diff --git a/.changeset/fix-oauth-quota-project-header.md b/.changeset/fix-oauth-quota-project-header.md
new file mode 100644
index 00000000..604dd5ae
--- /dev/null
+++ b/.changeset/fix-oauth-quota-project-header.md
@@ -0,0 +1,5 @@
+---
+"@googleworkspace/cli": patch
+---
+
+Skip x-goog-user-project header for OAuth auth to fix 403 errors for non-project-member users
diff --git a/crates/google-workspace-cli/src/auth.rs b/crates/google-workspace-cli/src/auth.rs
index 9d8847e4..66f8bdf4 100644
--- a/crates/google-workspace-cli/src/auth.rs
+++ b/crates/google-workspace-cli/src/auth.rs
@@ -126,6 +126,17 @@ fn adc_well_known_path() -> Option<PathBuf> {
     })
 }
 
+/// Tracks what authentication method was used for the request.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum AuthMethod {
+    /// OAuth2 bearer token from a user credential (`gws auth login`)
+    OAuth,
+    /// Bearer token from a service-account key
+    ServiceAccount,
+    /// No authentication was provided
+    None,
+}
+
 /// Types of credentials we support
 #[derive(Debug)]
 enum Credential {
@@ -229,6 +240,30 @@ pub async fn get_token(scopes: &[&str]) -> anyhow::Result<String> {
     get_token_inner(scopes, creds, &token_cache).await
 }
 
+/// Like [`get_token`] but also returns the [`AuthMethod`] so callers can
+/// decide whether to include the `x-goog-user-project` quota header.
+pub async fn get_token_with_kind(scopes: &[&str]) -> anyhow::Result<(String, AuthMethod)> {
+    if let Ok(token) = std::env::var("GOOGLE_WORKSPACE_CLI_TOKEN") {
+        if !token.is_empty() {
+            return Ok((token, AuthMethod::OAuth));
+        }
+    }
+
+    let creds_file = std::env::var("GOOGLE_WORKSPACE_CLI_CREDENTIALS_FILE").ok();
+    let config_dir = crate::auth_commands::config_dir();
+    let enc_path = credential_store::encrypted_credentials_path();
+    let default_path = config_dir.join("credentials.json");
+    let token_cache = config_dir.join("token_cache.json");
+
+    let creds = load_credentials_inner(creds_file.as_deref(), &enc_path, &default_path).await?;
+    let kind = match &creds {
+        Credential::ServiceAccount(_) => AuthMethod::ServiceAccount,
+        Credential::AuthorizedUser(_) => AuthMethod::OAuth,
+    };
+    let token = get_token_inner(scopes, creds, &token_cache).await?;
+    Ok((token, kind))
+}
+
 /// Check if HTTP proxy environment variables are set
 pub(crate) fn has_proxy_env() -> bool {
     PROXY_ENV_VARS
diff --git a/crates/google-workspace-cli/src/executor.rs b/crates/google-workspace-cli/src/executor.rs
index 46f31ac4..feac04d6 100644
--- a/crates/google-workspace-cli/src/executor.rs
+++ b/crates/google-workspace-cli/src/executor.rs
@@ -31,14 +31,7 @@ use crate::discovery::{RestDescription, RestMethod};
 use crate::error::GwsError;
 use crate::output::sanitize_for_terminal;
 
-/// Tracks what authentication method was used for the request.
-#[derive(Debug, Clone, PartialEq)]
-pub enum AuthMethod {
-    /// OAuth2 bearer token from credentials file
-    OAuth,
-    /// No authentication was provided
-    None,
-}
+pub use crate::auth::AuthMethod;
 
 /// Source for media upload content.
 ///
@@ -182,14 +175,17 @@ async fn build_http_request(
     };
 
     if let Some(token) = token {
-        if *auth_method == AuthMethod::OAuth {
+        if matches!(*auth_method, AuthMethod::OAuth | AuthMethod::ServiceAccount) {
             request = request.bearer_auth(token);
         }
     }
 
-    // Set quota project from ADC for billing/quota attribution
-    if let Some(quota_project) = crate::auth::get_quota_project() {
-        request = request.header("x-goog-user-project", quota_project);
+    // Only send quota project for service-account auth; OAuth users may not be
+    // IAM members of the project, so the header would trigger 403 errors.
+    if *auth_method == AuthMethod::ServiceAccount {
+        if let Some(quota_project) = crate::auth::get_quota_project() {
+            request = request.header("x-goog-user-project", quota_project);
+        }
     }
 
     let mut all_query_params = input.query_params.clone();
@@ -2399,3 +2395,42 @@ async fn test_get_does_not_set_content_length_zero() {
         "GET with no body should not have Content-Length header"
     );
 }
+
+#[tokio::test]
+async fn test_oauth_auth_does_not_set_quota_project_header() {
+    // Arrange: even if get_quota_project() would return a value, OAuth requests
+    // must NOT send x-goog-user-project because OAuth users are not necessarily
+    // IAM members of the project and the header would trigger 403 errors.
+    let client = reqwest::Client::new();
+    let method = RestMethod {
+        http_method: "GET".to_string(),
+        path: "files".to_string(),
+        ..Default::default()
+    };
+    let input = ExecutionInput {
+        full_url: "https://example.com/files".to_string(),
+        body: None,
+        params: Map::new(),
+        query_params: Vec::new(),
+        is_upload: false,
+    };
+
+    let request = build_http_request(
+        &client,
+        &method,
+        &input,
+        Some("fake-token"),
+        &AuthMethod::OAuth,
+        None,
+        0,
+        &None,
+    )
+    .await
+    .unwrap();
+
+    let built = request.build().unwrap();
+    assert!(
+        built.headers().get("x-goog-user-project").is_none(),
+        "OAuth requests must not include x-goog-user-project header"
+    );
+}
diff --git a/crates/google-workspace-cli/src/main.rs b/crates/google-workspace-cli/src/main.rs
index 41dcc1e1..5e9fb10c 100644
--- a/crates/google-workspace-cli/src/main.rs
+++ b/crates/google-workspace-cli/src/main.rs
@@ -262,8 +262,8 @@ async fn run() -> Result<(), GwsError> {
     let scopes: Vec<&str> = select_scope(&method.scopes).into_iter().collect();
 
     // Authenticate: try OAuth, fail with error if credentials exist but are broken
-    let (token, auth_method) = match auth::get_token(&scopes).await {
-        Ok(t) => (Some(t), executor::AuthMethod::OAuth),
+    let (token, auth_method) = match auth::get_token_with_kind(&scopes).await {
+        Ok((t, method)) => (Some(t), method),
         Err(e) => {
             // If credentials were found but failed (e.g. decryption error, invalid token),
             // propagate the error instead of silently falling back to unauthenticated.