feat(imagegen): face swap (identity transfer), a separate operation from Edit

Measured against the instruction-edit models on 2026-07-31: asking a diffusion
model to put a SPECIFIC person's face into a photo does not work by any route.
qwen-image-edit returns the picture essentially unchanged whether asked by
name, by attribute, or by supplying the portrait as a second reference image;
flux-kontext replaces the face with a different generic person. Identity
transfer is a detect/align/blend pipeline, not a better prompt, so it gets its
own interface rather than more Edit options.

imagegen.FaceSwapper is optional and type-asserted, like Editor — a provider
that cannot do this must not have Edit quietly stand in for it.

ListFaces is part of the interface, not a convenience: a caller asked to
change "the man on the right" needs a stable way to NAME one face, and pixel
boxes let it check its own choice. The llamaswap shim orders faces left to
right for exactly that reason (insightface's own order is score-ranked and
unstable between near-identical images), and a malformed box is a protocol
error rather than a zero-filled struct, because a wrong box aims the swap at
the wrong person.

The provider is the first here to POST more than one file, so buildMultipart
gained buildMultipartFiles and now delegates to it — one writer loop, so the
two cannot drift in how they escape names or terminate the body.

index and all are mutually exclusive ON THE WIRE: the shim ignores index under
all=true, and sending both would imply a precedence the caller cannot see.
A JSON body is refused rather than returned as image bytes — the shim answers
JSON on a semantic miss (no face in the source), and handing that back as a
picture would report success while delivering a file that is not one.

imagegen/faceswap.go

@@ -0,0 +1,74 @@
+package imagegen
+
+import "context"
+
+// FaceSwapRequest transfers an identity from Source into Target.
+//
+// This is a DIFFERENT OPERATION from Edit, not a better-tuned one. Measured
+// against the instruction-edit models on 2026-07-31, asking a diffusion model
+// to put a specific person's face into a photo does not work by any route —
+// by name, by description, or by supplying the portrait as a reference image.
+// Face swapping is a dedicated detect/align/blend pipeline; a provider that
+// cannot do it should not pretend Edit is a substitute.
+type FaceSwapRequest struct {
+	// Target is the photo to edit — the pose, expression, lighting and
+	// everything outside the face are preserved from it.
+	Target Image
+
+	// Source is a photo of the face to put in. Only the identity travels;
+	// the source's own pose and expression do not.
+	Source Image
+
+	// Index selects WHICH face in Target, in the provider's documented
+	// ordering (llamaswap: left to right by box centre, as reported by
+	// ListFaces). nil = the largest face, which is right for a portrait and
+	// wrong for a group — enumerate first when it matters.
+	Index *int
+
+	// All swaps every detected face and ignores Index.
+	All bool
+}
+
+// FaceSwapOption mutates a FaceSwapRequest before it is sent.
+type FaceSwapOption func(*FaceSwapRequest)
+
+// WithFaceIndex selects which face in the target to replace.
+func WithFaceIndex(i int) FaceSwapOption { return func(r *FaceSwapRequest) { r.Index = &i } }
+
+// WithAllFaces swaps every detected face.
+func WithAllFaces() FaceSwapOption { return func(r *FaceSwapRequest) { r.All = true } }
+
+// Apply returns a copy of the request with all options applied.
+func (r FaceSwapRequest) Apply(opts ...FaceSwapOption) FaceSwapRequest {
+	for _, opt := range opts {
+		opt(&r)
+	}
+	return r
+}
+
+// DetectedFace is one face located in an image, in PIXEL coordinates.
+type DetectedFace struct {
+	// Index is the face's position in the provider's stable ordering, and
+	// the value FaceSwapRequest.Index expects.
+	Index int
+	// Box is [x0, y0, x1, y1].
+	Box [4]int
+	// Score is the detector's confidence, 0-1.
+	Score float64
+	// Width and Height are the box dimensions, carried so a caller can pick
+	// "the big face" without recomputing them.
+	Width, Height int
+}
+
+// FaceSwapper is the optional face-transfer surface. Separate interface so
+// existing providers keep compiling; callers type-assert.
+type FaceSwapper interface {
+	// ListFaces enumerates the faces in an image, in the SAME ordering
+	// FaceSwapRequest.Index uses. Exposed because a caller asked to change
+	// "the man on the right" needs a way to name one face and to check its
+	// own choice against pixel boxes.
+	ListFaces(ctx context.Context, img Image) ([]DetectedFace, error)
+
+	// FaceSwap transfers Source's identity into Target.
+	FaceSwap(ctx context.Context, req FaceSwapRequest, opts ...FaceSwapOption) (*Result, error)
+}

provider/llamaswap/faceswap.go

@@ -0,0 +1,176 @@
+// faceswap.go implements imagegen.FaceSwapper against the InsightFace shim
+// (buffalo_l + inswapper_128) reached through llama-swap's /upstream
+// passthrough (ADR-0024):
+//
+//	POST /upstream/<id>/v1/faces     multipart file            -> JSON
+//	POST /upstream/<id>/v1/faceswap  multipart target, source  -> PNG
+package llamaswap
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"mime"
+	"net/http"
+	"strconv"
+	"strings"
+
+	"gitea.stevedudenhoeffer.com/steve/majordomo/imagegen"
+	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
+)
+
+// maxFaceSwapResponseBytes bounds the returned PNG. Generous: the shim echoes
+// the target's dimensions, and a 4K photo round-trips as a large lossless PNG.
+const maxFaceSwapResponseBytes = 64 << 20
+
+// FaceSwapModel implements the face-transfer surface. The id selects which
+// upstream llama-swap loads.
+func (p *Provider) FaceSwapModel(id string) (imagegen.FaceSwapper, error) {
+	if err := p.requireBaseURL(); err != nil {
+		return nil, err
+	}
+	return &faceSwapModel{p: p, id: id}, nil
+}
+
+type faceSwapModel struct {
+	p  *Provider
+	id string
+}
+
+// facesResponse mirrors the shim's /v1/faces body.
+type facesResponse struct {
+	Count int `json:"count"`
+	Faces []struct {
+		Index  int     `json:"index"`
+		Box    []int   `json:"box"`
+		Score  float64 `json:"score"`
+		Width  int     `json:"width"`
+		Height int     `json:"height"`
+	} `json:"faces"`
+}
+
+// ListFaces implements imagegen.FaceSwapper.
+func (m *faceSwapModel) ListFaces(ctx context.Context, img imagegen.Image) ([]imagegen.DetectedFace, error) {
+	if len(img.Data) == 0 {
+		return nil, fmt.Errorf("%w: face detection requires image bytes", llm.ErrUnsupported)
+	}
+	path, err := upstreamPath(m.id, "/v1/faces")
+	if err != nil {
+		return nil, err
+	}
+	body, contentType, err := buildMultipart("build faces form",
+		filePart{field: "file", filename: imageFilename(img.MIME, "image"), data: img.Data}, nil)
+	if err != nil {
+		return nil, err
+	}
+	raw, _, err := m.p.doRaw(ctx, http.MethodPost, path, m.id, contentType, body, maxFaceSwapResponseBytes)
+	if err != nil {
+		return nil, err
+	}
+	var parsed facesResponse
+	if err := json.Unmarshal(raw, &parsed); err != nil {
+		return nil, &llm.APIError{Provider: m.p.name, Model: m.id,
+			Message: fmt.Sprintf("faces response is not JSON: %s", truncateForError(raw))}
+	}
+	out := make([]imagegen.DetectedFace, 0, len(parsed.Faces))
+	for _, f := range parsed.Faces {
+		df := imagegen.DetectedFace{Index: f.Index, Score: f.Score, Width: f.Width, Height: f.Height}
+		// A short box would silently index out of range below; treat a
+		// malformed entry as a protocol error rather than zero-filling it,
+		// because a wrong box sends the caller at the wrong face.
+		if len(f.Box) != 4 {
+			return nil, &llm.APIError{Provider: m.p.name, Model: m.id,
+				Message: fmt.Sprintf("face %d has a %d-element box, want 4", f.Index, len(f.Box))}
+		}
+		copy(df.Box[:], f.Box)
+		out = append(out, df)
+	}
+	return out, nil
+}
+
+// FaceSwap implements imagegen.FaceSwapper. The endpoint always answers PNG.
+func (m *faceSwapModel) FaceSwap(ctx context.Context, req imagegen.FaceSwapRequest, opts ...imagegen.FaceSwapOption) (*imagegen.Result, error) {
+	req = req.Apply(opts...)
+	if len(req.Target.Data) == 0 {
+		return nil, fmt.Errorf("%w: face swap requires a target image", llm.ErrUnsupported)
+	}
+	if len(req.Source.Data) == 0 {
+		return nil, fmt.Errorf("%w: face swap requires a source image", llm.ErrUnsupported)
+	}
+	if req.Index != nil && *req.Index < 0 {
+		return nil, fmt.Errorf("%w: face index must be >= 0, got %d", llm.ErrUnsupported, *req.Index)
+	}
+	path, err := upstreamPath(m.id, "/v1/faceswap")
+	if err != nil {
+		return nil, err
+	}
+
+	var fields []formField
+	if req.All {
+		fields = append(fields, formField{key: "all", value: "true", required: true})
+	} else if req.Index != nil {
+		// Only sent when NOT swapping all: the shim ignores index under
+		// all=true, and sending both would imply a precedence the caller
+		// cannot see.
+		fields = append(fields, formField{key: "index", value: strconv.Itoa(*req.Index), required: true})
+	}
+
+	body, contentType, err := buildMultipartFiles("build faceswap form",
+		[]filePart{
+			{field: "target", filename: imageFilename(req.Target.MIME, "target"), data: req.Target.Data},
+			{field: "source", filename: imageFilename(req.Source.MIME, "source"), data: req.Source.Data},
+		}, fields)
+	if err != nil {
+		return nil, err
+	}
+	raw, respType, err := m.p.doRaw(ctx, http.MethodPost, path, m.id, contentType, body, maxFaceSwapResponseBytes)
+	if err != nil {
+		return nil, err
+	}
+	if len(raw) == 0 {
+		return nil, &llm.APIError{Provider: m.p.name, Model: m.id, Message: "face swap response contained no image"}
+	}
+	mime := sniffImageMIME(raw)
+	if respType != "" && !isImageContentType(respType) {
+		// A JSON error body sniffs as text, not image — say what came back
+		// rather than handing the caller bytes that are not a picture.
+		return nil, &llm.APIError{Provider: m.p.name, Model: m.id,
+			Message: fmt.Sprintf("face swap response is not an image (Content-Type %q): %s", respType, truncateForError(raw))}
+	}
+	return &imagegen.Result{Images: []llm.ImagePart{{MIME: mime, Data: raw}}}, nil
+}
+
+// imageFilename picks a multipart filename for an image part. The shim reads
+// bytes, not names, but a plausible extension keeps server-side sniffing and
+// request logs honest. base distinguishes the parts of a multi-file form
+// ("target"/"source") so a log line says which one was malformed.
+func imageFilename(mimeType, base string) string {
+	if base == "" {
+		base = "image"
+	}
+	mt := strings.ToLower(strings.TrimSpace(mimeType))
+	if parsed, _, err := mime.ParseMediaType(mt); err == nil {
+		mt = parsed
+	}
+	switch mt {
+	case "image/jpeg", "image/jpg":
+		return base + ".jpg"
+	case "image/webp":
+		return base + ".webp"
+	case "image/gif":
+		return base + ".gif"
+	case "image/bmp":
+		return base + ".bmp"
+	default:
+		// PNG is the safe default: every caller in this repo either sends PNG
+		// or sends something the decoder identifies by magic bytes anyway.
+		return base + ".png"
+	}
+}
+
+// isImageContentType reports whether a response Content-Type is an image.
+// Used to tell a returned picture apart from a JSON error body, which would
+// otherwise be handed back as "image" bytes.
+func isImageContentType(contentType string) bool {
+	return mimeFromContentType(contentType, "image/") != ""
+}

provider/llamaswap/faceswap_test.go

@@ -0,0 +1,214 @@
+package llamaswap
+
+import (
+	"context"
+	"encoding/base64"
+	"io"
+	"mime"
+	"mime/multipart"
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+
+	"gitea.stevedudenhoeffer.com/steve/majordomo/imagegen"
+	"gitea.stevedudenhoeffer.com/steve/majordomo/llm"
+)
+
+func swapImg(t *testing.T) imagegen.Image {
+	t.Helper()
+	raw, err := base64.StdEncoding.DecodeString(onePixelPNG)
+	if err != nil {
+		t.Fatalf("decode fixture: %v", err)
+	}
+	return imagegen.Image{MIME: "image/png", Data: raw}
+}
+
+// parseParts pulls the multipart form a handler received.
+func parseParts(t *testing.T, r *http.Request) (files map[string][]byte, fields map[string]string) {
+	t.Helper()
+	_, params, err := mime.ParseMediaType(r.Header.Get("Content-Type"))
+	if err != nil {
+		t.Fatalf("content type: %v", err)
+	}
+	mr := multipart.NewReader(r.Body, params["boundary"])
+	files, fields = map[string][]byte{}, map[string]string{}
+	for {
+		p, err := mr.NextPart()
+		if err == io.EOF {
+			break
+		}
+		if err != nil {
+			t.Fatalf("next part: %v", err)
+		}
+		body, _ := io.ReadAll(p)
+		if p.FileName() != "" {
+			files[p.FormName()] = body
+		} else {
+			fields[p.FormName()] = string(body)
+		}
+	}
+	return files, fields
+}
+
+// TestFaceSwapSendsBothFiles pins the two-file wire shape. A face swap is the
+// first endpoint in this provider taking more than one file, so buildMultipart
+// grew a sibling; getting the field NAMES wrong would reach the shim as a
+// missing-argument 422 rather than anything self-explanatory.
+func TestFaceSwapSendsBothFiles(t *testing.T) {
+	var gotPath string
+	var files map[string][]byte
+	var fields map[string]string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotPath = r.URL.Path
+		files, fields = parseParts(t, r)
+		w.Header().Set("Content-Type", "image/png")
+		raw, _ := base64.StdEncoding.DecodeString(onePixelPNG)
+		_, _ = w.Write(raw)
+	}))
+	defer srv.Close()
+
+	p := New(WithBaseURL(srv.URL), WithHTTPClient(srv.Client()))
+	m, err := p.FaceSwapModel("faceswap")
+	if err != nil {
+		t.Fatalf("model: %v", err)
+	}
+	img := swapImg(t)
+	res, err := m.FaceSwap(context.Background(), imagegen.FaceSwapRequest{Target: img, Source: img},
+		imagegen.WithFaceIndex(2))
+	if err != nil {
+		t.Fatalf("faceswap: %v", err)
+	}
+	if len(res.Images) != 1 {
+		t.Fatalf("images = %d, want 1", len(res.Images))
+	}
+	if !strings.HasSuffix(gotPath, "/upstream/faceswap/v1/faceswap") {
+		t.Errorf("path = %q", gotPath)
+	}
+	for _, want := range []string{"target", "source"} {
+		if len(files[want]) == 0 {
+			t.Errorf("no %q file part — the shim requires both", want)
+		}
+	}
+	if fields["index"] != "2" {
+		t.Errorf("index = %q, want 2", fields["index"])
+	}
+	if _, ok := fields["all"]; ok {
+		t.Error("all sent alongside index — the shim ignores index under all=true, so sending both implies a precedence the caller cannot see")
+	}
+}
+
+// TestFaceSwapAllSuppressesIndex: same reasoning from the other side.
+func TestFaceSwapAllSuppressesIndex(t *testing.T) {
+	var fields map[string]string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		_, fields = parseParts(t, r)
+		w.Header().Set("Content-Type", "image/png")
+		raw, _ := base64.StdEncoding.DecodeString(onePixelPNG)
+		_, _ = w.Write(raw)
+	}))
+	defer srv.Close()
+
+	p := New(WithBaseURL(srv.URL), WithHTTPClient(srv.Client()))
+	m, _ := p.FaceSwapModel("faceswap")
+	img := swapImg(t)
+	if _, err := m.FaceSwap(context.Background(),
+		imagegen.FaceSwapRequest{Target: img, Source: img, Index: new(int), All: true}); err != nil {
+		t.Fatalf("faceswap: %v", err)
+	}
+	if fields["all"] != "true" {
+		t.Errorf("all = %q, want true", fields["all"])
+	}
+	if _, ok := fields["index"]; ok {
+		t.Error("index sent under all=true")
+	}
+}
+
+// TestFaceSwapRejectsMissingImages: both are required, and the error should
+// name which one rather than surfacing a shim 422.
+func TestFaceSwapRejectsMissingImages(t *testing.T) {
+	p := New(WithBaseURL("http://example.invalid"))
+	m, _ := p.FaceSwapModel("faceswap")
+	img := swapImg(t)
+	for _, tc := range []struct {
+		name string
+		req  imagegen.FaceSwapRequest
+		want string
+	}{
+		{"no target", imagegen.FaceSwapRequest{Source: img}, "target"},
+		{"no source", imagegen.FaceSwapRequest{Target: img}, "source"},
+	} {
+		_, err := m.FaceSwap(context.Background(), tc.req)
+		if err == nil || !strings.Contains(err.Error(), tc.want) {
+			t.Errorf("%s: err = %v, want one naming %q", tc.name, err, tc.want)
+		}
+	}
+}
+
+// TestFaceSwapRejectsNonImageResponse: the shim answers JSON on a semantic
+// miss (no face found). Returning those bytes as an "image" would hand the
+// caller a file that is not a picture and call it success.
+func TestFaceSwapRejectsNonImageResponse(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"detail":{"error":"no_face_in_source"}}`))
+	}))
+	defer srv.Close()
+
+	p := New(WithBaseURL(srv.URL), WithHTTPClient(srv.Client()))
+	m, _ := p.FaceSwapModel("faceswap")
+	img := swapImg(t)
+	_, err := m.FaceSwap(context.Background(), imagegen.FaceSwapRequest{Target: img, Source: img})
+	if err == nil {
+		t.Fatal("a JSON body was accepted as an image")
+	}
+	var apiErr *llm.APIError
+	if !strings.Contains(err.Error(), "no_face_in_source") {
+		t.Errorf("err = %v, want it to relay the shim's reason", err)
+	}
+	_ = apiErr
+}
+
+// TestListFacesParsesOrdering: the shim's left-to-right index is the contract
+// callers select against, so it must survive decoding intact.
+func TestListFacesParsesOrdering(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if !strings.HasSuffix(r.URL.Path, "/upstream/faceswap/v1/faces") {
+			t.Errorf("path = %q", r.URL.Path)
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"count":2,"faces":[
+			{"index":0,"box":[10,20,30,40],"score":0.9,"width":20,"height":20},
+			{"index":1,"box":[50,20,90,60],"score":0.8,"width":40,"height":40}]}`))
+	}))
+	defer srv.Close()
+
+	p := New(WithBaseURL(srv.URL), WithHTTPClient(srv.Client()))
+	m, _ := p.FaceSwapModel("faceswap")
+	faces, err := m.ListFaces(context.Background(), swapImg(t))
+	if err != nil {
+		t.Fatalf("list: %v", err)
+	}
+	if len(faces) != 2 || faces[0].Index != 0 || faces[1].Index != 1 {
+		t.Fatalf("faces = %+v", faces)
+	}
+	if faces[1].Box != [4]int{50, 20, 90, 60} {
+		t.Errorf("box = %v", faces[1].Box)
+	}
+}
+
+// TestListFacesRejectsShortBox: a malformed box would send a caller at the
+// wrong face, which is worse than an error.
+func TestListFacesRejectsShortBox(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{"count":1,"faces":[{"index":0,"box":[1,2],"score":0.9}]}`))
+	}))
+	defer srv.Close()
+
+	p := New(WithBaseURL(srv.URL), WithHTTPClient(srv.Client()))
+	m, _ := p.FaceSwapModel("faceswap")
+	if _, err := m.ListFaces(context.Background(), swapImg(t)); err == nil {
+		t.Fatal("a 2-element box was accepted")
+	}
+}

provider/llamaswap/upstream.go

@@ -51,14 +51,27 @@ type filePart struct {
 // writeFormFields). wrap labels errors. Returns the body and its content
 // type.
 func buildMultipart(wrap string, file filePart, fields []formField) (*bytes.Buffer, string, error) {
+	return buildMultipartFiles(wrap, []filePart{file}, fields)
+}
+
+// buildMultipartFiles is buildMultipart for endpoints taking SEVERAL files
+// (face swap sends a target and a source). Files are written in the given
+// order, then the fields. One writer loop serves both so the two cannot drift
+// in how they escape names or terminate the body.
+func buildMultipartFiles(wrap string, files []filePart, fields []formField) (*bytes.Buffer, string, error) {
+	if len(files) == 0 {
+		return nil, "", fmt.Errorf("llama-swap: %s: no file parts", wrap)
+	}
 	var buf bytes.Buffer
 	w := multipart.NewWriter(&buf)
-	fw, err := w.CreateFormFile(file.field, file.filename)
-	if err != nil {
-		return nil, "", fmt.Errorf("llama-swap: %s: %w", wrap, err)
-	}
-	if _, err := fw.Write(file.data); err != nil {
-		return nil, "", fmt.Errorf("llama-swap: %s: %w", wrap, err)
+	for _, file := range files {
+		fw, err := w.CreateFormFile(file.field, file.filename)
+		if err != nil {
+			return nil, "", fmt.Errorf("llama-swap: %s: %w", wrap, err)
+		}
+		if _, err := fw.Write(file.data); err != nil {
+			return nil, "", fmt.Errorf("llama-swap: %s: %w", wrap, err)
+		}
 	}
 	if err := writeFormFields(w, wrap, fields); err != nil {
 		return nil, "", err