@Marvin if we set up the python environment in a docker image with

uv

for a prefect AWS ECS push pool, do we need to modify the entrypoint command to ensure that the correct executable is used at runtime? I didn't modify the entrypoint command but now the execution at runtime seems to be in the wrong python environment

✅ thought for 469.8 seconds

aw nice getting that agent workflow going

Short answer: Yes — with an ECS push work pool, Prefect will start your container and run a command without “activating” any virtual environment. If your image uses uv in a way that puts your deps in a venv that isn’t on PATH, the runtime will use the wrong Python. You don’t necessarily need a custom entrypoint, but you do need to ensure that the default python/prefect in PATH is the one with your deps. What’s happening - In Prefect 3.x ECS push pools, Prefect submits an ECS task that overrides the container command at runtime. It does not source/activate a venv for you. - If uv installed your packages into a venv (or cache) that is not the default interpreter, the command Prefect runs will hit the system python instead. Reliable patterns that work with ECS push pools Pick one of these; they avoid needing a special entrypoint: 1) Install into the system Python with uv (recommended) - This makes python and prefect resolve correctly without activation.

FROM prefecthq/prefect:3-latest

# Add uv
COPY --from=<http://ghcr.io/astral-sh/uv:latest|ghcr.io/astral-sh/uv:latest> /uv /usr/local/bin/uv

# Install into system Python
ENV UV_SYSTEM_PYTHON=1
COPY requirements.txt .
RUN uv pip install --system -r requirements.txt

2) Use a dedicated venv and put it first on PATH - No shell activation required; the right python is picked up by PATH.

FROM prefecthq/prefect:3-latest

# Optional: install uv
COPY --from=<http://ghcr.io/astral-sh/uv:latest|ghcr.io/astral-sh/uv:latest> /uv /usr/local/bin/uv

# Create venv and ensure it’s first on PATH
RUN python -m venv /opt/venv
ENV PATH="/opt/venv/bin:${PATH}"

# Install dependencies into that venv
COPY requirements.txt .
RUN uv pip install -r requirements.txt
# or: RUN pip install -r requirements.txt

3) If you must activate a venv via ENTRYPOINT - Keep ENTRYPOINT minimal and “exec”-based so Prefect’s command is passed through. - ECS push pools typically override CMD/command, not ENTRYPOINT. Your ENTRYPOINT can activate and then exec "$@". Example:

# entrypoint.sh
#!/usr/bin/env sh
. /opt/venv/bin/activate
exec "$@"

# Dockerfile
COPY entrypoint.sh /entrypoint.sh
RUN chmod +x /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]

Quick checks to confirm at runtime Add a tiny debug at the start of your flow to confirm the interpreter that’s running:

import sys, prefect
print("Python:", sys.executable)
print("Prefect:", prefect.__version__)

If it does not point to your intended env, fix PATH or switch to UV_SYSTEM_PYTHON. A couple of gotchas with uv - uvx is great for ad-hoc commands, but it resolves environments on the fly. For containers, prefer a stable install (UV_SYSTEM_PYTHON=1 or an explicit venv on PATH). - If you rely on uv-managed cached envs without adjusting PATH, ECS won’t “activate” them and you’ll see missing imports at runtime. If you can share your Dockerfile (base image, how you install uv and deps, and whether you set ENTRYPOINT), I can pinpoint the minimal change for your setup. Relevant docs - ECS guide: Prefect AWS ECS guide - Serverless/push infra overview: Serverless (push) infrastructure

my approach seems to have worked actually but thank you, good bot, that was a very good answer!