diff --git a/server/test/unit/validation/monitorValidation.test.ts b/server/test/unit/validation/monitorValidation.test.ts
index c08b8be84..08f516f99 100644
--- a/server/test/unit/validation/monitorValidation.test.ts
+++ b/server/test/unit/validation/monitorValidation.test.ts
@@ -243,6 +243,14 @@ describe("monitorValidation — customUpCodes", () => {
 			expect(parsed.customUpCodes).toEqual([200, 301, 404, 503]);
 		});
 
+		it("accepts non-standard HTTP status codes (Cloudflare, AWS ELB, etc.)", () => {
+			const parsed = createMonitorBodyValidation.parse({
+				...baseHttpBody,
+				customUpCodes: [419, 420, 440, 449, 460, 463, 497, 499, 509, 520, 521, 522, 523, 524, 525, 526, 527, 529, 530, 561],
+			});
+			expect(parsed.customUpCodes).toEqual([419, 420, 440, 449, 460, 463, 497, 499, 509, 520, 521, 522, 523, 524, 525, 526, 527, 529, 530, 561]);
+		});
+
 		it("rejects invalid HTTP status codes", () => {
 			expect(() =>
 				createMonitorBodyValidation.parse({
@@ -259,6 +267,33 @@ describe("monitorValidation — customUpCodes", () => {
 			).toThrow();
 		});
 
+		it("rejects fractional or floating-point status codes", () => {
+			expect(() =>
+				createMonitorBodyValidation.parse({
+					...baseHttpBody,
+					customUpCodes: [200.5],
+				})
+			).toThrow();
+		});
+
+		it("rejects status codes below 100", () => {
+			expect(() =>
+				createMonitorBodyValidation.parse({
+					...baseHttpBody,
+					customUpCodes: [99],
+				})
+			).toThrow();
+		});
+
+		it("rejects valid-looking but mathematically unsupported status codes (e.g. 599)", () => {
+			expect(() =>
+				createMonitorBodyValidation.parse({
+					...baseHttpBody,
+					customUpCodes: [599], // Not in Node's default list nor our expanded list
+				})
+			).toThrow();
+		});
+
 		it("defaults to an empty array when not provided", () => {
 			const parsed = createMonitorBodyValidation.parse(baseHttpBody);
 			expect(parsed.customUpCodes).toEqual([]);
@@ -273,6 +308,13 @@ describe("monitorValidation — customUpCodes", () => {
 			expect(parsed.customUpCodes).toEqual([200, 201]);
 		});
 
+		it("accepts non-standard HTTP status codes on edits", () => {
+			const parsed = editMonitorBodyValidation.parse({
+				customUpCodes: [419, 420, 440, 449, 460, 463, 497, 499, 509, 520, 521, 522, 523, 524, 525, 526, 527, 529, 530, 561],
+			});
+			expect(parsed.customUpCodes).toEqual([419, 420, 440, 449, 460, 463, 497, 499, 509, 520, 521, 522, 523, 524, 525, 526, 527, 529, 530, 561]);
+		});
+
 		it("rejects invalid HTTP status codes on edits", () => {
 			expect(() =>
 				editMonitorBodyValidation.parse({
@@ -295,6 +337,20 @@ describe("monitorValidation — customUpCodes", () => {
 			expect(parsed.monitors[0].customUpCodes).toEqual([404]);
 		});
 
+		it("accepts non-standard HTTP status codes on imported HTTP monitors", () => {
+			const parsed = importMonitorsBodyValidation.parse({
+				monitors: [
+					{
+						...baseHttpBody,
+						customUpCodes: [419, 420, 440, 449, 460, 463, 497, 499, 509, 520, 521, 522, 523, 524, 525, 526, 527, 529, 530, 561],
+					},
+				],
+			});
+			expect(parsed.monitors[0].customUpCodes).toEqual([
+				419, 420, 440, 449, 460, 463, 497, 499, 509, 520, 521, 522, 523, 524, 525, 526, 527, 529, 530, 561,
+			]);
+		});
+
 		it("rejects invalid HTTP status codes on imported HTTP monitors", () => {
 			expect(() =>
 				importMonitorsBodyValidation.parse({