diff --git a/sdk/src/__tests__/read-url.test.ts b/sdk/src/__tests__/read-url.test.ts
index 902d47c45a..5ae4c56299 100644
--- a/sdk/src/__tests__/read-url.test.ts
+++ b/sdk/src/__tests__/read-url.test.ts
@@ -369,4 +369,40 @@ describe('readUrl SSRF protection', () => {
       errorMessage: 'Invalid redirect location: http://',
     })
   })
+
+  it('refuses to fetch through a proxy instead of dropping DNS pinning', async () => {
+    const proxyVars = [
+      'HTTPS_PROXY',
+      'https_proxy',
+      'HTTP_PROXY',
+      'http_proxy',
+      'ALL_PROXY',
+      'all_proxy',
+    ] as const
+    const saved = new Map<string, string | undefined>(
+      proxyVars.map((name) => [name, process.env[name]]),
+    )
+    for (const name of proxyVars) delete process.env[name]
+    process.env.HTTPS_PROXY = 'http://proxy.example.com:8080'
+    try {
+      // No custom fetch: the default transport is under test. The hostname
+      // resolves public, so only the proxy refusal can stop the fetch.
+      const result = await readUrl({
+        url: 'https://example.com/page',
+        lookupHost: async () => ['93.184.216.34'],
+      })
+
+      expect(result[0].value).toEqual({
+        url: 'https://example.com/page',
+        errorMessage:
+          'read_url is unavailable while a proxy is configured (HTTP_PROXY, HTTPS_PROXY or ALL_PROXY is set): the proxy resolves hostnames itself, so the destination cannot be validated against the SSRF policy. Unset the proxy variables to use read_url.',
+      })
+    } finally {
+      for (const name of proxyVars) {
+        const value = saved.get(name)
+        if (value === undefined) delete process.env[name]
+        else process.env[name] = value
+      }
+    }
+  })
 })
diff --git a/sdk/src/tools/read-url.ts b/sdk/src/tools/read-url.ts
index cd09cd99b7..d34f6ab878 100644
--- a/sdk/src/tools/read-url.ts
+++ b/sdk/src/tools/read-url.ts
@@ -330,9 +330,12 @@ function truncateText(
  * The transport used when the caller supplies no `fetch`.
  *
  * Normally {@link pinnedFetch}: the connection goes only to addresses that
- * were validated in the same resolution. Under Bun with a proxy configured,
- * Bun's own `fetch` instead, because it honours the proxy variables and
- * `node:http` there does not; the proxy, not this process, resolves the name.
+ * were validated in the same resolution. Under Bun with a proxy configured
+ * that guarantee cannot be kept: the proxy, not this process, resolves the
+ * name, so the address {@link assertUrlAllowed} validated is not the
+ * address the connection would use. Rather than fetch unvalidated through
+ * `globalThis.fetch`, the transport refuses until the pinned transport can
+ * speak proxy. A clear error beats a silent downgrade of the SSRF check.
  */
 function defaultTransport(opts: {
   lookupHost?: HostLookup
@@ -340,7 +343,11 @@ function defaultTransport(opts: {
 }): FetchLike {
   const isBun = typeof process !== 'undefined' && !!process.versions?.bun
   if (isBun && isHttpProxyConfiguredFromEnv()) {
-    return globalThis.fetch
+    return async () => {
+      throw new Error(
+        'read_url is unavailable while a proxy is configured (HTTP_PROXY, HTTPS_PROXY or ALL_PROXY is set): the proxy resolves hostnames itself, so the destination cannot be validated against the SSRF policy. Unset the proxy variables to use read_url.',
+      )
+    }
   }
   return async (input, init) =>
     pinnedFetch(String(input), {