diff --git a/json/codec.go b/json/codec.go
index 77fe264..e4b6ab1 100644
--- a/json/codec.go
+++ b/json/codec.go
@@ -10,6 +10,7 @@ import (
 	"sort"
 	"strconv"
 	"strings"
+	"sync"
 	"sync/atomic"
 	"time"
 	"unicode"
@@ -32,13 +33,29 @@ type codec struct {
 
 type encoder struct {
 	flags AppendFlags
-	// ptrDepth tracks the depth of pointer cycles, when it reaches the value
+	// refDepth tracks the depth of pointer cycles, when it reaches the value
 	// of startDetectingCyclesAfter, the ptrSeen map is allocated and the
 	// encoder starts tracking pointers it has seen as an attempt to detect
 	// whether it has entered a pointer cycle and needs to error before the
 	// goroutine runs out of stack space.
-	ptrDepth uint32
-	ptrSeen  map[unsafe.Pointer]struct{}
+	//
+	// This relies on encoder being passed as a value,
+	// and encoder methods calling each other in a traditional stack
+	// (not using trampoline techniques),
+	// since refDepth is never decremented.
+	refDepth uint32
+	refSeen  cycleMap
+}
+
+type cycleKey struct {
+	ptr unsafe.Pointer
+	len int // 0 for pointers or maps; length for slices or array pointers.
+}
+
+type cycleMap map[cycleKey]struct{}
+
+var cycleMapPool = sync.Pool{
+	New: func() any { return make(cycleMap) },
 }
 
 type decoder struct {
@@ -63,6 +80,17 @@ type (
 // lookup time for simple types like bool, int, etc..
 var cache atomic.Pointer[map[unsafe.Pointer]codec]
 
+func cachedCodec(t reflect.Type) codec {
+	cache := cacheLoad()
+
+	c, found := cache[typeid(t)]
+	if !found {
+		c = constructCachedCodec(t, cache)
+	}
+
+	return c
+}
+
 func cacheLoad() map[unsafe.Pointer]codec {
 	p := cache.Load()
 	if p == nil {
diff --git a/json/encode.go b/json/encode.go
index 2a6da07..a65dc41 100644
--- a/json/encode.go
+++ b/json/encode.go
@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"math"
 	"reflect"
+	"runtime"
 	"sort"
 	"strconv"
 	"sync"
@@ -17,6 +18,23 @@ import (
 
 const hex = "0123456789abcdef"
 
+func (e encoder) appendAny(b []byte, x any) ([]byte, error) {
+	if x == nil {
+		// Special case for nil values because it makes the rest of the code
+		// simpler to assume that it won't be seeing nil pointers.
+		return e.encodeNull(b, nil)
+	}
+
+	t := reflect.TypeOf(x)
+	p := (*iface)(unsafe.Pointer(&x)).ptr
+	c := cachedCodec(t)
+
+	b, err := c.encode(e, b, p)
+	runtime.KeepAlive(x)
+
+	return b, err
+}
+
 func (e encoder) encodeNull(b []byte, p unsafe.Pointer) ([]byte, error) {
 	return append(b, "null"...), nil
 }
@@ -241,7 +259,7 @@ func (e encoder) encodeToString(b []byte, p unsafe.Pointer, encode encodeFunc) (
 func (e encoder) encodeBytes(b []byte, p unsafe.Pointer) ([]byte, error) {
 	v := *(*[]byte)(p)
 	if v == nil {
-		return append(b, "null"...), nil
+		return e.encodeNull(b, nil)
 	}
 
 	n := base64.StdEncoding.EncodedLen(len(v)) + 2
@@ -278,6 +296,15 @@ func (e encoder) encodeTime(b []byte, p unsafe.Pointer) ([]byte, error) {
 }
 
 func (e encoder) encodeArray(b []byte, p unsafe.Pointer, n int, size uintptr, t reflect.Type, encode encodeFunc) ([]byte, error) {
+	if shouldCheckForRefCycle(&e) {
+		key := cycleKey{ptr: p}
+		if hasRefCycle(&e, key) {
+			return b, refCycleError(t, p)
+		}
+
+		defer freeRefCycleInfo(&e, key)
+	}
+
 	start := len(b)
 	var err error
 	b = append(b, '[')
@@ -299,7 +326,7 @@ func (e encoder) encodeSlice(b []byte, p unsafe.Pointer, size uintptr, t reflect
 	s := (*slice)(p)
 
 	if s.data == nil && s.len == 0 && s.cap == 0 {
-		return append(b, "null"...), nil
+		return e.encodeNull(b, nil)
 	}
 
 	return e.encodeArray(b, s.data, s.len, size, t, encode)
@@ -308,7 +335,20 @@ func (e encoder) encodeSlice(b []byte, p unsafe.Pointer, size uintptr, t reflect
 func (e encoder) encodeMap(b []byte, p unsafe.Pointer, t reflect.Type, encodeKey, encodeValue encodeFunc, sortKeys sortFunc) ([]byte, error) {
 	m := reflect.NewAt(t, p).Elem()
 	if m.IsNil() {
-		return append(b, "null"...), nil
+		return e.encodeNull(b, nil)
+	}
+
+	// checkRefCycle/freeRefCycle expect the map header pointer itself,
+	// rather than a pointer to the header.
+	p = *(*unsafe.Pointer)(p)
+
+	if shouldCheckForRefCycle(&e) {
+		key := cycleKey{ptr: p}
+		if hasRefCycle(&e, key) {
+			return b, refCycleError(t, p)
+		}
+
+		defer freeRefCycleInfo(&e, key)
 	}
 
 	keys := m.MapKeys()
@@ -317,9 +357,10 @@ func (e encoder) encodeMap(b []byte, p unsafe.Pointer, t reflect.Type, encodeKey
 	}
 
 	start := len(b)
-	var err error
 	b = append(b, '{')
 
+	var err error
+
 	for i, k := range keys {
 		v := m.MapIndex(k)
 
@@ -363,7 +404,20 @@ var mapslicePool = sync.Pool{
 func (e encoder) encodeMapStringInterface(b []byte, p unsafe.Pointer) ([]byte, error) {
 	m := *(*map[string]any)(p)
 	if m == nil {
-		return append(b, "null"...), nil
+		return e.encodeNull(b, nil)
+	}
+
+	// checkRefCycle/freeRefCycle expect the map header pointer itself,
+	// rather than a pointer to the header.
+	p = *(*unsafe.Pointer)(p)
+
+	if shouldCheckForRefCycle(&e) {
+		key := cycleKey{ptr: p}
+		if hasRefCycle(&e, key) {
+			return b, refCycleError(mapStringInterfaceType, p)
+		}
+
+		defer freeRefCycleInfo(&e, key)
 	}
 
 	if (e.flags & SortMapKeys) == 0 {
@@ -383,7 +437,7 @@ func (e encoder) encodeMapStringInterface(b []byte, p unsafe.Pointer) ([]byte, e
 				b, _ = e.encodeString(b, unsafe.Pointer(&k))
 				b = append(b, ':')
 
-				b, err = Append(b, v, e.flags)
+				b, err = e.appendAny(b, v)
 				if err != nil {
 					return b, err
 				}
@@ -406,9 +460,10 @@ func (e encoder) encodeMapStringInterface(b []byte, p unsafe.Pointer) ([]byte, e
 	sort.Sort(s)
 
 	start := len(b)
-	var err error
 	b = append(b, '{')
 
+	var err error
+
 	for i, elem := range s.elements {
 		if i != 0 {
 			b = append(b, ',')
@@ -417,7 +472,7 @@ func (e encoder) encodeMapStringInterface(b []byte, p unsafe.Pointer) ([]byte, e
 		b, _ = e.encodeString(b, unsafe.Pointer(&elem.key))
 		b = append(b, ':')
 
-		b, err = Append(b, elem.val, e.flags)
+		b, err = e.appendAny(b, elem.val)
 		if err != nil {
 			break
 		}
@@ -441,7 +496,7 @@ func (e encoder) encodeMapStringInterface(b []byte, p unsafe.Pointer) ([]byte, e
 func (e encoder) encodeMapStringRawMessage(b []byte, p unsafe.Pointer) ([]byte, error) {
 	m := *(*map[string]RawMessage)(p)
 	if m == nil {
-		return append(b, "null"...), nil
+		return e.encodeNull(b, nil)
 	}
 
 	if (e.flags & SortMapKeys) == 0 {
@@ -520,7 +575,7 @@ func (e encoder) encodeMapStringRawMessage(b []byte, p unsafe.Pointer) ([]byte,
 func (e encoder) encodeMapStringString(b []byte, p unsafe.Pointer) ([]byte, error) {
 	m := *(*map[string]string)(p)
 	if m == nil {
-		return append(b, "null"...), nil
+		return e.encodeNull(b, nil)
 	}
 
 	if (e.flags & SortMapKeys) == 0 {
@@ -586,7 +641,7 @@ func (e encoder) encodeMapStringString(b []byte, p unsafe.Pointer) ([]byte, erro
 func (e encoder) encodeMapStringStringSlice(b []byte, p unsafe.Pointer) ([]byte, error) {
 	m := *(*map[string][]string)(p)
 	if m == nil {
-		return append(b, "null"...), nil
+		return e.encodeNull(b, nil)
 	}
 
 	stringSize := unsafe.Sizeof("")
@@ -667,7 +722,7 @@ func (e encoder) encodeMapStringStringSlice(b []byte, p unsafe.Pointer) ([]byte,
 func (e encoder) encodeMapStringBool(b []byte, p unsafe.Pointer) ([]byte, error) {
 	m := *(*map[string]bool)(p)
 	if m == nil {
-		return append(b, "null"...), nil
+		return e.encodeNull(b, nil)
 	}
 
 	if (e.flags & SortMapKeys) == 0 {
@@ -794,30 +849,31 @@ func (e encoder) encodeEmbeddedStructPointer(b []byte, p unsafe.Pointer, t refle
 }
 
 func (e encoder) encodePointer(b []byte, p unsafe.Pointer, t reflect.Type, encode encodeFunc) ([]byte, error) {
-	if p = *(*unsafe.Pointer)(p); p != nil {
-		if e.ptrDepth++; e.ptrDepth >= startDetectingCyclesAfter {
-			if _, seen := e.ptrSeen[p]; seen {
-				// TODO: reconstruct the reflect.Value from p + t so we can set
-				// the erorr's Value field?
-				return b, &UnsupportedValueError{Str: fmt.Sprintf("encountered a cycle via %s", t)}
-			}
-			if e.ptrSeen == nil {
-				e.ptrSeen = make(map[unsafe.Pointer]struct{})
-			}
-			e.ptrSeen[p] = struct{}{}
-			defer delete(e.ptrSeen, p)
+	// p was a pointer to the actual user data pointer:
+	// dereference it to operate on the user data pointer.
+	p = *(*unsafe.Pointer)(p)
+	if p == nil {
+		return e.encodeNull(b, nil)
+	}
+
+	if shouldCheckForRefCycle(&e) {
+		key := cycleKey{ptr: p}
+		if hasRefCycle(&e, key) {
+			return b, refCycleError(t, p)
 		}
-		return encode(e, b, p)
+
+		defer freeRefCycleInfo(&e, key)
 	}
-	return e.encodeNull(b, nil)
+
+	return encode(e, b, p)
 }
 
 func (e encoder) encodeInterface(b []byte, p unsafe.Pointer) ([]byte, error) {
-	return Append(b, *(*any)(p), e.flags)
+	return e.appendAny(b, *(*any)(p))
 }
 
 func (e encoder) encodeMaybeEmptyInterface(b []byte, p unsafe.Pointer, t reflect.Type) ([]byte, error) {
-	return Append(b, reflect.NewAt(t, p).Elem().Interface(), e.flags)
+	return e.appendAny(b, reflect.NewAt(t, p).Elem().Interface())
 }
 
 func (e encoder) encodeUnsupportedTypeError(b []byte, p unsafe.Pointer, t reflect.Type) ([]byte, error) {
@@ -828,7 +884,7 @@ func (e encoder) encodeRawMessage(b []byte, p unsafe.Pointer) ([]byte, error) {
 	v := *(*RawMessage)(p)
 
 	if v == nil {
-		return append(b, "null"...), nil
+		return e.encodeNull(b, nil)
 	}
 
 	var s []byte
@@ -862,7 +918,7 @@ func (e encoder) encodeJSONMarshaler(b []byte, p unsafe.Pointer, t reflect.Type,
 	switch v.Kind() {
 	case reflect.Ptr, reflect.Interface:
 		if v.IsNil() {
-			return append(b, "null"...), nil
+			return e.encodeNull(b, nil)
 		}
 	}
 
@@ -968,3 +1024,64 @@ func appendCompactEscapeHTML(dst []byte, src []byte) []byte {
 
 	return dst
 }
+
+// shouldCheckForRefCycle determines whether checking for reference cycles
+// is reasonable to do at this time.
+//
+// When true, checkRefCycle should be called and any error handled,
+// and then a deferred call to freeRefCycleInfo should be made.
+//
+// This should only be called from encoder methods that are possible points
+// that could directly contribute to a reference cycle.
+func shouldCheckForRefCycle(e *encoder) bool {
+	// Note: do not combine this with checkRefCycle,
+	// because checkRefCycle is too large to be inlined,
+	// and a non-inlined depth check leads to ~5%+ benchmark degradation.
+	e.refDepth++
+	return e.refDepth >= startDetectingCyclesAfter
+}
+
+// refCycleError constructs an [UnsupportedValueError].
+func refCycleError(t reflect.Type, p unsafe.Pointer) error {
+	v := reflect.NewAt(t, p)
+	return &UnsupportedValueError{
+		Value: v,
+		Str:   fmt.Sprintf("encountered a cycle via %s", t),
+	}
+}
+
+// hasRefCycle returns an error if a reference cycle was detected.
+// The data pointer passed in should be equivalent to one of:
+//
+//   - A normal Go pointer, e.g. `unsafe.Pointer(&T)`
+//   - The pointer to a map header, e.g. `*(*unsafe.Pointer)(&map[K]V)`
+//
+// Many [encoder] methods accept a pointer-to-a-pointer,
+// and so those may need to be derenced in order to safely pass them here.
+func hasRefCycle(e *encoder, key cycleKey) bool {
+	_, seen := e.refSeen[key]
+	if seen {
+		return true
+	}
+
+	if e.refSeen == nil {
+		e.refSeen = cycleMapPool.Get().(cycleMap)
+	}
+
+	e.refSeen[key] = struct{}{}
+
+	return false
+}
+
+// freeRefCycle performs the cleanup operation for [checkRefCycle].
+// p must be the same value passed into a prior call to checkRefCycle.
+func freeRefCycleInfo(e *encoder, key cycleKey) {
+	delete(e.refSeen, key)
+	if len(e.refSeen) == 0 {
+		// There are no remaining elements,
+		// so we can release this map for later reuse.
+		m := e.refSeen
+		e.refSeen = nil
+		cycleMapPool.Put(m)
+	}
+}
diff --git a/json/json.go b/json/json.go
index 028fd1f..a3138f8 100644
--- a/json/json.go
+++ b/json/json.go
@@ -6,7 +6,6 @@ import (
 	"io"
 	"math/bits"
 	"reflect"
-	"runtime"
 	"sync"
 	"unsafe"
 )
@@ -194,25 +193,9 @@ func (k Kind) Class() Kind { return Kind(1 << uint(bits.Len(uint(k))-1)) }
 // Append acts like Marshal but appends the json representation to b instead of
 // always reallocating a new slice.
 func Append(b []byte, x any, flags AppendFlags) ([]byte, error) {
-	if x == nil {
-		// Special case for nil values because it makes the rest of the code
-		// simpler to assume that it won't be seeing nil pointers.
-		return append(b, "null"...), nil
-	}
-
-	t := reflect.TypeOf(x)
-	p := (*iface)(unsafe.Pointer(&x)).ptr
-
-	cache := cacheLoad()
-	c, found := cache[typeid(t)]
-
-	if !found {
-		c = constructCachedCodec(t, cache)
-	}
+	e := encoder{flags: flags}
 
-	b, err := c.encode(encoder{flags: flags}, b, p)
-	runtime.KeepAlive(x)
-	return b, err
+	return e.appendAny(b, x)
 }
 
 // Escape is a convenience helper to construct an escaped JSON string from s.
@@ -330,14 +313,9 @@ func Parse(b []byte, x any, flags ParseFlags) ([]byte, error) {
 		}
 		return r, &InvalidUnmarshalError{Type: t}
 	}
-	t = t.Elem()
 
-	cache := cacheLoad()
-	c, found := cache[typeid(t)]
-
-	if !found {
-		c = constructCachedCodec(t, cache)
-	}
+	t = t.Elem()
+	c := cachedCodec(t)
 
 	r, err := c.decode(d, b, p)
 	return skipSpaces(r), err