core/agent+cli: wire system prompt and agent budgets

ChatAgent accepts system_prompt, result budget, and parallel_tools;
CLI loads them from config [agent].

src/plyngent/agent/chat.py

@@ -2,8 +2,9 @@ from __future__ import annotations
 
 from typing import TYPE_CHECKING
 
-from plyngent.lmproto.openai_compatible.model import UserChatMessage
+from plyngent.lmproto.openai_compatible.model import SystemChatMessage, UserChatMessage
 
+from .budget import DEFAULT_TOOL_RESULT_MAX_CHARS
 from .loop import DEFAULT_MAX_ROUNDS, run_chat_loop
 
 if TYPE_CHECKING:
@@ -31,6 +32,9 @@ class ChatAgent:
     temperature: float | None
     on_limit: LimitContinueHook | None
     stream: bool
+    system_prompt: str | None
+    max_tool_result_chars: int
+    parallel_tools: bool
     messages: list[AnyChatMessage]
     pending_retry_text: str | None
 
@@ -47,6 +51,9 @@ class ChatAgent:
         messages: Sequence[AnyChatMessage] | None = None,
         on_limit: LimitContinueHook | None = None,
         stream: bool = True,
+        system_prompt: str | None = None,
+        max_tool_result_chars: int = DEFAULT_TOOL_RESULT_MAX_CHARS,
+        parallel_tools: bool = True,
     ) -> None:
         self.client = client
         self.model = model
@@ -57,8 +64,20 @@ class ChatAgent:
         self.temperature = temperature
         self.on_limit = on_limit
         self.stream = stream
+        self.system_prompt = system_prompt
+        self.max_tool_result_chars = max_tool_result_chars
+        self.parallel_tools = parallel_tools
         self.messages = list(messages) if messages is not None else []
         self.pending_retry_text = None
+        self._ensure_system_prompt()
+
+    def _ensure_system_prompt(self) -> None:
+        """Prepend system prompt once when configured and history has none."""
+        if not self.system_prompt:
+            return
+        if self.messages and isinstance(self.messages[0], SystemChatMessage):
+            return
+        self.messages.insert(0, SystemChatMessage(content=self.system_prompt))
 
     async def load_history(self) -> None:
         """Replace in-memory messages from the bound memory session."""
@@ -67,6 +86,7 @@ class ChatAgent:
             raise RuntimeError(msg)
         self.messages = await self.memory.list_messages(self.session_id)
         self.pending_retry_text = None
+        self._ensure_system_prompt()
 
     async def bind_session(self, session_id: int, *, load: bool = True) -> None:
         """Attach a memory session id; optionally load existing messages."""
@@ -103,11 +123,12 @@ class ChatAgent:
                 temperature=self.temperature,
                 on_limit=self.on_limit,
                 stream=self.stream,
+                max_tool_result_chars=self.max_tool_result_chars,
+                parallel_tools=self.parallel_tools,
             ):
                 yield event
             completed = True
         except BaseException:
-            # Includes CancelledError / KeyboardInterrupt paths via task cancel.
             if not completed:
                 self._rollback_turn(pre_len, user_msg.content)
             raise
@@ -118,6 +139,7 @@ class ChatAgent:
 
     async def run(self, user_text: str) -> AsyncIterator[AgentEvent]:
         """Append a user message, run the tool loop, yield events, persist only on success."""
+        self._ensure_system_prompt()
         user_msg = UserChatMessage(content=user_text)
         self.messages.append(user_msg)
         async for event in self._run_from_user_message(user_msg):
@@ -129,6 +151,7 @@ class ChatAgent:
         if text is None:
             msg = "nothing to retry"
             raise RuntimeError(msg)
+        self._ensure_system_prompt()
         user_msg = UserChatMessage(content=text)
         self.messages.append(user_msg)
         async for event in self._run_from_user_message(user_msg):

src/plyngent/cli/editor.py

@@ -22,6 +22,11 @@ _MINIMAL_CONFIG = """\
 # implementation = "sqlite"
 # url = "/path/to/chat.db"
 
+# [agent]
+# system_prompt = "You are a careful coding assistant."
+# max_tool_result_chars = 32000
+# parallel_tools = true
+
 # [providers.example]
 # preset = "openai-compatible"
 # url = "https://api.openai.com/v1"

src/plyngent/cli/state.py

@@ -45,6 +45,8 @@ class ReplState:
     def _make_agent(self) -> ChatAgent:
         from plyngent.cli.limits import prompt_continue_limit
 
+        agent_cfg = self.config.agent_config
+        system_prompt = agent_cfg.system_prompt or None
         return ChatAgent(
             self.client,
             model=self.model,
@@ -54,6 +56,9 @@ class ReplState:
             max_rounds=self.max_rounds,
             on_limit=prompt_continue_limit,
             stream=True,
+            system_prompt=system_prompt,
+            max_tool_result_chars=agent_cfg.max_tool_result_chars,
+            parallel_tools=agent_cfg.parallel_tools,
         )
 
     def rebuild_client(self) -> None:

tests/test_agent/test_loop.py

@@ -391,3 +391,57 @@ async def test_chat_agent_retry_after_failure() -> None:
     assert isinstance(loaded[0], UserChatMessage)
     assert loaded[0].content == "ping"
     await store.close()
+
+
+async def test_chat_agent_system_prompt_prepended() -> None:
+    from plyngent.lmproto.openai_compatible.model import SystemChatMessage
+
+    client = ScriptedClient([_response(AssistantChatMessage(content="ok"))])
+    agent = ChatAgent(client, model="m", system_prompt="Be brief.", stream=False)
+    _ = [e async for e in agent.run("hi")]
+    assert isinstance(agent.messages[0], SystemChatMessage)
+    assert agent.messages[0].content == "Be brief."
+    assert isinstance(client.calls[0].messages[0], SystemChatMessage)
+
+
+async def test_tool_result_char_budget() -> None:
+    @tool
+    def big() -> str:
+        return "x" * 100
+
+    registry = ToolRegistry([big])
+    client = ScriptedClient(
+        [
+            _response(
+                AssistantChatMessage(
+                    content="",
+                    tool_calls=[
+                        AssistantFunctionToolCall(
+                            id="1",
+                            function=AssistantFunctionTool(name="big", arguments="{}"),
+                        )
+                    ],
+                )
+            ),
+            _response(AssistantChatMessage(content="done")),
+        ]
+    )
+    messages: list[AnyChatMessage] = [UserChatMessage(content="go")]
+    _ = [
+        e
+        async for e in run_chat_loop(
+            client,
+            messages,
+            model="m",
+            tools=registry,
+            stream=False,
+            max_tool_result_chars=20,
+            parallel_tools=False,
+        )
+    ]
+    from plyngent.lmproto.openai_compatible.model import ToolChatMessage
+
+    tool_msgs = [m for m in messages if isinstance(m, ToolChatMessage)]
+    assert len(tool_msgs) == 1
+    assert tool_msgs[0].content.startswith("x" * 20)
+    assert "truncated" in tool_msgs[0].content