diff --git a/app/api/affiliate/route.ts b/app/api/affiliate/route.ts
index cc1e3e7..bb34973 100644
--- a/app/api/affiliate/route.ts
+++ b/app/api/affiliate/route.ts
@@ -1,6 +1,7 @@
 import { NextRequest, NextResponse } from "next/server";
 import { readSession, authConfigured, SESSION_COOKIE } from "@/lib/session";
 import { getAffiliate, enrollAffiliate, setAffiliateCommission, listReferrals, AFFILIATE_FLOOR, findOrCreateAccountByEmail } from "@/lib/db";
+import { parseCommissionPercent } from "@/lib/affiliate";
 
 export const runtime = "nodejs";
 export const dynamic = "force-dynamic";
@@ -54,7 +55,9 @@ export async function POST(req: NextRequest) {
     if (aff.plan !== "paid") {
       return NextResponse.json({ error: `Free plan is floored at ${AFFILIATE_FLOOR}%. Upgrade to $1/mo to lower it.` }, { status: 403 });
     }
-    await setAffiliateCommission(id, Number(body.commission_pct));
+    const pct = parseCommissionPercent(body.commission_pct);
+    if (pct == null) return NextResponse.json({ error: "Enter a valid commission percentage." }, { status: 400 });
+    await setAffiliateCommission(id, pct);
   } else {
     // Default action: enroll (idempotent).
     await enrollAffiliate(id);
diff --git a/lib/affiliate.ts b/lib/affiliate.ts
new file mode 100644
index 0000000..2292a65
--- /dev/null
+++ b/lib/affiliate.ts
@@ -0,0 +1,10 @@
+const COMMISSION_RE = /^(?:\d+)(?:\.\d{1,2})?$/;
+
+export function parseCommissionPercent(value: unknown): number | null {
+  if (typeof value === "number") return Number.isFinite(value) ? value : null;
+  if (typeof value !== "string") return null;
+  const text = value.trim();
+  if (!COMMISSION_RE.test(text)) return null;
+  const pct = Number(text);
+  return Number.isFinite(pct) ? pct : null;
+}
diff --git a/tests/affiliate-commission-input.test.mjs b/tests/affiliate-commission-input.test.mjs
new file mode 100644
index 0000000..06ad3fb
--- /dev/null
+++ b/tests/affiliate-commission-input.test.mjs
@@ -0,0 +1,20 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import { parseCommissionPercent } from "../lib/affiliate.ts";
+
+test("parseCommissionPercent accepts finite numbers and decimal strings", () => {
+  assert.equal(parseCommissionPercent(25), 25);
+  assert.equal(parseCommissionPercent("80"), 80);
+  assert.equal(parseCommissionPercent(" 12.5 "), 12.5);
+});
+
+test("parseCommissionPercent rejects coerced or malformed values", () => {
+  assert.equal(parseCommissionPercent("1e2"), null);
+  assert.equal(parseCommissionPercent("0x10"), null);
+  assert.equal(parseCommissionPercent("80abc"), null);
+  assert.equal(parseCommissionPercent(["80"]), null);
+  assert.equal(parseCommissionPercent({ valueOf: () => 80 }), null);
+  assert.equal(parseCommissionPercent(Number.NaN), null);
+  assert.equal(parseCommissionPercent(Number.POSITIVE_INFINITY), null);
+});