diff --git a/apps/mcp/src/widget/views/Upload.tsx b/apps/mcp/src/widget/views/Upload.tsx
index ed71c856a..6e34490b2 100644
--- a/apps/mcp/src/widget/views/Upload.tsx
+++ b/apps/mcp/src/widget/views/Upload.tsx
@@ -29,15 +29,53 @@ function formatFileSize(bytes: number): string {
 
 const ACCEPT = ".txt,.pdf,.png,.jpg,.jpeg,.mp4"
 
+// The MCP transport rejects JSON-RPC bodies over 4 MiB with a bare 413
+// (MAXIMUM_MESSAGE_SIZE_BYTES in the agents SDK). The file travels base64
+// encoded (~4/3 inflation) inside that body, so cap the raw file size such
+// that the encoded payload plus the JSON-RPC envelope stays under the limit.
+const TRANSPORT_MESSAGE_LIMIT_BYTES = 4 * 1024 * 1024
+const ENVELOPE_ALLOWANCE_BYTES = 64 * 1024
+const MAX_UPLOAD_BYTES = Math.floor(
+	((TRANSPORT_MESSAGE_LIMIT_BYTES - ENVELOPE_ALLOWANCE_BYTES) * 3) / 4,
+)
+
+function fileTooLargeMessage(size: number): string {
+	return `This file is ${formatFileSize(size)}. The maximum upload size is ${formatFileSize(MAX_UPLOAD_BYTES)} — please choose a smaller file.`
+}
+
+// The transport-level 413 surfaces as an opaque error string; translate it
+// so the user sees the size limit instead of a generic failure.
+function friendlyUploadError(raw: string, fileSize: number): string {
+	if (/413|too large|payload/i.test(raw)) {
+		return fileTooLargeMessage(fileSize)
+	}
+	return raw
+}
+
 export function Upload({ activeTag, writableTags, onAdvance, onError }: Props) {
 	const { callTool } = useApp()
 	const log = useLog()
 	const [file, setFile] = useState<File | null>(null)
+	const [fileError, setFileError] = useState<string | null>(null)
 	const [selectedTag, setSelectedTag] = useState<string | null>(
 		activeTag ?? writableTags[0] ?? null,
 	)
 	const [uploading, setUploading] = useState(false)
 
+	function handleFileSelect(selected: File) {
+		if (selected.size > MAX_UPLOAD_BYTES) {
+			log(
+				"warning",
+				`[upload] rejected oversized file: ${selected.name} (${selected.size}B > ${MAX_UPLOAD_BYTES}B)`,
+			)
+			setFileError(fileTooLargeMessage(selected.size))
+			setFile(null)
+			return
+		}
+		setFileError(null)
+		setFile(selected)
+	}
+
 	const options = useMemo(
 		() => writableTags.map((tag) => ({ value: tag, label: tag })),
 		[writableTags],
@@ -59,13 +97,17 @@ export function Upload({ activeTag, writableTags, onAdvance, onError }: Props) {
 			})
 			if (!result.ok || !result.data) {
 				log("error", `[upload] failed: ${result.error}`)
-				onError(result.error ?? "Upload failed")
+				onError(
+					result.error
+						? friendlyUploadError(result.error, file.size)
+						: "Upload failed",
+				)
 				return
 			}
 			onAdvance(result.data)
 		} catch (err) {
 			log("error", `[upload] threw: ${err}`)
-			onError(String(err))
+			onError(friendlyUploadError(String(err), file.size))
 		} finally {
 			setUploading(false)
 		}
@@ -102,11 +144,21 @@ export function Upload({ activeTag, writableTags, onAdvance, onError }: Props) {
 								/>
 							</div>
 						) : (
-							<FileUpload
-								accept={ACCEPT}
-								description="Supports TXT, PDF, PNG, JPG, MP4"
-								onFile={setFile}
-							/>
+							<Stack gap="sm">
+								<FileUpload
+									accept={ACCEPT}
+									description={`Supports TXT, PDF, PNG, JPG, MP4 · up to ${formatFileSize(MAX_UPLOAD_BYTES)}`}
+									onFile={handleFileSelect}
+								/>
+								{fileError ? (
+									<p
+										className="text-(length:--text-xs) leading-relaxed text-error"
+										role="alert"
+									>
+										{fileError}
+									</p>
+								) : null}
+							</Stack>
 						)}
 
 						{writableTags.length > 0 ? (