diff --git a/packages/core/src/core/loggingContentGenerator.test.ts b/packages/core/src/core/loggingContentGenerator.test.ts
index 2b8249d539..7f3b1a9f33 100644
--- a/packages/core/src/core/loggingContentGenerator.test.ts
+++ b/packages/core/src/core/loggingContentGenerator.test.ts
@@ -315,6 +315,100 @@ describe('LoggingContentGenerator', () => {
           return true;
         });
       });
+
+      it('should decode Uint8Array data in Gaxios errors', async () => {
+        const req = { contents: [], model: 'gemini-pro' };
+
+        const gaxiosError = Object.assign(new Error('Gaxios Error'), {
+          response: { data: new Uint8Array([72, 101, 108, 108, 111]) },
+        });
+
+        vi.mocked(wrapped.generateContent).mockRejectedValue(gaxiosError);
+
+        await expect(
+          loggingContentGenerator.generateContent(
+            req,
+            'prompt-123',
+            LlmRole.MAIN,
+          ),
+        ).rejects.toSatisfy((error: unknown) => {
+          const gError = error as { response: { data: unknown } };
+          expect(gError.response.data).toBe('Hello');
+          return true;
+        });
+      });
+
+      it('should decode multi-byte UTF-8 from comma-separated byte strings', async () => {
+        const req = { contents: [], model: 'gemini-pro' };
+
+        // "Héllo" in UTF-8 bytes: H=72, é=195,169, l=108, l=108, o=111
+        const utf8Data = '72,195,169,108,108,111';
+        const gaxiosError = Object.assign(new Error('Gaxios Error'), {
+          response: { data: utf8Data },
+        });
+
+        vi.mocked(wrapped.generateContent).mockRejectedValue(gaxiosError);
+
+        await expect(
+          loggingContentGenerator.generateContent(
+            req,
+            'prompt-123',
+            LlmRole.MAIN,
+          ),
+        ).rejects.toSatisfy((error: unknown) => {
+          const gError = error as { response: { data: unknown } };
+          expect(gError.response.data).toBe('Héllo');
+          return true;
+        });
+      });
+
+      it('should decode 3-byte UTF-8 from comma-separated byte strings', async () => {
+        const req = { contents: [], model: 'gemini-pro' };
+
+        // "こんにちは" in UTF-8 bytes (3 bytes per character)
+        const utf8Data =
+          '227,129,147,227,130,147,227,129,171,227,129,161,227,129,175';
+        const gaxiosError = Object.assign(new Error('Gaxios Error'), {
+          response: { data: utf8Data },
+        });
+
+        vi.mocked(wrapped.generateContent).mockRejectedValue(gaxiosError);
+
+        await expect(
+          loggingContentGenerator.generateContent(
+            req,
+            'prompt-123',
+            LlmRole.MAIN,
+          ),
+        ).rejects.toSatisfy((error: unknown) => {
+          const gError = error as { response: { data: unknown } };
+          expect(gError.response.data).toBe('こんにちは');
+          return true;
+        });
+      });
+
+      it('should reject byte strings with values outside 0-255 range', async () => {
+        const req = { contents: [], model: 'gemini-pro' };
+
+        const outOfRange = '72,256,108';
+        const gaxiosError = Object.assign(new Error('Gaxios Error'), {
+          response: { data: outOfRange },
+        });
+
+        vi.mocked(wrapped.generateContent).mockRejectedValue(gaxiosError);
+
+        await expect(
+          loggingContentGenerator.generateContent(
+            req,
+            'prompt-123',
+            LlmRole.MAIN,
+          ),
+        ).rejects.toSatisfy((error: unknown) => {
+          const gError = error as { response: { data: unknown } };
+          expect(gError.response.data).toBe(outOfRange);
+          return true;
+        });
+      });
     });
 
     it('should NOT log error on AbortError (user cancellation)', async () => {
diff --git a/packages/core/src/core/loggingContentGenerator.ts b/packages/core/src/core/loggingContentGenerator.ts
index 027a7ae622..1c8579df9a 100644
--- a/packages/core/src/core/loggingContentGenerator.ts
+++ b/packages/core/src/core/loggingContentGenerator.ts
@@ -276,8 +276,10 @@ export class LoggingContentGenerator implements ContentGenerator {
   }
 
   private _fixGaxiosErrorData(error: unknown): void {
-    // Fix for raw ASCII buffer strings appearing in dev with the latest
-    // Gaxios updates.
+    // Fix for raw buffer data appearing in Gaxios errors.
+    // Gaxios may return the response body as a Uint8Array, a Buffer, or
+    // a string of comma-separated byte values (e.g. "72,101,108,108,111").
+    // All three forms need to be decoded as UTF-8.
     if (
       typeof error === 'object' &&
       error !== null &&
@@ -288,11 +290,20 @@ export class LoggingContentGenerator implements ContentGenerator {
     ) {
       const response = error.response as { data: unknown };
       const data = response.data;
-      if (typeof data === 'string' && data.includes(',')) {
+
+      if (data instanceof Uint8Array) {
+        // Gaxios returned raw bytes directly
+        response.data = new TextDecoder().decode(data);
+      } else if (typeof data === 'string' && data.includes(',')) {
+        // Gaxios returned bytes as a comma-separated string
         try {
-          const charCodes = data.split(',').map(Number);
-          if (charCodes.every((code) => !isNaN(code))) {
-            response.data = String.fromCharCode(...charCodes);
+          const byteValues = data.split(',').map(Number);
+          if (
+            byteValues.every((b) => Number.isInteger(b) && b >= 0 && b <= 255)
+          ) {
+            response.data = new TextDecoder().decode(
+              new Uint8Array(byteValues),
+            );
           }
         } catch {
           // If parsing fails, just leave it alone