diff --git a/library/src/compute/data.rs b/library/src/compute/data.rs
index 8dba62f54..fc81435ca 100644
--- a/library/src/compute/data.rs
+++ b/library/src/compute/data.rs
@@ -113,11 +113,11 @@ impl Default for Delimiter {
 }
 
 /// Format the user-facing CSV error message.
-fn format_csv_error(error: csv::Error, line: usize) -> String {
+fn format_csv_error(error: csv::Error, line: usize) -> EcoString {
     match error.kind() {
         csv::ErrorKind::Utf8 { .. } => "file is not valid utf-8".into(),
         csv::ErrorKind::UnequalLengths { expected_len, len, .. } => {
-            format!(
+            eco_format!(
                 "failed to parse csv file: found {len} instead of {expected_len} fields in line {line}"
             )
         }
@@ -202,9 +202,9 @@ fn convert_json(value: serde_json::Value) -> Value {
 
 /// Format the user-facing JSON error message.
 #[track_caller]
-fn format_json_error(error: serde_json::Error) -> String {
+fn format_json_error(error: serde_json::Error) -> EcoString {
     assert!(error.is_syntax() || error.is_eof());
-    format!("failed to parse json file: syntax error in line {}", error.line())
+    eco_format!("failed to parse json file: syntax error in line {}", error.line())
 }
 
 /// Read structured data from a YAML file.
@@ -293,8 +293,8 @@ fn convert_yaml_key(key: serde_yaml::Value) -> Option<Str> {
 
 /// Format the user-facing YAML error message.
 #[track_caller]
-fn format_yaml_error(error: serde_yaml::Error) -> String {
-    format!("failed to parse yaml file: {}", error.to_string().trim())
+fn format_yaml_error(error: serde_yaml::Error) -> EcoString {
+    eco_format!("failed to parse yaml file: {}", error.to_string().trim())
 }
 
 /// Read structured data from an XML file.
@@ -388,6 +388,6 @@ fn convert_xml(node: roxmltree::Node) -> Value {
 }
 
 /// Format the user-facing XML error message.
-fn format_xml_error(error: roxmltree::Error) -> String {
+fn format_xml_error(error: roxmltree::Error) -> EcoString {
     format_xml_like_error("xml file", error)
 }
diff --git a/src/diag.rs b/src/diag.rs
index c1a7c8346..88141062a 100644
--- a/src/diag.rs
+++ b/src/diag.rs
@@ -249,31 +249,31 @@ impl From<FileError> for EcoString {
 }
 
 /// Format a user-facing error message for an XML-like file format.
-pub fn format_xml_like_error(format: &str, error: roxmltree::Error) -> String {
+pub fn format_xml_like_error(format: &str, error: roxmltree::Error) -> EcoString {
     match error {
         roxmltree::Error::UnexpectedCloseTag { expected, actual, pos } => {
-            format!(
+            eco_format!(
                 "failed to parse {format}: found closing tag '{actual}' \
                  instead of '{expected}' in line {}",
                 pos.row
             )
         }
         roxmltree::Error::UnknownEntityReference(entity, pos) => {
-            format!(
+            eco_format!(
                 "failed to parse {format}: unknown entity '{entity}' in line {}",
                 pos.row
             )
         }
         roxmltree::Error::DuplicatedAttribute(attr, pos) => {
-            format!(
+            eco_format!(
                 "failed to parse {format}: duplicate attribute '{attr}' in line {}",
                 pos.row
             )
         }
         roxmltree::Error::NoRootNode => {
-            format!("failed to parse {format}: missing root node")
+            eco_format!("failed to parse {format}: missing root node")
         }
         roxmltree::Error::SizeLimit => "file is too large".into(),
-        _ => format!("failed to parse {format}"),
+        _ => eco_format!("failed to parse {format}"),
     }
 }
diff --git a/src/image.rs b/src/image.rs
index 44b00adb1..23ea60f58 100644
--- a/src/image.rs
+++ b/src/image.rs
@@ -3,6 +3,8 @@
 use std::io;
 use std::sync::Arc;
 
+use ecow::EcoString;
+
 use crate::diag::{format_xml_like_error, StrResult};
 use crate::util::Buffer;
 
@@ -152,7 +154,7 @@ fn determine_size(data: &Buffer, format: ImageFormat) -> StrResult<(u32, u32)> {
 }
 
 /// Format the user-facing raster graphic decoding error message.
-fn format_image_error(error: image::ImageError) -> String {
+fn format_image_error(error: image::ImageError) -> EcoString {
     match error {
         image::ImageError::Limits(_) => "file is too large".into(),
         _ => "failed to decode image".into(),
@@ -160,7 +162,7 @@ fn format_image_error(error: image::ImageError) -> String {
 }
 
 /// Format the user-facing SVG decoding error message.
-fn format_usvg_error(error: usvg::Error) -> String {
+fn format_usvg_error(error: usvg::Error) -> EcoString {
     match error {
         usvg::Error::NotAnUtf8Str => "file is not valid utf-8".into(),
         usvg::Error::MalformedGZip => "file is not compressed correctly".into(),