diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index a7e0481..6bd5ae0 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -18,6 +18,8 @@ jobs:
     steps:
       - uses: actions/checkout@v4
       - run: rustup update stable && rustup default stable
+      # hashgraph-like-consensus's build.rs shells out to protoc via prost-build.
+      - run: sudo apt-get update && sudo apt-get install -y protobuf-compiler
       # chat-cli's build.rs unconditionally links liblogosdelivery and requires
       # LOGOS_DELIVERY_LIB_DIR. The smoketest job builds and exercises it under
       # Nix; here we keep the toolchain-only job fast by skipping it.
@@ -31,6 +33,8 @@ jobs:
       - uses: actions/checkout@v4
       - run: rustup update stable && rustup default stable
       - run: rustup component add clippy
+      # hashgraph-like-consensus's build.rs shells out to protoc via prost-build.
+      - run: sudo apt-get update && sudo apt-get install -y protobuf-compiler
       - run: cargo clippy --all-targets --all-features --workspace --exclude chat-cli -- -D warnings
 
   fmt:
diff --git a/flake.nix b/flake.nix
index b31738a..9ca747d 100644
--- a/flake.nix
+++ b/flake.nix
@@ -59,7 +59,7 @@
               };
             };
 
-            nativeBuildInputs = [ pkgs.perl pkgs.pkg-config pkgs.cmake ];
+            nativeBuildInputs = [ pkgs.perl pkgs.pkg-config pkgs.cmake pkgs.protobuf ];
             buildType = "release";
             doCheck = false;
             cargoBuildFlags = [ "--workspace" "--exclude" "chat-cli" ];
@@ -95,6 +95,7 @@
               pkgs.pkg-config
               pkgs.cmake
               pkgs.perl
+              pkgs.protobuf
             ];
           };
         }