core/config: default system prompt for coding agent tools

src/plyngent/cli/editor.py

@@ -29,7 +29,8 @@ _MINIMAL_CONFIG = """\
 # # url = ":memory:"
 
 # [agent]
-# system_prompt = "You are a careful coding assistant."
+# # system_prompt defaults to the built-in coding-agent guide when omitted.
+# # system_prompt = ""  # disable; or multi-line '''...''' to override
 # max_tool_result_chars = 32000
 # parallel_tools = true
 # confirm_destructive = true

src/plyngent/config/__init__.py

@@ -5,6 +5,7 @@ from typing import TYPE_CHECKING
 import tomlkit
 from tomlkit.exceptions import TOMLKitError
 
+from .models import DEFAULT_SYSTEM_PROMPT as DEFAULT_SYSTEM_PROMPT
 from .models import AgentConfig as AgentConfig
 from .models import AnthropicProvider as AnthropicProvider
 from .models import DatabaseConfig as DatabaseConfig

src/plyngent/config/models.py

@@ -2,6 +2,44 @@ from typing import Any
 
 from msgspec import Struct, field
 
+# Built-in agent system prompt when ``[agent].system_prompt`` is omitted.
+# Set ``system_prompt = ""`` in TOML to disable. Override with a multi-line
+# TOML literal (prefer '''...''' so nested " quotes are fine).
+DEFAULT_SYSTEM_PROMPT = """\
+You are a professional coding agent in a workspace-bound tool environment.
+
+### Workspace
+- Explore with `tree`/`listdir`/`glob_paths`/`grep_files`/`read_file` when unsure.
+- Stay under the workspace (or a path from `new_temporary_workspace`). Respect path denylists.
+
+### Files
+- Prefer file tools over shell. `edit_replace` fails usually means a bad match — \
+fix `old_string` or `max_replaces`; shell workarounds rarely help.
+- `edit_replace` defaults to the first match; if remaining matches are reported, \
+raise `max_replaces` or narrow `old_string`.
+- Before `edit_lineno`, `read_file` with `with_lineno=true`.
+
+### Commands
+- Prefer `run_command` / `run_command_batch` (argv, no shell) over `bash -c` or similar.
+- Several `run_command` calls in one step may run in parallel; use `run_command_batch` \
+for ordered pipelines (`pipe_out` / `mix_stderr` as needed).
+- Prefer `vcs_*` for status/diff/log/branch when enough.
+
+### Humans & safety
+- Prefer `ask_user_line` / `ask_user_choice` / `ask_user_form` over waiting for the next free-form turn.
+- Overwrites, deletes, shells, and risky ops may require human confirm; hard denylists are not skipped by YOLO.
+- Temporary scratch: `new_temporary_workspace`.
+
+### PTY
+- Use PTY for interactive/TUI, sudo, ssh, or live servers.
+- Passwords and other secret input: only `ask_into_pty` (never echo secrets into `write_pty`).
+- Control keys: `write_pty_keys`, not literal `write_pty` data.
+
+### Todos
+- Use the todo stack for multi-step work (LIFO groups: push related items, finish/update, \
+pop the group). Open items mean unfinished work.
+"""
+
 
 class DatabaseConfig(Struct, omit_defaults=True):
     """Database connection configuration.
@@ -20,7 +58,7 @@ class DatabaseConfig(Struct, omit_defaults=True):
 class AgentConfig(Struct, omit_defaults=True):
     """Single-user agent profile defaults."""
 
-    system_prompt: str = ""
+    system_prompt: str = DEFAULT_SYSTEM_PROMPT
     max_tool_result_chars: int = 32_000
     parallel_tools: bool = True
     confirm_destructive: bool = True