diff --git a/.github/workflows/ci-nix.yaml b/.github/workflows/ci-nix.yaml
index ee45bc3068b..1781355e711 100644
--- a/.github/workflows/ci-nix.yaml
+++ b/.github/workflows/ci-nix.yaml
@@ -233,6 +233,31 @@ jobs:
         body-file: comment-body.md
         reactions: rocket
 
+  nix-flake-check:
+    name: "nix flake check"
+    runs-on: ubuntu-latest
+    steps:
+    - name: 📥 Checkout repository
+      uses: actions/checkout@v4
+
+    - name: ❄ Prepare nix
+      uses: cachix/install-nix-action@v26
+      with:
+        extra_nix_config: |
+          accept-flake-config = true
+          log-lines = 1000
+
+    - name: ❄ Cachix cache of nix derivations
+      uses: cachix/cachix-action@v14
+      with:
+        name: cardano-scaling
+        authToken: '${{ secrets.CACHIX_CARDANO_SCALING_AUTH_TOKEN }}'
+
+    - name: ❄ Nix Flake Check
+      run: |
+        nix flake check -L
+
+
   build-specification:
     name: "Build specification using nix"
     runs-on: ubuntu-latest
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 65e3dbacac6..3183c2998eb 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -112,9 +112,8 @@ Besides these general build instructions, some components might document
 additional steps and useful tools in their `README.md` files, e.g. the
 [docs](./docs/README.md) or the [hydra-cluster](./hydra-cluster/README.md)
 
-Warnings are treated as errors for the whole project. To ease development, it
-might be handy to it off locally in a `cabal.project.local` with: `cabal
-configure --ghc-options -Wwarn`.
+While warnings are not treated as errors during builds, CI will check for it
+before we merge any contributions.
 
 ### Coding standards
 
diff --git a/cabal.project b/cabal.project
index e995e5630b0..1bf666579ab 100644
--- a/cabal.project
+++ b/cabal.project
@@ -35,10 +35,6 @@ packages:
 package *
   ghc-options: -j8
 
--- Warnings as errors for local packages
-program-options
-  ghc-options: -Werror
-
 -- Always build tests and benchmarks of local packages
 tests: True
 benchmarks: True
diff --git a/flake.nix b/flake.nix
index 5a6f2985718..cd0f6e3e0d1 100644
--- a/flake.nix
+++ b/flake.nix
@@ -98,6 +98,8 @@
             with pkgs.lib.attrsets;
             mapAttrs' (name: value: nameValuePair (s + name) value) attrs;
 
+          addWerror = x: x.override { ghcOptions = [ "-Werror" ]; };
+
         in
         rec {
           legacyPackages = hsPkgs;
@@ -133,6 +135,15 @@
               src = self;
               exe = "${packages.plutus-merkle-tree-tests}/bin/tests";
             };
+            hydra-cardano-api-werror = addWerror hsPkgs.hydra-cardano-api.components.library;
+            hydra-cluster-werror = addWerror hsPkgs.hydra-cluster.components.library;
+            hydra-chain-observer-werror = addWerror hsPkgs.hydra-chain-observer.components.exes.hydra-chain-observer;
+            hydra-plutus-werror = addWerror hsPkgs.hydra-plutus.components.library;
+            hydra-plutus-tests-werror = addWerror hsPkgs.hydra-plutus.components.exes.tests;
+            hydra-node-werror = addWerror hsPkgs.hydra-node.components.library;
+            hydra-node-tests-werror = addWerror hsPkgs.hydra-node.components.tests.tests;
+            hydra-tui-werror = addWerror hsPkgs.hydra-tui.components.library;
+            hydra-tui-tests-werror = addWerror hsPkgs.hydra-tui.components.tests.tests;
           } //
           (if pkgs.stdenv.isLinux then
             {
diff --git a/hydra-node/test/Hydra/Model/Payment.hs b/hydra-node/test/Hydra/Model/Payment.hs
index 6be421cc140..b5ef6dab88f 100644
--- a/hydra-node/test/Hydra/Model/Payment.hs
+++ b/hydra-node/test/Hydra/Model/Payment.hs
@@ -86,6 +86,7 @@ instance IsTx Payment where
   type UTxOType Payment = [(CardanoSigningKey, Value)]
   type ValueType Payment = Value
   txId = error "undefined"
+  txSpendingUTxO = error "undefined"
   balance = foldMap snd
   hashUTxO = encodeUtf8 . show @Text