diff --git a/.github/workflows/publish.yaml b/.github/workflows/publish.yaml
index 0e401db634..c2ecb41863 100644
--- a/.github/workflows/publish.yaml
+++ b/.github/workflows/publish.yaml
@@ -215,7 +215,7 @@ jobs:
             --secret id=DEPOT_TOKEN,env=DEPOT_TOKEN \
             --build-arg BUILD_TARGET=${{ matrix.build_target }} \
             --build-arg BUILD_MODE=${{ steps.mode.outputs.build_mode }} \
-            --build-arg BUILD_FRONTEND=false \
+            --build-arg BUILD_FRONTEND=${{ matrix.build_target == 'engine' && 'true' || 'false' }} \
             -f ${{ matrix.docker }} \
             -t builder-${{ matrix.build_target }}-${{ matrix.platform }} \
             --load \
@@ -262,7 +262,7 @@ jobs:
             echo "build_frontend=true" >> $GITHUB_OUTPUT
           else
             echo "cargo_build_mode=debug" >> $GITHUB_OUTPUT
-            echo "build_frontend=false" >> $GITHUB_OUTPUT
+            echo "build_frontend=true" >> $GITHUB_OUTPUT
           fi
       - uses: ./.github/actions/docker-setup
         with:
diff --git a/docker/build/darwin-arm64.Dockerfile b/docker/build/darwin-arm64.Dockerfile
index 2e21d3e180..8f91c7b346 100644
--- a/docker/build/darwin-arm64.Dockerfile
+++ b/docker/build/darwin-arm64.Dockerfile
@@ -9,6 +9,7 @@ ARG BUILD_TARGET=engine
 ARG BUILD_MODE=release
 ARG BUILD_FRONTEND=false
 ARG VITE_APP_API_URL=__SAME__
+ARG VITE_FEATURE_FLAGS=
 
 ENV BINDGEN_EXTRA_CLANG_ARGS_aarch64_apple_darwin="--sysroot=/root/osxcross/target/SDK/MacOSX11.3.sdk -isystem /root/osxcross/target/SDK/MacOSX11.3.sdk/usr/include" \
     CFLAGS_aarch64_apple_darwin="-B/root/osxcross/target/bin" \
@@ -35,11 +36,7 @@ RUN if [ "$BUILD_TARGET" = "engine" ] && [ "$BUILD_FRONTEND" = "true" ]; then \
         export NODE_OPTIONS="--max-old-space-size=8192" && \
         export SKIP_NAPI_BUILD=1 && \
         pnpm install --ignore-scripts && \
-        if [ -n "$VITE_APP_API_URL" ]; then \
-            VITE_APP_API_URL="${VITE_APP_API_URL}" npx turbo build:engine -F @rivetkit/engine-frontend; \
-        else \
-            npx turbo build:engine -F @rivetkit/engine-frontend; \
-        fi; \
+        VITE_APP_API_URL="${VITE_APP_API_URL}" VITE_FEATURE_FLAGS="${VITE_FEATURE_FLAGS}" npx turbo build -F @rivetkit/engine-frontend; \
     fi
 
 RUN --mount=type=cache,id=cargo-registry-darwin-arm64,target=/usr/local/cargo/registry,sharing=locked \
diff --git a/docker/build/darwin-x64.Dockerfile b/docker/build/darwin-x64.Dockerfile
index 1482cb5b84..d862fa55a4 100644
--- a/docker/build/darwin-x64.Dockerfile
+++ b/docker/build/darwin-x64.Dockerfile
@@ -9,6 +9,7 @@ ARG BUILD_TARGET=engine
 ARG BUILD_MODE=release
 ARG BUILD_FRONTEND=false
 ARG VITE_APP_API_URL=__SAME__
+ARG VITE_FEATURE_FLAGS=
 
 ENV BINDGEN_EXTRA_CLANG_ARGS_x86_64_apple_darwin="--sysroot=/root/osxcross/target/SDK/MacOSX11.3.sdk -isystem /root/osxcross/target/SDK/MacOSX11.3.sdk/usr/include" \
     CFLAGS_x86_64_apple_darwin="-B/root/osxcross/target/bin" \
@@ -35,11 +36,7 @@ RUN if [ "$BUILD_TARGET" = "engine" ] && [ "$BUILD_FRONTEND" = "true" ]; then \
         export NODE_OPTIONS="--max-old-space-size=8192" && \
         export SKIP_NAPI_BUILD=1 && \
         pnpm install --ignore-scripts && \
-        if [ -n "$VITE_APP_API_URL" ]; then \
-            VITE_APP_API_URL="${VITE_APP_API_URL}" npx turbo build:engine -F @rivetkit/engine-frontend; \
-        else \
-            npx turbo build:engine -F @rivetkit/engine-frontend; \
-        fi; \
+        VITE_APP_API_URL="${VITE_APP_API_URL}" VITE_FEATURE_FLAGS="${VITE_FEATURE_FLAGS}" npx turbo build -F @rivetkit/engine-frontend; \
     fi
 
 RUN --mount=type=cache,id=cargo-registry-darwin-x64,target=/usr/local/cargo/registry,sharing=locked \
diff --git a/docker/build/linux-arm64-gnu.Dockerfile b/docker/build/linux-arm64-gnu.Dockerfile
index 4f8806ab2b..27f6ede6ad 100644
--- a/docker/build/linux-arm64-gnu.Dockerfile
+++ b/docker/build/linux-arm64-gnu.Dockerfile
@@ -9,6 +9,7 @@ ARG BUILD_TARGET=engine
 ARG BUILD_MODE=release
 ARG BUILD_FRONTEND=false
 ARG VITE_APP_API_URL=__SAME__
+ARG VITE_FEATURE_FLAGS=
 
 ENV RUSTFLAGS="--cfg tokio_unstable"
 ENV RUSTC_WRAPPER=sccache \
@@ -22,11 +23,7 @@ RUN if [ "$BUILD_TARGET" = "engine" ] && [ "$BUILD_FRONTEND" = "true" ]; then \
         export NODE_OPTIONS="--max-old-space-size=8192" && \
         export SKIP_NAPI_BUILD=1 && \
         pnpm install --ignore-scripts && \
-        if [ -n "$VITE_APP_API_URL" ]; then \
-            VITE_APP_API_URL="${VITE_APP_API_URL}" npx turbo build:engine -F @rivetkit/engine-frontend; \
-        else \
-            npx turbo build:engine -F @rivetkit/engine-frontend; \
-        fi; \
+        VITE_APP_API_URL="${VITE_APP_API_URL}" VITE_FEATURE_FLAGS="${VITE_FEATURE_FLAGS}" npx turbo build -F @rivetkit/engine-frontend; \
     fi
 
 RUN --mount=type=cache,id=cargo-registry-linux-arm64-gnu,target=/usr/local/cargo/registry,sharing=locked \
diff --git a/docker/build/linux-arm64-musl.Dockerfile b/docker/build/linux-arm64-musl.Dockerfile
index 2a2988e50d..ce59e540ad 100644
--- a/docker/build/linux-arm64-musl.Dockerfile
+++ b/docker/build/linux-arm64-musl.Dockerfile
@@ -9,6 +9,7 @@ ARG BUILD_TARGET=engine
 ARG BUILD_MODE=release
 ARG BUILD_FRONTEND=false
 ARG VITE_APP_API_URL=__SAME__
+ARG VITE_FEATURE_FLAGS=
 
 ENV OPENSSL_DIR=/musl-aarch64 \
     OPENSSL_INCLUDE_DIR=/musl-aarch64/include \
@@ -28,11 +29,7 @@ RUN if [ "$BUILD_TARGET" = "engine" ] && [ "$BUILD_FRONTEND" = "true" ]; then \
         export NODE_OPTIONS="--max-old-space-size=8192" && \
         export SKIP_NAPI_BUILD=1 && \
         pnpm install --ignore-scripts && \
-        if [ -n "$VITE_APP_API_URL" ]; then \
-            VITE_APP_API_URL="${VITE_APP_API_URL}" npx turbo build:engine -F @rivetkit/engine-frontend; \
-        else \
-            npx turbo build:engine -F @rivetkit/engine-frontend; \
-        fi; \
+        VITE_APP_API_URL="${VITE_APP_API_URL}" VITE_FEATURE_FLAGS="${VITE_FEATURE_FLAGS}" npx turbo build -F @rivetkit/engine-frontend; \
     fi
 
 RUN --mount=type=cache,id=cargo-registry-linux-arm64-musl,target=/usr/local/cargo/registry,sharing=locked \
diff --git a/docker/build/linux-x64-gnu.Dockerfile b/docker/build/linux-x64-gnu.Dockerfile
index 96d7133fc8..475cce5f7d 100644
--- a/docker/build/linux-x64-gnu.Dockerfile
+++ b/docker/build/linux-x64-gnu.Dockerfile
@@ -6,6 +6,7 @@
 #   BUILD_TARGET    - "engine" or "rivetkit-napi"
 #   BUILD_MODE      - "debug" (fast) or "release" (optimized)
 #   BUILD_FRONTEND  - "true" or "false" (engine only)
+#   VITE_FEATURE_FLAGS - comma-separated frontend features. Empty means engine UI.
 #
 # Base image: docker/builder-base/linux-gnu.Dockerfile
 # Rebuild base: scripts/docker-builder-base/build-push.sh linux-gnu --push
@@ -15,6 +16,7 @@ ARG BUILD_TARGET=engine
 ARG BUILD_MODE=release
 ARG BUILD_FRONTEND=false
 ARG VITE_APP_API_URL=__SAME__
+ARG VITE_FEATURE_FLAGS=
 
 ENV RUSTFLAGS="--cfg tokio_unstable"
 
@@ -30,11 +32,7 @@ RUN if [ "$BUILD_TARGET" = "engine" ] && [ "$BUILD_FRONTEND" = "true" ]; then \
         export NODE_OPTIONS="--max-old-space-size=8192" && \
         export SKIP_NAPI_BUILD=1 && \
         pnpm install --ignore-scripts && \
-        if [ -n "$VITE_APP_API_URL" ]; then \
-            VITE_APP_API_URL="${VITE_APP_API_URL}" npx turbo build:engine -F @rivetkit/engine-frontend; \
-        else \
-            npx turbo build:engine -F @rivetkit/engine-frontend; \
-        fi; \
+        VITE_APP_API_URL="${VITE_APP_API_URL}" VITE_FEATURE_FLAGS="${VITE_FEATURE_FLAGS}" npx turbo build -F @rivetkit/engine-frontend; \
     fi
 
 # Build binary.
diff --git a/docker/build/linux-x64-musl.Dockerfile b/docker/build/linux-x64-musl.Dockerfile
index 2c5b17a366..779efda303 100644
--- a/docker/build/linux-x64-musl.Dockerfile
+++ b/docker/build/linux-x64-musl.Dockerfile
@@ -9,6 +9,7 @@ ARG BUILD_TARGET=engine
 ARG BUILD_MODE=release
 ARG BUILD_FRONTEND=false
 ARG VITE_APP_API_URL=__SAME__
+ARG VITE_FEATURE_FLAGS=
 
 ENV OPENSSL_DIR=/musl-x86_64 \
     OPENSSL_INCLUDE_DIR=/musl-x86_64/include \
@@ -27,11 +28,7 @@ RUN if [ "$BUILD_TARGET" = "engine" ] && [ "$BUILD_FRONTEND" = "true" ]; then \
         export NODE_OPTIONS="--max-old-space-size=8192" && \
         export SKIP_NAPI_BUILD=1 && \
         pnpm install --ignore-scripts && \
-        if [ -n "$VITE_APP_API_URL" ]; then \
-            VITE_APP_API_URL="${VITE_APP_API_URL}" npx turbo build:engine -F @rivetkit/engine-frontend; \
-        else \
-            npx turbo build:engine -F @rivetkit/engine-frontend; \
-        fi; \
+        VITE_APP_API_URL="${VITE_APP_API_URL}" VITE_FEATURE_FLAGS="${VITE_FEATURE_FLAGS}" npx turbo build -F @rivetkit/engine-frontend; \
     fi
 
 RUN --mount=type=cache,id=cargo-registry-linux-x64-musl,target=/usr/local/cargo/registry,sharing=locked \
diff --git a/docker/build/windows-x64.Dockerfile b/docker/build/windows-x64.Dockerfile
index dd3faae1c5..c0017e240e 100644
--- a/docker/build/windows-x64.Dockerfile
+++ b/docker/build/windows-x64.Dockerfile
@@ -15,6 +15,7 @@ ARG BUILD_TARGET=engine
 ARG BUILD_MODE=release
 ARG BUILD_FRONTEND=false
 ARG VITE_APP_API_URL=__SAME__
+ARG VITE_FEATURE_FLAGS=
 
 # Windows-specific build flags:
 # - lld linker is ~5x faster than MinGW's default ld for big Rust binaries.
@@ -35,11 +36,7 @@ RUN if [ "$BUILD_TARGET" = "engine" ] && [ "$BUILD_FRONTEND" = "true" ]; then \
         export NODE_OPTIONS="--max-old-space-size=8192" && \
         export SKIP_NAPI_BUILD=1 && \
         pnpm install --ignore-scripts && \
-        if [ -n "$VITE_APP_API_URL" ]; then \
-            VITE_APP_API_URL="${VITE_APP_API_URL}" npx turbo build:engine -F @rivetkit/engine-frontend; \
-        else \
-            npx turbo build:engine -F @rivetkit/engine-frontend; \
-        fi; \
+        VITE_APP_API_URL="${VITE_APP_API_URL}" VITE_FEATURE_FLAGS="${VITE_FEATURE_FLAGS}" npx turbo build -F @rivetkit/engine-frontend; \
     fi
 
 RUN --mount=type=cache,id=cargo-registry-windows-x64,target=/usr/local/cargo/registry,sharing=locked \
diff --git a/docker/engine/Dockerfile b/docker/engine/Dockerfile
index 74c105bea4..6cbe4a83b7 100644
--- a/docker/engine/Dockerfile
+++ b/docker/engine/Dockerfile
@@ -7,6 +7,7 @@ FROM ghcr.io/rivet-dev/rivet/engine-base-builder:a36b881 AS builder
 # Docker automatically provides TARGETARCH
 ARG TARGETARCH
 
+ARG BUILD_FRONTEND=false
 ARG VITE_FEATURE_FLAGS=
 ARG CARGO_BUILD_MODE=debug
 ARG VITE_APP_API_URL=__SAME__
@@ -21,9 +22,9 @@ COPY . .
 # `lefthook install`, which needs a .git directory (excluded by
 # .dockerignore). lefthook is a dev-only git hook manager and has no
 # place inside the Docker build. SKIP_NAPI_BUILD=1 tells
-# @rivetkit/rivetkit-napi to skip its napi build — the frontend only
+# @rivetkit/rivetkit-napi to skip its napi build. The frontend only
 # consumes the TypeScript surface, not the runtime .node binary.
-RUN if [ -n "$VITE_FEATURE_FLAGS" ]; then \
+RUN if [ "$BUILD_FRONTEND" = "true" ]; then \
         export NODE_OPTIONS="--max-old-space-size=8192" && \
         export SKIP_NAPI_BUILD=1 && \
         pnpm install --ignore-scripts && \