diff --git a/cli/src/commands/fix.rs b/cli/src/commands/fix.rs
new file mode 100644
index 00000000000..dbc64ee411c
--- /dev/null
+++ b/cli/src/commands/fix.rs
@@ -0,0 +1,152 @@
+// Copyright 2024 The Jujutsu Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+use std::collections::{HashMap, HashSet};
+
+use futures::StreamExt;
+use itertools::Itertools;
+use jj_lib::backend::{BackendError, BackendResult, CommitId, FileId, TreeValue};
+use jj_lib::commit::{Commit, CommitIteratorExt};
+use jj_lib::matchers::EverythingMatcher;
+use jj_lib::merged_tree::MergedTreeBuilder;
+use jj_lib::repo::Repo;
+use jj_lib::repo_path::RepoPathBuf;
+use jj_lib::revset::{RevsetExpression, RevsetIteratorExt};
+use jj_lib::store::Store;
+use pollster::FutureExt;
+use tracing::instrument;
+
+use crate::cli_util::{CommandHelper, RevisionArg};
+use crate::command_error::CommandError;
+use crate::ui::Ui;
+
+/// Format files
+#[derive(clap::Args, Clone, Debug)]
+#[command(verbatim_doc_comment)]
+pub(crate) struct FixArgs {
+    /// Fix these commits and all their descendants
+    sources: Vec<RevisionArg>,
+}
+
+#[instrument(skip_all)]
+pub(crate) fn cmd_fix(
+    ui: &mut Ui,
+    command: &CommandHelper,
+    args: &FixArgs,
+) -> Result<(), CommandError> {
+    let mut workspace_command = command.workspace_helper(ui)?;
+    let root_commits: Vec<Commit> = workspace_command
+        .parse_union_revsets(&args.sources)?
+        .evaluate_to_commits()?
+        .try_collect()?;
+    workspace_command.check_rewritable(root_commits.iter().ids())?;
+
+    let mut tx = workspace_command.start_transaction();
+
+    // Collect all FileIds we're going to format and which commits they appear in
+    let commits: Vec<_> = RevsetExpression::commits(root_commits.iter().ids().cloned().collect())
+        .descendants()
+        .evaluate_programmatic(tx.base_repo().as_ref())?
+        .iter()
+        .commits(tx.repo().store())
+        .try_collect()?;
+    let mut file_ids = HashSet::new();
+    let mut commit_paths: HashMap<CommitId, Vec<RepoPathBuf>> = HashMap::new();
+    for commit in commits.iter().rev() {
+        let mut paths = vec![];
+        // Paths modified in parent commits in the set should also be updated in this
+        // commit
+        for parent_id in commit.parent_ids() {
+            if let Some(parent_paths) = commit_paths.get(parent_id) {
+                paths.extend_from_slice(parent_paths);
+            }
+        }
+        let parent_tree = commit.parent_tree(tx.repo())?;
+        let tree = commit.tree()?;
+        let mut diff_stream = parent_tree.diff_stream(&tree, &EverythingMatcher);
+        async {
+            while let Some((repo_path, diff)) = diff_stream.next().await {
+                let (_before, after) = diff?;
+                for term in after {
+                    if let Some(TreeValue::File { id, executable: _ }) = term {
+                        file_ids.insert((repo_path.clone(), id));
+                        paths.push(repo_path.clone());
+                    }
+                }
+            }
+            Ok::<(), BackendError>(())
+        }
+        .block_on()?;
+        commit_paths.insert(commit.id().clone(), paths);
+    }
+
+    let formatted = format_files(tx.repo().store().as_ref(), &file_ids)?;
+
+    tx.mut_repo().transform_descendants(
+        command.settings(),
+        root_commits.iter().ids().cloned().collect_vec(),
+        |mut rewriter| {
+            let paths = commit_paths.get(rewriter.old_commit().id()).unwrap();
+            let old_tree = rewriter.old_commit().tree()?;
+            let mut tree_builder = MergedTreeBuilder::new(old_tree.id().clone());
+            for path in paths {
+                let old_value = old_tree.path_value(path);
+                let new_value = old_value.map(|old_term| {
+                    if let Some(TreeValue::File { id, executable }) = old_term {
+                        if let Some(new_id) = formatted.get(&(path, id)) {
+                            Some(TreeValue::File {
+                                id: new_id.clone(),
+                                executable: *executable,
+                            })
+                        } else {
+                            old_term.clone()
+                        }
+                    } else {
+                        old_term.clone()
+                    }
+                });
+                if new_value != old_value {
+                    tree_builder.set_or_remove(path.clone(), new_value);
+                }
+            }
+            let new_tree = tree_builder.write_tree(rewriter.mut_repo().store())?;
+            let builder = rewriter.reparent(command.settings())?;
+            builder.set_tree_id(new_tree).write()?;
+            Ok(())
+        },
+    )?;
+
+    tx.finish(ui, format!("fixed {} commits", root_commits.len()))
+}
+
+fn format_files<'a>(
+    store: &Store,
+    file_ids: &'a HashSet<(RepoPathBuf, FileId)>,
+) -> BackendResult<HashMap<(&'a RepoPathBuf, &'a FileId), FileId>> {
+    let mut result = HashMap::new();
+    for (path, id) in file_ids {
+        // TODO: read asyncronously
+        let mut read = store.read_file(path, id)?;
+        let mut buf = vec![];
+        read.read_to_end(&mut buf).unwrap();
+        // TODO: Call a formatter instead of just uppercasing
+        for b in &mut buf {
+            b.make_ascii_uppercase();
+        }
+        // TODO: Don't write it if it didn't change
+        let formatted_id = store.write_file(path, &mut buf.as_slice())?;
+        result.insert((path, id), formatted_id);
+    }
+    Ok(result)
+}
diff --git a/cli/src/commands/mod.rs b/cli/src/commands/mod.rs
index 4007ca8259e..7a0af859775 100644
--- a/cli/src/commands/mod.rs
+++ b/cli/src/commands/mod.rs
@@ -29,6 +29,7 @@ mod diffedit;
 mod duplicate;
 mod edit;
 mod files;
+mod fix;
 mod git;
 mod init;
 mod interdiff;
@@ -92,6 +93,8 @@ enum Command {
     Duplicate(duplicate::DuplicateArgs),
     Edit(edit::EditArgs),
     Files(files::FilesArgs),
+    #[command(hide = true)]
+    Fix(fix::FixArgs),
     #[command(subcommand)]
     Git(git::GitCommand),
     Init(init::InitArgs),
@@ -199,6 +202,7 @@ pub fn run_command(ui: &mut Ui, command_helper: &CommandHelper) -> Result<(), Co
         Command::Merge(sub_args) => merge::cmd_merge(ui, command_helper, sub_args),
         Command::Rebase(sub_args) => rebase::cmd_rebase(ui, command_helper, sub_args),
         Command::Backout(sub_args) => backout::cmd_backout(ui, command_helper, sub_args),
+        Command::Fix(sub_args) => fix::cmd_fix(ui, command_helper, sub_args),
         Command::Resolve(sub_args) => resolve::cmd_resolve(ui, command_helper, sub_args),
         Command::Branch(sub_args) => branch::cmd_branch(ui, command_helper, sub_args),
         Command::Undo(sub_args) => operation::cmd_op_undo(ui, command_helper, sub_args),
diff --git a/lib/src/rewrite.rs b/lib/src/rewrite.rs
index cd6190753a0..5e79b87d6ec 100644
--- a/lib/src/rewrite.rs
+++ b/lib/src/rewrite.rs
@@ -262,6 +262,15 @@ impl<'repo> CommitRewriter<'repo> {
         let builder = self.rebase_with_empty_behavior(settings, EmptyBehaviour::Keep)?;
         Ok(builder.unwrap())
     }
+
+    /// Rewrite the old commit onto the new parents without changing its
+    /// contents. Returns a `CommitBuilder` for the new commit.
+    pub fn reparent(self, settings: &UserSettings) -> BackendResult<CommitBuilder<'repo>> {
+        Ok(self
+            .mut_repo
+            .rewrite_commit(settings, &self.old_commit)
+            .set_parents(self.new_parents))
+    }
 }
 
 pub enum RebasedCommit {