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Use structured output for prompt hooks via generate_object()

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Replace freeform LLM text generation with schema-constrained
generate_object() for prompt hooks, eliminating the need for
JSON formatting instructions in the system prompt and the
code-fence stripping workaround.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
This commit is contained in:
Bryan Helmkamp 2026-03-05 09:55:16 -05:00
parent e5354c3028
commit 3a7e7deb5a
1 changed files with 31 additions and 4 deletions
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35
crates/arc-workflows/src/hook/executor.rs
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@ -1,7 +1,7 @@
use std::borrow::Cow;
use std::collections::HashMap;
use std::path::Path;
use std::sync::Arc;
use std::sync::{Arc, LazyLock};
use std::time::Instant;
use async_trait::async_trait;
@ -11,7 +11,19 @@ use arc_agent::Sandbox;
use super::config::{HookDefinition, HookType, TlsMode};
use super::types::{HookContext, HookDecision, HookResult, PromptHookResponse};
const HOOK_EVALUATOR_SYSTEM_PROMPT: &str = "You are a hook evaluator for a workflow engine. Given context about a workflow event, evaluate the condition and respond with JSON: {\"ok\": true} or {\"ok\": false, \"reason\": \"...\"}. Respond ONLY with valid JSON.";
const HOOK_EVALUATOR_SYSTEM_PROMPT: &str = "You are a hook evaluator for a workflow engine. Given context about a workflow event, evaluate the condition.";
static HOOK_RESPONSE_SCHEMA: LazyLock<serde_json::Value> = LazyLock::new(|| {
serde_json::json!({
"type": "object",
"properties": {
"ok": { "type": "boolean" },
"reason": { "type": "string" }
},
"required": ["ok"],
"additionalProperties": false
})
});
/// Trait for executing hooks via different transports.
#[async_trait]
@ -252,8 +264,23 @@ impl HookExecutorImpl {
.prompt(user_msg)
.max_tokens(1024);
match arc_llm::generate::generate(params).await {
Ok(result) => Self::parse_prompt_response(&result.response.text()),
match arc_llm::generate::generate_object(params, HOOK_RESPONSE_SCHEMA.clone()).await {
Ok(result) => match result.output {
Some(obj) => match serde_json::from_value::<PromptHookResponse>(obj) {
Ok(resp) if resp.ok => HookDecision::Proceed,
Ok(resp) => HookDecision::Block {
reason: resp.reason,
},
Err(e) => {
tracing::warn!(error = %e, "prompt hook response deserialize failed, proceeding");
HookDecision::Proceed
}
},
None => {
tracing::warn!("prompt hook returned no structured output, proceeding");
HookDecision::Proceed
}
},
Err(e) => {
tracing::warn!(error = %e, "prompt hook LLM call failed, proceeding");
HookDecision::Proceed