fix(web): reject oversized prompts before provider turn start

apps/web/src/components/ChatView.tsx

@@ -4980,6 +4980,16 @@ function ChatViewContent(props: ChatViewProps) {
         draftText: trimmed,
         planMarkdown: activeProposedPlan.planMarkdown,
       });
+      const outgoingFollowUpText = formatOutgoingPrompt({
+        provider: ctxSelectedProvider,
+        model: ctxSelectedModel,
+        models: ctxSelectedProviderModels,
+        effort: ctxSelectedPromptEffort,
+        text: followUp.text.trim(),
+      });
+      if (composerRef.current?.validateProviderInput(outgoingFollowUpText) === false) {
+        return;
+      }
       promptRef.current = "";
       clearComposerDraftContent(composerDraftTarget);
       composerRef.current?.resetCursorState();
@@ -5049,24 +5059,6 @@ function ChatViewContent(props: ChatViewProps) {
       return;
     }
 
-    sendInFlightRef.current = true;
-    if (isDraftHeroState && activeThreadKey) {
-      let resolveDockStarted: (() => void) | undefined;
-      const dockStarted = new Promise<void>((resolve) => {
-        resolveDockStarted = resolve;
-      });
-      const dockTransition = runMobileComposerTransition(() => {
-        flushSync(() => {
-          captureDraftHeroComposerRect();
-          setDockedDraftHeroThreadKey(activeThreadKey);
-        });
-        resolveDockStarted?.();
-      });
-      void dockTransition.catch(() => resolveDockStarted?.());
-      await dockStarted;
-    }
-    beginLocalDispatch({ preparingWorktree: Boolean(baseBranchForWorktree) });
-
     const composerImagesSnapshot = [...composerImages];
     const composerTerminalContextsSnapshot = [...sendableComposerTerminalContexts];
     const composerElementContextsSnapshot = [...composerElementContexts];
@@ -5084,15 +5076,37 @@ function ChatViewContent(props: ChatViewProps) {
       messageTextWithPreviewAnnotations,
       composerReviewCommentsSnapshot,
     );
-    const messageIdForSend = newMessageId();
-    const messageCreatedAt = new Date().toISOString();
     const outgoingMessageText = formatOutgoingPrompt({
       provider: ctxSelectedProvider,
       model: ctxSelectedModel,
       models: ctxSelectedProviderModels,
       effort: ctxSelectedPromptEffort,
       text: messageTextForSend || IMAGE_ONLY_BOOTSTRAP_PROMPT,
     });
+    if (composerRef.current?.validateProviderInput(outgoingMessageText) === false) {
+      return;
+    }
+
+    sendInFlightRef.current = true;
+    if (isDraftHeroState && activeThreadKey) {
+      let resolveDockStarted: (() => void) | undefined;
+      const dockStarted = new Promise<void>((resolve) => {
+        resolveDockStarted = resolve;
+      });
+      const dockTransition = runMobileComposerTransition(() => {
+        flushSync(() => {
+          captureDraftHeroComposerRect();
+          setDockedDraftHeroThreadKey(activeThreadKey);
+        });
+        resolveDockStarted?.();
+      });
+      void dockTransition.catch(() => resolveDockStarted?.());
+      await dockStarted;
+    }
+    beginLocalDispatch({ preparingWorktree: Boolean(baseBranchForWorktree) });
+
+    const messageIdForSend = newMessageId();
+    const messageCreatedAt = new Date().toISOString();
     const turnAttachmentsPromise = Promise.all(
       composerImagesSnapshot.map(async (image) => ({
         type: "image" as const,
@@ -5709,6 +5723,9 @@ function ChatViewContent(props: ChatViewProps) {
       effort: ctxSelectedPromptEffort,
       text: implementationPrompt,
     });
+    if (composerRef.current?.validateProviderInput(outgoingImplementationPrompt) === false) {
+      return;
+    }
     const nextThreadTitle = truncate(buildPlanImplementationThreadTitle(planMarkdown));
     const nextThreadModelSelection: ModelSelection = ctxSelectedModelSelection;
 

apps/web/src/components/chat/ChatComposer.tsx

@@ -106,6 +106,12 @@ import { buildExpandedImagePreview, type ExpandedImagePreview } from "./Expanded
 import { basenameOfPath } from "../../pierre-icons";
 import { cn, randomUUID } from "~/lib/utils";
 import { Separator } from "../ui/separator";
+import {
+  getComposerPromptLengthValidationMessage,
+  getComposerSubmissionValidationMessage,
+  submitComposerDraft,
+} from "./composerSubmission";
+import { ComposerPromptLengthValidation } from "./ComposerPromptLengthValidation";
 
 type ComposerCommandMenuPosition = {
   bottom: number;
@@ -484,6 +490,8 @@ export interface ChatComposerHandle {
     selectedModel: string;
     selectedProviderModels: ReadonlyArray<ServerProvider["models"][number]>;
   };
+  /** Validate the fully composed text immediately before a provider turn starts. */
+  validateProviderInput: (providerInput: string) => boolean;
 }
 
 // --------------------------------------------------------------------------
@@ -947,6 +955,10 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
   const [isComposerPrimaryActionsCompact, setIsComposerPrimaryActionsCompact] = useState(false);
   const [isComposerModelPickerOpen, setIsComposerModelPickerOpen] = useState(false);
   const [isComposerFocused, setIsComposerFocused] = useState(false);
+  const [composerSubmissionError, setComposerSubmissionError] = useState<string | null>(null);
+  const [providerInputSubmissionError, setProviderInputSubmissionError] = useState<string | null>(
+    null,
+  );
   const [composerMenuAnchor, setComposerMenuAnchor] = useState<HTMLDivElement | null>(null);
   const [isStashMenuOpen, setIsStashMenuOpen] = useState(false);
   const [stashPulse, setStashPulse] = useState<{ key: number; active: boolean }>({
@@ -963,6 +975,7 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
   const composerEditorRef = useRef<ComposerPromptEditorHandle>(null);
   const composerFormRef = useRef<HTMLFormElement>(null);
   const composerSurfaceRef = useRef<HTMLDivElement>(null);
+  const providerInputRejectedRef = useRef(false);
   const composerSelectLockRef = useRef(false);
   const composerMenuOpenRef = useRef(false);
   const composerMenuItemsRef = useRef<ComposerCommandItem[]>([]);
@@ -1306,6 +1319,27 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
     setComposerCursor((existing) => clampCollapsedComposerCursor(prompt, existing));
   }, [prompt, promptRef]);
 
+  useEffect(() => {
+    if (composerSubmissionError === null) return;
+    const nextError = getComposerPromptLengthValidationMessage(prompt);
+    if (nextError !== composerSubmissionError) {
+      setComposerSubmissionError(nextError);
+    }
+  }, [composerSubmissionError, prompt]);
+
+  useEffect(() => {
+    setProviderInputSubmissionError(null);
+  }, [
+    composerElementContexts,
+    composerPreviewAnnotations,
+    composerReviewComments,
+    composerTerminalContexts,
+    prompt,
+    selectedModel,
+    selectedPromptEffort,
+    selectedProvider,
+  ]);
+
   useEffect(() => {
     composerImagesRef.current = composerImages;
   }, [composerImages, composerImagesRef]);
@@ -1397,6 +1431,8 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
   // ------------------------------------------------------------------
   useEffect(() => {
     setComposerHighlightedItemId(null);
+    setComposerSubmissionError(null);
+    setProviderInputSubmissionError(null);
     setComposerCursor(collapseExpandedComposerCursor(promptRef.current, promptRef.current.length));
     setComposerTrigger(detectComposerTrigger(promptRef.current, promptRef.current.length));
     dragDepthRef.current = 0;
@@ -1824,17 +1860,32 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
         });
         return;
       }
-      onSend(event);
+      const submission = submitComposerDraft({
+        prompt: promptRef.current,
+        submissionTarget: activePendingProgress ? "pending-user-input" : "provider-turn",
+        event,
+        onSend: (sendEvent) => {
+          // ChatView reports its final composed-input preflight through the
+          // composer handle before its first asynchronous send step.
+          providerInputRejectedRef.current = false;
+          onSend(sendEvent);
+          return !providerInputRejectedRef.current;
+        },
+      });
+      setComposerSubmissionError(submission.validationMessage);
+      if (!submission.didDispatch) return;
       if (shouldBlurMobileComposerOnSubmit()) {
         blurMobileComposerAfterSend();
       }
     },
     [
       activeThreadId,
+      activePendingProgress,
       blurMobileComposerAfterSend,
       isSendDisabled,
       noProviderAvailable,
       onSend,
+      promptRef,
       shouldBlurMobileComposerOnSubmit,
     ],
   );
@@ -2616,6 +2667,16 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
         selectedModel,
         selectedProviderModels,
       }),
+      validateProviderInput: (providerInput: string) => {
+        const validationMessage = getComposerSubmissionValidationMessage({
+          prompt: promptRef.current,
+          providerInput,
+          submissionTarget: "provider-turn",
+        });
+        providerInputRejectedRef.current = validationMessage !== null;
+        setProviderInputSubmissionError(validationMessage);
+        return validationMessage === null;
+      },
     }),
     [
       activeThread,
@@ -3086,6 +3147,10 @@ export const ChatComposer = memo(function ChatComposer(props: ChatComposerProps)
             </div>
           </div>
 
+          <ComposerPromptLengthValidation
+            message={providerInputSubmissionError ?? composerSubmissionError}
+          />
+
           {/* Bottom toolbar */}
           {isComposerCollapsedMobile ? null : activePendingApproval ? (
             <div className="flex items-center justify-end gap-2 px-3 pb-3 sm:px-4 sm:pb-4">

apps/web/src/components/chat/ComposerPromptLengthValidation.test.tsx

@@ -0,0 +1,23 @@
+import { PROVIDER_SEND_TURN_MAX_INPUT_CHARS } from "@t3tools/contracts";
+import { renderToStaticMarkup } from "react-dom/server";
+import { describe, expect, it } from "vite-plus/test";
+
+import { getComposerPromptLengthValidationMessage } from "./composerSubmission";
+import { ComposerPromptLengthValidation } from "./ComposerPromptLengthValidation";
+
+describe("ComposerPromptLengthValidation", () => {
+  it("renders oversized prompt feedback as an actionable composer alert", () => {
+    const message = getComposerPromptLengthValidationMessage(
+      "x".repeat(PROVIDER_SEND_TURN_MAX_INPUT_CHARS + 1),
+    );
+
+    const markup = renderToStaticMarkup(<ComposerPromptLengthValidation message={message} />);
+
+    expect(markup).toContain('role="alert"');
+    expect(markup).toContain('data-chat-composer-validation="prompt-length"');
+    expect(markup).toContain(
+      "Prompt is 1 character over the 120,000-character limit. Shorten or split it before sending.",
+    );
+    expect(markup).not.toContain("ProviderValidationError");
+  });
+});

apps/web/src/components/chat/ComposerPromptLengthValidation.tsx

@@ -0,0 +1,13 @@
+export function ComposerPromptLengthValidation({ message }: { message: string | null }) {
+  if (!message) return null;
+
+  return (
+    <p
+      role="alert"
+      className="px-3 pb-2 text-xs text-destructive sm:px-4"
+      data-chat-composer-validation="prompt-length"
+    >
+      {message}
+    </p>
+  );
+}