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World desk6 min

I Built a Magic Rules Agent, Then Tested Whether It Was Guessing

A blind ten-question test favored structured evidence gathering for a Magic rules agent, but a missing definition and flawed metric show why the result is only an early evaluation.

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Structured, multi-step evidence gathering performed better than a single keyword search in my small blind test of JudgeStack, a Magic: The Gathering rules agent—but the result is a promising project evaluation, not proof that the system can answer Magic questions reliably in general. The test also exposed a more basic lesson: retrieving a card’s status is not enough if the system does not know what that status means.

What JudgeStack is meant to prove

JudgeStack is a rules-question agent built to retrieve the source that supports each part of an answer. Its aim is not just to produce plausible rulings, but to identify whether a claim comes from card text, a rules definition, a format status, or a dated announcement.

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That distinction matters because superficially similar Magic questions can require different authorities. “Is this card legal in Vintage now?” is a current-status question. “When did the restriction take effect?” asks for a historical effective date. Current legality data may establish the first; it does not, by itself, establish the second. A dated announcement is needed for the effective-date claim. Combining those questions can tempt a system to invent a date while sounding certain.

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Match the evidence to the question

  • What does this card say now? Use its current Oracle wording.
  • How does an interaction work? Use the relevant Oracle text together with the current Comprehensive Rules.
  • What were the rules or wording at an earlier point? Use rules and card wording appropriate to that date.
  • Is the card legal in a format now? Use current format-legality data.
  • When did a ban or restriction begin? Find a dated announcement that establishes the effective date.
  • Why does a physical card differ from the current wording? Compare the printed text with current Oracle text.

Wizards of the Coast describes the Comprehensive Rules as a reference for rules and corner cases, intended for consultation on specific questions rather than cover-to-cover reading (Wizards of the Coast’s rules page). The Magic Judges rules resource lists the Comprehensive Rules version effective September 25, 2026 (Magic Judges rules resource). Rules documents are versioned, so the applicable version depends on the question’s date.

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How the evidence was organized

In a 2026 account, I reported that JudgeStack’s corpus contained 496 documents in ten types: card, printing, ruleParagraph, glossaryTerm, formatEvent, claim, decision, textDifference, adjudicationCase, and authoritySource. Reported component counts included 30 cards, 77 printings, 16 rule paragraphs, 208 legality claims, and 74 detected differences between printed wording and current Oracle text. These figures describe this project’s corpus, not a general Magic dataset.

The system used two Sanity Context MCP endpoints. One exposed structured documents through filtered GROQ queries; the other exposed the Comprehensive Rules as a knowledge-base file. They had to remain separate: an endpoint configured with a dataset source ignores its knowledge-base sources, so combining the sources would have removed access to the rules file.

The public structured dataset includes the 16 rule paragraphs cited by reviewed cases. The complete rules file is used as a retrieval source rather than republished as hundreds of individual dataset documents. This kept the duplicated public rules text narrower; it does not resolve the licensing question.

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What the comparison tested

The evaluation used 30 questions across three areas: printed wording versus current Oracle text, current format legality, and historical rules changes. Ten questions were held out and never used during development. Both answering conditions used the same answer model and prompt: DeepSeek Flash. Temperature and output-token limits were not set, so provider defaults applied.

Condition Evidence gathering Model-step budget
One-shot keyword retrieval One BM25 search over a flattened corpus; the top 12 chunks were supplied in one retrieval pass. One retrieval pass.
Structured evidence gathering Could query the Sanity dataset with GROQ, read the rules knowledge base, and follow references. Up to ten model steps.

This is a comparison of evidence-gathering architectures, not a controlled BM25-versus-GROQ test. The structured condition could take more model turns and choose its next query based on what it had found; the one-shot condition could not. The results therefore reflect both retrieval design and different interaction budgets.

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What the blind holdout showed

For the blind evaluation, the 20 answers—ten from each condition—were shuffled and stripped of condition labels. The judge received the questions, expected verdicts, and rubric, but not the labels or the number of answers from each condition. In my 2026 report, the results were:

Measure on ten held-out questions per condition One-shot keyword retrieval Structured evidence gathering
Verdict correct 1/10 9/10
Reasoning rested on something unretrieved 7/10 1/10

On this small holdout, structured evidence gathering produced more correct verdicts and less reasoning based on unretrieved material. Ten questions are too few to support a broad claim about Magic rules questions, and the setup does not isolate the effect of structured queries from the benefit of extra model steps.

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The full 30-question suite also produced useful diagnostics, but it includes questions used during development and is not an unbiased test set:

Full-suite diagnostic One-shot keyword retrieval Structured evidence gathering
Required rules cited 19/30 30/30
Required cards retrieved 22/30 30/30
Cited rules actually retrieved 24/30 30/30
Unsupported citations 6 0

Those counts suggest that retrieval coverage and citation support were weaknesses in the one-shot setup, but they should not be read as held-out performance estimates.

The failure that retrieval alone could not fix

The most instructive retained failure involved Sol Ring. JudgeStack retrieved format claims, including that Sol Ring was restricted in Vintage, yet concluded that the card could only be registered in Commander. The answer was wrong: the corpus did not explain what “restricted” means, and the model effectively treated the term as if it meant banned.

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This was a schema gap, not simply a missed card record. The system had the status but lacked the concept needed to interpret it. The proposed fix is a legality-term concept defining legal, banned, and restricted and explaining how restrictions apply. More retrieval cannot reliably supply a definition the corpus does not contain.

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Why one favorable metric was withdrawn

I withdrew an automated “date discipline” score of 10/10 for the structured condition. Its check only asked whether the system had retrieved any format event; it did not test whether that event concerned the card in the question. The two stored events were about an unrelated card, so the check could pass even when an answer invented an effective date.

The metric also failed to distinguish “banned as of” a date from “the ban became effective on” that date. Because the saved evaluation rows did not retain the retrieved IDs, I could not recompute the score against the relevant events. The original 10/10 score remains withdrawn.

This is a useful warning about evaluation design: a metric can reward an answer for retrieving something that looks relevant by type while missing the actual evidence relation the claim requires.

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Integration bugs that changed what the system could do

Several implementation problems initially obscured JudgeStack’s behavior. These are issues I reported while building the project, not independently reproduced tests:

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  • Incompatible AI SDK dependency versions caused tool-call validation failures.
  • Two endpoints exposed identical tool names. Merging their tool sets caused a name collision and dropped the dataset schema overview.
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These failures show why an apparently weak retrieval result may originate in tool wiring or response parsing rather than in the retrieval strategy or answer model. Debugging needs to check the whole path: tool registration, arguments, returned content, identifier parsing, and the evidence ultimately available to the answer step.

What happened with a local model

In three reported runs, a local Qwen3 configuration made no successful calls to the dataset endpoint and produced invalid arguments for parameterless tools. A separate workflow using a JSON retrieval plan and executing it externally could use the corpus. Three runs do not establish how local models perform generally; they show that this particular tool-calling setup failed and that separating planning from tool execution provided another route.

How reproducible is the result?

The answering model and judging model came from different vendors, but the exact judge build is not pinned because grading happened in the ChatGPT interface. The evaluation pack, rubric, and raw judgments are public, so readers can repeat the grading with another judge. That makes the reported comparison inspectable, but not identical to a fully version-pinned evaluation.

My conclusion is deliberately limited: JudgeStack’s structured condition did substantially better on this ten-question blind holdout, under a setup that gave it more opportunities to gather evidence. The Sol Ring case and withdrawn date score are as important as the accuracy numbers. One shows that an evidence store needs concepts as well as records; the other shows that a metric must verify relevance, not merely the presence of a plausible source type. JudgeStack is a rules laboratory, not a replacement for a judge.

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