fix(world-vercel): skip Dispatcher.compose() when undici can't compose (Bun)

[SandboxRD]

Fixes #4555.

Problem

Under Bun, import { Agent } from 'undici' resolves to Bun's built-in undici module even when the undici package is installed. Its dispatcher classes are stubs: they have no compose() and no dispatch(). Two factories in @workflow/world-vercel call compose() and throw:

• createQueueDispatcher(): new Agent(...).compose is not a function. This is the one reported in world-vercel: createQueueDispatcher crashes under Bun (Agent.compose is not a function), breaking SvelteKit builds since 4.7.4 #4555.
• createEventsDispatcher(): agent.compose is not a function. It's on the default HTTP/2 path (WORKFLOW_H2_MULTIPLEX unset), so the events path breaks under Bun too.

Bun's fetch also ignores the dispatcher option. I checked by passing a dispatcher whose dispatch() throws, and fetch went straight to the server. So under Bun there's nothing for an interceptor to wrap.

Fix

Add supportsCompose(dispatcher), which checks whether compose is a function, and skip composing when it isn't:

• Queue path: the RetryAgent wraps the plain Agent.
• Events path: return the RetryAgent without the H2 multiplex interceptor. This is the same shape as the existing WORKFLOW_H2_MULTIPLEX=0 branch.

Node behavior doesn't change: there, compose exists and both paths compose exactly as before.

One known gap: under Bun, the queue path's 30s deadline (deadlineInterceptor) doesn't apply. That's already true today wherever it doesn't crash, because Bun's fetch never consults the dispatcher. I noted it in the supportsCompose doc comment.

Tests

• New src/http-client-bun.test.ts mocks undici with Bun-style stub classes (no compose, no dispatch) and builds the queue dispatcher and both events dispatcher variants.
  • Without the fix: 3 of 4 tests fail, with the exact errors from world-vercel: createQueueDispatcher crashes under Bun (Agent.compose is not a function), breaking SvelteKit builds since 4.7.4 #4555.
  • With the fix: all 4 pass.
• src/http-client.test.ts: 49/49 pass on Node 24.21, so the composed Node path is unchanged.
• Whole package on Node 24.21: 847/849. The two failures are in http-core.test.ts ({"Connection":"invalid"} request-validation cases). They fail identically on main without this change on my machine, so they look environmental.

Checked under real Bun

I ran the repro from #4555 against this branch's built dist on Bun 1.3.11:

bun 1.3.11 | typeof new Agent().compose: undefined
createQueueDispatcher: ok
createEventsDispatcher: ok
createStreamDispatcher: ok
getQueueDispatcher: ok
getEventsDispatcher: ok

Before the fix, createQueueDispatcher and createEventsDispatcher both threw.

A changeset is included (@workflow/world-vercel patch).

🤖 Generated with Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 642e639

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 17 packages

Name	Type
@workflow/world-vercel	Patch
@workflow/cli	Patch
@workflow/core	Patch
@workflow/web	Patch
workflow	Patch
@workflow/world-testing	Patch
@workflow/builders	Patch
@workflow/next	Patch
@workflow/nitro	Patch
@workflow/vitest	Patch
@workflow/web-shared	Patch
@workflow/astro	Patch
@workflow/nest	Patch
@workflow/rollup	Patch
@workflow/sveltekit	Patch
@workflow/vite	Patch
@workflow/nuxt	Patch

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[vercel]

@SandboxRD is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[VaguelySerious]

AI review: no blocking issues

[VaguelySerious]

One more thing, could you make this check pass? https://github.com/vercel/workflow/pull/4557/checks?check_run_id=110513913363

[VaguelySerious]

Or ask your agent to do a DCO sign-off on this PR

[VaguelySerious]

Just saw you did that as I was commenting, thanks

[github-actions]

Backport PR opened against stable: #4567. Merge conflicts were resolved by AI — please review carefully. (backport job run)