diff --git a/src/app_error.rs b/src/app_error.rs
index 15fa3f2..1cbb79d 100644
--- a/src/app_error.rs
+++ b/src/app_error.rs
@@ -57,6 +57,8 @@
 //! `kind`, `code` and optional `message` fields. Prefer calling it at the
 //! transport boundary (e.g. in `IntoResponse`) to avoid duplicate logs.
 
+use std::borrow::Cow;
+
 use thiserror::Error;
 use tracing::error;
 
@@ -73,7 +75,7 @@ pub struct AppError {
     /// Semantic category of the error.
     pub kind:    AppErrorKind,
     /// Optional, public-friendly message.
-    pub message: Option<String>
+    pub message: Option<Cow<'static, str>>
 }
 
 /// Conventional result alias for application code.
@@ -92,7 +94,7 @@ impl AppError {
     /// let err = AppError::new(AppErrorKind::BadRequest, "invalid payload");
     /// assert!(err.message.is_some());
     /// ```
-    pub fn new(kind: AppErrorKind, msg: impl Into<String>) -> Self {
+    pub fn new(kind: AppErrorKind, msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(kind, msg)
     }
 
@@ -100,7 +102,7 @@ impl AppError {
     ///
     /// Prefer named helpers (e.g. [`AppError::not_found`]) where it clarifies
     /// intent.
-    pub fn with(kind: AppErrorKind, msg: impl Into<String>) -> Self {
+    pub fn with(kind: AppErrorKind, msg: impl Into<Cow<'static, str>>) -> Self {
         Self {
             kind,
             message: Some(msg.into())
@@ -142,103 +144,103 @@ impl AppError {
 
     // 4xx-ish
     /// Build a `NotFound` error.
-    pub fn not_found(msg: impl Into<String>) -> Self {
+    pub fn not_found(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::NotFound, msg)
     }
     /// Build a `Validation` error.
-    pub fn validation(msg: impl Into<String>) -> Self {
+    pub fn validation(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Validation, msg)
     }
     /// Build an `Unauthorized` error.
-    pub fn unauthorized(msg: impl Into<String>) -> Self {
+    pub fn unauthorized(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Unauthorized, msg)
     }
     /// Build a `Forbidden` error.
-    pub fn forbidden(msg: impl Into<String>) -> Self {
+    pub fn forbidden(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Forbidden, msg)
     }
     /// Build a `Conflict` error.
-    pub fn conflict(msg: impl Into<String>) -> Self {
+    pub fn conflict(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Conflict, msg)
     }
     /// Build a `BadRequest` error.
-    pub fn bad_request(msg: impl Into<String>) -> Self {
+    pub fn bad_request(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::BadRequest, msg)
     }
     /// Build a `RateLimited` error.
-    pub fn rate_limited(msg: impl Into<String>) -> Self {
+    pub fn rate_limited(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::RateLimited, msg)
     }
     /// Build a `TelegramAuth` error.
-    pub fn telegram_auth(msg: impl Into<String>) -> Self {
+    pub fn telegram_auth(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::TelegramAuth, msg)
     }
 
     // 5xx-ish
     /// Build an `Internal` error.
-    pub fn internal(msg: impl Into<String>) -> Self {
+    pub fn internal(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Internal, msg)
     }
     /// Build a `Service` error (generic server-side service failure).
-    pub fn service(msg: impl Into<String>) -> Self {
+    pub fn service(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Service, msg)
     }
     /// Build a `Database` error with an optional message.
     ///
     /// Accepts `Option` to avoid gratuitous `.map(|...| ...)` at call sites
     /// when you may or may not have a safe-to-print string at hand.
-    pub fn database(msg: Option<impl Into<String>>) -> Self {
+    pub fn database(msg: Option<impl Into<Cow<'static, str>>>) -> Self {
         Self {
             kind:    AppErrorKind::Database,
-            message: msg.map(|m| m.into())
+            message: msg.map(Into::into)
         }
     }
     /// Build a `Config` error.
-    pub fn config(msg: impl Into<String>) -> Self {
+    pub fn config(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Config, msg)
     }
     /// Build a `Turnkey` error.
-    pub fn turnkey(msg: impl Into<String>) -> Self {
+    pub fn turnkey(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Turnkey, msg)
     }
 
     // Infra / network
     /// Build a `Timeout` error.
-    pub fn timeout(msg: impl Into<String>) -> Self {
+    pub fn timeout(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Timeout, msg)
     }
     /// Build a `Network` error.
-    pub fn network(msg: impl Into<String>) -> Self {
+    pub fn network(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Network, msg)
     }
     /// Build a `DependencyUnavailable` error.
-    pub fn dependency_unavailable(msg: impl Into<String>) -> Self {
+    pub fn dependency_unavailable(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::DependencyUnavailable, msg)
     }
     /// Backward-compatible alias; routes to `DependencyUnavailable`.
-    pub fn service_unavailable(msg: impl Into<String>) -> Self {
+    pub fn service_unavailable(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::DependencyUnavailable, msg)
     }
 
     // Serialization / external API / subsystems
     /// Build a `Serialization` error.
-    pub fn serialization(msg: impl Into<String>) -> Self {
+    pub fn serialization(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Serialization, msg)
     }
     /// Build a `Deserialization` error.
-    pub fn deserialization(msg: impl Into<String>) -> Self {
+    pub fn deserialization(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Deserialization, msg)
     }
     /// Build an `ExternalApi` error.
-    pub fn external_api(msg: impl Into<String>) -> Self {
+    pub fn external_api(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::ExternalApi, msg)
     }
     /// Build a `Queue` error.
-    pub fn queue(msg: impl Into<String>) -> Self {
+    pub fn queue(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Queue, msg)
     }
     /// Build a `Cache` error.
-    pub fn cache(msg: impl Into<String>) -> Self {
+    pub fn cache(msg: impl Into<Cow<'static, str>>) -> Self {
         Self::with(AppErrorKind::Cache, msg)
     }
 }
diff --git a/src/convert/reqwest.rs b/src/convert/reqwest.rs
index 4a632bc..19256e2 100644
--- a/src/convert/reqwest.rs
+++ b/src/convert/reqwest.rs
@@ -104,7 +104,10 @@ mod tests {
         let err_str = err.to_string();
         let app_err: AppError = err.into();
         let msg = app_err.message.expect("app error message");
-        assert!(msg.contains(&err_str), "{msg} does not contain {err_str}");
+        assert!(
+            msg.contains(err_str.as_str()),
+            "{msg} does not contain {err_str}"
+        );
 
         server.abort();
     }
diff --git a/src/response.rs b/src/response.rs
index 391ca3f..edd79fe 100644
--- a/src/response.rs
+++ b/src/response.rs
@@ -60,8 +60,8 @@
 //! provided as a deprecated shim.
 
 use std::{
-    fmt::{Display, Formatter, Result as FmtResult},
-    time::Duration
+    borrow::Cow,
+    fmt::{Display, Formatter, Result as FmtResult}
 };
 
 use http::StatusCode;
@@ -297,9 +297,9 @@ impl From<&AppError> for ErrorResponse {
 
         let message = err
             .message
-            .as_deref()
-            .unwrap_or("An error occurred")
-            .to_owned();
+            .clone()
+            .unwrap_or(Cow::Borrowed("An error occurred"))
+            .into_owned();
 
         Self {
             status,