[Feature]: Unified Harness Configs — sync skills/plugins/MCP/subagents across all harnesses

[NK-Works]

Before submitting

• I searched existing issues/discussions and did not find a duplicate (Per-project MCP #276/Per-project MCP #277 per-project MCP, OpenCode provider: surface MCP servers, LSP servers, and plugins #11398 read-only OpenCode inventory, [Feature]: [RFC] Extension System / Marketplace for custom integrations #6714 T3 extension marketplace cover adjacent but not this).
• This is a concrete management/sync problem, not a vague idea.

Area

Providers / Settings / Projects (Codex, Claude, Cursor, Grok, OpenCode, Antigravity + common-format harnesses)

Problem

Each harness keeps its own skills/plugins/MCP/apps/subagents in its own native format and location (~/.claude/, Codex config.toml, opencode.json, etc.). Using 2+ harnesses via T3 Code means duplicate setup, silent drift, and broken tools when switching harnesses. There is no single place to see what is installed where, push one change everywhere, copy Codex -> Claude, or keep a Claude-only extra from leaking into other harnesses.

Proposed solution

T3 Code owns a Harness Config Library as source of truth, with 4 sections: Skills / Plugins (with nested Apps/connectors) / MCP servers / Subagents. Behavior:

1. Pull/import existing configs from all 6 adapters (+ common-format harnesses) into the library, dedupe by name+hash.
2. Push to all or copy harness->harness (e.g. Codex plugin -> Claude), generating native files per adapter. Incompatible fields fail closed with a warning, never silently drop permissions/hooks/env.
3. Per-harness-only flag (e.g. Claude-only tool stays Claude-only). Precedence: per-project > per-user > global.
4. Subagents section disabled by default, opt-in Enable button; per-harness custom subagents always allowed.
5. Secrets shape-only: sync command/args/URL/names, never values; show missing-secret indicator per harness.
6. Thin installer as V1 marketplace: install-from-git-URL / local-path into library, then sync out. No hosted store/sandboxing in V1.
7. Safety contract: diff preview + auto-backup + one-click rollback on every sync; never silently overwrite externally edited harness files.

Lives in Settings -> Harness Configs (Library / Overrides / Sync log) + per-project overrides in Project Settings. Local/remote/tunnel. No new T3-only plugin spec in V1 — bridge native formats.

Why this matters

Removes the main blocker to using multiple harnesses/projects in parallel. One edit, consistent tools everywhere, with escape hatches for harness-specific needs.

Smallest useful scope

1. Import from all 6 into library.
2. Push library -> all 6 + copy harness->harness with diff/backup/rollback.
3. Per-harness-only flag works.
4. Missing-secret warnings + project > user > global precedence hold.

Alternatives considered

1. Read-only status view (OpenCode provider: surface MCP servers, LSP servers, and plugins #11398) — helps debug, doesn't fix drift.
2. Per-project MCP only (Per-project MCP #276/Per-project MCP #277) — solves one axis, not cross-harness sync.
3. Full extension marketplace with hosting/sandboxing ([Feature]: [RFC] Extension System / Marketplace for custom integrations #6714) — much larger, orthogonal; this bridges existing formats instead.

Risks

• Translation loss across formats -> mitigate with fail-closed + explicit warnings.
• Secret leakage -> mitigate with shape-only sync.
• Clobbering user files -> mitigate with backup/diff/rollback + confirm on external edits.
• Scope creep (hosting, ratings, execution sandbox) -> explicitly out for V1.

[juliusmarminge]

Note

Grok responding on behalf of Julius.

Thanks for the detailed write-up. This is already being tracked, so I'm closing it as a duplicate of #6987 (one central list of skills and plugins shared across all providers) to keep the discussion in one place.

Parts of this proposal are also covered by these open threads:

• #7334: T3 as the source of truth for skills and agents, routed to each provider.
• #15823: a skill manager that can view, add, edit, and link skills across harnesses.
• #14118: MCP servers set once per environment and shared by every provider.

If you want to add the import, sync, rollback, or secret handling details, please post them on #6987 or #14118.