diff --git a/apps/docs/content/features/moderations.mdx b/apps/docs/content/features/moderations.mdx
index 0152a34e7c..a3fa22b138 100644
--- a/apps/docs/content/features/moderations.mdx
+++ b/apps/docs/content/features/moderations.mdx
@@ -34,7 +34,9 @@ The `input` field accepts:
 
 - A single string
 - An array of strings
-- An array of multimodal content items with `text` and `image_url`
+- An array of multimodal content items where each item is type-driven and
+  contains only the fields relevant to its type, such as `text` for text items
+  or `image_url` for image items
 
 The default model is `omni-moderation-latest`.
 
diff --git a/apps/gateway/src/api.spec.ts b/apps/gateway/src/api.spec.ts
index 3d48539141..5168747ab1 100644
--- a/apps/gateway/src/api.spec.ts
+++ b/apps/gateway/src/api.spec.ts
@@ -198,6 +198,83 @@ describe("api", () => {
 		}
 	});
 
+	test("/v1/moderations rejects invalid multimodal items before proxying upstream", async () => {
+		const fetchSpy = vi.spyOn(globalThis, "fetch");
+
+		const res = await app.request("/v1/moderations", {
+			method: "POST",
+			headers: {
+				"Content-Type": "application/json",
+			},
+			body: JSON.stringify({
+				input: [
+					{
+						type: "text",
+					},
+				],
+			}),
+		});
+
+		expect(res.status).toBe(400);
+		expect(await res.json()).toEqual({
+			error: {
+				message: "Invalid request parameters",
+				type: "invalid_request_error",
+				param: null,
+				code: "invalid_parameters",
+			},
+		});
+		expect(fetchSpy).not.toHaveBeenCalled();
+	});
+
+	test("/v1/moderations maps undocumented upstream statuses to 502", async () => {
+		await db.insert(tables.apiKey).values({
+			id: "token-id",
+			token: "real-token",
+			projectId: "project-id",
+			description: "Test API Key",
+			createdBy: "user-id",
+		});
+
+		await db.insert(tables.providerKey).values({
+			id: "provider-key-id",
+			token: "sk-test-key",
+			provider: "openai",
+			organizationId: "org-id",
+			baseUrl: mockServerUrl,
+		});
+
+		const requestId = "moderation-undocumented-status-request-id";
+		const res = await app.request("/v1/moderations", {
+			method: "POST",
+			headers: {
+				"Content-Type": "application/json",
+				Authorization: "Bearer real-token",
+				"x-request-id": requestId,
+			},
+			body: JSON.stringify({
+				input: "TRIGGER_STATUS_418 moderation upstream status",
+			}),
+		});
+
+		expect(res.status).toBe(502);
+		expect(await res.json()).toEqual({
+			error: {
+				message: "Mock error with status 418",
+				type: "server_error",
+				param: null,
+				code: "error_418",
+			},
+		});
+
+		const logs = await waitForLogs(1);
+		const moderationLog = logs.find((log) => log.requestId === requestId);
+
+		expect(moderationLog).toBeTruthy();
+		expect(moderationLog?.hasError).toBe(true);
+		expect(moderationLog?.errorDetails?.statusCode).toBe(418);
+	});
+
 	test("/v1/images/edits accepts Gemini size and aspect ratio", async () => {
 		await db.insert(tables.apiKey).values({
 			id: "token-id-image-edits",
diff --git a/apps/gateway/src/moderations/moderations.ts b/apps/gateway/src/moderations/moderations.ts
index b436e44e5a..4d4e39ebaf 100644
--- a/apps/gateway/src/moderations/moderations.ts
+++ b/apps/gateway/src/moderations/moderations.ts
@@ -27,16 +27,25 @@ const moderationInputTextSchema = z.string().openapi({
 	example: "I want to harm someone.",
 });
 
-const moderationInputContentSchema = z
+const moderationTextContentSchema = z
 	.object({
-		type: z.enum(["text", "image_url"]).openapi({
+		type: z.literal("text").openapi({
 			description: "Input item type.",
 			example: "text",
 		}),
-		text: z.string().optional().openapi({
+		text: z.string().openapi({
 			description: "Text content for `type: text` items.",
 			example: "Please review this sentence.",
 		}),
+	})
+	.strict();
+
+const moderationImageContentSchema = z
+	.object({
+		type: z.literal("image_url").openapi({
+			description: "Input item type.",
+			example: "image_url",
+		}),
 		image_url: z
 			.object({
 				url: z.string().openapi({
@@ -44,11 +53,17 @@ const moderationInputContentSchema = z
 					example: "https://example.com/image.png",
 				}),
 			})
-			.optional()
 			.openapi({
 				description: "Image payload for `type: image_url` items.",
 			}),
 	})
+	.strict();
+
+const moderationInputContentSchema = z
+	.discriminatedUnion("type", [
+		moderationTextContentSchema,
+		moderationImageContentSchema,
+	])
 	.openapi({
 		description: "Multimodal moderation input item.",
 	});
@@ -123,14 +138,60 @@ const moderationResponseSchema = z
 	});
 
 const moderationErrorSchema = z.object({
-	error: z.object({
-		message: z.string(),
-		type: z.string(),
-		param: z.string().nullable(),
-		code: z.string(),
-	}),
+	error: z
+		.object({
+			message: z.string(),
+			type: z.string().nullable().optional(),
+			param: z.string().nullable().optional(),
+			code: z.string().nullable().optional(),
+		})
+		.passthrough(),
 });
 
+const documentedErrorStatuses = [
+	400, 401, 403, 404, 410, 429, 500, 502, 503, 504,
+] as const;
+type DocumentedErrorStatus = (typeof documentedErrorStatuses)[number];
+
+function normalizeDocumentedErrorStatus(status: number): DocumentedErrorStatus {
+	return documentedErrorStatuses.includes(status as DocumentedErrorStatus)
+		? (status as DocumentedErrorStatus)
+		: 502;
+}
+
+function normalizeModerationErrorPayload(upstreamJson: unknown) {
+	if (typeof upstreamJson === "string") {
+		return {
+			error: {
+				message: upstreamJson,
+				type: "upstream_error",
+				param: null,
+				code: "upstream_error",
+			},
+		};
+	}
+
+	if (
+		upstreamJson &&
+		typeof upstreamJson === "object" &&
+		"error" in upstreamJson &&
+		upstreamJson.error &&
+		typeof upstreamJson.error === "object" &&
+		"message" in upstreamJson.error
+	) {
+		return upstreamJson;
+	}
+
+	return {
+		error: {
+			message: "Upstream request failed",
+			type: "upstream_error",
+			param: null,
+			code: "upstream_error",
+		},
+	};
+}
+
 const moderationRequestSchema = z.object({
 	input: moderationInputSchema,
 	model: z.string().optional().default("omni-moderation-latest").openapi({
@@ -599,20 +660,8 @@ moderations.openapi(createModeration, async (c): Promise<any> => {
 		});
 
 		return c.json(
-			(typeof upstreamJson === "string"
-				? { error: { message: upstreamJson } }
-				: upstreamJson) ?? { error: true },
-			upstreamResponse.status as
-				| 400
-				| 401
-				| 403
-				| 404
-				| 410
-				| 429
-				| 500
-				| 502
-				| 503
-				| 504,
+			normalizeModerationErrorPayload(upstreamJson),
+			normalizeDocumentedErrorStatus(upstreamResponse.status),
 		);
 	}