diff --git a/pkg/dagger.io/cue.mod/module.cue b/pkg/dagger.io/cue.mod/module.cue
new file mode 100644
index 00000000..b2757079
--- /dev/null
+++ b/pkg/dagger.io/cue.mod/module.cue
@@ -0,0 +1 @@
+module: "dagger.io"
diff --git a/pkg/dagger.io/dagger/engine/types.cue b/pkg/dagger.io/dagger/engine/types.cue
index 4c13245c..15670df0 100644
--- a/pkg/dagger.io/dagger/engine/types.cue
+++ b/pkg/dagger.io/dagger/engine/types.cue
@@ -7,7 +7,7 @@ package engine
 //  - A directory containing binary artifacts
 // Rule of thumb: if it fits in a tar archive, it fits in a #FS.
 #FS: {
-	$dagger: fs: _id: string | null
+	$dagger: fs: _id: !="" | null
 }
 
 // A reference to an external secret, for example:
@@ -17,7 +17,7 @@ package engine
 // Secrets are never merged in the Cue tree. They can only be used
 // by a special filesystem mount designed to minimize leak risk.
 #Secret: {
-	$dagger: secret: _id: string
+	$dagger: secret: _id: !=""
 }
 
 // A reference to a network service endpoint, for example:
@@ -25,5 +25,5 @@ package engine
 //  - A unix or npipe socket
 //  - An HTTPS endpoint
 #Service: {
-	$dagger: service: _id: string
+	$dagger: service: _id: !=""
 }
diff --git a/pkg/universe.dagger.io/cue.mod/pkg/.gitignore b/pkg/universe.dagger.io/cue.mod/pkg/.gitignore
new file mode 100644
index 00000000..331f139d
--- /dev/null
+++ b/pkg/universe.dagger.io/cue.mod/pkg/.gitignore
@@ -0,0 +1,5 @@
+# generated by dagger
+dagger.lock
+alpha.dagger.io
+dagger.io
+universe.dagger.io
\ No newline at end of file
diff --git a/pkg/universe.dagger.io/cue.mod/pkg/alpha.dagger.io b/pkg/universe.dagger.io/cue.mod/pkg/alpha.dagger.io
deleted file mode 120000
index 961afc64..00000000
--- a/pkg/universe.dagger.io/cue.mod/pkg/alpha.dagger.io
+++ /dev/null
@@ -1 +0,0 @@
-../../../stdlib
\ No newline at end of file
diff --git a/pkg/universe.dagger.io/docker/build.cue b/pkg/universe.dagger.io/docker/build.cue
index 16e0724b..7ff5c8a8 100644
--- a/pkg/universe.dagger.io/docker/build.cue
+++ b/pkg/universe.dagger.io/docker/build.cue
@@ -10,6 +10,7 @@ import (
 	steps: [#Step, ...#Step]
 	output: #Image
 
+
 	// Generate build DAG from linerar steps
 	_dag: {
 		for idx, step in steps {
diff --git a/pkg/universe.dagger.io/yarn/tests/testdata/package.json b/pkg/universe.dagger.io/yarn/test/testdata/package.json
similarity index 100%
rename from pkg/universe.dagger.io/yarn/tests/testdata/package.json
rename to pkg/universe.dagger.io/yarn/test/testdata/package.json
diff --git a/pkg/universe.dagger.io/yarn/tests/testdata2/package.json b/pkg/universe.dagger.io/yarn/test/testdata2/package.json
similarity index 100%
rename from pkg/universe.dagger.io/yarn/tests/testdata2/package.json
rename to pkg/universe.dagger.io/yarn/test/testdata2/package.json
diff --git a/pkg/universe.dagger.io/yarn/test/yarn-test.cue b/pkg/universe.dagger.io/yarn/test/yarn-test.cue
new file mode 100644
index 00000000..014f238a
--- /dev/null
+++ b/pkg/universe.dagger.io/yarn/test/yarn-test.cue
@@ -0,0 +1,82 @@
+package yarn
+
+import (
+	"dagger.io/dagger"
+	"dagger.io/dagger/engine"
+
+	"universe.dagger.io/yarn"
+	// "universe.dagger.io/alpine"
+	// "universe.dagger.io/bash"
+)
+
+dagger.#Plan & {
+	inputs: {
+		directories: {
+			testdata: path: "./testdata"
+			testdata2: path: "./testdata2"
+		}
+	}
+
+	actions: {
+		TestReact: {
+			cache: engine.#CacheDir & {
+				id: "yarn cache"
+			}
+
+			pkg: yarn.#Build & {
+				source: inputs.directories.testdata.contents
+				"cache": cache
+			}
+
+			_image: engine.#Pull & {
+            	source: "alpine:3.15.0@sha256:e7d88de73db3d3fd9b2d63aa7f447a10fd0220b7cbf39803c803f2af9ba256b3"
+	        }
+
+			// FIXME: use bash.#Script
+			test: engine.#Exec & {
+				input: _image.output
+				mounts: build: {
+					dest: "/build"
+					contents: pkg.output
+				}
+				args: [
+					"sh", "-c",
+					#"""
+					test "$(cat /build/test)" = "output"
+					"""#
+				]
+			}
+		}
+
+		// FIXME: re-enable?
+		// TestSecretsAndFile: {
+		// 	pkg: #Package & {
+		// 		source:       inputs.directories.testdata2
+		// 		writeEnvFile: "/.env"
+		// 		env: {
+		// 			one: "one"
+		// 			two: "two"
+		// 		}
+		// 		secrets: {
+		// 			secretone: dagger.#Secret @dagger(input)
+		// 			secretwo:  dagger.#Secret @dagger(input)
+		// 		}
+		// 	}
+
+		// 	test: os.#Container & {
+		// 		image: alpine.#Image & {
+		// 			package: bash: true
+		// 		}
+		// 		shell: path: "/bin/bash"
+		// 		mount: "/build": from: pkg.build
+		// 		command: """
+		// 			content="$(cat /build/env)"
+		// 			[[ "${content}" = *"SECRETONE="* ]] && \\
+		// 			[[ "${content}" = *"SECRETWO="* ]] && \\
+		// 			[[ "${content}" = *"ONE=one"* ]] && \\
+		// 			[[ "${content}" = *"TWO=two"* ]]
+		// 			"""
+		// 	}
+		// }
+	}
+}
\ No newline at end of file
diff --git a/pkg/universe.dagger.io/yarn/tests/simple/simple.cue b/pkg/universe.dagger.io/yarn/tests/simple/simple.cue
deleted file mode 100644
index 19d771b1..00000000
--- a/pkg/universe.dagger.io/yarn/tests/simple/simple.cue
+++ /dev/null
@@ -1,11 +0,0 @@
-package yarn
-
-import (
-	"dagger.io/dagger/engine"
-)
-
-b: #Build & {
-	source: engine.#Scratch
-}
-
-out: b.output
diff --git a/pkg/universe.dagger.io/yarn/yarn.cue b/pkg/universe.dagger.io/yarn/yarn.cue
index 6f0d839c..4cac4723 100644
--- a/pkg/universe.dagger.io/yarn/yarn.cue
+++ b/pkg/universe.dagger.io/yarn/yarn.cue
@@ -7,7 +7,8 @@ import (
 	"dagger.io/dagger"
 	"dagger.io/dagger/engine"
 
-	"universe.dagger.io/alpine"
+	// "universe.dagger.io/alpine"
+	"universe.dagger.io/docker"
 	"universe.dagger.io/bash"
 )
 
@@ -30,28 +31,54 @@ import (
 	// Run this yarn script
 	script: string | *"build"
 
+	// Fix for shadowing issues
+	let yarnScript = script
+
+	// FIXME: `CacheDir` must be passed by the caller?
+	cache: engine.#CacheDir
+
 	// Optional arguments for the script
 	args: [...string] | *[]
 
 	// Secret variables
+	// FIXME: not implemented. Are they needed?
 	secrets: [string]: dagger.#Secret
 
 	// Yarn version
 	yarnVersion: *"=~1.22" | string
 
+	// FIXME: trouble getting docker.#Build to work (cueflow task dependencies not working)
+	alpine: docker.#Pull & {
+		source: "index.docker.io/alpine:3.13.5@sha256:69e70a79f2d41ab5d637de98c1e0b055206ba40a8145e7bddb55ccc04e13cf8f"
+	}
+	yarnImage: docker.#Run & {
+		image: alpine.image
+		script: """
+		apk add -U --no-cache bash yarn
+		"""
+	}
+
 	// Run yarn in a containerized build environment
 	command: bash.#Run & {
-		*{
-			image: (alpine.#Build & {
-				bash: version: "=~5.1"
-				yarn: version: yarnVersion
-			}).image
-			env: CUSTOM_IMAGE: "0"
-		} | {
-			env: CUSTOM_IMAGE: "1"
-		}
+		// FIXME: not working?
+		// *{
+		// 	_image: alpine.#Build & {
+		// 		packages: {
+		// 			bash: version: "=~5.1"
+		// 			yarn: version: yarnVersion
+		// 		}
+		// 	}
 
-		script: """
+		// 	image: _image.output
+		// 	env: CUSTOM_IMAGE: "0"
+		// } | {
+		// 	env: CUSTOM_IMAGE: "1"
+		// }
+
+		image: docker.#Image & yarnImage.output 
+
+
+		script: #"""
 			# Create $ENVFILE_NAME file if set
 			[ -n "$ENVFILE_NAME" ] && echo "$ENVFILE" > "$ENVFILE_NAME"
 
@@ -60,12 +87,12 @@ import (
 			opts=( $(echo $YARN_ARGS) )
 			yarn --cwd "$YARN_CWD" run "$YARN_BUILD_SCRIPT" ${opts[@]}
 			mv "$YARN_BUILD_DIRECTORY" /build
-			"""
+			"""#
 
 		mounts: {
 			"yarn cache": {
 				dest:     "/cache/yarn"
-				contents: engine.#CacheDir
+				contents: cache
 			}
 			"package source": {
 				dest:     "/src"
@@ -74,10 +101,11 @@ import (
 			// FIXME: mount secrets
 		}
 
-		output: directories: "/build": _
+		// FIXME
+		directories: "/build": _
 
 		env: {
-			YARN_BUILD_SCRIPT:    script
+			YARN_BUILD_SCRIPT:    yarnScript
 			YARN_ARGS:            strings.Join(args, "\n")
 			YARN_CACHE_FOLDER:    "/cache/yarn"
 			YARN_CWD:             cwd
@@ -92,5 +120,5 @@ import (
 	}
 
 	// The final contents of the package after build
-	output: command.output.directories."/build".contents
+	output: command.directories."/build".contents
 }
diff --git a/plancontext/fs.go b/plancontext/fs.go
index ab6f09c6..f1213ce3 100644
--- a/plancontext/fs.go
+++ b/plancontext/fs.go
@@ -84,7 +84,7 @@ func (c *fsContext) FromValue(v *compiler.Value) (*FS, error) {
 
 	id, err := v.LookupPath(fsIDPath).String()
 	if err != nil {
-		return nil, fmt.Errorf("invalid FS %q: %w", v.Path(), err)
+		return nil, fmt.Errorf("invalid FS at path %q: %w", v.Path(), err)
 	}
 
 	fs, ok := c.store[id]