diff --git a/lib/crates/arc-cli/src/main.rs b/lib/crates/arc-cli/src/main.rs
index 1b902f0ff..a979eec30 100644
--- a/lib/crates/arc-cli/src/main.rs
+++ b/lib/crates/arc-cli/src/main.rs
@@ -78,6 +78,11 @@ enum Command {
     Setup,
     /// List workflow runs
     Ps(arc_workflows::cli::runs::RunsListArgs),
+    /// Pull request operations
+    Pr {
+        #[command(subcommand)]
+        command: PrCommand,
+    },
     /// System maintenance commands
     System {
         #[command(subcommand)]
@@ -85,6 +90,12 @@ enum Command {
     },
 }
 
+#[derive(Subcommand)]
+enum PrCommand {
+    /// Create a pull request from a completed run
+    Create(arc_workflows::cli::pr::PrCreateArgs),
+}
+
 #[derive(Subcommand)]
 enum SystemCommand {
     /// Delete old workflow runs
@@ -141,6 +152,7 @@ async fn main() -> Result<()> {
         Command::Doctor { .. } => "doctor",
         Command::Setup => "setup",
         Command::Ps(_) => "ps",
+        Command::Pr { .. } => "pr",
         Command::System { .. } => "system",
     };
 
@@ -331,6 +343,13 @@ async fn main() -> Result<()> {
         Command::Ps(args) => {
             arc_workflows::cli::runs::list_command(&args)?;
         }
+        Command::Pr { command } => match command {
+            PrCommand::Create(args) => {
+                let server_config = arc_api::server_config::load_server_config(None)?;
+                let github_app = build_github_app_credentials(&server_config);
+                arc_workflows::cli::pr::pr_create_command(args, github_app).await?;
+            }
+        },
         Command::System { command } => match command {
             SystemCommand::Prune(args) => {
                 arc_workflows::cli::runs::prune_command(&args)?;
diff --git a/lib/crates/arc-github/src/lib.rs b/lib/crates/arc-github/src/lib.rs
index 9f5b0f60a..b13cd81e9 100644
--- a/lib/crates/arc-github/src/lib.rs
+++ b/lib/crates/arc-github/src/lib.rs
@@ -313,6 +313,41 @@ pub fn ssh_url_to_https(url: &str) -> String {
     url.to_string()
 }
 
+/// Check whether a branch exists in a GitHub repository.
+///
+/// Uses a GitHub App installation token to query the branches API.
+/// Returns `true` if the branch exists, `false` if it doesn't (404).
+pub async fn branch_exists(
+    creds: &GitHubAppCredentials,
+    owner: &str,
+    repo: &str,
+    branch: &str,
+    base_url: &str,
+) -> Result<bool, String> {
+    let jwt = sign_app_jwt(&creds.app_id, &creds.private_key_pem)?;
+    let client = reqwest::Client::new();
+
+    let token = create_installation_access_token(&client, &jwt, owner, repo, base_url).await?;
+
+    let url = format!("{base_url}/repos/{owner}/{repo}/branches/{branch}");
+    let resp = client
+        .get(&url)
+        .header("Authorization", format!("Bearer {token}"))
+        .header("Accept", "application/vnd.github+json")
+        .header("User-Agent", "arc")
+        .send()
+        .await
+        .map_err(|e| format!("Failed to check branch existence: {e}"))?;
+
+    match resp.status().as_u16() {
+        200 => Ok(true),
+        404 => Ok(false),
+        status => Err(format!(
+            "Unexpected status {status} checking branch '{branch}'"
+        )),
+    }
+}
+
 /// Resolve git clone credentials for a GitHub repository.
 ///
 /// Returns `(username, password)` for authenticated cloning.
@@ -658,4 +693,102 @@ mod tests {
 
         token_mock.assert_async().await;
     }
+
+    // -----------------------------------------------------------------------
+    // branch_exists
+    // -----------------------------------------------------------------------
+
+    #[tokio::test]
+    async fn branch_exists_returns_true_on_200() {
+        let mut server = mockito::Server::new_async().await;
+
+        server
+            .mock("GET", "/repos/owner/repo/installation")
+            .with_status(200)
+            .with_body(r#"{"id": 1}"#)
+            .create_async()
+            .await;
+        server
+            .mock("POST", "/app/installations/1/access_tokens")
+            .with_status(201)
+            .with_body(r#"{"token": "ghs_test"}"#)
+            .create_async()
+            .await;
+        server
+            .mock("GET", "/repos/owner/repo/branches/my-branch")
+            .with_status(200)
+            .with_body(r#"{"name": "my-branch"}"#)
+            .create_async()
+            .await;
+
+        let pem = test_rsa_key();
+        let creds = GitHubAppCredentials {
+            app_id: "test".to_string(),
+            private_key_pem: pem,
+        };
+        let result = branch_exists(&creds, "owner", "repo", "my-branch", &server.url()).await;
+        assert_eq!(result.unwrap(), true);
+    }
+
+    #[tokio::test]
+    async fn branch_exists_returns_false_on_404() {
+        let mut server = mockito::Server::new_async().await;
+
+        server
+            .mock("GET", "/repos/owner/repo/installation")
+            .with_status(200)
+            .with_body(r#"{"id": 1}"#)
+            .create_async()
+            .await;
+        server
+            .mock("POST", "/app/installations/1/access_tokens")
+            .with_status(201)
+            .with_body(r#"{"token": "ghs_test"}"#)
+            .create_async()
+            .await;
+        server
+            .mock("GET", "/repos/owner/repo/branches/no-such-branch")
+            .with_status(404)
+            .create_async()
+            .await;
+
+        let pem = test_rsa_key();
+        let creds = GitHubAppCredentials {
+            app_id: "test".to_string(),
+            private_key_pem: pem,
+        };
+        let result = branch_exists(&creds, "owner", "repo", "no-such-branch", &server.url()).await;
+        assert_eq!(result.unwrap(), false);
+    }
+
+    #[tokio::test]
+    async fn branch_exists_returns_error_on_500() {
+        let mut server = mockito::Server::new_async().await;
+
+        server
+            .mock("GET", "/repos/owner/repo/installation")
+            .with_status(200)
+            .with_body(r#"{"id": 1}"#)
+            .create_async()
+            .await;
+        server
+            .mock("POST", "/app/installations/1/access_tokens")
+            .with_status(201)
+            .with_body(r#"{"token": "ghs_test"}"#)
+            .create_async()
+            .await;
+        server
+            .mock("GET", "/repos/owner/repo/branches/broken")
+            .with_status(500)
+            .create_async()
+            .await;
+
+        let pem = test_rsa_key();
+        let creds = GitHubAppCredentials {
+            app_id: "test".to_string(),
+            private_key_pem: pem,
+        };
+        let result = branch_exists(&creds, "owner", "repo", "broken", &server.url()).await;
+        assert!(result.is_err());
+    }
 }
diff --git a/lib/crates/arc-workflows/src/cli/cp.rs b/lib/crates/arc-workflows/src/cli/cp.rs
index b0f99d34f..99db75987 100644
--- a/lib/crates/arc-workflows/src/cli/cp.rs
+++ b/lib/crates/arc-workflows/src/cli/cp.rs
@@ -2,9 +2,9 @@ use std::path::{Path, PathBuf};
 
 use anyhow::{bail, Context, Result};
 use clap::Args;
-use tracing::{debug, info, warn};
+use tracing::{debug, info};
 
-use crate::cli::runs::{default_logs_base, scan_runs};
+use crate::cli::runs::{default_logs_base, find_run_by_prefix};
 use crate::sandbox_record::SandboxRecord;
 
 #[derive(Args)]
@@ -73,34 +73,6 @@ fn split_run_path(s: &str) -> Option<(&str, &str)> {
     s.split_once(':')
 }
 
-/// Find a run directory by prefix match against run IDs.
-fn find_run_by_prefix(base: &Path, prefix: &str) -> Result<PathBuf> {
-    let runs = scan_runs(base).context("Failed to scan runs")?;
-    let matches: Vec<_> = runs
-        .iter()
-        .filter(|r| r.run_id.starts_with(prefix))
-        .collect();
-
-    match matches.len() {
-        0 => {
-            warn!(run_id = %prefix, "No matching run found");
-            bail!("No run found matching prefix '{prefix}'")
-        }
-        1 => {
-            let run = &matches[0];
-            debug!(run_id = %prefix, matched = %run.run_id, "Resolved run by prefix");
-            Ok(run.path.clone())
-        }
-        n => {
-            let ids: Vec<&str> = matches.iter().map(|r| r.run_id.as_str()).collect();
-            bail!(
-                "Ambiguous prefix '{prefix}': {n} runs match: {}",
-                ids.join(", ")
-            )
-        }
-    }
-}
-
 /// Reconnect to a sandbox from a saved record.
 ///
 /// Returns a sandbox that can perform file operations.
@@ -396,63 +368,4 @@ mod tests {
             _ => panic!("Expected Upload"),
         }
     }
-
-    #[test]
-    fn find_run_by_prefix_no_match() {
-        let dir = tempfile::tempdir().unwrap();
-        let result = find_run_by_prefix(dir.path(), "nonexistent");
-        assert!(result.is_err());
-    }
-
-    #[test]
-    fn find_run_by_prefix_single_match() {
-        let dir = tempfile::tempdir().unwrap();
-        let run_dir = dir.path().join("20260101-ABC123");
-        std::fs::create_dir_all(&run_dir).unwrap();
-        std::fs::write(
-            run_dir.join("manifest.json"),
-            serde_json::to_string_pretty(&serde_json::json!({
-                "run_id": "abc123-full-id",
-                "workflow_name": "test",
-                "goal": "",
-                "start_time": "2026-01-01T12:00:00Z",
-                "node_count": 1,
-                "edge_count": 0
-            }))
-            .unwrap(),
-        )
-        .unwrap();
-
-        let result = find_run_by_prefix(dir.path(), "abc123").unwrap();
-        assert_eq!(result, run_dir);
-    }
-
-    #[test]
-    fn find_run_by_prefix_ambiguous() {
-        let dir = tempfile::tempdir().unwrap();
-        for (subdir, run_id) in [("d1", "abc-111"), ("d2", "abc-222")] {
-            let run_dir = dir.path().join(subdir);
-            std::fs::create_dir_all(&run_dir).unwrap();
-            std::fs::write(
-                run_dir.join("manifest.json"),
-                serde_json::to_string_pretty(&serde_json::json!({
-                    "run_id": run_id,
-                    "workflow_name": "test",
-                    "goal": "",
-                    "start_time": "2026-01-01T12:00:00Z",
-                    "node_count": 1,
-                    "edge_count": 0
-                }))
-                .unwrap(),
-            )
-            .unwrap();
-        }
-
-        let result = find_run_by_prefix(dir.path(), "abc");
-        assert!(result.is_err());
-        assert!(
-            result.unwrap_err().to_string().contains("Ambiguous"),
-            "Should mention ambiguity"
-        );
-    }
 }
diff --git a/lib/crates/arc-workflows/src/cli/mod.rs b/lib/crates/arc-workflows/src/cli/mod.rs
index 8be9cb766..7efe228f2 100644
--- a/lib/crates/arc-workflows/src/cli/mod.rs
+++ b/lib/crates/arc-workflows/src/cli/mod.rs
@@ -2,6 +2,7 @@ pub mod backend;
 pub mod cli_backend;
 pub mod cp;
 pub mod parse;
+pub mod pr;
 pub mod progress;
 pub mod run;
 pub mod run_config;
diff --git a/lib/crates/arc-workflows/src/cli/pr.rs b/lib/crates/arc-workflows/src/cli/pr.rs
new file mode 100644
index 000000000..31f4fcfe0
--- /dev/null
+++ b/lib/crates/arc-workflows/src/cli/pr.rs
@@ -0,0 +1,336 @@
+use std::path::Path;
+
+use anyhow::{bail, Context, Result};
+use clap::Args;
+use tracing::info;
+
+use crate::cli::runs::{default_logs_base, find_run_by_prefix};
+use crate::conclusion::Conclusion;
+use crate::manifest::Manifest;
+use crate::outcome::StageStatus;
+
+#[derive(Args)]
+pub struct PrCreateArgs {
+    /// Run ID or prefix
+    pub run_id: String,
+    /// LLM model for generating PR description
+    #[arg(long)]
+    pub model: Option<String>,
+}
+
+pub async fn pr_create_command(
+    args: PrCreateArgs,
+    github_app: Option<arc_github::GitHubAppCredentials>,
+) -> Result<()> {
+    let base = default_logs_base();
+    pr_create_from(&base, args, github_app).await
+}
+
+async fn pr_create_from(
+    base: &Path,
+    args: PrCreateArgs,
+    github_app: Option<arc_github::GitHubAppCredentials>,
+) -> Result<()> {
+    let run_dir = find_run_by_prefix(base, &args.run_id)?;
+
+    let manifest =
+        Manifest::load(&run_dir.join("manifest.json")).context("Failed to load manifest.json")?;
+
+    let conclusion = Conclusion::load(&run_dir.join("conclusion.json"))
+        .context("Failed to load conclusion.json — is the run finished?")?;
+
+    match conclusion.status {
+        StageStatus::Success | StageStatus::PartialSuccess => {}
+        status => bail!("Run status is '{status}', expected success or partial_success"),
+    }
+
+    let run_branch = manifest
+        .run_branch
+        .as_deref()
+        .context("Run has no run_branch — was it run with git push enabled?")?;
+
+    let diff = std::fs::read_to_string(run_dir.join("final.patch"))
+        .context("Failed to read final.patch — no diff available")?;
+    if diff.trim().is_empty() {
+        bail!("final.patch is empty — nothing to create a PR for");
+    }
+
+    let cwd = std::env::current_dir().context("Failed to get current directory")?;
+    let (origin_url, detected_branch) =
+        crate::daytona_sandbox::detect_repo_info(&cwd).map_err(|e| anyhow::anyhow!("{e}"))?;
+
+    let base_branch = manifest
+        .base_branch
+        .as_deref()
+        .or(detected_branch.as_deref())
+        .unwrap_or("main");
+
+    let https_url = arc_github::ssh_url_to_https(&origin_url);
+    let (owner, repo) =
+        arc_github::parse_github_owner_repo(&https_url).map_err(|e| anyhow::anyhow!("{e}"))?;
+
+    let creds = github_app.context(
+        "GitHub App credentials required — set GITHUB_APP_PRIVATE_KEY and configure app_id",
+    )?;
+
+    let branch_found =
+        arc_github::branch_exists(&creds, &owner, &repo, run_branch, "https://api.github.com")
+            .await
+            .map_err(|e| anyhow::anyhow!("{e}"))?;
+
+    if !branch_found {
+        bail!(
+            "Branch '{run_branch}' not found on GitHub. \
+             Was it pushed? Try: git push origin {run_branch}"
+        );
+    }
+
+    let model = args
+        .model
+        .unwrap_or_else(|| arc_llm::catalog::default_model().id.to_string());
+
+    let url = crate::pull_request::maybe_open_pull_request(
+        &creds,
+        &origin_url,
+        base_branch,
+        run_branch,
+        &manifest.goal,
+        &diff,
+        &model,
+    )
+    .await
+    .map_err(|e| anyhow::anyhow!("{e}"))?;
+
+    match url {
+        Some(url) => {
+            info!(pr_url = %url, "Pull request created");
+            println!("{url}");
+        }
+        None => {
+            println!("No pull request created (empty diff).");
+        }
+    }
+
+    Ok(())
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::fs;
+
+    fn make_test_run(
+        base: &Path,
+        manifest_json: serde_json::Value,
+        conclusion_json: Option<serde_json::Value>,
+        diff: Option<&str>,
+    ) -> String {
+        let run_id = manifest_json["run_id"].as_str().unwrap();
+        let dir_name = format!("20260101-{}", &run_id[..6].to_uppercase());
+        let run_dir = base.join(&dir_name);
+        fs::create_dir_all(&run_dir).unwrap();
+        fs::write(
+            run_dir.join("manifest.json"),
+            serde_json::to_string_pretty(&manifest_json).unwrap(),
+        )
+        .unwrap();
+        if let Some(c) = conclusion_json {
+            fs::write(
+                run_dir.join("conclusion.json"),
+                serde_json::to_string_pretty(&c).unwrap(),
+            )
+            .unwrap();
+        }
+        if let Some(d) = diff {
+            fs::write(run_dir.join("final.patch"), d).unwrap();
+        }
+        run_id.to_string()
+    }
+
+    #[tokio::test]
+    async fn pr_create_fails_missing_conclusion() {
+        let tmp = tempfile::tempdir().unwrap();
+        let run_id = make_test_run(
+            tmp.path(),
+            serde_json::json!({
+                "run_id": "abc123-test",
+                "workflow_name": "test",
+                "goal": "fix bug",
+                "start_time": "2026-01-01T12:00:00Z",
+                "node_count": 1,
+                "edge_count": 0,
+                "run_branch": "arc/run/abc123"
+            }),
+            None,
+            Some("diff content"),
+        );
+
+        let args = PrCreateArgs {
+            run_id,
+            model: None,
+        };
+        let result = pr_create_from(tmp.path(), args, None).await;
+        assert!(result.is_err());
+        let err = result.unwrap_err().to_string();
+        assert!(err.contains("conclusion"), "got: {err}");
+    }
+
+    #[tokio::test]
+    async fn pr_create_fails_on_failed_run() {
+        let tmp = tempfile::tempdir().unwrap();
+        let run_id = make_test_run(
+            tmp.path(),
+            serde_json::json!({
+                "run_id": "abc123-test",
+                "workflow_name": "test",
+                "goal": "fix bug",
+                "start_time": "2026-01-01T12:00:00Z",
+                "node_count": 1,
+                "edge_count": 0,
+                "run_branch": "arc/run/abc123"
+            }),
+            Some(serde_json::json!({
+                "timestamp": "2026-01-01T12:01:00Z",
+                "status": "fail",
+                "duration_ms": 60000
+            })),
+            Some("diff content"),
+        );
+
+        let args = PrCreateArgs {
+            run_id,
+            model: None,
+        };
+        let result = pr_create_from(tmp.path(), args, None).await;
+        assert!(result.is_err());
+        let err = result.unwrap_err().to_string();
+        assert!(err.contains("fail"), "got: {err}");
+    }
+
+    #[tokio::test]
+    async fn pr_create_fails_missing_run_branch() {
+        let tmp = tempfile::tempdir().unwrap();
+        let run_id = make_test_run(
+            tmp.path(),
+            serde_json::json!({
+                "run_id": "abc123-test",
+                "workflow_name": "test",
+                "goal": "fix bug",
+                "start_time": "2026-01-01T12:00:00Z",
+                "node_count": 1,
+                "edge_count": 0
+            }),
+            Some(serde_json::json!({
+                "timestamp": "2026-01-01T12:01:00Z",
+                "status": "success",
+                "duration_ms": 60000
+            })),
+            Some("diff content"),
+        );
+
+        let args = PrCreateArgs {
+            run_id,
+            model: None,
+        };
+        let result = pr_create_from(tmp.path(), args, None).await;
+        assert!(result.is_err());
+        let err = result.unwrap_err().to_string();
+        assert!(err.contains("run_branch"), "got: {err}");
+    }
+
+    #[tokio::test]
+    async fn pr_create_fails_missing_diff() {
+        let tmp = tempfile::tempdir().unwrap();
+        let run_id = make_test_run(
+            tmp.path(),
+            serde_json::json!({
+                "run_id": "abc123-test",
+                "workflow_name": "test",
+                "goal": "fix bug",
+                "start_time": "2026-01-01T12:00:00Z",
+                "node_count": 1,
+                "edge_count": 0,
+                "run_branch": "arc/run/abc123"
+            }),
+            Some(serde_json::json!({
+                "timestamp": "2026-01-01T12:01:00Z",
+                "status": "success",
+                "duration_ms": 60000
+            })),
+            None,
+        );
+
+        let args = PrCreateArgs {
+            run_id,
+            model: None,
+        };
+        let result = pr_create_from(tmp.path(), args, None).await;
+        assert!(result.is_err());
+        let err = result.unwrap_err().to_string();
+        assert!(err.contains("final.patch"), "got: {err}");
+    }
+
+    #[tokio::test]
+    async fn pr_create_fails_empty_diff() {
+        let tmp = tempfile::tempdir().unwrap();
+        let run_id = make_test_run(
+            tmp.path(),
+            serde_json::json!({
+                "run_id": "abc123-test",
+                "workflow_name": "test",
+                "goal": "fix bug",
+                "start_time": "2026-01-01T12:00:00Z",
+                "node_count": 1,
+                "edge_count": 0,
+                "run_branch": "arc/run/abc123"
+            }),
+            Some(serde_json::json!({
+                "timestamp": "2026-01-01T12:01:00Z",
+                "status": "success",
+                "duration_ms": 60000
+            })),
+            Some("  \n  "),
+        );
+
+        let args = PrCreateArgs {
+            run_id,
+            model: None,
+        };
+        let result = pr_create_from(tmp.path(), args, None).await;
+        assert!(result.is_err());
+        let err = result.unwrap_err().to_string();
+        assert!(err.contains("empty"), "got: {err}");
+    }
+
+    #[tokio::test]
+    async fn pr_create_fails_missing_github_creds() {
+        let tmp = tempfile::tempdir().unwrap();
+        let run_id = make_test_run(
+            tmp.path(),
+            serde_json::json!({
+                "run_id": "abc123-test",
+                "workflow_name": "test",
+                "goal": "fix bug",
+                "start_time": "2026-01-01T12:00:00Z",
+                "node_count": 1,
+                "edge_count": 0,
+                "run_branch": "arc/run/abc123"
+            }),
+            Some(serde_json::json!({
+                "timestamp": "2026-01-01T12:01:00Z",
+                "status": "success",
+                "duration_ms": 60000
+            })),
+            Some("diff content"),
+        );
+
+        let args = PrCreateArgs {
+            run_id,
+            model: None,
+        };
+        let result = pr_create_from(tmp.path(), args, None).await;
+        assert!(result.is_err());
+        let err = result.unwrap_err().to_string();
+        assert!(err.contains("GitHub App"), "got: {err}");
+    }
+}
diff --git a/lib/crates/arc-workflows/src/cli/runs.rs b/lib/crates/arc-workflows/src/cli/runs.rs
index afde94821..7ae94d749 100644
--- a/lib/crates/arc-workflows/src/cli/runs.rs
+++ b/lib/crates/arc-workflows/src/cli/runs.rs
@@ -1,10 +1,10 @@
 use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 
-use anyhow::Result;
+use anyhow::{bail, Context, Result};
 use clap::Args;
 use serde::Serialize;
-use tracing::{debug, info};
+use tracing::{debug, info, warn};
 
 #[derive(Args)]
 pub struct RunFilterArgs {
@@ -190,6 +190,34 @@ pub(crate) fn default_logs_base() -> PathBuf {
         .join("logs")
 }
 
+/// Find a run directory by prefix match against run IDs.
+pub fn find_run_by_prefix(base: &Path, prefix: &str) -> Result<PathBuf> {
+    let runs = scan_runs(base).context("Failed to scan runs")?;
+    let matches: Vec<_> = runs
+        .iter()
+        .filter(|r| r.run_id.starts_with(prefix))
+        .collect();
+
+    match matches.len() {
+        0 => {
+            warn!(run_id = %prefix, "No matching run found");
+            bail!("No run found matching prefix '{prefix}'")
+        }
+        1 => {
+            let run = &matches[0];
+            debug!(run_id = %prefix, matched = %run.run_id, "Resolved run by prefix");
+            Ok(run.path.clone())
+        }
+        n => {
+            let ids: Vec<&str> = matches.iter().map(|r| r.run_id.as_str()).collect();
+            bail!(
+                "Ambiguous prefix '{prefix}': {n} runs match: {}",
+                ids.join(", ")
+            )
+        }
+    }
+}
+
 pub fn list_command(args: &RunsListArgs) -> Result<()> {
     let base = default_logs_base();
     let runs = scan_runs(&base)?;
@@ -660,4 +688,64 @@ mod tests {
         let result = parse_label_filters(&args);
         assert!(result.is_empty());
     }
+
+    #[test]
+    fn find_run_by_prefix_no_match() {
+        let dir = tempfile::tempdir().unwrap();
+        let result = find_run_by_prefix(dir.path(), "nonexistent");
+        assert!(result.is_err());
+    }
+
+    #[test]
+    fn find_run_by_prefix_single_match() {
+        let dir = tempfile::tempdir().unwrap();
+        let run_dir = dir.path().join("20260101-ABC123");
+        fs::create_dir_all(&run_dir).unwrap();
+        fs::write(
+            run_dir.join("manifest.json"),
+            serde_json::to_string_pretty(&serde_json::json!({
+                "run_id": "abc123-full-id",
+                "workflow_name": "test",
+                "goal": "",
+                "start_time": "2026-01-01T12:00:00Z",
+                "node_count": 1,
+                "edge_count": 0
+            }))
+            .unwrap(),
+        )
+        .unwrap();
+
+        let result = find_run_by_prefix(dir.path(), "abc123").unwrap();
+        assert_eq!(result, run_dir);
+    }
+
+    #[test]
+    fn find_run_by_prefix_ambiguous() {
+        let dir = tempfile::tempdir().unwrap();
+        let subdirs = [("d1", "abc-111"), ("d2", "abc-222")];
+        for (subdir, run_id) in subdirs {
+            let run_dir = dir.path().join(subdir);
+            fs::create_dir_all(&run_dir).unwrap();
+            fs::write(
+                run_dir.join("manifest.json"),
+                serde_json::to_string_pretty(&serde_json::json!({
+                    "run_id": run_id,
+                    "workflow_name": "test",
+                    "goal": "",
+                    "start_time": "2026-01-01T12:00:00Z",
+                    "node_count": 1,
+                    "edge_count": 0
+                }))
+                .unwrap(),
+            )
+            .unwrap();
+        }
+
+        let result = find_run_by_prefix(dir.path(), "abc");
+        assert!(result.is_err());
+        assert!(
+            result.unwrap_err().to_string().contains("Ambiguous"),
+            "Should mention ambiguity"
+        );
+    }
 }