…s at the routing hop, pre-warmed hosts (#1494)

## The problem

A Slack message in prd took **~14s to get the 👀 reaction** and ~20s to
get a reply (example: `iterate` project, thread `ts-1781170058-112929`).
Hop-by-hop, from the message's Slack `ts`:

| Δ | what happened |
|---|---|
| +0.9s | Slack delivered the webhook — Slack was fast |
| **+6.5s** | nothing of ours executed anywhere: cold instantiation of
SlackIntegrationDO + the integration StreamDO (handler: 8.1s wall, **5ms
CPU**). Slack's 3s retry queued behind the same gate and doubled the
work |
| +2.1s | integration DO init + subscription + append + routing |
| **+3.0s** | cold instantiation of the new thread StreamDO |
| **+1.4s** | cold dial of the SLACK_AGENT host DO → input rendered →
eyes at ~14s |
| +6s | LLM leg (openai-ws connect 1.1s, gpt-5.5 ~2s, itx exec) → reply
at ~20s |

Two multiplying causes: **the deployed script was 89.1 MB** (50 MB
sourcemaps + browser-only modules uploaded as worker modules by
alchemy's noBundle glob over `dist/server`; the live server graph is ~34
MB, the entrypoint 1.75 MB) — and every cold DO isolate loads all of it
— times **3–4 distinct DOs chained serially** on the webhook path. The
warm path was always fine (webhook 1–6ms, appends 20–100ms): this is
cold-start tax, not stream-architecture tax.

## The fixes (no change to the streams/processors idea)

1. **`prune-server-bundle.ts`** (runs between build and asset
preupload): deletes every `dist/server` module unreachable from the
entrypoint via import/`new URL` literals (browser web workers + their
wasm that the SSR build emits), plus all sourcemaps **except the
entrypoint's own** (small; the one Cloudflare can symbolicate worker
stack traces with — chunk maps are browser code and pure ballast inside
a worker script). Validated against the extracted prd bundle: keeps
exactly the 186-module live graph, deletes the 3 browser-only modules +
chunk maps.
2. **Append-only webhook ack**: the handler no longer awaits
`SlackIntegrationDO.initialize()` before responding — only the durable
append gates the 200; initialize + catch-up moved to `waitUntil`.
Order-independent (existing integrations have their subscription on the
stream; new ones pick the webhook up via replay). Stops the >3s Slack
retry storm.
3. **👀 at the routing hop**: the slack router reports routed webhooks to
its host (`acknowledgeRoutedWebhook`) and SlackIntegrationDO adds the
reaction immediately — one hop from ingress instead of three cold DO
hops downstream — gated by the same payload-only rules the slack-agent
applies (no bot messages, no reaction events, no bot-user actions).
slack-agent still adds it on catch-up; `already_reacted` makes the pair
idempotent.
4. **Pre-warmed hosts** (`prewarmRoutedStreamHosts`): for a newly routed
thread, the SLACK_AGENT and AGENT host DOs `initialize()` concurrently
with the bootstrap append instead of serially after each dial.
Everything either side appends is idempotency-keyed and
order-independent (the anchor-skip recovery from #1481 covers trigger
ordering).

## Measured

Dev-stage deploys of this branch (`os-dev-jonas`):

- prd today: **89.1 MB**
- this branch pre-#1486 baseline: **34.1 MB**
- this branch on latest main (includes #1486's SSR-graph shrink, 186→178
live modules): **28.3 MB** — 3.1× smaller; app smoke-tested (sign-in
200)
- prune log on the real prd bundle: `kept 186 modules, deleted 3
unreachable modules + 180 sourcemaps (55.0 MB)`

Expected effect: each cold DO instantiation drops from multi-second to
sub-second, and the eyes ack stops depending on the deepest part of the
chain. Worth re-measuring the full message→eyes timing in prd after this
deploys.

## Trade-offs / notes

- Chunk-level deployed stack traces lose symbolication (entrypoint map
kept). Symbolicate locally against the build output if needed.
- The prune is conservative: anything referenced by a quoted relative
specifier (`from`, `import()`, `export from`, `new URL`) stays. The
unreachable set on the real bundle is exactly the browser-only web
workers + wasm.
- Follow-up idea (not this PR): split app-vs-platform workers so UI
deploys stop evicting agent/stream DOs (the 2026-06-10 deploy-race
incident), and consider per-DO-class workers for deploy isolation.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Changes production Slack webhook timing, adds best-effort Slack API
calls on the routing path, and alters deploy artifacts via bundle
pruning; behavior is designed to be idempotent but affects a critical
user-visible path.
> 
> **Overview**
> Cuts Slack cold-path latency by shrinking the deployed worker and
parallelizing work on the webhook path.
> 
> **Deploy:** Adds `prune-server-bundle` to the Alchemy build (after
Vite, before asset preupload). It strips unreachable `dist/server`
modules and most sourcemaps so each cold Durable Object isolate loads a
much smaller script.
> 
> **Webhook ingress:** The Slack webhook handler now returns `{ ok: true
}` after the durable stream append only;
`SlackIntegrationDO.initialize()` / `ensureReady()` run in `waitUntil`,
avoiding >3s acks and Slack retries.
> 
> **Routing hop:** `SlackProcessor` gains optional
`acknowledgeRoutedWebhook` and `prewarmRoutedStreamHosts`. The
integration DO adds the 👀 reaction at route time (via
`eyesReactionTargetFromWebhookPayload` + `reactions.add`) and
pre-initializes `SLACK_AGENT` and `AGENT` DOs in parallel with
new-thread bootstrap. Downstream slack-agent behavior stays idempotent
(`already_reacted`).
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
8cf05b1. Bugbot is set up for automated
code reviews on this repo. Configure
[here](https://www.cursor.com/dashboard/bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

<!-- CLOUDFLARE_PREVIEW -->
## Environment Config Lease
<!-- CLOUDFLARE_PREVIEW_STATE -->
<!--
{
  "apps": {
    "os": {
      "appDisplayName": "OS",
      "appSlug": "os",
      "status": "deployed",
      "updatedAt": "2026-06-11T11:02:23.553Z",
      "headSha": "8cf05b16d08e47333866be25d49508ddcf145a9b",
      "message": null,
      "publicUrl": "https://os.iterate-preview-3.com",
"runUrl": "https://github.com/iterate/iterate/actions/runs/27342005408",
      "shortSha": "8cf05b1"
    },
    "semaphore": {
      "appDisplayName": "Semaphore",
      "appSlug": "semaphore",
      "status": "deployed",
      "updatedAt": "2026-06-11T10:59:58.014Z",
      "headSha": "8cf05b16d08e47333866be25d49508ddcf145a9b",
      "message": null,
      "publicUrl": "https://semaphore.iterate-preview-3.com",
"runUrl": "https://github.com/iterate/iterate/actions/runs/27342005408",
      "shortSha": "8cf05b1"
    }
  },
  "environmentConfigLease": {
    "dopplerConfig": "preview_3",
    "leasedUntil": 1781179095710,
    "leaseId": "699c7e52-ad4d-4c0a-a337-a6b9397144b7",
    "slug": "preview-3",
    "type": "environment-config-lease"
  }
}
-->
<!-- /CLOUDFLARE_PREVIEW_STATE -->
Lease: `preview-3`
Doppler config: `preview_3`
Type: `environment-config-lease`
Leased until: 2026-06-11T11:58:15.710Z

### OS
Status: deployed
Commit: `8cf05b1`
Preview: https://os.iterate-preview-3.com
[Workflow
run](https://github.com/iterate/iterate/actions/runs/27342005408)
Updated: 2026-06-11T11:02:23.553Z

### Semaphore
Status: deployed
Commit: `8cf05b1`
Preview: https://semaphore.iterate-preview-3.com
[Workflow
run](https://github.com/iterate/iterate/actions/runs/27342005408)
Updated: 2026-06-11T10:59:58.014Z
<!-- /CLOUDFLARE_PREVIEW -->

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>