callguard

Guardrails for AI-agent tool / function calls. Framework-agnostic, zero heavy dependencies (only jsonschema). Makes agent tool calls reliable and cost-bounded — the two things every agent framework reinvents from scratch and gets wrong.

Why this exists

In production, agents don't fail with clean exceptions. They fail with:

• JSON that has single quotes, trailing commas, unquoted keys (LLM output)
• Wrong types ("123" instead of 123)
• Missing required args (the model "forgot")
• Typo'd tool names (send_emial vs send_email)
• $40 runaway loops because nothing capped spend
• Retry storms against a tool that's 5xx-ing

callguard handles all six in one decorator.

Install

pip install agent-callguard

(PyPI name is agent-callguard; you import it as callguard.)

Quick start

from callguard import Guard

g = Guard(
    budget=g.budget(max_cost=1.0, cost_per_call=0.002),  # stop the $40 loop
    breaker=g.breaker(failure_threshold=5, reset_after=30),  # halt retry storms
    cache=g.cache(ttl=120),                                # skip dup idempotent reads
)

@g.tool(schema={
    "type": "object",
    "properties": {
        "to":      {"type": "string"},
        "subject": {"type": "string"},
        "priority": {"type": "string", "enum": ["low", "normal", "high"]},
        "retry":   {"type": "integer"},
    },
    "required": ["to", "subject"],
})
def send_email(to, subject, body="", priority="normal", retry=0):
    ...  # your real tool body
    return f"sent:{to}"

# The LLM produced this MESSY blob — callguard fixes it automatically:
g.call("send_email", "{'to': 'a@b.com', 'subject': 'Hi', 'retry': '3'}")
#  -> single quotes repaired, '3' coerced to int 3, tool executed OK

# Typo'd tool name? Auto-recovered:
g.call("send_emial", {"to": "a@b.com", "subject": "Hi"})  # redirects to send_email

# Missing required arg? Structured, actionable error (not a stack trace):
try:
    g.call("send_email", {"to": "a@b.com"})
except ToolCallError as e:
    print(e.errors[0].suggestion)   # "provide 'subject'"

What it does

Lower-level pieces

Every component is importable and usable on its own:

from callguard import repair_json, validate, classify, suggest_tool, parse_args
from callguard import Budget, CircuitBreaker, Cache, tool, Guard

repair_json("{'a': 1,}")            # '{"a": 1}'
classify("503 Service Unavailable")  # CallError(kind='transient', recoverable=True)
suggest_tool("send_emial", ["send_email", "get_weather"])  # 'send_email'

Backtest

The repo ships a regression corpus of real-world malformed agent calls (callguard/backtest.py) — single-quote JSON, trailing commas, missing args, enum typos, tool-name typos, transient vs hard errors, budget enforcement, circuit-breaker trips. Run it:

python -m callguard.backtest

Current status: 18 / 18 cases passing.

License

MIT