Expand repository instruction discovery for ReAct prompts

src/prompts/react.rs

@@ -1,4 +1,6 @@
 use crate::thinker::ToolDescription;
+use std::fs;
+use std::sync::OnceLock;
 
 const INTRO: &str = "You are Golem, an AI agent that solves tasks using a ReAct loop.\n\nCRITICAL: Your entire response must be a single JSON object. No prose, no explanation, no markdown — just JSON.";
 
@@ -34,13 +36,38 @@ const RULES: &[&str] = &[
     "When you have enough information, respond with the answer format.",
 ];
 
+static AGENT_INSTRUCTIONS_CACHE: OnceLock<Option<String>> = OnceLock::new();
+// Explicit filename variants keep behavior predictable across platforms and
+// avoid case-insensitive filesystem assumptions.
+static INSTRUCTION_FILE_CANDIDATES: &[&str] = &[
+    "AGENTS.md",
+    "agents.md",
+    "CLAUDE.md",
+    "claude.md",
+    "GEMINI.md",
+    "gemini.md",
+    ".github/copilot-instructions.md",
+];
+
 pub fn build_react_system_prompt(tools: &[ToolDescription]) -> String {
-    build_react_system_prompt_with_session(tools, false)
+    build_react_system_prompt_with_session_and_agents(tools, false, load_agent_instructions())
 }
 
 pub fn build_react_system_prompt_with_session(
     tools: &[ToolDescription],
     has_session_history: bool,
+) -> String {
+    build_react_system_prompt_with_session_and_agents(
+        tools,
+        has_session_history,
+        load_agent_instructions(),
+    )
+}
+
+fn build_react_system_prompt_with_session_and_agents(
+    tools: &[ToolDescription],
+    has_session_history: bool,
+    agents_md: Option<&str>,
 ) -> String {
     let mut prompt = String::with_capacity(1024);
 
@@ -53,6 +80,13 @@ pub fn build_react_system_prompt_with_session(
         prompt.push('\n');
     }
 
+    if let Some(agents_md) = agents_md {
+        prompt.push('\n');
+        prompt.push_str("Repository agent instructions:\n");
+        prompt.push_str(&sanitize_agents_md(agents_md));
+        prompt.push('\n');
+    }
+
     // Tool list
     if !tools.is_empty() {
         prompt.push_str("\nAvailable tools:\n");
@@ -81,6 +115,32 @@ pub fn build_react_system_prompt_with_session(
     prompt
 }
 
+fn load_agent_instructions() -> Option<&'static str> {
+    AGENT_INSTRUCTIONS_CACHE
+        .get_or_init(|| {
+            // Ordered by precedence. Keep AGENTS/agents first, then common
+            // single-file alternatives used by other agents.
+            for path in INSTRUCTION_FILE_CANDIDATES {
+                // Missing/unreadable files are expected in many repos; ignore and
+                // continue scanning fallback candidates.
+                if let Ok(content) = fs::read_to_string(path) {
+                    let trimmed = content.trim();
+                    if !trimmed.is_empty() {
+                        return Some(trimmed.to_string());
+                    }
+                }
+            }
+            None
+        })
+        .as_deref()
+}
+
+fn sanitize_agents_md(content: &str) -> String {
+    // Keep sanitization minimal: strip markdown code fences so we preserve the
+    // prompt's no-fences constraint while leaving other text unchanged.
+    content.replace("```", "'''")
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -187,4 +247,29 @@ mod tests {
         let prompt = build_react_system_prompt(&[]);
         assert!(!prompt.contains("prior tasks"));
     }
+
+    #[test]
+    fn includes_agents_md_when_present() {
+        let prompt = build_react_system_prompt_with_session_and_agents(
+            &[],
+            false,
+            Some("# AGENTS\n```bash\nfollow repo instructions\n```"),
+        );
+        assert!(prompt.contains("Repository agent instructions:"));
+        assert!(prompt.contains("# AGENTS\n'''bash\nfollow repo instructions\n'''"));
+    }
+
+    #[test]
+    fn omits_agents_md_section_when_missing() {
+        let prompt = build_react_system_prompt_with_session_and_agents(&[], false, None);
+        assert!(!prompt.contains("Repository agent instructions:"));
+    }
+
+    #[test]
+    fn instruction_candidates_include_other_agent_files() {
+        let files = INSTRUCTION_FILE_CANDIDATES;
+        assert!(files.contains(&"CLAUDE.md"));
+        assert!(files.contains(&"GEMINI.md"));
+        assert!(files.contains(&".github/copilot-instructions.md"));
+    }
 }