diff --git a/.github/workflows/__global-proxy.yml b/.github/workflows/__global-proxy.yml
index e3ba6ff101..45df36f268 100644
--- a/.github/workflows/__global-proxy.yml
+++ b/.github/workflows/__global-proxy.yml
@@ -55,17 +55,45 @@ jobs:
           version: ${{ matrix.version }}
           use-all-platform-bundle: 'false'
           setup-kotlin: 'false'
+      - name: Block direct internet access to force proxy usage
+        run: |
+          apt-get update -qq && apt-get install -y -qq iptables >/dev/null 2>&1
+          PROXY_IP=$(getent hosts squid-proxy | awk '{ print $1 }')
+          echo "Squid proxy IP: $PROXY_IP"
+          # Allow all traffic to the proxy container
+          iptables -A OUTPUT -d "$PROXY_IP" -j ACCEPT
+          # Allow DNS resolution
+          iptables -A OUTPUT -p udp --dport 53 -j ACCEPT
+          iptables -A OUTPUT -p tcp --dport 53 -j ACCEPT
+          # Allow loopback
+          iptables -A OUTPUT -o lo -j ACCEPT
+          # Allow already-established connections (from checkout/prepare-test)
+          iptables -A OUTPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
+          # Block all other outbound HTTP and HTTPS, ensuring direct access fails
+          iptables -A OUTPUT -p tcp --dport 80 -j REJECT --reject-with tcp-reset
+          iptables -A OUTPUT -p tcp --dport 443 -j REJECT --reject-with tcp-reset
+          echo "Direct HTTP/HTTPS access is now blocked - all traffic must go through the proxy"
+
+      - name: Set proxy environment variables
+        shell: bash
+        run: |
+          echo "http_proxy=http://squid-proxy:3128" >> $GITHUB_ENV
+          echo "HTTP_PROXY=http://squid-proxy:3128" >> $GITHUB_ENV
+          echo "https_proxy=http://squid-proxy:3128" >> $GITHUB_ENV
+          echo "HTTPS_PROXY=http://squid-proxy:3128" >> $GITHUB_ENV
+
       - uses: ./../action/init
         with:
           languages: javascript
           tools: ${{ steps.prepare-test.outputs.tools-url }}
+
       - uses: ./../action/analyze
     env:
-      https_proxy: http://squid-proxy:3128
       CODEQL_ACTION_TOLERATE_MISSING_GIT_VERSION: true
       CODEQL_ACTION_TEST_MODE: true
     container:
       image: ubuntu:22.04
+      options: --cap-add=NET_ADMIN
     services:
       squid-proxy:
         image: ubuntu/squid:latest
diff --git a/lib/entry-points.js b/lib/entry-points.js
index 0183005a70..b91e8c1e06 100644
--- a/lib/entry-points.js
+++ b/lib/entry-points.js
@@ -145788,7 +145788,11 @@ function getRegistryProxy(action) {
   return void 0;
 }
 function makeProxyRequestOptions(dispatcher) {
+  if (dispatcher === void 0) {
+    return githubUtils.defaults.request;
+  }
   return {
+    ...githubUtils.defaults.request,
     fetch: (req, init2) => {
       return (0, import_undici.fetch)(req, { ...init2, dispatcher });
     }
@@ -145797,7 +145801,7 @@ function makeProxyRequestOptions(dispatcher) {
 function createApiClientWithDetails(apiDetails, { allowExternal = false, proxy = void 0 } = {}) {
   const auth2 = allowExternal && apiDetails.externalRepoAuth || apiDetails.auth;
   const retryingOctokit = githubUtils.GitHub.plugin(retry);
-  const requestOptions = proxy === void 0 ? githubUtils.defaults.request : makeProxyRequestOptions(proxy);
+  const requestOptions = makeProxyRequestOptions(proxy);
   return new retryingOctokit(
     githubUtils.getOctokitOptions(auth2, {
       baseUrl: apiDetails.apiURL,
diff --git a/pr-checks/checks/global-proxy.yml b/pr-checks/checks/global-proxy.yml
index 5f90022c04..9d9653c13c 100644
--- a/pr-checks/checks/global-proxy.yml
+++ b/pr-checks/checks/global-proxy.yml
@@ -5,17 +5,45 @@ versions:
   - nightly-latest
 container:
   image: ubuntu:22.04
+  options: --cap-add=NET_ADMIN
 services:
   squid-proxy:
     image: ubuntu/squid:latest
     ports:
       - 3128:3128
 env:
-  https_proxy: http://squid-proxy:3128
   CODEQL_ACTION_TOLERATE_MISSING_GIT_VERSION: true
 steps:
+  - name: Block direct internet access to force proxy usage
+    run: |
+      apt-get update -qq && apt-get install -y -qq iptables >/dev/null 2>&1
+      PROXY_IP=$(getent hosts squid-proxy | awk '{ print $1 }')
+      echo "Squid proxy IP: $PROXY_IP"
+      # Allow all traffic to the proxy container
+      iptables -A OUTPUT -d "$PROXY_IP" -j ACCEPT
+      # Allow DNS resolution
+      iptables -A OUTPUT -p udp --dport 53 -j ACCEPT
+      iptables -A OUTPUT -p tcp --dport 53 -j ACCEPT
+      # Allow loopback
+      iptables -A OUTPUT -o lo -j ACCEPT
+      # Allow already-established connections (from checkout/prepare-test)
+      iptables -A OUTPUT -m conntrack --ctstate ESTABLISHED,RELATED -j ACCEPT
+      # Block all other outbound HTTP and HTTPS, ensuring direct access fails
+      iptables -A OUTPUT -p tcp --dport 80 -j REJECT --reject-with tcp-reset
+      iptables -A OUTPUT -p tcp --dport 443 -j REJECT --reject-with tcp-reset
+      echo "Direct HTTP/HTTPS access is now blocked - all traffic must go through the proxy"
+
+  - name: Set proxy environment variables
+    shell: bash
+    run: |
+      echo "http_proxy=http://squid-proxy:3128" >> $GITHUB_ENV
+      echo "HTTP_PROXY=http://squid-proxy:3128" >> $GITHUB_ENV
+      echo "https_proxy=http://squid-proxy:3128" >> $GITHUB_ENV
+      echo "HTTPS_PROXY=http://squid-proxy:3128" >> $GITHUB_ENV
+
   - uses: ./../action/init
     with:
       languages: javascript
       tools: ${{ steps.prepare-test.outputs.tools-url }}
+
   - uses: ./../action/analyze
diff --git a/src/api-client.test.ts b/src/api-client.test.ts
index e43f16ef29..ae8c6269b1 100644
--- a/src/api-client.test.ts
+++ b/src/api-client.test.ts
@@ -2,6 +2,7 @@ import * as github from "@actions/github";
 import * as githubUtils from "@actions/github/lib/utils";
 import test from "ava";
 import * as sinon from "sinon";
+import { ProxyAgent } from "undici";
 
 import * as actionsUtil from "./actions-util";
 import * as api from "./api-client";
@@ -251,3 +252,23 @@ test("getRegistryProxyConfig - gets the configuration from the env vars", async
     )
     .passes(t.like, { host, port, ca });
 });
+
+test("makeProxyRequestOptions - returns defaults without custom proxy", async (t) => {
+  t.deepEqual(
+    api.makeProxyRequestOptions(undefined),
+    githubUtils.defaults.request,
+  );
+});
+
+test("makeProxyRequestOptions - returns fetch with custom proxy", async (t) => {
+  const opts = api.makeProxyRequestOptions(
+    new ProxyAgent("http://localhost:1080"),
+  );
+  // Fetch should be different from the defaults.
+  t.notDeepEqual(opts?.fetch, githubUtils.defaults.request?.fetch);
+  // The options should be the same aside from that.
+  t.deepEqual(
+    { ...opts, fetch: githubUtils.defaults.request?.fetch },
+    githubUtils.defaults.request,
+  );
+});
diff --git a/src/api-client.ts b/src/api-client.ts
index 7c63b5a6fe..ba800a2587 100644
--- a/src/api-client.ts
+++ b/src/api-client.ts
@@ -108,12 +108,19 @@ export function getRegistryProxy(
  * Constructs a `RequestRequestOptions` with a custom `fetch` implementation
  * that uses `dispatcher` as a proxy for requests.
  *
- * @param dispatcher The proxy to use.
+ * @param dispatcher The proxy to use, if any.
  */
 export function makeProxyRequestOptions(
-  dispatcher: ProxyAgent,
-): RequestRequestOptions {
+  dispatcher: ProxyAgent | undefined,
+): RequestRequestOptions | undefined {
+  // If we don't have a custom `ProxyAgent`, return the defaults.
+  if (dispatcher === undefined) {
+    return githubUtils.defaults.request;
+  }
+
+  // Otherwise, construct the custom `fetch` and add it onto the defaults.
   return {
+    ...githubUtils.defaults.request,
     fetch: (req: RequestInfo, init?: RequestInit) => {
       return undiciFetch(req, { ...init, dispatcher });
     },
@@ -136,10 +143,7 @@ function createApiClientWithDetails(
   const auth =
     (allowExternal && apiDetails.externalRepoAuth) || apiDetails.auth;
   const retryingOctokit = githubUtils.GitHub.plugin(retry.retry);
-  const requestOptions =
-    proxy === undefined
-      ? githubUtils.defaults.request
-      : makeProxyRequestOptions(proxy);
+  const requestOptions = makeProxyRequestOptions(proxy);
   return new retryingOctokit(
     githubUtils.getOctokitOptions(auth, {
       baseUrl: apiDetails.apiURL,