diff --git a/.changeset/fix-adc-quota-project.md b/.changeset/fix-adc-quota-project.md
new file mode 100644
index 0000000..4c42c53
--- /dev/null
+++ b/.changeset/fix-adc-quota-project.md
@@ -0,0 +1,5 @@
+---
+"@googleworkspace/cli": patch
+---
+
+Send x-goog-user-project header when using ADC with a quota_project_id
diff --git a/src/auth.rs b/src/auth.rs
index a14d48f..c368c5b 100644
--- a/src/auth.rs
+++ b/src/auth.rs
@@ -24,6 +24,20 @@ use anyhow::Context;
 
 use crate::credential_store;
 
+/// Returns the `quota_project_id` from Application Default Credentials, if present.
+/// This is used to set the `x-goog-user-project` header on API requests.
+pub fn get_quota_project() -> Option<String> {
+    let path = std::env::var("GOOGLE_APPLICATION_CREDENTIALS")
+        .ok()
+        .map(PathBuf::from)
+        .or_else(adc_well_known_path)?;
+    let content = std::fs::read_to_string(path).ok()?;
+    let json: serde_json::Value = serde_json::from_str(&content).ok()?;
+    json.get("quota_project_id")
+        .and_then(|v| v.as_str())
+        .map(|s| s.to_string())
+}
+
 /// Returns the well-known Application Default Credentials path:
 /// `~/.config/gcloud/application_default_credentials.json`.
 ///
@@ -637,4 +651,20 @@ mod tests {
             .to_string()
             .contains("No credentials found"));
     }
+
+    #[test]
+    #[serial_test::serial]
+    fn test_get_quota_project_reads_adc() {
+        let tmp = tempfile::tempdir().unwrap();
+        let adc_dir = tmp.path().join(".config").join("gcloud");
+        std::fs::create_dir_all(&adc_dir).unwrap();
+        std::fs::write(
+            adc_dir.join("application_default_credentials.json"),
+            r#"{"quota_project_id": "my-project-123"}"#,
+        )
+        .unwrap();
+
+        let _home_guard = EnvVarGuard::set("HOME", tmp.path());
+        assert_eq!(get_quota_project(), Some("my-project-123".to_string()));
+    }
 }
diff --git a/src/client.rs b/src/client.rs
index eb83885..421ffbb 100644
--- a/src/client.rs
+++ b/src/client.rs
@@ -11,6 +11,13 @@ pub fn build_client() -> Result<reqwest::Client, crate::error::GwsError> {
         headers.insert("x-goog-api-client", header_value);
     }
 
+    // Set quota project from ADC for billing/quota attribution
+    if let Some(quota_project) = crate::auth::get_quota_project() {
+        if let Ok(header_value) = HeaderValue::from_str(&quota_project) {
+            headers.insert("x-goog-user-project", header_value);
+        }
+    }
+
     reqwest::Client::builder()
         .default_headers(headers)
         .build()