diff --git a/internal/api/server_test.go b/internal/api/server_test.go
index e503fe71b..9f426686f 100644
--- a/internal/api/server_test.go
+++ b/internal/api/server_test.go
@@ -263,7 +263,7 @@ func TestModelsWithClientVersionReturnsCodexCatalog(t *testing.T) {
 			DisplayName:   "Custom Codex Model",
 			Description:   "Custom model from registry",
 			ContextLength: 123456,
-			Thinking:      &registry.ThinkingSupport{Levels: []string{"low", "medium"}},
+			Thinking:      &registry.ThinkingSupport{Levels: []string{"none", "minimal", "low", "medium", "unsupported", "high", "xhigh"}},
 		},
 		{ID: "grok-imagine-image-quality", Object: "model", OwnedBy: "xai", Type: "openai"},
 		{ID: "gpt-image-2", Object: "model", OwnedBy: "openai", Type: "openai"},
@@ -334,6 +334,7 @@ func TestModelsWithClientVersionReturnsCodexCatalog(t *testing.T) {
 	if got, _ := custom["context_window"].(float64); got != 123456 {
 		t.Fatalf("custom context_window = %v, want 123456", custom["context_window"])
 	}
+	assertCodexSupportedReasoningLevels(t, custom, []string{"none", "low", "medium", "high", "xhigh"})
 	if custom["base_instructions"] != gpt55["base_instructions"] {
 		t.Fatal("expected custom model to use gpt-5.5 base_instructions fallback")
 	}
@@ -376,6 +377,27 @@ func TestModelsWithClientVersionReturnsCodexCatalog(t *testing.T) {
 	}
 }
 
+func assertCodexSupportedReasoningLevels(t *testing.T, model map[string]any, want []string) {
+	t.Helper()
+
+	rawLevels, ok := model["supported_reasoning_levels"].([]any)
+	if !ok {
+		t.Fatalf("expected supported_reasoning_levels, got %#v", model["supported_reasoning_levels"])
+	}
+	if len(rawLevels) != len(want) {
+		t.Fatalf("supported_reasoning_levels length = %d, want %d: %#v", len(rawLevels), len(want), rawLevels)
+	}
+	for index, rawLevel := range rawLevels {
+		levelEntry, ok := rawLevel.(map[string]any)
+		if !ok {
+			t.Fatalf("supported_reasoning_levels[%d] = %#v, want object", index, rawLevel)
+		}
+		if got, _ := levelEntry["effort"].(string); got != want[index] {
+			t.Fatalf("supported_reasoning_levels[%d].effort = %q, want %q", index, got, want[index])
+		}
+	}
+}
+
 func TestDefaultRequestLoggerFactory_UsesResolvedLogDirectory(t *testing.T) {
 	t.Setenv("WRITABLE_PATH", "")
 	t.Setenv("writable_path", "")
diff --git a/sdk/api/handlers/openai/codex_client_models.go b/sdk/api/handlers/openai/codex_client_models.go
index e5b43bbae..5f9a254ee 100644
--- a/sdk/api/handlers/openai/codex_client_models.go
+++ b/sdk/api/handlers/openai/codex_client_models.go
@@ -20,6 +20,14 @@ var (
 	codexClientModelTemplatesErr  error
 )
 
+var codexClientAllowedReasoningLevels = map[string]struct{}{
+	"none":   {},
+	"low":    {},
+	"medium": {},
+	"high":   {},
+	"xhigh":  {},
+}
+
 func (h *OpenAIAPIHandler) codexClientModelsResponse() map[string]any {
 	return CodexClientModelsResponse(h.Models())
 }
@@ -45,6 +53,7 @@ func buildCodexClientModels(models []map[string]any) []map[string]any {
 
 		if template, ok := templates[id]; ok {
 			entry := cloneCodexClientModelMap(template)
+			sanitizeCodexClientReasoningMetadata(entry)
 			applyCodexClientVisibilityOverride(entry, id)
 			result = append(result, entry)
 			continue
@@ -52,6 +61,7 @@ func buildCodexClientModels(models []map[string]any) []map[string]any {
 
 		entry := cloneCodexClientModelMap(defaultTemplate)
 		applyCodexClientModelMetadata(entry, id, model)
+		sanitizeCodexClientReasoningMetadata(entry)
 		applyCodexClientVisibilityOverride(entry, id)
 		result = append(result, entry)
 	}
@@ -153,12 +163,16 @@ func applyCodexClientThinkingMetadata(entry map[string]any, thinking *registry.T
 
 	levels := make([]any, 0, len(thinking.Levels))
 	defaultLevel := ""
+	firstLevel := ""
 	for _, rawLevel := range thinking.Levels {
-		level := strings.ToLower(strings.TrimSpace(rawLevel))
-		if level == "" || level == "none" {
+		level := normalizeCodexClientReasoningLevel(rawLevel)
+		if level == "" {
 			continue
 		}
-		if defaultLevel == "" || level == "medium" {
+		if firstLevel == "" {
+			firstLevel = level
+		}
+		if (defaultLevel == "" && level != "none") || level == "medium" {
 			defaultLevel = level
 		}
 		levels = append(levels, map[string]any{
@@ -169,15 +183,64 @@ func applyCodexClientThinkingMetadata(entry map[string]any, thinking *registry.T
 	if len(levels) == 0 {
 		return
 	}
+	if defaultLevel == "" {
+		defaultLevel = firstLevel
+	}
 
 	entry["supported_reasoning_levels"] = levels
 	entry["default_reasoning_level"] = defaultLevel
 }
 
+func sanitizeCodexClientReasoningMetadata(entry map[string]any) {
+	rawLevels, ok := entry["supported_reasoning_levels"].([]any)
+	if !ok {
+		return
+	}
+
+	levels := make([]any, 0, len(rawLevels))
+	allowedDefaults := make(map[string]struct{}, len(rawLevels))
+	for _, rawLevelEntry := range rawLevels {
+		levelEntry, ok := rawLevelEntry.(map[string]any)
+		if !ok {
+			continue
+		}
+		level := normalizeCodexClientReasoningLevel(stringModelValue(levelEntry, "effort"))
+		if level == "" {
+			continue
+		}
+		clonedEntry := cloneCodexClientModelMap(levelEntry)
+		clonedEntry["effort"] = level
+		levels = append(levels, clonedEntry)
+		allowedDefaults[level] = struct{}{}
+	}
+
+	if len(levels) == 0 {
+		delete(entry, "supported_reasoning_levels")
+		delete(entry, "default_reasoning_level")
+		return
+	}
+
+	defaultLevel := normalizeCodexClientReasoningLevel(stringModelValue(entry, "default_reasoning_level"))
+	if _, ok := allowedDefaults[defaultLevel]; !ok {
+		defaultLevel = stringModelValue(levels[0].(map[string]any), "effort")
+	}
+
+	entry["supported_reasoning_levels"] = levels
+	entry["default_reasoning_level"] = defaultLevel
+}
+
+func normalizeCodexClientReasoningLevel(rawLevel string) string {
+	level := strings.ToLower(strings.TrimSpace(rawLevel))
+	if _, ok := codexClientAllowedReasoningLevels[level]; !ok {
+		return ""
+	}
+	return level
+}
+
 func codexClientReasoningDescription(level string) string {
 	switch level {
-	case "minimal":
-		return "Fastest responses with minimal reasoning"
+	case "none":
+		return "No reasoning"
 	case "low":
 		return "Fast responses with lighter reasoning"
 	case "medium":