diff --git a/packages/@aws-cdk/toolkit/lib/api/io/private/codes.ts b/packages/@aws-cdk/toolkit/lib/api/io/private/codes.ts
new file mode 100644
index 0000000000000..41988d0764f59
--- /dev/null
+++ b/packages/@aws-cdk/toolkit/lib/api/io/private/codes.ts
@@ -0,0 +1,43 @@
+import { IoMessageCode } from '../io-message';
+
+export const CODES = {
+  // Default codes -- all 0000 codes
+  CDK_TOOLKIT_I0000: 'Default toolkit info code',
+  CDK_TOOLKIT_E0000: 'Default toolkit error code',
+  CDK_TOOLKIT_W0000: 'Default toolkit warning code',
+  CDK_SDK_I0000: 'Default sdk info code',
+  CDK_SDK_E0000: 'Default sdk error code',
+  CDK_SDK_WOOOO: 'Default sdk warning code',
+  CDK_ASSETS_I0000: 'Default assets info code',
+  CDK_ASSETS_E0000: 'Default assets error code',
+  CDK_ASSETS_W0000: 'Default assets warning code',
+  CDK_ASSEMBLY_I0000: 'Default assembly info code',
+  CDK_ASSEMBLY_E0000: 'Default assembly error code',
+  CDK_ASSEMBLY_W0000: 'Default assembly warning code',
+
+  // Toolkit Info codes
+  CDK_TOOLKIT_I0001: 'Display stack data',
+  CDK_TOOLKIT_I0002: 'Successfully deployed stacks',
+  CDK_TOOLKIT_I5001: 'Display synthesis times',
+  CDK_TOOLKIT_I5050: 'Confirm rollback',
+  CDK_TOOLKIT_I5060: 'Confirm deploy security sensitive changes',
+  CDK_TOOLKIT_I7010: 'Confirm destroy stacks',
+
+  // Toolkit Warning codes
+
+  // Toolkit Error codes
+
+  // Assembly Info codes
+  CDK_ASSEMBLY_I0042: 'Writing updated context',
+  CDK_ASSEMBLY_I0241: 'Fetching missing context',
+
+  // Assembly Warning codes
+
+  // Assembly Error codes
+  CDK_ASSEMBLY_E1111: 'Incompatible CDK CLI version. Upgrade needed.',
+};
+
+// If we give CODES a type with key: IoMessageCode,
+// this dynamically generated type will generalize to allow all IoMessageCodes.
+// Instead, we will validate that VALID_CODE must be IoMessageCode with the '&'.
+export type VALID_CODE = keyof typeof CODES & IoMessageCode;
diff --git a/packages/@aws-cdk/toolkit/lib/api/io/private/index.ts b/packages/@aws-cdk/toolkit/lib/api/io/private/index.ts
index 476d4d9b28b60..392d2142cd9b5 100644
--- a/packages/@aws-cdk/toolkit/lib/api/io/private/index.ts
+++ b/packages/@aws-cdk/toolkit/lib/api/io/private/index.ts
@@ -2,3 +2,4 @@ export * from './logger';
 export * from './messages';
 export * from './timer';
 export * from './types';
+export * from './codes';
diff --git a/packages/@aws-cdk/toolkit/lib/api/io/private/logger.ts b/packages/@aws-cdk/toolkit/lib/api/io/private/logger.ts
index 04ab089d5fc42..f4ef260f0da34 100644
--- a/packages/@aws-cdk/toolkit/lib/api/io/private/logger.ts
+++ b/packages/@aws-cdk/toolkit/lib/api/io/private/logger.ts
@@ -1,7 +1,7 @@
 import * as util from 'node:util';
 import type { Logger } from '@smithy/types';
 import type { IoMessage, IoMessageCodeCategory, IoMessageLevel, IoRequest } from '../io-message';
-import { debug, error, info, messageCode, trace, warn } from './messages';
+import { debug, error, info, defaultMessageCode, trace, warn } from './messages';
 import type { ActionAwareIoHost } from './types';
 import type { ToolkitAction } from '../../../toolkit';
 import { formatSdkLoggerContent } from '../../aws-cdk';
@@ -118,7 +118,7 @@ export function asSdkLogger(ioHost: IIoHost, action: ToolkitAction): Logger {
  * Turn an ActionAwareIoHost into a logger that is compatible with older code, but doesn't support data
  */
 export function asLogger(ioHost: ActionAwareIoHost, category?: IoMessageCodeCategory) {
-  const code = (level: IoMessageLevel) => messageCode(level, category);
+  const code = (level: IoMessageLevel) => defaultMessageCode(level, category);
 
   return {
     trace: async (msg: string, ...args: any[]) => {
diff --git a/packages/@aws-cdk/toolkit/lib/api/io/private/messages.ts b/packages/@aws-cdk/toolkit/lib/api/io/private/messages.ts
index 9bf3a388c2bd5..e5fe8e8edaa9d 100644
--- a/packages/@aws-cdk/toolkit/lib/api/io/private/messages.ts
+++ b/packages/@aws-cdk/toolkit/lib/api/io/private/messages.ts
@@ -1,5 +1,6 @@
 import * as chalk from 'chalk';
-import type { IoMessageCode, IoMessageCodeCategory, IoMessageLevel } from '../io-message';
+import type { IoMessageCodeCategory, IoMessageLevel } from '../io-message';
+import { type VALID_CODE } from './codes';
 import type { ActionLessMessage, ActionLessRequest, Optional, SimplifiedMessage } from './types';
 
 /**
@@ -11,27 +12,28 @@ export function formatMessage<T>(msg: Optional<SimplifiedMessage<T>, 'code'>, ca
   return {
     time: new Date(),
     level: msg.level,
-    code: msg.code ?? messageCode(msg.level, category),
+    code: msg.code ?? defaultMessageCode(msg.level, category),
     message: msg.message,
     data: msg.data,
   };
 }
 
 /**
- * Build a message code from level and category
+ * Build a message code from level and category. The code must be valid for this function to pass.
+ * Otherwise it returns a ToolkitError.
  */
-export function messageCode(level: IoMessageLevel, category: IoMessageCodeCategory = 'TOOLKIT', number?: `${number}${number}${number}${number}`): IoMessageCode {
+export function defaultMessageCode(level: IoMessageLevel, category: IoMessageCodeCategory = 'TOOLKIT'): VALID_CODE {
   const levelIndicator = level === 'error' ? 'E' :
     level === 'warn' ? 'W' :
       'I';
-  return `CDK_${category}_${levelIndicator}${number ?? '0000'}`;
+  return `CDK_${category}_${levelIndicator}0000` as VALID_CODE;
 }
 
 /**
  * Requests a yes/no confirmation from the IoHost.
  */
 export const confirm = (
-  code: IoMessageCode,
+  code: VALID_CODE,
   question: string,
   motivation: string,
   defaultResponse: boolean,
@@ -49,7 +51,7 @@ export const confirm = (
 /**
  * Prompt for a a response from the IoHost.
  */
-export const prompt = <T, U>(code: IoMessageCode, message: string, defaultResponse: U, payload?: T): ActionLessRequest<T, U> => {
+export const prompt = <T, U>(code: VALID_CODE, message: string, defaultResponse: U, payload?: T): ActionLessRequest<T, U> => {
   return {
     defaultResponse,
     ...formatMessage({
@@ -64,7 +66,7 @@ export const prompt = <T, U>(code: IoMessageCode, message: string, defaultRespon
 /**
  * Logs an error level message.
  */
-export const error = <T>(message: string, code?: IoMessageCode, payload?: T) => {
+export const error = <T>(message: string, code?: VALID_CODE, payload?: T) => {
   return formatMessage({
     level: 'error',
     code,
@@ -76,7 +78,7 @@ export const error = <T>(message: string, code?: IoMessageCode, payload?: T) =>
 /**
  * Logs an warning level message.
  */
-export const warn = <T>(message: string, code?: IoMessageCode, payload?: T) => {
+export const warn = <T>(message: string, code?: VALID_CODE, payload?: T) => {
   return formatMessage({
     level: 'warn',
     code,
@@ -88,7 +90,7 @@ export const warn = <T>(message: string, code?: IoMessageCode, payload?: T) => {
 /**
  * Logs an info level message.
  */
-export const info = <T>(message: string, code?: IoMessageCode, payload?: T) => {
+export const info = <T>(message: string, code?: VALID_CODE, payload?: T) => {
   return formatMessage({
     level: 'info',
     code,
@@ -101,7 +103,7 @@ export const info = <T>(message: string, code?: IoMessageCode, payload?: T) => {
  * Logs an info level message to stdout.
  * @deprecated
  */
-export const data = <T>(message: string, code?: IoMessageCode, payload?: T) => {
+export const data = <T>(message: string, code?: VALID_CODE, payload?: T) => {
   return formatMessage({
     level: 'info',
     code,
@@ -113,7 +115,7 @@ export const data = <T>(message: string, code?: IoMessageCode, payload?: T) => {
 /**
  * Logs a debug level message.
  */
-export const debug = <T>(message: string, code?: IoMessageCode, payload?: T) => {
+export const debug = <T>(message: string, code?: VALID_CODE, payload?: T) => {
   return formatMessage({
     level: 'debug',
     code,
@@ -125,7 +127,7 @@ export const debug = <T>(message: string, code?: IoMessageCode, payload?: T) =>
 /**
  * Logs a trace level message.
  */
-export const trace = <T>(message: string, code?: IoMessageCode, payload?: T) => {
+export const trace = <T>(message: string, code?: VALID_CODE, payload?: T) => {
   return formatMessage({
     level: 'trace',
     code,
@@ -138,7 +140,7 @@ export const trace = <T>(message: string, code?: IoMessageCode, payload?: T) =>
  * Logs an info level success message in green text.
  * @deprecated
  */
-export const success = <T>(message: string, code?: IoMessageCode, payload?: T) => {
+export const success = <T>(message: string, code?: VALID_CODE, payload?: T) => {
   return formatMessage({
     level: 'info',
     code,
@@ -151,7 +153,7 @@ export const success = <T>(message: string, code?: IoMessageCode, payload?: T) =
  * Logs an info level message in bold text.
  * @deprecated
  */
-export const highlight = <T>(message: string, code?: IoMessageCode, payload?: T) => {
+export const highlight = <T>(message: string, code?: VALID_CODE, payload?: T) => {
   return formatMessage({
     level: 'info',
     code,