diff --git a/src/agents/tools/sessions-spawn-tool.test.ts b/src/agents/tools/sessions-spawn-tool.test.ts
index d452ec23a32..bd67d8eab7d 100644
--- a/src/agents/tools/sessions-spawn-tool.test.ts
+++ b/src/agents/tools/sessions-spawn-tool.test.ts
@@ -446,6 +446,24 @@ describe("sessions_spawn tool", () => {
     expect(spawnArgs.runTimeoutSeconds).toBe(2);
   });
 
+  it.each([
+    [{ runTimeoutSeconds: 1.5 }, "runTimeoutSeconds must be a non-negative integer"],
+    [{ runTimeoutSeconds: -1 }, "runTimeoutSeconds must be a non-negative integer"],
+    [{ timeoutSeconds: "1sec" }, "timeoutSeconds must be a non-negative integer"],
+  ])("rejects invalid timeout override %o", async (params, message) => {
+    const tool = createSessionsSpawnTool({
+      agentSessionKey: "agent:main:main",
+    });
+
+    await expect(
+      tool.execute("call-invalid-timeout", {
+        task: "do thing",
+        ...params,
+      }),
+    ).rejects.toThrow(message);
+    expect(hoisted.spawnSubagentDirectMock).not.toHaveBeenCalled();
+  });
+
   it("passes inherited workspaceDir from tool context, not from tool args", async () => {
     const tool = createSessionsSpawnTool({
       agentSessionKey: "agent:main:main",
diff --git a/src/agents/tools/sessions-spawn-tool.ts b/src/agents/tools/sessions-spawn-tool.ts
index a0fb89387d3..fbafe150a3a 100644
--- a/src/agents/tools/sessions-spawn-tool.ts
+++ b/src/agents/tools/sessions-spawn-tool.ts
@@ -35,6 +35,7 @@ import type { AnyAgentTool } from "./common.js";
 import {
   jsonResult,
   normalizeToolModelOverride,
+  readNonNegativeIntegerParam,
   readStringParam,
   ToolInputError,
 } from "./common.js";
@@ -169,9 +170,9 @@ function createSessionsSpawnToolSchema(params: {
     model: Type.Optional(Type.String()),
     thinking: Type.Optional(Type.String()),
     cwd: Type.Optional(Type.String()),
-    runTimeoutSeconds: Type.Optional(Type.Number({ minimum: 0 })),
+    runTimeoutSeconds: Type.Optional(Type.Integer({ minimum: 0 })),
     // Back-compat: older callers used timeoutSeconds for this tool.
-    timeoutSeconds: Type.Optional(Type.Number({ minimum: 0 })),
+    timeoutSeconds: Type.Optional(Type.Integer({ minimum: 0 })),
     ...(params.threadAvailable
       ? {
           thread: Type.Optional(
@@ -342,17 +343,10 @@ export function createSessionsSpawnTool(
       if (runtime === "acp" && context === "fork") {
         throw new Error('context="fork" is only supported for runtime="subagent".');
       }
-      // Back-compat: older callers used timeoutSeconds for this tool.
-      const timeoutSecondsCandidate =
-        typeof params.runTimeoutSeconds === "number"
-          ? params.runTimeoutSeconds
-          : typeof params.timeoutSeconds === "number"
-            ? params.timeoutSeconds
-            : undefined;
       const runTimeoutSeconds =
-        typeof timeoutSecondsCandidate === "number" && Number.isFinite(timeoutSecondsCandidate)
-          ? Math.max(0, Math.floor(timeoutSecondsCandidate))
-          : undefined;
+        readNonNegativeIntegerParam(params, "runTimeoutSeconds") ??
+        // Back-compat: older callers used timeoutSeconds for this tool.
+        readNonNegativeIntegerParam(params, "timeoutSeconds");
       const thread = params.thread === true;
       const attachments = Array.isArray(params.attachments)
         ? (params.attachments as Array<{