diff --git a/axumtest/src/auth/mod.rs b/axumtest/src/auth/mod.rs
index 879aaa9..d80f2cc 100644
--- a/axumtest/src/auth/mod.rs
+++ b/axumtest/src/auth/mod.rs
@@ -4,7 +4,11 @@ use axum::headers::HeaderName;
 use axum::http::Request;
 use axum::http::StatusCode;
 use axum::middleware::Next;
+use axum::response::IntoResponse;
 use axum::response::Response;
+use axum::Json;
+
+use crate::entities::ErrorResponse;
 
 static CIUSR: HeaderName = HeaderName::from_static("x-ciusr");
 static CIPWD: HeaderName = HeaderName::from_static("x-cipwd");
@@ -16,7 +20,7 @@ pub async fn ci_auth<B>(
     expected_usr: String,
     expected_pwd: String,
     expected_role: String,
-) -> Result<Response, StatusCode> {
+) -> Result<Response, (StatusCode, impl IntoResponse)> {
     let usr = req
         .headers()
         .get(&CIUSR)
@@ -38,6 +42,9 @@ pub async fn ci_auth<B>(
         {
             Ok(next.run(req).await)
         }
-        (_, _, _) => Err(StatusCode::UNAUTHORIZED),
+        (_, _, _) => Err((
+            StatusCode::UNAUTHORIZED,
+            Json(ErrorResponse::new("Invalid credentials")),
+        )),
     }
 }
diff --git a/axumtest/src/entities/mod.rs b/axumtest/src/entities/mod.rs
new file mode 100644
index 0000000..eedce79
--- /dev/null
+++ b/axumtest/src/entities/mod.rs
@@ -0,0 +1,48 @@
+//! Entities used in the system.
+
+use clap::Parser;
+use mongodb::Database;
+
+/// State shared between routes.
+pub struct State {
+    pub db: Database,
+}
+
+/// Command line options.
+#[derive(Parser)]
+pub struct Params {
+    /// Address to listen for requests.
+    #[clap(env = "SRV_ADDR", default_value = "0.0.0.0:3000")]
+    pub addr: String,
+
+    /// URI to connect to MongoDB.
+    #[clap(short, long, env = "MONGO_URI")]
+    pub mongo_addr: String,
+
+    /// Validation for the X-CIUSR header.
+    #[clap(long, env = "CIUSR")]
+    pub ci_usr: String,
+
+    /// Validation for the X-CIPWD header.
+    #[clap(long, env = "CIPWD")]
+    pub ci_pwd: String,
+
+    /// Validation for the X-CIROLE header.
+    #[clap(long, env = "CIROLE")]
+    pub ci_role: String,
+}
+
+#[derive(serde::Serialize)]
+pub struct ErrorResponse {
+    status: String,
+    reason: String,
+}
+
+impl ErrorResponse {
+    pub fn new(reason: &str) -> Self {
+        Self {
+            status: "ERR".into(),
+            reason: reason.into(),
+        }
+    }
+}
diff --git a/axumtest/src/main.rs b/axumtest/src/main.rs
index 79a802f..c5fb3cf 100644
--- a/axumtest/src/main.rs
+++ b/axumtest/src/main.rs
@@ -1,6 +1,7 @@
 mod auth;
 mod cases;
 mod collections;
+mod entities;
 
 use std::sync::Arc;
 
@@ -10,29 +11,9 @@ use clap::Parser;
 use dotenv::dotenv;
 use mongodb::options::ClientOptions;
 use mongodb::Client;
-use mongodb::Database;
 
-#[derive(Parser)]
-struct Params {
-    #[clap(env = "SRV_ADDR", default_value = "0.0.0.0:3000")]
-    addr: String,
-
-    #[clap(short, long, env = "MONGO_URI")]
-    mongo_addr: String,
-
-    #[clap(long, env = "CIUSR")]
-    ci_usr: String,
-
-    #[clap(long, env = "CIPWD")]
-    ci_pwd: String,
-
-    #[clap(long, env = "CIROLE")]
-    ci_role: String,
-}
-
-pub struct State {
-    db: Database,
-}
+use crate::entities::Params;
+use crate::entities::State;
 
 #[tokio::main]
 async fn main() {