runaway-guard — $-Cost is the Third Complexity Dimension

Catalog review modification (2026-10-23): corrected cost arithmetic, retry/idempotency claims and unsupported provider-cap guarantees. Upstream authorship and Apache-2.0 attribution are retained.

Every loop has time complexity and space complexity. A loop that calls a paid API has a third: dollars per execution. The model tracks the first two automatically. It does not track the third, so it ships code where a single bug — a retry without bound, a stream reconnect storm, an agent that re-queues itself, a webhook that fires the same job twice — silently spends real money.

Illustrative, unverified incident narrative: developer writes a Fal.ai image-generation loop. Loop "obviously terminates" because it iterates over a fixed list. The list comes from a callback that fires on every Inngest retry. Each retry doubles the list. By morning, the bill is $200. Tests pass. Code review passed. The bug is not in the loop body. The bug is that no one stated the wallet invariant.

runaway-guard helps identify this design risk. State the max calls. State the max dollars per run. State the max dollars per day. Set the same caps in the provider dashboard so a code bug cannot bypass them. Then write the code.

Violating the letter of these rules is violating the spirit of the skill. "I'm only testing locally" is the exact rationalization that ships the $200 bill — local code hits the same paid API as production.

When to Use This Skill

Use runaway-guard when: