[mcp-server] Expose fuzzy file search in MCP (#2677)

## Summary
Expose a simple fuzzy file search implementation for mcp clients to work
with

## Testing
- [x] Tested locally

codex-rs/mcp-server/src/codex_message_processor.rs

@@ -1,5 +1,6 @@
 use crate::error_code::INTERNAL_ERROR_CODE;
 use crate::error_code::INVALID_REQUEST_ERROR_CODE;
+use crate::fuzzy_file_search::run_fuzzy_file_search;
 use crate::json_to_toml::json_to_toml;
 use crate::outgoing_message::OutgoingMessageSender;
 use crate::outgoing_message::OutgoingNotification;
@@ -52,6 +53,8 @@ use codex_protocol::mcp_protocol::ExecArbitraryCommandResponse;
 use codex_protocol::mcp_protocol::ExecCommandApprovalParams;
 use codex_protocol::mcp_protocol::ExecCommandApprovalResponse;
 use codex_protocol::mcp_protocol::ExecOneOffCommandParams;
+use codex_protocol::mcp_protocol::FuzzyFileSearchParams;
+use codex_protocol::mcp_protocol::FuzzyFileSearchResponse;
 use codex_protocol::mcp_protocol::GetUserAgentResponse;
 use codex_protocol::mcp_protocol::GetUserSavedConfigResponse;
 use codex_protocol::mcp_protocol::GitDiffToRemoteResponse;
@@ -88,6 +91,8 @@ use std::collections::HashMap;
 use std::ffi::OsStr;
 use std::path::PathBuf;
 use std::sync::Arc;
+use std::sync::atomic::AtomicBool;
+use std::sync::atomic::Ordering;
 use std::time::Duration;
 use tokio::select;
 use tokio::sync::Mutex;
@@ -122,6 +127,7 @@ pub(crate) struct CodexMessageProcessor {
     active_login: Arc<Mutex<Option<ActiveLogin>>>,
     // Queue of pending interrupt requests per conversation. We reply when TurnAborted arrives.
     pending_interrupts: Arc<Mutex<HashMap<ConversationId, Vec<RequestId>>>>,
+    pending_fuzzy_searches: Arc<Mutex<HashMap<String, Arc<AtomicBool>>>>,
 }
 
 impl CodexMessageProcessor {
@@ -141,6 +147,7 @@ impl CodexMessageProcessor {
             conversation_listeners: HashMap::new(),
             active_login: Arc::new(Mutex::new(None)),
             pending_interrupts: Arc::new(Mutex::new(HashMap::new())),
+            pending_fuzzy_searches: Arc::new(Mutex::new(HashMap::new())),
         }
     }
 
@@ -206,6 +213,9 @@ impl CodexMessageProcessor {
             ClientRequest::UserInfo { request_id } => {
                 self.get_user_info(request_id).await;
             }
+            ClientRequest::FuzzyFileSearch { request_id, params } => {
+                self.fuzzy_file_search(request_id, params).await;
+            }
             ClientRequest::ExecOneOffCommand { request_id, params } => {
                 self.exec_one_off_command(request_id, params).await;
             }
@@ -1167,6 +1177,46 @@ impl CodexMessageProcessor {
             }
         }
     }
+
+    async fn fuzzy_file_search(&mut self, request_id: RequestId, params: FuzzyFileSearchParams) {
+        let FuzzyFileSearchParams {
+            query,
+            roots,
+            cancellation_token,
+        } = params;
+
+        let cancel_flag = match cancellation_token.clone() {
+            Some(token) => {
+                let mut pending_fuzzy_searches = self.pending_fuzzy_searches.lock().await;
+                // if a cancellation_token is provided and a pending_request exists for
+                // that token, cancel it
+                if let Some(existing) = pending_fuzzy_searches.get(&token) {
+                    existing.store(true, Ordering::Relaxed);
+                }
+                let flag = Arc::new(AtomicBool::new(false));
+                pending_fuzzy_searches.insert(token.clone(), flag.clone());
+                flag
+            }
+            None => Arc::new(AtomicBool::new(false)),
+        };
+
+        let results = match query.as_str() {
+            "" => vec![],
+            _ => run_fuzzy_file_search(query, roots, cancel_flag.clone()).await,
+        };
+
+        if let Some(token) = cancellation_token {
+            let mut pending_fuzzy_searches = self.pending_fuzzy_searches.lock().await;
+            if let Some(current_flag) = pending_fuzzy_searches.get(&token)
+                && Arc::ptr_eq(current_flag, &cancel_flag)
+            {
+                pending_fuzzy_searches.remove(&token);
+            }
+        }
+
+        let response = FuzzyFileSearchResponse { files: results };
+        self.outgoing.send_response(request_id, response).await;
+    }
 }
 
 async fn apply_bespoke_event_handling(

codex-rs/mcp-server/src/fuzzy_file_search.rs

@@ -0,0 +1,84 @@
+use std::num::NonZero;
+use std::num::NonZeroUsize;
+use std::path::PathBuf;
+use std::sync::Arc;
+use std::sync::atomic::AtomicBool;
+
+use codex_file_search as file_search;
+use codex_protocol::mcp_protocol::FuzzyFileSearchResult;
+use tokio::task::JoinSet;
+use tracing::warn;
+
+const LIMIT_PER_ROOT: usize = 50;
+const MAX_THREADS: usize = 12;
+const COMPUTE_INDICES: bool = true;
+
+pub(crate) async fn run_fuzzy_file_search(
+    query: String,
+    roots: Vec<String>,
+    cancellation_flag: Arc<AtomicBool>,
+) -> Vec<FuzzyFileSearchResult> {
+    #[expect(clippy::expect_used)]
+    let limit_per_root =
+        NonZero::new(LIMIT_PER_ROOT).expect("LIMIT_PER_ROOT should be a valid non-zero usize");
+
+    let cores = std::thread::available_parallelism()
+        .map(std::num::NonZero::get)
+        .unwrap_or(1);
+    let threads = cores.min(MAX_THREADS);
+    let threads_per_root = (threads / roots.len()).max(1);
+    let threads = NonZero::new(threads_per_root).unwrap_or(NonZeroUsize::MIN);
+
+    let mut files: Vec<FuzzyFileSearchResult> = Vec::new();
+    let mut join_set = JoinSet::new();
+
+    for root in roots {
+        let search_dir = PathBuf::from(&root);
+        let query = query.clone();
+        let cancel_flag = cancellation_flag.clone();
+        join_set.spawn_blocking(move || {
+            match file_search::run(
+                query.as_str(),
+                limit_per_root,
+                &search_dir,
+                Vec::new(),
+                threads,
+                cancel_flag,
+                COMPUTE_INDICES,
+            ) {
+                Ok(res) => Ok((root, res)),
+                Err(err) => Err((root, err)),
+            }
+        });
+    }
+
+    while let Some(res) = join_set.join_next().await {
+        match res {
+            Ok(Ok((root, res))) => {
+                for m in res.matches {
+                    let result = FuzzyFileSearchResult {
+                        root: root.clone(),
+                        path: m.path,
+                        score: m.score,
+                        indices: m.indices,
+                    };
+                    files.push(result);
+                }
+            }
+            Ok(Err((root, err))) => {
+                warn!("fuzzy-file-search in dir '{root}' failed: {err}");
+            }
+            Err(err) => {
+                warn!("fuzzy-file-search join_next failed: {err}");
+            }
+        }
+    }
+
+    files.sort_by(file_search::cmp_by_score_desc_then_path_asc::<
+        FuzzyFileSearchResult,
+        _,
+        _,
+    >(|f| f.score, |f| f.path.as_str()));
+
+    files
+}

codex-rs/mcp-server/src/lib.rs

@@ -25,6 +25,7 @@ mod codex_tool_config;
 mod codex_tool_runner;
 mod error_code;
 mod exec_approval;
+mod fuzzy_file_search;
 mod json_to_toml;
 pub(crate) mod message_processor;
 mod outgoing_message;