diff --git a/cli/src/merge_tools/builtin.rs b/cli/src/merge_tools/builtin.rs
index 2fe4c16be..8b33aa8c5 100644
--- a/cli/src/merge_tools/builtin.rs
+++ b/cli/src/merge_tools/builtin.rs
@@ -2,6 +2,7 @@ use std::borrow::Cow;
 use std::path::Path;
 use std::sync::Arc;
 
+use futures::{StreamExt, TryStreamExt};
 use itertools::Itertools;
 use jj_lib::backend::{BackendError, FileId, MergedTreeId, ObjectId, TreeValue};
 use jj_lib::diff::{find_line_ranges, Diff, DiffHunk};
@@ -11,6 +12,7 @@ use jj_lib::merge::Merge;
 use jj_lib::merged_tree::{MergedTree, MergedTreeBuilder};
 use jj_lib::repo_path::RepoPath;
 use jj_lib::store::Store;
+use pollster::FutureExt;
 use thiserror::Error;
 
 #[derive(Debug, Error)]
@@ -409,9 +411,10 @@ pub fn edit_diff_builtin(
 ) -> Result<MergedTreeId, BuiltinToolError> {
     let store = left_tree.store().clone();
     let changed_files: Vec<_> = left_tree
-        .diff(right_tree, matcher)
+        .diff_stream(right_tree, matcher)
         .map(|(path, diff)| diff.map(|_| path))
-        .try_collect()?;
+        .try_collect()
+        .block_on()?;
     let files = make_diff_files(&store, left_tree, right_tree, &changed_files)?;
     let recorder = scm_record::Recorder::new(
         scm_record::RecordState {
diff --git a/cli/src/merge_tools/external.rs b/cli/src/merge_tools/external.rs
index de592c97c..96dcf8fda 100644
--- a/cli/src/merge_tools/external.rs
+++ b/cli/src/merge_tools/external.rs
@@ -6,6 +6,7 @@ use std::process::{Command, ExitStatus, Stdio};
 use std::sync::Arc;
 
 use config::ConfigError;
+use futures::StreamExt;
 use itertools::Itertools;
 use jj_lib::backend::{FileId, MergedTreeId, TreeValue};
 use jj_lib::conflicts::{self, materialize_merge_result};
@@ -238,10 +239,11 @@ fn check_out_trees(
     matcher: &dyn Matcher,
     output_is: Option<DiffSide>,
 ) -> Result<DiffWorkingCopies, DiffCheckoutError> {
-    let changed_files = left_tree
-        .diff(right_tree, matcher)
+    let changed_files: Vec<_> = left_tree
+        .diff_stream(right_tree, matcher)
         .map(|(path, _diff)| path)
-        .collect_vec();
+        .collect()
+        .block_on();
 
     let temp_dir = new_utf8_temp_dir("jj-diff-").map_err(DiffCheckoutError::SetUpDir)?;
     let left_wc_dir = temp_dir.path().join("left");