diff --git a/scripts/resolve-upgrade-survivor-baselines.mjs b/scripts/resolve-upgrade-survivor-baselines.mjs
index 5636e694ed8..9325959fa32 100644
--- a/scripts/resolve-upgrade-survivor-baselines.mjs
+++ b/scripts/resolve-upgrade-survivor-baselines.mjs
@@ -33,6 +33,18 @@ function dedupeSpecs(specs) {
   return [...new Set(specs.map(normalizeUpgradeSurvivorBaselineSpec).filter(Boolean))];
 }
 
+function parsePositiveInteger(value, label) {
+  const text = String(value ?? "").trim();
+  if (!/^[1-9]\d*$/u.test(text)) {
+    throw new Error(`${label} must be a positive integer`);
+  }
+  const parsed = Number(text);
+  if (!Number.isSafeInteger(parsed)) {
+    throw new Error(`${label} must be a positive integer`);
+  }
+  return parsed;
+}
+
 function readPublishedVersions(file) {
   if (!file) {
     return undefined;
@@ -111,10 +123,7 @@ export function resolveReleaseHistory(args) {
   if (!releasesJson) {
     throw new Error("--releases-json is required when requested baselines include release-history");
   }
-  const historyCount = Number.parseInt(args.get("history-count") ?? "6", 10);
-  if (!Number.isInteger(historyCount) || historyCount < 1) {
-    throw new Error("--history-count must be a positive integer");
-  }
+  const historyCount = parsePositiveInteger(args.get("history-count") ?? "6", "--history-count");
   const includeVersion = args.get("include-version") ?? "2026.4.23";
   const preDate = args.get("pre-date") ?? "2026-03-15T00:00:00Z";
   const publishedVersions = readPublishedVersions(args.get("npm-versions-json"));
@@ -181,7 +190,10 @@ export function resolveBaselines(args) {
     if (token === "release-history") {
       resolved.push(...resolveReleaseHistory(args));
     } else if (token.startsWith("last-stable-")) {
-      const count = Number.parseInt(token.slice("last-stable-".length), 10);
+      const count = parsePositiveInteger(
+        token.slice("last-stable-".length),
+        "last-stable baseline count",
+      );
       resolved.push(...resolveLastStable(args, count));
     } else if (token.startsWith("all-since-")) {
       const minimumVersion = token.slice("all-since-".length);
diff --git a/test/scripts/upgrade-survivor-baselines.test.ts b/test/scripts/upgrade-survivor-baselines.test.ts
index 801787ccdc2..09bc5b1e4c1 100644
--- a/test/scripts/upgrade-survivor-baselines.test.ts
+++ b/test/scripts/upgrade-survivor-baselines.test.ts
@@ -159,6 +159,33 @@ describe("scripts/resolve-upgrade-survivor-baselines", () => {
     });
   });
 
+  it("rejects loose release-history count values", () => {
+    withReleaseFixture([], (file) => {
+      expect(() =>
+        resolveBaselines(
+          new Map([
+            ["requested", "release-history"],
+            ["releases-json", file],
+            ["history-count", "1e3"],
+          ]),
+        ),
+      ).toThrow("--history-count must be a positive integer");
+    });
+  });
+
+  it("rejects loose last-stable count tokens", () => {
+    withReleaseFixture([], (file) => {
+      expect(() =>
+        resolveBaselines(
+          new Map([
+            ["requested", "last-stable-1e3"],
+            ["releases-json", file],
+          ]),
+        ),
+      ).toThrow("last-stable baseline count must be a positive integer");
+    });
+  });
+
   it("maps release-history anchors to npm-published package versions when GitHub tags have republish suffixes", () => {
     const releases = (
       [