diff --git a/.mise.toml b/.mise.toml
index 01afb8d..4c58f64 100644
--- a/.mise.toml
+++ b/.mise.toml
@@ -1,5 +1,6 @@
 [tools]
 prek = "0.4.1"
+pre-commit = "4.6.0"
 zig = "0.16.0"
 zls = "0.16.0"
 "ubi:DonIsaac/zlint" = "0.7.9"
diff --git a/src/portfolio_loader.zig b/src/portfolio_loader.zig
index 7a1d34d..e782032 100644
--- a/src/portfolio_loader.zig
+++ b/src/portfolio_loader.zig
@@ -672,8 +672,57 @@ test "loadFromBytes: all-empty bytes returns empty portfolio" {
 // Kept minimal (one happy-path, one missing-file case); the
 // bulk of the logic is in `loadFromBytes`, covered above.
 
+/// Build an environment map from the parent process with the GIT_*
+/// variables stripped out.
+///
+/// When these tests run inside a git hook (pre-commit, prek, etc.),
+/// the hook runner sets `GIT_INDEX_FILE`, `GIT_DIR`, and
+/// `GIT_WORK_TREE` to point at the outer repo's staging state. Git
+/// inherits those env vars unconditionally - `git -C <tmpdir>`
+/// changes the CWD but does NOT clear these env vars. The result is
+/// that `git init` in our temp dir succeeds but subsequent `git
+/// add`/`git commit` operate against the OUTER repo's index, with
+/// blob references that don't exist in our temp dir's object store
+/// ("invalid object 100644 <hash> for '<outer-repo-path>'").
+///
+/// The hook runner can't fix this for us; per upstream guidance,
+/// hooks (and code shelled out by hooks) that run git against a
+/// different repo must explicitly opt out of the inherited env. See
+/// https://github.com/j178/prek/issues/1786 for the prek-specific
+/// instance and https://pre-commit.com/ for the analogous pre-commit
+/// guidance.
+fn buildScrubbedEnv(allocator: std.mem.Allocator) !std.process.Environ.Map {
+    var map = try testing.environ.createMap(allocator);
+    errdefer map.deinit();
+
+    // Strip every GIT_* variable. Iterating-while-removing isn't
+    // supported on the underlying ArrayHashMap, so collect first
+    // then remove. Keys are duped because `swapRemove` frees the
+    // map's owned key buffer, which would otherwise leave us with
+    // dangling pointers in `keys_to_remove`.
+    var keys_to_remove: std.ArrayList([]u8) = .empty;
+    defer {
+        for (keys_to_remove.items) |k| allocator.free(k);
+        keys_to_remove.deinit(allocator);
+    }
+
+    var it = map.iterator();
+    while (it.next()) |entry| {
+        if (std.mem.startsWith(u8, entry.key_ptr.*, "GIT_")) {
+            try keys_to_remove.append(allocator, try allocator.dupe(u8, entry.key_ptr.*));
+        }
+    }
+    for (keys_to_remove.items) |key| _ = map.swapRemove(key);
+
+    return map;
+}
+
 /// Run a one-shot git command in `cwd` for test setup. Panics on
-/// failure — these tests can't proceed without a working repo.
+/// failure - these tests can't proceed without a working repo.
+///
+/// Uses `buildScrubbedEnv` to drop inherited `GIT_*` env vars; see
+/// that function's doc comment for why this matters under git
+/// hooks.
 fn gitInTestRepo(allocator: std.mem.Allocator, cwd: []const u8, argv: []const []const u8) !void {
     const full_argv = try allocator.alloc([]const u8, argv.len + 3);
     defer allocator.free(full_argv);
@@ -681,8 +730,13 @@ fn gitInTestRepo(allocator: std.mem.Allocator, cwd: []const u8, argv: []const []
     full_argv[1] = "-C";
     full_argv[2] = cwd;
     @memcpy(full_argv[3..], argv);
+
+    var env_map = try buildScrubbedEnv(allocator);
+    defer env_map.deinit();
+
     const result = try std.process.run(allocator, testing.io, .{
         .argv = full_argv,
+        .environ_map = &env_map,
         .stdout_limit = .limited(64 * 1024),
     });
     defer allocator.free(result.stdout);