diff --git a/host/tests/it/common.rs b/host/tests/it/common.rs
index 77c57dc..c47c486 100644
--- a/host/tests/it/common.rs
+++ b/host/tests/it/common.rs
@@ -1,5 +1,7 @@
 //! Shared fixture copies and builds. A mutex protects cwd-init and the brush checkout.
 use std::{
+    hash::{Hash, Hasher},
+    os::unix::{ffi::OsStrExt, fs::PermissionsExt},
     path::{Path, PathBuf},
     process::{Command, ExitStatus},
     sync::{Mutex, OnceLock},
@@ -31,6 +33,79 @@ pub fn target_dir() -> PathBuf {
         })
 }
 
+/// Fingerprint the snapshot inputs and their current working-tree contents.
+pub fn bundle_key(repo: &Path) -> anyhow::Result<String> {
+    const INPUTS: &[&str] = &[
+        "host",
+        "vfs",
+        "userland",
+        "agents",
+        "skills",
+        "extensions",
+        "Cargo.toml",
+        "Cargo.lock",
+        "flake.nix",
+        "flake.lock",
+        "README.md",
+        "AGENTS.md",
+        ".gitignore",
+    ];
+    let tree = Command::new("git")
+        .current_dir(repo)
+        .args(["ls-tree", "HEAD", "--"])
+        .args(INPUTS)
+        .output()?;
+    anyhow::ensure!(
+        tree.status.success(),
+        "git ls-tree: {}",
+        String::from_utf8_lossy(&tree.stderr)
+    );
+    let files = Command::new("git")
+        .current_dir(repo)
+        .args([
+            "ls-files",
+            "-z",
+            "--cached",
+            "--others",
+            "--exclude-standard",
+            "--",
+        ])
+        .args(INPUTS)
+        .output()?;
+    anyhow::ensure!(
+        files.status.success(),
+        "git ls-files: {}",
+        String::from_utf8_lossy(&files.stderr)
+    );
+    let mut hash = std::collections::hash_map::DefaultHasher::new();
+    tree.stdout.hash(&mut hash);
+    let mut names: Vec<_> = files
+        .stdout
+        .split(|byte| *byte == 0)
+        .filter(|name| !name.is_empty())
+        .collect();
+    names.sort_unstable();
+    for name in names {
+        name.hash(&mut hash);
+        let path = repo.join(std::ffi::OsStr::from_bytes(name));
+        match std::fs::symlink_metadata(&path) {
+            Ok(metadata) => {
+                metadata.permissions().mode().hash(&mut hash);
+                if metadata.file_type().is_symlink() {
+                    std::fs::read_link(path)?.hash(&mut hash);
+                } else {
+                    std::fs::read(path)?.hash(&mut hash);
+                }
+            }
+            Err(error) if error.kind() == std::io::ErrorKind::NotFound => {
+                "deleted".hash(&mut hash);
+            }
+            Err(error) => return Err(error.into()),
+        }
+    }
+    Ok(format!("{:016x}\n", hash.finish()))
+}
+
 pub fn cwd_init() {
     static INIT: OnceLock<()> = OnceLock::new();
     INIT.get_or_init(|| {
diff --git a/host/tests/it/p1.rs b/host/tests/it/p1.rs
index 6c9c5d3..5e748c3 100644
--- a/host/tests/it/p1.rs
+++ b/host/tests/it/p1.rs
@@ -7,9 +7,15 @@ fn bundle() -> anyhow::Result<PathBuf> {
     let repo = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("..");
     let bundle = crate::common::target_dir().join("bundle");
     let _guard = crate::common::BUILD.lock().unwrap();
+    let key = crate::common::bundle_key(&repo)?;
+    let stamp = bundle.join(".source-key");
     if !bundle.join("bin/sh.wasm").exists()
         || !bundle.join("src/sausage/agents/models.json").exists()
+        || std::fs::read_to_string(&stamp).ok().as_deref() != Some(key.as_str())
     {
+        if stamp.exists() {
+            std::fs::remove_file(&stamp)?;
+        }
         assert!(
             std::process::Command::new("nu")
                 .arg(repo.join("userland/bundle.nu"))
@@ -17,6 +23,7 @@ fn bundle() -> anyhow::Result<PathBuf> {
                 .status()?
                 .success()
         );
+        std::fs::write(stamp, key)?;
     }
     Ok(bundle)
 }