agent: Do not send stale files notifications (#32974)

Removing it for two reasons:

1. We need a better implementation that doesn't hurt caching and doesn't
distracts the agent too much (see
https://github.com/zed-industries/zed/pull/32876 for more context)

2. Current insertion point of notifications doesn't play well with
Claude Thinking models (see
https://github.com/zed-industries/zed/issues/33000#issuecomment-2991709484)

I think we should get this code back in a form of a tool. But for now,
I'm dropping it to resolve recent issues.

Closes #33000

Release Notes:

- N/A

crates/agent/src/prompts/stale_files_prompt_header.txt

@@ -1,3 +0,0 @@
-[The following is an auto-generated notification; do not reply]
-
-These files have changed since the last read:

crates/agent/src/thread.rs

@@ -1,4 +1,3 @@
-use std::fmt::Write as _;
 use std::io::Write;
 use std::ops::Range;
 use std::sync::Arc;
@@ -37,6 +36,7 @@ use settings::Settings;
 use thiserror::Error;
 use ui::Window;
 use util::{ResultExt as _, post_inc};
+
 use uuid::Uuid;
 use zed_llm_client::{CompletionIntent, CompletionRequestStatus};
 
@@ -1389,8 +1389,6 @@ impl Thread {
             request.messages[message_ix_to_cache].cache = true;
         }
 
-        self.attach_tracked_files_state(&mut request.messages, cx);
-
         request.tools = available_tools;
         request.mode = if model.supports_max_mode() {
             Some(self.completion_mode.into())
@@ -1453,60 +1451,6 @@ impl Thread {
         request
     }
 
-    fn attach_tracked_files_state(
-        &self,
-        messages: &mut Vec<LanguageModelRequestMessage>,
-        cx: &App,
-    ) {
-        let mut stale_files = String::new();
-
-        let action_log = self.action_log.read(cx);
-
-        for stale_file in action_log.stale_buffers(cx) {
-            if let Some(file) = stale_file.read(cx).file() {
-                writeln!(&mut stale_files, "- {}", file.path().display()).ok();
-            }
-        }
-
-        if stale_files.is_empty() {
-            return;
-        }
-
-        // NOTE: Changes to this prompt require a symmetric update in the LLM Worker
-        const STALE_FILES_HEADER: &str = include_str!("./prompts/stale_files_prompt_header.txt");
-        let content = MessageContent::Text(
-            format!("{STALE_FILES_HEADER}{stale_files}").replace("\r\n", "\n"),
-        );
-
-        // Insert our message before the last Assistant message.
-        // Inserting it to the tail distracts the agent too much
-        let insert_position = messages
-            .iter()
-            .enumerate()
-            .rfind(|(_, message)| message.role == Role::Assistant)
-            .map_or(messages.len(), |(i, _)| i);
-
-        let request_message = LanguageModelRequestMessage {
-            role: Role::User,
-            content: vec![content],
-            cache: false,
-        };
-
-        messages.insert(insert_position, request_message);
-
-        // It makes no sense to cache messages after this one because
-        // the cache is invalidated when this message is gone.
-        // Move the cache marker before this message.
-        let has_cached_messages_after = messages
-            .iter()
-            .skip(insert_position + 1)
-            .any(|message| message.cache);
-
-        if has_cached_messages_after {
-            messages[insert_position - 1].cache = true;
-        }
-    }
-
     pub fn stream_completion(
         &mut self,
         request: LanguageModelRequest,
@@ -3229,106 +3173,6 @@ fn main() {{
         );
     }
 
-    #[gpui::test]
-    async fn test_stale_buffer_notification(cx: &mut TestAppContext) {
-        init_test_settings(cx);
-
-        let project = create_test_project(
-            cx,
-            json!({"code.rs": "fn main() {\n    println!(\"Hello, world!\");\n}"}),
-        )
-        .await;
-
-        let (_workspace, _thread_store, thread, context_store, model) =
-            setup_test_environment(cx, project.clone()).await;
-
-        // Open buffer and add it to context
-        let buffer = add_file_to_context(&project, &context_store, "test/code.rs", cx)
-            .await
-            .unwrap();
-
-        let context =
-            context_store.read_with(cx, |store, _| store.context().next().cloned().unwrap());
-        let loaded_context = cx
-            .update(|cx| load_context(vec![context], &project, &None, cx))
-            .await;
-
-        // Insert user message with the buffer as context
-        thread.update(cx, |thread, cx| {
-            thread.insert_user_message("Explain this code", loaded_context, None, Vec::new(), cx)
-        });
-
-        // Create a request and check that it doesn't have a stale buffer warning yet
-        let initial_request = thread.update(cx, |thread, cx| {
-            thread.to_completion_request(model.clone(), CompletionIntent::UserPrompt, cx)
-        });
-
-        // Make sure we don't have a stale file warning yet
-        let has_stale_warning = initial_request.messages.iter().any(|msg| {
-            msg.string_contents()
-                .contains("These files changed since last read:")
-        });
-        assert!(
-            !has_stale_warning,
-            "Should not have stale buffer warning before buffer is modified"
-        );
-
-        // Modify the buffer
-        buffer.update(cx, |buffer, cx| {
-            // Find a position at the end of line 1
-            buffer.edit(
-                [(1..1, "\n    println!(\"Added a new line\");\n")],
-                None,
-                cx,
-            );
-        });
-
-        // Insert another user message without context
-        thread.update(cx, |thread, cx| {
-            thread.insert_user_message(
-                "What does the code do now?",
-                ContextLoadResult::default(),
-                None,
-                Vec::new(),
-                cx,
-            )
-        });
-
-        // Create a new request and check for the stale buffer warning
-        let new_request = thread.update(cx, |thread, cx| {
-            thread.to_completion_request(model.clone(), CompletionIntent::UserPrompt, cx)
-        });
-
-        // We should have a stale file warning as the last message
-        let last_message = new_request
-            .messages
-            .last()
-            .expect("Request should have messages");
-
-        // The last message should be the stale buffer notification
-        assert_eq!(last_message.role, Role::User);
-
-        // Check the exact content of the message
-        let expected_content = "[The following is an auto-generated notification; do not reply]
-
-These files have changed since the last read:
-- code.rs
-";
-        assert_eq!(
-            last_message.string_contents(),
-            expected_content,
-            "Last message should be exactly the stale buffer notification"
-        );
-
-        // The message before the notification should be cached
-        let index = new_request.messages.len() - 2;
-        let previous_message = new_request.messages.get(index).unwrap();
-        assert!(
-            previous_message.cache,
-            "Message before the stale buffer notification should be cached"
-        );
-    }
-
     #[gpui::test]
     async fn test_storing_profile_setting_per_thread(cx: &mut TestAppContext) {
         init_test_settings(cx);