diff --git a/README.md b/README.md
index 68c7896..4a2b45c 100644
--- a/README.md
+++ b/README.md
@@ -3,8 +3,7 @@
 The application-facing API is:
 
 ```python
-from ipython_demo import App
-from ipython_demo import CallOptions
+from ipython_shell import App, CallOptions
 
 app = App()
 call = app.run_code(
diff --git a/src/ipython_shell/jupyter/app.py b/src/ipython_shell/jupyter/app.py
index f548c30..f20a1fe 100644
--- a/src/ipython_shell/jupyter/app.py
+++ b/src/ipython_shell/jupyter/app.py
@@ -1,3 +1,4 @@
+import asyncio
 from collections.abc import AsyncIterator
 from datetime import UTC, datetime
 from time import monotonic
@@ -97,6 +98,10 @@ class JupyterApp:
 
     async def shutdown(self) -> None:
         """Stop all kernel processes owned by this app."""
-        awaitables = [shell.shutdown() for shell in self.shells.values()]
-        for awaitable in awaitables:
-            await awaitable
+        results = await asyncio.gather(
+            *(shell.shutdown() for shell in self.shells.values()),
+            return_exceptions=True,
+        )
+        errors = [result for result in results if isinstance(result, BaseException)]
+        if errors:
+            raise BaseExceptionGroup("Kernel shutdown failed", errors)
diff --git a/src/ipython_shell/jupyter/messages.py b/src/ipython_shell/jupyter/messages.py
index 80d5c08..6530879 100644
--- a/src/ipython_shell/jupyter/messages.py
+++ b/src/ipython_shell/jupyter/messages.py
@@ -1,6 +1,6 @@
 from typing import Annotated, ClassVar, Final, Literal
 
-from pydantic import BaseModel, ConfigDict, Field, TypeAdapter
+from pydantic import BaseModel, ConfigDict, Field, TypeAdapter, ValidationError
 
 
 type JupyterChannel = Literal["iopub", "shell", "stdin"]
@@ -176,6 +176,9 @@ class InputRequestMessage(_TypedMessage):
 class UnknownJupyterMessage(JupyterMessage):
     """An extension or future message preserved without lossy parsing."""
 
+    # Allow any channel value so known-but-malformed fallbacks never fail
+    # on a strict Literal. The parser preserves the transport channel verbatim.
+    channel: str = "iopub"  # type: ignore[assignment]
     raw: dict[str, object] = Field(default_factory=dict)
 
 
@@ -252,7 +255,14 @@ def parse_jupyter_message(
 
     if values["msg_type"] not in _KNOWN_MESSAGE_TYPES:
         return UnknownJupyterMessage.model_validate({**values, "raw": raw})
-    return _KNOWN_MESSAGE_ADAPTER.validate_python(values)
+    try:
+        return _KNOWN_MESSAGE_ADAPTER.validate_python(values)
+    except ValidationError:
+        # Known msg_type but content or channel doesn't match the strict
+        # model (e.g., extra execution_state, wrong stdout/stderr name,
+        # or msg delivered on an unexpected channel). Degrade losslessly
+        # instead of aborting the whole call.
+        return UnknownJupyterMessage.model_validate({**values, "raw": raw})
 
 
 # Existing callers use this name for the transport-level message. Keep it as
diff --git a/src/ipython_shell/jupyter/transport.py b/src/ipython_shell/jupyter/transport.py
index 6d4ff11..4fbc4d5 100644
--- a/src/ipython_shell/jupyter/transport.py
+++ b/src/ipython_shell/jupyter/transport.py
@@ -48,6 +48,7 @@ class JupyterTransport:
             asyncio.Queue[tuple[JupyterChannel, dict[str, Any] | BaseException]] | None
         ) = None
         self._reader_tasks: set[asyncio.Task[None]] = set()
+        self._reader_failure: BaseException | None = None
 
     async def start(self) -> None:
         """Start the kernel and its persistent channel readers."""
@@ -100,6 +101,7 @@ class JupyterTransport:
             raise
         except BaseException as error:
             # Surface reader failures instead of leaving messages_for() stuck.
+            self._reader_failure = error
             await self._message_queue.put((channel, error))
 
     async def _stop_readers(self) -> None:
@@ -113,6 +115,8 @@ class JupyterTransport:
 
     async def execute(self, code: str) -> str:
         """Submit one cell and return its Jupyter message ID."""
+        if self._reader_failure is not None:
+            raise RuntimeError("JupyterTransport channel reader failed") from self._reader_failure
         if self.client is None:
             raise RuntimeError("JupyterTransport has not been started")
 
@@ -126,6 +130,8 @@ class JupyterTransport:
         self, call_id: str
     ) -> AsyncIterator[ParsedJupyterMessage | InputRequest]:
         """Yield decoded output, input, and completion messages for one call."""
+        if self._reader_failure is not None:
+            raise RuntimeError("JupyterTransport channel reader failed") from self._reader_failure
         if self.client is None:
             raise RuntimeError("JupyterTransport has not been started")
         if self._active_call != call_id:
diff --git a/src/ipython_shell/shell.py b/src/ipython_shell/shell.py
index 8cb0c4c..53509ea 100644
--- a/src/ipython_shell/shell.py
+++ b/src/ipython_shell/shell.py
@@ -87,7 +87,8 @@ class Shell:
         else:
             self.status = "ready"
             self.last_used_at = datetime.now(UTC).isoformat()
-            self._last_execution_count = call_result.execution_count or 0
+            if call_result.execution_count is not None:
+                self._last_execution_count = call_result.execution_count
             return call_result, events
 
     def describe(self) -> ShellInfo:
diff --git a/src/st_demo/app.py b/src/st_demo/app.py
index 272464c..6015ee3 100644
--- a/src/st_demo/app.py
+++ b/src/st_demo/app.py
@@ -227,6 +227,17 @@ def _render_active_call(controller: ShellController) -> None:
         events, finished, failed = controller.poll(handle)
         records = call_records.setdefault(handle, [])
         records.extend(event_to_record(event) for event in events)
+        # Restore persisted failure from a previous poll where the sentinel
+        # was already drained. Without this, the next poll returns
+        # failed=None, finished=False and shows "Running…" forever.
+        if failed is None:
+            failures = st.session_state.get("call_failures")
+            if isinstance(failures, dict):
+                cached = failures.get(handle)
+                if isinstance(cached, BaseException):
+                    failed = cached  # type: ignore[assignment]
+                elif isinstance(cached, Exception):
+                    failed = cached
 
     for record in visible_records(call_records):
         _render_record(record)
@@ -249,7 +260,17 @@ def _render_active_call(controller: ShellController) -> None:
             controller.reply_to_input(handle, call_id, value)
             st.rerun(scope="fragment")
     elif failed is not None:
+        failures = st.session_state.get("call_failures")
+        if not isinstance(failures, dict):
+            failures = {}
+            st.session_state.call_failures = failures
+        failures[handle] = failed
+        # Persist in call history so the error remains visible after clearing.
+        error_text = f"{type(failed).__name__}: {failed}"
+        if not records or records[-1].get("text") != error_text:
+            records.append({"kind": "error", "text": error_text})
         st.exception(failed)
+        st.session_state.active_handle = None
     elif not finished:
         st.caption("Running…")
     else:
@@ -276,7 +297,9 @@ def render() -> None:
         value="import matplotlib.pyplot as plt\nplt.plot([1, 2, 3], [4, 5, 6])",
         height=180,
     )
-    if st.button("Run", type="primary"):
+    active_handle = st.session_state.get("active_handle")
+    is_running = isinstance(active_handle, str)
+    if st.button("Run", type="primary", disabled=is_running):
         handle = controller.start_call(code, shell_name)
         st.session_state.active_handle = handle
         st.session_state.call_records = getattr(