diff --git a/backend/internal/pkg/antigravity/schema_cleaner.go b/backend/internal/pkg/antigravity/schema_cleaner.go
index 9ac4211e4a..73cf362561 100644
--- a/backend/internal/pkg/antigravity/schema_cleaner.go
+++ b/backend/internal/pkg/antigravity/schema_cleaner.go
@@ -258,6 +258,18 @@ func cleanJSONSchemaRecursive(value any) any {
 		if constVal, exists := schemaMap["const"]; exists {
 			if _, hasEnum := schemaMap["enum"]; !hasEnum {
 				schemaMap["enum"] = []any{constVal}
+			} else if constant, ok := constVal.(string); ok {
+				if existing, ok := schemaMap["enum"].([]any); ok {
+					// Both constraints apply; preserve their intersection before dropping const.
+					values := []any{}
+					for _, value := range existing {
+						if text, ok := value.(string); ok && text == constant {
+							values = append(values, constant)
+							break
+						}
+					}
+					schemaMap["enum"] = values
+				}
 			}
 			if _, hasType := schemaMap["type"]; !hasType {
 				switch constVal.(type) {
diff --git a/backend/internal/pkg/antigravity/schema_const_test.go b/backend/internal/pkg/antigravity/schema_const_test.go
new file mode 100644
index 0000000000..05551fbd83
--- /dev/null
+++ b/backend/internal/pkg/antigravity/schema_const_test.go
@@ -0,0 +1,45 @@
+package antigravity
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestBuildToolsPreservesStringConst(t *testing.T) {
+	for _, tc := range []struct {
+		name   string
+		schema string
+		want   string
+	}{
+		{"typed", `{"type":"string","const":"browser"}`, `{"type":"string","enum":["browser"]}`},
+		{"inferred", `{"const":"browser"}`, `{"type":"string","enum":["browser"]}`},
+		{"empty", `{"type":"string","const":""}`, `{"type":"string","enum":[""]}`},
+		{"existing enum", `{"type":"string","const":"browser","enum":["browser","shell"]}`, `{"type":"string","enum":["browser"]}`},
+		{"conflicting enum", `{"type":"string","const":"browser","enum":["shell"]}`, `{"type":"string","enum":[]}`},
+		{"ordinary enum", `{"type":"string","enum":["browser","shell"]}`, `{"type":"string","enum":["browser","shell"]}`},
+		{"array items", `{"type":"array","items":{"type":"string","const":"browser"}}`, `{"type":"array","items":{"type":"string","enum":["browser"]}}`},
+		{"nested property named const", `{"type":"object","properties":{"const":{"const":"browser"}}}`, `{"type":"object","properties":{"const":{"type":"string","enum":["browser"]}}}`},
+		{"schema metadata", `{"type":"string","const":"browser","$schema":"https://json-schema.org/draft/2020-12/schema","description":"Tool kind"}`, `{"type":"string","enum":["browser"],"description":"Tool kind"}`},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			var property map[string]any
+			require.NoError(t, json.Unmarshal([]byte(tc.schema), &property))
+			tools := buildTools([]ClaudeTool{{
+				Name: "dispatch",
+				InputSchema: map[string]any{
+					"type":       "object",
+					"properties": map[string]any{"action": property},
+				},
+			}})
+			require.Len(t, tools, 1)
+			require.Len(t, tools[0].FunctionDeclarations, 1)
+			properties, ok := tools[0].FunctionDeclarations[0].Parameters["properties"].(map[string]any)
+			require.True(t, ok, "tool parameters must contain a properties object")
+			got, err := json.Marshal(properties["action"])
+			require.NoError(t, err)
+			require.JSONEq(t, tc.want, string(got))
+		})
+	}
+}