fix: fix openai stream chunk marshaling (#35)

Fixes chunk marshaling for openai stream interceptions so no empty values are added.

Author: pawbana

go.mod

@@ -10,7 +10,7 @@ require (
 	github.com/mark3labs/mcp-go v0.38.0
 	github.com/stretchr/testify v1.10.0
 	github.com/tidwall/gjson v1.18.0 // indirect
-	github.com/tidwall/sjson v1.2.5 // indirect
+	github.com/tidwall/sjson v1.2.5
 	go.uber.org/goleak v1.3.0
 	go.uber.org/mock v0.6.0
 	golang.org/x/exp v0.0.0-20250819193227-8b4c13bb791b

intercept_openai_chat_streaming.go

@@ -14,6 +14,7 @@ import (
 	"github.com/google/uuid"
 	"github.com/openai/openai-go/v2"
 	"github.com/openai/openai-go/v2/packages/ssestream"
+	"github.com/tidwall/sjson"
 
 	"cdr.dev/slog"
 )
@@ -117,16 +118,8 @@ func (i *OpenAIStreamingChatInterception) ProcessRequest(w http.ResponseWriter,
 				continue
 			}
 
-			// If usage information is available, relay the cumulative usage once all tool invocations have completed.
-			if chunk.Usage.CompletionTokens > 0 {
-				chunk.Usage = processor.getCumulativeUsage()
-			}
-
-			// Overwrite response identifier since proxy obscures injected tool call invocations.
-			chunk.ID = i.ID().String()
-
 			// Marshal and relay chunk to client.
-			payload, err := i.marshal(chunk)
+			payload, err := i.marshalChunk(&chunk, i.ID(), processor)
 			if err != nil {
 				logger.Warn(ctx, "failed to marshal chunk", slog.Error(err), chunk.RawJSON())
 				lastErr = fmt.Errorf("marshal chunk: %w", err)
@@ -202,7 +195,7 @@ func (i *OpenAIStreamingChatInterception) ProcessRequest(w http.ResponseWriter,
 		}
 
 		if interceptionErr != nil {
-			payload, err := i.marshal(interceptionErr)
+			payload, err := i.marshalErr(interceptionErr)
 			if err != nil {
 				logger.Warn(ctx, "failed to marshal error", slog.Error(err), slog.F("error_payload", slog.F("%+v", interceptionErr)))
 			} else if err := events.Send(streamCtx, payload); err != nil {
@@ -291,10 +284,36 @@ func (i *OpenAIStreamingChatInterception) getInjectedToolByName(name string) *mc
 	return i.mcpProxy.GetTool(name)
 }
 
-func (i *OpenAIStreamingChatInterception) marshal(payload any) ([]byte, error) {
-	data, err := json.Marshal(payload)
+// Mashals received stream chunk.
+// Overrides id (since proxy obscures injected tool call invocations).
+// If usage field was set in original chunk overrides it to culminative usage.
+//
+// sjson is used instead of normal struct marshaling so forwarded data
+// is as close to the original as possible. Structs from openai library lack
+// `omitzero/omitempty` annotations which adds additional empty fields
+// when marshaling structs. Those additional empty fields can break Codex client.
+func (i *OpenAIStreamingChatInterception) marshalChunk(chunk *openai.ChatCompletionChunk, id uuid.UUID, prc *openAIStreamProcessor) ([]byte, error) {
+	sj, err := sjson.Set(chunk.RawJSON(), "id", id.String())
+	if err != nil {
+		return nil, fmt.Errorf("marshal chunk id failed: %w", err)
+	}
+
+	// If usage information is available, relay the cumulative usage once all tool invocations have completed.
+	if chunk.JSON.Usage.Valid() {
+		u := prc.getCumulativeUsage()
+		sj, err = sjson.Set(sj, "usage", u)
+		if err != nil {
+			return nil, fmt.Errorf("marshal chunk usage failed: %w", err)
+		}
+	}
+
+	return i.encodeForStream([]byte(sj)), nil
+}
+
+func (i *OpenAIStreamingChatInterception) marshalErr(err error) ([]byte, error) {
+	data, err := json.Marshal(err)
 	if err != nil {
-		return nil, fmt.Errorf("marshal payload: %w", err)
+		return nil, fmt.Errorf("marshal error failed: %w", err)
 	}
 
 	return i.encodeForStream(data), nil