Cancel obsolete command discovery without crashing the UI

src/toad/command_catalog.py

@@ -1,5 +1,6 @@
 """One derived command view joins ACP advertisements and local declarations."""
 
+import asyncio
 from dataclasses import dataclass
 from collections.abc import Awaitable, Sequence
 from typing import TYPE_CHECKING
@@ -11,6 +12,10 @@
     from toad.widgets.conversation import Conversation
 
 
+class CommandDiscoveryCancelled(asyncio.CancelledError):
+    """The view that requested commands changed before discovery completed."""
+
+
 @dataclass(frozen=True)
 class CommandCatalog:
     entries: tuple[CommandPresentation, ...]
@@ -60,10 +65,18 @@ def target_choices(self):
 
     @classmethod
     async def execute(cls, text: str, conversation: Conversation) -> bool:
-        catalog = await conversation.read_command_catalog()
-        name, _, arguments = text.partition(" ")
-        command = next((command for command in catalog.commands
-                        if command.command == name), None)
-        if command is None:
-            return False
-        return await command.parse_arguments(arguments).apply(conversation)
+        try:
+            catalog = await conversation.read_command_catalog()
+            name, _, arguments = text.partition(" ")
+            command = next((command for command in catalog.commands
+                            if command.command == name), None)
+            if command is None:
+                return False
+            return await command.parse_arguments(arguments).apply(conversation)
+        except CommandDiscoveryCancelled:
+            # A recognized UI command must never fall through to agent input
+            # because its original target disappeared during an awaited read.
+            conversation.app.notify(
+                "View changed; command was not run. Submit it again.", severity="warning"
+            )
+            return True

src/toad/target_commands.py

@@ -119,8 +119,10 @@ def parse_arguments(self, arguments: str) -> ContextualCommand:
         return self
 
     async def apply(self, conversation: Conversation, ctx: TargetContext) -> bool:
+        from toad.command_catalog import CommandDiscoveryCancelled
+
         if await conversation.command_target_context() != ctx:
-            raise ValueError("Command target changed; refresh this view")
+            raise CommandDiscoveryCancelled("Command target changed; refresh this view")
         if not self.available(ctx):
             raise ValueError("Action is no longer available for this target")
         self.execute(ctx)

src/toad/widgets/conversation.py

@@ -54,7 +54,7 @@
     UpCursor,
     require_blocks,
 )
-from toad.command_catalog import CommandCatalog
+from toad.command_catalog import CommandCatalog, CommandDiscoveryCancelled
 from toad.conversation_comms_handlers import ConversationCommsHandlers
 from toad.conversation_submission import ConversationSubmissions
 from toad.conversation_turn import ConversationTurn, TurnOwner
@@ -1515,7 +1515,7 @@ async def command_target_context(self):
             lambda: self.app.coordination_access.observation.observed().service
         )
         if not self.is_attached or screen.navigation_context != nav:
-            raise ValueError("Command view changed during discovery")
+            raise CommandDiscoveryCancelled("Command view changed during discovery")
         return screen.thread_identity.command_target(
             lambda actor: TargetContext(
                 self.app, comms, CommandTarget.named(actor), actor, nav.project_path, nav.owner_mode
@@ -1551,7 +1551,7 @@ async def read_command_catalog(self) -> CommandCatalog:
             or source.navigation_context != navigation
             or agent is not self.agent_binding
         ):
-            raise ValueError("Command view changed during discovery")
+            raise CommandDiscoveryCancelled("Command view changed during discovery")
         return catalog
 
     @handles(core_events.LinkContextChanged)