diff --git a/packages/app/src/composer/attachments/attachments.ts b/packages/app/src/composer/attachments/attachments.ts
index 8350ea46c49..4fa2a7a3ab4 100644
--- a/packages/app/src/composer/attachments/attachments.ts
+++ b/packages/app/src/composer/attachments/attachments.ts
@@ -52,14 +52,14 @@ export function createComposerAttachments(
   // Uploads this composer started; they finish (or fail) even if the composer unmounts.
   const [pending, setPending] = createStore<{ ids: string[] }>({ ids: [] })
 
-  // A file the model reads natively travels inline with the prompt, so its bytes live in the draft
-  // store. Anything else reaches the model as a path on the server and never enters the store:
-  // hashing and copying a large archive through it is what used to freeze the window.
+  // Media the model reads natively travels inline with the prompt, so its bytes live in the draft
+  // store. Everything else, including text, reaches the model as a path on the server that its
+  // tools open; those bytes never enter the store, and never get base64-encoded into the request.
   const add = async (file: File, target = capture(), clipboard = false) => {
     if (!target) return false
     const mime = await attachmentMime(file)
     const destination = input.destination()
-    if (native(mime, destination.input)) return addInline(file, mime, target, clipboard)
+    if (native(mime, destination.input) && file.size <= MAX_INLINE_BYTES) return addInline(file, mime, target, clipboard)
     const sourcePath = input.getPathForFile?.(file) || undefined
     if (destination.local && sourcePath) return addPath(target, { filename: file.name, mime, path: sourcePath })
     void stage(file, mime, target, destination)
@@ -213,9 +213,11 @@ export function createComposerAttachments(
 
 const imageMimes = new Set(["image/png", "image/jpeg", "image/gif", "image/webp"])
 
-// Mirrors the attachment kinds the server forwards to the model as message content.
+// The server rejects inline attachments above this size, so larger media takes the path route.
+const MAX_INLINE_BYTES = 20 * 1024 * 1024
+
+// Mirrors the media the server forwards to the model as message content.
 function native(mime: string, input: AttachmentDestination["input"]) {
-  if (mime === "text/plain") return true
   if (imageMimes.has(mime)) return input.image
   if (mime === "application/pdf") return input.pdf
   return false
@@ -238,8 +240,8 @@ const textMimes = new Set([
   "application/yaml",
 ])
 
-// Text-like files normalize to text/plain so the server inlines their content; every other
-// file keeps a binary type and is delivered to the model by path or as native media.
+// Text-like files normalize to text/plain so the chip labels them as text; every other file keeps
+// a binary type. Delivery is decided separately: native media inline, everything else by path.
 async function attachmentMime(file: File) {
   const type = file.type.split(";", 1)[0]?.trim().toLowerCase() ?? ""
   if (imageMimes.has(type) || type === "application/pdf") return type
diff --git a/packages/core/src/session/prompt.ts b/packages/core/src/session/prompt.ts
index bb552681e77..80878f354dd 100644
--- a/packages/core/src/session/prompt.ts
+++ b/packages/core/src/session/prompt.ts
@@ -93,9 +93,10 @@ export const prepare = Effect.fn("SessionPrompt.prepare")(function* (request: {
 const materializeAttachment = Effect.fn("SessionPrompt.materializeAttachment")(function* (
   input: PromptInput.FileAttachment,
 ) {
+  const label = attachmentLabel(input)
   const resolved = input.uri.startsWith("data:")
     ? {
-        bytes: yield* decodeDataURL(input.uri),
+        bytes: yield* decodeDataURL(input.uri, label),
         source: { type: "inline" as const },
         start: undefined,
         end: undefined,
@@ -105,8 +106,8 @@ const materializeAttachment = Effect.fn("SessionPrompt.materializeAttachment")(f
     : yield* readFileAttachment(input.uri)
   if (resolved.bytes.byteLength > MAX_ATTACHMENT_BYTES)
     return yield* new AttachmentError({
-      uri: input.uri,
-      message: `Attachment exceeds the ${MAX_ATTACHMENT_BYTES} byte limit: ${input.uri}`,
+      uri: label,
+      message: `Attachment exceeds the ${MAX_ATTACHMENT_BYTES} byte limit: ${label}`,
     })
 
   const mime = resolved.mime ?? Mime.detect(resolved.bytes)
@@ -138,7 +139,7 @@ const normalizeImageAttachment = Effect.fn("SessionPrompt.normalizeImageAttachme
 ) {
   if (!mime.startsWith("image/")) return { data: Base64.make(data), mime }
   const image = yield* Image.Service
-  const label = input.name ?? (input.uri.startsWith("data:") ? "inline attachment" : input.uri)
+  const label = attachmentLabel(input)
   const content = { uri: label, content: data, encoding: "base64" as const, mime }
   const normalized = yield* image.normalize(label, content).pipe(
     Effect.catchTag("Image.ResizerUnavailableError", () => Effect.succeed(content)),
@@ -201,7 +202,12 @@ const readFileAttachment = Effect.fn("SessionPrompt.readFileAttachment")(functio
 
 const MAX_ATTACHMENT_BYTES = 20 * 1024 * 1024
 
-function decodeDataURL(uri: string) {
+// A data URL is the whole file; errors and logs must name the attachment, not echo its bytes.
+function attachmentLabel(input: PromptInput.FileAttachment) {
+  return input.name ?? (input.uri.startsWith("data:") ? "inline attachment" : input.uri)
+}
+
+function decodeDataURL(uri: string, label: string) {
   return Effect.try({
     try: () => {
       const comma = uri.indexOf(",")
@@ -214,7 +220,7 @@ function decodeDataURL(uri: string) {
       if (bytes.toString("base64") !== payload) throw new Error("Non-canonical base64")
       return bytes
     },
-    catch: () => new AttachmentError({ uri, message: "Invalid attachment data URL" }),
+    catch: () => new AttachmentError({ uri: label, message: `Invalid attachment data URL: ${label}` }),
   })
 }
 
diff --git a/packages/core/test/session-prompt.test.ts b/packages/core/test/session-prompt.test.ts
index 16ca1e2c063..96a2cddcb56 100644
--- a/packages/core/test/session-prompt.test.ts
+++ b/packages/core/test/session-prompt.test.ts
@@ -501,8 +501,31 @@ describe("Session.prompt", () => {
 
       expect(error).toMatchObject({
         _tag: "Session.AttachmentError",
-        uri,
-        message: "Invalid attachment data URL",
+        uri: "image.png",
+        message: "Invalid attachment data URL: image.png",
+      })
+    }),
+  )
+
+  it.effect("rejects oversized inline attachments without echoing their bytes", () =>
+    Effect.gen(function* () {
+      yield* setup
+      const session = yield* Session.Service
+      const uri = `data:application/octet-stream;base64,${Buffer.alloc(20 * 1024 * 1024 + 1).toString("base64")}`
+
+      const error = yield* session
+        .prompt({
+          sessionID,
+          text: "Inspect this",
+          files: [{ uri }],
+          resume: false,
+        })
+        .pipe(Effect.flip)
+
+      expect(error).toMatchObject({
+        _tag: "Session.AttachmentError",
+        uri: "inline attachment",
+        message: "Attachment exceeds the 20971520 byte limit: inline attachment",
       })
     }),
   )