diff --git a/backend/src/billing.rs b/backend/src/billing.rs
index b9013ee..92d2c77 100644
--- a/backend/src/billing.rs
+++ b/backend/src/billing.rs
@@ -732,3 +732,112 @@ impl Billing {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use sqlx::SqlitePool;
+    use sqlx::sqlite::{SqliteConnectOptions, SqlitePoolOptions};
+    use std::str::FromStr;
+
+    use crate::models::Activity;
+
+    async fn test_pool() -> SqlitePool {
+        let connect_options = SqliteConnectOptions::from_str("sqlite::memory:")
+            .expect("valid sqlite memory url")
+            .create_if_missing(true);
+
+        let pool = SqlitePoolOptions::new()
+            .max_connections(1)
+            .connect_with(connect_options)
+            .await
+            .expect("connect sqlite memory db");
+
+        sqlx::migrate!("./migrations")
+            .run(&pool)
+            .await
+            .expect("run migrations");
+
+        pool
+    }
+
+    fn test_billing(pool: SqlitePool, stripe_secret_key: &str) -> Billing {
+        Billing {
+            nwc_url: String::new(),
+            stripe_secret_key: stripe_secret_key.to_string(),
+            stripe_webhook_secret: String::new(),
+            http: reqwest::Client::new(),
+            query: Query::new(pool.clone()),
+            command: Command::new(pool),
+            robot: Robot::test_stub(),
+        }
+    }
+
+    #[tokio::test]
+    async fn stripe_create_customer_requires_secret_key() {
+        let pool = test_pool().await;
+        let billing = test_billing(pool, "");
+
+        let err = billing
+            .stripe_create_customer("tenant_pubkey")
+            .await
+            .expect_err("missing key should fail before HTTP call");
+
+        assert!(
+            err.to_string().contains("missing STRIPE_SECRET_KEY"),
+            "unexpected error: {err}"
+        );
+    }
+
+    #[tokio::test]
+    async fn sync_relay_subscription_fails_for_empty_tenant_customer_id() {
+        let pool = test_pool().await;
+
+        sqlx::query(
+            "INSERT INTO tenant (pubkey, nwc_url, created_at, stripe_customer_id)
+             VALUES (?, ?, ?, ?)",
+        )
+        .bind("tenant_pubkey")
+        .bind("")
+        .bind(0_i64)
+        .bind("")
+        .execute(&pool)
+        .await
+        .expect("insert tenant");
+
+        sqlx::query(
+            "INSERT INTO relay (id, tenant, schema, subdomain, plan, status)
+             VALUES (?, ?, ?, ?, ?, ?)",
+        )
+        .bind("relay_1")
+        .bind("tenant_pubkey")
+        .bind("relay_1")
+        .bind("relay-1")
+        .bind("basic")
+        .bind("active")
+        .execute(&pool)
+        .await
+        .expect("insert relay");
+
+        let billing = test_billing(pool, "sk_test_dummy");
+        let activity = Activity {
+            id: "activity_1".to_string(),
+            tenant: "tenant_pubkey".to_string(),
+            created_at: 0,
+            activity_type: "create_relay".to_string(),
+            resource_type: "relay".to_string(),
+            resource_id: "relay_1".to_string(),
+        };
+
+        let err = billing
+            .sync_relay_subscription(&activity)
+            .await
+            .expect_err("empty tenant customer id should fail clearly");
+
+        assert!(
+            err.to_string()
+                .contains("tenant tenant_pubkey has no stripe_customer_id"),
+            "unexpected error: {err}"
+        );
+    }
+}
diff --git a/backend/src/command.rs b/backend/src/command.rs
index ce19174..c5df629 100644
--- a/backend/src/command.rs
+++ b/backend/src/command.rs
@@ -324,3 +324,56 @@ impl Command {
         Ok(())
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use sqlx::SqlitePool;
+    use sqlx::sqlite::{SqliteConnectOptions, SqlitePoolOptions};
+    use std::str::FromStr;
+
+    async fn test_pool() -> SqlitePool {
+        let connect_options = SqliteConnectOptions::from_str("sqlite::memory:")
+            .expect("valid sqlite memory url")
+            .create_if_missing(true);
+
+        let pool = SqlitePoolOptions::new()
+            .max_connections(1)
+            .connect_with(connect_options)
+            .await
+            .expect("connect sqlite memory db");
+
+        sqlx::migrate!("./migrations")
+            .run(&pool)
+            .await
+            .expect("run migrations");
+
+        pool
+    }
+
+    #[tokio::test]
+    async fn create_tenant_rejects_empty_stripe_customer_id() {
+        let pool = test_pool().await;
+        let command = Command::new(pool);
+
+        let tenant = Tenant {
+            pubkey: "tenant_pubkey".to_string(),
+            nwc_url: String::new(),
+            nwc_error: None,
+            created_at: 0,
+            stripe_customer_id: "   ".to_string(),
+            stripe_subscription_id: None,
+            past_due_at: None,
+        };
+
+        let err = command
+            .create_tenant(&tenant)
+            .await
+            .expect_err("empty customer id must be rejected");
+
+        assert!(
+            err.to_string().contains("stripe_customer_id is required"),
+            "unexpected error: {err}"
+        );
+    }
+}
diff --git a/backend/src/robot.rs b/backend/src/robot.rs
index e002a31..e8195db 100644
--- a/backend/src/robot.rs
+++ b/backend/src/robot.rs
@@ -254,3 +254,23 @@ async fn set_cached(
         },
     );
 }
+
+#[cfg(test)]
+impl Robot {
+    pub fn test_stub() -> Self {
+        let keys = Keys::generate();
+        let client = Client::new(keys);
+
+        Self {
+            secret: String::new(),
+            name: String::new(),
+            description: String::new(),
+            picture: String::new(),
+            outbox_client: client.clone(),
+            indexer_client: client.clone(),
+            messaging_client: client,
+            outbox_cache: std::sync::Arc::new(Mutex::new(HashMap::new())),
+            dm_cache: std::sync::Arc::new(Mutex::new(HashMap::new())),
+        }
+    }
+}