diff --git a/README.md b/README.md
index b99a221b..804c4d6c 100644
--- a/README.md
+++ b/README.md
@@ -180,6 +180,11 @@ transport; its revision and unpacked source hash are pinned in
 also verifies the native compatibility contract and runtime closure, runs
 the package tests, and checks installed completions. `nix develop` supplies
 the Rust toolchain and the same native artifact, without Zig.
+GitHub-hosted Zig dependency archives use fixed-output downloads with the original
+URL followed by an equivalent `codeload.github.com` fallback. URLs and archive
+checksums come from Ghostty's pinned package manifest; the Zig package-cache
+output hash is unchanged. This improves cold-cache build reliability without
+changing runtime behavior or dependency contents.
 
 With Cargo (see the build requirements below):
 
diff --git a/flake.nix b/flake.nix
index 2e193d70..220bc378 100644
--- a/flake.nix
+++ b/flake.nix
@@ -56,6 +56,29 @@
           hash = ghosttyContract.sourceHash;
         };
 
+        # Keep transport mirrors separate from Zig's package content pins.
+        # Upstream's manifest supplies both archive byte hashes and cache names.
+        ghosttyGithubArchives = map
+          (package:
+            let
+              archiveUrl = builtins.match
+                "https://github.com/([^/]+/[^/]+)/archive/(.+)\\.tar\\.gz"
+                package.url;
+            in package // {
+              archive = pkgs.fetchurl {
+                urls = [
+                  package.url
+                  "https://codeload.github.com/${builtins.elemAt archiveUrl 0}/tar.gz/${builtins.elemAt archiveUrl 1}"
+                ];
+                sha256 = package.sha256;
+              };
+            })
+          (builtins.filter
+            (package: package.type == "archive" && builtins.match
+              "https://github.com/([^/]+/[^/]+)/archive/(.+)\\.tar\\.gz"
+              package.url != null)
+            (builtins.fromJSON (builtins.readFile "${ghosttySrc}/flatpak/zig-packages.json")));
+
         # Ghostty's zig package cache, in the layout `zig build --system <dir>`
         # reads (one directory per package, named by its content hash). Fetched
         # with Ghostty's own script because `zig build --fetch` skips transitive
@@ -79,6 +102,10 @@
           buildPhase = ''
             runHook preBuild
             export ZIG_GLOBAL_CACHE_DIR="$TMPDIR/zig-global-cache"
+            ${lib.concatMapStringsSep "\n" (package: ''
+              substituteInPlace build.zig.zon.txt \
+                --replace-fail ${lib.escapeShellArg package.url} ${lib.escapeShellArg (toString package.archive)}
+            '') ghosttyGithubArchives}
             ./nix/build-support/fetch-zig-cache.sh
             runHook postBuild
           '';