fix(skill): customize-opencode off by default; promote browser-execute guide to a registered skill

[Alezander9]

Summary

Eval-agent's deep-dive on v0.1.6 regression traces (gemini-3-flash-preview pooled n=296, -7.5pp vs v0.1.2 baseline) surfaced two skill-registration problems on browser sessions:

1. customize-opencode is force-registered in every session. The skill ships from upstream (opencode-meta → customize-opencode, #26617 / #26899) and teaches the agent how to author opencode.json, opencode plugins, and opencode agent files. For BrowserCode browser-driving workflows it is pure system-prompt pollution: ~480 chars the model has to evaluate-and-discard every turn, with a negatively-correlated name (opencode ≠ what the binary is called now).
2. The actually-useful browser-execute-guide.md is not a registered skill. It sits at <dataDir>/skills/browser-execute-guide.md as a plain markdown file. The only reason agents find it is the browser_execute tool description's hard requirement: "you MUST use the Read tool first to read …/browser-execute-guide.md". Eval data shows 77% of tasks do eventually read it — but it has no presence in <available_skills> at planning time, so the agent only discovers it after committing to browser_execute.

Changes

Gate customize-opencode registration on BCODE_ENABLE_CUSTOMIZE_OPENCODE=1 (default off). The built-in registration in packages/opencode/src/skill/index.ts is now wrapped in an env check. When unset / falsy, the skill is not registered at all — it doesn't appear in <available_skills> and the skill tool can't load it. The gate runs before disk discovery, so a user who places their own customize-opencode/SKILL.md on disk still gets it. Opt-in via BCODE_ENABLE_CUSTOMIZE_OPENCODE=1 for sessions that are genuinely editing bcode.json or agent configs.

Promote browser-execute-guide.md to a real registered skill.

• File renamed: packages/bcode-browser/skills/browser-execute-guide.md → packages/bcode-browser/skills/browser-execute/SKILL.md (matches opencode's **/SKILL.md discovery glob).
• Frontmatter added: name: browser-execute, description front-loads "Use ONLY when calling the browser_execute tool or driving a real browser via the Chrome DevTools Protocol. Required reading before the first browser_execute call in a session."
• discoverSkills extended to scan the bcode-shipped skills materialization dir (<dataDir>/skills/) so the skill auto-registers without users needing a bcode.json entry.
• browser-execute.txt tool description updated: "you MUST use the skill tool first to load the browser-execute skill" — keeps the strong MUST wording verbatim (the eval-agent confirmed it materially improves scores).

Why this should help eval scores

• Planning-time visibility: agents see browser-execute in <available_skills> before they ever pick the tool, instead of discovering the guide reactively after the first browser_execute call.
• Cleaner system prompt on browser sessions: one relevant skill (browser-execute) instead of one irrelevant one (customize-opencode).
• Same strong MUST read first prompting on browser_execute, now routed through the canonical skill-loading path.

Diff size

8 files, +46 / -17. Yellow-zone modifications in packages/opencode/src/skill/index.ts (gate + new scan, ~20 lines net) and packages/opencode/src/tool/browser-execute.txt (1 line). Everything else is in packages/bcode-browser/ (Green zone). Logged in maintainer-side EXCEPTIONS.md.

Test plan

• bun typecheck from packages/opencode/ ✓
• bun typecheck from packages/bcode-browser/ ✓
• bun typecheck from repo root (filtered) ✓
• bun test in packages/bcode-browser/ ✓ (13 pass, 8 skip — the 8 skips are smoke tests gated on BCODE_SMOKE_CHROME=1)
• bun test test/skill in packages/opencode/: 14 pass, 5 fail — the 5 failures are pre-existing on main (verified by stashing my changes); unrelated to this PR.

Suggested next eval: re-run glm-5.1 and gemini-3-flash-preview baselines on a build of this branch and compare to v0.1.6.

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Summary by cubic

Removed the built-in customize-opencode skill and promoted the browser execution guide to a registered browser-execute skill. This cleans up prompt noise and makes the guide visible at planning time; browser_execute now instructs loading the skill first.

• New Features

  • Promoted the guide to a real skill: skills/browser-execute/SKILL.md with frontmatter (name: browser-execute). It auto-registers, appears in available skills, and is loaded via the skill tool. Tool text now says “MUST use the skill tool to load browser-execute.”
  • Skill discovery scans <dataDir>/skills/ so first‑party skills shipped by @browser-use/bcode-browser are picked up automatically.

• Bug Fixes

  • Removed forced registration of customize-opencode (no env gate; it no longer ships). A user skill with the same name on disk still loads via normal discovery.

^{Written for commit a6ac76a. Summary will update on new commits. Review in cubic}

[cubic-dev-ai]

No issues found across 8 files

_{Re-trigger cubic}