Dynamically refresh skill instructions each turn (#9217)

Signed-off-by: Douwe Osinga <douwe@squareup.com>
Co-authored-by: Douwe Osinga <douwe@squareup.com>

crates/goose-cli/src/scenario_tests/mock_client.rs

@@ -56,7 +56,7 @@ impl McpClientTrait for MockClient {
     }
 
     fn get_info(&self) -> std::option::Option<&rmcp::model::InitializeResult> {
-        todo!()
+        None
     }
 
     async fn read_resource(

crates/goose/src/agents/extension_manager.rs

@@ -106,9 +106,7 @@ impl Extension {
     }
 
     fn get_instructions(&self) -> Option<String> {
-        self.server_info
-            .as_ref()
-            .and_then(|info| info.instructions.clone())
+        self.client.get_instructions()
     }
 
     fn get_client(&self) -> McpClientBox {

crates/goose/src/agents/mcp_client.rs

@@ -84,6 +84,13 @@ pub trait McpClientTrait: Send + Sync {
 
     fn get_info(&self) -> Option<&InitializeResult>;
 
+    /// Return the extension's current instructions. The default reads from
+    /// `get_info()`, but platform extensions can override this to provide
+    /// dynamically computed instructions (e.g. freshly discovered skills).
+    fn get_instructions(&self) -> Option<String> {
+        self.get_info().and_then(|info| info.instructions.clone())
+    }
+
     async fn list_resources(
         &self,
         _session_id: &str,

crates/goose/src/agents/prompt_manager.rs

@@ -441,11 +441,9 @@ mod tests {
             .values()
             .map(|def| {
                 let client = (def.client_factory)(context.clone());
-                let info = client.get_info();
-                let instructions = info
-                    .and_then(|i| i.instructions.clone())
-                    .unwrap_or_default();
-                let has_resources = info
+                let instructions = client.get_instructions().unwrap_or_default();
+                let has_resources = client
+                    .get_info()
                     .and_then(|i| i.capabilities.resources.as_ref())
                     .is_some();
                 ExtensionInfo::new(def.name, &instructions, has_resources)

crates/goose/src/skills/client.rs

@@ -27,30 +27,8 @@ impl SkillsClient {
             .map(|s| s.working_dir.clone())
             .unwrap_or_else(|| std::env::current_dir().unwrap_or_default());
 
-        let mut instructions = String::new();
-        if context.session.is_some() {
-            let sources = discover_skills(Some(&working_dir));
-            let mut skills: Vec<&SourceEntry> = sources
-                .iter()
-                .filter(|s| {
-                    s.source_type == SourceType::Skill || s.source_type == SourceType::BuiltinSkill
-                })
-                .collect();
-            skills.sort_by(|a, b| (&a.name, &a.path).cmp(&(&b.name, &b.path)));
-
-            if !skills.is_empty() {
-                instructions.push_str(
-                    "\n\nYou have these skills at your disposal, when it is clear they can help you solve a problem or you are asked to use them:",
-                );
-                for skill in &skills {
-                    instructions.push_str(&format!("\n• {} - {}", skill.name, skill.description));
-                }
-            }
-        }
-
         let info = InitializeResult::new(ServerCapabilities::builder().enable_tools().build())
-            .with_server_info(Implementation::new(EXTENSION_NAME, "1.0.0").with_title("Skills"))
-            .with_instructions(instructions);
+            .with_server_info(Implementation::new(EXTENSION_NAME, "1.0.0").with_title("Skills"));
 
         Ok(Self { info, working_dir })
     }
@@ -252,6 +230,29 @@ impl McpClientTrait for SkillsClient {
         Some(&self.info)
     }
 
+    fn get_instructions(&self) -> Option<String> {
+        let sources = discover_skills(Some(&self.working_dir));
+        let mut skills: Vec<&SourceEntry> = sources
+            .iter()
+            .filter(|s| {
+                s.source_type == SourceType::Skill || s.source_type == SourceType::BuiltinSkill
+            })
+            .collect();
+        skills.sort_by(|a, b| (&a.name, &a.path).cmp(&(&b.name, &b.path)));
+
+        if skills.is_empty() {
+            return None;
+        }
+
+        let mut instructions = String::from(
+            "\n\nYou have these skills at your disposal, when it is clear they can help you solve a problem or you are asked to use them:",
+        );
+        for skill in &skills {
+            instructions.push_str(&format!("\n• {} - {}", skill.name, skill.description));
+        }
+        Some(instructions)
+    }
+
     async fn subscribe(&self) -> mpsc::Receiver<ServerNotification> {
         let (_tx, rx) = mpsc::channel(1);
         rx