diff --git a/internal/apiform/encoder.go b/internal/apiform/encoder.go
index fc963b2..adb5e78 100644
--- a/internal/apiform/encoder.go
+++ b/internal/apiform/encoder.go
@@ -49,6 +49,19 @@ func (e *encoder) encodeValue(key string, val reflect.Value, writer *multipart.W
 		return writer.WriteField(key, "")
 	}
 
+	// Unwrap interfaces before detecting io.Reader. A non-nil interface can
+	// contain a typed nil pointer, which must retain the encoder's empty-field
+	// semantics instead of being passed to io.Copy.
+	for val.Kind() == reflect.Interface {
+		if val.IsNil() {
+			return writer.WriteField(key, "")
+		}
+		val = val.Elem()
+	}
+	if val.Kind() == reflect.Pointer && val.IsNil() {
+		return writer.WriteField(key, "")
+	}
+
 	t := val.Type()
 
 	if t.Implements(reflect.TypeOf((*io.Reader)(nil)).Elem()) {
@@ -57,9 +70,6 @@ func (e *encoder) encodeValue(key string, val reflect.Value, writer *multipart.W
 
 	switch t.Kind() {
 	case reflect.Pointer:
-		if val.IsNil() || !val.IsValid() {
-			return writer.WriteField(key, "")
-		}
 		return e.encodeValue(key, val.Elem(), writer)
 
 	case reflect.Slice, reflect.Array:
@@ -68,12 +78,6 @@ func (e *encoder) encodeValue(key string, val reflect.Value, writer *multipart.W
 	case reflect.Map:
 		return e.encodeMap(key, val, writer)
 
-	case reflect.Interface:
-		if val.IsNil() {
-			return writer.WriteField(key, "")
-		}
-		return e.encodeValue(key, val.Elem(), writer)
-
 	case reflect.String:
 		return writer.WriteField(key, val.String())
 
diff --git a/internal/apiform/typed_nil_reader_test.go b/internal/apiform/typed_nil_reader_test.go
new file mode 100644
index 0000000..df243ad
--- /dev/null
+++ b/internal/apiform/typed_nil_reader_test.go
@@ -0,0 +1,48 @@
+package apiform
+
+import (
+	"bytes"
+	"io"
+	"mime/multipart"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+type panicOnRead struct{}
+
+func (*panicOnRead) Read([]byte) (int, error) {
+	panic("Read called on typed nil receiver")
+}
+
+func TestMarshalTreatsTypedNilReaderAsEmptyField(t *testing.T) {
+	t.Parallel()
+
+	var concrete *panicOnRead
+	var reader io.Reader = concrete
+	tests := map[string]any{
+		"concrete pointer in any map": map[string]any{"file": concrete},
+		"pointer in reader map":       map[string]io.Reader{"file": reader},
+		"nil reader interface":        map[string]io.Reader{"file": nil},
+	}
+
+	for name, value := range tests {
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			var buf bytes.Buffer
+			writer := multipart.NewWriter(&buf)
+			require.NoError(t, writer.SetBoundary("xxx"))
+
+			require.NotPanics(t, func() {
+				require.NoError(t, Marshal(value, writer))
+				require.NoError(t, writer.Close())
+			})
+
+			require.Equal(t,
+				"--xxx\r\nContent-Disposition: form-data; name=\"file\"\r\n\r\n\r\n--xxx--\r\n",
+				buf.String(),
+			)
+		})
+	}
+}
diff --git a/pkg/cmd/multipartbody.go b/pkg/cmd/multipartbody.go
index a9983e6..72c1b2d 100644
--- a/pkg/cmd/multipartbody.go
+++ b/pkg/cmd/multipartbody.go
@@ -219,6 +219,14 @@ type multipartBodyInfo struct {
 	knownLength bool
 }
 
+func isTypedNilReader(value any) bool {
+	if _, ok := value.(io.Reader); !ok {
+		return false
+	}
+	reflected := reflect.ValueOf(value)
+	return reflected.Kind() == reflect.Pointer && reflected.IsNil()
+}
+
 func inspectMultipartBody(value any) multipartBodyInfo {
 	switch value := value.(type) {
 	case map[string]any:
@@ -240,6 +248,9 @@ func inspectMultipartBody(value any) multipartBodyInfo {
 	case fileUpload:
 		return multipartBodyInfo{hasUpload: true, knownLength: value.hasKnownSize()}
 	default:
+		if isTypedNilReader(value) {
+			return multipartBodyInfo{knownLength: true}
+		}
 		_, isReader := value.(io.Reader)
 		return multipartBodyInfo{hasUpload: isReader, knownLength: !isReader}
 	}
@@ -311,6 +322,9 @@ func transformFileUploads(
 		}
 		return result, nil
 	default:
+		if isTypedNilReader(value) {
+			return value, nil
+		}
 		if _, isReader := value.(io.Reader); isReader {
 			return nil, errors.New("multipart body contains an unknown-size reader")
 		}
diff --git a/pkg/cmd/multipartbody_typed_nil_test.go b/pkg/cmd/multipartbody_typed_nil_test.go
new file mode 100644
index 0000000..46df970
--- /dev/null
+++ b/pkg/cmd/multipartbody_typed_nil_test.go
@@ -0,0 +1,146 @@
+package cmd
+
+import (
+	"context"
+	"io"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"strings"
+	"sync/atomic"
+	"testing"
+
+	"github.com/openai/openai-cli/internal/apiform"
+	"github.com/openai/openai-go/v3"
+	"github.com/openai/openai-go/v3/option"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func typedNilMultipartReader() io.Reader {
+	var reader *strings.Reader
+	return reader
+}
+
+func TestInspectMultipartBodyTreatsTypedNilReaderAsScalar(t *testing.T) {
+	info := inspectMultipartBody(map[string]any{"file": typedNilMultipartReader()})
+
+	require.False(t, info.hasUpload)
+	require.True(t, info.knownLength)
+}
+
+func TestMultipartRequestOptionsRetryTypedNilReaderAsScalar(t *testing.T) {
+	var requestCount atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount.Add(1)
+		if !assert.NoError(t, r.ParseMultipartForm(1<<20)) {
+			http.Error(w, "invalid multipart form", http.StatusBadRequest)
+			return
+		}
+		assert.Equal(t, []string{""}, r.MultipartForm.Value["file"])
+		assert.Empty(t, r.MultipartForm.File["file"])
+		assert.Positive(t, r.ContentLength)
+
+		w.Header().Set("Content-Type", "application/json")
+		if requestCount.Load() == 1 {
+			w.Header().Set("Retry-After", "0")
+			w.WriteHeader(http.StatusTooManyRequests)
+			_, _ = io.WriteString(w, `{"error":{"message":"retry me","type":"rate_limit_error"}}`)
+			return
+		}
+		_, _ = io.WriteString(w, `{"id":"video_123","object":"video","status":"queued"}`)
+	}))
+	t.Cleanup(server.Close)
+
+	options, err := multipartRequestOptions(map[string]any{
+		"file":   typedNilMultipartReader(),
+		"prompt": "hello",
+	}, apiform.FormatBrackets)
+	require.NoError(t, err)
+	client := openai.NewClient(
+		option.WithAPIKey("test-key"),
+		option.WithBaseURL(server.URL+"/"),
+	)
+
+	_, err = client.Videos.New(context.Background(), openai.VideoNewParams{}, options...)
+	require.NoError(t, err)
+	require.Equal(t, int32(2), requestCount.Load())
+}
+
+func TestMultipartRequestOptionsReplayTypedNilReaderAcrossRedirects(t *testing.T) {
+	for _, status := range []int{http.StatusTemporaryRedirect, http.StatusPermanentRedirect} {
+		t.Run(http.StatusText(status), func(t *testing.T) {
+			var requestCount atomic.Int32
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				requestCount.Add(1)
+				if r.URL.Path != "/redirected" {
+					http.Redirect(w, r, "/redirected", status)
+					return
+				}
+
+				if !assert.NoError(t, r.ParseMultipartForm(1<<20)) {
+					http.Error(w, "invalid multipart form", http.StatusBadRequest)
+					return
+				}
+				assert.Equal(t, []string{""}, r.MultipartForm.Value["file"])
+				assert.Empty(t, r.MultipartForm.File["file"])
+				assert.Positive(t, r.ContentLength)
+				w.Header().Set("Content-Type", "application/json")
+				_, _ = io.WriteString(w, `{"id":"video_123","object":"video","status":"queued"}`)
+			}))
+			t.Cleanup(server.Close)
+
+			options, err := multipartRequestOptions(map[string]any{
+				"file":   typedNilMultipartReader(),
+				"prompt": "hello",
+			}, apiform.FormatBrackets)
+			require.NoError(t, err)
+			client := openai.NewClient(
+				option.WithAPIKey("test-key"),
+				option.WithBaseURL(server.URL+"/"),
+			)
+
+			_, err = client.Videos.New(context.Background(), openai.VideoNewParams{}, options...)
+			require.NoError(t, err)
+			require.Equal(t, int32(2), requestCount.Load())
+		})
+	}
+}
+
+func TestMultipartRequestOptionsKnownUploadAllowsTypedNilReaderField(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "payload.txt")
+	require.NoError(t, os.WriteFile(path, []byte("payload"), 0o600))
+	upload, err := openFileUpload(path)
+	require.NoError(t, err)
+
+	var requestCount atomic.Int32
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		requestCount.Add(1)
+		assert.Positive(t, r.ContentLength)
+		if !assert.NoError(t, r.ParseMultipartForm(1<<20)) {
+			http.Error(w, "invalid multipart form", http.StatusBadRequest)
+			return
+		}
+		assert.Equal(t, []string{""}, r.MultipartForm.Value["optional"])
+		assert.Len(t, r.MultipartForm.File["file"], 1)
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = io.WriteString(w, `{"id":"file_123","object":"file","bytes":7,"filename":"payload.txt","purpose":"assistants"}`)
+	}))
+	t.Cleanup(server.Close)
+
+	options, err := multipartRequestOptions(map[string]any{
+		"file":     upload,
+		"optional": typedNilMultipartReader(),
+		"purpose":  "assistants",
+	}, apiform.FormatBrackets)
+	require.NoError(t, err)
+	client := openai.NewClient(
+		option.WithAPIKey("test-key"),
+		option.WithBaseURL(server.URL+"/"),
+	)
+
+	_, err = client.Files.New(context.Background(), openai.FileNewParams{}, options...)
+	require.NoError(t, err)
+	require.Equal(t, int32(1), requestCount.Load())
+}