diff --git a/internal/xmlrpc/xmlrpc.go b/internal/xmlrpc/xmlrpc.go
new file mode 100644
index 0000000..0fcd820
--- /dev/null
+++ b/internal/xmlrpc/xmlrpc.go
@@ -0,0 +1,525 @@
+// Package xmlrpc parses and writes XML-RPC calls and responses, keeping
+// value types and struct member order exactly.
+package xmlrpc
+
+import (
+	"bytes"
+	"encoding/base64"
+	"encoding/xml"
+	"errors"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+)
+
+// MaxSize is the largest document accepted.
+const MaxSize = 64 << 20
+
+// MaxDepth is the deepest nesting of arrays and structs accepted.
+const MaxDepth = 64
+
+// Value is one of: int64, bool, string, float64, []byte (base64), DateTime,
+// []Value (array), Struct, or nil (<nil/>).
+type Value = any
+
+// DateTime is dateTime.iso8601 text, kept as is.
+type DateTime string
+
+// Member is a struct member.
+type Member struct {
+	Name  string
+	Value Value
+}
+
+// Struct is an XML-RPC struct, in document order.
+type Struct []Member
+
+// Get returns the value of the first member with the name.
+func (s Struct) Get(name string) (Value, bool) {
+	for _, m := range s {
+		if m.Name == name {
+			return m.Value, true
+		}
+	}
+	return nil, false
+}
+
+// Call is a methodCall.
+type Call struct {
+	Method string
+	Params []Value
+}
+
+// Response is a methodResponse: Params, or Fault if it's set.
+type Response struct {
+	Params []Value
+	Fault  *Fault
+}
+
+// Fault is a fault response's code and message.
+type Fault struct {
+	Code   int64
+	String string
+}
+
+func (f *Fault) Error() string { return fmt.Sprintf("xmlrpc fault %d: %s", f.Code, f.String) }
+
+type parser struct{ d *xml.Decoder }
+
+func newParser(data []byte) (*parser, error) {
+	if len(data) > MaxSize {
+		return nil, errors.New("xmlrpc: document too large")
+	}
+	d := xml.NewDecoder(bytes.NewReader(data))
+	d.Strict = true
+	d.CharsetReader = func(label string, r io.Reader) (io.Reader, error) {
+		switch strings.ToLower(label) {
+		case "utf-8", "utf8", "us-ascii", "ascii":
+			return r, nil
+		}
+		return nil, fmt.Errorf("xmlrpc: unsupported encoding %q", label)
+	}
+	return &parser{d}, nil
+}
+
+func (p *parser) token() (xml.Token, error) {
+	t, err := p.d.Token()
+	if err == io.EOF {
+		err = io.ErrUnexpectedEOF
+	}
+	return t, err
+}
+
+// next returns the next start or end element, skipping whitespace,
+// comments and processing instructions.
+func (p *parser) next() (xml.Token, error) {
+	for {
+		t, err := p.token()
+		if err != nil {
+			return nil, err
+		}
+		switch t := t.(type) {
+		case xml.StartElement, xml.EndElement:
+			return t, nil
+		case xml.CharData:
+			if len(bytes.TrimSpace(t)) != 0 {
+				return nil, fmt.Errorf("xmlrpc: unexpected text %q", string(t))
+			}
+		}
+	}
+}
+
+func (p *parser) start(name string) error {
+	t, err := p.next()
+	if err != nil {
+		return err
+	}
+	if s, ok := t.(xml.StartElement); !ok || s.Name.Local != name {
+		return fmt.Errorf("xmlrpc: expected <%s>", name)
+	}
+	return nil
+}
+
+func (p *parser) end(name string) error {
+	t, err := p.next()
+	if err != nil {
+		return err
+	}
+	if e, ok := t.(xml.EndElement); !ok || e.Name.Local != name {
+		return fmt.Errorf("xmlrpc: expected </%s>", name)
+	}
+	return nil
+}
+
+// text reads character data up to the current element's end.
+func (p *parser) text() (string, error) {
+	var b strings.Builder
+	for {
+		t, err := p.token()
+		if err != nil {
+			return "", err
+		}
+		switch t := t.(type) {
+		case xml.CharData:
+			b.Write(t)
+		case xml.EndElement:
+			return b.String(), nil
+		case xml.StartElement:
+			return "", fmt.Errorf("xmlrpc: unexpected <%s>", t.Name.Local)
+		}
+	}
+}
+
+// eof checks that only whitespace follows the root element.
+func (p *parser) eof() error {
+	for {
+		t, err := p.d.Token()
+		if err == io.EOF {
+			return nil
+		}
+		if err != nil {
+			return err
+		}
+		switch t := t.(type) {
+		case xml.StartElement, xml.EndElement:
+			return errors.New("xmlrpc: content after document")
+		case xml.CharData:
+			if len(bytes.TrimSpace(t)) != 0 {
+				return errors.New("xmlrpc: content after document")
+			}
+		}
+	}
+}
+
+// value parses a value's content; its <value> start was read.
+// Bare text is a string, whitespace included.
+func (p *parser) value(depth int) (Value, error) {
+	var text strings.Builder
+	for {
+		t, err := p.token()
+		if err != nil {
+			return nil, err
+		}
+		switch t := t.(type) {
+		case xml.CharData:
+			text.Write(t)
+		case xml.EndElement:
+			return text.String(), nil
+		case xml.StartElement:
+			if strings.TrimSpace(text.String()) != "" {
+				return nil, errors.New("xmlrpc: text mixed with elements in <value>")
+			}
+			v, err := p.typed(t.Name.Local, depth)
+			if err != nil {
+				return nil, err
+			}
+			return v, p.end("value")
+		}
+	}
+}
+
+func (p *parser) typed(name string, depth int) (Value, error) {
+	switch name {
+	case "array", "struct":
+		if depth >= MaxDepth {
+			return nil, errors.New("xmlrpc: nested too deeply")
+		}
+		if name == "array" {
+			return p.array(depth + 1)
+		}
+		return p.strct(depth + 1)
+	}
+	s, err := p.text()
+	if err != nil {
+		return nil, err
+	}
+	switch name {
+	case "nil":
+		return nil, nil
+	case "string":
+		return s, nil
+	case "int", "i4", "i8":
+		n, err := strconv.ParseInt(strings.TrimSpace(s), 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("xmlrpc: bad <%s> %q", name, s)
+		}
+		return n, nil
+	case "boolean":
+		switch strings.TrimSpace(s) {
+		case "0":
+			return false, nil
+		case "1":
+			return true, nil
+		}
+		return nil, fmt.Errorf("xmlrpc: bad <boolean> %q", s)
+	case "double":
+		f, err := strconv.ParseFloat(strings.TrimSpace(s), 64)
+		if err != nil {
+			return nil, fmt.Errorf("xmlrpc: bad <double> %q", s)
+		}
+		return f, nil
+	case "dateTime.iso8601":
+		return DateTime(s), nil
+	case "base64":
+		b, err := base64.StdEncoding.DecodeString(strings.Join(strings.Fields(s), ""))
+		if err != nil {
+			return nil, fmt.Errorf("xmlrpc: bad <base64>: %v", err)
+		}
+		return b, nil
+	}
+	return nil, fmt.Errorf("xmlrpc: unknown type <%s>", name)
+}
+
+func (p *parser) array(depth int) (Value, error) {
+	if err := p.start("data"); err != nil {
+		return nil, err
+	}
+	a := []Value{}
+	for {
+		t, err := p.next()
+		if err != nil {
+			return nil, err
+		}
+		switch t := t.(type) {
+		case xml.EndElement:
+			return a, p.end("array")
+		case xml.StartElement:
+			if t.Name.Local != "value" {
+				return nil, fmt.Errorf("xmlrpc: unexpected <%s> in <data>", t.Name.Local)
+			}
+			v, err := p.value(depth)
+			if err != nil {
+				return nil, err
+			}
+			a = append(a, v)
+		}
+	}
+}
+
+func (p *parser) strct(depth int) (Value, error) {
+	s := Struct{}
+	for {
+		t, err := p.next()
+		if err != nil {
+			return nil, err
+		}
+		switch t := t.(type) {
+		case xml.EndElement:
+			return s, nil
+		case xml.StartElement:
+			if t.Name.Local != "member" {
+				return nil, fmt.Errorf("xmlrpc: unexpected <%s> in <struct>", t.Name.Local)
+			}
+			if err := p.start("name"); err != nil {
+				return nil, err
+			}
+			name, err := p.text()
+			if err != nil {
+				return nil, err
+			}
+			if err := p.start("value"); err != nil {
+				return nil, err
+			}
+			v, err := p.value(depth)
+			if err != nil {
+				return nil, err
+			}
+			if err := p.end("member"); err != nil {
+				return nil, err
+			}
+			s = append(s, Member{name, v})
+		}
+	}
+}
+
+// params parses <param> elements up to </params>.
+func (p *parser) params() ([]Value, error) {
+	ps := []Value{}
+	for {
+		t, err := p.next()
+		if err != nil {
+			return nil, err
+		}
+		switch t := t.(type) {
+		case xml.EndElement:
+			return ps, nil
+		case xml.StartElement:
+			if t.Name.Local != "param" {
+				return nil, fmt.Errorf("xmlrpc: unexpected <%s> in <params>", t.Name.Local)
+			}
+			if err := p.start("value"); err != nil {
+				return nil, err
+			}
+			v, err := p.value(0)
+			if err != nil {
+				return nil, err
+			}
+			if err := p.end("param"); err != nil {
+				return nil, err
+			}
+			ps = append(ps, v)
+		}
+	}
+}
+
+// ParseCall parses a methodCall document.
+func ParseCall(data []byte) (*Call, error) {
+	p, err := newParser(data)
+	if err != nil {
+		return nil, err
+	}
+	if err := p.start("methodCall"); err != nil {
+		return nil, err
+	}
+	if err := p.start("methodName"); err != nil {
+		return nil, err
+	}
+	method, err := p.text()
+	if err != nil {
+		return nil, err
+	}
+	c := &Call{Method: strings.TrimSpace(method), Params: []Value{}}
+	t, err := p.next()
+	if err != nil {
+		return nil, err
+	}
+	if s, ok := t.(xml.StartElement); ok && s.Name.Local == "params" {
+		if c.Params, err = p.params(); err != nil {
+			return nil, err
+		}
+		if err := p.end("methodCall"); err != nil {
+			return nil, err
+		}
+	} else if e, ok := t.(xml.EndElement); !ok || e.Name.Local != "methodCall" {
+		return nil, errors.New("xmlrpc: expected <params>")
+	}
+	return c, p.eof()
+}
+
+// ParseResponse parses a methodResponse document.
+func ParseResponse(data []byte) (*Response, error) {
+	p, err := newParser(data)
+	if err != nil {
+		return nil, err
+	}
+	if err := p.start("methodResponse"); err != nil {
+		return nil, err
+	}
+	t, err := p.next()
+	if err != nil {
+		return nil, err
+	}
+	s, _ := t.(xml.StartElement)
+	r := &Response{}
+	switch s.Name.Local {
+	case "params":
+		if r.Params, err = p.params(); err != nil {
+			return nil, err
+		}
+	case "fault":
+		if r.Fault, err = p.fault(); err != nil {
+			return nil, err
+		}
+	default:
+		return nil, errors.New("xmlrpc: expected <params> or <fault>")
+	}
+	if err := p.end("methodResponse"); err != nil {
+		return nil, err
+	}
+	return r, p.eof()
+}
+
+func (p *parser) fault() (*Fault, error) {
+	if err := p.start("value"); err != nil {
+		return nil, err
+	}
+	v, err := p.value(0)
+	if err != nil {
+		return nil, err
+	}
+	if err := p.end("fault"); err != nil {
+		return nil, err
+	}
+	st, _ := v.(Struct)
+	code, _ := st.Get("faultCode")
+	msg, _ := st.Get("faultString")
+	f := &Fault{}
+	var ok1, ok2 bool
+	f.Code, ok1 = code.(int64)
+	f.String, ok2 = msg.(string)
+	if !ok1 || !ok2 {
+		return nil, errors.New("xmlrpc: bad fault")
+	}
+	return f, nil
+}
+
+// Marshal writes the call. It panics on a value of an unsupported type.
+func (c *Call) Marshal() []byte {
+	var b bytes.Buffer
+	b.WriteString("<?xml version='1.0'?>\n<methodCall>\n<methodName>")
+	xml.EscapeText(&b, []byte(c.Method))
+	b.WriteString("</methodName>\n")
+	writeParams(&b, c.Params)
+	b.WriteString("</methodCall>\n")
+	return b.Bytes()
+}
+
+// Marshal writes the response. It panics on a value of an unsupported type.
+func (r *Response) Marshal() []byte {
+	var b bytes.Buffer
+	b.WriteString("<?xml version='1.0'?>\n<methodResponse>\n")
+	if r.Fault != nil {
+		b.WriteString("<fault>\n")
+		writeValue(&b, Struct{{"faultCode", r.Fault.Code}, {"faultString", r.Fault.String}})
+		b.WriteString("</fault>\n")
+	} else {
+		writeParams(&b, r.Params)
+	}
+	b.WriteString("</methodResponse>\n")
+	return b.Bytes()
+}
+
+func writeParams(b *bytes.Buffer, ps []Value) {
+	b.WriteString("<params>\n")
+	for _, v := range ps {
+		b.WriteString("<param>\n")
+		writeValue(b, v)
+		b.WriteString("</param>\n")
+	}
+	b.WriteString("</params>\n")
+}
+
+// writeText escapes s; xml.EscapeText also escapes quotes and \t\n\r as
+// character references, so whitespace survives exactly.
+func writeText(b *bytes.Buffer, s string) { xml.EscapeText(b, []byte(s)) }
+
+func writeValue(b *bytes.Buffer, v Value) {
+	b.WriteString("<value>")
+	switch v := v.(type) {
+	case nil:
+		b.WriteString("<nil/>")
+	case int64:
+		fmt.Fprintf(b, "<int>%d</int>", v)
+	case int:
+		fmt.Fprintf(b, "<int>%d</int>", v)
+	case bool:
+		if v {
+			b.WriteString("<boolean>1</boolean>")
+		} else {
+			b.WriteString("<boolean>0</boolean>")
+		}
+	case string:
+		b.WriteString("<string>")
+		writeText(b, v)
+		b.WriteString("</string>")
+	case float64:
+		// Shortest round-trip form; Python's float() parses it, NaN and Inf too.
+		fmt.Fprintf(b, "<double>%s</double>", strconv.FormatFloat(v, 'g', -1, 64))
+	case []byte:
+		fmt.Fprintf(b, "<base64>%s</base64>", base64.StdEncoding.EncodeToString(v))
+	case DateTime:
+		b.WriteString("<dateTime.iso8601>")
+		writeText(b, string(v))
+		b.WriteString("</dateTime.iso8601>")
+	case []Value:
+		b.WriteString("<array><data>\n")
+		for _, e := range v {
+			writeValue(b, e)
+		}
+		b.WriteString("</data></array>")
+	case Struct:
+		b.WriteString("<struct>\n")
+		for _, m := range v {
+			b.WriteString("<member>\n<name>")
+			writeText(b, m.Name)
+			b.WriteString("</name>\n")
+			writeValue(b, m.Value)
+			b.WriteString("</member>\n")
+		}
+		b.WriteString("</struct>")
+	default:
+		panic(fmt.Sprintf("xmlrpc: unsupported type %T", v))
+	}
+	b.WriteString("</value>\n")
+}
diff --git a/internal/xmlrpc/xmlrpc_test.go b/internal/xmlrpc/xmlrpc_test.go
new file mode 100644
index 0000000..0ebbd83
--- /dev/null
+++ b/internal/xmlrpc/xmlrpc_test.go
@@ -0,0 +1,303 @@
+package xmlrpc
+
+import (
+	"bytes"
+	"reflect"
+	"strings"
+	"testing"
+)
+
+// Python 2.7 xmlrpclib.dumps(..., methodname=...) output, synthetic values.
+const authorizeCall = `<?xml version='1.0'?>
+<methodCall>
+<methodName>test.authorizeProducts2</methodName>
+<params>
+<param>
+<value><struct>
+<member>
+<name>auths</name>
+<value><array><data>
+<value><array><data>
+<value><string>userId</string></value>
+<value><string>productId</string></value>
+</data></array></value>
+<value><array><data>
+<value><int>123</int></value>
+<value><int>456</int></value>
+</data></array></value>
+</data></array></value>
+</member>
+<member>
+<name>use_extracted_assets_if_available</name>
+<value><boolean>1</boolean></value>
+</member>
+<member>
+<name>max_supported_format</name>
+<value><int>9</int></value>
+</member>
+</struct></value>
+</param>
+</params>
+</methodCall>
+`
+
+// PHP-style response: declaration with encoding, indentation, i4, bare text.
+const phpResponse = `<?xml version="1.0" encoding="UTF-8"?>
+<methodResponse>
+  <params>
+    <param>
+      <value>
+        <array>
+          <data>
+            <value>
+              <array>
+                <data>
+                  <value><i4>1001</i4></value>
+                  <value><string>  Spaced &amp; &lt;name&gt;  </string></value>
+                  <value><boolean>0</boolean></value>
+                </data>
+              </array>
+            </value>
+            <value>
+              <array>
+                <data>
+                  <value><int>-7</int></value>
+                  <value>bare text</value>
+                  <value><boolean>1</boolean></value>
+                  <value><i8>9007199254740993</i8></value>
+                  <value><double>0.1</double></value>
+                  <value><dateTime.iso8601>20261029T12:34:56</dateTime.iso8601></value>
+                  <value><base64>aGVs
+bG8=</base64></value>
+                  <value><nil/></value>
+                  <value></value>
+                  <value/>
+                  <value><string/></value>
+                  <value><struct></struct></value>
+                  <value><array><data></data></array></value>
+                </data>
+              </array>
+            </value>
+          </data>
+        </array>
+      </value>
+    </param>
+  </params>
+</methodResponse>
+`
+
+const faultResponse = `<?xml version="1.0" encoding="UTF-8"?>
+<methodResponse>
+  <fault>
+    <value>
+      <struct>
+        <member><name>faultCode</name><value><int>4</int></value></member>
+        <member><name>faultString</name><value><string>Too many parameters.</string></value></member>
+      </struct>
+    </value>
+  </fault>
+</methodResponse>
+`
+
+const unicodeCall = "<?xml version='1.0'?>\n<methodCall><methodName>test.echo</methodName><params>" +
+	"<param><value><string>café 日本 \U0001F600 &#x263A;</string></value></param>" +
+	"</params></methodCall>"
+
+func TestParseCall(t *testing.T) {
+	c, err := ParseCall([]byte(authorizeCall))
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := &Call{Method: "test.authorizeProducts2", Params: []Value{Struct{
+		{"auths", []Value{[]Value{"userId", "productId"}, []Value{int64(123), int64(456)}}},
+		{"use_extracted_assets_if_available", true},
+		{"max_supported_format", int64(9)},
+	}}}
+	if !reflect.DeepEqual(c, want) {
+		t.Fatalf("got %#v", c)
+	}
+	roundTripCall(t, c)
+}
+
+func TestParseResponse(t *testing.T) {
+	r, err := ParseResponse([]byte(phpResponse))
+	if err != nil {
+		t.Fatal(err)
+	}
+	want := &Response{Params: []Value{[]Value{
+		[]Value{int64(1001), "  Spaced & <name>  ", false},
+		[]Value{int64(-7), "bare text", true, int64(9007199254740993), 0.1,
+			DateTime("20261029T12:34:56"), []byte("hello"), nil, "", "", "", Struct{}, []Value{}},
+	}}}
+	if !reflect.DeepEqual(r, want) {
+		t.Fatalf("got %#v", r)
+	}
+	roundTripResponse(t, r)
+}
+
+func TestFault(t *testing.T) {
+	r, err := ParseResponse([]byte(faultResponse))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if r.Fault == nil || *r.Fault != (Fault{4, "Too many parameters."}) || r.Params != nil {
+		t.Fatalf("got %#v", r)
+	}
+	out := r.Marshal()
+	if !bytes.Contains(out, []byte("<methodResponse>\n<fault>")) {
+		t.Fatalf("marshal: %s", out)
+	}
+	roundTripResponse(t, r)
+}
+
+func TestUnicode(t *testing.T) {
+	c, err := ParseCall([]byte(unicodeCall))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if s := c.Params[0]; s != "café 日本 \U0001F600 ☺" {
+		t.Fatalf("got %q", s)
+	}
+	roundTripCall(t, c)
+}
+
+func TestWhitespaceInString(t *testing.T) {
+	r := &Response{Params: []Value{" a\tb\r\nc ", Struct{{"k\n", "\r"}}, 1.5e300, -0.0}}
+	roundTripResponse(t, r)
+}
+
+func TestMarshalFormat(t *testing.T) {
+	c := &Call{Method: "m", Params: []Value{int64(1), true, "a<&>", nil, 2.5}}
+	want := "<?xml version='1.0'?>\n<methodCall>\n<methodName>m</methodName>\n<params>\n" +
+		"<param>\n<value><int>1</int></value>\n</param>\n" +
+		"<param>\n<value><boolean>1</boolean></value>\n</param>\n" +
+		"<param>\n<value><string>a&lt;&amp;&gt;</string></value>\n</param>\n" +
+		"<param>\n<value><nil/></value>\n</param>\n" +
+		"<param>\n<value><double>2.5</double></value>\n</param>\n" +
+		"</params>\n</methodCall>\n"
+	if got := string(c.Marshal()); got != want {
+		t.Fatalf("got\n%s", got)
+	}
+}
+
+func TestCallWithoutParams(t *testing.T) {
+	c, err := ParseCall([]byte("<methodCall><methodName>x</methodName></methodCall>"))
+	if err != nil || c.Method != "x" || len(c.Params) != 0 {
+		t.Fatal(c, err)
+	}
+}
+
+func TestErrors(t *testing.T) {
+	bad := []string{
+		"",
+		"<methodCall>",
+		"<methodResponse><params><param><value><int>x</int></value></param></params></methodResponse>",
+		"<methodResponse><params><param><value><boolean>2</boolean></value></param></params></methodResponse>",
+		"<methodResponse><params><param><value><double>z</double></value></param></params></methodResponse>",
+		"<methodResponse><params><param><value><base64>!!</base64></value></param></params></methodResponse>",
+		"<methodResponse><params><param><value><foo/></value></param></params></methodResponse>",
+		"<methodResponse><params><param><value>x<int>1</int></value></param></params></methodResponse>",
+		"<methodResponse><params><param><value><int>1</int></value></param></params></methodResponse><x/>",
+		"<methodResponse><fault><value><int>1</int></value></fault></methodResponse>",
+		"<methodResponse><params><param><value><string>a</value></param></params></methodResponse>",
+		"<methodResponse></methodResponse>",
+		"<methodCall><methodName>x</methodName><params>junk</params></methodCall>",
+		`<?xml version="1.0" encoding="ISO-8859-1"?><methodCall><methodName>x</methodName></methodCall>`,
+	}
+	for _, s := range bad {
+		_, err1 := ParseResponse([]byte(s))
+		_, err2 := ParseCall([]byte(s))
+		if err1 == nil && err2 == nil {
+			t.Errorf("accepted %q", s)
+		}
+	}
+}
+
+func nested(n int) string {
+	return "<methodResponse><params><param><value>" +
+		strings.Repeat("<array><data><value>", n) + "<int>1</int>" +
+		strings.Repeat("</value></data></array>", n) +
+		"</value></param></params></methodResponse>"
+}
+
+func TestDepth(t *testing.T) {
+	if _, err := ParseResponse([]byte(nested(MaxDepth))); err != nil {
+		t.Fatal(err)
+	}
+	if _, err := ParseResponse([]byte(nested(MaxDepth + 1))); err == nil {
+		t.Fatal("accepted too deep")
+	}
+}
+
+func TestSize(t *testing.T) {
+	big := make([]byte, MaxSize+1)
+	if _, err := ParseCall(big); err == nil {
+		t.Fatal("accepted too large")
+	}
+}
+
+func roundTripCall(t *testing.T, c *Call) {
+	t.Helper()
+	c2, err := ParseCall(c.Marshal())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !reflect.DeepEqual(c, c2) {
+		t.Fatalf("round trip:\n%#v\n%#v", c, c2)
+	}
+}
+
+func roundTripResponse(t *testing.T, r *Response) {
+	t.Helper()
+	r2, err := ParseResponse(r.Marshal())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if r.Params == nil && r.Fault == nil {
+		r.Params = []Value{}
+	}
+	if !reflect.DeepEqual(r, r2) {
+		t.Fatalf("round trip:\n%#v\n%#v", r, r2)
+	}
+}
+
+// Fuzzed documents must not panic, and whatever parses must re-marshal stably
+// (bytes, not values, so NaN compares equal).
+func FuzzParseCall(f *testing.F) {
+	f.Add([]byte(authorizeCall))
+	f.Add([]byte(unicodeCall))
+	f.Fuzz(func(t *testing.T, data []byte) {
+		c, err := ParseCall(data)
+		if err != nil {
+			return
+		}
+		out := c.Marshal()
+		c2, err := ParseCall(out)
+		if err != nil {
+			t.Fatalf("reparse: %v\n%s", err, out)
+		}
+		if !bytes.Equal(out, c2.Marshal()) {
+			t.Fatalf("unstable:\n%s", out)
+		}
+	})
+}
+
+func FuzzParseResponse(f *testing.F) {
+	f.Add([]byte(phpResponse))
+	f.Add([]byte(faultResponse))
+	f.Add([]byte(nested(3)))
+	f.Fuzz(func(t *testing.T, data []byte) {
+		r, err := ParseResponse(data)
+		if err != nil {
+			return
+		}
+		out := r.Marshal()
+		r2, err := ParseResponse(out)
+		if err != nil {
+			t.Fatalf("reparse: %v\n%s", err, out)
+		}
+		if !bytes.Equal(out, r2.Marshal()) {
+			t.Fatalf("unstable:\n%s", out)
+		}
+	})
+}