389 Tests Passed. NIST Still Caught the Bug.

A developer testing an AI agent's calculator tool found that a mutation changing multiplication to addition in a Rust library still passed 389 of 390 tests, with the sole failure caught by NIST's certified Longley regression dataset. The experiment highlights that deterministic computation is not inherently trustworthy, and that independent reference data is crucial for verifying tool behavior in AI systems.

I gave an AI agent a calculator because I wanted one hard, inspectable point inside a probabilistic workflow. The model could interpret the request and explain the result. The calculator would perform the computation. It seemed like a clean division of labor. Then I changed one multiplication sign into addition. The calculator still passed 389 of the 390 tests in its Rust library harness. The sole failure compared its answer with NIST's certified results for the Longley regression dataset. That bothered me more than a completely broken build would have. I had treated deterministic computation as safer than asking a language model to improvise arithmetic. But deterministic does not mean trustworthy. A program can return the same wrong answer forever. “Source of truth” suddenly felt too comfortable. Before an AI agent delegates authority to a tool, that authority should be challenged—and remain revocable by evidence. The calculator is only the specimen. The larger idea is a way to place inspectable, replayable instruments inside probabilistic systems. The interesting distinction is not model weights versus a “real CPU.” Model inference also runs on processors, and language models can learn genuine arithmetic procedures. The useful boundary is between generating an answer and executing a defined operation under a tested contract . Research on Program-Aided Language Models PAL https://proceedings.mlr.press/v202/gao23f.html makes a related split: That is the division I want in an agent: This does not make the whole agent deterministic, and it does not make the model unnecessary. The agent can still choose the wrong tool, supply the wrong arguments, misunderstand units, or misread the result. The promise is smaller: one claim becomes inspectable, replayable, and independently testable. The opposite of probabilistic is not trustworthy. It is repeatable. A CPU can be precisely wrong. Calibration cannot be entirely self-referential. The implementation should not be the sole author of its own expected answers. That is why I chose NIST—not because government authority turns a result into mathematical truth, but because NIST has a long institutional practice of building shared, independently evaluated references. Congress established the National Bureau of Standards in 1901 https://www.nist.gov/pml/owm/about-owm/owm-background-and-history to StRD pairs datasets with certified expected values for specific statistical procedures. Its collection includes generated and real-world cases of varying difficulty. For linear procedures, NIST carried 500 digits through its calculations https://www.nist.gov/itl/sed/products-services/statistical-reference-data-sets-strd Longley is a small but numerically challenging linear-regression dataset in that collection. Its certified results gave my tests something the implementation could not manufacture for itself: an expected answer produced outside the code under test. That qualification matters. StRD does not certify Oddly Exact, prove the statistics engine correct, or make the software traceable to NIST. NIST explicitly says the datasets are an aid for evaluating software and that no mechanism establishes software traceability https://www.itl.nist.gov/div898/strd/general/faq.html . The reference data was not an oracle for the entire program. It was an independent witness for the calculations it covered. Example-based tests ask whether familiar inputs still produce familiar outputs. Mutation testing asks a more uncomfortable question: If I introduce a small, plausible defect, does the suite notice? I used cargo-mutants to replace multiplication with addition in the multiple-regression standard-error calculation: - residual std dev sum sq.sqrt + residual std dev + sum sq.sqrt The mutation preserved valid Rust, valid types, and a plausible-looking numeric result. In the library harness, 389 of 390 tests still passed. The assertion against NIST's Longley values was the sole failure. The public mutation record https://gist.github.com/copyleftdev/277e48079658f6f89deb2254701ebcf7 I cannot claim Longley was the only test in the entire repository capable of catching the mutation; the run stopped after the failed library harness. I can claim something narrower and more useful: hundreds of tests tolerated a semantically broken formula, while an assertion anchored to independently produced values rejected it. A green test count is evidence only to the extent that those tests would turn red when the implementation meaningfully changes. Mutation testing measures that sensitivity instead of admiring the count. The formula mutation challenged numerical meaning. My next audit challenged the promises around the calculation: what the tool accepts, what it refuses, and how much work it will perform. The first crack was a contradiction between the advertised contract and the executable one. The generated JSON Schema forbade additional properties, but the Rust deserializer silently accepted an unknown field at the request root and another inside an expression node. The answer was still numerically correct. That did not make the behavior harmless. A caller validating against the schema saw a stricter instrument than a caller speaking directly to the binary. A misspelled or misunderstood field could disappear without warning. For an agent-facing tool, silently interpreting a different contract is itself a correctness defect. The second crack was a resource boundary. The expression engine limited things such as integer size, expression depth, and precision. The optimization interface, however, accepted grid resolution and iteration counts without upper ceilings. I requested a grid search with one billion sample points. The pre-repair binary produced no JSON before an external one-second watchdog terminated it. The tool had input validation, but it had not earned the bounded-work guarantee I thought I had built. Those failures became permanent regression requests. The deserializer now rejects unknown fields at both levels. The optimizer now publishes and enforces ceilings of 100,000 iterations and 1,000,000 grid points. The same billion-point request returns a typed resource limit response before entering the search. The contract challenge Gist https://gist.github.com/copyleftdev/28ed6931c6174099b21db54846f72b65 That script is more valuable than a screenshot of a green run. It lets another observer cross-examine the boundary directly. A new guard is only another claim until the tests prove they care about it. After repairing the optimizer limits, I selected every mutation cargo-mutants generated for the two new validators. Eleven mutations tried to remove the checks, replace them with unconditional success, or alter their boundary comparisons. The tests caught all eleven. The focused optimizer mutation record https://gist.github.com/copyleftdev/9b9afe1b1173217d92a4cb0ede2278d0 That completed the loop: claim → attack → fail → repair → mutate → replay If I published only the final green suite, you would see confidence. By preserving the changed operator, the request that hung, the contract contradiction, the repair, and the mutations that tried to undo it, I can show a reason for confidence. The scar is part of the calibration record. Oddly Exact is one calculator, but the method travels. Before giving a narrow tool authority inside an agent workflow, I now ask five questions: Is the contract explicit and executable? The schema, parser, runtime, limits, and failure modes must agree. Documentation the executable does not enforce is only a suggestion. Is there an independent reference? In this statistical case, NIST StRD moved selected expected answers outside my implementation. Another domain might use a standards specification, a reference implementation, a verified corpus, or a separately derived test oracle. Which properties should survive across many inputs? Property-based tests generate cases and check laws rather than memorizing individual examples. One Oddly Exact property checks that translating every value in a sample by a large exact offset leaves sample variance unchanged within a tight floating-point tolerance. Can the tests detect plausible corruption? Mutation testing changes operators, comparisons, return values, and guards. A surviving mutation does not prove the implementation is wrong; it exposes a behavior change the suite cannot currently distinguish. Are failures bounded, typed, and replayable? An agent must distinguish a rejected computation from an unavailable tool. Division by zero, resource exhaustion, malformed JSON, and a crashed process should not collapse into the same ambiguous failure. These layers do different jobs. External references challenge specific answers. Properties challenge behavior across an input space. Mutations challenge the tests themselves. Contract and resource attacks challenge the boundary around all of it. None provides universal correctness. Together they create a narrower and more useful result: evidence that a particular tool deserves provisional authority for validated requests inside a declared contract. For a validated request inside that contract, the tool's structured result can serve as the agent's operational source of truth. That is a runtime role, not a claim of infallibility. If the agent encounters a disagreement, it can inspect the arguments, surface the conflict, or consult another independently trusted tool. It should not quietly replace a structured result with fresh prose arithmetic. The trust relationship runs in opposite directions at different times: New evidence can always return the tool to the first phase. That is a source under challenge : trusted in operation, open to appeal, and always one The beautiful thing is not that source code is truth. The beautiful thing is that source code can be cross-examined. I did not end this experiment with a calculator an AI agent can trust forever. I ended it with something more useful: a calculator whose authority can be revoked by evidence https://github.com/copyleftdev/agent-calc . AI tools assisted with editorial research and revision. I verified the technical claims and stand behind the final text.