This is genuinely great work, Parth. The multi-brain teamwork, Claude leading a team of Codex workers, is the most "AgentNet" idea in the whole product, and you built it cleanly (the engineBinary identity trick and the automatic depth guard are really nice). I fully want this to ship.

One idea on how to roll it out, building on what you have rather than changing it:

Ship Claudex as a discoverable easter egg first, with a clear test window, then promote it to a main feature.

Instead of adding a third engine tab right away, keep the surface as Claude / Codex, and make Claudex something you find: drag the Claude chip onto the Codex chip, a fusion effect plays, and that session becomes Claudex. It's eye-catching enough that people will notice it and talk about it, and because it only activates when both engines are signed in, it naturally scopes who's exercising it.

Two reasons I like this rollout:

1. A clearer test period. Landing it as an easter egg first gives us a defined window to watch the write-capable team on real sessions (parallel writes, the one-approval flow, rate-limit handling) before it becomes a headline feature everyone leans on. It's the same code you wrote, just introduced with room to observe it.

2. Easter egg to main feature is a stronger arc. Discoverable, then promoted, concentrates attention: people find it, share it, and when we flip it to first-class it already has a story and some buzz behind it. A third tab on day one spends all of that at once.

The reason underneath: we're a vibe-coding app, and Claude + Codex are just the starting pair. I want headroom to test more engines and combos, and introducing Claudex this way lets us trial the multi-engine direction with a real feedback loop first.

Everything in packages/core can stay exactly as you built it. The only surface change would be swapping the visible Claudex chip and onboarding card for the drag-to-fuse gesture on the two existing chips. Happy to take that part if it helps.

Follow-up: implementation spec for the "drag to fuse" easter egg

Sharing a concrete build plan so this is easy to pick up. It is purely additive UI on top of your PR. Nothing in packages/core changes, so the runtime, approval gate, depth guard, and disallowedTools all stay exactly as you built them.

Files touched (all in surfaces/webview, the shared React app)

• src/chat/Composer.tsx - the an-term-seg engine strip. Render only ["claude", "codex"]; add the pointer-drag gesture, the Codex drop target, the fusion animation, and a fused "Claudex" pill with an un-fuse affordance.
• src/onboarding/PickEngine.tsx - drop the Claudex card, back to Claude / Codex only.
• src/index.css - fusion keyframes and drag styles (reusing the --claudex accent token this PR already added).
• src/state/store.tsx - no reducer change needed. selectEngine("claudex") already maps to the claude binary and sets cli: "claudex", so we reuse it as-is. Only a tiny local component state for the in-progress drag.

Because the UI lives in the shared webview, this ships to VSCode, the Android app, and localhost from one change. surfaces/cli has no DOM, so Claudex stays reachable there only programmatically, which is fine for now.

Interaction spec

Use Pointer Events (pointerdown / pointermove / pointerup), not HTML5 drag-and-drop, because native DnD is unreliable in the Android WebView. Set touch-action: none on the Claude chip only while a drag is armed so it does not fight page swipe or the chip strip's own scroll.

• Desktop (VSCode / browser): press and drag the Claude chip.
• Mobile (Android): long-press (~250 ms) to pick up Claude, then drag. The threshold keeps a normal tap = select and a horizontal move = scroll.
• While dragging: a lightweight clone of the Claude chip follows the pointer. The Codex chip becomes a drop target and glows --claudex only when both engines are signed in (state.cliReport?.claude === "ok" && state.cliReport?.codex === "ok", the same condition you already compute as locked, inverted).
• Drop on Codex (hit-test the pointer against the Codex chip's bounding rect): play the fusion effect, then call selectEngine("claudex").
• Drop anywhere else / released early: the clone snaps back, no-op.
• Un-fuse: when state.cli === "claudex", the strip collapses to a single fused Claudex pill. Tap it (or long-press to split) to call selectEngine("claude") and return to the two-chip strip.

Discoverability (hidden, but eye-catching)

No label, tooltip, or onboarding copy advertises it. The single reveal is the Codex chip lighting up --claudex the instant you start dragging Claude, and only if both engines are ready. That is enough to be found by anyone who experiments, and invisible otherwise. If we want a fainter breadcrumb later, an occasional subtle magnet shimmer between the two chips is a one-line addition, but I would start with nothing.

The fusion effect

Short and physical, roughly 500 to 700 ms, pure CSS keyframes plus a transient fusing state, no library:

1. The two chips ease toward each other and merge into one pill.
2. A brief purple burst (radial --claudex glow / particle ring) fires at the merge point.
3. The node-graph glyph from PickEngine (the three-circle vector) pops in on the fused pill, then settles into the steady Claudex label.

Respect prefers-reduced-motion: fall back to a simple crossfade into the fused pill with no burst.

Gating and safety (reused, not rebuilt)

The both-engines requirement is enforced physically: with only one engine signed in, the Codex target never glows and the drop is a no-op, so you literally cannot fuse without both. Everything downstream (the write-capable coder gate, the one plain-language approval, rate-limit handling) is unchanged from this PR.

Acceptance criteria

• Desktop: drag Claude onto Codex fuses; dropping elsewhere is a no-op.
• Mobile: long-press drag fuses and does not break chip scroll or page swipe.
• Only one engine signed in: no glow, no fuse.
• prefers-reduced-motion: crossfade path, no burst.
• Un-fuse returns to Claude / Codex.
• No visible "Claudex" anywhere else (no third chip, no PickEngine card).
• A fused session is stored and badged as claudex exactly as it is today.

Why this is low risk

One component's worth of gesture logic plus CSS, no new dependencies, no core changes, and fully reversible: promoting Claudex to a main feature later is just re-adding the visible chip. The easter egg and the clear test window both come for free from the gate that is already in your PR.

Happy to take this part on top of your branch if that helps.