diff --git a/apps/web/next.config.ts b/apps/web/next.config.ts
index d26a944..7e59214 100644
--- a/apps/web/next.config.ts
+++ b/apps/web/next.config.ts
@@ -19,6 +19,15 @@ const nextConfig: NextConfig = {
   allowedDevOrigins: ["192.168.0.253", "test.04080616.xyz", "rexwa.04080616.xyz"],
   experimental: {
     typedRoutes: true,
+    serverActions: {
+      // Default Server Action body limit is 1 MB — way under WhatsApp's
+      // 100 MB document cap. Lifted to 100 MB so document uploads reach
+      // the action; the per-kind WhatsApp validator
+      // (lib/whatsapp-media.ts) then enforces the actual limit
+      // (5 MB image / 16 MB video/audio / 100 MB document) and returns
+      // a useful error for the rest.
+      bodySizeLimit: "100mb",
+    },
   },
   turbopack: {
     root: workspaceRoot,
diff --git a/apps/web/src/actions/media.ts b/apps/web/src/actions/media.ts
index d64a746..69db040 100644
--- a/apps/web/src/actions/media.ts
+++ b/apps/web/src/actions/media.ts
@@ -10,8 +10,7 @@ import { db } from "@/lib/db";
 import { env } from "@/env";
 import { getSeededOperator } from "@/lib/operator";
 import { checkRateLimit } from "@/lib/rate-limit";
-
-const MAX_BYTES = 50 * 1024 * 1024;
+import { validateForWhatsApp } from "@/lib/whatsapp-media";
 
 async function rateLimit(key: string) {
   const h = await headers();
@@ -31,8 +30,12 @@ export async function uploadMediaAction(
   await rateLimit("media-upload");
   const file = formData.get("file");
   if (!(file instanceof File)) return { ok: false, error: "No file uploaded" };
-  if (file.size === 0) return { ok: false, error: "Empty file" };
-  if (file.size > MAX_BYTES) return { ok: false, error: "File too large (>50MB)" };
+  // Per-kind WhatsApp size validation: 5 MB images, 16 MB video/audio,
+  // 100 MB documents. Fall-through everything we don't recognise to
+  // "document" (matches the bot's sender path).
+  const mimeType = file.type || "application/octet-stream";
+  const sizeCheck = validateForWhatsApp(mimeType, file.size);
+  if (!sizeCheck.ok) return { ok: false, error: sizeCheck.error };
 
   const op = await getSeededOperator();
   const buffer = Buffer.from(await file.arrayBuffer());
@@ -47,7 +50,7 @@ export async function uploadMediaAction(
     .values({
       operatorId: op.id,
       filenameOriginal: file.name,
-      mimeType: file.type || "application/octet-stream",
+      mimeType,
       sizeBytes: buffer.byteLength,
       sha256,
       storagePath,
@@ -58,6 +61,6 @@ export async function uploadMediaAction(
     ok: true,
     mediaId: row!.id,
     filename: file.name,
-    mimeType: file.type || "application/octet-stream",
+    mimeType,
   };
 }
diff --git a/apps/web/src/components/message-stack.tsx b/apps/web/src/components/message-stack.tsx
index 4dc824e..649dcc1 100644
--- a/apps/web/src/components/message-stack.tsx
+++ b/apps/web/src/components/message-stack.tsx
@@ -377,7 +377,7 @@ function MediaBlock({ part, onChange }: MediaBlockProps) {
               <div>
                 <p className="text-xs font-medium">Click to upload or drag & drop</p>
                 <p className="text-[11px] text-muted-foreground mt-0.5">
-                  Images, documents, audio — up to 50 MB
+                  Image up to 5 MB · video / audio up to 16 MB · document up to 100 MB
                 </p>
               </div>
             </>
diff --git a/apps/web/src/lib/whatsapp-media.test.ts b/apps/web/src/lib/whatsapp-media.test.ts
new file mode 100644
index 0000000..40cda6d
--- /dev/null
+++ b/apps/web/src/lib/whatsapp-media.test.ts
@@ -0,0 +1,130 @@
+import { describe, it, expect } from "vitest";
+import {
+  classifyMediaKind,
+  formatBytes,
+  validateForWhatsApp,
+  WA_LIMITS,
+  WA_MAX_BYTES,
+} from "./whatsapp-media";
+
+const MB = 1024 * 1024;
+
+describe("classifyMediaKind", () => {
+  it("classifies image/* as image", () => {
+    expect(classifyMediaKind("image/jpeg")).toBe("image");
+    expect(classifyMediaKind("image/png")).toBe("image");
+    expect(classifyMediaKind("image/webp")).toBe("image");
+  });
+
+  it("classifies video/* as video", () => {
+    expect(classifyMediaKind("video/mp4")).toBe("video");
+    expect(classifyMediaKind("video/3gpp")).toBe("video");
+  });
+
+  it("classifies audio/* as audio", () => {
+    expect(classifyMediaKind("audio/aac")).toBe("audio");
+    expect(classifyMediaKind("audio/ogg")).toBe("audio");
+    expect(classifyMediaKind("audio/mpeg")).toBe("audio");
+  });
+
+  it("classifies anything else as document (PDFs, office, plain text…)", () => {
+    expect(classifyMediaKind("application/pdf")).toBe("document");
+    expect(classifyMediaKind("application/zip")).toBe("document");
+    expect(classifyMediaKind("text/plain")).toBe("document");
+    expect(classifyMediaKind("application/octet-stream")).toBe("document");
+    expect(classifyMediaKind("")).toBe("document");
+  });
+});
+
+describe("validateForWhatsApp — per-kind WA caps", () => {
+  it("accepts an image just under the 5 MB cap", () => {
+    const r = validateForWhatsApp("image/jpeg", 5 * MB - 1);
+    expect(r.ok).toBe(true);
+    if (r.ok) {
+      expect(r.kind).toBe("image");
+      expect(r.limitBytes).toBe(WA_LIMITS.image);
+    }
+  });
+
+  it("accepts an image at exactly the 5 MB cap", () => {
+    const r = validateForWhatsApp("image/png", 5 * MB);
+    expect(r.ok).toBe(true);
+  });
+
+  it("rejects a 6 MB image with a useful WhatsApp-flavoured message", () => {
+    const r = validateForWhatsApp("image/jpeg", 6 * MB);
+    expect(r.ok).toBe(false);
+    if (!r.ok) {
+      expect(r.kind).toBe("image");
+      expect(r.error).toMatch(/Image too large/);
+      expect(r.error).toMatch(/6\.0 MB/);
+      expect(r.error).toMatch(/5\.0 MB/);
+      expect(r.error).toMatch(/WhatsApp/);
+    }
+  });
+
+  it("rejects a 16 MB + 1 byte video", () => {
+    const r = validateForWhatsApp("video/mp4", 16 * MB + 1);
+    expect(r.ok).toBe(false);
+    if (!r.ok) {
+      expect(r.kind).toBe("video");
+      expect(r.error).toMatch(/Video too large/);
+    }
+  });
+
+  it("accepts a 16 MB audio file (boundary)", () => {
+    const r = validateForWhatsApp("audio/mpeg", 16 * MB);
+    expect(r.ok).toBe(true);
+  });
+
+  it("accepts a 99 MB document and rejects 101 MB", () => {
+    expect(validateForWhatsApp("application/pdf", 99 * MB).ok).toBe(true);
+    const r = validateForWhatsApp("application/pdf", 101 * MB);
+    expect(r.ok).toBe(false);
+    if (!r.ok) expect(r.error).toMatch(/Document too large/);
+  });
+
+  it("rejects empty / zero-byte uploads regardless of kind", () => {
+    expect(validateForWhatsApp("image/png", 0).ok).toBe(false);
+    expect(validateForWhatsApp("application/pdf", 0).ok).toBe(false);
+  });
+
+  it("a 60 MB unknown-mime upload is treated as a document and accepted", () => {
+    // The "anything not image/video/audio is a document" fall-through
+    // is what makes this lenient — that's intentional, mirrors how
+    // the bot's sender path delivers it.
+    const r = validateForWhatsApp("application/octet-stream", 60 * MB);
+    expect(r.ok).toBe(true);
+    if (r.ok) expect(r.kind).toBe("document");
+  });
+});
+
+describe("WA_MAX_BYTES is the largest single-kind cap", () => {
+  it("equals the document cap (100 MB)", () => {
+    expect(WA_MAX_BYTES).toBe(WA_LIMITS.document);
+    expect(WA_MAX_BYTES).toBe(100 * MB);
+  });
+
+  it("is at least as large as every per-kind limit", () => {
+    for (const limit of Object.values(WA_LIMITS)) {
+      expect(WA_MAX_BYTES).toBeGreaterThanOrEqual(limit);
+    }
+  });
+});
+
+describe("formatBytes", () => {
+  it("renders bytes < 1 KB plainly", () => {
+    expect(formatBytes(0)).toBe("0 B");
+    expect(formatBytes(512)).toBe("512 B");
+  });
+
+  it("renders KB rounded to no decimals", () => {
+    expect(formatBytes(1024)).toBe("1 KB");
+    expect(formatBytes(102_400)).toBe("100 KB");
+  });
+
+  it("renders MB to one decimal", () => {
+    expect(formatBytes(5 * MB)).toBe("5.0 MB");
+    expect(formatBytes(5_400_000)).toBe("5.1 MB");
+  });
+});
diff --git a/apps/web/src/lib/whatsapp-media.ts b/apps/web/src/lib/whatsapp-media.ts
new file mode 100644
index 0000000..111001a
--- /dev/null
+++ b/apps/web/src/lib/whatsapp-media.ts
@@ -0,0 +1,105 @@
+/**
+ * WhatsApp media size limits, by deliverable type.
+ *
+ * These are the practical WA Web upload limits Baileys passes through
+ * — anything over the cap is either re-encoded by WA's server (image)
+ * or rejected outright (everything else). Sourced from WA Business
+ * docs and Baileys' own validation paths; aligned to the most
+ * conservative documented value where two specs disagree.
+ *
+ *   image   :  5 MB     image/jpeg, image/png, image/webp (non-sticker)
+ *   video   : 16 MB     video/mp4, video/3gpp
+ *   audio   : 16 MB     audio/aac, audio/ogg, audio/amr, audio/mp4, audio/mpeg
+ *   document: 100 MB    everything else (PDFs, office docs, plain text…)
+ *   sticker : 100 KB    image/webp marked as a sticker (we don't support
+ *                       sticker-mode uploads yet — kept here for future)
+ */
+const MB = 1024 * 1024;
+const KB = 1024;
+
+export const WA_LIMITS = {
+  image: 5 * MB,
+  video: 16 * MB,
+  audio: 16 * MB,
+  document: 100 * MB,
+  sticker: 100 * KB,
+} as const;
+
+export type WaMediaKind = keyof typeof WA_LIMITS;
+
+/** The largest single-file upload WA will accept across all kinds.
+ *  Doubles as the Next Server Action body-size cap. */
+export const WA_MAX_BYTES = WA_LIMITS.document;
+
+/**
+ * Map an uploaded file's MIME type to the WhatsApp delivery kind. We
+ * use the kind both to pick the right size limit AND to decide which
+ * Baileys sender path the bot will use (image / video / document).
+ *
+ * Anything we don't recognise as image/video/audio falls through to
+ * "document", which is also how the bot delivers it (file with a
+ * filename and the original mime type preserved).
+ */
+export function classifyMediaKind(mimeType: string): WaMediaKind {
+  if (mimeType.startsWith("image/")) return "image";
+  if (mimeType.startsWith("video/")) return "video";
+  if (mimeType.startsWith("audio/")) return "audio";
+  return "document";
+}
+
+export type WaSizeCheck =
+  | { ok: true; kind: WaMediaKind; limitBytes: number }
+  | { ok: false; kind: WaMediaKind; limitBytes: number; error: string };
+
+/**
+ * Validate an uploaded file against WhatsApp's per-kind size cap.
+ *
+ * Returns the resolved kind and the cap regardless of pass/fail so
+ * the UI can show "Image: 4.8 MB / 5 MB" or "Image too large
+ * (5.2 MB > 5 MB)" without re-deriving the kind itself.
+ */
+export function validateForWhatsApp(
+  mimeType: string,
+  sizeBytes: number,
+): WaSizeCheck {
+  const kind = classifyMediaKind(mimeType);
+  const limitBytes = WA_LIMITS[kind];
+  if (sizeBytes <= 0) {
+    return { ok: false, kind, limitBytes, error: "Empty file" };
+  }
+  if (sizeBytes > limitBytes) {
+    return {
+      ok: false,
+      kind,
+      limitBytes,
+      error: `${labelFor(kind)} too large (${formatBytes(sizeBytes)} > ${formatBytes(limitBytes)} limit on WhatsApp)`,
+    };
+  }
+  return { ok: true, kind, limitBytes };
+}
+
+function labelFor(kind: WaMediaKind): string {
+  switch (kind) {
+    case "image":
+      return "Image";
+    case "video":
+      return "Video";
+    case "audio":
+      return "Audio";
+    case "document":
+      return "Document";
+    case "sticker":
+      return "Sticker";
+  }
+}
+
+/**
+ * Render a byte count with a sensible WhatsApp-style unit. Keeps a
+ * single decimal for MB so 5.0 MB and 4.8 MB read consistently in the
+ * error message.
+ */
+export function formatBytes(n: number): string {
+  if (n < KB) return `${n} B`;
+  if (n < MB) return `${(n / KB).toFixed(0)} KB`;
+  return `${(n / MB).toFixed(1)} MB`;
+}