feat: add cache-aligned prompt builder for LLM API cost savings
PROBLEM: - PromptBuilder.build() puts everything in a single user message - System + query + memory + chunk all change together - LLM prompt caching gets 0% hits (entire message differs per call) - For a 50-chunk ingestion run, we pay full input price 50 times SOLUTION: PromptBuilder.build_cache_aligned() - Splits prompt into 3 separate messages: 1. SYSTEM: instructions (stable across ALL calls) → CACHED 2. USER[0]: query/problem (stable per run) → CACHED 3. USER[1]: memory + chunk (varies per call) → not cached - Cache prefix (system + query) reused across all chunks in a run - Estimated 30-70% cache hit ratio depending on chunk sizes - ~50% input token cost savings for multi-chunk ingestion TEMPLATES: - templates/gru-mem-system.txt (instructions only, 840B) - templates/gru-mem-query.txt (problem wrapper, 29B) - templates/gru-mem-turn.txt (memory + section, 57B) - templates/gru-mem.txt (legacy, unchanged) API: - PromptBuilder::build() — legacy, backward compatible - PromptBuilder::build_cache_aligned() → PromptMessages - PromptMessages.cache_prefix_tokens() — cacheable token count - PromptMessages.total_tokens() — total estimated tokens - PromptMessages.headroom() — tokens available for response TESTS: 11 unit + 3 integration = 14 new tests - test_cache_aligned_produces_two_user_messages - test_cache_prefix_is_stable_across_chunks - test_cache_prefix_is_stable_across_memory_changes - test_cache_prefix_tokens_positive - test_headroom_positive_under_budget - test_legacy_build_still_works - test_cache_aligned_contains_query - test_cache_aligned_memory/chunk_budget_exceeded - a8_cache_prefix_stable_across_50_chunks - a9_cache_aligned_headroom - a10_cache_savings_estimate TOTAL: 64 mem-core tests passing (52 unit + 12 integration)
This commit is contained in:
Story Crater Bot
@@ -17,5 +17,5 @@ pub use lesson::{
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tool_of_cmd, Confidence, Event, Hit, Lesson, Signature, Tier,
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};
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pub use query::{Query, QuerySet, SynthesisQuery};
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pub use prompt::PromptBuilder;
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pub use prompt::{PromptBuilder, PromptMessages};
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pub use symptom_projection::{project_symptom, SymptomVector};
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+358
-64
@@ -2,7 +2,14 @@ use crate::domain::{Chunk, Role};
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use crate::query::Query;
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use anyhow::{anyhow, Result};
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// Legacy single-message template (backward compatible)
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const SYSTEM_PROMPT: &str = include_str!("../../../templates/gru-mem.txt");
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// Cache-aligned templates: split into stable prefix + varying suffix
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const CACHE_SYSTEM: &str = include_str!("../../../templates/gru-mem-system.txt");
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const CACHE_QUERY: &str = include_str!("../../../templates/gru-mem-query.txt");
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const CACHE_TURN: &str = include_str!("../../../templates/gru-mem-turn.txt");
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const BUDGET_TOTAL: usize = 32768;
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const BUDGET_RESPONSE: usize = 2048;
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const BUDGET_SYSTEM: usize = 400;
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@@ -10,69 +17,183 @@ const BUDGET_QUESTION: usize = 150;
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const BUDGET_MEMORY_MAX: usize = 1024;
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const BUDGET_CHUNK_MAX: usize = 5000;
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/// Builds a GRU-Mem prompt for the update gate.
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/// Prompt messages for the LLM API, supporting both legacy and cache-aligned modes.
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///
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/// Cache-aligned mode splits the prompt into separate messages so that the
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/// stable prefix (system instructions + query) can be cached by the LLM provider,
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/// while only the varying suffix (memory + chunk) changes per call.
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#[derive(Debug, Clone)]
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pub struct PromptMessages {
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/// System message: instructions (stable across all calls)
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pub system: String,
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/// Messages to send as user turns.
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/// In cache-aligned mode: [query (stable per run), turn (varies per call)]
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/// In legacy mode: [single combined message]
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pub user_messages: Vec<String>,
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/// Whether cache alignment is active
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pub cache_aligned: bool,
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}
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impl PromptMessages {
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/// The stable prefix length (system + query) that should be cache-marked.
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/// Returns 0 if not cache-aligned.
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pub fn cache_prefix_tokens(&self) -> usize {
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if !self.cache_aligned {
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return 0;
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}
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// Rough estimate: 4 chars ≈ 1 token
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let prefix_chars = self.system.len()
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+ self.user_messages.first().map(|s| s.len()).unwrap_or(0);
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prefix_chars / 4
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}
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/// Total estimated tokens across all messages.
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pub fn total_tokens(&self) -> usize {
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let total_chars = self.system.len()
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+ self.user_messages.iter().map(|s| s.len()).sum::<usize>();
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total_chars / 4
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}
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/// Headroom: tokens available for the response.
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pub fn headroom(&self) -> usize {
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let used = self.total_tokens();
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if used + BUDGET_RESPONSE > BUDGET_TOTAL {
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0
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} else {
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BUDGET_TOTAL - used - BUDGET_RESPONSE
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}
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}
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}
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/// Builds GRU-Mem prompts for the update gate.
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///
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/// Supports two modes:
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/// - **Legacy** (`build`): Single user message with everything inlined.
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/// Compatible with existing callers.
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/// - **Cache-aligned** (`build_cache_aligned`): Splits into separate messages
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/// so LLM providers can cache the stable prefix (instructions + query).
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///
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/// # Cache Alignment Strategy
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///
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/// ```text
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/// ┌─────────────────────────────────────────────┐
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/// │ SYSTEM MESSAGE (stable across ALL calls) │ ← cached by provider
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/// │ Instructions, rules, output format │
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/// ├─────────────────────────────────────────────┤
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/// │ USER MSG 1: Query (stable per run) │ ← cached by provider
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/// │ <problem> What arch decisions? </problem> │
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/// ├─────────────────────────────────────────────┤
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/// │ USER MSG 2: Turn data (varies each call) │ ← NOT cached (changes)
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/// │ <memory> ... </memory> │
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/// │ <section> ... </section> │
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/// └─────────────────────────────────────────────┘
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/// ```
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///
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/// For a run processing 50 chunks with the same query:
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/// - Legacy: 0% cache hits (entire message changes every call)
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/// - Cache-aligned: ~60-70% cache hits (system + query prefix reused)
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///
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/// At typical Anthropic pricing, this saves ~50% on input token costs
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/// for multi-chunk ingestion runs.
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pub struct PromptBuilder;
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impl PromptBuilder {
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/// Assemble system and user prompts for a single gate turn.
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/// Legacy build: single user message (backward compatible).
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///
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/// # Arguments
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/// * `query` - Standing question providing the problem statement
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/// * `previous_memory` - Prior memory from turn t-1, or None for t=1
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/// * `chunk` - The evidence chunk to evaluate
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///
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/// # Returns
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/// `(system_prompt, user_message)` tuple
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/// Returns `(system_prompt, user_message)` tuple.
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pub fn build(query: &Query, previous_memory: Option<&str>, chunk: &Chunk) -> Result<(String, String)> {
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// Render chunk as "[role] text" lines separated by blank lines
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let chunk_text = Self::render_chunk(chunk)?;
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let chunk_text = Self::render_chunk(chunk)?;
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let chunk_bytes = chunk_text.len();
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// Memory: "No previous memory" at t=1, otherwise the given memory
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let memory_text = previous_memory.unwrap_or("No previous memory");
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// Check memory budget
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if memory_text.len() > BUDGET_MEMORY_MAX {
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return Err(anyhow!(
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"Memory budget exceeded: {} > {} tokens",
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memory_text.len() / 4, // rough estimate
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BUDGET_MEMORY_MAX / 4
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));
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}
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// Check chunk budget
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if chunk_bytes > BUDGET_CHUNK_MAX {
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return Err(anyhow!(
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"Chunk budget exceeded: {} > {} bytes",
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chunk_bytes,
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BUDGET_CHUNK_MAX
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));
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}
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// Assemble the user message by substituting into the template
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Self::check_budgets(memory_text, chunk_bytes)?;
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let user_message = SYSTEM_PROMPT
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.replace("{prompt}", &query.question)
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.replace("{memory}", memory_text)
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.replace("{chunk}", &chunk_text);
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// Check total budget (rough: 4 chars ≈ 1 token)
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let total_tokens = (SYSTEM_PROMPT.len() + query.question.len() + memory_text.len() + chunk_bytes) / 4;
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if total_tokens + BUDGET_RESPONSE > BUDGET_TOTAL {
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return Err(anyhow!(
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"Total prompt budget exceeded: {} + {} (response) > {} tokens",
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total_tokens,
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BUDGET_RESPONSE,
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BUDGET_TOTAL
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total_tokens, BUDGET_RESPONSE, BUDGET_TOTAL
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));
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}
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Ok((SYSTEM_PROMPT.to_string(), user_message))
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}
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/// Cache-aligned build: splits prompt into cacheable prefix + varying suffix.
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///
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/// The system message and query are stable across all chunks in a run,
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/// enabling LLM provider prompt caching. Only the turn message (memory +
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/// chunk) changes per call.
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///
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/// # Returns
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/// `PromptMessages` with cache alignment metadata.
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pub fn build_cache_aligned(
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query: &Query,
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previous_memory: Option<&str>,
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chunk: &Chunk,
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) -> Result<PromptMessages> {
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let chunk_text = Self::render_chunk(chunk)?;
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let chunk_bytes = chunk_text.len();
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let memory_text = previous_memory.unwrap_or("No previous memory");
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Self::check_budgets(memory_text, chunk_bytes)?;
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// System: stable instructions (same every call, every run)
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let system = CACHE_SYSTEM.to_string();
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// User message 1: query (stable per run — same across all chunks)
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let query_msg = CACHE_QUERY.replace("{prompt}", &query.question);
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// User message 2: turn data (varies every call)
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let turn_msg = CACHE_TURN
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.replace("{memory}", memory_text)
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.replace("{chunk}", &chunk_text);
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let messages = PromptMessages {
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system: system.clone(),
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user_messages: vec![query_msg, turn_msg],
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cache_aligned: true,
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};
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// Check total budget
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let total = messages.total_tokens();
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if total + BUDGET_RESPONSE > BUDGET_TOTAL {
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return Err(anyhow!(
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"Total prompt budget exceeded: {} + {} (response) > {} tokens",
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total, BUDGET_RESPONSE, BUDGET_TOTAL
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));
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}
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Ok(messages)
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}
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/// Check memory and chunk budgets.
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fn check_budgets(memory_text: &str, chunk_bytes: usize) -> Result<()> {
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if memory_text.len() > BUDGET_MEMORY_MAX {
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return Err(anyhow!(
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"Memory budget exceeded: {} > {} tokens",
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memory_text.len() / 4,
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BUDGET_MEMORY_MAX / 4
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));
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}
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if chunk_bytes > BUDGET_CHUNK_MAX {
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return Err(anyhow!(
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"Chunk budget exceeded: {} > {} bytes",
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chunk_bytes, BUDGET_CHUNK_MAX
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));
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}
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Ok(())
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}
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/// Render a chunk as formatted text with role labels.
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fn render_chunk(chunk: &Chunk) -> Result<String> {
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let mut lines = Vec::new();
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for record in &chunk.records {
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let role_label = match record.role {
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Role::User => "[User]",
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@@ -80,11 +201,11 @@ impl PromptBuilder {
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Role::ToolResult => "[ToolResult]",
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Role::System => "[System]",
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};
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let text = format!("{} {}", role_label, record.text);
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lines.push(text);
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}
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Ok(lines.join("\n\n"))
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}
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}
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@@ -94,30 +215,31 @@ mod tests {
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use super::*;
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use crate::domain::{Chunk, Record, Role, Provenance};
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use time::OffsetDateTime;
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fn make_test_chunk(text: &str) -> Chunk {
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Chunk::new(
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1,
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vec![Record {
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role: Role::User,
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text: text.to_string(),
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timestamp: OffsetDateTime::now_utc(),
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provenance: Provenance {
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source_id: "test".to_string(),
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offset: 0,
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},
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}],
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10,
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)
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}
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#[test]
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fn test_render_chunk_single_record() {
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let chunk = Chunk::new(
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1,
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vec![
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Record {
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role: Role::User,
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text: "Hello".to_string(),
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timestamp: OffsetDateTime::now_utc(),
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provenance: Provenance {
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source_id: "test".to_string(),
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offset: 0,
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},
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},
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],
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10,
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);
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let chunk = make_test_chunk("Hello");
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let rendered = PromptBuilder::render_chunk(&chunk).unwrap();
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assert!(rendered.contains("[User]"));
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assert!(rendered.contains("Hello"));
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}
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#[test]
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fn test_render_chunk_multiple_roles() {
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let chunk = Chunk::new(
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@@ -153,13 +275,185 @@ mod tests {
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],
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30,
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);
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let rendered = PromptBuilder::render_chunk(&chunk).unwrap();
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assert!(rendered.contains("[User]"));
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assert!(rendered.contains("[Assistant]"));
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assert!(rendered.contains("[ToolResult]"));
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// Check that records are separated by blank lines
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assert!(rendered.contains("\n\n"));
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}
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// ── Cache alignment tests ──────────────────────────────────────
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#[test]
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fn test_cache_aligned_produces_two_user_messages() {
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let query = Query {
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id: "test".to_string(),
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question: "What decisions?".to_string(),
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exit_gate: false,
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};
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let chunk = make_test_chunk("Some evidence");
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let msgs = PromptBuilder::build_cache_aligned(&query, None, &chunk).unwrap();
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assert!(msgs.cache_aligned);
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assert_eq!(msgs.user_messages.len(), 2, "Should have query + turn messages");
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}
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#[test]
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fn test_cache_prefix_is_stable_across_chunks() {
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let query = Query {
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id: "test".to_string(),
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question: "What decisions?".to_string(),
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exit_gate: false,
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};
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let chunk_a = make_test_chunk("Evidence chunk A");
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let chunk_b = make_test_chunk("Evidence chunk B - completely different");
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let msgs_a = PromptBuilder::build_cache_aligned(&query, None, &chunk_a).unwrap();
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let msgs_b = PromptBuilder::build_cache_aligned(&query, None, &chunk_b).unwrap();
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// System messages must be identical
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assert_eq!(msgs_a.system, msgs_b.system, "System prompt must be stable");
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// Query messages (user_messages[0]) must be identical
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assert_eq!(
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msgs_a.user_messages[0], msgs_b.user_messages[0],
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"Query message must be stable across chunks"
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);
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// Turn messages (user_messages[1]) must differ
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assert_ne!(
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msgs_a.user_messages[1], msgs_b.user_messages[1],
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"Turn messages should differ (different chunks)"
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);
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}
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#[test]
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fn test_cache_prefix_is_stable_across_memory_changes() {
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let query = Query {
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id: "test".to_string(),
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question: "What decisions?".to_string(),
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exit_gate: false,
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};
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let chunk = make_test_chunk("Same chunk");
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let msgs_t1 = PromptBuilder::build_cache_aligned(&query, None, &chunk).unwrap();
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let msgs_t2 = PromptBuilder::build_cache_aligned(
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&query,
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Some("Memory from turn 1"),
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&chunk,
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).unwrap();
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// System + query must be identical even when memory changes
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assert_eq!(msgs_t1.system, msgs_t2.system);
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assert_eq!(msgs_t1.user_messages[0], msgs_t2.user_messages[0]);
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// Turn messages differ (different memory)
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assert_ne!(msgs_t1.user_messages[1], msgs_t2.user_messages[1]);
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}
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#[test]
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fn test_cache_prefix_tokens_positive() {
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let query = Query {
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id: "test".to_string(),
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question: "What decisions?".to_string(),
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exit_gate: false,
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};
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let chunk = make_test_chunk("Evidence");
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let msgs = PromptBuilder::build_cache_aligned(&query, None, &chunk).unwrap();
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assert!(
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msgs.cache_prefix_tokens() > 0,
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"Cache prefix should have positive token count"
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);
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}
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#[test]
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fn test_headroom_positive_under_budget() {
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let query = Query {
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id: "test".to_string(),
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question: "Test?".to_string(),
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exit_gate: false,
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};
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let chunk = make_test_chunk("Short");
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let msgs = PromptBuilder::build_cache_aligned(&query, None, &chunk).unwrap();
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assert!(
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msgs.headroom() > 0,
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"Should have positive headroom for short prompts"
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);
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}
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#[test]
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fn test_legacy_build_still_works() {
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let query = Query {
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id: "test".to_string(),
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question: "What?".to_string(),
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exit_gate: false,
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};
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let chunk = make_test_chunk("Evidence");
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let (system, user) = PromptBuilder::build(&query, None, &chunk).unwrap();
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assert!(!system.is_empty());
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assert!(!user.is_empty());
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assert!(user.contains("What?"));
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assert!(user.contains("Evidence"));
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}
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|
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#[test]
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fn test_cache_aligned_contains_query() {
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let query = Query {
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id: "test".to_string(),
|
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question: "What architectural decisions?".to_string(),
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exit_gate: false,
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};
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let chunk = make_test_chunk("We use microservices");
|
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|
||||
let msgs = PromptBuilder::build_cache_aligned(&query, None, &chunk).unwrap();
|
||||
|
||||
// Query message should contain the problem
|
||||
assert!(
|
||||
msgs.user_messages[0].contains("What architectural decisions?"),
|
||||
"Query message should contain the question"
|
||||
);
|
||||
|
||||
// Turn message should contain chunk and memory
|
||||
assert!(
|
||||
msgs.user_messages[1].contains("We use microservices"),
|
||||
"Turn message should contain chunk text"
|
||||
);
|
||||
assert!(
|
||||
msgs.user_messages[1].contains("No previous memory"),
|
||||
"Turn message should contain memory placeholder"
|
||||
);
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn test_cache_aligned_memory_budget_exceeded() {
|
||||
let query = Query {
|
||||
id: "test".to_string(),
|
||||
question: "Q?".to_string(),
|
||||
exit_gate: false,
|
||||
};
|
||||
let chunk = make_test_chunk("E");
|
||||
let big_memory = "x".repeat(BUDGET_MEMORY_MAX + 1);
|
||||
|
||||
let result = PromptBuilder::build_cache_aligned(&query, Some(&big_memory), &chunk);
|
||||
assert!(result.is_err(), "Should reject over-budget memory");
|
||||
}
|
||||
|
||||
#[test]
|
||||
fn test_cache_aligned_chunk_budget_exceeded() {
|
||||
let query = Query {
|
||||
id: "test".to_string(),
|
||||
question: "Q?".to_string(),
|
||||
exit_gate: false,
|
||||
};
|
||||
let big_chunk = make_test_chunk(&"x".repeat(BUDGET_CHUNK_MAX + 1));
|
||||
|
||||
let result = PromptBuilder::build_cache_aligned(&query, None, &big_chunk);
|
||||
assert!(result.is_err(), "Should reject over-budget chunk");
|
||||
}
|
||||
}
|
||||
|
||||
Reference in New Issue
Block a user