diff --git a/pkgs/by-name/ga/gaphor/fix-dulwich-api.patch b/pkgs/by-name/ga/gaphor/fix-dulwich-api.patch
new file mode 100644
index 000000000000..8af56ca1aa8e
--- /dev/null
+++ b/pkgs/by-name/ga/gaphor/fix-dulwich-api.patch
@@ -0,0 +1,29 @@
+From 7f4f7fcd97acef841e046191d98c900b61b78c17 Mon Sep 17 00:00:00 2001
+From: Dan Yeaw <dan@yeaw.me>
+Date: Sat, 4 Apr 2026 15:46:25 -0400
+Subject: [PATCH] Fix dulwich porcelain API
+
+---
+ gaphor/storage/tests/fixtures.py | 6 ++++--
+ 1 file changed, 4 insertions(+), 2 deletions(-)
+
+diff --git a/gaphor/storage/tests/fixtures.py b/gaphor/storage/tests/fixtures.py
+index d071e4556f..4dd0c60bf1 100644
+--- a/gaphor/storage/tests/fixtures.py
++++ b/gaphor/storage/tests/fixtures.py
+@@ -16,11 +16,13 @@ def commit_all(message):
+     main_ref = commit_all("Initial commit")
+ 
+     porcelain.branch_create(repo, "branch")
+-    porcelain.checkout_branch(repo, "branch")
++    porcelain.switch(repo, "branch")
+     filename.write_text(their_text, encoding="utf-8")
+     branch_oid = commit_all("Branch commit")
+ 
+-    porcelain.checkout_branch(repo, main_ref)
++    # OID + detach=True: works when default branch is not "main" (e.g. older
++    # Dulwich uses "master"); "git merge" still runs on HEAD.
++    porcelain.switch(repo, main_ref, detach=True)
+     filename.write_text(our_text, encoding="utf-8")
+     commit_all("Second commit")
+ 
diff --git a/pkgs/by-name/ga/gaphor/package.nix b/pkgs/by-name/ga/gaphor/package.nix
index a414571cfcad..6922fc74d7b8 100644
--- a/pkgs/by-name/ga/gaphor/package.nix
+++ b/pkgs/by-name/ga/gaphor/package.nix
@@ -23,6 +23,12 @@ python3Packages.buildPythonApplication (finalAttrs: {
     hash = "sha256-0Z0RFQrN2g0beV2konZBfMroeNtbT+sPRsWlRvQFYBk=";
   };
 
+  patches = [
+    # from https://github.com/gaphor/gaphor/pull/4236
+    # please remove after next update
+    ./fix-dulwich-api.patch
+  ];
+
   pythonRelaxDeps = [
     "pydot"
     "pygobject"