diff --git a/default.nix b/default.nix
index f26de54eab39ba3adff824db3daff7b3eedf52ed..bc6a0884559432f6b8c0c65473530de99154bbd5 100644
--- a/default.nix
+++ b/default.nix
@@ -72,7 +72,8 @@ in
           pyrsistent
           configparser
           eliot
-          foolscap >= 21.7.0
+          foolscap
+          collections-extended >= 2.0.2
 
           # undetected cryptography build dependency
           # https://github.com/DavHau/mach-nix/issues/305
diff --git a/nix/sources.json b/nix/sources.json
index 5e23721d3393d58299acef4677b15b68c33cef21..658a129cece37115b20115201342083508e51732 100644
--- a/nix/sources.json
+++ b/nix/sources.json
@@ -29,10 +29,10 @@
         "homepage": "",
         "owner": "DavHau",
         "repo": "pypi-deps-db",
-        "rev": "856d67ab093a68425c3896ec2961cea3b95ae93f",
-        "sha256": "0a7avm4xvm454gxy4dq0fc19j3f9ik8gf3kjpsqhszfkamrn9y0p",
+        "rev": "a44853469d0d6bc65cd382f24e4e60ff59bd60b2",
+        "sha256": "1v5ynbkyl8x7nm28wxijhsgqdzxp84h36wa0xdnfz7fhk6ayfbf4",
         "type": "tarball",
-        "url": "https://github.com/DavHau/pypi-deps-db/archive/856d67ab093a68425c3896ec2961cea3b95ae93f.tar.gz",
+        "url": "https://github.com/DavHau/pypi-deps-db/archive/a44853469d0d6bc65cd382f24e4e60ff59bd60b2.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     },
     "release2105": {
@@ -71,10 +71,10 @@
         "homepage": "https://tahoe-lafs.org/",
         "owner": "tahoe-lafs",
         "repo": "tahoe-lafs",
-        "rev": "766a53b5208c03c45ca0a98e97eee76870276aa1",
-        "sha256": "16mvh5ypgv9a5vhjnc78jjf4v5yzyyz19af2vixrc4zqb0ikxrxa",
+        "rev": "5e8cc06e93e7b23dae0f1c3e0bc158b49a9e14e6",
+        "sha256": "0lkdchnjr9nzpqsr0vphgx7ihn2ilvayi4almnlmv0n0hmgjrgjr",
         "type": "tarball",
-        "url": "https://github.com/tahoe-lafs/tahoe-lafs/archive/766a53b5208c03c45ca0a98e97eee76870276aa1.tar.gz",
+        "url": "https://github.com/tahoe-lafs/tahoe-lafs/archive/5e8cc06e93e7b23dae0f1c3e0bc158b49a9e14e6.tar.gz",
         "url_template": "https://github.com/<owner>/<repo>/archive/<rev>.tar.gz"
     }
 }
diff --git a/tests.nix b/tests.nix
index 0d7f6a7750c73da64c8f4619503394234497d3f9..1e6330666c9006d1a4d8d7161e2154bf45f652bf 100644
--- a/tests.nix
+++ b/tests.nix
@@ -36,10 +36,12 @@ let
 
     lint-python = mach-nix.mkPython {
       python = "python39";
+      # Pin these narrowly so that lint rules only change when we specifically
+      # want them to.
       requirements = ''
-        isort
-        black
-        flake8
+        isort == 5.10.1
+        black == 21.12b0
+        flake8 == 4.0.1
       '';
     };