diff --git a/src/lib/onboard.ts b/src/lib/onboard.ts
index ad724b274d8..fe4511fb65d 100644
--- a/src/lib/onboard.ts
+++ b/src/lib/onboard.ts
@@ -1270,6 +1270,26 @@ async function configureWebSearch(
 
 // getSandboxInferenceConfig — moved to onboard-providers.ts
 
+// Shared validators for NEMOCLAW_PROXY_HOST / NEMOCLAW_PROXY_PORT.
+// Both `patchStagedDockerfile()` (build-time Dockerfile ARG override) and
+// `createSandbox()` (runtime sandbox env whitelist) must reject the same
+// inputs, otherwise the build and runtime paths can diverge — e.g. a
+// build-time-accepted value silently no-ops at runtime, leaving the
+// container running with the default proxy. Hostname regex deliberately
+// excludes `:` so raw IPv6 literals are rejected: the runtime
+// `http://${HOST}:${PORT}` template does not bracket them and would
+// produce a malformed URL. Port is range-checked because a 5-digit
+// length filter alone would accept out-of-range values like 70000.
+const PROXY_HOST_RE = /^[A-Za-z0-9._-]+$/;
+function isValidProxyHost(value: string): boolean {
+  return PROXY_HOST_RE.test(value);
+}
+function isValidProxyPort(value: string): boolean {
+  if (!/^[0-9]{1,5}$/.test(value)) return false;
+  const port = Number(value);
+  return port >= 1 && port <= 65535;
+}
+
 function patchStagedDockerfile(
   dockerfilePath: string,
   model: string,
@@ -1381,17 +1401,15 @@ function patchStagedDockerfile(
   // shell so the sandbox-side nemoclaw-start.sh sees them via $ENV at runtime.
   // Without this, the host export is silently dropped at image build time and
   // the sandbox falls back to the default 10.200.0.1:3128 proxy. See #1409.
-  const PROXY_HOST_RE = /^[A-Za-z0-9._:-]+$/;
-  const PROXY_PORT_RE = /^[0-9]{1,5}$/;
   const proxyHostEnv = process.env.NEMOCLAW_PROXY_HOST;
-  if (proxyHostEnv && PROXY_HOST_RE.test(proxyHostEnv)) {
+  if (proxyHostEnv && isValidProxyHost(proxyHostEnv)) {
     dockerfile = dockerfile.replace(
       /^ARG NEMOCLAW_PROXY_HOST=.*$/m,
       `ARG NEMOCLAW_PROXY_HOST=${proxyHostEnv}`,
     );
   }
   const proxyPortEnv = process.env.NEMOCLAW_PROXY_PORT;
-  if (proxyPortEnv && PROXY_PORT_RE.test(proxyPortEnv)) {
+  if (proxyPortEnv && isValidProxyPort(proxyPortEnv)) {
     dockerfile = dockerfile.replace(
       /^ARG NEMOCLAW_PROXY_PORT=.*$/m,
       `ARG NEMOCLAW_PROXY_PORT=${proxyPortEnv}`,
@@ -3588,6 +3606,24 @@ async function createSandbox(
   // 18789 and the gateway listens on the wrong port. (#2267, #1925)
   const effectiveDashboardPort = getDashboardForwardPort(chatUiUrl);
   envArgs.push(formatEnvAssignment("NEMOCLAW_DASHBOARD_PORT", effectiveDashboardPort));
+  // Propagate NEMOCLAW_PROXY_HOST / NEMOCLAW_PROXY_PORT to the runtime
+  // sandbox container. patchStagedDockerfile() already substitutes them
+  // into the build-time Dockerfile ARG/ENV, but `openshell sandbox create
+  // -- env … nemoclaw-start` only forwards the explicitly listed env vars
+  // — image-baked ENV does not propagate into the running pod. Without
+  // this, nemoclaw-start.sh:898 falls back to the default 10.200.0.1:3128
+  // and `HTTPS_PROXY` inside the sandbox ignores the host override. The
+  // build-time substitution and runtime env stay in sync as a result.
+  // Fixes #2424. Uses the shared isValidProxyHost / isValidProxyPort
+  // helpers so build-time and runtime validation stay aligned.
+  const sandboxProxyHost = process.env.NEMOCLAW_PROXY_HOST;
+  if (sandboxProxyHost && isValidProxyHost(sandboxProxyHost)) {
+    envArgs.push(formatEnvAssignment("NEMOCLAW_PROXY_HOST", sandboxProxyHost));
+  }
+  const sandboxProxyPort = process.env.NEMOCLAW_PROXY_PORT;
+  if (sandboxProxyPort && isValidProxyPort(sandboxProxyPort)) {
+    envArgs.push(formatEnvAssignment("NEMOCLAW_PROXY_PORT", sandboxProxyPort));
+  }
   if (webSearchConfig?.fetchEnabled) {
     const braveKey =
       getCredential(webSearch.BRAVE_API_KEY_ENV) || process.env[webSearch.BRAVE_API_KEY_ENV];