Proofs that find real production bugs

TrustedRouter added property-based testing and TLA+ model checking to its existing 50,000-line Python control plane and CI pipeline, finding fifteen substantive defects across twenty-three pull requests despite an already extensive conventional test suite. Property tests exposed an attestation bypass caused by empty accepted-digest policies, while a log-scrubbing property initially missed API keys used as dictionary keys because its generator only placed secrets in values, illustrating that coverage depends on what generators actually explore. A TLA+ model of an unimplemented regional quota-lease system explored millions of interleavings and revealed that quarantined, expired leases could permanently strand escrowed funds, a liveness failure that unit tests could not have found before implementation. The work also showed the limits of formal verification: a model can appear to prove a claim while omitting the behavior that makes the claim relevant, so specifications and CI checks must prevent vacuous successes. Integrated deployment-gate checks require model configurations and explicit properties, while simpler safeguards such as bounding untrusted retry values, testing durable-data round trips, and forcing classification decisions for new data fields delivered especially high value. One missing bound on a Retry-After header, for example, crashed the Swift SDK, hung the Python client indefinitely, and caused the Go client to wait centuries, demonstrating how broadly small specification gaps can affect production systems.

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