diff --git a/packages/core/src/tools/agent/agent.test.ts b/packages/core/src/tools/agent/agent.test.ts
index caf17360d57..0f1c57efcf7 100644
--- a/packages/core/src/tools/agent/agent.test.ts
+++ b/packages/core/src/tools/agent/agent.test.ts
@@ -898,6 +898,16 @@ describe('AgentTool', () => {
       ).toMatch(/fork/i);
     });
 
+    it('rejects a non-boolean run_in_background value', () => {
+      expect(
+        agentTool.validateToolParams({
+          ...validParams,
+          // @ts-expect-error: raw model parameters are untrusted
+          run_in_background: 'true',
+        }),
+      ).toMatch(/run_in_background.*boolean/i);
+    });
+
     it('rejects working_dir combined with run_in_background', () => {
       expect(
         agentTool.validateToolParams({
diff --git a/packages/core/src/tools/agent/agent.ts b/packages/core/src/tools/agent/agent.ts
index 003a8ae4bb5..ca1148b4fb2 100644
--- a/packages/core/src/tools/agent/agent.ts
+++ b/packages/core/src/tools/agent/agent.ts
@@ -1007,6 +1007,13 @@ assistant: Uses the ${ToolNames.AGENT} tool to launch the test-runner agent
       return 'Parameter "prompt" must be a non-empty string.';
     }
 
+    if (
+      params.run_in_background !== undefined &&
+      typeof params.run_in_background !== 'boolean'
+    ) {
+      return 'Parameter "run_in_background" must be a boolean when set.';
+    }
+
     if (params.subagent_type !== undefined) {
       if (
         typeof params.subagent_type !== 'string' ||