diff --git a/packages/app/src/components/prompt-input/attachments.test.ts b/packages/app/src/components/prompt-input/attachments.test.ts
index 43f7d425bd14..a9376a3b94d2 100644
--- a/packages/app/src/components/prompt-input/attachments.test.ts
+++ b/packages/app/src/components/prompt-input/attachments.test.ts
@@ -1,5 +1,6 @@
 import { describe, expect, test } from "bun:test"
 import { attachmentMime } from "./files"
+import { MAX_ATTACHMENT_BYTES, estimateAttachment, totalAttachments, wouldExceedAttachmentLimit } from "./limit"
 import { pasteMode } from "./paste"
 
 describe("attachmentMime", () => {
@@ -42,3 +43,19 @@ describe("pasteMode", () => {
     expect(pasteMode("x".repeat(8000))).toBe("manual")
   })
 })
+
+describe("attachment limit", () => {
+  test("estimates encoded attachment size", () => {
+    expect(estimateAttachment({ size: 3 }, "image/png")).toBe("data:image/png;base64,".length + 4)
+  })
+
+  test("totals current attachments", () => {
+    expect(totalAttachments([{ dataUrl: "abc" }, { dataUrl: "de" }])).toBe(5)
+  })
+
+  test("flags uploads that exceed the total limit", () => {
+    const list = [{ dataUrl: "a".repeat(MAX_ATTACHMENT_BYTES - 4) }]
+    expect(wouldExceedAttachmentLimit(list, { size: 3 }, "image/png")).toBe(true)
+    expect(wouldExceedAttachmentLimit([], { size: 3 }, "image/png")).toBe(false)
+  })
+})
diff --git a/packages/app/src/components/prompt-input/attachments.ts b/packages/app/src/components/prompt-input/attachments.ts
index fa9930f6839a..0ff123bf0586 100644
--- a/packages/app/src/components/prompt-input/attachments.ts
+++ b/packages/app/src/components/prompt-input/attachments.ts
@@ -5,6 +5,7 @@ import { useLanguage } from "@/context/language"
 import { uuid } from "@/utils/uuid"
 import { getCursorPosition } from "./editor-dom"
 import { attachmentMime } from "./files"
+import { wouldExceedAttachmentLimit } from "./limit"
 import { normalizePaste, pasteMode } from "./paste"
 
 function dataUrl(file: File, mime: string) {
@@ -33,6 +34,8 @@ type PromptAttachmentsInput = {
   readClipboardImage?: () => Promise<File | null>
 }
 
+type AddState = "added" | "failed" | "unsupported" | "limit"
+
 export function createPromptAttachments(input: PromptAttachmentsInput) {
   const prompt = usePrompt()
   const language = useLanguage()
@@ -44,18 +47,27 @@ export function createPromptAttachments(input: PromptAttachmentsInput) {
     })
   }
 
-  const add = async (file: File, toast = true) => {
+  const warnLimit = () => {
+    showToast({
+      title: language.t("prompt.toast.attachmentLimit.title"),
+      description: language.t("prompt.toast.attachmentLimit.description"),
+    })
+  }
+
+  const add = async (file: File): Promise<AddState> => {
     const mime = await attachmentMime(file)
     if (!mime) {
-      if (toast) warn()
-      return false
+      return "unsupported" as const
     }
 
     const editor = input.editor()
-    if (!editor) return false
+    if (!editor) return "failed" as const
+
+    const images = prompt.current().filter((part): part is ImageAttachmentPart => part.type === "image")
+    if (wouldExceedAttachmentLimit(images, file, mime)) return "limit" as const
 
     const url = await dataUrl(file, mime)
-    if (!url) return false
+    if (!url) return "failed" as const
 
     const attachment: ImageAttachmentPart = {
       type: "image",
@@ -66,23 +78,26 @@ export function createPromptAttachments(input: PromptAttachmentsInput) {
     }
     const cursor = prompt.cursor() ?? getCursorPosition(editor)
     prompt.set([...prompt.current(), attachment], cursor)
-    return true
+    return "added" as const
   }
 
-  const addAttachment = (file: File) => add(file)
-
-  const addAttachments = async (files: File[], toast = true) => {
-    let found = false
-
-    for (const file of files) {
-      const ok = await add(file, false)
-      if (ok) found = true
+  const addAttachments = async (list: File[]) => {
+    const result = { added: false, unsupported: false }
+    for (const file of list) {
+      const state = await add(file)
+      if (state === "limit") {
+        warnLimit()
+        return result.added
+      }
+      result.added = result.added || state === "added"
+      result.unsupported = result.unsupported || state === "unsupported"
     }
-
-    if (!found && files.length > 0 && toast) warn()
-    return found
+    if (!result.added && result.unsupported) warn()
+    return result.added
   }
 
+  const addAttachment = (file: File) => addAttachments([file])
+
   const removeAttachment = (id: string) => {
     const current = prompt.current()
     const next = current.filter((part) => part.type !== "image" || part.id !== id)
diff --git a/packages/app/src/components/prompt-input/limit.ts b/packages/app/src/components/prompt-input/limit.ts
new file mode 100644
index 000000000000..3a90262a5f73
--- /dev/null
+++ b/packages/app/src/components/prompt-input/limit.ts
@@ -0,0 +1,15 @@
+const MB = 1024 * 1024
+
+export const MAX_ATTACHMENT_BYTES = 5 * MB
+
+export function estimateAttachment(file: { size: number }, mime: string) {
+  return `data:${mime};base64,`.length + Math.ceil(file.size / 3) * 4
+}
+
+export function totalAttachments(list: Array<{ dataUrl: string }>) {
+  return list.reduce((sum, part) => sum + part.dataUrl.length, 0)
+}
+
+export function wouldExceedAttachmentLimit(list: Array<{ dataUrl: string }>, file: { size: number }, mime: string) {
+  return totalAttachments(list) + estimateAttachment(file, mime) > MAX_ATTACHMENT_BYTES
+}
diff --git a/packages/app/src/i18n/en.ts b/packages/app/src/i18n/en.ts
index 579b740d3aee..58089ffd8136 100644
--- a/packages/app/src/i18n/en.ts
+++ b/packages/app/src/i18n/en.ts
@@ -281,6 +281,8 @@ export const dict = {
   "prompt.action.send": "Send",
   "prompt.action.stop": "Stop",
 
+  "prompt.toast.attachmentLimit.title": "Attachment limit reached",
+  "prompt.toast.attachmentLimit.description": "Attachments can total up to 5 MB. Try smaller or fewer files.",
   "prompt.toast.pasteUnsupported.title": "Unsupported attachment",
   "prompt.toast.pasteUnsupported.description": "Only images, PDFs, or text files can be attached here.",
   "prompt.toast.modelAgentRequired.title": "Select an agent and model",