diff --git a/.gitignore b/.gitignore
index 2b39066..d402255 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,5 +10,14 @@ flamegraph.svg
 CLAUDE.md
 memory/
 
-# Local e2e test fixtures (real, proprietary containers — never committed)
-/fixtures/
+# e2e fixtures: ignore anything dropped into fixtures/ (your own real containers
+# stay private) EXCEPT the committed synthetic test fixtures and their README.
+/fixtures/*
+!/fixtures/README.md
+!/fixtures/TEST.bin
+!/fixtures/TEST_1.00.00_20240101120000_0.app
+!/fixtures/TEST_1.01.00_20240102120000_1_1.00.00.app
+!/fixtures/TEST_T001_20240101120000_0.opt
+
+# Fixture generator (kept local, not committed)
+/tests/fixtures_gen/
diff --git a/fixtures/README.md b/fixtures/README.md
new file mode 100644
index 0000000..22b91b1
--- /dev/null
+++ b/fixtures/README.md
@@ -0,0 +1,34 @@
+# Test fixtures
+
+These are **synthetic** SEGA fscrypt containers used by `tests/e2e.rs`. They
+contain only dummy files (a few text/XML/stub binaries), so they are safe to
+commit — there is no proprietary game content inside.
+
+| File | Type | What it exercises |
+|------|------|-------------------|
+| `TEST_T001_20240101120000_0.opt` | OPTION | exFAT decrypt + extract (built-in OPTION key) |
+| `TEST_1.00.00_20240101120000_0.app` | APP (base) | outer NTFS → `internal_0.vhd` (fixed VHD) → inner NTFS extract |
+| `TEST_1.01.00_20240102120000_1_1.00.00.app` | APP (delta) | differencing `internal_1.vhd`, auto-merged against the base by VHD GUID |
+| `TEST.bin` | key | external key (16-byte key + 16-byte IV) for game id `TEST` |
+
+The delta is a real differencing VHD linked to the base; decrypting it (with the
+base alongside) merges the two and yields the base files plus the delta's added
+`data/patch_notes.txt` and a modified `readme.txt`.
+
+The `.app` fixtures use the synthetic game id `TEST`, which is **not** in the
+built-in key table — so they are decrypted via the external-key-file fallback
+using the committed `TEST.bin` (this also exercises that fallback path).
+`tests/e2e.rs` copies `TEST.bin` next to the containers and runs from there so
+the binary finds it. The OPTION fixture uses the built-in OPTION key. Everything
+here — keys and filesystem data alike — is synthetic.
+
+You can also drop your own **real** `.app`/`.opt` files in this folder to test
+against them; everything here except these committed fixtures is git-ignored.
+
+## Regenerating
+
+These are generated by a local, uncommitted script set (NTFS/exFAT image
+creation in Docker + a small SEGA fscrypt packer adapted from
+[`beerpsi/x`](https://gitea.tendokyu.moe/beerpsi/x)). The byte output is not
+reproducible (NTFS/exFAT embed creation timestamps), but regenerated fixtures are
+functionally equivalent.
diff --git a/fixtures/TEST.bin b/fixtures/TEST.bin
new file mode 100644
index 0000000..fefa1cc
Binary files /dev/null and b/fixtures/TEST.bin differ
diff --git a/fixtures/TEST_1.00.00_20240101120000_0.app b/fixtures/TEST_1.00.00_20240101120000_0.app
new file mode 100644
index 0000000..45e5bd4
Binary files /dev/null and b/fixtures/TEST_1.00.00_20240101120000_0.app differ
diff --git a/fixtures/TEST_1.01.00_20240102120000_1_1.00.00.app b/fixtures/TEST_1.01.00_20240102120000_1_1.00.00.app
new file mode 100644
index 0000000..f98fd79
Binary files /dev/null and b/fixtures/TEST_1.01.00_20240102120000_1_1.00.00.app differ
diff --git a/fixtures/TEST_T001_20240101120000_0.opt b/fixtures/TEST_T001_20240101120000_0.opt
new file mode 100644
index 0000000..456096f
Binary files /dev/null and b/fixtures/TEST_T001_20240101120000_0.opt differ
diff --git a/tests/e2e.rs b/tests/e2e.rs
index b95cad4..33fbaf7 100644
--- a/tests/e2e.rs
+++ b/tests/e2e.rs
@@ -1,10 +1,11 @@
-//! End-to-end decryption test driven by local fixtures.
+//! End-to-end decryption test driven by the fixtures in `fixtures/`.
 //!
-//! Drop real `.app` / `.opt` containers into the git-ignored `fixtures/`
-//! directory at the crate root. Each one is copied to a temp dir, run through
-//! the built binary, and checked for a non-empty extraction output. When
-//! `fixtures/` is absent or empty the test is a no-op, so the suite stays green
-//! without the (proprietary) sample data.
+//! The committed fixtures are small, fully synthetic containers (see
+//! `fixtures/README.md`) — a base APP, a delta APP, and an OPTION — so this runs
+//! the whole decrypt-and-extract path, including the delta/base VHD merge, with
+//! no proprietary data. You can also drop your own real `.app`/`.opt` files in
+//! `fixtures/` (they are git-ignored) and they'll be exercised too. When
+//! `fixtures/` is empty the test is a no-op, so the suite stays green regardless.
 
 use std::fs;
 use std::path::{Path, PathBuf};
@@ -41,18 +42,34 @@ fn decrypts_fixture_containers() {
     }
 
     // Work in a temp dir so extraction never touches the fixtures folder. The
-    // tool extracts next to its input, so we copy each fixture in first.
+    // tool extracts next to its input, so we copy every fixture in *first* — a
+    // delta APP needs its base APP sitting alongside for the auto-merge.
     let workdir = std::env::temp_dir().join(format!("fsdecrypt-e2e-{}", std::process::id()));
     let _ = fs::remove_dir_all(&workdir);
     fs::create_dir_all(&workdir).expect("create work dir");
 
-    for src in &files {
-        let name = src.file_name().unwrap().to_os_string();
-        let input = workdir.join(&name);
-        fs::copy(src, &input).expect("copy fixture into work dir");
+    // Copy *every* fixture file in — the containers plus any external
+    // `{game_id}.bin` key files an unlisted game needs.
+    let dir = Path::new(env!("CARGO_MANIFEST_DIR")).join("fixtures");
+    if let Ok(entries) = fs::read_dir(&dir) {
+        for e in entries.filter_map(|e| e.ok()) {
+            let p = e.path();
+            if p.is_file() {
+                let _ = fs::copy(&p, workdir.join(p.file_name().unwrap()));
+            }
+        }
+    }
+    let inputs: Vec<PathBuf> = files
+        .iter()
+        .map(|f| workdir.join(f.file_name().unwrap()))
+        .collect();
 
+    for input in &inputs {
+        let name = input.file_name().unwrap().to_os_string();
+        // Run from the work dir so the binary resolves `{game_id}.bin` keys there.
         let output = Command::new(env!("CARGO_BIN_EXE_fsdecrypt"))
-            .arg(&input)
+            .current_dir(&workdir)
+            .arg(input)
             .output()
             .expect("failed to run fsdecrypt");
 
@@ -69,7 +86,7 @@ fn decrypts_fixture_containers() {
         let produced = fs::read_dir(&out_dir).map(|rd| rd.count()).unwrap_or(0);
         assert!(
             produced > 0,
-            "{name:?}: expected extracted entries in {}",
+            "{name:?}: expected extracted entries in {}\n--- stdout ---\n{stdout}",
             out_dir.display()
         );
         println!("ok: {name:?} -> {produced} top-level entr(y/ies) extracted");