diff --git a/actions/setup/js/action_setup_otlp.cjs b/actions/setup/js/action_setup_otlp.cjs
index 3de1e20783..5af21e42e8 100644
--- a/actions/setup/js/action_setup_otlp.cjs
+++ b/actions/setup/js/action_setup_otlp.cjs
@@ -44,8 +44,9 @@ async function run() {
 
   const { sendJobSetupSpan, isValidTraceId, isValidSpanId } = require(path.join(__dirname, "send_otlp_span.cjs"));
 
-  const parsedStartMs = parseInt(process.env.SETUP_START_MS || "0", 10);
-  const startMs = Number.isFinite(parsedStartMs) ? parsedStartMs : 0;
+  const rawStartMs = process.env.SETUP_START_MS;
+  const parsedMs = /^\d+$/.test(rawStartMs ?? "") ? Number(rawStartMs) : NaN;
+  const startMs = Number.isSafeInteger(parsedMs) ? parsedMs : 0;
 
   // Explicitly read INPUT_TRACE_ID and pass it as options.traceId so the
   // activation job's trace ID is used even when process.env propagation
diff --git a/actions/setup/js/action_setup_otlp.test.cjs b/actions/setup/js/action_setup_otlp.test.cjs
index ffdf3e20ef..6527246f4e 100644
--- a/actions/setup/js/action_setup_otlp.test.cjs
+++ b/actions/setup/js/action_setup_otlp.test.cjs
@@ -164,6 +164,30 @@ describe("action_setup_otlp.cjs", () => {
 
       expect(mockSendJobSetupSpan).toHaveBeenCalledWith(expect.objectContaining({ startMs: 0 }));
     });
+
+    it("should pass startMs=0 when SETUP_START_MS is a partial number string like '123abc'", async () => {
+      process.env.SETUP_START_MS = "123abc";
+
+      await run();
+
+      expect(mockSendJobSetupSpan).toHaveBeenCalledWith(expect.objectContaining({ startMs: 0 }));
+    });
+
+    it("should pass startMs=0 when SETUP_START_MS is scientific notation like '1e3'", async () => {
+      process.env.SETUP_START_MS = "1e3";
+
+      await run();
+
+      expect(mockSendJobSetupSpan).toHaveBeenCalledWith(expect.objectContaining({ startMs: 0 }));
+    });
+
+    it("should pass startMs=0 when SETUP_START_MS is hex notation like '0x10'", async () => {
+      process.env.SETUP_START_MS = "0x10";
+
+      await run();
+
+      expect(mockSendJobSetupSpan).toHaveBeenCalledWith(expect.objectContaining({ startMs: 0 }));
+    });
   });
 
   describe("INPUT_TRACE_ID handling", () => {