fix(openai): harden GPT-5.6 billing and usage

backend/internal/pkg/apicompat/chatcompletions_responses_test.go

@@ -53,6 +53,26 @@ func TestUsageConversionsPreserveCacheWriteTokens(t *testing.T) {
 	require.Equal(t, 200, roundTrip.InputTokensDetails.CacheWriteTokens)
 }
 
+func TestResponsesUsageNestedCacheWritePresenceOverridesTopLevelAlias(t *testing.T) {
+	tests := []struct {
+		name       string
+		nestedJSON string
+		want       int
+	}{
+		{name: "explicit zero", nestedJSON: `{"cache_write_tokens":0}`, want: 0},
+		{name: "nonzero", nestedJSON: `{"cache_write_tokens":7}`, want: 7},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var usage ResponsesUsage
+			payload := []byte(`{"input_tokens":20,"output_tokens":2,"cache_creation_input_tokens":19,"input_tokens_details":` + tt.nestedJSON + `}`)
+			require.NoError(t, json.Unmarshal(payload, &usage))
+			require.Equal(t, tt.want, usage.CacheCreationInputTokens)
+		})
+	}
+}
+
 func TestChatCompletionsToResponses_SystemMessage(t *testing.T) {
 	req := &ChatCompletionsRequest{
 		Model: "gpt-4o",

backend/internal/pkg/apicompat/types.go

@@ -332,6 +332,10 @@ type ResponsesUsage struct {
 
 func (u *ResponsesUsage) UnmarshalJSON(data []byte) error {
 	type responsesUsageAlias ResponsesUsage
+	type cacheTokenPresence struct {
+		CacheCreationTokens *int `json:"cache_creation_tokens"`
+		CacheWriteTokens    *int `json:"cache_write_tokens"`
+	}
 	var aux struct {
 		responsesUsageAlias
 		PromptTokens            int                           `json:"prompt_tokens"`
@@ -345,6 +349,13 @@ func (u *ResponsesUsage) UnmarshalJSON(data []byte) error {
 	if err := json.Unmarshal(data, &aux); err != nil {
 		return err
 	}
+	var nestedPresence struct {
+		InputTokensDetails  *cacheTokenPresence `json:"input_tokens_details"`
+		PromptTokensDetails *cacheTokenPresence `json:"prompt_tokens_details"`
+	}
+	if err := json.Unmarshal(data, &nestedPresence); err != nil {
+		return err
+	}
 	*u = ResponsesUsage(aux.responsesUsageAlias)
 	if u.InputTokens == 0 && aux.PromptTokens != 0 {
 		u.InputTokens = aux.PromptTokens
@@ -368,6 +379,20 @@ func (u *ResponsesUsage) UnmarshalJSON(data []byte) error {
 	if u.OutputTokensDetails == nil && aux.CompletionTokensDetails != nil {
 		u.OutputTokensDetails = aux.CompletionTokensDetails
 	}
+	var canonicalCacheCreationTokens *int
+	switch {
+	case nestedPresence.InputTokensDetails != nil && nestedPresence.InputTokensDetails.CacheWriteTokens != nil:
+		canonicalCacheCreationTokens = nestedPresence.InputTokensDetails.CacheWriteTokens
+	case nestedPresence.PromptTokensDetails != nil && nestedPresence.PromptTokensDetails.CacheWriteTokens != nil:
+		canonicalCacheCreationTokens = nestedPresence.PromptTokensDetails.CacheWriteTokens
+	case nestedPresence.InputTokensDetails != nil && nestedPresence.InputTokensDetails.CacheCreationTokens != nil:
+		canonicalCacheCreationTokens = nestedPresence.InputTokensDetails.CacheCreationTokens
+	case nestedPresence.PromptTokensDetails != nil && nestedPresence.PromptTokensDetails.CacheCreationTokens != nil:
+		canonicalCacheCreationTokens = nestedPresence.PromptTokensDetails.CacheCreationTokens
+	}
+	if canonicalCacheCreationTokens != nil {
+		u.CacheCreationInputTokens = max(*canonicalCacheCreationTokens, 0)
+	}
 	if u.TotalTokens == 0 && (u.InputTokens != 0 || u.OutputTokens != 0) {
 		u.TotalTokens = u.InputTokens + u.OutputTokens
 	}

backend/internal/pkg/openai/constants.go

@@ -18,6 +18,7 @@ type Model struct {
 
 // DefaultModels OpenAI models list
 var DefaultModels = []Model{
+	{ID: "gpt-5.6", Object: "model", Created: 1780876800, OwnedBy: "openai", Type: "model", DisplayName: "GPT-5.6 (Sol)"},
 	{ID: "gpt-5.6-sol", Object: "model", Created: 1780876800, OwnedBy: "openai", Type: "model", DisplayName: "GPT-5.6 Sol"},
 	{ID: "gpt-5.6-terra", Object: "model", Created: 1780876800, OwnedBy: "openai", Type: "model", DisplayName: "GPT-5.6 Terra"},
 	{ID: "gpt-5.6-luna", Object: "model", Created: 1780876800, OwnedBy: "openai", Type: "model", DisplayName: "GPT-5.6 Luna"},

backend/internal/pkg/openai/constants_test.go

@@ -0,0 +1,11 @@
+package openai
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDefaultModelsIncludeBareGPT56Alias(t *testing.T) {
+	require.Contains(t, DefaultModelIDs(), "gpt-5.6")
+}