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llama.cpp
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fix: fail closed when grammar parsing fails (#19051) 

When response_format: json_schema is requested but grammar parsing
fails (e.g., invalid regex pattern), the server now returns HTTP 400
error instead of silently generating unconstrained output.

Previously, common_sampler_init() would return a valid sampler even
when grammar parsing failed (grmr=nullptr). This caused the server
to continue with unconstrained generation, violating the structured
output contract.

Now, if grammar was requested but parsing failed, common_sampler_init()
returns nullptr, triggering the existing error handling path.

Fixes #19051
This commit is contained in:
ingyukoh 2026-02-05 12:02:25 +09:00
parent 25f40ca65f
commit 76363cd0af
1 changed files with 11 additions and 0 deletions
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11
common/sampling.cpp
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@ -191,6 +191,11 @@ struct common_sampler * common_sampler_init(const struct llama_model * model, st
if (params.grammar.compare(0, 11, "%llguidance") == 0) {
#ifdef LLAMA_USE_LLGUIDANCE
grmr = llama_sampler_init_llg(vocab, "lark", params.grammar.c_str());
// fail closed: if llguidance grammar parsing failed, return nullptr
if (grmr == nullptr) {
llama_sampler_free(chain);
return nullptr;
}
#else
GGML_ABORT("llguidance (cmake -DLLAMA_LLGUIDANCE=ON) is not enabled");
#endif // LLAMA_USE_LLGUIDANCE
@ -247,6 +252,12 @@ struct common_sampler * common_sampler_init(const struct llama_model * model, st
} else {
grmr = llama_sampler_init_grammar(vocab, params.grammar.c_str(), "root");
}
// fail closed: if grammar was requested but parsing failed, return nullptr
// this ensures the server returns an error instead of generating unconstrained output
if (grmr == nullptr) {
llama_sampler_free(chain);
return nullptr;
}
}
}