diff --git a/packages/core/src/tools/web-search.test.ts b/packages/core/src/tools/web-search.test.ts
index 0f7c1d6c..54c4b4b3 100644
--- a/packages/core/src/tools/web-search.test.ts
+++ b/packages/core/src/tools/web-search.test.ts
@@ -173,5 +173,77 @@ Sources:
       );
       expect(result.sources).toHaveLength(2);
     });
+
+    it('should insert markers at correct byte positions for multibyte text', async () => {
+      const params: WebSearchToolParams = { query: 'multibyte query' };
+      (mockGeminiClient.generateContent as Mock).mockResolvedValue({
+        candidates: [
+          {
+            content: {
+              role: 'model',
+              parts: [{ text: 'こんにちは! Gemini CLI✨️' }],
+            },
+            groundingMetadata: {
+              groundingChunks: [
+                {
+                  web: {
+                    title: 'Japanese Greeting',
+                    uri: 'https://example.test/japanese-greeting',
+                  },
+                },
+                {
+                  web: {
+                    title: 'google-gemini/gemini-cli',
+                    uri: 'https://github.com/google-gemini/gemini-cli',
+                  },
+                },
+                {
+                  web: {
+                    title: 'Gemini CLI: your open-source AI agent',
+                    uri: 'https://blog.google/technology/developers/introducing-gemini-cli-open-source-ai-agent/',
+                  },
+                },
+              ],
+              groundingSupports: [
+                {
+                  segment: {
+                    // Byte range of "こんにちは!" (utf-8 encoded)
+                    startIndex: 0,
+                    endIndex: 16,
+                  },
+                  groundingChunkIndices: [0],
+                },
+                {
+                  segment: {
+                    // Byte range of "Gemini CLI✨️" (utf-8 encoded)
+                    startIndex: 17,
+                    endIndex: 33,
+                  },
+                  groundingChunkIndices: [1, 2],
+                },
+              ],
+            },
+          },
+        ],
+      });
+
+      const invocation = tool.build(params);
+      const result = await invocation.execute(abortSignal);
+
+      const expectedLlmContent = `Web search results for "multibyte query":
+
+こんにちは![1] Gemini CLI✨️[2][3]
+
+Sources:
+[1] Japanese Greeting (https://example.test/japanese-greeting)
+[2] google-gemini/gemini-cli (https://github.com/google-gemini/gemini-cli)
+[3] Gemini CLI: your open-source AI agent (https://blog.google/technology/developers/introducing-gemini-cli-open-source-ai-agent/)`;
+
+      expect(result.llmContent).toBe(expectedLlmContent);
+      expect(result.returnDisplay).toBe(
+        'Search results for "multibyte query" returned.',
+      );
+      expect(result.sources).toHaveLength(3);
+    });
   });
 });
diff --git a/packages/core/src/tools/web-search.ts b/packages/core/src/tools/web-search.ts
index 442fac4f..03404e0a 100644
--- a/packages/core/src/tools/web-search.ts
+++ b/packages/core/src/tools/web-search.ts
@@ -128,11 +128,28 @@ class WebSearchToolInvocation extends BaseToolInvocation<
           // Sort insertions by index in descending order to avoid shifting subsequent indices
           insertions.sort((a, b) => b.index - a.index);
 
-          const responseChars = modifiedResponseText.split(''); // Use new variable
-          insertions.forEach((insertion) => {
-            responseChars.splice(insertion.index, 0, insertion.marker);
-          });
-          modifiedResponseText = responseChars.join(''); // Assign back to modifiedResponseText
+          // Use TextEncoder/TextDecoder since segment indices are UTF-8 byte positions
+          const encoder = new TextEncoder();
+          const responseBytes = encoder.encode(modifiedResponseText);
+          const parts: Uint8Array[] = [];
+          let lastIndex = responseBytes.length;
+          for (const ins of insertions) {
+            const pos = Math.min(ins.index, lastIndex);
+            parts.unshift(responseBytes.subarray(pos, lastIndex));
+            parts.unshift(encoder.encode(ins.marker));
+            lastIndex = pos;
+          }
+          parts.unshift(responseBytes.subarray(0, lastIndex));
+
+          // Concatenate all parts into a single buffer
+          const totalLength = parts.reduce((sum, part) => sum + part.length, 0);
+          const finalBytes = new Uint8Array(totalLength);
+          let offset = 0;
+          for (const part of parts) {
+            finalBytes.set(part, offset);
+            offset += part.length;
+          }
+          modifiedResponseText = new TextDecoder().decode(finalBytes);
         }
 
         if (sourceListFormatted.length > 0) {