fix: proxy input always fails due to an undici version mismatch

[githubdev58yashi]

Summary

Setting the proxy input always makes requests fail. It doesn't matter whether the proxy or the destination is actually reachable.

TypeError: fetch failed
Cause: InvalidArgumentError: invalid onRequestStart method

The cause is a version mismatch inside undici. src/proxies.js builds a ProxyAgent dispatcher from the npm undici package (^8.9.0), but drives it with globalThis.fetch. On Node 24, globalThis.fetch is backed by a different, older bundled undici (7.21.0). undici made a breaking change to its request handler interface between v7 and v8 (see the v8.0.0 release notes), so the mismatch throws before any network I/O even starts.

The fix is to use undici's own exported fetch instead of the global one. That keeps the dispatcher and the fetch implementation on the same undici version.

-import { ProxyAgent } from "undici";
+import { ProxyAgent, fetch as undiciFetch } from "undici";
-  return (url, init) => globalThis.fetch(url, { ...init, dispatcher });
+  return (url, init) => undiciFetch(url, { ...init, dispatcher });

This only affects requests made while the proxy input is set. The call path is src/client.js#L26 / src/webhook.js#L22 → proxies.js.

This mismatch was introduced in v4, when both SDKs switched from axios to the Fetch API and this proxy dispatcher started depending on undici (#619). It went unnoticed until a later undici v7 → v8 bump (#653) actually broke it.

Testing

Added a test to test/proxies.spec.js that actually calls the function fetch() returns, instead of only checking its type. That gap is why the bug went unnoticed — the existing tests never called the returned function. No real server is needed: the mismatch happens synchronously, before any network I/O, so pointing the proxy at a closed local port is enough to catch a regression.

Also verified by running the actual packaged action (dist/index.js, runs.using: node24) through act (Docker). Before the fix, the underlying cause is InvalidArgumentError UND_ERR_INVALID_ARG invalid onRequestStart method. After the fix, it's Error ECONNREFUSED — a normal network error, since nothing listens on the test proxy port.

Log excerpt (before / after)

Before:

❗  ::error::SlackError: A request error occurred: fetch failed
error.message: fetch failed
cause: InvalidArgumentError UND_ERR_INVALID_ARG invalid onRequestStart method

After:

❗  ::error::SlackError: A request error occurred: fetch failed
error.message: fetch failed
cause: Error ECONNREFUSED connect ECONNREFUSED 127.0.0.1:1

The action's own warning/error text looks almost the same before and after, since SlackError doesn't propagate .cause. The cause error type is the reliable way to tell them apart, and that's exactly what the new test checks.

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[changeset-bot]

🦋 Changeset detected

Latest commit: 83c2ce0

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

This PR includes changesets to release 1 package

Name	Type
@slack/slack-github-action	Patch

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.86%. Comparing base (faf8f8f) to head (83c2ce0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #675   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files           8        8           
  Lines         732      735    +3     
=======================================
+ Hits          731      734    +3     
  Misses          1        1           

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