diff --git a/parser/metadecoders/decoder.go b/parser/metadecoders/decoder.go
index 419fbf4d2..83b4b04cd 100644
--- a/parser/metadecoders/decoder.go
+++ b/parser/metadecoders/decoder.go
@@ -36,6 +36,10 @@ import (
 
 // Decoder provides some configuration options for the decoders.
 type Decoder struct {
+	// Format specifies a specific format to decode from. If empty or
+	// unspecified, it's inferred from the contents or the filename.
+	Format string
+
 	// Delimiter is the field delimiter. Used in the CSV decoder. Default is
 	// ','.
 	Delimiter rune
@@ -57,6 +61,7 @@ type Decoder struct {
 // OptionsKey is used in cache keys.
 func (d Decoder) OptionsKey() string {
 	var sb strings.Builder
+	sb.WriteString(d.Format)
 	sb.WriteRune(d.Delimiter)
 	sb.WriteRune(d.Comment)
 	sb.WriteString(strconv.FormatBool(d.LazyQuotes))
diff --git a/tpl/transform/unmarshal.go b/tpl/transform/unmarshal.go
index a3a9c040a..d0edda453 100644
--- a/tpl/transform/unmarshal.go
+++ b/tpl/transform/unmarshal.go
@@ -72,9 +72,17 @@ func (ns *Namespace) Unmarshal(args ...any) (any, error) {
 		}
 
 		v, err := ns.cacheUnmarshal.GetOrCreate(key, func(string) (*resources.StaleValue[any], error) {
-			f := metadecoders.FormatFromStrings(r.MediaType().Suffixes()...)
-			if f == "" {
-				return nil, fmt.Errorf("MIME %q not supported", r.MediaType())
+			var f metadecoders.Format
+			if decoder.Format != "" {
+				f = metadecoders.FormatFromString(decoder.Format)
+				if f == "" {
+					return nil, fmt.Errorf("format %q not supported", decoder.Format)
+				}
+			} else {
+				f = metadecoders.FormatFromStrings(r.MediaType().Suffixes()...)
+				if f == "" {
+					return nil, fmt.Errorf("MIME %q not supported", r.MediaType())
+				}
 			}
 
 			reader, err := r.ReadSeekCloser()
@@ -119,10 +127,22 @@ func (ns *Namespace) Unmarshal(args ...any) (any, error) {
 
 	key := hashing.MD5FromStringHexEncoded(dataStr)
 
+	if decoder != metadecoders.Default {
+		key += decoder.OptionsKey()
+	}
+
 	v, err := ns.cacheUnmarshal.GetOrCreate(key, func(string) (*resources.StaleValue[any], error) {
-		f := decoder.FormatFromContentString(dataStr)
-		if f == "" {
-			return nil, errors.New("unknown format")
+		var f metadecoders.Format
+		if decoder.Format != "" {
+			f = metadecoders.FormatFromString(decoder.Format)
+			if f == "" {
+				return nil, fmt.Errorf("format %q not supported", decoder.Format)
+			}
+		} else {
+			f = decoder.FormatFromContentString(dataStr)
+			if f == "" {
+				return nil, errors.New("unknown format")
+			}
 		}
 
 		v, err := decoder.Unmarshal([]byte(dataStr), f)
diff --git a/tpl/transform/unmarshal_test.go b/tpl/transform/unmarshal_test.go
index 9b34e1daa..934b29ee1 100644
--- a/tpl/transform/unmarshal_test.go
+++ b/tpl/transform/unmarshal_test.go
@@ -128,6 +128,9 @@ func TestUnmarshal(t *testing.T) {
 		{testContentResource{key: "r1", content: `a;b;c`, mime: media.Builtin.CSVType}, map[string]any{"delimiter": ";"}, func(r [][]string) {
 			b.Assert([][]string{{"a", "b", "c"}}, qt.DeepEquals, r)
 		}},
+		{testContentResource{key: "r1", content: `{p: [a, b]}`, mime: media.Builtin.JSONType}, map[string]any{"format": "yaml"}, func(m map[string]any) {
+			b.Assert(m, qt.DeepEquals, map[string]any{"p": []any{"a", "b"}})
+		}},
 		{"a,b,c", nil, func(r [][]string) {
 			b.Assert([][]string{{"a", "b", "c"}}, qt.DeepEquals, r)
 		}},
@@ -141,6 +144,9 @@ a;b;c`, mime: media.Builtin.CSVType}, map[string]any{"DElimiter": ";", "Comment"
 		}},
 		{``, nil, nil},
 		{`   `, nil, nil},
+		{`{p: [a, b]}`, map[string]any{"format": "yaml"}, func(m map[string]any) {
+			b.Assert(m, qt.DeepEquals, map[string]any{"p": []any{"a", "b"}})
+		}},
 		// errors
 		{"thisisnotavaliddataformat", nil, false},
 		{testContentResource{key: "r1", content: `invalid&toml"`, mime: media.Builtin.TOMLType}, nil, false},
@@ -148,6 +154,8 @@ a;b;c`, mime: media.Builtin.CSVType}, map[string]any{"DElimiter": ";", "Comment"
 		{"thisisnotavaliddataformat", nil, false},
 		{`{ notjson }`, nil, false},
 		{tstNoStringer{}, nil, false},
+		{"<root><a>notjson</a></root>", map[string]any{"format": "json"}, false},
+		{"anything", map[string]any{"format": "invalidformat"}, false},
 	} {
 
 		ns.Reset()