diff --git a/lib/entry-points.js b/lib/entry-points.js
index 5a8e49bc9..73417126b 100644
--- a/lib/entry-points.js
+++ b/lib/entry-points.js
@@ -163637,8 +163637,7 @@ function getCredentials(logger, registrySecrets, registriesCredentials, language
   }
   return out;
 }
-function getProxyPackage() {
-  const platform2 = process.platform === "win32" ? "win64" : process.platform === "darwin" ? "osx64" : "linux64";
+function getProxyPackage(platform2) {
   return `${UPDATEJOB_PROXY}-${platform2}.tar.gz`;
 }
 function getFallbackUrl(proxyPackage) {
@@ -163656,7 +163655,14 @@ async function getCliVersionFromFeatures(features) {
   return await features.getEnabledDefaultCliVersions(gitHubVersion.type);
 }
 async function getDownloadUrl(logger, features) {
-  const proxyPackage = getProxyPackage();
+  let platform2 = getBundlePlatform();
+  if (platform2 === void 0) {
+    logger.warning(
+      `Unsupported platform ${process.platform} on architecture ${process.arch}, defaulting to ${"linux64" /* Linux64 */}`
+    );
+    platform2 = "linux64" /* Linux64 */;
+  }
+  const proxyPackage = getProxyPackage(platform2);
   try {
     const useFeaturesToDetermineCLI = await features.getValue(
       "start_proxy_use_features_release" /* StartProxyUseFeaturesRelease */
diff --git a/src/start-proxy.test.ts b/src/start-proxy.test.ts
index ee953798b..6e22aa85b 100644
--- a/src/start-proxy.test.ts
+++ b/src/start-proxy.test.ts
@@ -12,6 +12,7 @@ import { UnvalidatedObject, validateSchema } from "./json";
 import { makeFromSchema } from "./json/testing-util";
 import { BuiltInLanguage } from "./languages";
 import { getRunnerLogger, Logger } from "./logging";
+import { BundlePlatform, getBundlePlatform } from "./platform";
 import * as startProxyExports from "./start-proxy";
 import * as statusReport from "./status-report";
 import {
@@ -721,6 +722,10 @@ function mockOfflineFeatures(tempDir: string, logger: Logger) {
   return setUpFeatureFlagTests(tempDir, logger, gitHubVersion);
 }
 
+/** The result of `getBundlePlatform` or `Linux64` if `undefined`. */
+const testPlatform: BundlePlatform =
+  getBundlePlatform() ?? BundlePlatform.Linux64;
+
 test.serial(
   "getDownloadUrl returns fallback when `getReleaseByVersion` rejects",
   async (t) => {
@@ -737,7 +742,9 @@ test.serial(
       t.is(info.version, startProxyExports.UPDATEJOB_PROXY_VERSION);
       t.is(
         info.url,
-        startProxyExports.getFallbackUrl(startProxyExports.getProxyPackage()),
+        startProxyExports.getFallbackUrl(
+          startProxyExports.getProxyPackage(testPlatform),
+        ),
       );
     });
   },
@@ -762,7 +769,9 @@ test.serial(
         t.is(info.version, startProxyExports.UPDATEJOB_PROXY_VERSION);
         t.is(
           info.url,
-          startProxyExports.getFallbackUrl(startProxyExports.getProxyPackage()),
+          startProxyExports.getFallbackUrl(
+            startProxyExports.getProxyPackage(testPlatform),
+          ),
         );
 
         stub.restore();
@@ -775,7 +784,10 @@ test.serial("getDownloadUrl returns matching release asset", async (t) => {
   const logger = new RecordingLogger();
   const assets = [
     { name: "foo", url: "other-url" },
-    { name: startProxyExports.getProxyPackage(), url: "url-we-want" },
+    {
+      name: startProxyExports.getProxyPackage(testPlatform),
+      url: "url-we-want",
+    },
   ];
   mockGetReleaseByTag(assets);
 
@@ -935,7 +947,10 @@ test.serial(
     const logger = new RecordingLogger();
     const downloadUrl = "url-we-want";
     mockGetReleaseByTag([
-      { name: startProxyExports.getProxyPackage(), url: downloadUrl },
+      {
+        name: startProxyExports.getProxyPackage(testPlatform),
+        url: downloadUrl,
+      },
     ]);
 
     const toolcachePath = "/path/to/proxy/dir";
@@ -976,7 +991,7 @@ test.serial(
     );
 
     checkExpectedLogMessages(t, logger.messages, [
-      `Found '${startProxyExports.getProxyPackage()}' in release '${defaults.bundleVersion}' at '${downloadUrl}'`,
+      `Found '${startProxyExports.getProxyPackage(testPlatform)}' in release '${defaults.bundleVersion}' at '${downloadUrl}'`,
     ]);
   },
 );
@@ -994,7 +1009,7 @@ test.serial(
     const downloadUrl = "url-we-want";
     const assets = [
       {
-        name: startProxyExports.getProxyPackage(),
+        name: startProxyExports.getProxyPackage(testPlatform),
         url: downloadUrl,
       },
     ];
@@ -1067,7 +1082,7 @@ test.serial(
     });
 
     checkExpectedLogMessages(t, logger.messages, [
-      `Found '${startProxyExports.getProxyPackage()}' in release '${expectedTag}' at '${downloadUrl}'`,
+      `Found '${startProxyExports.getProxyPackage(testPlatform)}' in release '${expectedTag}' at '${downloadUrl}'`,
     ]);
   },
 );
diff --git a/src/start-proxy.ts b/src/start-proxy.ts
index caa1b3054..b01f35222 100644
--- a/src/start-proxy.ts
+++ b/src/start-proxy.ts
@@ -3,6 +3,7 @@ import * as path from "path";
 import * as core from "@actions/core";
 import * as toolcache from "@actions/tool-cache";
 
+import { ActionState } from "./action-common";
 import {
   getApiClient,
   getApiDetails,
@@ -20,6 +21,7 @@ import {
 import * as json from "./json";
 import { BuiltInLanguage } from "./languages";
 import { Logger } from "./logging";
+import { BundlePlatform, getBundlePlatform } from "./platform";
 import {
   Address,
   Registry,
@@ -370,14 +372,10 @@ export function getCredentials(
 
 /**
  * Gets the name of the proxy release asset for the current platform.
+ *
+ * @param platform The platform to get the asset name for.
  */
-export function getProxyPackage(): string {
-  const platform =
-    process.platform === "win32"
-      ? "win64"
-      : process.platform === "darwin"
-        ? "osx64"
-        : "linux64";
+export function getProxyPackage(platform: BundlePlatform): string {
   return `${UPDATEJOB_PROXY}-${platform}.tar.gz`;
 }
 
@@ -425,7 +423,17 @@ export async function getDownloadUrl(
   logger: Logger,
   features: FeatureEnablement,
 ): Promise<{ url: string; version: string }> {
-  const proxyPackage = getProxyPackage();
+  // Default to linux64 if we don't recognise the platform+arch pair.
+  // This maintains the behaviour we had before switching to `getBundlePlatform` here.
+  let platform = getBundlePlatform();
+  if (platform === undefined) {
+    logger.warning(
+      `Unsupported platform ${process.platform} on architecture ${process.arch}, defaulting to ${BundlePlatform.Linux64}`,
+    );
+    platform = BundlePlatform.Linux64;
+  }
+
+  const proxyPackage = getProxyPackage(platform);
 
   try {
     const useFeaturesToDetermineCLI = await features.getValue(