diff --git a/build.zig b/build.zig
index 149eac8..4ca331c 100644
--- a/build.zig
+++ b/build.zig
@@ -11,6 +11,7 @@ pub fn build(b: *std.Build) void {
     const build_options = b.addOptions();
     build_options.addOption([]const u8, "version", version);
     build_options.addOption(bool, "download_geoip", download_geoip);
+    build_options.addOption([]const u8, "cache_dir", resolveCacheDir(b));
 
     const httpz = b.dependency("httpz", .{
         .target = target,
@@ -170,3 +171,26 @@ fn configureCompilationUnit(compile: *std.Build.Step.Compile, libs: []const *std
     // option set where `root_module` is created.
     for (libs) |lib| compile.root_module.linkLibrary(lib);
 }
+
+/// Resolves the runtime cache directory, mirroring `Config.load`'s precedence
+/// (`WTTR_CACHE_DIR`, else `${XDG_CACHE_HOME:-$HOME/.cache}/wttr`).
+///
+/// Tests need this because Zig 0.16 removed process-environment access outside
+/// of `main`; build scripts kept it. Without this, `Config.loadForTest` would
+/// fall back to `HOME = "/tmp"` and write test artifacts -- including the
+/// GeoLite2 database -- into `/tmp`.
+fn resolveCacheDir(b: *std.Build) []const u8 {
+    const env = &b.graph.environ_map;
+    if (env.get("WTTR_CACHE_DIR")) |dir| return b.dupe(dir);
+
+    const xdg_cache = if (env.get("XDG_CACHE_HOME")) |x|
+        b.dupe(x)
+    else if (env.get("HOME")) |home|
+        b.pathJoin(&.{ home, ".cache" })
+    else
+        // No home directory to work from; keep artifacts inside the build
+        // cache rather than falling back to a world-writable location.
+        b.pathFromRoot(".zig-cache");
+
+    return b.pathJoin(&.{ xdg_cache, "wttr" });
+}
diff --git a/src/Config.zig b/src/Config.zig
index 1916f77..4f66aec 100644
--- a/src/Config.zig
+++ b/src/Config.zig
@@ -90,14 +90,22 @@ pub fn load(allocator: std.mem.Allocator, env: *const std.process.Environ.Map) !
     };
 }
 
-/// Loads configuration from an empty environment, for tests.
+/// Loads configuration for tests.
 ///
 /// Zig 0.16 removed any way to reach the process environment outside of `main`,
-/// and tests are better off not depending on ambient variables anyway: this
-/// keeps them reproducible regardless of the developer's shell.
+/// so tests cannot resolve `HOME`/`XDG_CACHE_HOME` themselves. Left to an empty
+/// environment, `load` would fall back to `HOME = "/tmp"` and scatter test
+/// artifacts (including the ~63 MB GeoLite2 database that `-Ddownload-geoip`
+/// fetches) into `/tmp`.
+///
+/// Instead `build.zig` resolves the cache directory from its own environment --
+/// build scripts still have access -- and passes it through `build_options`, so
+/// tests read and write the same location the server uses. Everything else is
+/// left unset so tests do not depend on the developer's shell.
 pub fn loadForTest(allocator: std.mem.Allocator) !Config {
     var env: std.process.Environ.Map = .init(allocator);
     defer env.deinit();
+    try env.put("WTTR_CACHE_DIR", @import("build_options").cache_dir);
     return load(allocator, &env);
 }