diff --git a/apps/web/app/api/upload/[...route]/multipart.ts b/apps/web/app/api/upload/[...route]/multipart.ts
index 5947b0775b5..0541bfcbd3a 100644
--- a/apps/web/app/api/upload/[...route]/multipart.ts
+++ b/apps/web/app/api/upload/[...route]/multipart.ts
@@ -4,6 +4,7 @@ import { serverEnv } from "@cap/env";
 import { userIsPro } from "@cap/utils";
 import {
 	Database,
+	MAX_UPLOAD_BYTES,
 	makeCurrentUserLayer,
 	provideOptionalAuth,
 	Storage,
@@ -290,7 +291,10 @@ app.post(
 					z.object({
 						partNumber: z.number(),
 						etag: z.string(),
-						size: z.number(),
+						// A non-negative integer (bytes) so neither a negative nor a
+						// fractional size can drag the summed total below the cap and
+						// bypass the upload-size limit.
+						size: z.number().int().nonnegative(),
 					}),
 				),
 				durationInSecs: stringOrNumberOptional,
@@ -399,6 +403,63 @@ app.post(
 				}
 			}
 
+			// Server-side backstop for the maximum upload size. Presigned POST URLs
+			// enforce a content-length-range policy, but presigned PUT part URLs
+			// cannot enforce a total size, so reject an oversized assembled upload
+			// here before persisting (and before paying to assemble it). Part sizes
+			// are client-reported, so this raises the bar rather than enforcing
+			// authoritatively.
+			let totalUploadSize = 0;
+			for (const part of parts) {
+				totalUploadSize += part.size;
+				if (totalUploadSize > MAX_UPLOAD_BYTES) break;
+			}
+			if (totalUploadSize > MAX_UPLOAD_BYTES) {
+				// Avoid leaving the parts as incomplete-MPU storage and a stale
+				// videoUploads row, mirroring the free-plan rejection cleanup. Each
+				// step is caught independently so a failed abort doesn't skip the DB
+				// cleanup (and vice versa). The 413 stands regardless of either.
+				yield* Effect.gen(function* () {
+					const [bucket] = yield* Storage.getAccessForVideo(video);
+					yield* bucket.multipart
+						.abort(fileKey, uploadId)
+						.pipe(
+							Effect.catchAll((error) =>
+								Effect.logError(
+									"Failed to abort rejected oversized multipart upload",
+									error,
+								),
+							),
+						);
+					yield* db
+						.use((db) =>
+							db
+								.delete(Db.videoUploads)
+								.where(eq(Db.videoUploads.videoId, videoId)),
+						)
+						.pipe(
+							Effect.catchAll((error) =>
+								Effect.logError(
+									"Failed to delete videoUploads row for rejected upload",
+									error,
+								),
+							),
+						);
+				}).pipe(
+					Effect.catchAll((error) =>
+						Effect.logError(
+							"Failed to clean up rejected oversized multipart upload",
+							error,
+						),
+					),
+				);
+
+				c.status(413);
+				return c.text(
+					"Upload exceeds the maximum allowed size and cannot be completed.",
+				);
+			}
+
 			return yield* Effect.gen(function* () {
 				const [bucket] = yield* Storage.getAccessForVideo(video);
 
diff --git a/packages/web-backend/src/S3Buckets/S3BucketAccess.ts b/packages/web-backend/src/S3Buckets/S3BucketAccess.ts
index 106e98985a3..b5b7c11a0dc 100644
--- a/packages/web-backend/src/S3Buckets/S3BucketAccess.ts
+++ b/packages/web-backend/src/S3Buckets/S3BucketAccess.ts
@@ -14,6 +14,11 @@ import { S3BucketClientProvider } from "./S3BucketClientProvider.ts";
 const DEFAULT_PRESIGNED_GET_EXPIRES_SECONDS = 3600;
 const DEFAULT_PRESIGNED_PUT_EXPIRES_SECONDS = 3600;
 
+// Upper bound on a single upload to prevent unbounded storage abuse. Generous
+// on purpose so legitimate long/high-bitrate recordings are never blocked;
+// tune here if the product ever needs a larger ceiling.
+export const MAX_UPLOAD_BYTES = 100 * 1024 * 1024 * 1024; // 100 GiB
+
 type NodeReadableWebStream = Parameters<typeof Readable.fromWeb>[0];
 
 const wrapS3Promise = <T>(
@@ -266,13 +271,49 @@ export const createS3BucketAccess = Effect.gen(function* () {
 		) =>
 			wrapS3Promise(
 				provider.getPublic.pipe(
-					Effect.map((client) =>
-						createPresignedPost(client, {
+					Effect.map((client) => {
+						// Enforce an upper bound on the uploaded object size. The POST
+						// policy rejects the upload at S3 if the body exceeds this,
+						// closing the unbounded-storage hole for presigned POSTs. We emit
+						// exactly one content-length-range whose max never exceeds
+						// MAX_UPLOAD_BYTES, even if a caller supplied a looser one, so a
+						// caller can tighten but never raise/disable the cap.
+						const callerConditions = args.Conditions ?? [];
+						const isLengthRange = (condition: unknown): boolean =>
+							Array.isArray(condition) &&
+							condition[0] === "content-length-range";
+						const callerRange = callerConditions.find(isLengthRange) as
+							| [unknown, unknown, unknown]
+							| undefined;
+						const callerMin =
+							callerRange &&
+							typeof callerRange[1] === "number" &&
+							Number.isFinite(callerRange[1])
+								? Math.max(0, callerRange[1])
+								: 0;
+						const callerMax =
+							callerRange &&
+							typeof callerRange[2] === "number" &&
+							Number.isFinite(callerRange[2])
+								? Math.max(0, callerRange[2])
+								: MAX_UPLOAD_BYTES;
+						const otherConditions = callerConditions.filter(
+							(condition) => !isLengthRange(condition),
+						);
+						return createPresignedPost(client, {
 							...args,
+							Conditions: [
+								[
+									"content-length-range",
+									callerMin,
+									Math.min(callerMax, MAX_UPLOAD_BYTES),
+								],
+								...otherConditions,
+							],
 							Bucket: provider.bucket,
 							Key: key,
-						}),
-					),
+						});
+					}),
 				),
 			),
 		multipart: {
diff --git a/packages/web-backend/src/index.ts b/packages/web-backend/src/index.ts
index 7cd60bc8fb6..2c235b94692 100644
--- a/packages/web-backend/src/index.ts
+++ b/packages/web-backend/src/index.ts
@@ -10,6 +10,7 @@ export { Organisations } from "./Organisations/index.ts";
 export { OrganisationsPolicy } from "./Organisations/OrganisationsPolicy.ts";
 export * from "./Rpcs.ts";
 export { S3Buckets } from "./S3Buckets/index.ts";
+export { MAX_UPLOAD_BYTES } from "./S3Buckets/S3BucketAccess.ts";
 export { Spaces } from "./Spaces/index.ts";
 export { SpacesPolicy } from "./Spaces/SpacesPolicy.ts";
 export * from "./Storage/GoogleDrive.ts";