diff --git a/packages/core/src/utils/errors.ts b/packages/core/src/utils/errors.ts
index dc7d5b33c5..5465977ff2 100644
--- a/packages/core/src/utils/errors.ts
+++ b/packages/core/src/utils/errors.ts
@@ -10,6 +10,16 @@ interface GaxiosError {
   };
 }
 
+function isGaxiosError(error: unknown): error is GaxiosError {
+  return (
+    typeof error === 'object' &&
+    error !== null &&
+    'response' in error &&
+    typeof (error as { response: unknown }).response === 'object' &&
+    (error as { response: unknown }).response !== null
+  );
+}
+
 export function isNodeError(error: unknown): error is NodeJS.ErrnoException {
   return error instanceof Error && 'code' in error;
 }
@@ -105,11 +115,41 @@ interface ResponseData {
   };
 }
 
+function isResponseData(data: unknown): data is ResponseData {
+  if (typeof data !== 'object' || data === null) {
+    return false;
+  }
+  const candidate = data as ResponseData;
+  if (!('error' in candidate)) {
+    return false;
+  }
+  const error = candidate.error;
+  if (typeof error !== 'object' || error === null) {
+    return false; // error property exists but is not an object (could be undefined, but we checked 'in')
+  }
+
+  // Optional properties check
+  if (
+    'code' in error &&
+    typeof error.code !== 'number' &&
+    error.code !== undefined
+  ) {
+    return false;
+  }
+  if (
+    'message' in error &&
+    typeof error.message !== 'string' &&
+    error.message !== undefined
+  ) {
+    return false;
+  }
+
+  return true;
+}
+
 export function toFriendlyError(error: unknown): unknown {
-  if (error && typeof error === 'object' && 'response' in error) {
-    // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-    const gaxiosError = error as GaxiosError;
-    const data = parseResponseData(gaxiosError);
+  if (isGaxiosError(error)) {
+    const data = parseResponseData(error);
     if (data && data.error && data.error.message && data.error.code) {
       switch (data.error.code) {
         case 400:
@@ -129,17 +169,20 @@ export function toFriendlyError(error: unknown): unknown {
 }
 
 function parseResponseData(error: GaxiosError): ResponseData | undefined {
+  let data = error.response?.data;
   // Inexplicably, Gaxios sometimes doesn't JSONify the response data.
-  if (typeof error.response?.data === 'string') {
+  if (typeof data === 'string') {
     try {
-      // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-      return JSON.parse(error.response?.data) as ResponseData;
+      data = JSON.parse(data);
     } catch {
       return undefined;
     }
   }
-  // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-  return error.response?.data as ResponseData | undefined;
+
+  if (isResponseData(data)) {
+    return data;
+  }
+  return undefined;
 }
 
 /**
@@ -152,8 +195,15 @@ function parseResponseData(error: GaxiosError): ResponseData | undefined {
 export function isAuthenticationError(error: unknown): boolean {
   // Check for MCP SDK errors with code property
   // (SseError and StreamableHTTPError both have numeric 'code' property)
-  if (error && typeof error === 'object' && 'code' in error) {
-    const errorCode = (error as { code: unknown }).code;
+  if (
+    error &&
+    typeof error === 'object' &&
+    'code' in error &&
+    typeof (error as { code: unknown }).code === 'number'
+  ) {
+    // Safe access after check
+    // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
+    const errorCode = (error as { code: number }).code;
     if (errorCode === 401) {
       return true;
     }
diff --git a/packages/core/src/utils/googleErrors.ts b/packages/core/src/utils/googleErrors.ts
index 3ae74f0de8..88f78e35d6 100644
--- a/packages/core/src/utils/googleErrors.ts
+++ b/packages/core/src/utils/googleErrors.ts
@@ -207,9 +207,13 @@ export function parseGoogleApiError(error: unknown): GoogleApiError | null {
               detailObj['@type'] = detailObj[typeKey];
               delete detailObj[typeKey];
             }
-            // We can just cast it; the consumer will have to switch on @type
-            // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-            details.push(detailObj as unknown as GoogleApiErrorDetail);
+            // Basic structural check before casting.
+            // Since the proto definitions are loose, we primarily rely on @type presence.
+            if (typeof detailObj['@type'] === 'string') {
+              // We can just cast it; the consumer will have to switch on @type
+              // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
+              details.push(detailObj as unknown as GoogleApiErrorDetail);
+            }
           }
         }
       }
@@ -225,6 +229,16 @@ export function parseGoogleApiError(error: unknown): GoogleApiError | null {
   return null;
 }
 
+function isErrorShape(obj: unknown): obj is ErrorShape {
+  return (
+    typeof obj === 'object' &&
+    obj !== null &&
+    (('message' in obj &&
+      typeof (obj as { message: unknown }).message === 'string') ||
+      ('code' in obj && typeof (obj as { code: unknown }).code === 'number'))
+  );
+}
+
 function fromGaxiosError(errorObj: object): ErrorShape | undefined {
   const gaxiosError = errorObj as {
     response?: {
@@ -260,7 +274,10 @@ function fromGaxiosError(errorObj: object): ErrorShape | undefined {
     if (typeof data === 'object' && data !== null) {
       if ('error' in data) {
         // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-        outerError = (data as { error: ErrorShape }).error;
+        const potentialError = (data as { error: unknown }).error;
+        if (isErrorShape(potentialError)) {
+          outerError = potentialError;
+        }
       }
     }
   }
@@ -320,7 +337,10 @@ function fromApiError(errorObj: object): ErrorShape | undefined {
     if (typeof data === 'object' && data !== null) {
       if ('error' in data) {
         // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
-        outerError = (data as { error: ErrorShape }).error;
+        const potentialError = (data as { error: unknown }).error;
+        if (isErrorShape(potentialError)) {
+          outerError = potentialError;
+        }
       }
     }
   }