diff --git a/src/server.ts b/src/server.ts
index 5a884c20b..375b791ad 100644
--- a/src/server.ts
+++ b/src/server.ts
@@ -2864,6 +2864,20 @@ export function buildFetchCode(url: string, outputPath: string): string {
       ? `var classifyIp = ${classifyIpInner};`
       : `var ${classifyIpFnName} = ${classifyIpInner};\nvar classifyIp = ${classifyIpFnName};`;
   const strictMode = process.env.CTX_FETCH_STRICT === "1";
+  // Default strips proxy env so the connect-time rebinding guard runs (#476/#1039).
+  // Opt-in is exact string "1" only (any other value keeps the strip path).
+  const allowProxy = process.env.CTX_FETCH_ALLOW_PROXY === "1";
+  const proxyEnvBlock = allowProxy
+    ? `// Proxy env vars preserved under CTX_FETCH_ALLOW_PROXY=1 (issue #1039).`
+    : `// Strip proxy env by default so DNS rebinding guard sees connect-time IPs (#1039).
+delete process.env.HTTP_PROXY;
+delete process.env.HTTPS_PROXY;
+delete process.env.ALL_PROXY;
+delete process.env.http_proxy;
+delete process.env.https_proxy;
+delete process.env.all_proxy;
+delete process.env.npm_config_proxy;
+delete process.env.npm_config_https_proxy;`;
   return `
 const TurndownService = require(${turndownPath});
 const { gfm } = require(${gfmPath});
@@ -2873,19 +2887,7 @@ const dnsPromises = require('no' + 'de:dns/promises');
 const url = ${JSON.stringify(url)};
 const outputPath = ${escapedOutputPath};
 
-// Strip proxy env vars from this subprocess only. A configured outbound
-// proxy (HTTP_PROXY / HTTPS_PROXY / ALL_PROXY) would route fetch through
-// an arbitrary target — DNS resolution happens at the proxy and the
-// in-subprocess DNS rebinding guard never sees the rebound IP. The
-// sandbox fetch path has no legitimate need for an upstream proxy.
-delete process.env.HTTP_PROXY;
-delete process.env.HTTPS_PROXY;
-delete process.env.ALL_PROXY;
-delete process.env.http_proxy;
-delete process.env.https_proxy;
-delete process.env.all_proxy;
-delete process.env.npm_config_proxy;
-delete process.env.npm_config_https_proxy;
+${proxyEnvBlock}
 
 ${classifyIpSrc}
 
diff --git a/tests/core/server.test.ts b/tests/core/server.test.ts
index 661053b72..9fb83e34a 100644
--- a/tests/core/server.test.ts
+++ b/tests/core/server.test.ts
@@ -3830,17 +3830,69 @@ import { buildFetchCode } from "../../src/server.js";
 describe("buildFetchCode — embedded SSRF guard contract", () => {
   const generated = buildFetchCode("https://example.com/x", "/tmp/x");
 
-  test("strips proxy env vars (HTTP_PROXY / HTTPS_PROXY / ALL_PROXY)", () => {
-    // A configured outbound proxy would route fetch through an arbitrary
-    // target; DNS resolution would happen at the proxy and the in-subprocess
-    // DNS guard would never see the rebound IP. The generated subprocess
-    // source must delete every proxy env var before any fetch can run.
-    expect(generated).toMatch(/delete process\.env\.HTTP_PROXY/);
-    expect(generated).toMatch(/delete process\.env\.HTTPS_PROXY/);
-    expect(generated).toMatch(/delete process\.env\.ALL_PROXY/);
-    expect(generated).toMatch(/delete process\.env\.http_proxy/);
-    expect(generated).toMatch(/delete process\.env\.https_proxy/);
-    expect(generated).toMatch(/delete process\.env\.all_proxy/);
+  test("strips proxy env vars (HTTP_PROXY / HTTPS_PROXY / ALL_PROXY) by default (#476 pinning)", () => {
+    // Default must delete proxy env vars so the in-subprocess DNS guard runs (#476).
+    const prev = process.env.CTX_FETCH_ALLOW_PROXY;
+    delete process.env.CTX_FETCH_ALLOW_PROXY;
+    try {
+      const src = buildFetchCode("https://example.com/x", "/tmp/x");
+      expect(src).toMatch(/delete process\.env\.HTTP_PROXY/);
+      expect(src).toMatch(/delete process\.env\.HTTPS_PROXY/);
+      expect(src).toMatch(/delete process\.env\.ALL_PROXY/);
+      expect(src).toMatch(/delete process\.env\.http_proxy/);
+      expect(src).toMatch(/delete process\.env\.https_proxy/);
+      expect(src).toMatch(/delete process\.env\.all_proxy/);
+      expect(src).toMatch(/delete process\.env\.npm_config_proxy/);
+      expect(src).toMatch(/delete process\.env\.npm_config_https_proxy/);
+    } finally {
+      if (prev === undefined) delete process.env.CTX_FETCH_ALLOW_PROXY;
+      else process.env.CTX_FETCH_ALLOW_PROXY = prev;
+    }
+  });
+
+  test("preserves proxy env vars when CTX_FETCH_ALLOW_PROXY=1 (#1039 opt-in)", () => {
+    // Exact "1" opt-in preserves proxy env for corporate egress; parent ssrfGuard remains.
+    const prev = process.env.CTX_FETCH_ALLOW_PROXY;
+    process.env.CTX_FETCH_ALLOW_PROXY = "1";
+    try {
+      const src = buildFetchCode("https://example.com/x", "/tmp/x");
+      expect(src).not.toMatch(/delete process\.env\.HTTP_PROXY/);
+      expect(src).not.toMatch(/delete process\.env\.HTTPS_PROXY/);
+      expect(src).not.toMatch(/delete process\.env\.ALL_PROXY/);
+      expect(src).not.toMatch(/delete process\.env\.http_proxy/);
+      expect(src).not.toMatch(/delete process\.env\.https_proxy/);
+      expect(src).not.toMatch(/delete process\.env\.all_proxy/);
+      expect(src).not.toMatch(/delete process\.env\.npm_config_proxy/);
+      expect(src).not.toMatch(/delete process\.env\.npm_config_https_proxy/);
+      expect(src).not.toMatch(/delete process\.env\[[^\]]*PROXY/i);
+      expect(src).not.toMatch(
+        /process\.env\.(HTTP|HTTPS|ALL)_?PROXY\s*=\s*(undefined|null|['"]{2})/i,
+      );
+      expect(src).toMatch(/CTX_FETCH_ALLOW_PROXY=1/);
+    } finally {
+      if (prev === undefined) delete process.env.CTX_FETCH_ALLOW_PROXY;
+      else process.env.CTX_FETCH_ALLOW_PROXY = prev;
+    }
+  });
+
+  test('rejects non-"1" truthy CTX_FETCH_ALLOW_PROXY (fail-secure; still strips)', () => {
+    // Non-"1" values (e.g. "true") must still strip — fail-secure vs accidental truthy env.
+    const prev = process.env.CTX_FETCH_ALLOW_PROXY;
+    process.env.CTX_FETCH_ALLOW_PROXY = "true";
+    try {
+      const src = buildFetchCode("https://example.com/x", "/tmp/x");
+      expect(src).toMatch(/delete process\.env\.HTTP_PROXY/);
+      expect(src).toMatch(/delete process\.env\.HTTPS_PROXY/);
+      expect(src).toMatch(/delete process\.env\.ALL_PROXY/);
+      expect(src).toMatch(/delete process\.env\.http_proxy/);
+      expect(src).toMatch(/delete process\.env\.https_proxy/);
+      expect(src).toMatch(/delete process\.env\.all_proxy/);
+      expect(src).toMatch(/delete process\.env\.npm_config_proxy/);
+      expect(src).toMatch(/delete process\.env\.npm_config_https_proxy/);
+    } finally {
+      if (prev === undefined) delete process.env.CTX_FETCH_ALLOW_PROXY;
+      else process.env.CTX_FETCH_ALLOW_PROXY = prev;
+    }
   });
 
   test("embedded SSRF classifier is callable as `classifyIp` even when bundler renames the export (#bug-v1.0.133)", () => {