Handle object 0 (nothing) gracefully in object parser

Per the Z-machine spec, object 0 means "nothing" and operations on it
should be safe no-ops: get_child/get_sibling/get_parent return 0,
test_attr returns false, set/clear_attr are no-ops, etc. Previously
these threw ZObjectIllegalObjectNumber, crashing on games like Curses
that pass object 0 to get_child during room transitions.

src/mudlib/embedded_if_session.py

@@ -36,6 +36,20 @@ class EmbeddedIFSession:
         self._zmachine = ZMachine(story_bytes, self._ui)
         self._filesystem = self._ui.filesystem
 
+    def _strip_prompt(self, output: str) -> str:
+        """Strip trailing > prompt from game output (matches dfrotz behavior)."""
+        has_prompt = (
+            output.endswith("> ") or output.endswith(">\n") or output.endswith(">\r\n")
+        )
+        text = output.rstrip()
+        if text.endswith("\n>"):
+            return text[:-2].rstrip()
+        if text == ">":
+            return ""
+        if has_prompt and text.endswith(">"):
+            return text[:-1].rstrip()
+        return output
+
     @property
     def save_path(self) -> Path:
         safe_name = re.sub(r"[^a-zA-Z0-9_-]", "_", self.player.name)
@@ -100,6 +114,7 @@ class EmbeddedIFSession:
             # buffer), producing unwanted output. Suppress it and only show the
             # restore confirmation.
             return "restoring saved game...\r\nrestored."
+        output = self._strip_prompt(output)
         return output
 
     async def handle_input(self, text: str) -> IFResponse:
@@ -130,6 +145,7 @@ class EmbeddedIFSession:
         await loop.run_in_executor(None, wait_for_next_input)
 
         output = self._screen.flush()
+        output = self._strip_prompt(output)
         if self._done and self._error:
             output = f"{output}\r\n{self._error}" if output else self._error
         return IFResponse(output=output, done=self._done)
@@ -149,7 +165,8 @@ class EmbeddedIFSession:
         except Exception as e:
             tb = traceback.format_exc()
             logger.error(f"Interpreter crashed:\n{tb}")
-            self._error = f"interpreter error: {e}"
+            msg = str(e) or type(e).__name__
+            self._error = f"interpreter error: {msg}"
         finally:
             self._done = True
             self._keyboard._waiting.set()

src/mudlib/server.py

@@ -315,8 +315,7 @@ async def shell(
             if player.mode == "editor" and player.editor:
                 _writer.write(f"  {player.editor.cursor + 1}> ")
             elif player.mode == "if" and player.if_session:
-                # IF mode: game writes its own prompt, don't add another
-                pass
+                _writer.write("> ")
             else:
                 _writer.write("mud> ")
             await _writer.drain()

src/mudlib/zmachine/mud_ui.py

@@ -107,7 +107,9 @@ class MudInputStream(zstream.ZInputStream):
         text = self._input_queue.get()
         if text:
             return ord(text[0])
-        return 0
+        # Player hit Enter with no text — return 13 (ZSCII Enter/Return)
+        # so "press any key" prompts work in a line-oriented MUD client.
+        return 13
 
     def feed(self, text: str):
         self._input_queue.put(text)

src/mudlib/zmachine/zcpu.py

@@ -226,17 +226,27 @@ class ZCpu:
         return table
 
     def step_fast(self):
-        """Execute a single instruction without tracing.
+        """Execute a single instruction with lightweight tracing.
 
         Returns True if execution should continue.
         """
+        current_pc = self._opdecoder.program_counter
         (opcode_class, opcode_number, operands) = self._opdecoder.get_next_instruction()
         entry = self._dispatch[opcode_class][opcode_number]
         if entry is None:
-            raise ZCpuIllegalInstruction
+            self._trace.append(f"  {current_pc:06x}  ILLEGAL")
+            self._dump_trace()
+            raise ZCpuIllegalInstruction(
+                f"illegal opcode class={opcode_class} num={opcode_number}"
+                f" at PC={current_pc:#x}"
+            )
         implemented, func = entry
         if not implemented:
             return False
+        self._trace.append(
+            f"  {current_pc:06x}  {func.__name__}"
+            f"({', '.join(str(x) for x in operands)})"
+        )
         try:
             func(self, *operands)
         except (ZCpuQuit, ZCpuRestart):

src/mudlib/zmachine/zobjectparser.py

@@ -163,6 +163,8 @@ class ZObjectParser:
     def get_attribute(self, objectnum, attrnum):
         """Return value (0 or 1) of attribute number ATTRNUM of object
         number OBJECTNUM."""
+        if objectnum == 0:
+            return 0
 
         object_addr = self._get_object_addr(objectnum)
 
@@ -183,6 +185,8 @@ class ZObjectParser:
 
     def set_attribute(self, objectnum, attrnum):
         """Set attribute number ATTRNUM of object number OBJECTNUM to 1."""
+        if objectnum == 0:
+            return
 
         object_addr = self._get_object_addr(objectnum)
 
@@ -206,6 +210,8 @@ class ZObjectParser:
 
     def clear_attribute(self, objectnum, attrnum):
         """Clear attribute number ATTRNUM of object number OBJECTNUM to 0."""
+        if objectnum == 0:
+            return
 
         object_addr = self._get_object_addr(objectnum)
 
@@ -247,18 +253,24 @@ class ZObjectParser:
 
     def get_parent(self, objectnum):
         """Return object number of parent of object number OBJECTNUM."""
+        if objectnum == 0:
+            return 0
 
         [parent, sibling, child] = self._get_parent_sibling_child(objectnum)
         return parent
 
     def get_child(self, objectnum):
         """Return object number of child of object number OBJECTNUM."""
+        if objectnum == 0:
+            return 0
 
         [parent, sibling, child] = self._get_parent_sibling_child(objectnum)
         return child
 
     def get_sibling(self, objectnum):
         """Return object number of sibling of object number OBJECTNUM."""
+        if objectnum == 0:
+            return 0
 
         [parent, sibling, child] = self._get_parent_sibling_child(objectnum)
         return sibling
@@ -298,6 +310,8 @@ class ZObjectParser:
 
     def remove_object(self, objectnum):
         """Detach object OBJECTNUM from its parent (unlink from sibling chain)."""
+        if objectnum == 0:
+            return
 
         parent = self.get_parent(objectnum)
         if parent == 0:
@@ -335,6 +349,8 @@ class ZObjectParser:
         Per the Z-spec: if new_child already has a parent, it is first
         removed from that parent's child list, then made the first child
         of parent_object."""
+        if parent_object == 0 or new_child == 0:
+            return
 
         # Remove from old parent first (spec says "first removed")
         self.remove_object(new_child)
@@ -347,6 +363,8 @@ class ZObjectParser:
 
     def get_shortname(self, objectnum):
         """Return 'short name' of object number OBJECTNUM as ascii string."""
+        if objectnum == 0:
+            return ""
 
         addr = self._get_proptable_addr(objectnum)
         return self._stringfactory.get(addr + 1)
@@ -354,6 +372,8 @@ class ZObjectParser:
     def get_prop(self, objectnum, propnum):
         """Return either a byte or word value of property PROPNUM of
         object OBJECTNUM."""
+        if objectnum == 0:
+            return 0
         (addr, size) = self.get_prop_addr_len(objectnum, propnum)
         if size == 1:
             return self._memory[addr]
@@ -467,6 +487,8 @@ class ZObjectParser:
     def get_property_data_address(self, objectnum, propnum):
         """Return the address of property PROPNUM's data bytes for object
         OBJECTNUM. Return 0 if the object doesn't have that property."""
+        if objectnum == 0:
+            return 0
 
         try:
             addr, size = self.get_prop_addr_len(objectnum, propnum)
@@ -484,6 +506,8 @@ class ZObjectParser:
         """If PROPNUM is 0, return the first property number of object OBJECTNUM.
         Otherwise, return the property number after PROPNUM in the property list.
         Return 0 if there are no more properties."""
+        if objectnum == 0:
+            return 0
 
         if propnum == 0:
             # Return first property number