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10. When the model fails

on_backend_error picks what a failed check does: raise, block, or allow. Limits(max_input_bytes=...) blocks oversized text before any model call. The backend is down on purpose, so this lesson needs no key and no Ollama.

See Failures and limits.

Needs: nothing (the backend is down on purpose).

uv run --group examples python -m examples.10_failures.jev

View examples/10_failures/jev.py on GitHub

examples/10_failures/jev.py
"""Lesson 10 (Jev): When the decision model fails.

on_backend_error picks what a failed check does: raise, block, or allow.
Limits(max_input_bytes=...) blocks oversized text before any model call.

Run:   uv run --group examples python -m examples.10_failures.jev
Needs: nothing (the backend is down on purpose)
"""

from langchain_typesafe import TypeSafeClassifier

from examples._common import print_check
from jes import Guard, Limits
from jes.errors import BackendError
from jes.policies import injection

# Nothing listens on port 9, so every check fails like a backend that is down.
MODEL = TypeSafeClassifier(
    model="jev-latest", base_url="http://localhost:9", api_key="unused", timeout=5
)
THRESHOLD = 0.72
POLICIES = [injection(threshold=THRESHOLD)]


def main() -> None:
    print("== raise (the default)")
    try:
        Guard(POLICIES, model=MODEL, on_backend_error="raise").check_input("hello")
    except BackendError as error:
        print(f"  BackendError: {error}")

    # complete=False means the text was not fully checked.
    print("== block")
    blocked = Guard(POLICIES, model=MODEL, on_backend_error="block").check_input("hello")
    print_check("input", blocked)
    print(f"  complete={blocked.complete} ok={blocked.ok}")

    # The decision is "allow", but ok is False, so your code chooses what to do.
    print("== allow")
    allowed = Guard(POLICIES, model=MODEL, on_backend_error="allow").check_input("hello")
    print_check("input", allowed)
    print(f"  complete={allowed.complete} ok={allowed.ok}")

    print("== byte cap")
    capped = Guard(POLICIES, model=MODEL, limits=Limits(max_input_bytes=4)).check_input(
        "too long for the cap"
    )
    print_check("input", capped)


if __name__ == "__main__":
    main()