feat: bump Opus experiments to Claude Opus 5

[Rodriguespn]

Swaps the two Opus 4.8 experiments to Opus 5 and updates the matching README reference, mirroring the earlier Sonnet 4.6 → Sonnet 5 bump.

According to Claude models overview this is a stable ID:

Every Claude model ID is a pinned snapshot. Models with a date in the ID (for example, 20250929) are fixed to that specific release. Starting with the Claude 4.6 generation, model IDs use a dateless format that is also a pinned snapshot, not an evergreen pointer

Pricing is unchanged from Opus 4.8 ($5/$25 per MTok), so this stays within the same GitHub Actions budget.

Claude Code compatibility

Checked the Claude Code changelog between our pinned CLI (2.1.191) and latest (2.1.220): no breaking changes to the --output-format stream-json transcript structure that packages/core/src/agents/claude-code/parser.ts depends on — only additive changes:

• mcp_server_errors on the init event in 2.1.219
• opt-in --forward-subagent-text in 2.1.212. feat(claude-code): capture subagent text and thinking in transcripts #137 adds support to capture subagents steps from the transcript

Claude Code added Opus 5 as its default Opus alias in 2.1.219, but the pinned 2.1.191 passes the explicit claude-opus-5 ID through to the API fine (same as Sonnet 5 did in #73, which predated CLI-side Sonnet 5 support), so no CLI bump is needed.

Opus 5's only API breaking change vs 4.8 (what's new: thinking: disabled with effort xhigh/max now 400s) doesn't affect these experiments — they run reasoningEffort: 'high' with default thinking.

Note: high is also Claude Code's default effort for Opus 5 (model config docs.

Testing

Sanity-checked claude-code-opus-5 against resolve-dataapi-001-empty-results: 7/7 checks passed (1 attempt, 230s, 43 tool calls, stoppedReason: stop, transcript parsed cleanly with skill-load detection working).

Closes AI-968

[linear-code]

AI-968 Replace Opus 4.8 with Opus 5 in evals

Update the evals to use Opus 5 instead of Opus 4.8 ahead of launch. The thread discussed either replacing 4.8 or running both side by side, and the decision was to replace 4.8 with 5 because of the GitHub Actions jobs budget, similar to the earlier Sonnet 4.6 → 5 update.

Review in Linear

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
evals	Ignored		Jul 28, 2026 3:20pm

[mattrossman]

Tested the updated experiments locally and they seemed to work fine!