diff --git a/src/utils/websearch/claude-tool-args.ts b/src/utils/websearch/claude-tool-args.ts
index 8e9b7584..62de05ce 100644
--- a/src/utils/websearch/claude-tool-args.ts
+++ b/src/utils/websearch/claude-tool-args.ts
@@ -12,6 +12,14 @@ function parseToolValue(rawValue: string): string[] {
     .filter((value) => value.length > 0);
 }
 
+function mergeToolValues(rawValues: string[], toolName: string): string {
+  const merged = rawValues.flatMap(parseToolValue);
+  if (!merged.includes(toolName)) {
+    merged.push(toolName);
+  }
+  return merged.join(',');
+}
+
 function hasToolInFlag(args: string[], flag: string, toolName: string): boolean {
   for (let index = 0; index < args.length; index += 1) {
     const arg = args[index];
@@ -47,5 +55,34 @@ export function appendThirdPartyWebSearchToolArgs(args: string[]): string[] {
     return args;
   }
 
+  for (let index = 0; index < args.length; index += 1) {
+    const arg = args[index];
+
+    if (arg === DISALLOWED_TOOLS_FLAG) {
+      let cursor = index + 1;
+      const rawValues: string[] = [];
+
+      while (cursor < args.length && !args[cursor].startsWith('--')) {
+        rawValues.push(args[cursor]);
+        cursor += 1;
+      }
+
+      return [
+        ...args.slice(0, index + 1),
+        mergeToolValues(rawValues, NATIVE_WEBSEARCH_TOOL),
+        ...args.slice(cursor),
+      ];
+    }
+
+    if (arg.startsWith(`${DISALLOWED_TOOLS_FLAG}=`)) {
+      const rawValue = arg.slice(DISALLOWED_TOOLS_FLAG.length + 1);
+      return [
+        ...args.slice(0, index),
+        `${DISALLOWED_TOOLS_FLAG}=${mergeToolValues([rawValue], NATIVE_WEBSEARCH_TOOL)}`,
+        ...args.slice(index + 1),
+      ];
+    }
+  }
+
   return [...args, DISALLOWED_TOOLS_FLAG, NATIVE_WEBSEARCH_TOOL];
 }
diff --git a/tests/unit/utils/websearch/claude-tool-args.test.ts b/tests/unit/utils/websearch/claude-tool-args.test.ts
index e96d4de0..6eb2367c 100644
--- a/tests/unit/utils/websearch/claude-tool-args.test.ts
+++ b/tests/unit/utils/websearch/claude-tool-args.test.ts
@@ -21,4 +21,19 @@ describe('appendThirdPartyWebSearchToolArgs', () => {
       ['smoke', '--disallowedTools=Read,WebSearch']
     );
   });
+
+  it('merges WebSearch into an existing space-separated disallowed tool flag', () => {
+    expect(appendThirdPartyWebSearchToolArgs(['smoke', '--disallowedTools', 'Read'])).toEqual([
+      'smoke',
+      '--disallowedTools',
+      'Read,WebSearch',
+    ]);
+  });
+
+  it('merges WebSearch into an existing equals-form disallowed tool flag', () => {
+    expect(appendThirdPartyWebSearchToolArgs(['smoke', '--disallowedTools=Read'])).toEqual([
+      'smoke',
+      '--disallowedTools=Read,WebSearch',
+    ]);
+  });
 });