diff --git a/crates/wasm_interp/src/lib.rs b/crates/wasm_interp/src/lib.rs
index 8cc98d3e39..c3add8e661 100644
--- a/crates/wasm_interp/src/lib.rs
+++ b/crates/wasm_interp/src/lib.rs
@@ -1,6 +1,5 @@
 mod frame;
 mod instance;
-mod tests;
 mod value_store;
 pub mod wasi;
 
diff --git a/flake.lock b/flake.lock
index d39bc639e0..3d5c0cf6f7 100644
--- a/flake.lock
+++ b/flake.lock
@@ -68,6 +68,7 @@
       },
       "original": {
         "owner": "nixos",
+        "ref": "nixos-unstable",
         "repo": "nixpkgs",
         "rev": "886c9aee6ca9324e127f9c2c4e6f68c2641c8256",
         "type": "github"
diff --git a/nix/builder.nix b/nix/builder.nix
index 265df5a310..262d3bc203 100644
--- a/nix/builder.nix
+++ b/nix/builder.nix
@@ -3,8 +3,8 @@ let
   inherit (compile-deps) zigPkg llvmPkgs llvmVersion llvmMajorMinorStr glibcPath libGccSPath;
 
   subPackagePath = if subPackage != null then "crates/${subPackage}" else null;
-
   mainBin = if subPackage == "lang_srv" then "roc_ls" else "roc";
+  filteredSource = pkgs.callPackage ./fileFilter.nix { };
 in
 rustPlatform.buildRustPackage {
   pname = "roc" + lib.optionalString (subPackage != null) "_${subPackage}";
@@ -12,7 +12,7 @@ rustPlatform.buildRustPackage {
 
   buildAndTestSubdir = subPackagePath;
 
-  src = pkgs.nix-gitignore.gitignoreSource [ ] ../.;
+  src = filteredSource;
 
   cargoLock = {
     lockFile = ../Cargo.lock;
diff --git a/nix/fileFilter.nix b/nix/fileFilter.nix
new file mode 100644
index 0000000000..8f974f85b2
--- /dev/null
+++ b/nix/fileFilter.nix
@@ -0,0 +1,80 @@
+{ lib, nix-gitignore }:
+let
+  # See https://nix.dev/tutorials/file-sets for a guide on how the file set api works
+
+  fs = lib.fileset;
+
+  fileDoesNotHaveExt = fileExts: file: (!lib.lists.any (ext: file.hasExt ext) fileExts);
+
+  repoRoot = ../.;
+
+  # The file set api does not currently have a way to easily remove folders dynamically.
+  # The nix build does not run tests, so we try to remove any folders with only tests.
+  removedTests =
+    let
+      dirFilter = pathStr: (
+        let dirName = baseNameOf pathStr; in !(
+          # remove any folder whos name is `tests` or starts with `test_`
+          dirName == "tests"
+        )
+      );
+      removeTestFilter =
+        path: type:
+        # only do a "real" check on directory, allow everything else through
+        (type == "directory" && dirFilter path)
+        || type != "directory";
+    in
+    lib.sources.cleanSourceWith { src = repoRoot; filter = removeTestFilter; };
+  fsBase = fs.fromSource removedTests;
+
+  # fsBase = fs.fromSource repoRoot;
+
+  # only look at files in the crates folder
+  onlyCratesFolder = fs.intersection ../crates fsBase;
+
+  # the above filter only has the subfolder, put some needed files from the root back in
+  includeCargoRootFiles = fs.unions [
+    ../Cargo.toml
+    ../Cargo.lock
+    ../version.txt
+    onlyCratesFolder
+  ];
+
+  # Remove any "simple" files like markdown/pictures since they probably wont be used in the actual code
+  removedSimpleFiles =
+    let
+      extensionsToRemove = [ "md" "svg" "png" ];
+    in
+    fs.intersection
+      includeCargoRootFiles
+      (fs.fileFilter (fileDoesNotHaveExt extensionsToRemove) repoRoot);
+
+  # the above filter can make the doc crate sad since it has pictures
+  docsAddedBack = fs.unions [
+    ../crates/docs
+    removedSimpleFiles
+  ];
+
+  # ===============================
+  # If you are trying to see what is ok to exclude from the "main" builds (cli/lang_server)
+  # use `cargo tree` https://doc.rust-lang.org/cargo/commands/cargo-tree.html
+  # 
+  # Ex: `cargo tree -i roc_build` will show all deps of the `roc_build` crate
+  # if only the package passed with `-i` is shown, nothing depends on it
+  # ===============================
+
+  # remove www folder from checkmate crate since it's not built with nix
+  removedWWW = fs.difference docsAddedBack ../crates/compiler/checkmate/www;
+
+  # potential packages/folders that could be removed 
+  # repl_* -> I don't think nix will build those
+
+  filteredSrc = fs.toSource {
+    root = repoRoot;
+    # to debug you can switch to
+    # fileset = fs.traceVal <file set>
+    fileset = removedWWW;
+  };
+
+in
+filteredSrc