feat: support Claude Sonnet 5.5

backend/internal/pkg/claude/constants.go

@@ -190,6 +190,12 @@ var DefaultModels = []Model{
 		DisplayName: "Claude Opus 5",
 		CreatedAt:   "2026-07-25T00:00:00Z",
 	},
+	{
+		ID:          "claude-sonnet-5-5",
+		Type:        "model",
+		DisplayName: "Claude Sonnet 5.5",
+		CreatedAt:   "2026-09-28T00:00:00Z",
+	},
 	{
 		ID:          "claude-sonnet-5",
 		Type:        "model",

backend/internal/pkg/claude/constants_model_test.go

@@ -30,3 +30,16 @@ func TestDefaultModelsContainsOpus55(t *testing.T) {
 	}
 	t.Fatal("claude-opus-5-5 missing")
 }
+
+func TestDefaultModelsContainsSonnet55(t *testing.T) {
+	t.Parallel()
+
+	for _, model := range DefaultModels {
+		if model.ID == "claude-sonnet-5-5" {
+			require.Equal(t, "Claude Sonnet 5.5", model.DisplayName)
+			require.Equal(t, "2026-09-28T00:00:00Z", model.CreatedAt)
+			return
+		}
+	}
+	t.Fatal("claude-sonnet-5-5 missing from DefaultModels")
+}

backend/internal/pkg/claude/effort_catalog.go

@@ -19,6 +19,7 @@ var effortFamilies = []struct {
 	{family: "claude-mythos-5", levels: effortLowMediumHighXHighMax},
 	{family: "claude-fable-5", levels: effortLowMediumHighXHighMax},
 	{family: "claude-sonnet-4-6", levels: effortLowMediumHighMax},
+	{family: "claude-sonnet-5-5", levels: effortLowMediumHighXHighMax},
 	{family: "claude-sonnet-5", levels: effortLowMediumHighXHighMax},
 	{family: "claude-opus-4-8", levels: effortLowMediumHighXHighMax},
 	{family: "claude-opus-4-7", levels: effortLowMediumHighXHighMax},
@@ -45,19 +46,30 @@ func IsOpus55(model string) bool {
 	return normalizeEffortModelID(model) == "claude-opus-5-5"
 }
 
+// IsSonnet55 identifies the fixed Sonnet 5.5 ID after provider/local suffix normalization.
+func IsSonnet55(model string) bool {
+	return normalizeEffortModelID(model) == "claude-sonnet-5-5"
+}
+
 func normalizeEffortModelID(model string) string {
 	id := strings.ToLower(strings.TrimSpace(model))
 	id = strings.TrimPrefix(id, "models/")
 	if slash := strings.IndexByte(id, '/'); slash >= 0 {
 		id = strings.TrimPrefix(strings.TrimSpace(id[slash+1:]), "models/")
 	}
+	for _, prefix := range []string{"us.", "eu.", "apac.", "jp.", "au.", "us-gov.", "global."} {
+		id = strings.TrimPrefix(id, prefix)
+	}
 	id = strings.TrimPrefix(id, "anthropic.")
 	id = strings.TrimSuffix(id, "-thinking")
-	// OpenRouter uses a dotted minor version for this exact Opus 5.5 ID.
-	// Normalize it before effort, thinking, and billing family lookups.
+	// OpenRouter uses dotted minor versions for some models. Normalize them
+	// before effort, thinking, and billing family lookups.
 	if id == "claude-opus-5.5" {
 		id = "claude-opus-5-5"
 	}
+	if id == "claude-sonnet-5.5" {
+		id = "claude-sonnet-5-5"
+	}
 	if mapped, ok := ModelIDReverseOverrides[id]; ok {
 		id = mapped
 	}

backend/internal/pkg/claude/effort_catalog_test.go

@@ -17,6 +17,8 @@ func TestEffortLevelsForModel(t *testing.T) {
 		{model: "anthropic/claude-sonnet-4-6", want: []string{"low", "medium", "high", "max"}},
 		{model: "claude-opus-5", want: []string{"low", "medium", "high", "xhigh", "max"}},
 		{model: "anthropic/claude-opus-5.5", want: []string{"low", "medium", "high", "xhigh", "max"}},
+		{model: "claude-sonnet-5-5", want: []string{"low", "medium", "high", "xhigh", "max"}},
+		{model: "us.anthropic.claude-sonnet-5-5", want: []string{"low", "medium", "high", "xhigh", "max"}},
 		{model: "claude-opus-4-5-20251101", want: []string{"low", "medium", "high"}},
 		{model: "claude-haiku-4-5-20251001", want: nil},
 		{model: "gpt-5.6", want: nil},
@@ -38,3 +40,20 @@ func TestIsOpus55OpenRouterExactAlias(t *testing.T) {
 		require.False(t, IsOpus55(model), model)
 	}
 }
+
+func TestIsSonnet55(t *testing.T) {
+	t.Parallel()
+	for _, model := range []string{
+		"claude-sonnet-5-5",
+		"anthropic/claude-sonnet-5.5",
+		"anthropic.claude-sonnet-5-5",
+		"us.anthropic.claude-sonnet-5-5",
+		"us-gov.anthropic.claude-sonnet-5-5",
+		"global.anthropic.claude-sonnet-5-5-thinking",
+	} {
+		require.True(t, IsSonnet55(model), model)
+	}
+	for _, model := range []string{"claude-sonnet-5", "claude-sonnet-5-5-preview", "claude-opus-5-5"} {
+		require.False(t, IsSonnet55(model), model)
+	}
+}