GPT-5.6 Sol vs Claude Mythos 5: Cybersecurity AI (June 2026)
GPT-5.6 Sol vs Claude Mythos 5: Cybersecurity AI (June 2026)
Two announcements on June 26-27, 2026 reshaped the cybersecurity-AI landscape: OpenAI previewed GPT-5.6 Sol with major efficiency gains on security benchmarks, and the US government restored Anthropic Claude Mythos 5 to vetted US critical-infrastructure organizations. Both are gated behind government-controlled access lists. This comparison covers benchmarks, access, capabilities, and how to choose for real security workflows.
Last verified: June 27, 2026.
TL;DR
- Terminal-Bench 2.1: Sol Ultra 91.9%, base Sol 88.8%, Mythos 5 88.0%, GPT-5.5 83.4%
- ExploitBench: Sol matches Mythos Preview-class with ~1/3 the output tokens
- Pricing: Sol $5/$30 per 1M tokens (same as GPT-5.5); Mythos 5 pricing not publicly disclosed (estimated $15+/M input)
- Access: Both gated; Sol via US gov cleared partner list, Mythos 5 via US critical-infrastructure cleared list
- Lifted safeguards: Mythos 5 has tuned-for-defensive-cyber posture; Sol has strengthened layered safeguards
- Best for raw benchmarks and cost: Sol (once accessible)
- Best for lifted-safeguards defensive cyber work: Mythos 5 (if cleared)
- Available alternative for most teams: GPT-5.5-Cyber
The two models head-to-head
GPT-5.6 Sol (OpenAI)
- Released: June 26, 2026 (limited preview only)
- Pricing: $5.00 input / $30.00 output per 1M tokens
- Context: Large (GPT-5.6 family standard)
- Specialization: Long-horizon coding, agentic security workflows, frontier reasoning
- Safeguards: Strengthened layered safeguards for high-risk activities (cyber, bio)
- Coding benchmark (Terminal-Bench 2.1): 91.9% Sol Ultra, 88.8% base Sol
- Security benchmark (ExploitBench): Competitive with Mythos Preview at ~1/3 output tokens
- Access: US-government-cleared trusted partner list
Claude Mythos 5 (Anthropic)
- Released: Initial GA in earlier 2026; suspended June 12; partially restored June 27, 2026
- Pricing: Not publicly disclosed (estimated $15+ input / $75+ output per 1M tokens for cleared customers)
- Context: 1M tokens
- Specialization: Defensive cyber operations, vulnerability research, threat triage, IR workflows
- Safeguards: Lifted in cybersecurity contexts for vetted defensive use
- Coding benchmark (Terminal-Bench 2.1): 88.0%
- Security benchmark (ExploitBench): Prior frontier (until Sol’s June 26 release)
- Access: US critical-infrastructure cleared organization list (100+ orgs initial wave); EU access via Project Glasswing with ENISA
The benchmark story
OpenAI’s June 26 GPT-5.6 announcement included direct comparisons against Mythos 5 on two benchmarks:
Terminal-Bench 2.1 (agentic command-line coding — tests planning, iteration, and tool coordination):
| Model | Score |
|---|---|
| GPT-5.6 Sol Ultra | 91.9% |
| GPT-5.6 Sol (base) | 88.8% |
| Claude Mythos 5 | 88.0% |
| GPT-5.5 | 83.4% |
Sol Ultra is the new record. Base Sol edges Mythos 5 by 0.8 points. The gap from GPT-5.5 to base Sol (5.4 points) is larger than typical between flagship versions, suggesting Sol is a genuine capability jump, not just refinement.
ExploitBench (UC Berkeley / OpenAI / other labs joint benchmark for vulnerability research):
OpenAI’s headline result: Sol is competitive with the prior Mythos Preview frontier while generating ~1/3 the output tokens. Specific scores: Mythos 5 leads at ~80% but without comparable efficiency data; Sol matches that level with one-third the tokens consumed.
What the efficiency claim means in practice. Agentic security work (vulnerability hunting across a codebase, multi-step exploit reasoning, IR investigation chains) is dominated by output token cost. If Sol can produce the same conclusions with 1/3 the output tokens, the per-task cost drops by roughly 67%. For a security team running 10,000 agentic security tasks per month at $30 average Mythos 5 output cost per task ($300K/month), Sol at the same effectiveness could be ~$100K/month — $200K/month savings.
The lifted-safeguards story
Benchmarks aren’t the whole picture. Mythos 5 has a unique posture that even a higher-scoring Sol may not replicate:
Lifted safeguards means Mythos 5 is tuned to engage with exploit techniques, malware behavior analysis, offensive tactics reasoning, and red-team scenarios when the use case is defensive cyber operations. General-purpose models — including Sol, even with its strengthened safeguards — typically refuse or hedge on such requests because they can’t distinguish defender from attacker.
For a defender, this is the difference between:
- “Help me understand how this RCE works so I can detect attempts” — Mythos 5 engages fully, Sol may engage with caveats
- “Analyze this attack chain and predict the next stage” — Mythos 5 reasons forward, Sol may decline some inferences
- “Generate a detection rule for this evasion technique” — both can help, but Mythos 5 reasons about the technique itself more freely
The lifted-safeguards posture is exactly why Mythos 5 access is restricted to vetted organizations. The government’s concern is misuse; the same posture that makes the model uniquely useful for defenders also makes it uniquely capable in attacker hands.
The access story (June 27, 2026 reality)
Both models are gated. Here’s what each access regime looks like:
| Access dimension | GPT-5.6 Sol | Claude Mythos 5 |
|---|---|---|
| Cleared by | OpenAI + US government | US government → Anthropic |
| Selection criterion | ”Trusted partners” (not public) | US critical-infrastructure organizations (not fully public) |
| Estimated size | Likely 50-200 orgs initial | 100+ orgs initial wave |
| Public availability target | ”Coming weeks” (July 2026 likely) | No public-availability target announced |
| EU access | Not yet defined | Project Glasswing with ENISA (June 18, 2026) |
| Application process | Through OpenAI enterprise account team | Through Anthropic enterprise account team |
| Likely overlap with other list | High (similar org categories) | High (similar org categories) |
For most security teams, neither model is reachable as of June 27, 2026. The realistic fallback set:
- GPT-5.5-Cyber — OpenAI’s domain-tuned security model, generally available (announced June 22, 2026)
- Claude Sonnet 4.x / Opus 4.x — General-purpose Claude, generally available, weaker security-specific tuning
- Gemini 3.5 Pro — Long-context analysis, generally available
- Open-weight Llama 4 + security fine-tunes — self-hosted, no access restrictions
Decision framework
Use Mythos 5 if cleared, when:
- The task requires lifted safeguards (offensive technique reasoning, exploit analysis, red-team-aware defense)
- You need the 1M token context for full-attack-chain analysis
- Cost-per-task is acceptable for the quality and posture benefit
- You’re inside the cleared US critical-infrastructure organization list or Project Glasswing in EU
Use Sol if cleared, when:
- The task is agentic security work where output-token efficiency matters (vulnerability hunts, IR loops)
- Coding-style security tasks (Terminal-Bench-shaped workflows) where Sol’s benchmark lead matters
- You need to run many security tasks per day and per-task cost compression is the deciding factor
- You can work within Sol’s strengthened (but not lifted) safeguards
Use both, with routing:
- Mythos 5 for tasks requiring lifted safeguards
- Sol for high-volume agentic security work where efficiency wins
- GPT-5.5-Cyber as the always-available fallback
- Build behind a router so model selection is a config decision
Use neither (most teams today):
- Default to GPT-5.5-Cyber for security-tuned work
- Claude Sonnet for general security reasoning where lifted-safeguards isn’t required
- Gemini 3.5 Pro for long-context analysis
- Apply for cleared access through both Anthropic and OpenAI enterprise account teams if your organization is a candidate
What to watch over the next 30 days
- Sol public availability — likely July 2026 for ChatGPT and API
- Mythos 5 cleared-org list expansion — additional waves expected
- Fable 5 restoration — Anthropic’s Fable 5 remains restricted as of June 27; restoration timing impacts the general-purpose Mythos alternative
- GPT-5.5-Cyber enhancements — OpenAI is likely to add Sol-derived capabilities to GPT-5.5-Cyber as a generally-available equivalent
- EU access decisions — Project Glasswing expansion and any equivalent Sol EU access path