JFryy · ChrisJr404 · Aug 25, 2026
diff --git a/README.md b/README.md
@@ -15,12 +15,15 @@ Read-only: `.proto`, `.env`
 
 ## Transcoding Between Formats
 
-Any input format can be transcoded to any output format in a single command. Use `-i`/`--input` and `-o`/`--output` to specify formats explicitly, or let `qq` detect them from the file extension.
+Any input format can be transcoded to any output format in a single command. Use `-i`/`--input` and `-o`/`--output` to specify formats explicitly, or let `qq` detect them from the file extension. When reading from stdin and you don't know the format up front, pass `-i auto` to detect it from the content (json, yaml, toml and xml/html).
 
 ```sh
 # YAML → JSON
 qq '.' config.yaml -o json
 
+# Detect the input format from content (handy for stdin)
+kubectl get pod my-pod -o yaml | qq -i auto '.metadata.name'
+
 # TOML → YAML
 qq '.' pyproject.toml -o yaml
 

diff --git a/cli/qq.go b/cli/qq.go
@@ -49,7 +49,7 @@ func CreateRootCmd() *cobra.Command {
 			handleCommand(cmd, args, inputType, outputType, rawOutput, help, interactive, monochrome, stream, slurp, exitStatus)
 		},
 	}
-	cmd.Flags().StringVarP(&inputType, "input", "i", "json", "specify input file type, only required on parsing stdin.")
+	cmd.Flags().StringVarP(&inputType, "input", "i", "json", "specify input file type, only required on parsing stdin. use \"auto\" to detect the format from the content.")
 	cmd.Flags().StringVarP(&outputType, "output", "o", "json", "specify output file type by extension name. This is inferred from extension if passing file position argument.")
 	cmd.Flags().BoolVarP(&rawOutput, "raw-output", "r", false, "output strings without escapes and quotes.")
 	cmd.Flags().BoolVarP(&help, "help", "h", false, "help for qq")
@@ -161,7 +161,16 @@ func handleCommand(cmd *cobra.Command, args []string, inputtype string, outputty
 	// Check if -i flag was explicitly set by user
 	inputFlagSet := cmd.Flags().Changed("input")
 
-	if inputFlagSet {
+	if strings.EqualFold(inputtype, "auto") {
+		// Content based detection, opt-in via -i auto so it never changes the
+		// default behaviour. Detection needs the whole input buffered, which is
+		// at odds with streaming, so ask the user to be explicit there.
+		if stream {
+			fmt.Println("Error: input type \"auto\" cannot be used with --stream; specify the input type explicitly")
+			os.Exit(1)
+		}
+		inputCodec, err = codec.Detect(input)
+	} else if inputFlagSet {
 		// -i flag takes precedence over file extension
 		inputCodec, err = codec.GetEncodingType(inputtype)
 	} else if filename != "" {

diff --git a/codec/detect.go b/codec/detect.go
@@ -0,0 +1,77 @@
+package codec
+
+import (
+	"bytes"
+	"fmt"
+	"reflect"
+
+	"github.com/goccy/go-json"
+)
+
+// Detect infers the encoding of input from its content. It backs the "auto"
+// input type and is intentionally conservative: it only reports a format when a
+// strict parse confirms it, and returns an error otherwise so callers can ask
+// the user to be explicit rather than guess wrong.
+//
+// Detection covers the common structured text formats (json, xml/html, toml,
+// yaml). Binary formats and ambiguous line based formats (csv, ini, env,
+// properties, ...) are deliberately left out because their inputs overlap too
+// much to tell apart reliably.
+func Detect(input []byte) (EncodingType, error) {
+	trimmed := bytes.TrimSpace(input)
+	if len(trimmed) == 0 {
+		return JSON, fmt.Errorf("cannot detect input format: empty input")
+	}
+
+	switch trimmed[0] {
+	case '{', '[':
+		if json.Valid(input) {
+			return JSON, nil
+		}
+	case '<':
+		lower := bytes.ToLower(trimmed)
+		if bytes.HasPrefix(lower, []byte("<!doctype html")) || bytes.Contains(lower, []byte("<html")) {
+			return HTML, nil
+		}
+		if _, err := tryDecode(input, XML); err == nil {
+			return XML, nil
+		}
+		if _, err := tryDecode(input, HTML); err == nil {
+			return HTML, nil
+		}
+	}
+
+	// TOML is strict enough that arbitrary text will not parse, so try it before
+	// falling back to YAML which accepts almost anything.
+	if v, err := tryDecode(input, TOML); err == nil && isStructured(v) {
+		return TOML, nil
+	}
+
+	// YAML is the catch all for structured config. Require a map or sequence so
+	// that plain scalars and prose are not claimed as YAML.
+	if v, err := tryDecode(input, YAML); err == nil && isStructured(v) {
+		return YAML, nil
+	}
+
+	return JSON, fmt.Errorf("could not detect input format, please specify it with -i/--input")
+}
+
+func tryDecode(input []byte, t EncodingType) (any, error) {
+	var v any
+	if err := Codecs[t].Unmarshal(input, &v); err != nil {
+		return nil, err
+	}
+	return v, nil
+}
+
+func isStructured(v any) bool {
+	if v == nil {
+		return false
+	}
+	switch reflect.ValueOf(v).Kind() {
+	case reflect.Map, reflect.Slice, reflect.Array:
+		return true
+	default:
+		return false
+	}
+}
diff --git a/codec/detect_test.go b/codec/detect_test.go
@@ -0,0 +1,56 @@
+package codec
+
+import "testing"
+
+func TestDetect(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected EncodingType
+	}{
+		{"json object", `{"name":"Alice","age":30}`, JSON},
+		{"json array", `[1,2,3]`, JSON},
+		{"json with leading space", "  \n{\"a\":1}", JSON},
+		{"yaml map", "name: Alice\nage: 30\n", YAML},
+		{"yaml doc marker", "---\nname: Bob\n", YAML},
+		{"yaml list", "- one\n- two\n", YAML},
+		{"toml", "title = \"hi\"\n[owner]\nname = \"x\"\n", TOML},
+		{"xml", "<root><a>1</a></root>", XML},
+		{"xml with declaration", "<?xml version=\"1.0\"?><note><to>x</to></note>", XML},
+		{"html doctype", "<!DOCTYPE html><html><body><p>hi</p></body></html>", HTML},
+		{"html tag", "<html><head></head></html>", HTML},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			got, err := Detect([]byte(tt.input))
+			if err != nil {
+				t.Fatalf("Detect(%q) returned error: %v", tt.input, err)
+			}
+			if got != tt.expected {
+				t.Errorf("Detect(%q) = %v, want %v", tt.input, got, tt.expected)
+			}
+		})
+	}
+}
+
+func TestDetectUndetectable(t *testing.T) {
+	// Inputs that Detect should refuse rather than guess at.
+	cases := []struct {
+		name  string
+		input string
+	}{
+		{"empty", ""},
+		{"whitespace only", "   \n\t"},
+		{"plain scalar", "just some words"},
+		{"bare number", "42"},
+	}
+
+	for _, tt := range cases {
+		t.Run(tt.name, func(t *testing.T) {
+			if got, err := Detect([]byte(tt.input)); err == nil {
+				t.Errorf("Detect(%q) = %v, expected an error", tt.input, got)
+			}
+		})
+	}
+}