diff --git a/frontend/src/app/pages/AgentChat/ChatInput.tsx b/frontend/src/app/pages/AgentChat/ChatInput.tsx
index f9f77918..701105b8 100644
--- a/frontend/src/app/pages/AgentChat/ChatInput.tsx
+++ b/frontend/src/app/pages/AgentChat/ChatInput.tsx
@@ -149,12 +149,17 @@ const ChatInput = forwardRef<ChatInputHandle, Props>(({ onSend, disabled, mode,
       contextPaths, sessionFrameworkOverhead,
     });
     if (block) {
-      // Auto-compact instead of prompting. Conversation history is the only
-      // overflow source we can shrink without losing user content — files were
-      // already auto-shrunk above, the prompt itself is the message the user
-      // just wrote, MCPs are framework. So if we're over, hit /compact, capture
-      // the send intent, and let the next-message effect fire it after the
-      // server-side compaction acks. User did nothing; problem solved silently.
+      if (block.kind === 'too_long') {
+        // This one message is too big to send even with zero history, so
+        // compaction can't save it. Hard-block and tell the user plainly; they
+        // shorten it and the block clears on the next send attempt. Don't fire
+        // /compact (pointless) and don't queue a retry (it'd just re-block).
+        pendingSendRef.current = null;
+        setSendBlock(block);
+        return;
+      }
+      // kind === 'compacting': history is the overflow source, which we CAN
+      // shrink. Auto-compact, capture the send intent, flash a status banner.
       if (sessionId) {
         pendingSendRef.current = () => { handleSend(); };
         try {
@@ -164,14 +169,14 @@ const ChatInput = forwardRef<ChatInputHandle, Props>(({ onSend, disabled, mode,
           await fetch(`${API_BASE}/agents/sessions/${sessionId}/compact`, { method: 'POST', headers });
         } catch (err) { console.error('[auto-compact] failed:', err); pendingSendRef.current = null; }
       }
-      // Briefly flash the banner as a status (not a prompt) so the user sees
-      // something happened. Auto-clear after 2s; in 99% of cases the auto-retry
-      // send has already fired by then.
       setSendBlock(block);
       setTimeout(() => setSendBlock(null), 2000);
       return;
     }
 
+    // Fits now (e.g. user shortened a too-long message): clear any lingering block banner.
+    setSendBlock(null);
+
     onboardingBus.emit('chat:message_sent');
     if (window.location.hash.includes('/apps/')) {
       onboardingBus.emit('app:generation_started');
diff --git a/frontend/src/app/pages/AgentChat/ChatInput/hooks/useContextFiles.ts b/frontend/src/app/pages/AgentChat/ChatInput/hooks/useContextFiles.ts
index 28f76581..dfc2dd90 100644
--- a/frontend/src/app/pages/AgentChat/ChatInput/hooks/useContextFiles.ts
+++ b/frontend/src/app/pages/AgentChat/ChatInput/hooks/useContextFiles.ts
@@ -17,6 +17,9 @@ function shrinkThreshold(modelCtx: number): number {
 }
 
 export type SendBlock = null | {
+  // 'compacting' = history overflow, auto-compact can fix it.
+  // 'too_long'   = this single message exceeds the window on its own; hard block.
+  kind: 'compacting' | 'too_long';
   estimate: number;
   window: number;
   history: number;
diff --git a/frontend/src/app/pages/AgentChat/ChatInput/sendHelpers.ts b/frontend/src/app/pages/AgentChat/ChatInput/sendHelpers.ts
index e6c82776..34288810 100644
--- a/frontend/src/app/pages/AgentChat/ChatInput/sendHelpers.ts
+++ b/frontend/src/app/pages/AgentChat/ChatInput/sendHelpers.ts
@@ -35,7 +35,16 @@ export function computeSendBlock({ trimmed, currentModelCtx, historyUsed, contex
     for (const cp of contextPaths) {
       if ((cp.tokens || 0) > (largest?.tokens || 0)) largest = { path: cp.path, tokens: cp.tokens || 0 };
     }
+    // Distinguish "history is the culprit" (compaction can fix it) from "this one
+    // message is too big on its own" (compaction can't help: dropping all prior
+    // turns still leaves framework+files+prompt over the window). The latter only
+    // happens with a giant pasted prompt, since attached files auto-shrink on
+    // attach. We surface that as a hard block instead of a doomed compact loop.
+    const nonHistory = framework + filesSum + promptTokens + systemTokens;
+    const kind: 'compacting' | 'too_long' =
+      nonHistory > Math.floor(win * 0.95) ? 'too_long' : 'compacting';
     return {
+      kind,
       estimate, window: win,
       history, system: systemTokens, framework, files: filesSum, prompt: promptTokens,
       largestFile: largest,
diff --git a/frontend/src/app/pages/AgentChat/ChatInput/view/SendBlockBanner.tsx b/frontend/src/app/pages/AgentChat/ChatInput/view/SendBlockBanner.tsx
index 0666c653..01fa2dc8 100644
--- a/frontend/src/app/pages/AgentChat/ChatInput/view/SendBlockBanner.tsx
+++ b/frontend/src/app/pages/AgentChat/ChatInput/view/SendBlockBanner.tsx
@@ -9,9 +9,26 @@ interface Props {
   c: ClaudeTokens;
 }
 
-/** Status indicator, not a prompt. Auto-compact already fired from handleSend; this
- *  just lets the user know we're freeing up space so the send doesn't look frozen. */
+/** Two states:
+ *  - 'compacting': auto-compact already fired; pulsing-dot status so the send
+ *    doesn't look frozen while we free up room.
+ *  - 'too_long': this single message can't fit no matter what; a plain, friendly
+ *    block telling the user to shorten it (no jargon, no spinner). */
 export const SendBlockBanner: React.FC<Props> = ({ sendBlock, c }) => {
+  if (sendBlock.kind === 'too_long') {
+    return (
+      <Box sx={{
+        mx: 1.5, mt: 1, mb: 0.5, px: 2, py: 1.25,
+        borderRadius: '12px',
+        bgcolor: c.bg.surface,
+        border: `1px solid ${c.border.medium}`,
+      }}>
+        <Typography sx={{ fontSize: '0.88rem', color: c.text.primary, lineHeight: 1.45 }}>
+          That message is too long to send. Try shortening it or splitting it into a few smaller ones.
+        </Typography>
+      </Box>
+    );
+  }
   return (
     <Box sx={{
       mx: 1.5, mt: 1, mb: 0.5, px: 2, py: 1.25,