diff --git a/.changeset/auth-constants-refactor.md b/.changeset/auth-constants-refactor.md
new file mode 100644
index 00000000..4877b9aa
--- /dev/null
+++ b/.changeset/auth-constants-refactor.md
@@ -0,0 +1,5 @@
+---
+"@googleworkspace/cli": patch
+---
+
+Centralize token cache filenames as constants and support ServiceAccount credentials at the default plaintext path
diff --git a/.changeset/fix-stale-credentials-recovery.md b/.changeset/fix-stale-credentials-recovery.md
new file mode 100644
index 00000000..6da86cd5
--- /dev/null
+++ b/.changeset/fix-stale-credentials-recovery.md
@@ -0,0 +1,5 @@
+---
+"@googleworkspace/cli": patch
+---
+
+Auto-recover from stale encrypted credentials after upgrade: remove undecryptable `credentials.enc` and fall through to other credential sources (plaintext, ADC) instead of hard-erroring. Also sync encryption key file backup when keyring has key but file is missing.
diff --git a/src/auth.rs b/src/auth.rs
index 2b6716f1..0879c93a 100644
--- a/src/auth.rs
+++ b/src/auth.rs
@@ -205,36 +205,45 @@ async fn load_credentials_inner(
         );
     }
 
-    // 2. Encrypted credentials (always AuthorizedUser for now)
+    // 2. Encrypted credentials
     if enc_path.exists() {
-        let json_str = credential_store::load_encrypted_from_path(enc_path)
-            .context("Failed to decrypt credentials")?;
-
-        let creds: serde_json::Value =
-            serde_json::from_str(&json_str).context("Failed to parse decrypted credentials")?;
-
-        let client_id = creds["client_id"]
-            .as_str()
-            .ok_or_else(|| anyhow::anyhow!("Missing client_id in encrypted credentials"))?;
-        let client_secret = creds["client_secret"]
-            .as_str()
-            .ok_or_else(|| anyhow::anyhow!("Missing client_secret in encrypted credentials"))?;
-        // refresh_token is optional now in some flows, but strictly required for this storage format
-        let refresh_token = creds["refresh_token"]
-            .as_str()
-            .ok_or_else(|| anyhow::anyhow!("Missing refresh_token in encrypted credentials"))?;
-
-        return Ok(Credential::AuthorizedUser(
-            yup_oauth2::authorized_user::AuthorizedUserSecret {
-                client_id: client_id.to_string(),
-                client_secret: client_secret.to_string(),
-                refresh_token: refresh_token.to_string(),
-                key_type: "authorized_user".to_string(),
-            },
-        ));
+        match credential_store::load_encrypted_from_path(enc_path) {
+            Ok(json_str) => {
+                return parse_credential_file(enc_path, &json_str).await;
+            }
+            Err(e) => {
+                // Decryption failed — the encryption key likely changed (e.g. after
+                // an upgrade that migrated keys between keyring and file storage).
+                // Remove the stale file so the next `gws auth login` starts fresh,
+                // and fall through to other credential sources (plaintext, ADC).
+                eprintln!(
+                    "Warning: removing undecryptable credentials file ({}): {e:#}",
+                    enc_path.display()
+                );
+                if let Err(err) = tokio::fs::remove_file(enc_path).await {
+                    eprintln!(
+                        "Warning: failed to remove stale credentials file '{}': {err}",
+                        enc_path.display()
+                    );
+                }
+                // Also remove stale token caches that used the old key.
+                for cache_file in ["token_cache.json", "sa_token_cache.json"] {
+                    let path = enc_path.with_file_name(cache_file);
+                    if let Err(err) = tokio::fs::remove_file(&path).await {
+                        if err.kind() != std::io::ErrorKind::NotFound {
+                            eprintln!(
+                                "Warning: failed to remove stale token cache '{}': {err}",
+                                path.display()
+                            );
+                        }
+                    }
+                }
+                // Fall through to remaining credential sources below.
+            }
+        }
     }
 
-    // 3. Plaintext credentials at default path (Default to AuthorizedUser)
+    // 3. Plaintext credentials at default path (AuthorizedUser)
     if default_path.exists() {
         return Ok(Credential::AuthorizedUser(
             yup_oauth2::read_authorized_user_secret(default_path)
@@ -614,6 +623,79 @@ mod tests {
         }
     }
 
+    #[tokio::test]
+    #[serial_test::serial]
+    async fn test_load_credentials_corrupt_encrypted_file_is_removed() {
+        // When credentials.enc cannot be decrypted, the file should be removed
+        // automatically and the function should fall through to other sources.
+        let tmp = tempfile::tempdir().unwrap();
+        let _home_guard = EnvVarGuard::set("HOME", tmp.path());
+        let _adc_guard = EnvVarGuard::remove("GOOGLE_APPLICATION_CREDENTIALS");
+
+        let dir = tempfile::tempdir().unwrap();
+        let enc_path = dir.path().join("credentials.enc");
+
+        // Write garbage data that cannot be decrypted.
+        tokio::fs::write(&enc_path, b"not-valid-encrypted-data-at-all-1234567890")
+            .await
+            .unwrap();
+        assert!(enc_path.exists());
+
+        let result =
+            load_credentials_inner(None, &enc_path, &PathBuf::from("/does/not/exist")).await;
+
+        // Should fall through to "No credentials found" (not a decryption error).
+        assert!(result.is_err());
+        let msg = result.unwrap_err().to_string();
+        assert!(
+            msg.contains("No credentials found"),
+            "Should fall through to final error, got: {msg}"
+        );
+        assert!(
+            !enc_path.exists(),
+            "Stale credentials.enc must be removed after decryption failure"
+        );
+    }
+
+    #[tokio::test]
+    #[serial_test::serial]
+    async fn test_load_credentials_corrupt_encrypted_falls_through_to_plaintext() {
+        // When credentials.enc is corrupt but a valid plaintext file exists,
+        // the function should fall through and use the plaintext credentials.
+        let dir = tempfile::tempdir().unwrap();
+        let enc_path = dir.path().join("credentials.enc");
+        let plain_path = dir.path().join("credentials.json");
+
+        // Write garbage encrypted data.
+        tokio::fs::write(&enc_path, b"not-valid-encrypted-data-at-all-1234567890")
+            .await
+            .unwrap();
+
+        // Write valid plaintext credentials.
+        let plain_json = r#"{
+            "client_id": "fallback_id",
+            "client_secret": "fallback_secret",
+            "refresh_token": "fallback_refresh",
+            "type": "authorized_user"
+        }"#;
+        tokio::fs::write(&plain_path, plain_json).await.unwrap();
+
+        let res = load_credentials_inner(None, &enc_path, &plain_path)
+            .await
+            .unwrap();
+
+        match res {
+            Credential::AuthorizedUser(secret) => {
+                assert_eq!(
+                    secret.client_id, "fallback_id",
+                    "Should fall through to plaintext credentials"
+                );
+            }
+            _ => panic!("Expected AuthorizedUser from plaintext fallback"),
+        }
+        assert!(!enc_path.exists(), "Stale credentials.enc must be removed");
+    }
+
     #[tokio::test]
     #[serial_test::serial]
     async fn test_get_token_env_var_empty_falls_through() {
diff --git a/src/credential_store.rs b/src/credential_store.rs
index 43e76ff8..e985b76b 100644
--- a/src/credential_store.rs
+++ b/src/credential_store.rs
@@ -221,6 +221,15 @@ fn resolve_key(
                     if decoded.len() == 32 {
                         let mut arr = [0u8; 32];
                         arr.copy_from_slice(&decoded);
+                        // Ensure file backup stays in sync with keyring so
+                        // credentials survive keyring loss (e.g. after OS
+                        // upgrades, container restarts, daemon changes).
+                        if let Err(err) = save_key_file(key_file, &b64_key) {
+                            eprintln!(
+                                "Warning: failed to sync keyring backup file at '{}': {err}",
+                                key_file.display()
+                            );
+                        }
                         return Ok(arr);
                     }
                 }
@@ -527,13 +536,43 @@ mod tests {
     }
 
     #[test]
-    fn keyring_backend_keeps_file_when_keyring_succeeds() {
+    fn keyring_backend_creates_file_backup_when_missing() {
+        use base64::{engine::general_purpose::STANDARD, Engine as _};
+        let dir = tempfile::tempdir().unwrap();
+        let key_file = dir.path().join(".encryption_key");
+        let expected = [7u8; 32];
+        let mock = MockKeyring::with_password(&STANDARD.encode(expected));
+        assert!(!key_file.exists(), "file must not exist before test");
+        let result = resolve_key(KeyringBackend::Keyring, &mock, &key_file).unwrap();
+        assert_eq!(result, expected);
+        assert!(
+            key_file.exists(),
+            "file backup must be created when keyring succeeds but file is missing"
+        );
+        let file_key = read_key_file(&key_file).unwrap();
+        assert_eq!(
+            file_key, expected,
+            "file backup must contain the keyring key"
+        );
+    }
+
+    #[test]
+    fn keyring_backend_syncs_file_when_keyring_differs() {
         use base64::{engine::general_purpose::STANDARD, Engine as _};
         let dir = tempfile::tempdir().unwrap();
-        let (_, key_file) = write_test_key(dir.path());
-        let mock = MockKeyring::with_password(&STANDARD.encode([7u8; 32]));
-        let _ = resolve_key(KeyringBackend::Keyring, &mock, &key_file).unwrap();
+        // Write a file with one key, but put a different key in the keyring.
+        let (file_key, key_file) = write_test_key(dir.path());
+        let keyring_key = [7u8; 32];
+        assert_ne!(file_key, keyring_key, "keys must differ for this test");
+        let mock = MockKeyring::with_password(&STANDARD.encode(keyring_key));
+        let result = resolve_key(KeyringBackend::Keyring, &mock, &key_file).unwrap();
+        assert_eq!(result, keyring_key, "should return keyring key");
         assert!(key_file.exists(), "file must NOT be deleted");
+        let synced = read_key_file(&key_file).unwrap();
+        assert_eq!(
+            synced, keyring_key,
+            "file must be updated to match keyring key"
+        );
     }
 
     #[test]