diff --git a/app/routers/chat.py b/app/routers/chat.py
index b7436ee..79e61e1 100644
--- a/app/routers/chat.py
+++ b/app/routers/chat.py
@@ -1,3 +1,4 @@
+import logging
 from datetime import datetime, timezone
 import uuid
 from fastapi import APIRouter, Request, Depends, Form
@@ -12,7 +13,7 @@ from app.models.workout import Workout
 from app.models.checkin import Checkin
 from app.models.measurement import Measurement, MeasurementType
 from app.auth import get_current_user
-from app.services.opencode_proxy import query_opencode
+from app.services.opencode_proxy import query_opencode, OpenCodeUnavailableError
 
 router = APIRouter()
 templates = Jinja2Templates(directory="app/templates")
@@ -26,6 +27,9 @@ ONBOARDING_PROMPT = (
 )
 
 
+logger = logging.getLogger("chat")
+
+
 @router.get("/api/chat/messages")
 async def get_chat_messages(request: Request, user: User = Depends(get_current_user)):
     session_id = request.cookies.get("chat_session_id")
@@ -159,9 +163,15 @@ async def chat_send(
             await session.commit()
 
         assistant_content = ""
-        async for chunk in query_opencode(message, session_id, user_context):
-            assistant_content += chunk
-            yield f"data: {chunk}\n\n"
+        try:
+            async for chunk in query_opencode(message, session_id, user_context):
+                assistant_content += chunk
+                yield f"data: {chunk}\n\n"
+        except OpenCodeUnavailableError as e:
+            logger.error("Chat failed for user %s: %s", user.username, e)
+            yield f"data: [error] {e}\n\n"
+            # Don't save an error assistant message
+            return
 
         async with async_session() as session:
             assistant_msg = ChatMessage(
diff --git a/app/services/opencode_proxy.py b/app/services/opencode_proxy.py
index c800985..be60913 100644
--- a/app/services/opencode_proxy.py
+++ b/app/services/opencode_proxy.py
@@ -1,8 +1,15 @@
 import asyncio
 import json
+import logging
 from typing import AsyncGenerator
 from app.config import OPENCODE_SERVE_URL
 
+logger = logging.getLogger("opencode_proxy")
+
+
+class OpenCodeUnavailableError(Exception):
+    pass
+
 
 async def query_opencode(
     message: str,
@@ -37,6 +44,10 @@ async def query_opencode(
 
         await proc.wait()
     except FileNotFoundError:
-        yield "AI coach is not available. Install opencode and run `opencode serve` to enable."
+        logger.error("opencode binary not found in PATH")
+        raise OpenCodeUnavailableError(
+            "opencode binary not found. Install opencode or run `opencode serve`."
+        )
     except Exception as e:
-        yield f"Error connecting to AI coach: {e}"
+        logger.error("opencode proxy error: %s", e)
+        raise OpenCodeUnavailableError(f"AI coach unavailable: {e}")
diff --git a/app/static/chat-sidebar.js b/app/static/chat-sidebar.js
index f3606f2..267c63b 100644
--- a/app/static/chat-sidebar.js
+++ b/app/static/chat-sidebar.js
@@ -79,7 +79,12 @@
                 buffer = lines.pop() || '';
                 for (const line of lines) {
                     if (line.startsWith('data: ')) {
-                        p.innerHTML += line.slice(6);
+                        const chunk = line.slice(6);
+                        if (chunk.startsWith('[error] ')) {
+                            p.innerHTML = '<strong style="color: var(--del-color, #b33)">' + chunk.slice(8) + '</strong>';
+                            break;
+                        }
+                        p.innerHTML += chunk;
                     }
                 }
                 messagesEl.scrollTop = messagesEl.scrollHeight;
@@ -104,7 +109,9 @@
         input.addEventListener('keydown', function(e) {
             if (e.key === 'Enter' && !e.shiftKey) {
                 e.preventDefault();
-                form.dispatchEvent(new Event('submit'));
+                const event = new Event('submit', {cancelable: true});
+                form.dispatchEvent(event);
+                if (!event.defaultPrevented) sendMessage(event);
             }
         });
     }
diff --git a/app/templates/chat.html b/app/templates/chat.html
index 0e343e9..a6ee7b3 100644
--- a/app/templates/chat.html
+++ b/app/templates/chat.html
@@ -33,7 +33,9 @@ const ONBOARDING_PROMPT = chatForm.dataset.onboarding;
 chatInput.addEventListener('keydown', function(e) {
     if (e.key === 'Enter' && !e.shiftKey) {
         e.preventDefault();
-        chatForm.dispatchEvent(new Event('submit'));
+        const event = new Event('submit', {cancelable: true});
+        chatForm.dispatchEvent(event);
+        if (!event.defaultPrevented) sendMessage(event);
     }
 });
 
@@ -90,7 +92,12 @@ async function sendMessage(event) {
             buffer = lines.pop() || '';
             for (const line of lines) {
                 if (line.startsWith('data: ')) {
-                    p.innerHTML += line.slice(6);
+                    const chunk = line.slice(6);
+                    if (chunk.startsWith('[error] ')) {
+                        p.innerHTML = '<strong style="color: var(--del-color, #b33)">' + chunk.slice(8) + '</strong>';
+                        break;
+                    }
+                    p.innerHTML += chunk;
                 }
             }
             chatMessages.scrollTop = chatMessages.scrollHeight;
@@ -114,7 +121,9 @@ async function sendMessage(event) {
 
 if (IS_FIRST && ONBOARDING_PROMPT) {
     chatInput.value = ONBOARDING_PROMPT;
-    chatForm.dispatchEvent(new Event('submit'));
+    const event = new Event('submit', {cancelable: true});
+    chatForm.dispatchEvent(event);
+    if (!event.defaultPrevented) sendMessage(event);
 }
 </script>
 {% endblock %}