Add -g for specifying a GBNF grammar

[oleksandrvoyager]

#1

[isaac-asimov-bot]

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[artob]

@race-of-sloths score 5

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Current status: executed

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@artob	5

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[Copilot]

Pull Request Overview

This PR adds support for specifying a GBNF grammar file to constrain model decoding via the -g flag. The implementation removes the bon builder pattern dependency in favor of a simpler struct initialization approach.

Key changes:

• Added -g/--grammar CLI option to specify GBNF grammar files
• Added grammar field to Options struct with file reading logic
• Removed bon dependency and its builder pattern
• Added documentation showing GBNF usage example

Reviewed Changes

Copilot reviewed 4 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/prompter/main.rs	Added CLI options for grammar, input, and output files; implemented grammar file reading
src/lib.rs	Removed bon builder, added grammar field to Options struct, modified request to include grammar parameter
Cargo.toml	Removed bon dependency
Cargo.lock	Removed bon and related transitive dependencies
README.md	Added GBNF grammar usage example with documentation

---

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[Copilot]

[nitpick] The json! macro creates an immutable Value, but you're attempting to mutate it. The let mut req declaration is necessary, but the pattern match on line 32 is checking the type unnecessarily since json!({...}) always produces a Value::Object.

Consider simplifying to:

let mut req = json!({
    "model": options.model,
    "messages": [{ "role": "user", "content": input.as_ref() }],
    "stream": false
});

if let Some(g) = &options.grammar {
    req["grammar"] = Value::String(g.clone());
}

This is cleaner and avoids the unnecessary type check.

Suggested change

	if let Value::Object(obj) = &mut req {
	obj.insert("grammar".into(), Value::String(g.clone()));
	}
	req["grammar"] = Value::String(g.clone());