diff --git a/pkgs/applications/networking/p2p/transmission/4.nix b/pkgs/applications/networking/p2p/transmission/4.nix
index 77585c18484c..8382a0c7d52e 100644
--- a/pkgs/applications/networking/p2p/transmission/4.nix
+++ b/pkgs/applications/networking/p2p/transmission/4.nix
@@ -49,7 +49,7 @@
 }:
 
 let
-  inherit (lib) cmakeBool optional optionals;
+  inherit (lib) cmakeBool optional optionals optionalString;
 
   apparmorRules = apparmorRulesFromClosure { name = "transmission-daemon"; } (
     [
@@ -204,6 +204,17 @@ stdenv.mkDerivation (finalAttrs: {
     install -Dm0444 -t $out/share/icons ../icons/hicolor_apps_scalable_transmission.svg
   '';
 
+  postFixup = optionalString enableMac ''
+    # The macOS bundle fixup collapses different store paths that share the
+    # basename libiconv.2.dylib into a single bundled file. libintl is linked
+    # against Darwin libiconv's ABI (_iconv), while libpsl pulls GNU libiconv
+    # (_libiconv) into the closure. Point libintl back at the Darwin store path
+    # to avoid the basename collision inside the app bundle.
+    install_name_tool \
+      -change @rpath/libiconv.2.dylib ${libiconv}/lib/libiconv.2.dylib \
+      $out/Applications/Transmission.app/Contents/MacOS/libintl.8.dylib
+  '';
+
   passthru.tests = {
     apparmor = nixosTests.transmission_4; # starts the service with apparmor enabled
     smoke-test = nixosTests.bittorrent;

Here's Gemini's version, seems a bit longer (needlessly?)

--- gemini/transmission.nix.orig	2026-03-20 00:57:41
+++ gemini/transmission.nix	2026-03-20 00:55:01
@@ -25,6 +25,7 @@
   dht,
   libnatpmp,
   libiconv,
+  gettext,
   # Build options
   enableGTK3 ? false,
   gtkmm3,
@@ -111,7 +112,7 @@
     # Excluding gtest since it is hardcoded to vendored version. The rest of the listed libraries are not packaged.
     pushd third-party
     for f in *; do
-        if [[ ! $f =~ googletest|wildmat|wide-integer|jsonsl|madler-crcany ]]; then
+        if [[ ! $f =~ googletest|wildmat|wide-integer|jsonsl|madler-crcany|libutp|dht|natpmp ]]; then
             rm -r "$f"
         fi
     done
@@ -178,9 +179,12 @@
   ]
   ++ optionals enableSystemd [ systemd ]
   ++ optionals stdenv.hostPlatform.isLinux [ inotify-tools ]
-  ++ optionals enableMac [ libiconv ];
+  ++ optionals enableMac [ 
+    darwin.libiconv
+    gettext
+  ];
 
-  postInstall = optional stdenv.hostPlatform.isLinux ''
+  postInstall = lib.optionalString stdenv.hostPlatform.isLinux ''
     mkdir $apparmor
     cat >$apparmor/bin.transmission-daemon <<EOF
     abi <abi/4.0>,
@@ -202,6 +206,14 @@
     }
     EOF
     install -Dm0444 -t $out/share/icons ../icons/hicolor_apps_scalable_transmission.svg
+  '' + lib.optionalString stdenv.hostPlatform.isDarwin ''
+    # The build system's bundling logic is flawed and picks up the wrong libiconv.
+    # We manually replace it with the Apple-compatible one from nixpkgs.
+    APP_MACOS="$out/Applications/Transmission.app/Contents/MacOS"
+    rm "$APP_MACOS/libiconv.2.dylib"
+    cp "${darwin.libiconv}/lib/libiconv.2.dylib" "$APP_MACOS/"
+    chmod +w "$APP_MACOS/libiconv.2.dylib"
+    install_name_tool -id "@rpath/libiconv.2.dylib" "$APP_MACOS/libiconv.2.dylib"
   '';
 
   passthru.tests = {

Good initiative!
(shouldn't we be using

badPlatforms = [
  "x86_64-darwin"
];

instead?)

Good initiative!
(shouldn't we be using

badPlatforms = [
   # https://hydra.nixos.org/job/nixpkgs/unstable/arrow-cpp.x86_64-darwin/all
  "x86_64-darwin"
];

instead?)

Any workaround besides pinning:

nixpkgs.url = "github:nixos/nixpkgs/9cf7092bdd603554bd8b63c216e8943cf9b12512"; # pin: direnv 2.37.1 cgo breakage on newer unstable