diff --git a/Cargo.lock b/Cargo.lock
index 611165099c6e615174f437f0768f00850b43f14f..ccf1edcdbf85a11e5c874813a229ada281071fbc 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -5308,6 +5308,7 @@ dependencies = [
  "smol",
  "sqlez",
  "sqlez_macros",
+ "tempfile",
  "terminal_view",
  "util",
  "wasmtime",
diff --git a/crates/edit_prediction_cli/Cargo.toml b/crates/edit_prediction_cli/Cargo.toml
index 36f264c70ed579865b3af6f25ac1d6690c89603d..874f344865e2517f3622f44ae0e08462cf40df5a 100644
--- a/crates/edit_prediction_cli/Cargo.toml
+++ b/crates/edit_prediction_cli/Cargo.toml
@@ -71,3 +71,4 @@ indoc.workspace = true
 gpui = { workspace = true, features = ["test-support"] }
 project = { workspace = true, features = ["test-support"] }
 pretty_assertions.workspace = true
+tempfile.workspace = true
diff --git a/crates/edit_prediction_cli/src/main.rs b/crates/edit_prediction_cli/src/main.rs
index c12c2ae2aadb1f308da4db9a5682379392287223..e64246a2af7fb278a6ff5f0d6bfa1db6943d64fe 100644
--- a/crates/edit_prediction_cli/src/main.rs
+++ b/crates/edit_prediction_cli/src/main.rs
@@ -14,6 +14,7 @@ mod reorder_patch;
 mod retrieve_context;
 mod score;
 mod split_commit;
+mod split_dataset;
 mod synthesize;
 use clap::{Args, CommandFactory, Parser, Subcommand, ValueEnum};
 use collections::HashSet;
@@ -40,6 +41,7 @@ use crate::progress::Progress;
 use crate::retrieve_context::run_context_retrieval;
 use crate::score::run_scoring;
 use crate::split_commit::SplitCommitArgs;
+use crate::split_dataset::SplitArgs;
 use crate::synthesize::{SynthesizeConfig, run_synthesize};
 
 #[derive(Parser, Debug)]
@@ -124,6 +126,8 @@ enum Command {
     Clean,
     /// Generate an evaluation example by splitting a chronologically-ordered commit
     SplitCommit(SplitCommitArgs),
+    /// Split a JSONL dataset into multiple files (stratified by repository_url if present)
+    Split(SplitArgs),
 }
 
 impl Display for Command {
@@ -178,6 +182,7 @@ impl Display for Command {
             }
             Command::Clean => write!(f, "clean"),
             Command::SplitCommit(_) => write!(f, "split-commit"),
+            Command::Split(_) => write!(f, "split"),
         }
     }
 }
@@ -416,6 +421,13 @@ fn main() {
             }
             return;
         }
+        Command::Split(split_args) => {
+            if let Err(error) = split_dataset::run_split(split_args, &args.inputs) {
+                eprintln!("{error:#}");
+                std::process::exit(1);
+            }
+            return;
+        }
         _ => {}
     }
 
@@ -509,7 +521,8 @@ fn main() {
                                     }
                                     Command::Clean
                                     | Command::Synthesize(_)
-                                    | Command::SplitCommit(_) => {
+                                    | Command::SplitCommit(_)
+                                    | Command::Split(_) => {
                                         unreachable!()
                                     }
                                 }
@@ -603,17 +616,12 @@ async fn handle_error(
         indoc::indoc! {"
             While processing \"{}\":
 
-            {:?}
-
-            Written to: \x1b[36m{}\x1b[0m
-
-            Cursor File: \x1b[36m{}\x1b[0m
-
-            Explore this example data with:
-            fx \x1b[36m{}\x1b[0m
+            \x1b[31m{:?}\x1b[0m
 
-            Re-run this example with:
-            cargo run -p edit_prediction_cli -- {} \x1b[36m{}\x1b[0m
+            Example:        \x1b[36m{}\x1b[0m
+            Error file:     \x1b[36m{}\x1b[0m
+            Cursor file:    \x1b[36m{}\x1b[0m
+            Re-run:         cargo run -p edit_prediction_cli -- {} \x1b[36m{}\x1b[0m
         "},
         example.spec.name,
         error,
diff --git a/crates/edit_prediction_cli/src/split_dataset.rs b/crates/edit_prediction_cli/src/split_dataset.rs
new file mode 100644
index 0000000000000000000000000000000000000000..d4ff39be87d341ec1204bb6f668006050d1afac3
--- /dev/null
+++ b/crates/edit_prediction_cli/src/split_dataset.rs
@@ -0,0 +1,516 @@
+//! `ep split` implementation.
+//!
+//! This command splits a JSONL dataset into multiple files based on size specifications,
+//! with stratification by repository URL (if the field is present).
+//!
+//! # Usage
+//!
+//! ```text
+//! ep split [input.jsonl] <out1>=<size1> <out2>=<size2> ...
+//! ```
+//!
+//! If `input.jsonl` is not provided or is `-`, reads from stdin.
+//!
+//! # Size specifications
+//!
+//! - `80%` - percentage of total (repositories if stratified, examples otherwise)
+//! - `100` - absolute count of repositories (if stratified) or examples
+//! - `rest` - all remaining items (only one split can use this)
+//!
+//! # Stratification
+//!
+//! When examples have a `repository_url` field, the split is stratified by repository.
+//! This ensures each output file contains examples from non-overlapping repositories.
+//! Size specifications apply to the number of repositories, not individual examples.
+//!
+//! Examples without `repository_url` are distributed proportionally across all outputs.
+
+use anyhow::{Context as _, Result, bail};
+use clap::Args;
+use rand::SeedableRng;
+use rand::seq::SliceRandom;
+use serde_json::Value;
+use std::collections::HashMap;
+use std::fs::File;
+use std::io::{self, BufRead, BufReader, BufWriter, Write};
+use std::path::{Path, PathBuf};
+
+/// `ep split` CLI args.
+#[derive(Debug, Args, Clone)]
+#[command(
+    about = "Split a JSONL dataset into multiple files (stratified by repository_url if present)",
+    after_help = r#"SIZE SPECIFICATIONS:
+  <percentage>%    Percentage of total (e.g., 80%)
+  <count>          Absolute number (e.g., 100)
+  rest             All remaining items (only one output can use this)
+
+  When stratifying by repository_url, sizes apply to repositories, not examples.
+
+EXAMPLES:
+  # Split 80% train, 20% validation
+  ep split input.jsonl train.jsonl=80% valid.jsonl=rest
+
+  # Split into train/valid/test
+  ep split input.jsonl train.jsonl=80% valid.jsonl=10% test.jsonl=rest
+
+  # Use absolute counts (100 repos to train, rest to valid)
+  ep split input.jsonl train.jsonl=100 valid.jsonl=rest
+
+  # Read from stdin
+  cat input.jsonl | ep split train.jsonl=80% valid.jsonl=rest
+
+  # Reproducible split with seed
+  ep split --seed 42 input.jsonl train.jsonl=80% valid.jsonl=rest
+
+STRATIFICATION:
+  When examples have a "repository_url" field, the split ensures each output
+  file contains examples from non-overlapping repositories. This prevents
+  data leakage between train/test splits.
+"#
+)]
+pub struct SplitArgs {
+    /// Random seed for reproducibility
+    #[arg(long)]
+    pub seed: Option<u64>,
+}
+
+#[derive(Debug, Clone)]
+pub enum SplitSize {
+    Percentage(f64),
+    Absolute(usize),
+    Rest,
+}
+
+#[derive(Debug, Clone)]
+pub struct SplitSpec {
+    pub path: PathBuf,
+    pub size: SplitSize,
+}
+
+fn parse_split_spec(spec: &str) -> Result<SplitSpec> {
+    let (path, size_str) = spec
+        .rsplit_once('=')
+        .with_context(|| format!("invalid split spec '{}': expected <path>=<size>", spec))?;
+
+    let size = if size_str == "rest" {
+        SplitSize::Rest
+    } else if size_str.ends_with('%') {
+        let pct_str = size_str.trim_end_matches('%');
+        let pct: f64 = pct_str
+            .parse()
+            .with_context(|| format!("invalid percentage '{}' in '{}'", pct_str, spec))?;
+        if !(0.0..=100.0).contains(&pct) {
+            bail!("percentage must be between 0 and 100, got {}", pct);
+        }
+        SplitSize::Percentage(pct / 100.0)
+    } else {
+        let count: usize = size_str
+            .parse()
+            .with_context(|| format!("invalid count '{}' in '{}'", size_str, spec))?;
+        SplitSize::Absolute(count)
+    };
+
+    Ok(SplitSpec {
+        path: PathBuf::from(path),
+        size,
+    })
+}
+
+fn read_lines_from_input(input: Option<&Path>) -> Result<Vec<String>> {
+    let reader: Box<dyn BufRead> = match input {
+        Some(path) => {
+            let file =
+                File::open(path).with_context(|| format!("failed to open '{}'", path.display()))?;
+            Box::new(BufReader::new(file))
+        }
+        None => Box::new(BufReader::new(io::stdin())),
+    };
+
+    let lines: Vec<String> = reader
+        .lines()
+        .collect::<io::Result<Vec<_>>>()
+        .context("failed to read input lines")?;
+
+    Ok(lines)
+}
+
+fn get_repository_url(line: &str) -> Option<String> {
+    let value: Value = serde_json::from_str(line).ok()?;
+    value
+        .get("repository_url")
+        .and_then(|v| v.as_str())
+        .map(|s| s.to_string())
+}
+
+fn group_lines_by_repo(lines: Vec<String>) -> (HashMap<String, Vec<String>>, Vec<String>) {
+    let mut by_repo: HashMap<String, Vec<String>> = HashMap::new();
+    let mut without_repo: Vec<String> = Vec::new();
+
+    for line in lines {
+        if let Some(repo_url) = get_repository_url(&line) {
+            by_repo.entry(repo_url).or_default().push(line);
+        } else {
+            without_repo.push(line);
+        }
+    }
+
+    (by_repo, without_repo)
+}
+
+fn compute_split_counts(specs: &[SplitSpec], total: usize) -> Result<Vec<usize>> {
+    let mut counts = vec![0usize; specs.len()];
+    let mut remaining = total;
+    let mut rest_index: Option<usize> = None;
+
+    for (i, spec) in specs.iter().enumerate() {
+        match &spec.size {
+            SplitSize::Percentage(pct) => {
+                let count = (total as f64 * pct).round() as usize;
+                counts[i] = count.min(remaining);
+                remaining = remaining.saturating_sub(counts[i]);
+            }
+            SplitSize::Absolute(count) => {
+                counts[i] = (*count).min(remaining);
+                remaining = remaining.saturating_sub(counts[i]);
+            }
+            SplitSize::Rest => {
+                if rest_index.is_some() {
+                    bail!("only one split can use 'rest'");
+                }
+                rest_index = Some(i);
+            }
+        }
+    }
+
+    if let Some(idx) = rest_index {
+        counts[idx] = remaining;
+    }
+
+    Ok(counts)
+}
+
+fn write_lines_to_file(path: &Path, lines: &[String]) -> Result<()> {
+    if let Some(parent) = path.parent() {
+        if !parent.as_os_str().is_empty() {
+            std::fs::create_dir_all(parent)
+                .with_context(|| format!("failed to create directory '{}'", parent.display()))?;
+        }
+    }
+
+    let file =
+        File::create(path).with_context(|| format!("failed to create '{}'", path.display()))?;
+    let mut writer = BufWriter::new(file);
+
+    for line in lines {
+        writeln!(writer, "{}", line)
+            .with_context(|| format!("failed to write to '{}'", path.display()))?;
+    }
+
+    writer
+        .flush()
+        .with_context(|| format!("failed to flush '{}'", path.display()))?;
+
+    Ok(())
+}
+
+pub fn run_split(args: &SplitArgs, inputs: &[PathBuf]) -> Result<()> {
+    if inputs.is_empty() {
+        bail!("usage: ep split [input.jsonl] train.jsonl=80% valid.jsonl=rest");
+    }
+
+    let (input_path, split_specs_raw): (Option<&Path>, &[PathBuf]) =
+        if inputs.first().is_some_and(|p| {
+            let s = p.to_string_lossy();
+            !s.contains('=')
+        }) {
+            let first = inputs.first().map(|p| p.as_path());
+            let first = if first == Some(Path::new("-")) {
+                None
+            } else {
+                first
+            };
+            (first, &inputs[1..])
+        } else {
+            (None, inputs)
+        };
+
+    if split_specs_raw.is_empty() {
+        bail!("no split specifications provided");
+    }
+
+    let specs: Vec<SplitSpec> = split_specs_raw
+        .iter()
+        .map(|p| parse_split_spec(&p.to_string_lossy()))
+        .collect::<Result<Vec<_>>>()?;
+
+    let lines = read_lines_from_input(input_path)?;
+    let total_lines = lines.len();
+
+    if total_lines == 0 {
+        for spec in &specs {
+            write_lines_to_file(&spec.path, &[])?;
+        }
+        return Ok(());
+    }
+
+    let (by_repo, without_repo) = group_lines_by_repo(lines);
+    let has_repos = !by_repo.is_empty();
+
+    if has_repos {
+        eprintln!(
+            "Stratifying by repository_url ({} unique repositories, {} examples)",
+            by_repo.len(),
+            total_lines - without_repo.len()
+        );
+        if !without_repo.is_empty() {
+            eprintln!(
+                "  + {} examples without repository_url (distributed proportionally)",
+                without_repo.len()
+            );
+        }
+    }
+
+    let mut rng = match args.seed {
+        Some(seed) => rand::rngs::StdRng::seed_from_u64(seed),
+        None => rand::rngs::StdRng::from_os_rng(),
+    };
+
+    let mut split_outputs: Vec<Vec<String>> = vec![Vec::new(); specs.len()];
+
+    if has_repos {
+        let mut repos: Vec<String> = by_repo.keys().cloned().collect();
+        repos.shuffle(&mut rng);
+
+        let repo_counts = compute_split_counts(&specs, repos.len())?;
+
+        let mut repo_iter = repos.into_iter();
+        for (split_idx, &count) in repo_counts.iter().enumerate() {
+            for _ in 0..count {
+                if let Some(repo) = repo_iter.next() {
+                    if let Some(repo_lines) = by_repo.get(&repo) {
+                        split_outputs[split_idx].extend(repo_lines.iter().cloned());
+                    }
+                }
+            }
+        }
+
+        if !without_repo.is_empty() {
+            let no_repo_counts = compute_split_counts(&specs, without_repo.len())?;
+            let mut no_repo_shuffled = without_repo;
+            no_repo_shuffled.shuffle(&mut rng);
+
+            let mut line_iter = no_repo_shuffled.into_iter();
+            for (split_idx, &count) in no_repo_counts.iter().enumerate() {
+                for _ in 0..count {
+                    if let Some(line) = line_iter.next() {
+                        split_outputs[split_idx].push(line);
+                    }
+                }
+            }
+        }
+    } else {
+        let line_counts = compute_split_counts(&specs, total_lines)?;
+        let mut shuffled_lines = without_repo;
+        shuffled_lines.shuffle(&mut rng);
+
+        let mut line_iter = shuffled_lines.into_iter();
+        for (split_idx, &count) in line_counts.iter().enumerate() {
+            for _ in 0..count {
+                if let Some(line) = line_iter.next() {
+                    split_outputs[split_idx].push(line);
+                }
+            }
+        }
+    }
+
+    for (spec, output_lines) in specs.iter().zip(split_outputs.iter()) {
+        write_lines_to_file(&spec.path, output_lines)?;
+        eprintln!("{}: {} examples", spec.path.display(), output_lines.len());
+    }
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::io::Write;
+    use tempfile::NamedTempFile;
+
+    fn create_temp_jsonl(lines: &[&str]) -> NamedTempFile {
+        let mut file = NamedTempFile::new().unwrap();
+        for line in lines {
+            writeln!(file, "{}", line).unwrap();
+        }
+        file.flush().unwrap();
+        file
+    }
+
+    #[test]
+    fn test_parse_split_spec_percentage() {
+        let spec = parse_split_spec("train.jsonl=80%").unwrap();
+        assert_eq!(spec.path, PathBuf::from("train.jsonl"));
+        match spec.size {
+            SplitSize::Percentage(p) => assert!((p - 0.8).abs() < 0.001),
+            _ => panic!("expected percentage"),
+        }
+    }
+
+    #[test]
+    fn test_parse_split_spec_absolute() {
+        let spec = parse_split_spec("test.jsonl=100").unwrap();
+        assert_eq!(spec.path, PathBuf::from("test.jsonl"));
+        match spec.size {
+            SplitSize::Absolute(n) => assert_eq!(n, 100),
+            _ => panic!("expected absolute"),
+        }
+    }
+
+    #[test]
+    fn test_parse_split_spec_rest() {
+        let spec = parse_split_spec("valid.jsonl=rest").unwrap();
+        assert_eq!(spec.path, PathBuf::from("valid.jsonl"));
+        assert!(matches!(spec.size, SplitSize::Rest));
+    }
+
+    #[test]
+    fn test_get_repository_url() {
+        let line = r#"{"repository_url": "https://github.com/example/repo", "data": 123}"#;
+        assert_eq!(
+            get_repository_url(line),
+            Some("https://github.com/example/repo".to_string())
+        );
+
+        let line_no_repo = r#"{"data": 123}"#;
+        assert_eq!(get_repository_url(line_no_repo), None);
+    }
+
+    #[test]
+    fn test_compute_split_counts_percentage() {
+        let specs = vec![
+            SplitSpec {
+                path: PathBuf::from("a"),
+                size: SplitSize::Percentage(0.8),
+            },
+            SplitSpec {
+                path: PathBuf::from("b"),
+                size: SplitSize::Percentage(0.2),
+            },
+        ];
+        let counts = compute_split_counts(&specs, 100).unwrap();
+        assert_eq!(counts, vec![80, 20]);
+    }
+
+    #[test]
+    fn test_compute_split_counts_with_rest() {
+        let specs = vec![
+            SplitSpec {
+                path: PathBuf::from("a"),
+                size: SplitSize::Percentage(0.8),
+            },
+            SplitSpec {
+                path: PathBuf::from("b"),
+                size: SplitSize::Rest,
+            },
+        ];
+        let counts = compute_split_counts(&specs, 100).unwrap();
+        assert_eq!(counts, vec![80, 20]);
+    }
+
+    #[test]
+    fn test_compute_split_counts_absolute() {
+        let specs = vec![
+            SplitSpec {
+                path: PathBuf::from("a"),
+                size: SplitSize::Absolute(50),
+            },
+            SplitSpec {
+                path: PathBuf::from("b"),
+                size: SplitSize::Rest,
+            },
+        ];
+        let counts = compute_split_counts(&specs, 100).unwrap();
+        assert_eq!(counts, vec![50, 50]);
+    }
+
+    #[test]
+    fn test_group_lines_by_repo() {
+        let lines = vec![
+            r#"{"repository_url": "repo1", "id": 1}"#.to_string(),
+            r#"{"repository_url": "repo1", "id": 2}"#.to_string(),
+            r#"{"repository_url": "repo2", "id": 3}"#.to_string(),
+            r#"{"id": 4}"#.to_string(),
+        ];
+
+        let (by_repo, without_repo) = group_lines_by_repo(lines);
+
+        assert_eq!(by_repo.len(), 2);
+        assert_eq!(by_repo.get("repo1").unwrap().len(), 2);
+        assert_eq!(by_repo.get("repo2").unwrap().len(), 1);
+        assert_eq!(without_repo.len(), 1);
+    }
+
+    #[test]
+    fn test_run_split_basic() {
+        let input = create_temp_jsonl(&[
+            r#"{"repository_url": "repo1", "id": 1}"#,
+            r#"{"repository_url": "repo1", "id": 2}"#,
+            r#"{"repository_url": "repo2", "id": 3}"#,
+            r#"{"repository_url": "repo2", "id": 4}"#,
+            r#"{"repository_url": "repo3", "id": 5}"#,
+            r#"{"repository_url": "repo3", "id": 6}"#,
+            r#"{"repository_url": "repo4", "id": 7}"#,
+            r#"{"repository_url": "repo4", "id": 8}"#,
+        ]);
+
+        let temp_dir = tempfile::tempdir().unwrap();
+        let train_path = temp_dir.path().join("train.jsonl");
+        let valid_path = temp_dir.path().join("valid.jsonl");
+
+        let args = SplitArgs { seed: Some(42) };
+        let inputs = vec![
+            input.path().to_path_buf(),
+            PathBuf::from(format!("{}=50%", train_path.display())),
+            PathBuf::from(format!("{}=rest", valid_path.display())),
+        ];
+
+        run_split(&args, &inputs).unwrap();
+
+        let train_content = std::fs::read_to_string(&train_path).unwrap();
+        let valid_content = std::fs::read_to_string(&valid_path).unwrap();
+
+        let train_lines: Vec<&str> = train_content.lines().collect();
+        let valid_lines: Vec<&str> = valid_content.lines().collect();
+
+        assert_eq!(train_lines.len() + valid_lines.len(), 8);
+
+        let train_repos: std::collections::HashSet<_> = train_lines
+            .iter()
+            .filter_map(|l| get_repository_url(l))
+            .collect();
+        let valid_repos: std::collections::HashSet<_> = valid_lines
+            .iter()
+            .filter_map(|l| get_repository_url(l))
+            .collect();
+
+        assert!(
+            train_repos.is_disjoint(&valid_repos),
+            "train and valid should have non-overlapping repos"
+        );
+    }
+
+    #[test]
+    fn test_multiple_rest_fails() {
+        let specs = vec![
+            SplitSpec {
+                path: PathBuf::from("a"),
+                size: SplitSize::Rest,
+            },
+            SplitSpec {
+                path: PathBuf::from("b"),
+                size: SplitSize::Rest,
+            },
+        ];
+        assert!(compute_split_counts(&specs, 100).is_err());
+    }
+}