Frontier Model Routing Strategy
July 27, 2026 · View on GitHub
Status: accepted and implemented Decision date: 2026-07-27
Decision
Claude Octopus uses Opus 5 as its premium lead model, GPT-5.6 Sol as the independent coding/review peer, and Sonnet 5 as the standard Claude seat. Fable 5 remains an explicit capability escalation rather than an automatic default.
Fresh configurations adopt the new roster. Existing environment pins, session
overrides, and providers.json settings retain precedence and are not silently
rewritten.
Roster
| Model | Default job | Standard price per MTok (input/output) |
|---|---|---|
| Claude Opus 5 | architecture, planning, security reasoning, final judgment | $5 / $25 |
| GPT-5.6 Sol | implementation, terminal work, independent code review | $5 / $30 |
| GPT-5.6 Terra | balanced Codex alternative | $2.50 / $15 |
| GPT-5.6 Luna | budget Codex alternative | $1 / $6 |
| Claude Sonnet 5 | standard Claude orchestration and synthesis | $3 / $15 |
| Claude Haiku 4.5 | budget Claude work | $1 / $5 |
| Claude Fable 5 | opt-in judgment-class escalation | $10 / $50 |
Opus 5 and Fable 5 are both Anthropic-family models. Agreement between them does not count as independent provider diversity.
Routing rules
- Start generic, mergeable work with one capable owner.
- Add another model only for a distinct job: research, implementation, adversarial review, security review, or final verification.
- Explicit debate, council, squeeze, and multi-model commands still fan out because disagreement is their intended output.
- Model choice never changes permissions, repository rules, or quality gates.
- User and project configuration always beats release defaults.
Role defaults:
architect,strategist,security-reviewer,implementer-heavy: current Opus, preferring Opus 5 on Claude Code v2.1.219+.implementer,code-reviewer: GPT-5.6 Sol.synthesizer: current Sonnet, preferring Sonnet 5 on Claude Code v2.1.197+.researcher: Antigravity, retaining an independent research role.
Fallbacks
- Opus: Opus 5 → Opus 4.8 → Opus 4.7 → Opus 4.6.
- Sonnet: Sonnet 5 → Sonnet 4.6.
- Fable refusal/security fallback: Opus 5, overridable with
OCTOPUS_FABLE5_FALLBACK_MODEL. - GPT-5.6 requires Codex CLI v0.144.0 or newer.
Prompt policy
For Opus 5 and Sonnet 5, prompts should state the goal, relevant context,
boundaries, reasons for unusual constraints, and checkable acceptance criteria.
Avoid duplicated reminders, all-caps emphasis without a real compliance need,
token countdowns, and requests to reveal hidden reasoning. The runtime policy
block is skills/blocks/frontier-model-routing.md; Fable-specific constraints
remain in skills/blocks/fable5-prompting.md.
Sources
- Fable optimizer v2.0.0: https://github.com/nyldn/fable5-optimizer/tree/v2.0.0
- Anthropic model selection: https://platform.claude.com/docs/en/about-claude/models/choosing-a-model
- Opus 5 prompting: https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-opus-5
- Claude Code v2.1.219: https://github.com/anthropics/claude-code/releases/tag/v2.1.219
- GPT-5.6 model guide: https://developers.openai.com/api/docs/guides/latest-model