diff --git a/src-tauri/src/acp/binary_cache.rs b/src-tauri/src/acp/binary_cache.rs
index 2c71e21..471c0ec 100644
--- a/src-tauri/src/acp/binary_cache.rs
+++ b/src-tauri/src/acp/binary_cache.rs
@@ -1,11 +1,18 @@
 use std::collections::HashSet;
 use std::io::Read;
 use std::path::{Path, PathBuf};
+use std::sync::atomic::{AtomicU64, Ordering};
 
 use crate::acp::error::AcpError;
 use crate::acp::registry;
 use crate::models::agent::AgentType;
 
+/// Process-local counter appended to rename-aside trash directory names. Guards
+/// against the rare case where two `clear_agent_cache` calls land in the same
+/// `SystemTime::now()` tick (Windows `GetSystemTimePreciseAsFileTime` has ~100ns
+/// resolution) and would otherwise collide on the rename target.
+static TRASH_COUNTER: AtomicU64 = AtomicU64::new(0);
+
 pub(crate) fn cache_dir() -> Result<PathBuf, AcpError> {
     let base = dirs::cache_dir()
         .ok_or_else(|| AcpError::DownloadFailed("cannot determine cache directory".into()))?;
@@ -44,14 +51,56 @@ pub(crate) fn binary_dir(agent_id: &str, version: &str) -> Result<PathBuf, AcpEr
 
 pub fn clear_agent_cache(agent_type: AgentType) -> Result<(), AcpError> {
     let agent_id = agent_cache_key(agent_type);
-    let dir = cache_dir()?.join(agent_id);
-    if dir.exists() {
-        std::fs::remove_dir_all(&dir)
-            .map_err(|e| AcpError::DownloadFailed(format!("failed to clear cache: {e}")))?;
+    let dir = cache_dir()?.join(&agent_id);
+    if !dir.exists() {
+        return Ok(());
     }
+
+    if std::fs::remove_dir_all(&dir).is_ok() {
+        return Ok(());
+    }
+
+    // Windows: a running `<cmd>.exe` (ours or anti-virus scanning it) keeps the
+    // file locked, so `remove_dir_all` returns ERROR_ACCESS_DENIED. NTFS allows
+    // renaming a directory whose children are locked because rename only
+    // updates the parent directory entry; the locked file's FILE_OBJECT keeps
+    // working under the new path. The aside is swept on next startup.
+    let trash_root = cache_dir()?.join(".trash");
+    let _ = std::fs::create_dir_all(&trash_root);
+    let stamp = std::time::SystemTime::now()
+        .duration_since(std::time::UNIX_EPOCH)
+        .map(|d| d.as_nanos())
+        .unwrap_or(0);
+    let counter = TRASH_COUNTER.fetch_add(1, Ordering::Relaxed);
+    let aside = trash_root.join(format!("{agent_id}-{stamp}-{counter}"));
+    std::fs::rename(&dir, &aside)
+        .map_err(|e| AcpError::DownloadFailed(format!("failed to clear cache: {e}")))?;
+
+    let _ = std::fs::remove_dir_all(&aside);
     Ok(())
 }
 
+/// Best-effort cleanup of trash directories left behind by
+/// `clear_agent_cache`'s rename-aside fallback. Designed to be run from a
+/// detached OS thread at startup: every error path is silently swallowed,
+/// no logs, no panics escape, no subprocesses spawned. Whatever cannot be
+/// removed (e.g. a binary still locked by an external process) is left for
+/// the next startup.
+///
+/// Iterates children rather than nuking the parent so that a concurrent
+/// `clear_agent_cache` racing to rename a fresh entry into `.trash/` cannot
+/// have its target directory yanked out from under it.
+pub fn sweep_trash() {
+    let Ok(base) = cache_dir() else { return };
+    let trash = base.join(".trash");
+    let Ok(entries) = std::fs::read_dir(&trash) else {
+        return;
+    };
+    for entry in entries.flatten() {
+        let _ = std::fs::remove_dir_all(entry.path());
+    }
+}
+
 fn installed_binary_path(agent_id: &str, version: &str, cmd_name: &str) -> Option<PathBuf> {
     let bin_name = if cfg!(target_os = "windows") {
         format!("{cmd_name}.exe")
diff --git a/src-tauri/src/bin/codeg_server.rs b/src-tauri/src/bin/codeg_server.rs
index 291c461..e6f9885 100644
--- a/src-tauri/src/bin/codeg_server.rs
+++ b/src-tauri/src/bin/codeg_server.rs
@@ -29,6 +29,15 @@ fn main() {
 }
 
 async fn async_main() {
+    // Sweep stale ACP binary cache trash (rename-aside fallback artifacts).
+    // Detached OS thread: cannot block startup, panics are caught and dropped,
+    // errors are silenced, no subprocesses spawned.
+    std::thread::spawn(|| {
+        let _ = std::panic::catch_unwind(|| {
+            codeg_lib::sweep_acp_binary_trash();
+        });
+    });
+
     let port: u16 = std::env::var("CODEG_PORT")
         .ok()
         .and_then(|v| v.parse().ok())
diff --git a/src-tauri/src/lib.rs b/src-tauri/src/lib.rs
index e8594f2..0757f43 100644
--- a/src-tauri/src/lib.rs
+++ b/src-tauri/src/lib.rs
@@ -17,6 +17,14 @@ mod terminal;
 pub mod web;
 pub mod workspace_state;
 
+/// Sweep stale ACP binary cache trash created by the rename-aside fallback in
+/// `acp::binary_cache::clear_agent_cache`. Safe to call any time; intended to
+/// be invoked once at startup from a detached OS thread. Does not block, does
+/// not panic, errors are silently dropped.
+pub fn sweep_acp_binary_trash() {
+    crate::acp::binary_cache::sweep_trash();
+}
+
 #[cfg(feature = "tauri-runtime")]
 mod tauri_app {
     use std::sync::atomic::{AtomicBool, Ordering};
@@ -124,6 +132,16 @@ mod tauri_app {
                 tauri::async_runtime::block_on(windows::load_saved_zoom(&db.conn));
                 tauri::async_runtime::block_on(windows::load_saved_appearance_mode(&db.conn));
 
+                // Sweep stale ACP binary cache trash (rename-aside fallback
+                // artifacts). Detached OS thread: cannot block startup, panics
+                // are caught and dropped, errors are silenced, no subprocesses
+                // spawned. Anything still locked is left for next startup.
+                std::thread::spawn(|| {
+                    let _ = std::panic::catch_unwind(|| {
+                        crate::sweep_acp_binary_trash();
+                    });
+                });
+
                 // Install bundled expert skills into the central store
                 // (`~/.codeg/skills/`). Runs in the background and does
                 // not block startup; failures are logged but non-fatal.