diff --git a/src/cli/__tests__/global-config.test.ts b/src/cli/__tests__/global-config.test.ts
index 6e2038973..ba00fb421 100644
--- a/src/cli/__tests__/global-config.test.ts
+++ b/src/cli/__tests__/global-config.test.ts
@@ -82,9 +82,9 @@ describe('global-config', () => {
     it('creates directory and writes config when none exists', async () => {
       const fresh = createTempConfig('gc-fresh');
 
-      const ok = await updateGlobalConfig({ telemetry: { enabled: false } }, fresh.configDir, fresh.configFile);
+      const result = await updateGlobalConfig({ telemetry: { enabled: false } }, fresh.configDir, fresh.configFile);
 
-      expect(ok).toBe(true);
+      expect(result.success).toBe(true);
       const written = JSON.parse(await readFile(fresh.configFile, 'utf-8'));
       expect(written).toEqual({ telemetry: { enabled: false } });
 
@@ -106,14 +106,24 @@ describe('global-config', () => {
       });
     });
 
-    it('returns false on write failures', async () => {
-      const ok = await updateGlobalConfig(
+    it('does not overwrite malformed config and reports why', async () => {
+      await writeFile(tmp.configFile, '{ invalid json');
+
+      const result = await updateGlobalConfig({ telemetry: { enabled: true } }, tmp.configDir, tmp.configFile);
+
+      expect(result.success).toBe(false);
+      expect(result.success ? '' : result.error).toMatch(/malformed/);
+      expect(await readFile(tmp.configFile, 'utf-8')).toBe('{ invalid json');
+    });
+
+    it('returns an error on write failures', async () => {
+      const result = await updateGlobalConfig(
         { telemetry: { enabled: true } },
         tmp.testDir + '/\0invalid',
         tmp.testDir + '/\0invalid/config.json'
       );
 
-      expect(ok).toBe(false);
+      expect(result.success).toBe(false);
     });
   });
 
diff --git a/src/cli/commands/telemetry/actions.ts b/src/cli/commands/telemetry/actions.ts
index c83a6f425..aa8887f1a 100644
--- a/src/cli/commands/telemetry/actions.ts
+++ b/src/cli/commands/telemetry/actions.ts
@@ -10,18 +10,18 @@ export async function handleTelemetryDisable(
   configDir = GLOBAL_CONFIG_DIR,
   configFile = GLOBAL_CONFIG_FILE
 ): Promise<boolean> {
-  const ok = await updateGlobalConfig({ telemetry: { enabled: false } }, configDir, configFile);
-  console.log(ok ? 'Telemetry has been disabled.' : `Warning: could not write config to ${configFile}`);
-  return ok;
+  const result = await updateGlobalConfig({ telemetry: { enabled: false } }, configDir, configFile);
+  console.log(result.success ? 'Telemetry has been disabled.' : `Warning: ${result.error}`);
+  return result.success;
 }
 
 export async function handleTelemetryEnable(
   configDir = GLOBAL_CONFIG_DIR,
   configFile = GLOBAL_CONFIG_FILE
 ): Promise<boolean> {
-  const ok = await updateGlobalConfig({ telemetry: { enabled: true } }, configDir, configFile);
-  console.log(ok ? 'Telemetry has been enabled.' : `Warning: could not write config to ${configFile}`);
-  return ok;
+  const result = await updateGlobalConfig({ telemetry: { enabled: true } }, configDir, configFile);
+  console.log(result.success ? 'Telemetry has been enabled.' : `Warning: ${result.error}`);
+  return result.success;
 }
 
 export async function handleTelemetryStatus(configFile = GLOBAL_CONFIG_FILE): Promise<void> {
diff --git a/src/lib/schemas/io/global-config.ts b/src/lib/schemas/io/global-config.ts
index fc64eb39b..6452a1df2 100644
--- a/src/lib/schemas/io/global-config.ts
+++ b/src/lib/schemas/io/global-config.ts
@@ -4,6 +4,7 @@ import { randomUUID } from 'node:crypto';
 import { homedir } from 'os';
 import { join } from 'path';
 import { z } from 'zod';
+import { toError } from '../../errors/types.js';
 
 export const GLOBAL_CONFIG_DIR = process.env.AGENTCORE_CONFIG_DIR ?? join(homedir(), '.agentcore');
 export const GLOBAL_CONFIG_FILE = join(GLOBAL_CONFIG_DIR, 'config.json');
@@ -46,20 +47,55 @@ export function readGlobalConfigSync(configFile = GLOBAL_CONFIG_FILE): GlobalCon
   }
 }
 
+export type UpdateGlobalConfigResult = { success: true } | { success: false; error: string };
+
 export async function updateGlobalConfig(
   partial: GlobalConfig,
   configDir = GLOBAL_CONFIG_DIR,
   configFile = GLOBAL_CONFIG_FILE
-): Promise<boolean> {
-  try {
-    const existing = await readGlobalConfig(configFile);
-    const merged: GlobalConfig = mergeConfig(existing, partial);
+): Promise<UpdateGlobalConfigResult> {
+  // Read the existing config strictly: a missing file is fine (start fresh), but a
+  // malformed file must not be silently overwritten with merged-in defaults.
+  const existing = await loadConfigForUpdate(configFile);
+  if (!existing.success) {
+    return existing;
+  }
 
+  try {
+    const merged: GlobalConfig = mergeConfig(existing.config, partial);
     await mkdir(configDir, { recursive: true });
     await writeFile(configFile, JSON.stringify(merged, null, 2), 'utf-8');
-    return true;
-  } catch {
-    return false;
+    return { success: true };
+  } catch (error) {
+    return { success: false, error: `Failed to write config to ${configFile}: ${toError(error).message}` };
+  }
+}
+
+type LoadConfigResult = { success: true; config: GlobalConfig } | { success: false; error: string };
+
+/**
+ * Reads the existing global config for an update. Distinguishes a missing file
+ * (treated as an empty config) from a malformed one (read/parse/schema failure),
+ * so the caller can avoid clobbering a config it could not understand.
+ */
+async function loadConfigForUpdate(configFile: string): Promise<LoadConfigResult> {
+  let data: string;
+  try {
+    data = await readFile(configFile, 'utf-8');
+  } catch (error) {
+    if ((error as NodeJS.ErrnoException).code === 'ENOENT') {
+      return { success: true, config: {} };
+    }
+    return { success: false, error: `Could not read config at ${configFile}: ${toError(error).message}` };
+  }
+
+  try {
+    return { success: true, config: GlobalConfigSchema.parse(JSON.parse(data)) };
+  } catch (error) {
+    return {
+      success: false,
+      error: `Config at ${configFile} is malformed and was left unchanged: ${toError(error).message}`,
+    };
   }
 }