diff --git a/packages/core/src/tools/agent/agent.test.ts b/packages/core/src/tools/agent/agent.test.ts
index 3fdb9356ba7..65b1d9b00fe 100644
--- a/packages/core/src/tools/agent/agent.test.ts
+++ b/packages/core/src/tools/agent/agent.test.ts
@@ -635,22 +635,46 @@ describe('AgentTool', () => {
       ).toBeNull();
     });
 
-    it('rejects an empty working_dir', () => {
-      expect(
-        agentTool.validateToolParams({
-          ...validParams,
-          working_dir: '',
-        }),
-      ).toMatch(/working_dir/i);
+    it('treats an empty working_dir as unset', () => {
+      const params = {
+        ...validParams,
+        working_dir: '',
+      };
+
+      expect(agentTool.validateToolParams(params)).toBeNull();
+      expect(params.working_dir).toBeUndefined();
     });
 
-    it('rejects a whitespace-only working_dir', () => {
-      expect(
-        agentTool.validateToolParams({
-          ...validParams,
-          working_dir: '   ',
-        }),
-      ).toMatch(/working_dir/i);
+    it('treats a whitespace-only working_dir as unset', () => {
+      const params = {
+        ...validParams,
+        working_dir: '   ',
+      };
+
+      expect(agentTool.validateToolParams(params)).toBeNull();
+      expect(params.working_dir).toBeUndefined();
+    });
+
+    it('treats an empty working_dir as unset when isolation is set', () => {
+      const params = {
+        ...validParams,
+        isolation: 'worktree' as const,
+        working_dir: '',
+      };
+
+      expect(agentTool.validateToolParams(params)).toBeNull();
+      expect(params.working_dir).toBeUndefined();
+    });
+
+    it('treats a whitespace-only working_dir as unset when isolation is set', () => {
+      const params = {
+        ...validParams,
+        isolation: 'worktree' as const,
+        working_dir: '   ',
+      };
+
+      expect(agentTool.validateToolParams(params)).toBeNull();
+      expect(params.working_dir).toBeUndefined();
     });
 
     it('rejects working_dir combined with isolation', () => {
diff --git a/packages/core/src/tools/agent/agent.ts b/packages/core/src/tools/agent/agent.ts
index 685f547efb4..33e29ee0920 100644
--- a/packages/core/src/tools/agent/agent.ts
+++ b/packages/core/src/tools/agent/agent.ts
@@ -1028,6 +1028,15 @@ assistant: Uses the ${ToolNames.AGENT} tool to launch the test-runner agent
         }
       }
     }
+    // Some models emit an empty placeholder for the unused optional field.
+    // With isolation selected, normalize it away before downstream routing.
+    if (
+      (typeof params.working_dir === 'string' &&
+        params.working_dir.trim().length === 0) ||
+      params.working_dir === null
+    ) {
+      params.working_dir = undefined;
+    }
 
     if (params.isolation !== undefined) {
       if (params.isolation !== 'worktree') {