diff --git a/api/streaming.py b/api/streaming.py
index 3bf8bd9..385a59a 100644
--- a/api/streaming.py
+++ b/api/streaming.py
@@ -189,7 +189,8 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
             else: os.environ['HERMES_SESSION_KEY'] = old_session_key
 
     except Exception as e:
-        put('error', {'message': str(e), 'trace': traceback.format_exc()})
+        print('[webui] stream error:\n' + traceback.format_exc(), flush=True)
+        put('error', {'message': str(e)})
     finally:
         _clear_thread_env()  # TD1: always clear thread-local context
         with STREAMS_LOCK:
diff --git a/api/upload.py b/api/upload.py
index a74f9de..5cfe4d2 100644
--- a/api/upload.py
+++ b/api/upload.py
@@ -74,4 +74,5 @@ def handle_upload(handler):
         dest.write_bytes(file_bytes)
         return j(handler, {'filename': safe_name, 'path': str(dest), 'size': dest.stat().st_size})
     except Exception as e:
-        return j(handler, {'error': str(e), 'trace': _tb.format_exc()}, status=500)
+        print('[webui] upload error: ' + _tb.format_exc(), flush=True)
+        return j(handler, {'error': 'Upload failed'}, status=500)
diff --git a/server.py b/server.py
index b6706fc..d9b4fc6 100644
--- a/server.py
+++ b/server.py
@@ -40,7 +40,8 @@ class Handler(BaseHTTPRequestHandler):
             if result is False:
                 return j(self, {'error': 'not found'}, status=404)
         except Exception as e:
-            return j(self, {'error': str(e), 'trace': traceback.format_exc()}, status=500)
+            print(f'[webui] ERROR {self.command} {self.path}\n' + traceback.format_exc(), flush=True)
+            return j(self, {'error': 'Internal server error'}, status=500)
 
     def do_POST(self):
         self._req_t0 = time.time()
@@ -51,7 +52,8 @@ class Handler(BaseHTTPRequestHandler):
             if result is False:
                 return j(self, {'error': 'not found'}, status=404)
         except Exception as e:
-            return j(self, {'error': str(e), 'trace': traceback.format_exc()}, status=500)
+            print(f'[webui] ERROR {self.command} {self.path}\n' + traceback.format_exc(), flush=True)
+            return j(self, {'error': 'Internal server error'}, status=500)
 
 
 def main():
diff --git a/tests/test_regressions.py b/tests/test_regressions.py
index 4e396b6..c38481a 100644
--- a/tests/test_regressions.py
+++ b/tests/test_regressions.py
@@ -438,3 +438,37 @@ def test_newSession_clears_live_tool_cards(cleanup_test_sessions):
     next_fn = src.find("async function ", new_sess_idx + 10)
     new_sess_body = src[new_sess_idx:next_fn]
     assert "clearLiveToolCards" in new_sess_body,         "newSession() must call clearLiveToolCards() to clear stale live cards"
+
+
+# ── R16: Stack traces must not leak to clients in 500 responses ────────────
+
+def test_500_response_has_no_trace_field():
+    """R16: HTTP 500 responses must not include a 'trace' field.
+    Leaking tracebacks exposes file paths, module names, and potentially
+    secret values from local variables.
+    """
+    # POST to /api/chat/start with missing required fields to trigger an error
+    data, status = post("/api/chat/start", {})
+    # Should be an error response (4xx or 5xx)
+    assert "trace" not in data, \
+        "Server must not leak stack traces to clients"
+
+def test_upload_error_has_no_trace_field():
+    """R16b: Upload 500 responses must not include a 'trace' field."""
+    # Send a POST to /api/upload with invalid content to trigger the error handler
+    req = urllib.request.Request(
+        BASE + "/api/upload",
+        data=b"not-multipart-data",
+        headers={"Content-Type": "text/plain", "Content-Length": "18"},
+    )
+    try:
+        with urllib.request.urlopen(req, timeout=10) as r:
+            body = json.loads(r.read())
+            code = r.status
+    except urllib.error.HTTPError as e:
+        body = json.loads(e.read())
+        code = e.code
+    assert code >= 400, "Invalid upload should return an error status"
+    assert "trace" not in body, \
+        "Upload errors must not leak stack traces to clients"
+    assert "error" in body, "Error responses must include an 'error' key"