diff --git a/internal/registry/model_registry.go b/internal/registry/model_registry.go
index 8f56c43d01..588355827b 100644
--- a/internal/registry/model_registry.go
+++ b/internal/registry/model_registry.go
@@ -742,9 +742,33 @@ func (r *ModelRegistry) ClientSupportsModel(clientID, modelID string) bool {
 		}
 	}
 
+	// Fallback: strip known model variant suffixes (e.g. "-customtools").
+	// These variants share the same provider as the base model.
+	if stripped := stripModelVariantSuffix(modelID); stripped != modelID {
+		for _, id := range models {
+			if strings.EqualFold(strings.TrimSpace(id), stripped) {
+				return true
+			}
+		}
+	}
+
 	return false
 }
 
+// knownModelVariantSuffixes lists suffixes that Google appends to model names
+// for specialised fine-tune variants (e.g. "-customtools" for tool-calling).
+// These variants share the same provider/executor as the base model.
+var knownModelVariantSuffixes = []string{"-customtools"}
+
+func stripModelVariantSuffix(model string) string {
+	for _, suffix := range knownModelVariantSuffixes {
+		if strings.HasSuffix(model, suffix) {
+			return strings.TrimSuffix(model, suffix)
+		}
+	}
+	return model
+}
+
 // GetAvailableModels returns all models that have at least one available client
 // Parameters:
 //   - handlerType: The handler type to filter models for (e.g., "openai", "claude", "gemini")
diff --git a/sdk/api/handlers/handlers.go b/sdk/api/handlers/handlers.go
index 0e490e3202..a3991ee126 100644
--- a/sdk/api/handlers/handlers.go
+++ b/sdk/api/handlers/handlers.go
@@ -801,6 +801,19 @@ func (h *BaseAPIHandler) getRequestDetails(modelName string) (providers []string
 		providers = util.GetProviderName(resolvedModelName)
 	}
 
+	// Fallback: strip known model variant suffixes (e.g. "-customtools") for
+	// provider lookup. The suffix is also stripped from resolvedModelName
+	// because CPA routes gemini-cli through cloudcode-pa/v1internal (OAuth)
+	// which does not accept variant suffixes (returns 404).
+	// The behavioral difference is minor: -customtools only changes tool
+	// selection priority (prefers custom tools over bash), not model capabilities.
+	if len(providers) == 0 {
+		if stripped := stripModelVariantSuffix(baseModel); stripped != baseModel {
+			providers = util.GetProviderName(stripped)
+			resolvedModelName = stripModelVariantSuffix(resolvedModelName)
+		}
+	}
+
 	if len(providers) == 0 {
 		return nil, "", &interfaces.ErrorMessage{StatusCode: http.StatusBadGateway, Error: fmt.Errorf("unknown provider for model %s", modelName)}
 	}
@@ -810,6 +823,20 @@ func (h *BaseAPIHandler) getRequestDetails(modelName string) (providers []string
 	return providers, resolvedModelName, nil
 }
 
+// knownModelVariantSuffixes lists suffixes that Google appends to model names
+// for specialised fine-tune variants (e.g. "-customtools" for tool-calling).
+// These variants share the same provider/executor as the base model.
+var knownModelVariantSuffixes = []string{"-customtools"}
+
+func stripModelVariantSuffix(model string) string {
+	for _, suffix := range knownModelVariantSuffixes {
+		if strings.HasSuffix(model, suffix) {
+			return strings.TrimSuffix(model, suffix)
+		}
+	}
+	return model
+}
+
 func cloneBytes(src []byte) []byte {
 	if len(src) == 0 {
 		return nil