feat: add support for GBNF grammar - #20
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fix: update comment on GBNF grammar
fix: update comments in model.h for GBNF grammar
test: update test suite for GBNF grammar support
refactor: clean up grammar test formatting
feat: update GBNF grammar support with example and reset fix
Update GBNF grammar support to ModelConfig via a `grammar` string and
`grammar_root` rule name, applied to the sampler chain in
initialize_context(). load_grammar_file() reads a .gbnf file into
ModelConfig::grammar.
The grammar sampler retains its parse position across calls to
generate_from_tokens(), so a completed grammar from one turn forces
EOS immediately on the next. Reset only the grammar sampler (not the
whole chain) at the start of each turn, via a non-owning pointer
kept on Model, so the dist sampler's RNG and other stateful samplers
are left untouched when an explicit seed is configured.
Add examples/grammar, a minimal sentiment classifier constrained to
{"sentiment": "positive"|"negative"|"neutral"} via sentiment.gbnf,
demonstrating the feature end-to-end. Document grammar-constrained
output in the README and register the example in CMakeLists.txt.
fix: allow custom grammar roots in grammar example
Add a -r flag to the grammar demo, update the README usage examples,
and polish the grammar documentation wording.
fix: comment formatting in model.cpp
fix: update formatting for comments in model.h
An empty grammar file (or a path that opens but reads nothing) made load_grammar_file return an empty string, which initialize_context then silently treated as "no grammar" - unconstrained output with no error. Throw instead. Also document that a grammar constrains every token, so it blocks tool calls unless the grammar allows the tool-call format. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Thanks @oglego 🚀 ! |
daavoo
approved these changes
Aug 25, 2026
Contributor
Author
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Awesome! Thanks again for the review and also for the fix! |
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Summary
This PR partially addresses #18 - adding support for GBNF grammar. I believe that I have everything for this wired in correctly, I also tried to follow the current coding style as best as I could, but if anything needs to be changed please let me know.
Changes
The main items that I have added on this PR are:
ModelConfig gains grammar (GBNF string) and grammar_root (rule name, defaults to "root").
load_grammar_file() reads a .gbnf file into ModelConfig::grammar.
initialize_context() applies the grammar to the sampler chain when set.
examples/grammar/ — minimal sentiment classifier demonstrating grammar-constrained output.
tests/test_grammar.cpp — config defaults, file loading, missing-file error handling.
For testing this locally and interacting with it I just used the recommended model (granite-4.0-micro-Q8_0.gguf).
Thanks in advance for the review!