diff --git a/src/cli/__tests__/errors.test.ts b/src/cli/__tests__/errors.test.ts
index 663cd3508..9be22167a 100644
--- a/src/cli/__tests__/errors.test.ts
+++ b/src/cli/__tests__/errors.test.ts
@@ -1,5 +1,6 @@
 import { AgentAlreadyExistsError, toError } from '../../lib';
 import {
+  formatError,
   getErrorMessage,
   isChangesetInProgressError,
   isExpiredTokenError,
@@ -39,6 +40,18 @@ describe('errors', () => {
     });
   });
 
+  describe('formatError', () => {
+    it('surfaces the cause chain that getErrorMessage drops (CDK synth stderr)', () => {
+      const causeText = 'AgentCore CDK synthesis failed: memory strategy "foo" is invalid';
+      const err = new Error('CDK synth failed: node dist/bin/cdk.js: Subprocess exited with error 1', {
+        cause: new Error(causeText),
+      });
+
+      expect(getErrorMessage(err)).not.toContain(causeText);
+      expect(formatError(err)).toContain(causeText);
+    });
+  });
+
   describe('isExpiredTokenError', () => {
     // Test ALL error codes in EXPIRED_TOKEN_ERROR_CODES via error.name
     const allErrorCodes = [
diff --git a/src/cli/commands/deploy/command.tsx b/src/cli/commands/deploy/command.tsx
index 0af261125..1a37c3366 100644
--- a/src/cli/commands/deploy/command.tsx
+++ b/src/cli/commands/deploy/command.tsx
@@ -1,6 +1,6 @@
 import { ConfigIO, serializeResult } from '../../../lib';
 import { COMMAND_DESCRIPTIONS } from '../../constants';
-import { getErrorMessage } from '../../errors';
+import { formatError, getErrorMessage } from '../../errors';
 import { withCommandRunTelemetry } from '../../telemetry/cli-command-run.js';
 import { renderTUI } from '../../tui';
 import { requireProject, requireTTY } from '../../tui/guards';
@@ -57,7 +57,7 @@ async function handleDeployCLI(options: DeployOptions): Promise<void> {
     if (options.json) {
       console.log(JSON.stringify(serializeResult(deployResult)));
     } else {
-      console.error(deployResult.error.message);
+      console.error(formatError(deployResult.error));
       if (deployResult.logPath) {
         console.error(`Log: ${deployResult.logPath}`);
       }
diff --git a/src/cli/commands/import/command.ts b/src/cli/commands/import/command.ts
index 965fc8767..b9223bb8f 100644
--- a/src/cli/commands/import/command.ts
+++ b/src/cli/commands/import/command.ts
@@ -1,5 +1,6 @@
 import { ValidationError } from '../../../lib';
 import { ANSI } from '../../constants';
+import { formatError } from '../../errors';
 import { withCommandRunTelemetry } from '../../telemetry/cli-command-run.js';
 import { handleImport } from './actions';
 import { registerImportEvaluator } from './import-evaluator';
@@ -144,7 +145,7 @@ export const registerImport = (program: Command) => {
           console.log(`Log: ${result.logPath}`);
         }
       } else {
-        console.error(`\n${red}[error]${reset} Import failed: ${result.error.message}`);
+        console.error(`\n${red}[error]${reset} Import failed: ${formatError(result.error)}`);
         if (result.logPath) {
           console.error(`Log: ${result.logPath}`);
         }
diff --git a/src/cli/commands/import/import-evaluator.ts b/src/cli/commands/import/import-evaluator.ts
index 71a60d833..863ed2a45 100644
--- a/src/cli/commands/import/import-evaluator.ts
+++ b/src/cli/commands/import/import-evaluator.ts
@@ -9,6 +9,7 @@ import {
   listAllOnlineEvaluationConfigs,
 } from '../../aws/agentcore-control';
 import { ANSI } from '../../constants';
+import { formatError } from '../../errors';
 import { withCommandRunTelemetry } from '../../telemetry/cli-command-run.js';
 import { failResult, parseAndValidateArn } from './import-utils';
 import { executeResourceImport } from './resource-import';
@@ -164,7 +165,7 @@ export function registerImportEvaluator(importCmd: Command): void {
         console.log(`  ID: ${result.resourceId}`);
         console.log('');
       } else {
-        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${result.error.message}`);
+        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${formatError(result.error)}`);
         if (result.logPath) {
           console.error(`Log: ${result.logPath}`);
         }
diff --git a/src/cli/commands/import/import-gateway.ts b/src/cli/commands/import/import-gateway.ts
index 163a6ede2..736b7b61f 100644
--- a/src/cli/commands/import/import-gateway.ts
+++ b/src/cli/commands/import/import-gateway.ts
@@ -19,7 +19,7 @@ import {
   listAllGateways,
 } from '../../aws/agentcore-control';
 import { ANSI } from '../../constants';
-import { isAccessDeniedError } from '../../errors';
+import { formatError, isAccessDeniedError } from '../../errors';
 import { withCommandRunTelemetry } from '../../telemetry/cli-command-run.js';
 import { executeCdkImportPipeline } from './import-pipeline';
 import {
@@ -746,7 +746,7 @@ export function registerImportGateway(importCmd: Command): void {
         console.log(`  agentcore fetch access  ${ANSI.dim}Get gateway URL and token${ANSI.reset}`);
         console.log('');
       } else {
-        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${result.error.message}`);
+        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${formatError(result.error)}`);
         if (result.logPath) {
           console.error(`Log: ${result.logPath}`);
         }
diff --git a/src/cli/commands/import/import-memory.ts b/src/cli/commands/import/import-memory.ts
index 90a799633..d559f2d76 100644
--- a/src/cli/commands/import/import-memory.ts
+++ b/src/cli/commands/import/import-memory.ts
@@ -3,6 +3,7 @@ import { IndexedKeyTypeSchema } from '../../../schema';
 import type { MemoryDetail, MemorySummary } from '../../aws/agentcore-control';
 import { getMemoryDetail, listAllMemories } from '../../aws/agentcore-control';
 import { ANSI } from '../../constants';
+import { formatError } from '../../errors';
 import { withCommandRunTelemetry } from '../../telemetry/cli-command-run.js';
 import { parseAndValidateArn } from './import-utils';
 import { executeResourceImport } from './resource-import';
@@ -142,7 +143,7 @@ export function registerImportMemory(importCmd: Command): void {
         console.log(`  ID: ${result.resourceId}`);
         console.log('');
       } else {
-        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${result.error.message}`);
+        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${formatError(result.error)}`);
         if (result.logPath) {
           console.error(`Log: ${result.logPath}`);
         }
diff --git a/src/cli/commands/import/import-online-eval.ts b/src/cli/commands/import/import-online-eval.ts
index 94426ac14..febb93d7e 100644
--- a/src/cli/commands/import/import-online-eval.ts
+++ b/src/cli/commands/import/import-online-eval.ts
@@ -9,6 +9,7 @@ import {
 } from '../../aws/agentcore-control';
 import { arnPrefix } from '../../aws/partition';
 import { ANSI } from '../../constants';
+import { formatError } from '../../errors';
 import { withCommandRunTelemetry } from '../../telemetry/cli-command-run.js';
 import { failResult, findResourceInDeployedState, parseAndValidateArn } from './import-utils';
 import { executeResourceImport } from './resource-import';
@@ -219,7 +220,7 @@ export function registerImportOnlineEval(importCmd: Command): void {
         console.log(`  ID: ${result.resourceId}`);
         console.log('');
       } else {
-        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${result.error.message}`);
+        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${formatError(result.error)}`);
         if (result.logPath) {
           console.error(`Log: ${result.logPath}`);
         }
diff --git a/src/cli/commands/import/import-runtime.ts b/src/cli/commands/import/import-runtime.ts
index 58aa53fad..460b9b7dd 100644
--- a/src/cli/commands/import/import-runtime.ts
+++ b/src/cli/commands/import/import-runtime.ts
@@ -2,6 +2,7 @@ import type { AgentEnvSpec } from '../../../schema';
 import type { AgentRuntimeDetail, AgentRuntimeSummary } from '../../aws/agentcore-control';
 import { getAgentRuntimeDetail, listAllAgentRuntimes } from '../../aws/agentcore-control';
 import { ANSI } from '../../constants';
+import { formatError } from '../../errors';
 import { withCommandRunTelemetry } from '../../telemetry/cli-command-run.js';
 import { copyAgentSource, failResult, parseAndValidateArn } from './import-utils';
 import { executeResourceImport } from './resource-import';
@@ -227,7 +228,7 @@ export function registerImportRuntime(importCmd: Command): void {
         console.log(`  agentcore invoke     ${ANSI.dim}Test your agent${ANSI.reset}`);
         console.log('');
       } else {
-        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${result.error.message}`);
+        console.error(`\n${ANSI.red}[error]${ANSI.reset} ${formatError(result.error)}`);
         if (result.logPath) {
           console.error(`Log: ${result.logPath}`);
         }
diff --git a/src/cli/errors.ts b/src/cli/errors.ts
index ccca6fa16..e4adcea34 100644
--- a/src/cli/errors.ts
+++ b/src/cli/errors.ts
@@ -1,11 +1,38 @@
 /**
  * Converts an unknown error to a string message.
  * Handles Error instances and other thrown values consistently.
+ *
+ * Note: this intentionally returns only `err.message` and does NOT walk the
+ * cause chain. Use `formatError` when displaying an error to the user so the
+ * underlying cause (e.g. CDK synth child stderr) is surfaced.
  */
 export function getErrorMessage(err: unknown): string {
   return err instanceof Error ? err.message : String(err);
 }
 
+/**
+ * Format an error for user display. Shows the message and the full cause chain, but NOT the raw JS
+ * stack trace — dumping `err.stack` (minified dist frames) to users is noise for the common case
+ * (config/validation errors). Set AGENTCORE_DEBUG=1 to include the stack trace for debugging.
+ *
+ * Unlike `getErrorMessage`, this recurses into `err.cause`, so actionable detail attached by lower
+ * layers (e.g. the CDK toolkit wrapper preserving the synth child's stderr on `err.cause`) reaches
+ * the user instead of being dropped behind a generic top-level message.
+ */
+export function formatError(err: unknown): string {
+  if (err instanceof Error) {
+    const lines = [err.message];
+    if (err.stack && process.env.AGENTCORE_DEBUG) {
+      lines.push('', 'Stack trace:', err.stack);
+    }
+    if (err.cause) {
+      lines.push('', 'Caused by:', formatError(err.cause));
+    }
+    return lines.join('\n');
+  }
+  return String(err);
+}
+
 /**
  * Checks if an error is an AWS access denied error.
  * Returns true for AccessDeniedException or AccessDenied error codes.
diff --git a/src/cli/operations/deploy/preflight.ts b/src/cli/operations/deploy/preflight.ts
index a06321578..21b04b667 100644
--- a/src/cli/operations/deploy/preflight.ts
+++ b/src/cli/operations/deploy/preflight.ts
@@ -5,6 +5,7 @@ import { validateAwsCredentials } from '../../aws/account';
 import { LocalCdkProject } from '../../cdk/local-cdk-project';
 import { CdkToolkitWrapper, createCdkToolkitWrapper, silentIoHost } from '../../cdk/toolkit-lib';
 import { checkBootstrapStatus, checkStacksStatus, formatCdkEnvironment } from '../../cloudformation';
+import { formatError } from '../../errors';
 import { cleanupStaleLockFiles } from '../../tui/utils';
 import type { IIoHost } from '@aws-cdk/toolkit-lib';
 import { existsSync, readFileSync } from 'node:fs';
@@ -39,24 +40,10 @@ export interface StackStatusCheckResult {
   message?: string;
 }
 
-/**
- * Format an error for user display. Shows the message and the cause chain, but NOT the raw JS stack
- * trace — dumping `err.stack` (minified dist/cli/index.mjs frames) to users is noise for the common
- * case (config/validation errors). Set AGENTCORE_DEBUG=1 to include the stack trace for debugging.
- */
-export function formatError(err: unknown): string {
-  if (err instanceof Error) {
-    const lines = [err.message];
-    if (err.stack && process.env.AGENTCORE_DEBUG) {
-      lines.push('', 'Stack trace:', err.stack);
-    }
-    if (err.cause) {
-      lines.push('', 'Caused by:', formatError(err.cause));
-    }
-    return lines.join('\n');
-  }
-  return String(err);
-}
+// Re-exported from src/cli/errors.ts so existing importers (TUI hooks, operations/deploy/index.ts,
+// preflight tests) keep their import path. Lives in errors.ts so the thin command/display layers can
+// reuse it without pulling in this heavy deploy module.
+export { formatError };
 
 /**
  * Validates the CDK project and loads configuration.