diff --git a/client/src/Pages/StatusPage/Status/themes/bold/styles.ts b/client/src/Pages/StatusPage/Status/themes/bold/styles.ts
index 86208d41b..197310a7f 100644
--- a/client/src/Pages/StatusPage/Status/themes/bold/styles.ts
+++ b/client/src/Pages/StatusPage/Status/themes/bold/styles.ts
@@ -1,5 +1,5 @@
 import type { SxProps, Theme } from "@mui/material/styles";
-import type { StatusPageThemeTokens } from "../tokens";
+import { mixSeverityColor, type StatusPageThemeTokens } from "../tokens";
 import { type OverallTone, toneColor, toneSoft } from "../shared/overallStatus";
 import { BOLD_SANS_STACK, MONO_STACK } from "../shared/fontStacks";
 import { MAX_RECENT_CHECKS } from "@/Types/Monitor";
@@ -34,9 +34,9 @@ export interface BoldStyles {
 	monitorUrl: SxProps<Theme>;
 	badge: (tone: OverallTone) => SxProps<Theme>;
 	heatmap: SxProps<Theme>;
-	heatmapCell: (kind: BoldHeatCell) => SxProps<Theme>;
+	heatmapCell: (kind: BoldHeatCell, severity?: number) => SxProps<Theme>;
 	histogram: SxProps<Theme>;
-	bar: (kind: BoldBarKind, heightPct: number) => SxProps<Theme>;
+	bar: (kind: BoldBarKind, heightPct: number, severity?: number) => SxProps<Theme>;
 	chartStats: SxProps<Theme>;
 	infra: SxProps<Theme>;
 	infraEmpty: SxProps<Theme>;
@@ -327,9 +327,12 @@ export const boldStyles = (
 			gap: { xs: "1px", md: "3px" },
 			height: 48,
 		},
-		heatmapCell: (kind) => ({
+		heatmapCell: (kind, severity = 1) => ({
 			borderRadius: "3px",
-			background: heatCellBg[kind],
+			background:
+				kind === "degraded"
+					? mixSeverityColor(tokens.up, tokens.degraded, severity)
+					: heatCellBg[kind],
 			boxShadow: heatCellShadow[kind],
 			opacity: kind === "empty" ? 0.4 : 1,
 			transition: "transform 0.15s",
@@ -344,8 +347,11 @@ export const boldStyles = (
 			alignItems: "flex-end",
 			height: 48,
 		},
-		bar: (kind, heightPct) => ({
-			background: barBg[kind],
+		bar: (kind, heightPct, severity = 1) => ({
+			background:
+				kind === "degraded"
+					? mixSeverityColor(tokens.up, tokens.degraded, severity)
+					: barBg[kind],
 			borderRadius: "3px",
 			minHeight: 4,
 			opacity: kind === "empty" ? 0.4 : 1,
diff --git a/client/src/Pages/StatusPage/Status/themes/editorial/styles.ts b/client/src/Pages/StatusPage/Status/themes/editorial/styles.ts
index 9de2c96fb..759f856a0 100644
--- a/client/src/Pages/StatusPage/Status/themes/editorial/styles.ts
+++ b/client/src/Pages/StatusPage/Status/themes/editorial/styles.ts
@@ -1,5 +1,5 @@
 import type { SxProps, Theme } from "@mui/material/styles";
-import type { StatusPageThemeTokens } from "../tokens";
+import { mixSeverityColor, type StatusPageThemeTokens } from "../tokens";
 import { type OverallTone, toneColor, toneSoft } from "../shared/overallStatus";
 import {
 	EDITORIAL_SECONDARY_SANS_STACK,
@@ -38,9 +38,9 @@ export interface EditorialStyles {
 	monitorUrl: SxProps<Theme>;
 	badge: (tone: OverallTone) => SxProps<Theme>;
 	heatmap: SxProps<Theme>;
-	heatmapCell: (kind: EditorialHeatCell) => SxProps<Theme>;
+	heatmapCell: (kind: EditorialHeatCell, severity?: number) => SxProps<Theme>;
 	histogram: SxProps<Theme>;
-	bar: (kind: EditorialBarKind, heightPct: number) => SxProps<Theme>;
+	bar: (kind: EditorialBarKind, heightPct: number, severity?: number) => SxProps<Theme>;
 	chartStats: SxProps<Theme>;
 	infra: SxProps<Theme>;
 	infraEmpty: SxProps<Theme>;
@@ -272,8 +272,11 @@ export const editorialStyles = (
 			gap: { xs: "1px", md: "2px" },
 			height: 40,
 		},
-		heatmapCell: (kind) => ({
-			background: heatCellBg[kind],
+		heatmapCell: (kind, severity = 1) => ({
+			background:
+				kind === "degraded"
+					? mixSeverityColor(tokens.up, tokens.degraded, severity)
+					: heatCellBg[kind],
 			opacity: kind === "empty" ? 0.5 : 1,
 		}),
 
@@ -285,8 +288,11 @@ export const editorialStyles = (
 			alignItems: "flex-end",
 			height: 40,
 		},
-		bar: (kind, heightPct) => ({
-			background: barBg[kind],
+		bar: (kind, heightPct, severity = 1) => ({
+			background:
+				kind === "degraded"
+					? mixSeverityColor(tokens.up, tokens.degraded, severity)
+					: barBg[kind],
 			minHeight: 3,
 			opacity: kind === "empty" ? 0.5 : 1,
 			height: `${heightPct}%`,
diff --git a/client/src/Pages/StatusPage/Status/themes/modern/styles.ts b/client/src/Pages/StatusPage/Status/themes/modern/styles.ts
index 7bb108f51..710da1777 100644
--- a/client/src/Pages/StatusPage/Status/themes/modern/styles.ts
+++ b/client/src/Pages/StatusPage/Status/themes/modern/styles.ts
@@ -1,6 +1,6 @@
 import type { SxProps, Theme } from "@mui/material/styles";
 import { keyframes } from "@mui/system";
-import type { StatusPageThemeTokens } from "../tokens";
+import { mixSeverityColor, type StatusPageThemeTokens } from "../tokens";
 import { type OverallTone, toneColor, toneSoft } from "../shared/overallStatus";
 import { MONO_STACK, SANS_STACK } from "../shared/fontStacks";
 import { MAX_RECENT_CHECKS } from "@/Types/Monitor";
@@ -37,9 +37,9 @@ export interface ModernStyles {
 	monitorUrl: SxProps<Theme>;
 	badge: (tone: OverallTone) => SxProps<Theme>;
 	heatmap: SxProps<Theme>;
-	heatmapCell: (kind: ModernHeatCell) => SxProps<Theme>;
+	heatmapCell: (kind: ModernHeatCell, severity?: number) => SxProps<Theme>;
 	histogram: SxProps<Theme>;
-	bar: (kind: ModernBarKind, heightPct: number) => SxProps<Theme>;
+	bar: (kind: ModernBarKind, heightPct: number, severity?: number) => SxProps<Theme>;
 	chartStats: SxProps<Theme>;
 	infra: SxProps<Theme>;
 	infraEmpty: SxProps<Theme>;
@@ -83,13 +83,14 @@ export const modernStyles = (
 ): ModernStyles => {
 	const cardShadow = isDark ? darkShadow : lightShadow;
 	const cardHoverShadow = isDark ? darkShadowHover : lightShadowHover;
+	const darken = (color: string) => `color-mix(in srgb, ${color} 70%, #000000 30%)`;
 
 	const heatCellBg: Record<ModernHeatCell, string> = {
 		fast: `linear-gradient(180deg, ${tokens.up}, ${tokens.upStrong})`,
 		med: `linear-gradient(180deg, color-mix(in srgb, ${tokens.up} 70%, #ffffff 30%), ${tokens.up})`,
 		slow: `linear-gradient(180deg, ${tokens.warn}, ${tokens.degraded})`,
-		degraded: `linear-gradient(180deg, ${tokens.degraded}, color-mix(in srgb, ${tokens.degraded} 70%, #000000 30%))`,
-		down: `linear-gradient(180deg, ${tokens.down}, color-mix(in srgb, ${tokens.down} 70%, #000000 30%))`,
+		degraded: `linear-gradient(180deg, ${tokens.degraded}, ${darken(tokens.degraded)})`,
+		down: `linear-gradient(180deg, ${tokens.down}, ${darken(tokens.down)})`,
 		empty: tokens.border,
 	};
 
@@ -103,7 +104,7 @@ export const modernStyles = (
 	const gaugeFillBg: Record<ModernGaugeFill, string> = {
 		ok: `linear-gradient(90deg, ${tokens.up}, ${tokens.upStrong})`,
 		warm: `linear-gradient(90deg, ${tokens.warn}, ${tokens.degraded})`,
-		hot: `linear-gradient(90deg, ${tokens.down}, color-mix(in srgb, ${tokens.down} 70%, #000000 30%))`,
+		hot: `linear-gradient(90deg, ${tokens.down}, color-mix(in srgb, ${tokens.down}, ${darken(tokens.down)})`,
 	};
 
 	return {
@@ -354,13 +355,19 @@ export const modernStyles = (
 			gap: { xs: "1px", md: "3px" },
 			height: 46,
 		},
-		heatmapCell: (kind) => ({
-			borderRadius: "3px",
-			background: heatCellBg[kind],
-			opacity: kind === "empty" ? 0.4 : 1,
-			transition: "transform 0.15s",
-			"&:hover": { transform: "scaleY(1.2)" },
-		}),
+		heatmapCell: (kind, severity = 1) => {
+			const mixed = mixSeverityColor(tokens.up, tokens.degraded, severity);
+			return {
+				borderRadius: "3px",
+				background:
+					kind === "degraded"
+						? `linear-gradient(180deg, ${mixed}, ${darken(mixed)})`
+						: heatCellBg[kind],
+				opacity: kind === "empty" ? 0.4 : 1,
+				transition: "transform 0.15s",
+				"&:hover": { transform: "scaleY(1.2)" },
+			};
+		},
 
 		histogram: {
 			padding: { xs: "0 12px", md: "0 24px" },
@@ -370,8 +377,11 @@ export const modernStyles = (
 			alignItems: "flex-end",
 			height: 46,
 		},
-		bar: (kind, heightPct) => ({
-			background: barBg[kind],
+		bar: (kind, heightPct, severity = 1) => ({
+			background:
+				kind === "degraded"
+					? mixSeverityColor(tokens.up, tokens.degraded, severity)
+					: barBg[kind],
 			borderRadius: "3px",
 			minHeight: 4,
 			opacity: kind === "empty" ? 0.4 : 1,
diff --git a/client/src/Pages/StatusPage/Status/themes/refined/styles.ts b/client/src/Pages/StatusPage/Status/themes/refined/styles.ts
index 38f972c95..2655ab555 100644
--- a/client/src/Pages/StatusPage/Status/themes/refined/styles.ts
+++ b/client/src/Pages/StatusPage/Status/themes/refined/styles.ts
@@ -1,5 +1,5 @@
 import type { SxProps, Theme } from "@mui/material/styles";
-import type { StatusPageThemeTokens } from "../tokens";
+import { mixSeverityColor, type StatusPageThemeTokens } from "../tokens";
 import { type OverallTone, toneColor, toneSoft } from "../shared/overallStatus";
 import { MONO_STACK, SANS_STACK } from "../shared/fontStacks";
 
@@ -37,9 +37,9 @@ export interface RefinedStyles {
 	monitorUrl: SxProps<Theme>;
 	badge: (tone: OverallTone) => SxProps<Theme>;
 	heatmap: SxProps<Theme>;
-	heatmapCell: (kind: RefinedHeatCell) => SxProps<Theme>;
+	heatmapCell: (kind: RefinedHeatCell, severity?: number) => SxProps<Theme>;
 	histogram: SxProps<Theme>;
-	bar: (kind: RefinedBarKind, heightPct: number) => SxProps<Theme>;
+	bar: (kind: RefinedBarKind, heightPct: number, severity?: number) => SxProps<Theme>;
 	chartStats: SxProps<Theme>;
 	infra: SxProps<Theme>;
 	infraEmpty: SxProps<Theme>;
@@ -296,9 +296,12 @@ export const refinedStyles = (
 			gap: { xs: "1px", md: "3px" },
 			height: 42,
 		},
-		heatmapCell: (kind) => ({
+		heatmapCell: (kind, severity = 1) => ({
 			borderRadius: "2px",
-			background: heatCellBg[kind],
+			background:
+				kind === "degraded"
+					? mixSeverityColor(tokens.up, tokens.degraded, severity)
+					: heatCellBg[kind],
 			opacity: kind === "empty" ? 0.4 : 1,
 			transition: "transform 0.15s",
 			"&:hover": { transform: "scaleY(1.15)" },
@@ -312,8 +315,11 @@ export const refinedStyles = (
 			alignItems: "flex-end",
 			height: 42,
 		},
-		bar: (kind, heightPct) => ({
-			background: barBg[kind],
+		bar: (kind, heightPct, severity = 1) => ({
+			background:
+				kind === "degraded"
+					? mixSeverityColor(tokens.up, tokens.degraded, severity)
+					: barBg[kind],
 			borderRadius: "2px",
 			minHeight: 3,
 			opacity: kind === "empty" ? 0.4 : 1,
diff --git a/client/src/Pages/StatusPage/Status/themes/shared/BaseStatusPage.tsx b/client/src/Pages/StatusPage/Status/themes/shared/BaseStatusPage.tsx
index cf08ca62f..807eed742 100644
--- a/client/src/Pages/StatusPage/Status/themes/shared/BaseStatusPage.tsx
+++ b/client/src/Pages/StatusPage/Status/themes/shared/BaseStatusPage.tsx
@@ -59,9 +59,9 @@ export interface BaseStyles {
 	monitorUrl: SxProps<Theme>;
 	badge: (tone: OverallTone) => SxProps<Theme>;
 	heatmap: SxProps<Theme>;
-	heatmapCell: (kind: HeatCellKind) => SxProps<Theme>;
+	heatmapCell: (kind: HeatCellKind, severity?: number) => SxProps<Theme>;
 	histogram: SxProps<Theme>;
-	bar: (kind: BarKind, heightPct: number) => SxProps<Theme>;
+	bar: (kind: BarKind, heightPct: number, severity?: number) => SxProps<Theme>;
 	chartStats: SxProps<Theme>;
 	infra: SxProps<Theme>;
 	infraEmpty: SxProps<Theme>;
diff --git a/client/src/Pages/StatusPage/Status/themes/shared/ChartCells.tsx b/client/src/Pages/StatusPage/Status/themes/shared/ChartCells.tsx
index e45d2e6b6..8d8a6a675 100644
--- a/client/src/Pages/StatusPage/Status/themes/shared/ChartCells.tsx
+++ b/client/src/Pages/StatusPage/Status/themes/shared/ChartCells.tsx
@@ -18,6 +18,7 @@ export type HeatCellKind = "fast" | "med" | "slow" | "degraded" | "down" | "empt
 
 export interface ChartCell {
 	key: string;
+	severity: number;
 	barKind: BarKind;
 	heatKind: HeatCellKind;
 	heightPct: number; // histogram bar height
@@ -28,9 +29,22 @@ export interface ChartCell {
 
 const DAY_DOWN_THRESHOLD = 0.5;
 
-const classifyDay = (bucket: DailyCheckBucket): BarKind => {
+// Color interpolation
+// 0  = perfect
+// 1 = at or below the down threshold
+const SEVERITY_FLOOR = 0.15; // A degraded server should never be green
+const SEVERITY_GAMMA = 2; // >1 biases toward green so high-uptime days stay visibly distinct from low ones
+
+export const severityFromUpRatio = (upRatio: number): number => {
+	const failed = 1 - upRatio;
+	if (failed <= 0) return 0;
+	const scaled = (Math.log10(failed) + 3) / (Math.log10(1 - DAY_DOWN_THRESHOLD) + 3);
+	const clamped = Math.min(1, Math.max(0, scaled));
+	return Math.max(SEVERITY_FLOOR, clamped ** SEVERITY_GAMMA);
+};
+
+const classifyDay = (bucket: DailyCheckBucket, upRatio: number): BarKind => {
 	if (bucket.totalChecks === 0) return "empty";
-	const upRatio = bucket.upChecks / bucket.totalChecks;
 	if (upRatio >= 1) return "up";
 	if (upRatio < DAY_DOWN_THRESHOLD) return "down";
 	return "degraded";
@@ -55,6 +69,7 @@ const emptyCell = (key: string): ChartCell => ({
 	responseTime: 0,
 	tooltip: null,
 	ariaLabel: "",
+	severity: 0,
 });
 
 const padCells = (cells: ChartCell[], length: number): ChartCell[] => [
@@ -76,6 +91,7 @@ export const checksToCells = (checks: CheckSnapshot[]): ChartCell[] => {
 			responseTime: check.responseTime ?? 0,
 			tooltip: <ThemedChartTooltip check={check} />,
 			ariaLabel: `${formatMs(check.responseTime)}, ${check.status ? "up" : "down"}`,
+			severity: 0,
 		})
 	);
 	return padCells(cells, MAX_RECENT_CHECKS);
@@ -93,9 +109,17 @@ export const dailyBucketsToCells = (
 	return enumerated.map((date) => {
 		const bucket = byDate.get(date);
 		if (!bucket) return emptyCell(`day-${date}`);
-		const barKind = classifyDay(bucket);
+		const upRatio = bucket.upChecks / bucket.totalChecks;
+
+		const barKind = classifyDay(bucket, upRatio);
 		return {
 			key: `day-${date}`,
+			severity:
+				barKind === "degraded"
+					? severityFromUpRatio(upRatio)
+					: barKind === "down"
+						? 1
+						: 0,
 			barKind,
 			heatKind:
 				barKind === "up"
diff --git a/client/src/Pages/StatusPage/Status/themes/shared/ThemedHeatmap.tsx b/client/src/Pages/StatusPage/Status/themes/shared/ThemedHeatmap.tsx
index 390f70667..7ddc7424d 100644
--- a/client/src/Pages/StatusPage/Status/themes/shared/ThemedHeatmap.tsx
+++ b/client/src/Pages/StatusPage/Status/themes/shared/ThemedHeatmap.tsx
@@ -10,7 +10,7 @@ import type {
 interface Props {
 	cells: ChartCell[];
 	containerSx: SxProps<Theme>;
-	cellSx: (kind: HeatCellKind) => SxProps<Theme>;
+	cellSx: (kind: HeatCellKind, severity?: number) => SxProps<Theme>;
 }
 
 export const ThemedHeatmap = ({ cells, containerSx, cellSx }: Props) => {
@@ -42,7 +42,7 @@ export const ThemedHeatmap = ({ cells, containerSx, cellSx }: Props) => {
 						placement="top"
 					>
 						<Box
-							sx={cellSx(cell.heatKind)}
+							sx={cellSx(cell.heatKind, cell.severity)}
 							aria-label={cell.ariaLabel}
 						/>
 					</Tooltip>
diff --git a/client/src/Pages/StatusPage/Status/themes/shared/ThemedHistogram.tsx b/client/src/Pages/StatusPage/Status/themes/shared/ThemedHistogram.tsx
index 3643d901c..ea5fa649a 100644
--- a/client/src/Pages/StatusPage/Status/themes/shared/ThemedHistogram.tsx
+++ b/client/src/Pages/StatusPage/Status/themes/shared/ThemedHistogram.tsx
@@ -13,7 +13,7 @@ import type {
 interface Props {
 	cells: ChartCell[];
 	containerSx: SxProps<Theme>;
-	barSx: (kind: BarKind, heightPct: number) => SxProps<Theme>;
+	barSx: (kind: BarKind, heightPct: number, severity?: number) => SxProps<Theme>;
 	statsSx: SxProps<Theme>;
 	statsGap?: number;
 }
@@ -61,7 +61,7 @@ export const ThemedHistogram = ({
 							placement="top"
 						>
 							<Box
-								sx={barSx(cell.barKind, cell.heightPct)}
+								sx={barSx(cell.barKind, cell.heightPct, cell.severity)}
 								aria-label={cell.ariaLabel}
 							/>
 						</Tooltip>
diff --git a/client/src/Pages/StatusPage/Status/themes/tokens.ts b/client/src/Pages/StatusPage/Status/themes/tokens.ts
index 3ec90abde..644bad8d2 100644
--- a/client/src/Pages/StatusPage/Status/themes/tokens.ts
+++ b/client/src/Pages/StatusPage/Status/themes/tokens.ts
@@ -243,3 +243,10 @@ export const themeTokens: Record<StatusPageTheme, ThemeVariants> = {
 	editorial,
 	minimal,
 };
+
+// Interpolate between a theme's up and degraded colors by severity (0..1).
+export const mixSeverityColor = (
+	up: string,
+	degraded: string,
+	severity: number
+): string => `color-mix(in oklch, ${degraded} ${Math.round(severity * 100)}%, ${up})`;
diff --git a/client/src/Validation/monitor.ts b/client/src/Validation/monitor.ts
index 5b53ff3a5..00ab3aaff 100644
--- a/client/src/Validation/monitor.ts
+++ b/client/src/Validation/monitor.ts
@@ -69,7 +69,12 @@ const httpSchema = baseSchema.extend({
 	type: z.literal("http"),
 	url: urlSchema,
 	proxyMode: z.enum(ProxyModes),
-	proxyId: z.string().optional(),
+	// The form uses "" for "no proxy selected". drop it from the payload so the
+	// server never receives an empty ObjectId
+	proxyId: z
+		.string()
+		.transform((value) => (value === "" ? undefined : value))
+		.optional(),
 	method: z.enum(HttpMethods).optional().register(monitorStepRegistry, { step: 2 }),
 	ignoreTlsErrors: z.boolean().register(monitorStepRegistry, { step: 2 }),
 	useAdvancedMatching: z.boolean().register(monitorStepRegistry, { step: 2 }),
diff --git a/server/src/api/validation/monitorValidation.ts b/server/src/api/validation/monitorValidation.ts
index 65a64ed10..e694a216d 100644
--- a/server/src/api/validation/monitorValidation.ts
+++ b/server/src/api/validation/monitorValidation.ts
@@ -16,6 +16,13 @@ import { DateRanges, SortOrders } from "@/types/query.js";
 
 const httpStatusCode = z.number().refine((code) => HttpStatusCodeSet.has(code), { message: "Must be a valid HTTP status code" });
 
+// The client form submits proxyId: "" when no proxy is selected, set it to undefined
+const proxyIdValidation = z
+	.string()
+	.optional()
+	.transform((value) => (value === "" ? undefined : value))
+	.refine((value) => value === undefined || /^[0-9a-f]{24}$/i.test(value), { message: "Invalid proxy ID" });
+
 export const getMonitorByIdParamValidation = z.object({
 	monitorId: z.string().min(1, "Monitor ID is required"),
 });
@@ -122,7 +129,7 @@ export const createMonitorBodyValidation = z
 		url: z.string().min(1, "URL is required"),
 		ignoreTlsErrors: z.boolean().default(false),
 		proxyMode: z.enum(ProxyModes).default("inherit"),
-		proxyId: z.string().optional(),
+		proxyId: proxyIdValidation,
 		useAdvancedMatching: z.boolean().default(false),
 		port: z.number().optional(),
 		isActive: z.boolean().optional(),
@@ -171,7 +178,7 @@ export const editMonitorBodyValidation = z
 		secret: z.string().optional(),
 		ignoreTlsErrors: z.boolean().optional(),
 		proxyMode: z.enum(ProxyModes).optional(),
-		proxyId: z.string().optional(),
+		proxyId: proxyIdValidation,
 		useAdvancedMatching: z.boolean().optional(),
 		jsonPath: z.union([z.string(), z.literal("")]).optional(),
 		expectedValue: z.union([z.string(), z.literal("")]).optional(),
@@ -233,7 +240,7 @@ const importedMonitorSchema = z
 		type: z.enum(MonitorTypes, "Invalid monitor type"),
 		ignoreTlsErrors: z.boolean().default(false),
 		proxyMode: z.enum(ProxyModes).default("inherit"),
-		proxyId: z.string().optional(),
+		proxyId: proxyIdValidation,
 		useAdvancedMatching: z.boolean().default(false),
 		jsonPath: z.union([z.string(), z.literal("")]).optional(),
 		expectedValue: z.union([z.string(), z.literal("")]).optional(),
@@ -306,7 +313,7 @@ export const monitorResponseSchema = z
 		statusWindowThreshold: z.number(),
 		ignoreTlsErrors: z.boolean(),
 		proxyMode: z.enum(ProxyModes),
-		proxyId: z.string().optional(),
+		proxyId: proxyIdValidation,
 		useAdvancedMatching: z.boolean(),
 		jsonPath: z.string().optional(),
 		expectedValue: z.string().optional(),
diff --git a/server/src/domain/monitors/monitor.repository.interface.ts b/server/src/domain/monitors/monitor.repository.interface.ts
index 1ce2b8fa8..1b12fef71 100644
--- a/server/src/domain/monitors/monitor.repository.interface.ts
+++ b/server/src/domain/monitors/monitor.repository.interface.ts
@@ -34,7 +34,7 @@ export interface IMonitorsRepository {
 	findByIds(monitorIds: string[], options?: { recentChecks?: RecentChecksMode }): Promise<Monitor[]>;
 
 	// update
-	updateById(monitorId: string, teamId: string, updates: Partial<Monitor>): Promise<Monitor>;
+	updateById(monitorId: string, teamId: string, updates: Partial<Monitor>, options?: { unsetProxyId?: boolean }): Promise<Monitor>;
 	updateByIds(monitorIds: string[], teamId: string, updates: Partial<Monitor>, excludeStatuses?: MonitorStatus[]): Promise<number>;
 	updateStatusWindowAndChecks(
 		monitorId: string,
diff --git a/server/src/domain/monitors/monitor.repository.mongo.ts b/server/src/domain/monitors/monitor.repository.mongo.ts
index 6cf2e0ecf..8f773f983 100644
--- a/server/src/domain/monitors/monitor.repository.mongo.ts
+++ b/server/src/domain/monitors/monitor.repository.mongo.ts
@@ -177,16 +177,14 @@ class MongoMonitorsRepository implements IMonitorsRepository {
 		return count;
 	};
 
-	updateById = async (monitorId: string, teamId: string, patch: Partial<Monitor>) => {
-		const updatedMonitor = await MonitorModel.findOneAndUpdate(
-			{ _id: monitorId, teamId },
-			{
-				$set: {
-					...patch,
-				},
-			},
-			{ new: true, runValidators: true }
-		);
+	updateById = async (monitorId: string, teamId: string, patch: Partial<Monitor>, options?: { unsetProxyId?: boolean }) => {
+		const fields: Partial<Monitor> = { ...patch };
+		const update: Record<string, unknown> = { $set: fields };
+		if (options?.unsetProxyId) {
+			delete fields.proxyId;
+			update.$unset = { proxyId: "" };
+		}
+		const updatedMonitor = await MonitorModel.findOneAndUpdate({ _id: monitorId, teamId }, update, { new: true, runValidators: true });
 		if (!updatedMonitor) {
 			throw new AppError({ message: `Failed to update monitor with id ${monitorId}`, status: 500 });
 		}
diff --git a/server/src/domain/monitors/monitor.service.ts b/server/src/domain/monitors/monitor.service.ts
index 31488c378..6cf9af54b 100644
--- a/server/src/domain/monitors/monitor.service.ts
+++ b/server/src/domain/monitors/monitor.service.ts
@@ -133,6 +133,10 @@ export class MonitorService implements IMonitorService {
 	}
 
 	createMonitor = async (teamId: string, userId: string, body: Monitor): Promise<void> => {
+		// proxyId is only needed in custom mode
+		if (body.proxyMode !== "custom") {
+			delete body.proxyId;
+		}
 		const monitor = await this.monitorsRepository.create(body, teamId, userId);
 		if (!monitor) {
 			throw new AppError({ message: "Failed to create monitor", status: 500, service: SERVICE_NAME, method: "createMonitor" });
@@ -375,7 +379,12 @@ export class MonitorService implements IMonitorService {
 	};
 
 	editMonitor = async ({ teamId, monitorId, body }: { teamId: string; monitorId: string; body: Partial<Monitor> }) => {
-		const editedMonitor = await this.monitorsRepository.updateById(monitorId, teamId, body);
+		// Moving off custom mode orphans the stored proxyId. Unset it so the stale reference doesn't block deleting that proxy
+		const unsetProxyId = body.proxyMode !== undefined && body.proxyMode !== "custom";
+		if (unsetProxyId) {
+			delete body.proxyId;
+		}
+		const editedMonitor = await this.monitorsRepository.updateById(monitorId, teamId, body, { unsetProxyId });
 		await this.scheduler.updateJob(editedMonitor);
 		return editedMonitor;
 	};
diff --git a/server/test/unit/services/monitorService.test.ts b/server/test/unit/services/monitorService.test.ts
index 0f56e5d2b..fcfdf533d 100644
--- a/server/test/unit/services/monitorService.test.ts
+++ b/server/test/unit/services/monitorService.test.ts
@@ -124,6 +124,38 @@ describe("MonitorService", () => {
 			expect(jobQueue.addJob).toHaveBeenCalledWith(MONITOR_ID, monitor);
 		});
 
+		it("strips a stray proxyId when proxyMode is not custom", async () => {
+			const monitorsRepository = createMonitorsRepositoryMock();
+			const monitor = makeMonitor();
+			(monitorsRepository.create as jest.Mock).mockResolvedValue(monitor);
+			const { service } = createService({ monitorsRepository });
+
+			await service.createMonitor(TEAM_ID, USER_ID, {
+				...monitor,
+				proxyMode: "inherit",
+				proxyId: "64b7a1f2c9e77a001a2b3c4d",
+			} as any);
+
+			const createdBody = (monitorsRepository.create as jest.Mock).mock.calls[0][0] as Record<string, unknown>;
+			expect(createdBody.proxyId).toBeUndefined();
+		});
+
+		it("keeps proxyId when proxyMode is custom", async () => {
+			const monitorsRepository = createMonitorsRepositoryMock();
+			const monitor = makeMonitor();
+			(monitorsRepository.create as jest.Mock).mockResolvedValue(monitor);
+			const { service } = createService({ monitorsRepository });
+
+			await service.createMonitor(TEAM_ID, USER_ID, {
+				...monitor,
+				proxyMode: "custom",
+				proxyId: "64b7a1f2c9e77a001a2b3c4d",
+			} as any);
+
+			const createdBody = (monitorsRepository.create as jest.Mock).mock.calls[0][0] as Record<string, unknown>;
+			expect(createdBody.proxyId).toBe("64b7a1f2c9e77a001a2b3c4d");
+		});
+
 		it("throws when repository returns null", async () => {
 			const monitorsRepository = createMonitorsRepositoryMock();
 			(monitorsRepository.create as jest.Mock).mockResolvedValue(null);
@@ -750,9 +782,42 @@ describe("MonitorService", () => {
 			const result = await service.editMonitor({ teamId: TEAM_ID, monitorId: MONITOR_ID, body: { name: "Updated" } });
 
 			expect(result).toMatchObject({ name: "Updated" });
-			expect(monitorsRepository.updateById).toHaveBeenCalledWith(MONITOR_ID, TEAM_ID, { name: "Updated" });
+			expect(monitorsRepository.updateById).toHaveBeenCalledWith(MONITOR_ID, TEAM_ID, { name: "Updated" }, { unsetProxyId: false });
 			expect(jobQueue.updateJob).toHaveBeenCalledWith(updatedMonitor);
 		});
+
+		it("keeps proxyId and does not unset it when proxyMode is custom", async () => {
+			const monitorsRepository = createMonitorsRepositoryMock();
+			(monitorsRepository.updateById as jest.Mock).mockResolvedValue(makeMonitor());
+			const { service } = createService({ monitorsRepository });
+
+			await service.editMonitor({
+				teamId: TEAM_ID,
+				monitorId: MONITOR_ID,
+				body: { proxyMode: "custom", proxyId: "64b7a1f2c9e77a001a2b3c4d" } as any,
+			});
+
+			expect(monitorsRepository.updateById).toHaveBeenCalledWith(
+				MONITOR_ID,
+				TEAM_ID,
+				{ proxyMode: "custom", proxyId: "64b7a1f2c9e77a001a2b3c4d" },
+				{ unsetProxyId: false }
+			);
+		});
+
+		it("strips proxyId and unsets it when proxyMode moves off custom", async () => {
+			const monitorsRepository = createMonitorsRepositoryMock();
+			(monitorsRepository.updateById as jest.Mock).mockResolvedValue(makeMonitor());
+			const { service } = createService({ monitorsRepository });
+
+			await service.editMonitor({
+				teamId: TEAM_ID,
+				monitorId: MONITOR_ID,
+				body: { proxyMode: "inherit", proxyId: "64b7a1f2c9e77a001a2b3c4d" } as any,
+			});
+
+			expect(monitorsRepository.updateById).toHaveBeenCalledWith(MONITOR_ID, TEAM_ID, { proxyMode: "inherit" }, { unsetProxyId: true });
+		});
 	});
 
 	describe("updateNotifications", () => {
diff --git a/server/test/unit/validation/monitorValidation.test.ts b/server/test/unit/validation/monitorValidation.test.ts
index 4de4538e1..ea2257072 100644
--- a/server/test/unit/validation/monitorValidation.test.ts
+++ b/server/test/unit/validation/monitorValidation.test.ts
@@ -477,6 +477,7 @@ describe("monitorValidation — proxy fields", () => {
 		type: "http" as const,
 		url: "https://example.com",
 	};
+	const PROXY_ID = "64b7a1f2c9e77a001a2b3c4d";
 
 	describe("createMonitorBodyValidation", () => {
 		it("defaults proxyMode to inherit", () => {
@@ -487,23 +488,38 @@ describe("monitorValidation — proxy fields", () => {
 		});
 
 		it("accepts custom mode with a proxyId", () => {
-			const parsed = createMonitorBodyValidation.parse({ ...baseHttpBody, proxyMode: "custom", proxyId: "proxy-1" });
+			const parsed = createMonitorBodyValidation.parse({ ...baseHttpBody, proxyMode: "custom", proxyId: PROXY_ID });
 
 			expect(parsed.proxyMode).toBe("custom");
-			expect(parsed.proxyId).toBe("proxy-1");
+			expect(parsed.proxyId).toBe(PROXY_ID);
 		});
 
 		it("rejects custom mode without a proxyId", () => {
 			expect(() => createMonitorBodyValidation.parse({ ...baseHttpBody, proxyMode: "custom" })).toThrow();
 		});
 
+		it("rejects custom mode with an empty-string proxyId", () => {
+			expect(() => createMonitorBodyValidation.parse({ ...baseHttpBody, proxyMode: "custom", proxyId: "" })).toThrow();
+		});
+
+		it("normalizes an empty-string proxyId to undefined on non-custom modes", () => {
+			for (const proxyMode of ["inherit", "none"] as const) {
+				const parsed = createMonitorBodyValidation.parse({ ...baseHttpBody, proxyMode, proxyId: "" });
+				expect(parsed.proxyId).toBeUndefined();
+			}
+		});
+
+		it("rejects a proxyId that is not a valid ObjectId", () => {
+			expect(() => createMonitorBodyValidation.parse({ ...baseHttpBody, proxyMode: "custom", proxyId: "proxy-1" })).toThrow();
+		});
+
 		it("rejects an unknown proxyMode", () => {
 			expect(() => createMonitorBodyValidation.parse({ ...baseHttpBody, proxyMode: "global" })).toThrow();
 		});
 
 		it("allows a stray proxyId on non-custom modes", () => {
 			for (const proxyMode of ["inherit", "none"] as const) {
-				const parsed = createMonitorBodyValidation.parse({ ...baseHttpBody, proxyMode, proxyId: "proxy-1" });
+				const parsed = createMonitorBodyValidation.parse({ ...baseHttpBody, proxyMode, proxyId: PROXY_ID });
 				expect(parsed.proxyMode).toBe(proxyMode);
 			}
 		});
@@ -521,9 +537,19 @@ describe("monitorValidation — proxy fields", () => {
 		});
 
 		it("accepts custom mode with a proxyId", () => {
-			const parsed = editMonitorBodyValidation.parse({ proxyMode: "custom", proxyId: "proxy-1" });
+			const parsed = editMonitorBodyValidation.parse({ proxyMode: "custom", proxyId: PROXY_ID });
 
-			expect(parsed.proxyId).toBe("proxy-1");
+			expect(parsed.proxyId).toBe(PROXY_ID);
+		});
+
+		it("normalizes an empty-string proxyId to undefined", () => {
+			const parsed = editMonitorBodyValidation.parse({ proxyMode: "inherit", proxyId: "" });
+
+			expect(parsed.proxyId).toBeUndefined();
+		});
+
+		it("rejects a proxyId that is not a valid ObjectId", () => {
+			expect(() => editMonitorBodyValidation.parse({ proxyMode: "custom", proxyId: "not-an-object-id" })).toThrow();
 		});
 	});