diff --git a/dagger/compiler.go b/dagger/compiler.go
index 9ca9d271..4aa30856 100644
--- a/dagger/compiler.go
+++ b/dagger/compiler.go
@@ -18,7 +18,7 @@ import (
 // (we call it compiler to avoid confusion with dagger runtime)
 // Use this instead of cue.Runtime
 type Compiler struct {
-	sync.Mutex
+	sync.RWMutex
 	cue.Runtime
 	spec *Spec
 }
@@ -27,20 +27,19 @@ func (cc *Compiler) Cue() *cue.Runtime {
 	return &(cc.Runtime)
 }
 
-func (cc *Compiler) Spec() (*Spec, error) {
+func (cc *Compiler) Spec() *Spec {
 	if cc.spec != nil {
-		return cc.spec, nil
+		return cc.spec
 	}
 	v, err := cc.Compile("spec.cue", DaggerSpec)
 	if err != nil {
-		return nil, err
+		panic(err)
 	}
-	spec, err := v.Spec()
+	cc.spec, err = v.Spec()
 	if err != nil {
-		return nil, err
+		panic(err)
 	}
-	cc.spec = spec
-	return spec, nil
+	return cc.spec
 }
 
 // Compile an empty struct
diff --git a/dagger/env.go b/dagger/env.go
index 0ab8e9f6..cdeea27b 100644
--- a/dagger/env.go
+++ b/dagger/env.go
@@ -3,7 +3,6 @@ package dagger
 import (
 	"context"
 	"os"
-	"sync"
 
 	"cuelang.org/go/cue"
 	cueflow "cuelang.org/go/tools/flow"
@@ -144,14 +143,9 @@ func (env *Env) Walk(ctx context.Context, fn EnvWalkFunc) (*Value, error) {
 		return nil, err
 	}
 
-	l := sync.Mutex{}
-
 	// Cueflow config
 	flowCfg := &cueflow.Config{
 		UpdateFunc: func(c *cueflow.Controller, t *cueflow.Task) error {
-			l.Lock()
-			defer l.Unlock()
-
 			if t == nil {
 				return nil
 			}
@@ -180,9 +174,6 @@ func (env *Env) Walk(ctx context.Context, fn EnvWalkFunc) (*Value, error) {
 	}
 	// Cueflow match func
 	flowMatchFn := func(v cue.Value) (cueflow.Runner, error) {
-		l.Lock()
-		defer l.Unlock()
-
 		lg := lg.
 			With().
 			Str("path", v.Path().String()).
@@ -200,9 +191,6 @@ func (env *Env) Walk(ctx context.Context, fn EnvWalkFunc) (*Value, error) {
 			return nil, err
 		}
 		return cueflow.RunnerFunc(func(t *cueflow.Task) error {
-			l.Lock()
-			defer l.Unlock()
-
 			return fn(ctx, c, t)
 		}), nil
 	}
diff --git a/dagger/op.go b/dagger/op.go
index 72ea0b02..7a376bc2 100644
--- a/dagger/op.go
+++ b/dagger/op.go
@@ -202,6 +202,12 @@ func (op *Op) Export(ctx context.Context, fs FS, out Fillable) (FS, error) {
 	}
 	switch format {
 	case "string":
+		log.
+			Ctx(ctx).
+			Debug().
+			Bytes("contents", contents).
+			Msg("exporting string")
+
 		if err := out.Fill(string(contents)); err != nil {
 			return fs, err
 		}
@@ -210,6 +216,13 @@ func (op *Op) Export(ctx context.Context, fs FS, out Fillable) (FS, error) {
 		if err := json.Unmarshal(contents, &o); err != nil {
 			return fs, err
 		}
+
+		log.
+			Ctx(ctx).
+			Debug().
+			Interface("contents", o).
+			Msg("exporting json")
+
 		if err := out.Fill(o); err != nil {
 			return fs, err
 		}
diff --git a/dagger/spec.go b/dagger/spec.go
index 295d89b4..89f426c3 100644
--- a/dagger/spec.go
+++ b/dagger/spec.go
@@ -1,7 +1,6 @@
 package dagger
 
 import (
-	"cuelang.org/go/cue"
 	cueerrors "cuelang.org/go/cue/errors"
 	"github.com/pkg/errors"
 )
@@ -12,28 +11,14 @@ type Spec struct {
 }
 
 // eg. Validate(op, "#Op")
-func (s Spec) Validate(v *Value, defpath string) (err error) {
-	// Expand cue errors to get full details
-	// FIXME: there is probably a cleaner way to do this.
-	defer func() {
-		if err != nil {
-			err = errors.New(cueerrors.Details(err, nil))
-		}
-	}()
-
+func (s Spec) Validate(v *Value, defpath string) error {
 	// Lookup def by name, eg. "#Script" or "#Copy"
 	// See dagger/spec.cue
 	def := s.root.Get(defpath)
-	if err := def.Validate(); err != nil {
-		return err
-	}
-	merged := def.Unwrap().Fill(v.Value)
-	if err := merged.Err(); err != nil {
-		return err
-	}
-	if err := merged.Validate(cue.Final()); err != nil {
-		return err
+	if err := def.Fill(v); err != nil {
+		return errors.New(cueerrors.Details(err, nil))
 	}
+
 	return nil
 }
 
diff --git a/dagger/value.go b/dagger/value.go
index d4ee3d62..623ce007 100644
--- a/dagger/value.go
+++ b/dagger/value.go
@@ -9,51 +9,261 @@ import (
 	"github.com/moby/buildkit/client/llb"
 )
 
-// Polyfill for cue.Value.
+// Value is a wrapper around cue.Value.
 // Use instead of cue.Value and cue.Instance
 type Value struct {
-	// FIXME: don't embed, cleaner API
-	cue.Value
+	val  cue.Value
 	cc   *Compiler
 	inst *cue.Instance
 }
 
-func (v *Value) Lock() {
-	if v.cc == nil {
-		return
+func (v *Value) CueInst() *cue.Instance {
+	return v.inst
+}
+
+func (v *Value) Compiler() *Compiler {
+	return v.cc
+}
+
+func (v *Value) Wrap(v2 cue.Value) *Value {
+	return wrapValue(v2, v.inst, v.cc)
+}
+
+func wrapValue(v cue.Value, inst *cue.Instance, cc *Compiler) *Value {
+	return &Value{
+		val:  v,
+		cc:   cc,
+		inst: inst,
 	}
+}
+
+// Fill is a concurrency safe wrapper around cue.Value.Fill()
+// This is the only method which changes the value in-place.
+func (v *Value) Fill(x interface{}) error {
 	v.cc.Lock()
-}
+	defer v.cc.Unlock()
 
-func (v *Value) Unlock() {
-	if v.cc == nil {
-		return
+	// If calling Fill() with a Value, we want to use the underlying
+	// cue.Value to fill.
+	if val, ok := x.(*Value); ok {
+		v.val = v.val.Fill(val.val)
+	} else {
+		v.val = v.val.Fill(x)
 	}
-	v.cc.Unlock()
-}
-
-func (v *Value) Lookup(path ...string) *Value {
-	v.Lock()
-	defer v.Unlock()
-
-	return v.Wrap(v.Unwrap().LookupPath(cueStringsToCuePath(path...)))
+	return v.Validate()
 }
 
+// LookupPath is a concurrency safe wrapper around cue.Value.LookupPath
 func (v *Value) LookupPath(p cue.Path) *Value {
-	v.Lock()
-	defer v.Unlock()
-	return v.Wrap(v.Unwrap().LookupPath(p))
+	v.cc.RLock()
+	defer v.cc.RUnlock()
+
+	return v.Wrap(v.val.LookupPath(p))
 }
 
-// FIXME: deprecated by Get()
-func (v *Value) LookupTarget(target string) *Value {
-	return v.LookupPath(cue.ParsePath(target))
+// Lookup is a helper function to lookup by path parts.
+func (v *Value) Lookup(path ...string) *Value {
+	return v.LookupPath(cueStringsToCuePath(path...))
 }
 
+// Get is a helper function to lookup by path string
 func (v *Value) Get(target string) *Value {
 	return v.LookupPath(cue.ParsePath(target))
 }
 
+// Proxy function to the underlying cue.Value
+func (v *Value) Len() cue.Value {
+	return v.val.Len()
+}
+
+// Proxy function to the underlying cue.Value
+func (v *Value) List() (cue.Iterator, error) {
+	return v.val.List()
+}
+
+// Proxy function to the underlying cue.Value
+func (v *Value) Fields() (*cue.Iterator, error) {
+	return v.val.Fields()
+}
+
+// Proxy function to the underlying cue.Value
+func (v *Value) Struct() (*cue.Struct, error) {
+	return v.val.Struct()
+}
+
+// Proxy function to the underlying cue.Value
+func (v *Value) Exists() bool {
+	return v.val.Exists()
+}
+
+// Proxy function to the underlying cue.Value
+func (v *Value) String() (string, error) {
+	return v.val.String()
+}
+
+// Proxy function to the underlying cue.Value
+func (v *Value) Path() cue.Path {
+	return v.val.Path()
+}
+
+// Proxy function to the underlying cue.Value
+func (v *Value) Decode(x interface{}) error {
+	return v.val.Decode(x)
+}
+
+func (v *Value) RangeList(fn func(int, *Value) error) error {
+	it, err := v.List()
+	if err != nil {
+		return err
+	}
+	i := 0
+	for it.Next() {
+		if err := fn(i, v.Wrap(it.Value())); err != nil {
+			return err
+		}
+		i++
+	}
+	return nil
+}
+
+func (v *Value) RangeStruct(fn func(string, *Value) error) error {
+	it, err := v.Fields()
+	if err != nil {
+		return err
+	}
+	for it.Next() {
+		if err := fn(it.Label(), v.Wrap(it.Value())); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+// FIXME: receive string path?
+func (v *Value) Merge(x interface{}, path ...string) (*Value, error) {
+	if xval, ok := x.(*Value); ok {
+		if xval.Compiler() != v.Compiler() {
+			return nil, fmt.Errorf("can't merge values from different compilers")
+		}
+		x = xval.val
+	}
+
+	v.cc.Lock()
+	result := v.Wrap(v.val.Fill(x, path...))
+	v.cc.Unlock()
+
+	return result, result.Validate()
+}
+
+func (v *Value) MergePath(x interface{}, p cue.Path) (*Value, error) {
+	// FIXME: array indexes and defs are not supported,
+	//  they will be silently converted to regular fields.
+	//  eg.  `foo.#bar[0]` will become `foo["#bar"]["0"]`
+	return v.Merge(x, cuePathToStrings(p)...)
+}
+
+func (v *Value) MergeTarget(x interface{}, target string) (*Value, error) {
+	return v.MergePath(x, cue.ParsePath(target))
+}
+
+// Recursive concreteness check.
+// Return false if v is not concrete, or contains any
+// non-concrete fields or items.
+func (v *Value) IsConcreteR() bool {
+	// FIXME: use Value.Walk
+	if it, err := v.Fields(); err == nil {
+		for it.Next() {
+			w := v.Wrap(it.Value())
+			if !w.IsConcreteR() {
+				return false
+			}
+		}
+		return true
+	}
+	if it, err := v.List(); err == nil {
+		for it.Next() {
+			w := v.Wrap(it.Value())
+			if !w.IsConcreteR() {
+				return false
+			}
+		}
+		return true
+	}
+	dv, _ := v.val.Default()
+	return v.val.IsConcrete() || dv.IsConcrete()
+}
+
+// Export concrete values to JSON. ignoring non-concrete values.
+// Contrast with cue.Value.MarshalJSON which requires all values
+// to be concrete.
+func (v *Value) JSON() JSON {
+	var out JSON
+	v.val.Walk(
+		func(v cue.Value) bool {
+			b, err := v.MarshalJSON()
+			if err == nil {
+				newOut, err := out.Set(b, cuePathToStrings(v.Path())...)
+				if err == nil {
+					out = newOut
+				}
+				return false
+			}
+			return true
+		},
+		nil,
+	)
+	return out
+}
+
+func (v *Value) SaveJSON(fs FS, filename string) FS {
+	return fs.Change(func(st llb.State) llb.State {
+		return st.File(
+			llb.Mkfile(filename, 0600, v.JSON()),
+		)
+	})
+}
+
+func (v *Value) Save(fs FS, filename string) (FS, error) {
+	src, err := v.Source()
+	if err != nil {
+		return fs, err
+	}
+	return fs.Change(func(st llb.State) llb.State {
+		return st.File(
+			llb.Mkfile(filename, 0600, src),
+		)
+	}), nil
+}
+
+func (v *Value) Validate(defs ...string) error {
+	if err := v.val.Validate(); err != nil {
+		return err
+	}
+	if len(defs) == 0 {
+		return nil
+	}
+	spec := v.Compiler().Spec()
+	for _, def := range defs {
+		if err := spec.Validate(v, def); err != nil {
+			return err
+		}
+	}
+	return nil
+}
+
+func (v *Value) Source() ([]byte, error) {
+	return cueformat.Node(v.val.Eval().Syntax())
+}
+
+func (v *Value) IsEmptyStruct() bool {
+	if st, err := v.Struct(); err == nil {
+		if st.Len() == 0 {
+			return true
+		}
+	}
+	return false
+}
+
 // Component returns the component value if v is a valid dagger component or an error otherwise.
 // If no '#dagger' annotation is present, os.ErrNotExist
 // is returned.
@@ -111,11 +321,8 @@ func (v *Value) ScriptOrComponent() (interface{}, error) {
 
 func (v *Value) Op() (*Op, error) {
 	// Merge #Op definition from spec to get default values
-	spec, err := v.Compiler().Spec()
-	if err != nil {
-		return nil, err
-	}
-	v, err = spec.Get("#Op").Merge(v)
+	spec := v.Compiler().Spec()
+	v, err := spec.Get("#Op").Merge(v)
 	if err != nil {
 		return nil, err
 	}
@@ -143,194 +350,3 @@ func (v *Value) Spec() (*Spec, error) {
 		root: v,
 	}, nil
 }
-
-// FIXME: receive string path?
-func (v *Value) Merge(x interface{}, path ...string) (*Value, error) {
-	if xval, ok := x.(*Value); ok {
-		if xval.Compiler() != v.Compiler() {
-			return nil, fmt.Errorf("can't merge values from different compilers")
-		}
-		x = xval.Unwrap()
-	}
-	result := v.Wrap(v.Unwrap().Fill(x, path...))
-	return result, result.Validate()
-}
-
-func (v *Value) MergePath(x interface{}, p cue.Path) (*Value, error) {
-	// FIXME: array indexes and defs are not supported,
-	//  they will be silently converted to regular fields.
-	//  eg.  `foo.#bar[0]` will become `foo["#bar"]["0"]`
-	return v.Merge(x, cuePathToStrings(p)...)
-}
-
-func (v *Value) MergeTarget(x interface{}, target string) (*Value, error) {
-	return v.MergePath(x, cue.ParsePath(target))
-}
-
-func (v *Value) RangeList(fn func(int, *Value) error) error {
-	it, err := v.List()
-	if err != nil {
-		return err
-	}
-	i := 0
-	for it.Next() {
-		if err := fn(i, v.Wrap(it.Value())); err != nil {
-			return err
-		}
-		i++
-	}
-	return nil
-}
-
-func (v *Value) RangeStruct(fn func(string, *Value) error) error {
-	it, err := v.Fields()
-	if err != nil {
-		return err
-	}
-	for it.Next() {
-		if err := fn(it.Label(), v.Wrap(it.Value())); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
-// Recursive concreteness check.
-// Return false if v is not concrete, or contains any
-// non-concrete fields or items.
-func (v *Value) IsConcreteR() bool {
-	// FIXME: use Value.Walk
-	if it, err := v.Fields(); err == nil {
-		for it.Next() {
-			w := v.Wrap(it.Value())
-			if !w.IsConcreteR() {
-				return false
-			}
-		}
-		return true
-	}
-	if it, err := v.List(); err == nil {
-		for it.Next() {
-			w := v.Wrap(it.Value())
-			if !w.IsConcreteR() {
-				return false
-			}
-		}
-		return true
-	}
-	dv, _ := v.Default()
-	return v.IsConcrete() || dv.IsConcrete()
-}
-
-// Export concrete values to JSON. ignoring non-concrete values.
-// Contrast with cue.Value.MarshalJSON which requires all values
-// to be concrete.
-func (v *Value) JSON() JSON {
-	v.Lock()
-	defer v.Unlock()
-	var out JSON
-	v.Walk(
-		func(v cue.Value) bool {
-			b, err := v.MarshalJSON()
-			if err == nil {
-				newOut, err := out.Set(b, cuePathToStrings(v.Path())...)
-				if err == nil {
-					out = newOut
-				}
-				return false
-			}
-			return true
-		},
-		nil,
-	)
-	return out
-}
-
-func (v *Value) SaveJSON(fs FS, filename string) FS {
-	return fs.Change(func(st llb.State) llb.State {
-		return st.File(
-			llb.Mkfile(filename, 0600, v.JSON()),
-		)
-	})
-}
-
-func (v *Value) Save(fs FS, filename string) (FS, error) {
-	src, err := v.Source()
-	if err != nil {
-		return fs, err
-	}
-	return fs.Change(func(st llb.State) llb.State {
-		return st.File(
-			llb.Mkfile(filename, 0600, src),
-		)
-	}), nil
-}
-
-func (v *Value) Validate(defs ...string) error {
-	if err := v.Unwrap().Validate(); err != nil {
-		return err
-	}
-	if len(defs) == 0 {
-		return nil
-	}
-	spec, err := v.Compiler().Spec()
-	if err != nil {
-		return err
-	}
-	for _, def := range defs {
-		if err := spec.Validate(v, def); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
-// Value implements Fillable.
-// This is the only method which changes the value in-place.
-// FIXME this co-exists awkwardly with the rest of Value.
-func (v *Value) Fill(x interface{}) error {
-	v.Value = v.Value.Fill(x)
-	return v.Validate()
-}
-
-func (v *Value) Source() ([]byte, error) {
-	v.Lock()
-	defer v.Unlock()
-	return cueformat.Node(v.Eval().Syntax())
-}
-
-func (v *Value) IsEmptyStruct() bool {
-	if st, err := v.Struct(); err == nil {
-		if st.Len() == 0 {
-			return true
-		}
-	}
-	return false
-}
-
-func (v *Value) CueInst() *cue.Instance {
-	return v.inst
-}
-
-func (v *Value) Compiler() *Compiler {
-	//	if v.cc == nil {
-	//		return &Compiler{}
-	//	}
-	return v.cc
-}
-
-func (v *Value) Wrap(v2 cue.Value) *Value {
-	return wrapValue(v2, v.inst, v.cc)
-}
-
-func (v *Value) Unwrap() cue.Value {
-	return v.Value
-}
-
-func wrapValue(v cue.Value, inst *cue.Instance, cc *Compiler) *Value {
-	return &Value{
-		Value: v,
-		cc:    cc,
-		inst:  inst,
-	}
-}
diff --git a/examples/tests/fetch-container/exist/main.cue b/examples/tests/fetch-container/exist/main.cue
index 641411c1..bfd36bee 100644
--- a/examples/tests/fetch-container/exist/main.cue
+++ b/examples/tests/fetch-container/exist/main.cue
@@ -31,7 +31,7 @@ busybox4: {
 	#dagger: compute: [
 		{
 			do: "fetch-container"
-			ref: "busyboxa@sha256:e2af53705b841ace3ab3a44998663d4251d33ee8a9acaf71b66df4ae01c3bbe7"
+			ref: "busybox@sha256:e2af53705b841ace3ab3a44998663d4251d33ee8a9acaf71b66df4ae01c3bbe7"
 		},
 	]
 }