diff --git a/internal/util/gemini_schema.go b/internal/util/gemini_schema.go
index b8d07bf4d9..e0caab8222 100644
--- a/internal/util/gemini_schema.go
+++ b/internal/util/gemini_schema.go
@@ -431,6 +431,7 @@ func removeUnsupportedKeywords(jsonStr string) string {
 		"$schema", "$defs", "definitions", "const", "$ref", "$id", "additionalProperties",
 		"propertyNames", "patternProperties", // Gemini doesn't support these schema keywords
 		"enumTitles", "prefill", // Claude/OpenCode schema metadata fields unsupported by Gemini
+		"deprecated", // JSON Schema keyword not supported by Gemini API
 	)
 
 	deletePaths := make([]string, 0)
diff --git a/internal/util/gemini_schema_test.go b/internal/util/gemini_schema_test.go
index bb06e95673..d8f318c3bd 100644
--- a/internal/util/gemini_schema_test.go
+++ b/internal/util/gemini_schema_test.go
@@ -921,6 +921,38 @@ func TestCleanJSONSchemaForGemini_RemovesGeminiUnsupportedMetadataFields(t *test
 	compareJSON(t, expected, result)
 }
 
+func TestCleanJSONSchemaForGemini_RemovesDeprecatedField(t *testing.T) {
+	input := `{
+		"type": "object",
+		"properties": {
+			"old_field": {
+				"type": "string",
+				"deprecated": true
+			},
+			"deprecated": {
+				"type": "string",
+				"description": "property named deprecated should not be removed"
+			}
+		}
+	}`
+
+	expected := `{
+		"type": "object",
+		"properties": {
+			"old_field": {
+				"type": "string"
+			},
+			"deprecated": {
+				"type": "string",
+				"description": "property named deprecated should not be removed"
+			}
+		}
+	}`
+
+	result := CleanJSONSchemaForGemini(input)
+	compareJSON(t, expected, result)
+}
+
 func TestRemoveExtensionFields(t *testing.T) {
 	tests := []struct {
 		name     string