diff --git a/src/run.rs b/src/run.rs
index d68e62d..ff4008d 100644
--- a/src/run.rs
+++ b/src/run.rs
@@ -2,65 +2,102 @@ use std::cmp::max;
 use std::future::Future;
 use std::sync::{Arc, LazyLock};
 
-use tokio::task::JoinSet;
+use futures::StreamExt;
 use tracing::trace;
 
 use crate::hook::Hook;
 
-pub static CONCURRENCY: LazyLock<usize> = LazyLock::new(|| target_concurrency(false));
-
-fn target_concurrency(serial: bool) -> usize {
-    if serial || std::env::var_os("PRE_COMMIT_NO_CONCURRENCY").is_some() {
+pub static CONCURRENCY: LazyLock<usize> = LazyLock::new(|| {
+    if std::env::var_os("PRE_COMMIT_NO_CONCURRENCY").is_some() {
         1
     } else {
         std::thread::available_parallelism()
             .map(std::num::NonZero::get)
             .unwrap_or(1)
     }
+});
+
+fn target_concurrency(serial: bool) -> usize {
+    if serial {
+        1
+    } else {
+        *CONCURRENCY
+    }
 }
 
-// TODO: do a more accurate calculation
-fn partitions<'a>(
+/// Iterator that yields partitions of filenames that fit within the maximum command line length.
+struct Partitions<'a> {
     hook: &'a Hook,
-    filenames: &'a [&String],
+    filenames: &'a [&'a String],
     concurrency: usize,
-) -> Vec<Vec<&'a String>> {
-    // If there are no filenames, we still want to run the hook once.
-    if filenames.is_empty() {
-        return vec![vec![]];
-    }
+    current_index: usize,
+    command_length: usize,
+    max_per_batch: usize,
+    max_cli_length: usize,
+}
 
-    let max_per_batch = max(4, filenames.len().div_ceil(concurrency));
-    // TODO: subtract the env size
-    let max_cli_length = if cfg!(unix) {
-        1 << 12
-    } else {
-        (1 << 15) - 2048 // UNICODE_STRING max - headroom
-    };
-
-    let command_length =
-        hook.entry.len() + hook.args.iter().map(String::len).sum::<usize>() + hook.args.len();
-
-    let mut partitions = Vec::new();
-    let mut current = Vec::new();
-    let mut current_length = command_length + 1;
-
-    for &filename in filenames {
-        let length = filename.len() + 1;
-        if current_length + length > max_cli_length || current.len() >= max_per_batch {
-            partitions.push(current);
-            current = Vec::new();
-            current_length = command_length + 1;
+// TODO: do a more accurate calculation
+impl<'a> Partitions<'a> {
+    fn new(hook: &'a Hook, filenames: &'a [&'a String], concurrency: usize) -> Self {
+        let max_per_batch = max(4, filenames.len().div_ceil(concurrency));
+        // TODO: subtract the env size
+        let max_cli_length = if cfg!(unix) {
+            1 << 12
+        } else {
+            (1 << 15) - 2048 // UNICODE_STRING max - headroom
+        };
+        let command_length =
+            hook.entry.len() + hook.args.iter().map(String::len).sum::<usize>() + hook.args.len();
+
+        Self {
+            hook,
+            filenames,
+            concurrency,
+            current_index: 0,
+            command_length,
+            max_per_batch,
+            max_cli_length,
         }
-        current.push(filename);
-        current_length += length;
     }
+}
 
-    if !current.is_empty() {
-        partitions.push(current);
-    }
+impl<'a> Iterator for Partitions<'a> {
+    type Item = Vec<&'a String>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        // Handle empty filenames case
+        if self.filenames.is_empty() && self.current_index == 0 {
+            self.current_index = 1;
+            return Some(vec![]);
+        }
+
+        if self.current_index >= self.filenames.len() {
+            return None;
+        }
+
+        let mut current = Vec::new();
+        let mut current_length = self.command_length + 1;
+
+        while self.current_index < self.filenames.len() {
+            let filename = self.filenames[self.current_index];
+            let length = filename.len() + 1;
+
+            if current_length + length > self.max_cli_length || current.len() >= self.max_per_batch
+            {
+                break;
+            }
+
+            current.push(filename);
+            current_length += length;
+            self.current_index += 1;
+        }
 
-    partitions
+        if current.is_empty() {
+            None
+        } else {
+            Some(current)
+        }
+    }
 }
 
 pub async fn run_by_batch<T, F, Fut>(
@@ -74,15 +111,12 @@ where
     Fut: Future<Output = anyhow::Result<T>> + Send + 'static,
     T: Send + 'static,
 {
-    let mut concurrency = target_concurrency(hook.require_serial);
+    let concurrency = target_concurrency(hook.require_serial);
 
     // Split files into batches
-    let partitions = partitions(hook, filenames, concurrency);
-    concurrency = concurrency.min(partitions.len());
-    let semaphore = Arc::new(tokio::sync::Semaphore::new(concurrency));
+    let partitions = Partitions::new(hook, filenames, concurrency);
     trace!(
         total_files = filenames.len(),
-        partitions = partitions.len(),
         concurrency = concurrency,
         "Running {}",
         hook.id,
@@ -90,28 +124,17 @@ where
 
     let run = Arc::new(run);
 
-    // Spawn tasks for each batch
-    let mut tasks = JoinSet::new();
-
-    for batch in partitions {
-        let semaphore = semaphore.clone();
-        let run = run.clone();
-
-        let batch: Vec<_> = batch.into_iter().map(ToString::to_string).collect();
-
-        tasks.spawn(async move {
-            let _permit = semaphore
-                .acquire()
-                .await
-                .map_err(|_| anyhow::anyhow!("Failed to acquire semaphore"))?;
-
-            run(batch).await
-        });
-    }
+    let mut tasks = futures::stream::iter(partitions)
+        .map(|batch| {
+            let run = run.clone();
+            let batch: Vec<_> = batch.into_iter().map(ToString::to_string).collect();
+            run(batch)
+        })
+        .buffer_unordered(concurrency);
 
     let mut results = Vec::new();
-    while let Some(result) = tasks.join_next().await {
-        results.push(result??);
+    while let Some(result) = tasks.next().await {
+        results.push(result?);
     }
 
     Ok(results)