Step 40: JSON Schema — schema-compiled constrained decoding #78

Summary

Implement JsonSchemaConstraint — schema-compiled constrained decoding that guarantees output conforms to a specific JSON Schema. Corresponds to response_format: {"type": "json_schema", "json_schema": {...}} in the OpenAI API.

Builds on Step 39 (JSON mode, #76) by layering a SchemaTracker on top of the existing JsonCharParser to enforce schema semantics during generation.

Acceptance Criteria

• JsonSchemaConstraint : IDecodingConstraint enforces JSON Schema constraints via per-token vocabulary masking
• Schema compiled into immutable CompiledSchema graph at request time
• Property name enforcement via character trie (only valid property names allowed)
• Type enforcement: restrict value types to schema-specified types
• Required property enforcement: } blocked until all required properties emitted
• additionalProperties: false support
• Array items schema applied to each element
• String enum and const value constraints
• $ref / $defs resolution (non-recursive)
• anyOf union type support
• Integer type enforcement (no ./e/E in number generation)
• ResponseFormat.JsonSchema variant added
• TextGenerator dispatches to JsonSchemaConstraint for JsonSchema response format
• CLI --response-format json_schema --schema <json|@file> support
• Zero-alloc Clone() via struct-copy (SchemaTracker + JsonCharParser are value types)
• Token mask caching with bounded cache (default 4096 entries)
• Comprehensive unit tests for compiler, trie, tracker, and constraint

Supported Schema Features (MVP)

type, properties, required, additionalProperties, items, enum, const, $ref/$defs, anyOf, nested objects/arrays

Depends On

• Step 39: JSON mode (Step 39: JSON mode — FSM-based constrained decoding for valid JSON output #76) ✅

Key Files

• src/DotLLM.Engine/Constraints/Schema/ (new directory)
• src/DotLLM.Engine/Constraints/JsonSchemaConstraint.cs
• src/DotLLM.Core/Configuration/ResponseFormat.cs
• src/DotLLM.Engine/TextGenerator.cs

References

• docs/CONSTRAINED_DECODING.md
• docs/ROADMAP.md — Step 40