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Trie-based grammar constraints for Gemma force_tools #747

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Merged
jakmro merged 1 commit into from
Jul 1, 2026
Merged

Trie-based grammar constraints for Gemma force_tools #747

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41 changes: 28 additions & 13 deletions cactus-engine/src/complete.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,8 @@ namespace {

std::vector<std::pair<std::string, std::string>> extract_schema_property_types(const std::string& schema);
std::vector<std::string> extract_schema_required(const std::string& schema);
std::vector<std::string> extract_string_array(const std::string& json, const std::string& key);
std::string extract_json_object_field(const std::string& json, const std::string& key);

std::string extract_last_user_query(const std::vector<ChatMessage>& messages) {
for (auto it = messages.rbegin(); it != messages.rend(); ++it) {
Expand Down Expand Up @@ -65,9 +67,13 @@ std::vector<ToolConstraintSpec> build_tool_constraint_specs(const std::vector<To
auto schema_it = tool.parameters.find("schema");
if (schema_it != tool.parameters.end()) {
auto properties = extract_schema_property_types(schema_it->second);
std::string properties_object = extract_json_object_field(schema_it->second, "properties");
spec.parameter_names.reserve(properties.size());
spec.parameter_enums.reserve(properties.size());
for (const auto& [name, _] : properties) {
spec.parameter_names.push_back(name);
spec.parameter_enums.push_back(
extract_string_array(extract_json_object_field(properties_object, name), "enum"));
}
spec.required_parameter_names = extract_schema_required(schema_it->second);
}
Expand Down Expand Up @@ -194,25 +200,30 @@ std::vector<std::pair<std::string, std::string>> extract_schema_property_types(c
return properties;
}

std::vector<std::string> extract_schema_required(const std::string& schema) {
std::vector<std::string> required;
std::string key = "\"required\"";
size_t key_pos = schema.find(key);
if (key_pos == std::string::npos) return required;
size_t arr_start = schema.find('[', key_pos + key.size());
if (arr_start == std::string::npos) return required;
size_t arr_end = schema.find(']', arr_start);
if (arr_end == std::string::npos) return required;
std::vector<std::string> extract_string_array(const std::string& json, const std::string& key) {
std::vector<std::string> values;
if (json.empty()) return values;
std::string pattern = "\"" + key + "\"";
size_t key_pos = json.find(pattern);
if (key_pos == std::string::npos) return values;
size_t arr_start = json.find('[', key_pos + pattern.size());
if (arr_start == std::string::npos) return values;
size_t arr_end = json.find(']', arr_start);
if (arr_end == std::string::npos) return values;
size_t pos = arr_start + 1;
while (pos < arr_end) {
size_t qs = schema.find('"', pos);
size_t qs = json.find('"', pos);
if (qs == std::string::npos || qs >= arr_end) break;
size_t qe = schema.find('"', qs + 1);
size_t qe = json.find('"', qs + 1);
if (qe == std::string::npos || qe > arr_end) break;
required.push_back(schema.substr(qs + 1, qe - qs - 1));
values.push_back(json.substr(qs + 1, qe - qs - 1));
pos = qe + 1;
}
return required;
return values;
}

std::vector<std::string> extract_schema_required(const std::string& schema) {
return extract_string_array(schema, "required");
}

static bool json_object_has_key(const std::string& obj, const std::string& key) {
Expand Down Expand Up @@ -1141,6 +1152,10 @@ int cactus_complete(
: "kept local";
}

if (prompt.options.force_tools && !prompt.tools.empty() && primary_function_calls.empty()) {
CACTUS_LOG_WARN("force_tools", "force_tools was set but no tool call was parsed from the constrained output");
}

std::string result = construct_response_json(primary_response, primary_function_calls, time_to_first_token,
total_time_ms, prefill_tps, decode_tps, prompt_tokens,
completion_tokens, confidence, handoff_succeeded,
Expand Down
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