Cloudflare Mbt Worker Bundle

Cloudflare Workers + MoonBit bundle pipeline

The canonical way to ship a Cloudflare Worker where the bulk of the request handler is MoonBit code (compiled to JS via moon build --target js --release) but the entry shim is hand-written TypeScript.

When to invoke

Use when you're:

• Setting up a new MoonBit → Cloudflare Workers project.
• Migrating an existing project from a hand-written dist/worker.mjs shim to wrangler-native TS bundling.
• Diagnosing a Worker that hangs on the first await after startup (FFI rewrite missed; see references).

Key facts about wrangler + MoonBit

1. wrangler bundles TS natively. As of wrangler 4.x, main: "src/worker.ts" in wrangler.jsonc is enough — wrangler's built-in esbuild integration transpiles + bundles + emits a single JS file on every wrangler dev / wrangler deploy. No separate tsc emit step is needed for the worker.
2. MoonBit output is plain ESM JS. moon build --target js --release writes _build/js/release/build/<package>.js. wrangler can import it from a TS entry — no bridging needed except for the side-effect import (the moon module registers globals at module init).
3. Two MoonBit-specific source rewrites are mandatory. These are not optional and cannot move into TS:
   • moonbitlang$async$internal$event_loop$$reschedule() → the mangled _M0FP...event__loop10reschedule() name. The legacy hook only drains the deque once; the new name re-pumps via setTimeout(0). Without the rewrite, every await past startup hangs.
   • Module-scope random seed → a constant. Workers reject random in module init. Both must happen between moon build and wrangler deploy, applied to the moon JS output. See assets/scripts/prepare-worker.ts.template.

Pipeline shape

clean → db:verify → vite build (if frontend) → moon build → prepare-worker → wrangler deploy
                                                            ↓
                                                            (writes src/_generated/<pkg>-core.js
                                                             with FFI rewrites applied; wrangler's
                                                             esbuild picks it up as part of the src/ tree)

For a project without frontend / FFI rewrites (no MoonBit), the pipeline is even simpler: wrangler deploy against src/worker.ts is sufficient. The starter kit cloudflare-starterkit-mbt ships with a slim version of this.

What's in here

assets/templates/worker.ts.template

A minimal src/worker.ts entry that:

1. Side-effect imports the prepared moon core (registers globalThis.__appServerFetch).
2. Imports telemetry + utels wrappers from ./telemetry-runtime.ts.
3. Exports { fetch, scheduled } for wrangler.

Rename __appServerFetch to match your moon module's register_cloudflare_fetch call.

assets/scripts/prepare-worker.ts.template

The pre-bundle step. Reads moon output, applies the two FFI rewrites with requiredReplace (fails loudly if the target string isn't found — silent no-op was a real production hang), writes to src/_generated/<pkg>-core.js.

assets/scripts/check-worker-bundle.ts

Post-build sanity check on the final bundle. Catches:

• Stray \x1f control bytes (wasm-host text corruption, sqlc-gen-moonbit #17 family).
• Bundle too small (moon emitted a stub).
• Required markers missing (extend REQUIRED_MARKERS per project — e.g. globalThis.__appCronTick for scheduled handlers).

assets/templates/wrangler.jsonc.template

Skeleton with env.staging block, the name/main/compatibility_date shape, and inline comments on where to paste D1 IDs / R2 / Vectorize bindings.

Why this pipeline (over alternatives)

References

• references/wrangler-traps.md — deployments list ordering, the nonexistent --yes flag, etc.
• references/moonbit-ffi-rewrites.md — what each rewrite does, why, and how to recognize when one stops landing.

Source

The runtime reference is in mizchi/cloudflare-starterkit-mbt. A larger example with the FFI rewrites + editor-assets runtime is in mizchi/mnemo under mnemo-server/.