---
name: prismer-python-debugpy
scope: coding
description: "Debug Python: pdb REPL + debugpy remote (DAP)."
version: 1.0.0
author: Hermes Agent
license: MIT
platforms: [ linux, macos ]
metadata:
  nativeReplaces: [ python-debugpy ]
  hermes:
    tags: [ debugging, python, pdb, debugpy, breakpoints, dap, post-mortem ]
    related_skills: [ systematic-debugging, node-inspect-debugger ]
  requiresExplicitGrant: true
---

## Prismer execution contract

This is the uniquely named `prismer-python-debugpy` skill, adapted from Hermes.
Use the actual tools exposed by the executing host; examples using terminal,
process, delegate_task, vision_analyze or browser_* are not tool registrations.
Missing dependencies do not hide this skill. Report command startup, version,
account/permissions and task-specific live verification separately. Use task-owned
artifact paths and existing user authorization; do not change shared accounts,
Runtime/provider configuration, global security settings or unrelated work.
See NOTICE.md and LICENSE for resource provenance. Runtime availability and
upstream-entry suppression are owned by the integration layer.


# Python Debugger (pdb + debugpy)

## Overview

Three tools, picked by situation:

| Tool | When |
|---|---|
| **`breakpoint()` + pdb** | Local, interactive, simplest. Add `breakpoint()` in the source, run normally, get a REPL at that line. |
| **`python -m pdb`** | Launch an existing script under pdb with no source edits. Useful for quick poking. |
| **`debugpy`** | Remote / headless / "attach to already-running process." Talks DAP, scriptable from terminal, works for long-lived processes (gateway, daemon, PTY children). |

**Start with `breakpoint()`.** It's the cheapest thing that works.

## When to Use

- A test fails and the traceback doesn't reveal why a value is wrong
- You need to step through a function and watch a collection mutate
- A long-running process (hermes gateway, tui_gateway) misbehaves and you can't restart it
- Post-mortem: an exception fired in prod-ish code and you want to inspect locals at the crash site
- A subprocess / child (Python `_SlashWorker`, PTY bridge worker) is the actual bug site

**Don't use for:** things `print()` / `logging.debug` solve in under a minute, or things `pytest -vv --tb=long --showlocals` already reveals.

## pdb Quick Reference

Inside any pdb prompt (`(Pdb)`):

| Command | Action |
|---|---|
| `h` / `h cmd` | help |
| `n` | next line (step over) |
| `s` | step into |
| `r` | return from current function |
| `c` | continue |
| `unt N` | continue until line N |
| `j N` | jump to line N (same function only) |
| `l` / `ll` | list source around current line / full function |
| `w` | where (stack trace) |
| `u` / `d` | move up / down in the stack |
| `a` | print args of the current function |
| `p expr` / `pp expr` | print / pretty-print expression |
| `display expr` | auto-print expr on every stop |
| `b file:line` | set breakpoint |
| `b func` | break on function entry |
| `b file:line, cond` | conditional breakpoint |
| `cl N` | clear breakpoint N |
| `tbreak file:line` | one-shot breakpoint |
| `!stmt` | execute arbitrary Python (assignments included) |
| `interact` | drop into full Python REPL in current scope (Ctrl+D to exit) |
| `q` | quit |

The `interact` command is the most powerful — you can import anything, inspect complex objects, even call methods that mutate state. Locals are read-only by default; use `!x = 42` from the `(Pdb)` prompt to mutate.

## Recipe 1: Local breakpoint

Easiest. Edit the file:

```python
def compute(x, y):
    result = some_helper(x)
    breakpoint()           # <-- drops into pdb here
    return result + y
```

Run the code normally. You land at the `breakpoint()` line with full access to locals.

**Don't forget to remove `breakpoint()` before committing.** Use `git diff` or a pre-commit grep:
```bash
rg -n 'breakpoint\(\)' --type py
```

## Recipe 2: Launch a script under pdb (no source edits)

```bash
python -m pdb path/to/script.py arg1 arg2
# Lands at first line of script
(Pdb) b path/to/script.py:42
(Pdb) c
```

## Recipe 3: Debug a pytest test

Use raw pytest in a scoped test environment for an interactive debugger:

```bash
# Drop to pdb on failure (or on any raised exception):
python -m pytest <exact-test-path> -x --pdb

# Drop to pdb at the START of the test:
python -m pytest tests/path/to/test_file.py::test_name --trace

# Show locals in tracebacks without pdb:
python -m pytest tests/path/to/test_file.py --showlocals --tb=long
```

The repository runner may capture output and cannot always support pdb. For an
interactive session use a TTY and select one test, then rerun the actual canonical
runner non-interactively for regression verification:

```bash
source .venv/bin/activate
python -m pytest tests/foo_test.py::test_bar --pdb
```

Keep the test environment isolated; do not import shared production credentials.

## Recipe 4: Post-mortem on any exception

```python
import pdb, sys
try:
    run_the_thing()
except Exception:
    pdb.post_mortem(sys.exc_info()[2])
```

Or wrap a whole script:

```bash
python -m pdb -c continue script.py
# When it crashes, pdb catches it and you're in the frame of the exception
```

Or set a global hook in a repl/jupyter:

```python
import sys
def excepthook(etype, value, tb):
    import pdb; pdb.post_mortem(tb)
sys.excepthook = excepthook
```

## Recipe 5: Remote debug with debugpy (attach to running process)

For long-lived processes: Hermes gateway, tui_gateway, a daemon, a process that's already misbehaving and can't be restarted clean.

### Setup

```bash
source <hermes-agent-repo>/.venv/bin/activate
pip install debugpy
```

### Pattern A: Source-edit — process waits for debugger at launch

Add near the top of the entry point (or inside the function you want to debug):

```python
import debugpy
debugpy.listen(("127.0.0.1", 5678))
print("debugpy listening on 5678, waiting for client...", flush=True)
debugpy.wait_for_client()
debugpy.breakpoint()       # optional: pause immediately once attached
```

Start the process; it blocks on `wait_for_client()`.

### Pattern B: No source edit — launch with `-m debugpy`

```bash
python -m debugpy --listen 127.0.0.1:5678 --wait-for-client your_script.py arg1
```

Equivalent for module entry:

```bash
python -m debugpy --listen 127.0.0.1:5678 --wait-for-client -m your.module
```

### Pattern C: Attach to an already-running process

Needs the PID and debugpy preinstalled in the target's environment:

```bash
python -m debugpy --listen 127.0.0.1:5678 --pid <pid>
# debugpy injects itself into the process. Then attach a client as below.
```

If attach is blocked by OS policy, do not lower global ptrace protections.
Start a new task-owned process under debugpy, or ask the system owner for an
approved debugging environment.

### Connecting a client from the terminal

The easiest terminal-side DAP client is VS Code CLI or a small script. From inside Hermes you have two practical options:

**Option 1: `debugpy`'s own CLI REPL** — not an official feature, but a tiny DAP client script:

Use the bundled `scripts/dap_client.py` (Python stdlib); resolve its directory
from the installed skill, not cwd. With that directory on sys.path:

```python
import socket
from dap_client import DapClient
with socket.create_connection(("127.0.0.1", 5678), timeout=5) as sock:
    client = DapClient(sock, timeout=5)
    client.request("initialize", {"adapterID": "python"})
    attach_seq = client.send("attach", {})
    client.wait(lambda p: p.get("event") == "initialized")
    # Configure the exact authorized source and line here.
    client.request("setBreakpoints", {"source": {"path": SOURCE},
                                     "breakpoints": [{"line": LINE}]})
    client.request("configurationDone")
    client.response(attach_seq)
    # Continue/step only the authorized target; disconnect when finished.
    client.request("disconnect", {"terminateDebuggee": False})
```

The transport handles EOF, timeout, size limits, events and request_seq matching.
It is not a full IDE, and live debugpy handshake compatibility still needs a real
session. Do not leave the target paused or enable remote/public listeners.

**Option 2: Attach from VS Code / Cursor / Zed** — if the user has one open, they can add a `launch.json`:

```json
{
  "name": "Attach to Hermes",
  "type": "debugpy",
  "request": "attach",
  "connect": { "host": "127.0.0.1", "port": 5678 },
  "justMyCode": false,
  "pathMappings": [
    { "localRoot": "${workspaceFolder}", "remoteRoot": "<hermes-agent-repo>" }
  ]
}
```

**Option 3: Ditch DAP, use `remote-pdb`** — usually what you actually want from a terminal agent:

```bash
pip install remote-pdb
```

In your code:
```python
from remote_pdb import set_trace
set_trace(host="127.0.0.1", port=4444)   # blocks until connection
```

Then from the terminal:
```bash
nc 127.0.0.1 4444
# You get a (Pdb) prompt exactly as if debugging locally.
```

`remote-pdb` is the cleanest agent-friendly choice when `debugpy`'s DAP protocol is overkill. Use `debugpy` only when you actually need IDE integration.

## Debugging Hermes-specific Processes

### Tests
See Recipe 3. The wrapper captures subprocess output, so run pytest directly for interactive pdb.

### `run_agent.py` / CLI — one-shot
Easiest: add `breakpoint()` near the suspect line, then run `hermes` normally. Control returns to your terminal at the pause point.

### `tui_gateway` subprocess (spawned by `hermes --tui`)
The gateway runs as a child of the Node TUI. Options:

**A. Source-edit the gateway:**
```python
# tui_gateway/server.py near the top of serve()
import debugpy
debugpy.listen(("127.0.0.1", 5678))
debugpy.wait_for_client()
```
Start `hermes --tui`. The TUI will appear frozen (its backend is waiting). Attach a client; execution resumes when you `continue`.

**B. Use `remote-pdb` at a specific handler:**
```python
from remote_pdb import set_trace
set_trace(host="127.0.0.1", port=4444)   # in the RPC handler you want to trap
```
Trigger the matching slash command from the TUI, then `nc 127.0.0.1 4444` in another terminal.

### `_SlashWorker` subprocess
Same pattern — `remote-pdb` with `set_trace()` inside the worker's `exec` path. The worker is persistent across slash commands, so the first trigger blocks until you connect; subsequent slash commands pass through normally unless you re-arm.

### Gateway (`gateway/run.py`)
Long-lived. Use `remote-pdb` at a handler, or `debugpy` with `--wait-for-client` if you're restarting the gateway anyway.

## Common Pitfalls

1. **pdb under a parallel/output-capturing runner silently does nothing.** You won't see the prompt, the test just hangs (true of pytest-xdist and of `scripts/run_tests.sh`'s captured per-file subprocesses). Run pytest directly on a single file for interactive debugging.

2. **`breakpoint()` in CI / non-TTY contexts hangs the process.** Safe locally; never commit it. Add a pre-commit grep as a safety net.

3. **`PYTHONBREAKPOINT=0`** disables all `breakpoint()` calls. Check the env if your breakpoint isn't hitting:
   ```bash
   echo $PYTHONBREAKPOINT
   ```

4. **`debugpy.listen` blocks only if you also call `wait_for_client()`.** Without it, execution continues and your first breakpoint may fire before the client is attached.

5. Do not change global ptrace policy; use a task-owned debugpy child.

6. **Threads.** `pdb` only debugs the current thread. For multithreaded code, use `debugpy` (thread-aware DAP) or set `threading.settrace()` per thread.

7. **asyncio.** `pdb` works in coroutines but `await` inside pdb requires Python 3.13+ or `await` from `interact` mode on older versions. For 3.11/3.12, use `asyncio.run_coroutine_threadsafe` tricks or `!stmt`-based awaits via `asyncio.ensure_future`.

8. **Repository runners may isolate HOME/credentials or capture output.** Inspect the actual runner before choosing an interactive invocation. Do not import real API keys into a debug session; use a scoped test credential only when authorized.

9. **Forking / multiprocessing.** pdb does not follow forks. Each child needs its own `breakpoint()` or `set_trace()`. For Hermes subagents, debug one process at a time.

## Verification Checklist

- [ ] After `pip install debugpy`, confirm: `python -c "import debugpy; print(debugpy.__version__)"`
- [ ] For remote debug, confirm the port is actually listening: `ss -tlnp | grep 5678`
- [ ] First breakpoint actually hits (if it doesn't, you likely have `PYTHONBREAKPOINT=0`, you're under a parallel/capturing runner, or execution finished before attach)
- [ ] `where` / `w` shows the expected call stack
- [ ] Post-debug cleanup: no stray `breakpoint()` / `set_trace()` in committed code
  ```bash
  rg -n 'breakpoint\(\)|set_trace\(|debugpy\.listen' --type py
  ```

## One-Shot Recipes

**"Why is this dict missing a key?"**
```python
# add above the KeyError site
breakpoint()
# then in pdb:
(Pdb) pp d
(Pdb) pp list(d.keys())
(Pdb) w                # how did we get here
```

**"This test passes in isolation but fails in the suite."**
```bash
python -m pytest tests/the_test.py   # replace with the project's actual canonical runner
# For interactive debugging, or if it only fails WITH other tests:
source .venv/bin/activate
python -m pytest tests/ -x --pdb
# Now it pdb-traps at the exact failing test after state accumulated.
```

**"My async handler deadlocks."**
```python
# Add at handler entry
import remote_pdb; remote_pdb.set_trace(host="127.0.0.1", port=4444)
```
Trigger the handler. `nc 127.0.0.1 4444`, then `w` to see the suspended frame, `!import asyncio; asyncio.all_tasks()` to see what else is pending.

**"Post-mortem on a crash in an Ink child process / subprocess."**
```bash
PYTHONFAULTHANDLER=1 python -m pdb -c continue path/to/entrypoint.py
# On crash, pdb lands at the frame of the exception with full locals
```