diff --git a/cli/src/commands/hatch.ts b/cli/src/commands/hatch.ts
index 7caaa5f571e..3c201f357c0 100644
--- a/cli/src/commands/hatch.ts
+++ b/cli/src/commands/hatch.ts
@@ -538,6 +538,16 @@ async function hatchLocal(species: Species, name: string | null, daemonOnly: boo
     }
   }
 
+  const baseDataDir = join(process.env.BASE_DATA_DIR?.trim() || (process.env.HOME ?? userInfo().homedir), ".vellum");
+
+  if (existsSync(baseDataDir)) {
+    throw new Error(
+      `Base data directory already exists: ${baseDataDir}\n` +
+        "  Another assistant may already be using this directory.\n" +
+        "  To use a different directory, set the BASE_DATA_DIR environment variable.",
+    );
+  }
+
   console.log(`🥚 Hatching local assistant: ${instanceName}`);
   console.log(`   Species: ${species}`);
   console.log("");
@@ -555,8 +565,6 @@ async function hatchLocal(species: Species, name: string | null, daemonOnly: boo
     throw error;
   }
 
-  const baseDataDir = join(process.env.BASE_DATA_DIR?.trim() || (process.env.HOME ?? userInfo().homedir), ".vellum");
-
   // Read the bearer token written by the daemon so the client can authenticate
   // with the gateway (which requires auth by default).
   let bearerToken: string | undefined;