diff --git a/playground/impl.go b/playground/impl.go
index 9858bd1ef..fc1dcb5e5 100644
--- a/playground/impl.go
+++ b/playground/impl.go
@@ -26,6 +26,7 @@ import (
 	"cuelang.org/go/cue/format"
 	"cuelang.org/go/cue/load"
 	"cuelang.org/go/cue/token"
+	"cuelang.org/go/pkg/encoding/json"
 	"github.com/cue-lang/cuelang.org/playground/internal/cuelang_org_go_internal/encoding"
 	"github.com/cue-lang/cuelang.org/playground/internal/cuelang_org_go_internal/filetypes"
 )
@@ -36,6 +37,7 @@ const (
 	functionExport function = "export"
 	functionEval   function = "eval"
 	functionDef    function = "def"
+	functionFmt    function = "fmt"
 )
 
 type input string
@@ -57,7 +59,7 @@ const (
 func handleCUECompile(in input, fn function, out output, inputVal string) (string, error) {
 	// TODO implement more functions
 	switch fn {
-	case functionExport, functionEval, functionDef:
+	case functionExport, functionEval, functionDef, functionFmt:
 	default:
 		return "", fmt.Errorf("function %q is not implemented", fn)
 	}
@@ -67,6 +69,29 @@ func handleCUECompile(in input, fn function, out output, inputVal string) (strin
 	default:
 		return "", fmt.Errorf("unknown input type: %v", in)
 	}
+
+	switch out {
+	case outputCUE, outputJSON, outputYaml:
+	default:
+		return "", fmt.Errorf("unknown ouput type: %v", out)
+	}
+
+	if fn == functionFmt {
+		// For Fmt, we require in and out to be the same.
+		if in != input(out) {
+			return "", fmt.Errorf("fmt input output mismatch: input = %q, output = %q", in, out)
+		}
+
+		// Error in case we are trying to format something we can't
+		switch in {
+		case inputCUE, inputJSON:
+		default:
+			return "", fmt.Errorf("don't (yet) know how to format %q", in)
+		}
+
+		return formatInput(in, inputVal), nil
+	}
+
 	loadCfg := &load.Config{
 		Stdin:      strings.NewReader(inputVal),
 		Dir:        "/",
@@ -89,11 +114,6 @@ func handleCUECompile(in input, fn function, out output, inputVal string) (strin
 	if err := v.Err(); err != nil {
 		return "", fmt.Errorf("failed to build: %w", err)
 	}
-	switch out {
-	case outputCUE, outputJSON, outputYaml:
-	default:
-		return "", fmt.Errorf("unknown ouput type: %v", out)
-	}
 	cueOpts := []cue.Option{
 		cue.Docs(true),
 		cue.Attributes(true),
@@ -126,6 +146,9 @@ func handleCUECompile(in input, fn function, out output, inputVal string) (strin
 
 	// TODO(mvdan): Note that formatOpts appear to do nothing at all.
 	// For instance, the tests indent JSON with four spaces instead of two.
+	//
+	// TODO(myitcv): make the sharing of format options consistent with
+	// formatInput.
 	var formatOpts []format.Option
 	switch out {
 	case outputCUE:
@@ -159,3 +182,27 @@ func getSyntax(v cue.Value, opts []cue.Option) *ast.File {
 		panic("unreachable")
 	}
 }
+
+// formatInput attempts to format s according to the filetype in. On success it
+// returns the formatted result, otherwise it returns s. formatInput panics on
+// an unsupported in filetype.
+func formatInput(in input, s string) string {
+	switch in {
+	case inputCUE:
+		// TODO(myitcv): make the sharing of format options consistent with
+		// handleCUECompile.
+		b, err := format.Source([]byte(s), format.TabIndent(true))
+		if err != nil {
+			return s
+		}
+		return string(b)
+	case inputJSON:
+		res, err := json.Indent([]byte(s), "", "  ")
+		if err != nil {
+			return s
+		}
+		return res
+	default:
+		panic(fmt.Errorf("don't know how to format %q", in))
+	}
+}
diff --git a/playground/impl_test.go b/playground/impl_test.go
index aa1efba25..05e015d97 100644
--- a/playground/impl_test.go
+++ b/playground/impl_test.go
@@ -89,6 +89,42 @@ var testTable = []struct {
 		"x: {\n\tx2: \"foo\"\n}\ny: x.missing\n",
 		"",
 	},
+
+	// Simple format checks; no errors
+	{
+		inputCUE, functionFmt, outputCUE,
+		"  package foo\n",
+		"package foo\n",
+		"",
+	},
+	{
+		inputJSON, functionFmt, outputJSON,
+
+		// Preserve double-declaration; this is format-only, not evaluation
+		"{\n\"x\":  5,\n\"x\":  5\n}\n",
+		"{\n  \"x\": 5,\n  \"x\": 5\n}\n",
+		"",
+	},
+
+	// Error format checks
+	{
+		inputCUE, functionFmt, outputCUE,
+		"  {\n",
+		"  {\n",
+		"",
+	},
+	{
+		inputJSON, functionFmt, outputJSON,
+		"{\nx:  5\n}\n",
+		"{\nx:  5\n}\n",
+		"",
+	},
+	{
+		inputYaml, functionFmt, outputYaml,
+		"x:  5\n",
+		"",
+		"don't (yet) know how to format \"yaml\"",
+	},
 }
 
 func TestHandleCUECompile(t *testing.T) {