diff --git a/.envrc b/.envrc
index 1f15539fb..edebaae71 100644
--- a/.envrc
+++ b/.envrc
@@ -1,10 +1,14 @@
 # Create a gcroot that keeps all third_party.sources alive
-nix-build --out-link .gcroots/sources -E '
+nix-build --show-trace --out-link .gcroots/sources -E '
 with import ./. {};
 third_party.nixpkgs.writeText "depot-3p-sources.txt" (
   toString (
     builtins.map (s: s.outPath or null) (
-      builtins.attrValues third_party.sources
+      # Only gcroot sources that are explicitly marked using
+      # niv modify --attribute gcroot=true <source>
+      builtins.filter (source: source.gcroot or false) (
+        builtins.attrValues third_party.sources
+      )
     )
   )
 )'
diff --git a/third_party/sources/sources.json b/third_party/sources/sources.json
index 043034e98..690694635 100644
--- a/third_party/sources/sources.json
+++ b/third_party/sources/sources.json
@@ -14,6 +14,7 @@
     "gitignore.nix": {
         "branch": "master",
         "description": "Nix functions for filtering local git sources",
+        "gcroot": true,
         "homepage": "",
         "owner": "hercules-ci",
         "repo": "gitignore.nix",
@@ -26,6 +27,7 @@
     "home-manager": {
         "branch": "master",
         "description": "Manage a user environment using Nix  [maintainer=@rycee] ",
+        "gcroot": true,
         "homepage": "https://nix-community.github.io/home-manager/",
         "owner": "nix-community",
         "repo": "home-manager",
@@ -74,6 +76,7 @@
     "nixpkgs": {
         "branch": "nixos-unstable",
         "description": "Nix Packages collection",
+        "gcroot": true,
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
@@ -86,6 +89,7 @@
     "nixpkgs-stable": {
         "branch": "nixos-23.11",
         "description": "Nix Packages collection",
+        "gcroot": true,
         "homepage": "",
         "owner": "NixOS",
         "repo": "nixpkgs",
@@ -98,6 +102,7 @@
     "rust-overlay": {
         "branch": "master",
         "description": "Pure and reproducible nix overlay of binary distributed rust toolchains",
+        "gcroot": true,
         "homepage": "",
         "owner": "oxalica",
         "repo": "rust-overlay",