diff --git a/bin/lib/onboard.js b/bin/lib/onboard.js
index 5ab4e8629..5616e1507 100644
--- a/bin/lib/onboard.js
+++ b/bin/lib/onboard.js
@@ -2052,23 +2052,28 @@ async function recoverGatewayRuntime() {
 // ── Step 3: Sandbox ──────────────────────────────────────────────
 
 async function promptValidatedSandboxName() {
-  while (true) {
+  const MAX_ATTEMPTS = 3;
+  for (let attempt = 0; attempt < MAX_ATTEMPTS; attempt++) {
     const nameAnswer = await promptOrDefault(
-      "  Sandbox name (lowercase, numbers, hyphens) [my-assistant]: ",
+      "  Sandbox name (lowercase, starts with letter, hyphens ok) [my-assistant]: ",
       "NEMOCLAW_SANDBOX_NAME",
       "my-assistant",
     );
     const sandboxName = (nameAnswer || "my-assistant").trim().toLowerCase();
 
     // Validate: RFC 1123 subdomain — lowercase alphanumeric and hyphens,
-    // must start and end with alphanumeric (required by Kubernetes/OpenShell)
-    if (/^[a-z0-9]([a-z0-9-]*[a-z0-9])?$/.test(sandboxName)) {
+    // must start with a letter (not a digit) to satisfy Kubernetes naming.
+    if (/^[a-z]([a-z0-9-]*[a-z0-9])?$/.test(sandboxName)) {
       return sandboxName;
     }
 
     console.error(`  Invalid sandbox name: '${sandboxName}'`);
-    console.error("  Names must be lowercase, contain only letters, numbers, and hyphens,");
-    console.error("  and must start and end with a letter or number.");
+    if (/^[0-9]/.test(sandboxName)) {
+      console.error("  Names must start with a letter, not a digit.");
+    } else {
+      console.error("  Names must be lowercase, contain only letters, numbers, and hyphens,");
+      console.error("  must start with a letter, and end with a letter or number.");
+    }
 
     // Non-interactive runs cannot re-prompt — abort so the caller can fix the
     // NEMOCLAW_SANDBOX_NAME env var and retry.
@@ -2076,8 +2081,13 @@ async function promptValidatedSandboxName() {
       process.exit(1);
     }
 
-    console.error("  Please try again.\n");
+    if (attempt < MAX_ATTEMPTS - 1) {
+      console.error("  Please try again.\n");
+    }
   }
+
+  console.error("  Too many invalid attempts.");
+  process.exit(1);
 }
 
 // ── Step 5: Sandbox ──────────────────────────────────────────────
diff --git a/test/onboard.test.js b/test/onboard.test.js
index 267696119..b8bdbe2a5 100644
--- a/test/onboard.test.js
+++ b/test/onboard.test.js
@@ -507,6 +507,24 @@ describe("onboard helpers", () => {
     }
   });
 
+  it("rejects sandbox names starting with a digit", () => {
+    // The validation regex must require names to start with a letter,
+    // not a digit — Kubernetes rejects digit-prefixed names downstream.
+    const SANDBOX_NAME_REGEX = /^[a-z]([a-z0-9-]*[a-z0-9])?$/;
+
+    expect(SANDBOX_NAME_REGEX.test("my-assistant")).toBe(true);
+    expect(SANDBOX_NAME_REGEX.test("a")).toBe(true);
+    expect(SANDBOX_NAME_REGEX.test("agent-1")).toBe(true);
+    expect(SANDBOX_NAME_REGEX.test("test-sandbox-v2")).toBe(true);
+
+    expect(SANDBOX_NAME_REGEX.test("7racii")).toBe(false);
+    expect(SANDBOX_NAME_REGEX.test("1sandbox")).toBe(false);
+    expect(SANDBOX_NAME_REGEX.test("123")).toBe(false);
+    expect(SANDBOX_NAME_REGEX.test("-start-hyphen")).toBe(false);
+    expect(SANDBOX_NAME_REGEX.test("end-hyphen-")).toBe(false);
+    expect(SANDBOX_NAME_REGEX.test("")).toBe(false);
+  });
+
   it("passes credential names to openshell without embedding secret values in argv", () => {
     const repoRoot = path.join(import.meta.dirname, "..");
     const tmpDir = fs.mkdtempSync(path.join(os.tmpdir(), "nemoclaw-onboard-inference-"));
@@ -1817,9 +1835,12 @@ const { setupInference } = require(${onboardPath});
     );
     assert.ok(fnMatch, "promptValidatedSandboxName function not found");
     const fnBody = fnMatch[1];
-    // Verify the retry loop exists within this function
-    assert.match(fnBody, /while\s*\(true\)/);
+    // Verify the bounded retry loop exists within this function
+    assert.match(fnBody, /MAX_ATTEMPTS/);
+    assert.match(fnBody, /for\s*\(let attempt/);
     assert.match(fnBody, /Please try again/);
+    // Exits after too many invalid attempts
+    assert.match(fnBody, /Too many invalid attempts/);
     // Non-interactive still exits within this function
     assert.match(fnBody, /isNonInteractive\(\)/);
     assert.match(fnBody, /process\.exit\(1\)/);