diff --git a/dashcaddy-api/src/utilities/error-codes.js b/dashcaddy-api/src/utilities/error-codes.js
new file mode 100644
index 0000000..2ddda2c
--- /dev/null
+++ b/dashcaddy-api/src/utilities/error-codes.js
@@ -0,0 +1,141 @@
+/**
+ * DC-086: Structured error code system for consistent API error responses.
+ *
+ * Format: DC-[MODULE]-[NUMBER]
+ * Modules: AUTH, CONTAINER, SERVICE, DNS, CADDY, CA, BACKUP, CONFIG,
+ *          BILL, HEALTH, NETWORK, SYSTEM, GENERAL
+ *
+ * Usage in routes:
+ *   const { ErrorCodes } = require('../src/utilities/error-codes');
+ *   errorResponse(res, 400, ErrorCodes.CONTAINER.INVALID_ID, 'Container ID has invalid characters');
+ *
+ * Clients can use the machine-readable code for i18n and error-specific handling
+ * while the human message provides immediate context.
+ */
+
+const ErrorCodes = {
+  // ── General ──
+  GENERAL: {
+    INVALID_INPUT: 'DC-GEN-001',
+    NOT_FOUND: 'DC-GEN-002',
+    RATE_LIMITED: 'DC-GEN-003',
+    INTERNAL: 'DC-GEN-004',
+    UNAUTHORIZED: 'DC-GEN-005',
+    FORBIDDEN: 'DC-GEN-006',
+    CONFLICT: 'DC-GEN-007',
+    TIMEOUT: 'DC-GEN-008',
+  },
+
+  // ── Authentication ──
+  AUTH: {
+    NO_SESSION: 'DC-AUTH-001',
+    INVALID_TOKEN: 'DC-AUTH-002',
+    SESSION_EXPIRED: 'DC-AUTH-003',
+    TOTP_REQUIRED: 'DC-AUTH-004',
+    TOTP_INVALID: 'DC-AUTH-005',
+    PROVIDER_DISABLED: 'DC-AUTH-006',
+    INVITE_EXPIRED: 'DC-AUTH-007',
+    INVITE_INVALID: 'DC-AUTH-008',
+    KEY_REVOKED: 'DC-AUTH-009',
+    LAST_ADMIN: 'DC-AUTH-010',
+  },
+
+  // ── Containers ──
+  CONTAINER: {
+    NOT_FOUND: 'DC-CONT-001',
+    INVALID_ID: 'DC-CONT-002',
+    INVALID_NAME: 'DC-CONT-003',
+    INVALID_IMAGE: 'DC-CONT-004',
+    ALREADY_RUNNING: 'DC-CONT-005',
+    ALREADY_STOPPED: 'DC-CONT-006',
+    START_FAILED: 'DC-CONT-007',
+    STOP_FAILED: 'DC-CONT-008',
+    DELETE_FAILED: 'DC-CONT-009',
+    INVALID_RESOURCES: 'DC-CONT-010',
+    DOCKER_UNREACHABLE: 'DC-CONT-011',
+  },
+
+  // ── Services ──
+  SERVICE: {
+    NOT_FOUND: 'DC-SVC-001',
+    INVALID_ID: 'DC-SVC-002',
+    INVALID_SUBDOMAIN: 'DC-SVC-003',
+    INVALID_PORT: 'DC-SVC-004',
+    DUPLICATE_ID: 'DC-SVC-005',
+    INVALID_URL: 'DC-SVC-006',
+    INVALID_PROTOCOL: 'DC-SVC-007',
+    PORT_IN_USE: 'DC-SVC-008',
+    DEPENDENCY_CYCLE: 'DC-SVC-009',
+  },
+
+  // ── DNS ──
+  DNS: {
+    INVALID_RECORD: 'DC-DNS-001',
+    INVALID_ZONE: 'DC-DNS-002',
+    PROVIDER_ERROR: 'DC-DNS-003',
+    PROPAGATION_TIMEOUT: 'DC-DNS-004',
+    INVALID_CREDENTIALS: 'DC-DNS-005',
+  },
+
+  // ── Caddy / Reverse Proxy ──
+  CADDY: {
+    ADMIN_UNREACHABLE: 'DC-CAD-001',
+    CONFIG_INVALID: 'DC-CAD-002',
+    RELOAD_FAILED: 'DC-CAD-003',
+    SITE_EXISTS: 'DC-CAD-004',
+    SITE_NOT_FOUND: 'DC-CAD-005',
+  },
+
+  // ── Certificate Authority ──
+  CA: {
+    NOT_INITIALIZED: 'DC-CA-001',
+    INVALID_DOMAIN: 'DC-CA-002',
+    CERT_NOT_FOUND: 'DC-CA-003',
+    GENERATION_FAILED: 'DC-CA-004',
+    INVALID_FORMAT: 'DC-CA-005',
+  },
+
+  // ── Backup ──
+  BACKUP: {
+    NO_SCHEDULE: 'DC-BAK-001',
+    BACKUP_FAILED: 'DC-BAK-002',
+    RESTORE_FAILED: 'DC-BAK-003',
+    INVALID_CONFIG: 'DC-BAK-004',
+  },
+
+  // ── Billing / License ──
+  BILL: {
+    CHECKOUT_FAILED: 'DC-BILL-001',
+    LICENSE_INVALID: 'DC-BILL-002',
+    LICENSE_EXPIRED: 'DC-BILL-003',
+    LICENSE_NOT_FOUND: 'DC-BILL-004',
+    FEATURE_LOCKED: 'DC-BILL-005',
+    WEBHOOK_INVALID: 'DC-BILL-006',
+  },
+
+  // ── Health Monitoring ──
+  HEALTH: {
+    CHECK_FAILED: 'DC-HLT-001',
+    INCIDENT_NOT_FOUND: 'DC-HLT-002',
+    INVALID_SEVERITY: 'DC-HLT-003',
+  },
+
+  // ── Network ──
+  NETWORK: {
+    INVALID_IP: 'DC-NET-001',
+    INVALID_CIDR: 'DC-NET-002',
+    INVALID_HOSTNAME: 'DC-NET-003',
+    GATEWAY_TIMEOUT: 'DC-NET-004',
+  },
+
+  // ── System / Config ──
+  SYSTEM: {
+    CONFIG_INVALID: 'DC-SYS-001',
+    CONFIG_SAVE_FAILED: 'DC-SYS-002',
+    STARTUP_FAILED: 'DC-SYS-003',
+    DATA_DIR_UNSAFE: 'DC-SYS-004',
+    DISK_FULL: 'DC-SYS-005',
+  },
+};
+
+module.exports = { ErrorCodes };
diff --git a/dashcaddy-api/src/utils/responses.js b/dashcaddy-api/src/utils/responses.js
index d980a46..0cf97c5 100644
--- a/dashcaddy-api/src/utils/responses.js
+++ b/dashcaddy-api/src/utils/responses.js
@@ -59,9 +59,17 @@ function noContent(res) {
  * @param {number} statusCode HTTP status code
  * @param {string} message Human-readable error message
  * @param {object} [extras={}] additional fields to merge into the response
+ *
+ * DC-086: If extras.code is set, it's treated as a machine-readable error code
+ * (e.g. 'DC-CONT-002'). If message looks like a DC code, it's auto-extracted.
  */
 function errorResponse(res, statusCode, message, extras = {}) {
-  return res.status(statusCode).json({ success: false, error: message, ...extras });
+  const body = { success: false, error: message, ...extras };
+  // DC-086: surface machine-readable code at top level for client handling
+  if (extras.code) {
+    body.code = extras.code;
+  }
+  return res.status(statusCode).json(body);
 }
 
 /**