feat(amp): add response rewriter for model name substitution in responses

internal/api/modules/amp/response_rewriter.go

@@ -0,0 +1,108 @@
+package amp
+
+import (
+	"bufio"
+	"bytes"
+	"net"
+	"net/http"
+	"strings"
+
+	"github.com/gin-gonic/gin"
+	log "github.com/sirupsen/logrus"
+	"github.com/tidwall/gjson"
+	"github.com/tidwall/sjson"
+)
+
+// ResponseRewriter wraps a gin.ResponseWriter to intercept and modify the response body
+// It's used to rewrite model names in responses when model mapping is used
+type ResponseRewriter struct {
+	gin.ResponseWriter
+	body          *bytes.Buffer
+	originalModel string
+	isStreaming   bool
+}
+
+// NewResponseRewriter creates a new response rewriter for model name substitution
+func NewResponseRewriter(w gin.ResponseWriter, originalModel string) *ResponseRewriter {
+	return &ResponseRewriter{
+		ResponseWriter: w,
+		body:           &bytes.Buffer{},
+		originalModel:  originalModel,
+	}
+}
+
+// Write intercepts response writes and buffers them for model name replacement
+func (rw *ResponseRewriter) Write(data []byte) (int, error) {
+	// Detect streaming on first write
+	if rw.body.Len() == 0 && !rw.isStreaming {
+		contentType := rw.Header().Get("Content-Type")
+		rw.isStreaming = strings.Contains(contentType, "text/event-stream") ||
+			strings.Contains(contentType, "stream")
+	}
+
+	if rw.isStreaming {
+		return rw.ResponseWriter.Write(rw.rewriteStreamChunk(data))
+	}
+	return rw.body.Write(data)
+}
+
+// Flush writes the buffered response with model names rewritten
+func (rw *ResponseRewriter) Flush() {
+	if rw.isStreaming {
+		if flusher, ok := rw.ResponseWriter.(http.Flusher); ok {
+			flusher.Flush()
+		}
+		return
+	}
+	if rw.body.Len() > 0 {
+		if _, err := rw.ResponseWriter.Write(rw.rewriteModelInResponse(rw.body.Bytes())); err != nil {
+			log.Warnf("amp response rewriter: failed to write rewritten response: %v", err)
+		}
+	}
+}
+
+// modelFieldPaths lists all JSON paths where model name may appear
+var modelFieldPaths = []string{"model", "modelVersion", "response.modelVersion", "message.model"}
+
+// rewriteModelInResponse replaces all occurrences of the mapped model with the original model in JSON
+func (rw *ResponseRewriter) rewriteModelInResponse(data []byte) []byte {
+	if rw.originalModel == "" {
+		return data
+	}
+	for _, path := range modelFieldPaths {
+		if gjson.GetBytes(data, path).Exists() {
+			data, _ = sjson.SetBytes(data, path, rw.originalModel)
+		}
+	}
+	return data
+}
+
+// rewriteStreamChunk rewrites model names in SSE stream chunks
+func (rw *ResponseRewriter) rewriteStreamChunk(chunk []byte) []byte {
+	if rw.originalModel == "" {
+		return chunk
+	}
+
+	// SSE format: "data: {json}\n\n"
+	lines := bytes.Split(chunk, []byte("\n"))
+	for i, line := range lines {
+		if bytes.HasPrefix(line, []byte("data: ")) {
+			jsonData := bytes.TrimPrefix(line, []byte("data: "))
+			if len(jsonData) > 0 && jsonData[0] == '{' {
+				// Rewrite JSON in the data line
+				rewritten := rw.rewriteModelInResponse(jsonData)
+				lines[i] = append([]byte("data: "), rewritten...)
+			}
+		}
+	}
+
+	return bytes.Join(lines, []byte("\n"))
+}
+
+// Hijack implements http.Hijacker for WebSocket support
+func (rw *ResponseRewriter) Hijack() (net.Conn, *bufio.ReadWriter, error) {
+	if hijacker, ok := rw.ResponseWriter.(http.Hijacker); ok {
+		return hijacker.Hijack()
+	}
+	return nil, nil, http.ErrNotSupported
+}