude Fable 5, Mythos 5's generally available sibling with safeguards on, or GPT-5.6's eventual general availability.

If you do qualify for both, decide on safety posture, not token price. GPT-5.6 ships a layered safeguard stack, while Mythos 5 removes Fable 5's dual-use safety measures and its Opus 4.8 fallback for approved organizations. GPT-5.6 Sol is cheaper at $5 input and $30 output versus Mythos 5's $10 and $50, but access should lead the decision.

Compliance is the tiebreaker for regulated teams. Confirm GPT-5.6 is named in your BAA before sending regulated data, and note that the Mythos-class model carries a mandatory 30-day safety retention with no full zero-retention option.