diff --git a/src/lib/debug.ts b/src/lib/debug.ts
index 19bb7d64e3e..aa898cf443f 100644
--- a/src/lib/debug.ts
+++ b/src/lib/debug.ts
@@ -7,6 +7,7 @@ import { platform, tmpdir } from "node:os";
 import { basename, dirname, join } from "node:path";
 
 import { DASHBOARD_PORT } from "./ports";
+import { listSandboxes } from "./registry";
 
 // ---------------------------------------------------------------------------
 // Types
@@ -141,6 +142,22 @@ function collectShell(collectDir: string, label: string, shellCmd: string): void
 // ---------------------------------------------------------------------------
 
 function detectSandboxName(): string {
+  // First, check the local registry for the default sandbox. This is
+  // the authoritative source — it reflects the user's actual onboard
+  // choices and survives gateway restarts. Falling back to "default"
+  // without checking the registry was the bug in #1728: debug always
+  // targeted a sandbox named "default" even though the user's sandbox
+  // was named something else (e.g. "my-assistant").
+  try {
+    const registry = listSandboxes();
+    if (registry.defaultSandbox) return registry.defaultSandbox;
+    const names = registry.sandboxes.map((s) => s.name).filter(Boolean);
+    if (names.length > 0) return names[0];
+  } catch {
+    /* registry unreadable — fall through to openshell probe */
+  }
+
+  // Fallback: ask the live gateway directly
   if (!commandExists("openshell")) return "default";
   try {
     const output = execFileSync("openshell", ["sandbox", "list"], {