fix(python/omp-rpc): accepted string or array systemPrompt values in session state parsing

- Added `_optional_str_list` to normalize `systemPrompt` payloads into a tuple when absent, a bare string, or a string array, and to reject invalid shapes.
- Updated `SessionState` to hold `system_prompt` as a tuple and to parse daemon payloads with the new helper in `parse_session_state`.
- Extended protocol tests to verify array and legacy string parsing, default empty tuple behavior, and invalid `systemPrompt` values.

python/omp-rpc/src/omp_rpc/protocol.py

@@ -164,6 +164,29 @@ def _optional_str(payload: JsonObject, field: str) -> str | None:
     return value
 
 
+def _optional_str_list(payload: JsonObject, field: str) -> tuple[str, ...]:
+    """Parse an optional string-or-array-of-strings field.
+
+    The agent's `systemPrompt` (and similar) became `string[]` server-side
+    when multi-prompt support landed. Older daemons still emit a bare string,
+    so we accept either shape. Returns an empty tuple when the field is
+    absent or null.
+    """
+    value = payload.get(field)
+    if value is None:
+        return ()
+    if isinstance(value, str):
+        return (value,)
+    if isinstance(value, list):
+        items: list[str] = []
+        for index, item in enumerate(value):
+            if not isinstance(item, str):
+                raise ValueError(f"{field}[{index}] must be a string")
+            items.append(item)
+        return tuple(items)
+    raise ValueError(f"{field} must be a string or an array of strings")
+
+
 def _optional_bool(payload: JsonObject, field: str) -> bool | None:
     value = payload.get(field)
     if value is None:
@@ -669,7 +692,7 @@ class SessionState:
     message_count: int
     queued_message_count: int
     todo_phases: tuple[TodoPhase, ...] = ()
-    system_prompt: str | None = None
+    system_prompt: tuple[str, ...] = ()
     dump_tools: tuple[ToolDescriptor, ...] = ()
 
 
@@ -1133,7 +1156,7 @@ def parse_session_state(payload: JsonObject) -> SessionState:
         message_count=int(payload.get("messageCount", 0)),
         queued_message_count=int(payload.get("queuedMessageCount", 0)),
         todo_phases=parse_todo_phases(cast(JsonValue | None, payload.get("todoPhases"))),
-        system_prompt=_optional_str(payload, "systemPrompt"),
+        system_prompt=_optional_str_list(payload, "systemPrompt"),
         dump_tools=dump_tools,
     )
 

python/omp-rpc/tests/test_protocol.py

@@ -82,6 +82,8 @@ class ProtocolParsingTests(unittest.TestCase):
         self.assertEqual(state.follow_up_mode, "all")
         self.assertEqual(state.model.id if state.model else None, "claude-sonnet-4-5")
         self.assertEqual(state.todo_phases[0].tasks[0].status, "in_progress")
+        # Legacy bare-string systemPrompt is accepted and wrapped to a tuple.
+        self.assertEqual(state.system_prompt, ("You are useful.",))
         self.assertEqual(state.dump_tools[0].name, "read")
 
     def test_parse_agent_end_notification(self) -> None:
@@ -182,6 +184,53 @@ class ProtocolParsingTests(unittest.TestCase):
                 }
             )
 
+    def test_parse_session_state_accepts_system_prompt_array(self) -> None:
+        state = parse_session_state(
+            {
+                "sessionId": "session-abc",
+                "steeringMode": "one-at-a-time",
+                "followUpMode": "one-at-a-time",
+                "interruptMode": "immediate",
+                "systemPrompt": ["base instructions", "extra policy"],
+            }
+        )
+        self.assertEqual(state.system_prompt, ("base instructions", "extra policy"))
+
+    def test_parse_session_state_defaults_system_prompt_to_empty_tuple(self) -> None:
+        state = parse_session_state(
+            {
+                "sessionId": "session-abc",
+                "steeringMode": "one-at-a-time",
+                "followUpMode": "one-at-a-time",
+                "interruptMode": "immediate",
+            }
+        )
+        self.assertEqual(state.system_prompt, ())
+
+    def test_parse_session_state_rejects_non_string_in_system_prompt_array(self) -> None:
+        with self.assertRaises(ValueError):
+            parse_session_state(
+                {
+                    "sessionId": "session-abc",
+                    "steeringMode": "one-at-a-time",
+                    "followUpMode": "one-at-a-time",
+                    "interruptMode": "immediate",
+                    "systemPrompt": ["ok", 42],
+                }
+            )
+
+    def test_parse_session_state_rejects_invalid_system_prompt_shape(self) -> None:
+        with self.assertRaises(ValueError):
+            parse_session_state(
+                {
+                    "sessionId": "session-abc",
+                    "steeringMode": "one-at-a-time",
+                    "followUpMode": "one-at-a-time",
+                    "interruptMode": "immediate",
+                    "systemPrompt": {"unexpected": "object"},
+                }
+            )
+
     def test_parse_extension_ui_request_rejects_invalid_method(self) -> None:
         with self.assertRaises(ValueError):
             parse_notification({"type": "extension_ui_request", "id": "ui-1", "method": "launch"})