cel_brief/sources/
system_prompt.rs

1//! [`SystemPromptSource`] — static system-prompt text at [`Priority::Critical`].
2//!
3//! The simplest possible [`Source`]: holds an owned [`String`] and emits it as
4//! a [`Contribution::system`] every turn. Critical-priority, never redactable.
5//!
6//! For dynamic system prompts (per-user personalisation, A/B-tested copy)
7//! wrap [`Source`] yourself and template the text from [`BriefContext`] inside
8//! `contribute`.
9
10use async_trait::async_trait;
11
12use crate::source::{Contribution, Source, SourceError};
13use crate::types::{BriefContext, Priority, SourceId};
14
15/// A [`Source`] that emits a fixed system-prompt string every turn.
16///
17/// Critical-priority, marked non-redactable so governance can't quietly
18/// rewrite the system prompt. The tokenizer hint defaults to
19/// `text.len() / 4`; override with [`Self::with_estimated_tokens`] when you
20/// have a tighter number from your tokenizer.
21#[derive(Debug, Clone)]
22pub struct SystemPromptSource {
23    id: SourceId,
24    text: String,
25    estimated_tokens: usize,
26}
27
28impl SystemPromptSource {
29    /// Construct a source with the default ID `"system_prompt"`.
30    pub fn new(text: impl Into<String>) -> Self {
31        let text = text.into();
32        let estimated_tokens = text.len().div_ceil(4);
33        SystemPromptSource {
34            id: SourceId::new("system_prompt"),
35            text,
36            estimated_tokens,
37        }
38    }
39
40    /// Override the default [`SourceId`] (useful when wiring multiple system
41    /// prompts — e.g. a base prompt plus a user-specific personality layer —
42    /// and you want them attributed separately in the receipt).
43    pub fn with_id(mut self, id: impl Into<SourceId>) -> Self {
44        self.id = id.into();
45        self
46    }
47
48    /// Override the source-side token estimate. The builder always
49    /// re-tokenizes ground-truth, so this is a hint only.
50    pub fn with_estimated_tokens(mut self, tokens: usize) -> Self {
51        self.estimated_tokens = tokens;
52        self
53    }
54
55    /// The raw system-prompt text.
56    pub fn text(&self) -> &str {
57        &self.text
58    }
59}
60
61#[async_trait]
62impl Source for SystemPromptSource {
63    fn id(&self) -> SourceId {
64        self.id.clone()
65    }
66
67    fn priority(&self) -> Priority {
68        Priority::Critical
69    }
70
71    async fn contribute(&self, _ctx: &BriefContext) -> Result<Vec<Contribution>, SourceError> {
72        Ok(vec![Contribution::system(
73            self.text.clone(),
74            self.estimated_tokens,
75        )])
76    }
77}
78
79#[cfg(test)]
80mod tests {
81    use super::*;
82    use crate::source::ContributionContent;
83    use crate::types::TokenBudget;
84
85    #[tokio::test]
86    async fn emits_one_critical_system_contribution() {
87        let src = SystemPromptSource::new("You are a helpful assistant.");
88        assert_eq!(src.priority(), Priority::Critical);
89        assert_eq!(src.id(), SourceId::new("system_prompt"));
90
91        let ctx = BriefContext::new(TokenBudget::default());
92        let contributions = src.contribute(&ctx).await.expect("contribute ok");
93        assert_eq!(contributions.len(), 1);
94        let c = &contributions[0];
95        assert!(!c.redactable);
96        assert_eq!(c.importance, 1.0);
97        match &c.content {
98            ContributionContent::System { text } => {
99                assert_eq!(text, "You are a helpful assistant.");
100            }
101            other => panic!("expected System, got {other:?}"),
102        }
103    }
104
105    #[tokio::test]
106    async fn with_id_overrides_default() {
107        let src = SystemPromptSource::new("hi").with_id("personality");
108        assert_eq!(src.id(), SourceId::new("personality"));
109    }
110
111    #[tokio::test]
112    async fn with_estimated_tokens_overrides_default() {
113        let src = SystemPromptSource::new("hi").with_estimated_tokens(99);
114        let ctx = BriefContext::new(TokenBudget::default());
115        let c = src.contribute(&ctx).await.expect("contribute ok");
116        assert_eq!(c[0].estimated_tokens, 99);
117    }
118
119    #[test]
120    fn default_estimate_is_four_chars_per_token() {
121        // 12 chars → ceil(12/4) = 3 tokens.
122        let src = SystemPromptSource::new("hello, world");
123        assert_eq!(src.estimated_tokens, 3);
124    }
125}
cel_brief/sources/system_prompt.rs

cel_brief/sources/
system_prompt.rs
