diff --git a/src/app/state/rest.rs b/src/app/state/rest.rs
index e88ddaf..1c44318 100644
--- a/src/app/state/rest.rs
+++ b/src/app/state/rest.rs
@@ -9,7 +9,7 @@ use std::path::PathBuf;
 use std::sync::{Arc, Mutex};
 use tokio::sync::RwLock;
 
-use super::misc::{DirCache, InputState, MiscState, ScrollState};
+use super::misc::{DirCache, InputState, MentionIndex, MiscState, ScrollState};
 use super::runtime::{SessionRuntime, TurnEvent};
 use super::types::{Origin, Toast, TranscriptCache};
 use crate::app::lsp::LspManager;
@@ -54,6 +54,7 @@ pub struct AppStateRest {
     pub memory_dir: PathBuf,
     pub worktrees_dir: PathBuf,
     pub dir_cache: Arc<RwLock<DirCache>>,
+    pub mention_index: MentionIndex,
     pub edit_log: EditLog,
     pub session_runtime: Option<SessionRuntime>,
     pub sessions: Vec<crate::model::session::Session>,
@@ -110,6 +111,7 @@ impl AppStateRest {
             turn_in_flight: Arc::new(Mutex::new(false)),
             abort_flag: Arc::new(std::sync::atomic::AtomicBool::new(false)),
             dir_cache: Arc::new(RwLock::new(dir_cache)),
+            mention_index: MentionIndex::new(),
             edit_log: EditLog::new(session_dir),
             session_runtime: Some(SessionRuntime::new(session_dir.to_path_buf())),
             workflow_engine: WorkflowEngine::new(),
@@ -204,6 +206,44 @@ impl AppStateRest {
             });
         }
 
+        // Fire-and-forget background file index build for `@file` mention
+        // autocomplete.
+        //
+        // Flow: spawn OS thread -> `ignore::Walk` each workspace root,
+        // collecting file paths (workspace-index-prefixed for roots beyond
+        // the first, matching `resolve_path`'s `[N]path` convention) -> stop
+        // once 50,000 entries are collected -> store the result in
+        // `mention_index`.
+        //
+        // Why a raw thread and not a background tokio task: there is no
+        // persistent async runtime driving the render loop, and this is
+        // blocking filesystem I/O -- a dedicated thread keeps startup
+        // non-blocking. Not joined, same rationale as the LSP provisioning
+        // thread above: a slow/huge repo must not delay the TUI appearing.
+        {
+            let mention_index = state.mention_index.clone();
+            let roots = state.workspace_roots.clone();
+            std::thread::spawn(move || {
+                const MAX_MENTION_ENTRIES: usize = 50_000;
+                let mut paths = Vec::new();
+                'roots: for (i, root) in roots.iter().enumerate() {
+                    for entry in ignore::Walk::new(root).flatten() {
+                        if !entry.path().is_file() {
+                            continue;
+                        }
+                        let rel = entry.path().strip_prefix(root).unwrap_or(entry.path());
+                        let rel_str = rel.display().to_string();
+                        let formatted = if i == 0 { rel_str } else { format!("[{i}]{rel_str}") };
+                        paths.push(formatted);
+                        if paths.len() >= MAX_MENTION_ENTRIES {
+                            break 'roots;
+                        }
+                    }
+                }
+                mention_index.set(paths);
+            });
+        }
+
         state
     }
 
@@ -278,6 +318,7 @@ impl AppStateRest {
             memory_dir: self.memory_dir.clone(),
             worktrees_dir: self.worktrees_dir.clone(),
             dir_cache: self.dir_cache.clone(),
+            mention_index: self.mention_index.clone(),
             origin,
             graduated_checks: Vec::new(),
             lsp_manager: self.lsp_manager.clone(),
diff --git a/src/tool/mod.rs b/src/tool/mod.rs
index 69b7d35..6b7c190 100644
--- a/src/tool/mod.rs
+++ b/src/tool/mod.rs
@@ -47,6 +47,7 @@ pub struct ToolCtx {
     pub memory_dir: PathBuf,
     pub worktrees_dir: PathBuf,
     pub dir_cache: std::sync::Arc<tokio::sync::RwLock<super::app::state::misc::DirCache>>,
+    pub mention_index: super::app::state::misc::MentionIndex,
     pub origin: crate::app::state::types::Origin,
     pub graduated_checks: Vec<GraduatedCheck>,
     pub lsp_manager: Arc<Mutex<crate::app::lsp::LspManager>>,
@@ -94,6 +95,7 @@ pub struct ToolCtxBuilder {
     pub memory_dir: PathBuf,
     pub worktrees_dir: PathBuf,
     pub dir_cache: std::sync::Arc<tokio::sync::RwLock<super::app::state::misc::DirCache>>,
+    pub mention_index: super::app::state::misc::MentionIndex,
     pub origin: crate::app::state::types::Origin,
     pub graduated_checks: Vec<GraduatedCheck>,
     pub lsp_manager: Arc<Mutex<crate::app::lsp::LspManager>>,
@@ -110,6 +112,7 @@ impl Default for ToolCtxBuilder {
             memory_dir: PathBuf::new(),
             worktrees_dir: PathBuf::new(),
             dir_cache: std::sync::Arc::new(tokio::sync::RwLock::new(super::app::state::misc::DirCache::new())),
+            mention_index: super::app::state::misc::MentionIndex::new(),
             origin: crate::app::state::types::Origin::Main,
             graduated_checks: Vec::new(),
             lsp_manager: Arc::new(Mutex::new(crate::app::lsp::LspManager::new())),
@@ -138,6 +141,7 @@ impl ToolCtxBuilder {
             memory_dir: self.memory_dir,
             worktrees_dir: self.worktrees_dir,
             dir_cache: self.dir_cache,
+            mention_index: self.mention_index,
             origin: self.origin,
             graduated_checks: self.graduated_checks,
             lsp_manager: self.lsp_manager,