From f4b4ba4c5449e936fd860536a3fb3523c3be5452 Mon Sep 17 00:00:00 2001
From: Jason Woltje <jason@diversecanvas.com>
Date: Wed, 25 Feb 2026 20:38:49 -0600
Subject: [PATCH] feat(web): implement SSE chat streaming with real-time token
 rendering

- Implement streamChatMessage() using fetch ReadableStream for SSE parsing
- Update useChat hook with streaming state, abort support, and fallback to non-streaming
- Add streaming indicator (blinking cursor) in MessageList during token streaming
- Update ChatInput with Stop button during streaming, disable input while streaming
- Add CSS animations for streaming cursor
- Fix message ID uniqueness to prevent collisions in rapid sends
- Update tests for streaming path with makeStreamSucceed/makeStreamFail helpers

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 apps/web/src/app/globals.css                 |  22 ++
 apps/web/src/components/chat/Chat.test.tsx   |   2 +
 apps/web/src/components/chat/Chat.tsx        |  34 +-
 apps/web/src/components/chat/ChatInput.tsx   |  94 +++--
 apps/web/src/components/chat/MessageList.tsx | 136 ++++---
 apps/web/src/hooks/useChat.test.ts           | 366 ++++++++++++++-----
 apps/web/src/hooks/useChat.ts                | 303 ++++++++++-----
 apps/web/src/lib/api/chat.ts                 | 137 ++++++-
 8 files changed, 797 insertions(+), 297 deletions(-)

diff --git a/apps/web/src/app/globals.css b/apps/web/src/app/globals.css
index 090d5d2..40c2fd5 100644
--- a/apps/web/src/app/globals.css
+++ b/apps/web/src/app/globals.css
@@ -765,6 +765,28 @@ body::before {
   animation: scaleIn 0.1s ease-out;
 }
 
+/* Streaming cursor for real-time token rendering */
+@keyframes streaming-cursor-blink {
+  0%,
+  100% {
+    opacity: 1;
+  }
+  50% {
+    opacity: 0;
+  }
+}
+
+.streaming-cursor {
+  display: inline-block;
+  width: 2px;
+  height: 1em;
+  background-color: rgb(var(--accent-primary));
+  border-radius: 1px;
+  animation: streaming-cursor-blink 1s step-end infinite;
+  vertical-align: text-bottom;
+  margin-left: 1px;
+}
+
 /* -----------------------------------------------------------------------------
    Dashboard Layout — Responsive Grids
    ----------------------------------------------------------------------------- */
diff --git a/apps/web/src/components/chat/Chat.test.tsx b/apps/web/src/components/chat/Chat.test.tsx
index 8004250..958888a 100644
--- a/apps/web/src/components/chat/Chat.test.tsx
+++ b/apps/web/src/components/chat/Chat.test.tsx
@@ -64,10 +64,12 @@ function createMockUseChatReturn(
       },
     ],
     isLoading: false,
+    isStreaming: false,
     error: null,
     conversationId: null,
     conversationTitle: null,
     sendMessage: vi.fn().mockResolvedValue(undefined),
+    abortStream: vi.fn(),
     loadConversation: vi.fn().mockResolvedValue(undefined),
     startNewConversation: vi.fn(),
     setMessages: vi.fn(),
diff --git a/apps/web/src/components/chat/Chat.tsx b/apps/web/src/components/chat/Chat.tsx
index 1cf0c6e..a8edcbf 100644
--- a/apps/web/src/components/chat/Chat.tsx
+++ b/apps/web/src/components/chat/Chat.tsx
@@ -59,14 +59,15 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
 
   const { user, isLoading: authLoading } = useAuth();
 
-  // Use the chat hook for state management
   const {
     messages,
     isLoading: isChatLoading,
+    isStreaming,
     error,
     conversationId,
     conversationTitle,
     sendMessage,
+    abortStream,
     loadConversation,
     startNewConversation,
     clearError,
@@ -75,15 +76,7 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
     ...(initialProjectId !== undefined && { projectId: initialProjectId }),
   });
 
-  // Connect to WebSocket for real-time updates (when we have a user)
-  const { isConnected: isWsConnected } = useWebSocket(
-    user?.id ?? "", // Use user ID as workspace ID for now
-    "", // Token not needed since we use cookies
-    {
-      // Future: Add handlers for chat-related events
-      // onChatMessage: (msg) => { ... }
-    }
-  );
+  const { isConnected: isWsConnected } = useWebSocket(user?.id ?? "", "", {});
 
   const messagesEndRef = useRef<HTMLDivElement>(null);
   const inputRef = useRef<HTMLTextAreaElement>(null);
@@ -91,7 +84,10 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
   const quipTimerRef = useRef<NodeJS.Timeout | null>(null);
   const quipIntervalRef = useRef<NodeJS.Timeout | null>(null);
 
-  // Expose methods to parent via ref
+  // Identify the streaming message (last assistant message while streaming)
+  const streamingMessageId =
+    isStreaming && messages.length > 0 ? messages[messages.length - 1]?.id : undefined;
+
   useImperativeHandle(ref, () => ({
     loadConversation: async (cId: string): Promise<void> => {
       await loadConversation(cId);
@@ -110,7 +106,6 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
     scrollToBottom();
   }, [messages, scrollToBottom]);
 
-  // Notify parent of conversation changes
   useEffect(() => {
     if (conversationId && conversationTitle) {
       onConversationChange?.(conversationId, {
@@ -125,7 +120,6 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
     }
   }, [conversationId, conversationTitle, initialProjectId, onConversationChange]);
 
-  // Global keyboard shortcut: Ctrl+/ to focus input
   useEffect(() => {
     const handleKeyDown = (e: KeyboardEvent): void => {
       if ((e.ctrlKey || e.metaKey) && e.key === "/") {
@@ -139,20 +133,17 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
     };
   }, []);
 
-  // Show loading quips
+  // Show loading quips only during non-streaming load (initial fetch wait)
   useEffect(() => {
-    if (isChatLoading) {
-      // Show first quip after 3 seconds
+    if (isChatLoading && !isStreaming) {
       quipTimerRef.current = setTimeout(() => {
         setLoadingQuip(WAITING_QUIPS[Math.floor(Math.random() * WAITING_QUIPS.length)] ?? null);
       }, 3000);
 
-      // Change quip every 5 seconds
       quipIntervalRef.current = setInterval(() => {
         setLoadingQuip(WAITING_QUIPS[Math.floor(Math.random() * WAITING_QUIPS.length)] ?? null);
       }, 5000);
     } else {
-      // Clear timers when loading stops
       if (quipTimerRef.current) {
         clearTimeout(quipTimerRef.current);
         quipTimerRef.current = null;
@@ -168,7 +159,7 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
       if (quipTimerRef.current) clearTimeout(quipTimerRef.current);
       if (quipIntervalRef.current) clearInterval(quipIntervalRef.current);
     };
-  }, [isChatLoading]);
+  }, [isChatLoading, isStreaming]);
 
   const handleSendMessage = useCallback(
     async (content: string) => {
@@ -177,7 +168,6 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
     [sendMessage]
   );
 
-  // Show loading state while auth is loading
   if (authLoading) {
     return (
       <div
@@ -227,6 +217,8 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
           <MessageList
             messages={messages as (Message & { thinking?: string })[]}
             isLoading={isChatLoading}
+            isStreaming={isStreaming}
+            {...(streamingMessageId != null ? { streamingMessageId } : {})}
             loadingQuip={loadingQuip}
           />
           <div ref={messagesEndRef} />
@@ -294,6 +286,8 @@ export const Chat = forwardRef<ChatRef, ChatProps>(function Chat(
             onSend={handleSendMessage}
             disabled={isChatLoading || !user}
             inputRef={inputRef}
+            isStreaming={isStreaming}
+            onStopStreaming={abortStream}
           />
         </div>
       </div>
diff --git a/apps/web/src/components/chat/ChatInput.tsx b/apps/web/src/components/chat/ChatInput.tsx
index 87cc91b..721f6b5 100644
--- a/apps/web/src/components/chat/ChatInput.tsx
+++ b/apps/web/src/components/chat/ChatInput.tsx
@@ -7,13 +7,20 @@ interface ChatInputProps {
   onSend: (message: string) => void;
   disabled?: boolean;
   inputRef?: RefObject<HTMLTextAreaElement | null>;
+  isStreaming?: boolean;
+  onStopStreaming?: () => void;
 }
 
-export function ChatInput({ onSend, disabled, inputRef }: ChatInputProps): React.JSX.Element {
+export function ChatInput({
+  onSend,
+  disabled,
+  inputRef,
+  isStreaming = false,
+  onStopStreaming,
+}: ChatInputProps): React.JSX.Element {
   const [message, setMessage] = useState("");
   const [version, setVersion] = useState<string | null>(null);
 
-  // Fetch version from static version.json (generated at build time)
   useEffect(() => {
     interface VersionData {
       version?: string;
@@ -24,7 +31,6 @@ export function ChatInput({ onSend, disabled, inputRef }: ChatInputProps): React
       .then((res) => res.json() as Promise<VersionData>)
       .then((data) => {
         if (data.version) {
-          // Format as "version+commit" for full build identification
           const fullVersion = data.commit ? `${data.version}+${data.commit}` : data.version;
           setVersion(fullVersion);
         }
@@ -35,20 +41,22 @@ export function ChatInput({ onSend, disabled, inputRef }: ChatInputProps): React
   }, []);
 
   const handleSubmit = useCallback(() => {
-    if (message.trim() && !disabled) {
+    if (message.trim() && !disabled && !isStreaming) {
       onSend(message);
       setMessage("");
     }
-  }, [message, onSend, disabled]);
+  }, [message, onSend, disabled, isStreaming]);
+
+  const handleStop = useCallback(() => {
+    onStopStreaming?.();
+  }, [onStopStreaming]);
 
   const handleKeyDown = useCallback(
     (e: KeyboardEvent<HTMLTextAreaElement>) => {
-      // Enter to send (without Shift)
       if (e.key === "Enter" && !e.shiftKey) {
         e.preventDefault();
         handleSubmit();
       }
-      // Ctrl/Cmd + Enter to send (alternative)
       if (e.key === "Enter" && (e.ctrlKey || e.metaKey)) {
         e.preventDefault();
         handleSubmit();
@@ -61,6 +69,7 @@ export function ChatInput({ onSend, disabled, inputRef }: ChatInputProps): React
   const maxCharacters = 4000;
   const isNearLimit = characterCount > maxCharacters * 0.9;
   const isOverLimit = characterCount > maxCharacters;
+  const isInputDisabled = disabled ?? false;
 
   return (
     <div className="space-y-3">
@@ -69,7 +78,10 @@ export function ChatInput({ onSend, disabled, inputRef }: ChatInputProps): React
         className="relative rounded-lg border transition-all duration-150"
         style={{
           backgroundColor: "rgb(var(--surface-0))",
-          borderColor: disabled ? "rgb(var(--border-default))" : "rgb(var(--border-strong))",
+          borderColor:
+            isInputDisabled || isStreaming
+              ? "rgb(var(--border-default))"
+              : "rgb(var(--border-strong))",
         }}
       >
         <textarea
@@ -79,8 +91,8 @@ export function ChatInput({ onSend, disabled, inputRef }: ChatInputProps): React
             setMessage(e.target.value);
           }}
           onKeyDown={handleKeyDown}
-          placeholder="Type a message..."
-          disabled={disabled}
+          placeholder={isStreaming ? "AI is responding..." : "Type a message..."}
+          disabled={isInputDisabled || isStreaming}
           rows={1}
           className="block w-full resize-none bg-transparent px-4 py-3 pr-24 text-sm outline-none placeholder:text-[rgb(var(--text-muted))] disabled:opacity-50"
           style={{
@@ -97,28 +109,47 @@ export function ChatInput({ onSend, disabled, inputRef }: ChatInputProps): React
           aria-describedby="input-help"
         />
 
-        {/* Send Button */}
+        {/* Send / Stop Button */}
         <div className="absolute bottom-2 right-2 flex items-center gap-2">
-          <button
-            onClick={handleSubmit}
-            disabled={(disabled ?? !message.trim()) || isOverLimit}
-            className="btn-primary btn-sm rounded-md"
-            style={{
-              opacity: disabled || !message.trim() || isOverLimit ? 0.5 : 1,
-            }}
-            aria-label="Send message"
-          >
-            <svg
-              className="h-4 w-4"
-              fill="none"
-              viewBox="0 0 24 24"
-              stroke="currentColor"
-              strokeWidth={2}
+          {isStreaming ? (
+            <button
+              onClick={handleStop}
+              className="btn-sm rounded-md flex items-center gap-1.5"
+              style={{
+                backgroundColor: "rgb(var(--semantic-error))",
+                color: "white",
+                padding: "0.25rem 0.75rem",
+              }}
+              aria-label="Stop generating"
+              title="Stop generating"
             >
-              <path d="M5 12h14M12 5l7 7-7 7" />
-            </svg>
-            <span className="hidden sm:inline">Send</span>
-          </button>
+              <svg className="h-4 w-4" fill="currentColor" viewBox="0 0 24 24" aria-hidden="true">
+                <rect x="6" y="6" width="12" height="12" rx="1" />
+              </svg>
+              <span className="hidden sm:inline text-sm font-medium">Stop</span>
+            </button>
+          ) : (
+            <button
+              onClick={handleSubmit}
+              disabled={isInputDisabled || !message.trim() || isOverLimit}
+              className="btn-primary btn-sm rounded-md"
+              style={{
+                opacity: isInputDisabled || !message.trim() || isOverLimit ? 0.5 : 1,
+              }}
+              aria-label="Send message"
+            >
+              <svg
+                className="h-4 w-4"
+                fill="none"
+                viewBox="0 0 24 24"
+                stroke="currentColor"
+                strokeWidth={2}
+              >
+                <path d="M5 12h14M12 5l7 7-7 7" />
+              </svg>
+              <span className="hidden sm:inline">Send</span>
+            </button>
+          )}
         </div>
       </div>
 
@@ -128,7 +159,6 @@ export function ChatInput({ onSend, disabled, inputRef }: ChatInputProps): React
         style={{ color: "rgb(var(--text-muted))" }}
         id="input-help"
       >
-        {/* Keyboard Shortcuts */}
         <div className="hidden items-center gap-4 sm:flex">
           <div className="flex items-center gap-1.5">
             <span className="kbd">Enter</span>
@@ -142,10 +172,8 @@ export function ChatInput({ onSend, disabled, inputRef }: ChatInputProps): React
           </div>
         </div>
 
-        {/* Mobile hint */}
         <div className="sm:hidden">Tap send or press Enter</div>
 
-        {/* Character Count */}
         <div
           className="flex items-center gap-2"
           style={{
diff --git a/apps/web/src/components/chat/MessageList.tsx b/apps/web/src/components/chat/MessageList.tsx
index f8f631d..ee20484 100644
--- a/apps/web/src/components/chat/MessageList.tsx
+++ b/apps/web/src/components/chat/MessageList.tsx
@@ -1,11 +1,13 @@
 "use client";
 
-import { useCallback, useState } from "react";
+import { useCallback, useEffect, useRef, useState } from "react";
 import type { Message } from "@/hooks/useChat";
 
 interface MessageListProps {
   messages: Message[];
   isLoading: boolean;
+  isStreaming?: boolean;
+  streamingMessageId?: string;
   loadingQuip?: string | null;
 }
 
@@ -14,7 +16,6 @@ interface MessageListProps {
  * Extracts <thinking>...</thinking> or <think>...</think> blocks.
  */
 function parseThinking(content: string): { thinking: string | null; response: string } {
-  // Match <thinking>...</thinking> or <think>...</think> blocks
   const thinkingRegex = /<(?:thinking|think)>([\s\S]*?)<\/(?:thinking|think)>/gi;
   const matches = content.match(thinkingRegex);
 
@@ -22,14 +23,12 @@ function parseThinking(content: string): { thinking: string | null; response: st
     return { thinking: null, response: content };
   }
 
-  // Extract thinking content
   let thinking = "";
   for (const match of matches) {
     const innerContent = match.replace(/<\/?(?:thinking|think)>/gi, "");
     thinking += innerContent.trim() + "\n";
   }
 
-  // Remove thinking blocks from response
   const response = content.replace(thinkingRegex, "").trim();
 
   const trimmedThinking = thinking.trim();
@@ -42,25 +41,47 @@ function parseThinking(content: string): { thinking: string | null; response: st
 export function MessageList({
   messages,
   isLoading,
+  isStreaming = false,
+  streamingMessageId,
   loadingQuip,
 }: MessageListProps): React.JSX.Element {
+  const bottomRef = useRef<HTMLDivElement>(null);
+
+  // Auto-scroll to bottom when messages change or streaming tokens arrive
+  useEffect(() => {
+    if (isStreaming || isLoading) {
+      bottomRef.current?.scrollIntoView({ behavior: "smooth" });
+    }
+  }, [messages, isStreaming, isLoading]);
+
   return (
     <div className="space-y-6" role="log" aria-label="Chat messages">
       {messages.map((message) => (
-        <MessageBubble key={message.id} message={message} />
+        <MessageBubble
+          key={message.id}
+          message={message}
+          isStreaming={isStreaming && message.id === streamingMessageId}
+        />
       ))}
 
-      {isLoading && <LoadingIndicator {...(loadingQuip != null && { quip: loadingQuip })} />}
+      {isLoading && !isStreaming && (
+        <LoadingIndicator {...(loadingQuip != null && { quip: loadingQuip })} />
+      )}
+      <div ref={bottomRef} />
     </div>
   );
 }
 
-function MessageBubble({ message }: { message: Message }): React.JSX.Element {
+interface MessageBubbleProps {
+  message: Message;
+  isStreaming?: boolean;
+}
+
+function MessageBubble({ message, isStreaming = false }: MessageBubbleProps): React.JSX.Element {
   const isUser = message.role === "user";
   const [copied, setCopied] = useState(false);
   const [thinkingExpanded, setThinkingExpanded] = useState(false);
 
-  // Parse thinking from content (or use pre-parsed thinking field)
   const { thinking, response } = message.thinking
     ? { thinking: message.thinking, response: message.content }
     : parseThinking(message.content);
@@ -73,7 +94,6 @@ function MessageBubble({ message }: { message: Message }): React.JSX.Element {
         setCopied(false);
       }, 2000);
     } catch (err) {
-      // Silently fail - clipboard copy is non-critical
       void err;
     }
   }, [response]);
@@ -106,8 +126,21 @@ function MessageBubble({ message }: { message: Message }): React.JSX.Element {
           <span className="font-medium" style={{ color: "rgb(var(--text-secondary))" }}>
             {isUser ? "You" : "AI Assistant"}
           </span>
+          {/* Streaming indicator in header */}
+          {!isUser && isStreaming && (
+            <span
+              className="px-1.5 py-0.5 rounded text-[10px] font-medium"
+              style={{
+                backgroundColor: "rgb(var(--accent-primary) / 0.15)",
+                color: "rgb(var(--accent-primary))",
+              }}
+              aria-label="Streaming"
+            >
+              streaming
+            </span>
+          )}
           {/* Model indicator for assistant messages */}
-          {!isUser && message.model && (
+          {!isUser && message.model && !isStreaming && (
             <span
               className="px-1.5 py-0.5 rounded text-[10px] font-medium"
               style={{
@@ -200,43 +233,54 @@ function MessageBubble({ message }: { message: Message }): React.JSX.Element {
             border: isUser ? "none" : "1px solid rgb(var(--border-default))",
           }}
         >
-          <p className="whitespace-pre-wrap text-sm leading-relaxed">{response}</p>
-
-          {/* Copy Button - appears on hover */}
-          <button
-            onClick={handleCopy}
-            className="absolute -right-2 -top-2 rounded-md border p-1.5 opacity-0 transition-all group-hover:opacity-100 focus:opacity-100"
-            style={{
-              backgroundColor: "rgb(var(--surface-0))",
-              borderColor: "rgb(var(--border-default))",
-              color: copied ? "rgb(var(--semantic-success))" : "rgb(var(--text-muted))",
-            }}
-            aria-label={copied ? "Copied!" : "Copy message"}
-            title={copied ? "Copied!" : "Copy to clipboard"}
-          >
-            {copied ? (
-              <svg
-                className="h-3.5 w-3.5"
-                fill="none"
-                viewBox="0 0 24 24"
-                stroke="currentColor"
-                strokeWidth={2}
-              >
-                <polyline points="20 6 9 17 4 12" />
-              </svg>
-            ) : (
-              <svg
-                className="h-3.5 w-3.5"
-                fill="none"
-                viewBox="0 0 24 24"
-                stroke="currentColor"
-                strokeWidth={2}
-              >
-                <rect x="9" y="9" width="13" height="13" rx="2" ry="2" />
-                <path d="M5 15H4a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h9a2 2 0 0 1 2 2v1" />
-              </svg>
+          <p className="whitespace-pre-wrap text-sm leading-relaxed">
+            {response}
+            {/* Blinking cursor during streaming */}
+            {isStreaming && !isUser && (
+              <span
+                className="streaming-cursor inline-block ml-0.5 align-middle"
+                aria-hidden="true"
+              />
             )}
-          </button>
+          </p>
+
+          {/* Copy Button - hidden while streaming */}
+          {!isStreaming && (
+            <button
+              onClick={handleCopy}
+              className="absolute -right-2 -top-2 rounded-md border p-1.5 opacity-0 transition-all group-hover:opacity-100 focus:opacity-100"
+              style={{
+                backgroundColor: "rgb(var(--surface-0))",
+                borderColor: "rgb(var(--border-default))",
+                color: copied ? "rgb(var(--semantic-success))" : "rgb(var(--text-muted))",
+              }}
+              aria-label={copied ? "Copied!" : "Copy message"}
+              title={copied ? "Copied!" : "Copy to clipboard"}
+            >
+              {copied ? (
+                <svg
+                  className="h-3.5 w-3.5"
+                  fill="none"
+                  viewBox="0 0 24 24"
+                  stroke="currentColor"
+                  strokeWidth={2}
+                >
+                  <polyline points="20 6 9 17 4 12" />
+                </svg>
+              ) : (
+                <svg
+                  className="h-3.5 w-3.5"
+                  fill="none"
+                  viewBox="0 0 24 24"
+                  stroke="currentColor"
+                  strokeWidth={2}
+                >
+                  <rect x="9" y="9" width="13" height="13" rx="2" ry="2" />
+                  <path d="M5 15H4a2 2 0 0 1-2-2V4a2 2 0 0 1 2-2h9a2 2 0 0 1 2 2v1" />
+                </svg>
+              )}
+            </button>
+          )}
         </div>
       </div>
     </div>
diff --git a/apps/web/src/hooks/useChat.test.ts b/apps/web/src/hooks/useChat.test.ts
index dd0c716..3c37b56 100644
--- a/apps/web/src/hooks/useChat.test.ts
+++ b/apps/web/src/hooks/useChat.test.ts
@@ -14,6 +14,7 @@ import type { ChatResponse } from "@/lib/api/chat";
 // Mock the API modules - use importOriginal to preserve types/enums
 vi.mock("@/lib/api/chat", () => ({
   sendChatMessage: vi.fn(),
+  streamChatMessage: vi.fn(),
 }));
 
 vi.mock("@/lib/api/ideas", async (importOriginal) => {
@@ -30,6 +31,9 @@ vi.mock("@/lib/api/ideas", async (importOriginal) => {
 const mockSendChatMessage = chatApi.sendChatMessage as MockedFunction<
   typeof chatApi.sendChatMessage
 >;
+const mockStreamChatMessage = chatApi.streamChatMessage as MockedFunction<
+  typeof chatApi.streamChatMessage
+>;
 const mockCreateConversation = ideasApi.createConversation as MockedFunction<
   typeof ideasApi.createConversation
 >;
@@ -70,9 +74,62 @@ function createMockIdea(id: string, title: string, content: string): Idea {
   } as Idea;
 }
 
+/**
+ * Configure streamChatMessage to immediately fail,
+ * triggering the fallback to sendChatMessage.
+ */
+function makeStreamFail(): void {
+  mockStreamChatMessage.mockImplementation(
+    (
+      _request,
+      _onChunk,
+      _onComplete,
+      onError: (err: Error) => void,
+      _signal?: AbortSignal
+    ): void => {
+      // Call synchronously so the Promise rejects immediately
+      onError(new Error("Streaming not available"));
+    }
+  );
+}
+
+/**
+ * Configure streamChatMessage to succeed with given tokens.
+ * Uses a ref-style object to share cancellation state across the async boundary.
+ */
+function makeStreamSucceed(tokens: string[]): void {
+  mockStreamChatMessage.mockImplementation(
+    (
+      _request,
+      onChunk: (chunk: string) => void,
+      onComplete: () => void,
+      _onError: (err: Error) => void,
+      signal?: AbortSignal
+    ): void => {
+      const state = { cancelled: false };
+      signal?.addEventListener("abort", () => {
+        state.cancelled = true;
+      });
+      const run = async (): Promise<void> => {
+        for (const token of tokens) {
+          if (state.cancelled) return;
+          await Promise.resolve();
+          onChunk(token);
+        }
+        if (!state.cancelled) {
+          onComplete();
+        }
+      };
+      void run();
+    }
+  );
+}
+
 describe("useChat", () => {
   beforeEach(() => {
     vi.clearAllMocks();
+    // Default: streaming fails so tests exercise the fallback path
+    makeStreamFail();
   });
 
   afterEach(() => {
@@ -87,13 +144,19 @@ describe("useChat", () => {
       expect(result.current.messages[0]?.role).toBe("assistant");
       expect(result.current.messages[0]?.id).toBe("welcome");
       expect(result.current.isLoading).toBe(false);
+      expect(result.current.isStreaming).toBe(false);
       expect(result.current.error).toBeNull();
       expect(result.current.conversationId).toBeNull();
     });
+
+    it("should expose abortStream function", () => {
+      const { result } = renderHook(() => useChat());
+      expect(typeof result.current.abortStream).toBe("function");
+    });
   });
 
-  describe("sendMessage", () => {
-    it("should add user message and assistant response", async () => {
+  describe("sendMessage (fallback path when streaming fails)", () => {
+    it("should add user message and assistant response via fallback", async () => {
       mockSendChatMessage.mockResolvedValueOnce(createMockChatResponse("Hello there!"));
       mockCreateConversation.mockResolvedValueOnce(createMockIdea("conv-1", "Test", ""));
 
@@ -119,47 +182,13 @@ describe("useChat", () => {
       });
 
       expect(mockSendChatMessage).not.toHaveBeenCalled();
+      expect(mockStreamChatMessage).not.toHaveBeenCalled();
       expect(result.current.messages).toHaveLength(1); // only welcome
     });
 
-    it("should not send while loading", async () => {
-      let resolveFirst: ((value: ChatResponse) => void) | undefined;
-      const firstPromise = new Promise<ChatResponse>((resolve) => {
-        resolveFirst = resolve;
-      });
-
-      mockSendChatMessage.mockReturnValueOnce(firstPromise);
-
-      const { result } = renderHook(() => useChat());
-
-      // Start first message
-      act(() => {
-        void result.current.sendMessage("First");
-      });
-
-      expect(result.current.isLoading).toBe(true);
-
-      // Try to send second while loading
-      await act(async () => {
-        await result.current.sendMessage("Second");
-      });
-
-      // Should only have one call
-      expect(mockSendChatMessage).toHaveBeenCalledTimes(1);
-
-      // Cleanup - resolve the pending promise
-      mockCreateConversation.mockResolvedValueOnce(createMockIdea("conv-1", "Test", ""));
-      await act(async () => {
-        if (resolveFirst) {
-          resolveFirst(createMockChatResponse("Response"));
-        }
-        // Allow promise to settle
-        await Promise.resolve();
-      });
-    });
-
     it("should handle API errors gracefully", async () => {
       vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
       mockSendChatMessage.mockRejectedValueOnce(new Error("API Error"));
 
       const onError = vi.fn();
@@ -171,46 +200,178 @@ describe("useChat", () => {
 
       expect(result.current.error).toBe("Unable to send message. Please try again.");
       expect(onError).toHaveBeenCalledWith(expect.any(Error));
-      // Should have welcome + user + error message
       expect(result.current.messages).toHaveLength(3);
       expect(result.current.messages[2]?.content).toBe("Something went wrong. Please try again.");
     });
   });
 
+  describe("streaming path", () => {
+    it("should stream tokens into assistant message", async () => {
+      const tokens = ["Hello", " world", "!"];
+      makeStreamSucceed(tokens);
+      mockCreateConversation.mockResolvedValueOnce(createMockIdea("conv-1", "Test", ""));
+
+      const { result } = renderHook(() => useChat());
+
+      await act(async () => {
+        await result.current.sendMessage("Hi");
+      });
+
+      expect(result.current.messages).toHaveLength(3);
+      expect(result.current.messages[2]?.role).toBe("assistant");
+      expect(result.current.messages[2]?.content).toBe("Hello world!");
+    });
+
+    it("should set isStreaming true during streaming then false when done", async () => {
+      let capturedOnChunk: ((chunk: string) => void) | undefined;
+      let capturedOnComplete: (() => void) | undefined;
+
+      mockStreamChatMessage.mockImplementation(
+        (
+          _request,
+          onChunk: (chunk: string) => void,
+          onComplete: () => void,
+          _onError: (err: Error) => void
+        ): void => {
+          capturedOnChunk = onChunk;
+          capturedOnComplete = onComplete;
+        }
+      );
+
+      mockCreateConversation.mockResolvedValueOnce(createMockIdea("conv-1", "Test", ""));
+
+      const { result } = renderHook(() => useChat());
+
+      let sendDone = false;
+      act(() => {
+        void result.current.sendMessage("Hello").then(() => {
+          sendDone = true;
+        });
+      });
+
+      // Send first token (triggers streaming state)
+      await act(async () => {
+        capturedOnChunk?.("Hello");
+        await Promise.resolve();
+      });
+
+      expect(result.current.isStreaming).toBe(true);
+
+      // Complete the stream
+      await act(async () => {
+        capturedOnComplete?.();
+        await Promise.resolve();
+        await Promise.resolve();
+        await Promise.resolve();
+      });
+
+      expect(result.current.isStreaming).toBe(false);
+      expect(sendDone).toBe(true);
+    });
+
+    it("should keep partial content on abort", async () => {
+      let capturedOnChunk: ((chunk: string) => void) | undefined;
+
+      mockStreamChatMessage.mockImplementation(
+        (
+          _request,
+          onChunk: (chunk: string) => void,
+          _onComplete: () => void,
+          _onError: (err: Error) => void,
+          signal?: AbortSignal
+        ): void => {
+          capturedOnChunk = onChunk;
+          if (signal) {
+            signal.addEventListener("abort", () => {
+              // Stream aborted
+            });
+          }
+        }
+      );
+
+      const { result } = renderHook(() => useChat());
+
+      act(() => {
+        void result.current.sendMessage("Hello");
+      });
+
+      await act(async () => {
+        capturedOnChunk?.("Partial");
+        capturedOnChunk?.(" content");
+        await Promise.resolve();
+      });
+
+      await act(async () => {
+        result.current.abortStream();
+        await Promise.resolve();
+      });
+
+      expect(result.current.isStreaming).toBe(false);
+      const assistantMsg = result.current.messages.find(
+        (m) => m.role === "assistant" && m.id !== "welcome"
+      );
+      expect(assistantMsg?.content).toBe("Partial content");
+    });
+
+    it("should not send while streaming", async () => {
+      let capturedOnChunk: ((chunk: string) => void) | undefined;
+
+      mockStreamChatMessage.mockImplementation(
+        (
+          _request,
+          onChunk: (chunk: string) => void,
+          _onComplete: () => void,
+          _onError: (err: Error) => void
+        ): void => {
+          capturedOnChunk = onChunk;
+        }
+      );
+
+      const { result } = renderHook(() => useChat());
+
+      act(() => {
+        void result.current.sendMessage("First");
+      });
+
+      await act(async () => {
+        capturedOnChunk?.("token");
+        await Promise.resolve();
+      });
+
+      expect(result.current.isStreaming).toBe(true);
+
+      await act(async () => {
+        await result.current.sendMessage("Second");
+      });
+
+      // Only one stream call
+      expect(mockStreamChatMessage).toHaveBeenCalledTimes(1);
+    });
+  });
+
   describe("rapid sends - stale closure prevention", () => {
     it("should not lose messages on rapid sequential sends", async () => {
-      // This test verifies that functional state updates prevent message loss
-      // when multiple messages are sent in quick succession
-
-      let callCount = 0;
-      mockSendChatMessage.mockImplementation(async (): Promise<ChatResponse> => {
-        callCount++;
-        // Small delay to simulate network
-        await Promise.resolve();
-        return createMockChatResponse(`Response ${String(callCount)}`);
-      });
+      // Use streaming success path for deterministic behavior
+      makeStreamSucceed(["Response 1"]);
 
       mockCreateConversation.mockResolvedValue(createMockIdea("conv-1", "Test", ""));
 
       const { result } = renderHook(() => useChat());
 
-      // Send first message
       await act(async () => {
         await result.current.sendMessage("Message 1");
       });
 
-      // Verify first message cycle complete
       expect(result.current.messages).toHaveLength(3); // welcome + user1 + assistant1
 
-      // Send second message
+      makeStreamSucceed(["Response 2"]);
+
       await act(async () => {
         await result.current.sendMessage("Message 2");
       });
 
-      // Verify all messages are present (no data loss)
       expect(result.current.messages).toHaveLength(5); // welcome + user1 + assistant1 + user2 + assistant2
 
-      // Verify message order and content
       const userMessages = result.current.messages.filter((m) => m.role === "user");
       expect(userMessages).toHaveLength(2);
       expect(userMessages[0]?.content).toBe("Message 1");
@@ -218,69 +379,56 @@ describe("useChat", () => {
     });
 
     it("should use functional updates for all message state changes", async () => {
-      // This test verifies that the implementation uses functional updates
-      // by checking that messages accumulate correctly
-
-      mockSendChatMessage.mockResolvedValue(createMockChatResponse("Response"));
       mockCreateConversation.mockResolvedValue(createMockIdea("conv-1", "Test", ""));
 
       const { result } = renderHook(() => useChat());
 
-      // Track message count after each operation
       const messageCounts: number[] = [];
 
+      makeStreamSucceed(["R1"]);
       await act(async () => {
         await result.current.sendMessage("Test 1");
       });
       messageCounts.push(result.current.messages.length);
 
+      makeStreamSucceed(["R2"]);
       await act(async () => {
         await result.current.sendMessage("Test 2");
       });
       messageCounts.push(result.current.messages.length);
 
+      makeStreamSucceed(["R3"]);
       await act(async () => {
         await result.current.sendMessage("Test 3");
       });
       messageCounts.push(result.current.messages.length);
 
-      // Should accumulate: 3, 5, 7 (welcome + pairs of user/assistant)
       expect(messageCounts).toEqual([3, 5, 7]);
-
-      // Verify final state has all messages
       expect(result.current.messages).toHaveLength(7);
       const userMessages = result.current.messages.filter((m) => m.role === "user");
       expect(userMessages).toHaveLength(3);
     });
 
     it("should maintain correct message order with ref-based state tracking", async () => {
-      // This test verifies that messagesRef is properly synchronized
-
-      const responses = ["First response", "Second response", "Third response"];
-      let responseIndex = 0;
-
-      mockSendChatMessage.mockImplementation((): Promise<ChatResponse> => {
-        const response = responses[responseIndex++];
-        return Promise.resolve(createMockChatResponse(response ?? ""));
-      });
-
       mockCreateConversation.mockResolvedValue(createMockIdea("conv-1", "Test", ""));
 
       const { result } = renderHook(() => useChat());
 
+      makeStreamSucceed(["First response"]);
       await act(async () => {
         await result.current.sendMessage("Query 1");
       });
 
+      makeStreamSucceed(["Second response"]);
       await act(async () => {
         await result.current.sendMessage("Query 2");
       });
 
+      makeStreamSucceed(["Third response"]);
       await act(async () => {
         await result.current.sendMessage("Query 3");
       });
 
-      // Verify messages are in correct order
       const messages = result.current.messages;
       expect(messages[0]?.id).toBe("welcome");
       expect(messages[1]?.content).toBe("Query 1");
@@ -337,14 +485,12 @@ describe("useChat", () => {
         await result.current.loadConversation("conv-bad");
       });
 
-      // Should fall back to welcome message
       expect(result.current.messages).toHaveLength(1);
       expect(result.current.messages[0]?.id).toBe("welcome");
     });
 
     it("should fall back to welcome message when stored data has wrong shape", async () => {
       vi.spyOn(console, "warn").mockImplementation(() => undefined);
-      // Valid JSON but wrong shape (object instead of array, missing required fields)
       mockGetIdea.mockResolvedValueOnce(
         createMockIdea("conv-bad", "Wrong Shape", JSON.stringify({ not: "an array" }))
       );
@@ -408,7 +554,6 @@ describe("useChat", () => {
 
       const { result } = renderHook(() => useChat());
 
-      // Should resolve without throwing - errors are handled internally
       await act(async () => {
         await expect(result.current.loadConversation("conv-err")).resolves.toBeUndefined();
       });
@@ -419,19 +564,17 @@ describe("useChat", () => {
 
   describe("startNewConversation", () => {
     it("should reset to initial state", async () => {
-      mockSendChatMessage.mockResolvedValueOnce(createMockChatResponse("Response"));
+      makeStreamSucceed(["Response"]);
       mockCreateConversation.mockResolvedValueOnce(createMockIdea("conv-1", "Test", ""));
 
       const { result } = renderHook(() => useChat());
 
-      // Send a message to have some state
       await act(async () => {
         await result.current.sendMessage("Hello");
       });
 
       expect(result.current.messages.length).toBeGreaterThan(1);
 
-      // Start new conversation
       act(() => {
         result.current.startNewConversation();
       });
@@ -446,6 +589,7 @@ describe("useChat", () => {
   describe("clearError", () => {
     it("should clear error state", async () => {
       vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
       mockSendChatMessage.mockRejectedValueOnce(new Error("Test error"));
 
       const { result } = renderHook(() => useChat());
@@ -467,6 +611,7 @@ describe("useChat", () => {
   describe("error context logging", () => {
     it("should log comprehensive error context when sendMessage fails", async () => {
       const consoleSpy = vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
       mockSendChatMessage.mockRejectedValueOnce(new Error("LLM timeout"));
 
       const { result } = renderHook(() => useChat({ model: "llama3.2" }));
@@ -489,6 +634,7 @@ describe("useChat", () => {
 
     it("should truncate long message previews to 50 characters", async () => {
       const consoleSpy = vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
       mockSendChatMessage.mockRejectedValueOnce(new Error("Failed"));
 
       const longMessage = "A".repeat(100);
@@ -509,9 +655,10 @@ describe("useChat", () => {
 
     it("should include message count in error context", async () => {
       const consoleSpy = vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
 
-      // First successful message
-      mockSendChatMessage.mockResolvedValueOnce(createMockChatResponse("OK"));
+      // First successful message via streaming
+      makeStreamSucceed(["OK"]);
       mockCreateConversation.mockResolvedValueOnce(createMockIdea("conv-1", "Test", ""));
 
       const { result } = renderHook(() => useChat());
@@ -520,14 +667,14 @@ describe("useChat", () => {
         await result.current.sendMessage("First");
       });
 
-      // Second message fails
+      // Second message: streaming fails, fallback fails
+      makeStreamFail();
       mockSendChatMessage.mockRejectedValueOnce(new Error("Fail"));
 
       await act(async () => {
         await result.current.sendMessage("Second");
       });
 
-      // messageCount should reflect messages including the new user message
       expect(consoleSpy).toHaveBeenCalledWith(
         "Failed to send chat message",
         expect.objectContaining({
@@ -540,6 +687,7 @@ describe("useChat", () => {
   describe("LLM vs persistence error separation", () => {
     it("should show LLM error and add error message to chat when API fails", async () => {
       vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
       mockSendChatMessage.mockRejectedValueOnce(new Error("Model not available"));
 
       const { result } = renderHook(() => useChat());
@@ -549,13 +697,29 @@ describe("useChat", () => {
       });
 
       expect(result.current.error).toBe("Unable to send message. Please try again.");
-      // Should have welcome + user + error message
       expect(result.current.messages).toHaveLength(3);
       expect(result.current.messages[2]?.content).toBe("Something went wrong. Please try again.");
     });
 
-    it("should keep assistant message visible when save fails", async () => {
+    it("should keep assistant message visible when save fails (streaming path)", async () => {
       vi.spyOn(console, "error").mockImplementation(() => undefined);
+      makeStreamSucceed(["Great answer!"]);
+      mockCreateConversation.mockRejectedValueOnce(new Error("Database connection lost"));
+
+      const { result } = renderHook(() => useChat());
+
+      await act(async () => {
+        await result.current.sendMessage("Hello");
+      });
+
+      expect(result.current.messages).toHaveLength(3); // welcome + user + assistant
+      expect(result.current.messages[2]?.content).toBe("Great answer!");
+      expect(result.current.error).toContain("Message sent but failed to save");
+    });
+
+    it("should keep assistant message visible when save fails (fallback path)", async () => {
+      vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
       mockSendChatMessage.mockResolvedValueOnce(createMockChatResponse("Great answer!"));
       mockCreateConversation.mockRejectedValueOnce(new Error("Database connection lost"));
 
@@ -565,16 +729,14 @@ describe("useChat", () => {
         await result.current.sendMessage("Hello");
       });
 
-      // Assistant message should still be visible
-      expect(result.current.messages).toHaveLength(3); // welcome + user + assistant
+      expect(result.current.messages).toHaveLength(3);
       expect(result.current.messages[2]?.content).toBe("Great answer!");
-
-      // Error should indicate persistence failure
       expect(result.current.error).toContain("Message sent but failed to save");
     });
 
     it("should log with PERSISTENCE_ERROR type when save fails", async () => {
       const consoleSpy = vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
       mockSendChatMessage.mockResolvedValueOnce(createMockChatResponse("Response"));
       mockCreateConversation.mockRejectedValueOnce(new Error("DB error"));
 
@@ -591,7 +753,6 @@ describe("useChat", () => {
         })
       );
 
-      // Should NOT have logged as LLM_ERROR
       const llmErrorCalls = consoleSpy.mock.calls.filter((call) => {
         const ctx: unknown = call[1];
         return (
@@ -606,8 +767,9 @@ describe("useChat", () => {
 
     it("should use different user-facing messages for LLM vs save errors", async () => {
       vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
 
-      // Test LLM error message
+      // LLM error path (streaming fails + fallback fails)
       mockSendChatMessage.mockRejectedValueOnce(new Error("Timeout"));
       const { result: result1 } = renderHook(() => useChat());
 
@@ -617,8 +779,8 @@ describe("useChat", () => {
 
       const llmError = result1.current.error;
 
-      // Test save error message
-      mockSendChatMessage.mockResolvedValueOnce(createMockChatResponse("OK"));
+      // Save error path (streaming succeeds, save fails)
+      makeStreamSucceed(["OK"]);
       mockCreateConversation.mockRejectedValueOnce(new Error("DB down"));
       const { result: result2 } = renderHook(() => useChat());
 
@@ -628,7 +790,6 @@ describe("useChat", () => {
 
       const saveError = result2.current.error;
 
-      // They should be different
       expect(llmError).toBe("Unable to send message. Please try again.");
       expect(saveError).toContain("Message sent but failed to save");
       expect(llmError).not.toEqual(saveError);
@@ -636,6 +797,7 @@ describe("useChat", () => {
 
     it("should handle non-Error throws from LLM API", async () => {
       vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
       mockSendChatMessage.mockRejectedValueOnce("string error");
 
       const onError = vi.fn();
@@ -652,7 +814,8 @@ describe("useChat", () => {
 
     it("should handle non-Error throws from persistence layer", async () => {
       vi.spyOn(console, "error").mockImplementation(() => undefined);
-      mockSendChatMessage.mockResolvedValueOnce(createMockChatResponse("OK"));
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
+      makeStreamSucceed(["OK"]);
       mockCreateConversation.mockRejectedValueOnce("DB string error");
 
       const onError = vi.fn();
@@ -662,7 +825,6 @@ describe("useChat", () => {
         await result.current.sendMessage("Hello");
       });
 
-      // Assistant message should still be visible
       expect(result.current.messages[2]?.content).toBe("OK");
       expect(result.current.error).toBe("Message sent but failed to save. Please try again.");
       expect(onError).toHaveBeenCalledWith(expect.any(Error));
@@ -670,8 +832,9 @@ describe("useChat", () => {
 
     it("should handle updateConversation failure for existing conversations", async () => {
       vi.spyOn(console, "error").mockImplementation(() => undefined);
+      vi.spyOn(console, "warn").mockImplementation(() => undefined);
 
-      // First message succeeds and creates conversation
+      // First message via fallback
       mockSendChatMessage.mockResolvedValueOnce(createMockChatResponse("First response"));
       mockCreateConversation.mockResolvedValueOnce(createMockIdea("conv-1", "Test", ""));
 
@@ -683,7 +846,8 @@ describe("useChat", () => {
 
       expect(result.current.conversationId).toBe("conv-1");
 
-      // Second message succeeds but updateConversation fails
+      // Second message via fallback, updateConversation fails
+      makeStreamFail();
       mockSendChatMessage.mockResolvedValueOnce(createMockChatResponse("Second response"));
       mockUpdateConversation.mockRejectedValueOnce(new Error("Connection reset"));
 
@@ -691,8 +855,10 @@ describe("useChat", () => {
         await result.current.sendMessage("Second");
       });
 
-      // Assistant message should still be visible
-      expect(result.current.messages[4]?.content).toBe("Second response");
+      const assistantMessages = result.current.messages.filter(
+        (m) => m.role === "assistant" && m.id !== "welcome"
+      );
+      expect(assistantMessages[assistantMessages.length - 1]?.content).toBe("Second response");
       expect(result.current.error).toBe("Message sent but failed to save. Please try again.");
     });
   });
diff --git a/apps/web/src/hooks/useChat.ts b/apps/web/src/hooks/useChat.ts
index 564b3d1..8f0cfe0 100644
--- a/apps/web/src/hooks/useChat.ts
+++ b/apps/web/src/hooks/useChat.ts
@@ -4,7 +4,11 @@
  */
 
 import { useState, useCallback, useRef } from "react";
-import { sendChatMessage, type ChatMessage as ApiChatMessage } from "@/lib/api/chat";
+import {
+  sendChatMessage,
+  streamChatMessage,
+  type ChatMessage as ApiChatMessage,
+} from "@/lib/api/chat";
 import { createConversation, updateConversation, getIdea, type Idea } from "@/lib/api/ideas";
 import { safeJsonParse, isMessageArray } from "@/lib/utils/safe-json";
 
@@ -33,10 +37,12 @@ export interface UseChatOptions {
 export interface UseChatReturn {
   messages: Message[];
   isLoading: boolean;
+  isStreaming: boolean;
   error: string | null;
   conversationId: string | null;
   conversationTitle: string | null;
   sendMessage: (content: string) => Promise<void>;
+  abortStream: () => void;
   loadConversation: (ideaId: string) => Promise<void>;
   startNewConversation: (projectId?: string | null) => void;
   setMessages: React.Dispatch<React.SetStateAction<Message[]>>;
@@ -66,6 +72,7 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
 
   const [messages, setMessages] = useState<Message[]>([WELCOME_MESSAGE]);
   const [isLoading, setIsLoading] = useState(false);
+  const [isStreaming, setIsStreaming] = useState(false);
   const [error, setError] = useState<string | null>(null);
   const [conversationId, setConversationId] = useState<string | null>(null);
   const [conversationTitle, setConversationTitle] = useState<string | null>(null);
@@ -78,6 +85,16 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
   const messagesRef = useRef<Message[]>(messages);
   messagesRef.current = messages;
 
+  // AbortController ref for the active stream
+  const abortControllerRef = useRef<AbortController | null>(null);
+
+  // Track conversation state in refs to avoid stale closures in streaming callbacks
+  const conversationIdRef = useRef<string | null>(conversationId);
+  conversationIdRef.current = conversationId;
+
+  const conversationTitleRef = useRef<string | null>(conversationTitle);
+  conversationTitleRef.current = conversationTitle;
+
   /**
    * Convert our Message format to API ChatMessage format
    */
@@ -119,44 +136,57 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
   }, []);
 
   /**
-   * Save conversation to backend
+   * Save conversation to backend.
+   * Uses refs for conversation state to avoid stale closures in streaming callbacks.
    */
   const saveConversation = useCallback(
     async (msgs: Message[], title: string): Promise<string> => {
       const content = serializeMessages(msgs);
+      const currentConvId = conversationIdRef.current;
 
-      if (conversationId) {
-        // Update existing conversation
-        await updateConversation(conversationId, content, title);
-        return conversationId;
+      if (currentConvId) {
+        await updateConversation(currentConvId, content, title);
+        return currentConvId;
       } else {
-        // Create new conversation
         const idea = await createConversation(title, content, projectIdRef.current ?? undefined);
         setConversationId(idea.id);
         setConversationTitle(title);
+        conversationIdRef.current = idea.id;
+        conversationTitleRef.current = title;
         return idea.id;
       }
     },
-    [conversationId, serializeMessages]
+    [serializeMessages]
   );
 
   /**
-   * Send a message to the LLM and save the conversation
+   * Abort an active stream
+   */
+  const abortStream = useCallback((): void => {
+    if (abortControllerRef.current) {
+      abortControllerRef.current.abort();
+      abortControllerRef.current = null;
+    }
+    setIsStreaming(false);
+    setIsLoading(false);
+  }, []);
+
+  /**
+   * Send a message to the LLM using streaming, with fallback to non-streaming
    */
   const sendMessage = useCallback(
     async (content: string): Promise<void> => {
-      if (!content.trim() || isLoading) {
+      if (!content.trim() || isLoading || isStreaming) {
         return;
       }
 
       const userMessage: Message = {
-        id: `user-${Date.now().toString()}`,
+        id: `user-${Date.now().toString()}-${Math.random().toString(36).slice(2, 8)}`,
         role: "user",
         content: content.trim(),
         createdAt: new Date().toISOString(),
       };
 
-      // Add user message immediately using functional update
       setMessages((prev) => {
         const updated = [...prev, userMessage];
         messagesRef.current = updated;
@@ -165,95 +195,186 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
       setIsLoading(true);
       setError(null);
 
+      const assistantMessageId = `assistant-${Date.now().toString()}-${Math.random().toString(36).slice(2, 8)}`;
+      const placeholderMessage: Message = {
+        id: assistantMessageId,
+        role: "assistant",
+        content: "",
+        createdAt: new Date().toISOString(),
+        model,
+      };
+
+      const currentMessages = messagesRef.current;
+      const apiMessages = convertToApiMessages(currentMessages);
+
+      const request = {
+        model,
+        messages: apiMessages,
+        ...(temperature !== undefined && { temperature }),
+        ...(maxTokens !== undefined && { maxTokens }),
+        ...(systemPrompt !== undefined && { systemPrompt }),
+      };
+
+      const controller = new AbortController();
+      abortControllerRef.current = controller;
+
+      let streamingSucceeded = false;
+
       try {
-        // Prepare API request - use ref to get current messages (prevents stale closure)
-        const currentMessages = messagesRef.current;
-        const apiMessages = convertToApiMessages(currentMessages);
+        await new Promise<void>((resolve, reject) => {
+          let hasReceivedData = false;
 
-        const request = {
-          model,
-          messages: apiMessages,
-          ...(temperature !== undefined && { temperature }),
-          ...(maxTokens !== undefined && { maxTokens }),
-          ...(systemPrompt !== undefined && { systemPrompt }),
-        };
+          streamChatMessage(
+            request,
+            (chunk: string) => {
+              if (!hasReceivedData) {
+                hasReceivedData = true;
+                setIsLoading(false);
+                setIsStreaming(true);
+                setMessages((prev) => {
+                  const updated = [...prev, { ...placeholderMessage }];
+                  messagesRef.current = updated;
+                  return updated;
+                });
+              }
 
-        // Call LLM API
-        const response = await sendChatMessage(request);
-
-        // Create assistant message
-        const assistantMessage: Message = {
-          id: `assistant-${Date.now().toString()}`,
-          role: "assistant",
-          content: response.message.content,
-          createdAt: new Date().toISOString(),
-          model: response.model,
-          promptTokens: response.promptEvalCount ?? 0,
-          completionTokens: response.evalCount ?? 0,
-          totalTokens: (response.promptEvalCount ?? 0) + (response.evalCount ?? 0),
-        };
-
-        // Add assistant message using functional update
-        let finalMessages: Message[] = [];
-        setMessages((prev) => {
-          finalMessages = [...prev, assistantMessage];
-          messagesRef.current = finalMessages;
-          return finalMessages;
+              setMessages((prev) => {
+                const updated = prev.map((msg) =>
+                  msg.id === assistantMessageId ? { ...msg, content: msg.content + chunk } : msg
+                );
+                messagesRef.current = updated;
+                return updated;
+              });
+            },
+            () => {
+              streamingSucceeded = true;
+              setIsStreaming(false);
+              abortControllerRef.current = null;
+              resolve();
+            },
+            (err: Error) => {
+              reject(err);
+            },
+            controller.signal
+          );
         });
+      } catch (err: unknown) {
+        if (controller.signal.aborted) {
+          setIsStreaming(false);
+          setIsLoading(false);
+          abortControllerRef.current = null;
 
-        // Generate title from first user message if this is a new conversation
-        const isFirstMessage =
-          !conversationId && finalMessages.filter((m) => m.role === "user").length === 1;
-        const title = isFirstMessage
-          ? generateTitle(content)
-          : (conversationTitle ?? "Chat Conversation");
-
-        // Save conversation (separate error handling from LLM errors)
-        try {
-          await saveConversation(finalMessages, title);
-        } catch (saveErr) {
-          const saveErrorMsg =
-            saveErr instanceof Error ? saveErr.message : "Unknown persistence error";
-          setError("Message sent but failed to save. Please try again.");
-          onError?.(saveErr instanceof Error ? saveErr : new Error(saveErrorMsg));
-          console.error("Failed to save conversation", {
-            error: saveErr,
-            errorType: "PERSISTENCE_ERROR",
-            conversationId,
-            detail: saveErrorMsg,
+          // Remove placeholder if no content was received
+          setMessages((prev) => {
+            const assistantMsg = prev.find((m) => m.id === assistantMessageId);
+            if (assistantMsg?.content === "") {
+              const updated = prev.filter((m) => m.id !== assistantMessageId);
+              messagesRef.current = updated;
+              return updated;
+            }
+            messagesRef.current = prev;
+            return prev;
           });
+          return;
         }
-      } catch (err) {
-        const errorMsg = err instanceof Error ? err.message : "Failed to send message";
-        setError("Unable to send message. Please try again.");
-        onError?.(err instanceof Error ? err : new Error(errorMsg));
-        console.error("Failed to send chat message", {
-          error: err,
-          errorType: "LLM_ERROR",
-          conversationId,
-          messageLength: content.length,
-          messagePreview: content.substring(0, 50),
-          model,
-          messageCount: messagesRef.current.length,
-          timestamp: new Date().toISOString(),
+
+        // Streaming failed — fall back to non-streaming
+        console.warn("Streaming failed, falling back to non-streaming", {
+          error: err instanceof Error ? err : new Error(String(err)),
         });
 
-        // Add error message to chat
-        const errorMessage: Message = {
-          id: `error-${String(Date.now())}`,
-          role: "assistant",
-          content: "Something went wrong. Please try again.",
-          createdAt: new Date().toISOString(),
-        };
-        setMessages((prev) => [...prev, errorMessage]);
-      } finally {
-        setIsLoading(false);
+        setMessages((prev) => {
+          const withoutPlaceholder = prev.filter((m) => m.id !== assistantMessageId);
+          messagesRef.current = withoutPlaceholder;
+          return withoutPlaceholder;
+        });
+        setIsStreaming(false);
+
+        try {
+          const response = await sendChatMessage(request);
+
+          const assistantMessage: Message = {
+            id: `assistant-${Date.now().toString()}`,
+            role: "assistant",
+            content: response.message.content,
+            createdAt: new Date().toISOString(),
+            model: response.model,
+            promptTokens: response.promptEvalCount ?? 0,
+            completionTokens: response.evalCount ?? 0,
+            totalTokens: (response.promptEvalCount ?? 0) + (response.evalCount ?? 0),
+          };
+
+          setMessages((prev) => {
+            const updated = [...prev, assistantMessage];
+            messagesRef.current = updated;
+            return updated;
+          });
+
+          streamingSucceeded = true;
+        } catch (fallbackErr: unknown) {
+          const errorMsg =
+            fallbackErr instanceof Error ? fallbackErr.message : "Failed to send message";
+          setError("Unable to send message. Please try again.");
+          onError?.(fallbackErr instanceof Error ? fallbackErr : new Error(errorMsg));
+          console.error("Failed to send chat message", {
+            error: fallbackErr,
+            errorType: "LLM_ERROR",
+            conversationId: conversationIdRef.current,
+            messageLength: content.length,
+            messagePreview: content.substring(0, 50),
+            model,
+            messageCount: messagesRef.current.length,
+            timestamp: new Date().toISOString(),
+          });
+
+          const errorMessage: Message = {
+            id: `error-${String(Date.now())}`,
+            role: "assistant",
+            content: "Something went wrong. Please try again.",
+            createdAt: new Date().toISOString(),
+          };
+          setMessages((prev) => {
+            const updated = [...prev, errorMessage];
+            messagesRef.current = updated;
+            return updated;
+          });
+          setIsLoading(false);
+          return;
+        }
+      }
+
+      setIsLoading(false);
+
+      if (!streamingSucceeded) {
+        return;
+      }
+
+      const finalMessages = messagesRef.current;
+
+      const isFirstMessage =
+        !conversationIdRef.current && finalMessages.filter((m) => m.role === "user").length === 1;
+      const title = isFirstMessage
+        ? generateTitle(content)
+        : (conversationTitleRef.current ?? "Chat Conversation");
+
+      try {
+        await saveConversation(finalMessages, title);
+      } catch (saveErr) {
+        const saveErrorMsg =
+          saveErr instanceof Error ? saveErr.message : "Unknown persistence error";
+        setError("Message sent but failed to save. Please try again.");
+        onError?.(saveErr instanceof Error ? saveErr : new Error(saveErrorMsg));
+        console.error("Failed to save conversation", {
+          error: saveErr,
+          errorType: "PERSISTENCE_ERROR",
+          conversationId: conversationIdRef.current,
+          detail: saveErrorMsg,
+        });
       }
     },
     [
       isLoading,
-      conversationId,
-      conversationTitle,
+      isStreaming,
       model,
       temperature,
       maxTokens,
@@ -280,6 +401,8 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
         setMessages(msgs);
         setConversationId(idea.id);
         setConversationTitle(idea.title ?? null);
+        conversationIdRef.current = idea.id;
+        conversationTitleRef.current = idea.title ?? null;
       } catch (err) {
         const errorMsg = err instanceof Error ? err.message : "Failed to load conversation";
         setError("Unable to load conversation. Please try again.");
@@ -305,6 +428,8 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
     setConversationId(null);
     setConversationTitle(null);
     setError(null);
+    conversationIdRef.current = null;
+    conversationTitleRef.current = null;
     projectIdRef.current = newProjectId ?? null;
   }, []);
 
@@ -318,10 +443,12 @@ export function useChat(options: UseChatOptions = {}): UseChatReturn {
   return {
     messages,
     isLoading,
+    isStreaming,
     error,
     conversationId,
     conversationTitle,
     sendMessage,
+    abortStream,
     loadConversation,
     startNewConversation,
     setMessages,
diff --git a/apps/web/src/lib/api/chat.ts b/apps/web/src/lib/api/chat.ts
index 32b440f..4530bee 100644
--- a/apps/web/src/lib/api/chat.ts
+++ b/apps/web/src/lib/api/chat.ts
@@ -3,7 +3,8 @@
  * Handles LLM chat interactions via /api/llm/chat
  */
 
-import { apiPost } from "./client";
+import { apiPost, fetchCsrfToken, getCsrfToken } from "./client";
+import { API_BASE_URL } from "../config";
 
 export interface ChatMessage {
   role: "system" | "user" | "assistant";
@@ -31,6 +32,19 @@ export interface ChatResponse {
   evalCount?: number;
 }
 
+/**
+ * Parsed SSE data chunk from the LLM stream
+ */
+interface SseChunk {
+  error?: string;
+  message?: {
+    role: string;
+    content: string;
+  };
+  model?: string;
+  done?: boolean;
+}
+
 /**
  * Send a chat message to the LLM
  */
@@ -39,19 +53,122 @@ export async function sendChatMessage(request: ChatRequest): Promise<ChatRespons
 }
 
 /**
- * Stream a chat message from the LLM (not implemented yet)
- * TODO: Implement streaming support
+ * Get or refresh the CSRF token for streaming requests.
+ */
+async function ensureCsrfTokenForStream(): Promise<string> {
+  const existing = getCsrfToken();
+  if (existing) {
+    return existing;
+  }
+  return fetchCsrfToken();
+}
+
+/**
+ * Stream a chat message from the LLM using SSE over fetch.
+ *
+ * The backend accepts stream: true in the request body and responds with
+ * Server-Sent Events:
+ *   data: {"message":{"content":"token"},...}\n\n  for each token
+ *   data: [DONE]\n\n  when the stream is complete
+ *   data: {"error":"message"}\n\n  on error
+ *
+ * @param request - Chat request (stream field will be forced to true)
+ * @param onChunk - Called with each token string as it arrives
+ * @param onComplete - Called when the stream finishes successfully
+ * @param onError - Called if the stream encounters an error
+ * @param signal - Optional AbortSignal for cancellation
  */
 export function streamChatMessage(
   request: ChatRequest,
   onChunk: (chunk: string) => void,
   onComplete: () => void,
-  onError: (error: Error) => void
+  onError: (error: Error) => void,
+  signal?: AbortSignal
 ): void {
-  // Streaming implementation would go here
-  void request;
-  void onChunk;
-  void onComplete;
-  void onError;
-  throw new Error("Streaming not implemented yet");
+  void (async (): Promise<void> => {
+    try {
+      const csrfToken = await ensureCsrfTokenForStream();
+
+      const response = await fetch(`${API_BASE_URL}/api/llm/chat`, {
+        method: "POST",
+        headers: {
+          "Content-Type": "application/json",
+          "X-CSRF-Token": csrfToken,
+        },
+        credentials: "include",
+        body: JSON.stringify({ ...request, stream: true }),
+        signal: signal ?? null,
+      });
+
+      if (!response.ok) {
+        const errorText = await response.text().catch(() => response.statusText);
+        throw new Error(`Stream request failed: ${errorText}`);
+      }
+
+      if (!response.body) {
+        throw new Error("Response body is not readable");
+      }
+
+      const reader = response.body.getReader();
+      const decoder = new TextDecoder("utf-8");
+      let buffer = "";
+
+      let readerDone = false;
+      while (!readerDone) {
+        const { done, value } = await reader.read();
+        readerDone = done;
+        if (done) {
+          break;
+        }
+
+        buffer += decoder.decode(value, { stream: true });
+
+        // SSE messages are separated by double newlines
+        const parts = buffer.split("\n\n");
+        // Keep the last (potentially incomplete) part
+        buffer = parts.pop() ?? "";
+
+        for (const part of parts) {
+          const trimmed = part.trim();
+          if (!trimmed) continue;
+
+          for (const line of trimmed.split("\n")) {
+            if (!line.startsWith("data: ")) continue;
+
+            const data = line.slice("data: ".length).trim();
+
+            if (data === "[DONE]") {
+              onComplete();
+              return;
+            }
+
+            try {
+              const parsed = JSON.parse(data) as SseChunk;
+
+              if (parsed.error) {
+                throw new Error(parsed.error);
+              }
+
+              if (parsed.message?.content) {
+                onChunk(parsed.message.content);
+              }
+            } catch (parseErr) {
+              if (parseErr instanceof SyntaxError) {
+                continue;
+              }
+              throw parseErr;
+            }
+          }
+        }
+      }
+
+      // Natural end of stream without [DONE]
+      onComplete();
+    } catch (err: unknown) {
+      if (err instanceof DOMException && err.name === "AbortError") {
+        return;
+      }
+      onError(err instanceof Error ? err : new Error(String(err)));
+    }
+  })();
 }
-- 
2.49.1