diff --git a/internal/util/gemini_schema.go b/internal/util/gemini_schema.go
index bd4dae893..8c97b72cc 100644
--- a/internal/util/gemini_schema.go
+++ b/internal/util/gemini_schema.go
@@ -1322,6 +1322,7 @@ func removeUnsupportedKeywords(jsonStr string, options jsonSchemaCleanOptions) s
 		"$schema", "$defs", "definitions", "const", "$ref", "$id", "id", "additionalProperties",
 		"$anchor", "$vocabulary", "$dynamicRef", "$dynamicAnchor",
 		"propertyNames", "patternProperties", // Gemini doesn't support these schema keywords
+		"dependentRequired", "dependentSchemas", "dependencies",
 		"if", "then", "else",
 		"$comment", "enumDescriptions", "enumTitles", "prefill", "deprecated", "encrypted", // Schema metadata fields unsupported by Gemini
 		"additionalItems", "unevaluatedProperties", "unevaluatedItems", "contentSchema",
diff --git a/internal/util/gemini_schema_test.go b/internal/util/gemini_schema_test.go
index f31410813..5da8f3f4d 100644
--- a/internal/util/gemini_schema_test.go
+++ b/internal/util/gemini_schema_test.go
@@ -3039,3 +3039,99 @@ func TestCleanJSONSchema_EnforcesObjectTypeForProperties_Issue6394(t *testing.T)
 		}
 	})
 }
+
+// TestCleanJSONSchema_RemovesDependentKeywords_Issue6539 covers Issue #6539:
+// JSON Schema keywords dependentRequired, dependentSchemas, and dependencies are rejected by
+// the Gemini/Antigravity backend with 400 INVALID_ARGUMENT ("Unknown name \"dependentRequired\" ... Cannot find field")
+// and must be stripped while preserving legitimate tool properties with those names.
+func TestCleanJSONSchema_RemovesDependentKeywords_Issue6539(t *testing.T) {
+	input := `{
+		"type": "object",
+		"properties": {
+			"baseline_url": {"type": "string"},
+			"candidate_url": {"type": "string"},
+			"options": {
+				"type": "object",
+				"properties": {
+					"threshold": {"type": "number"},
+					"diff_color": {"type": "string"}
+				},
+				"dependentRequired": {
+					"threshold": ["diff_color"]
+				},
+				"dependentSchemas": {
+					"threshold": {
+						"properties": {
+							"diff_color": {"type": "string"}
+						}
+					}
+				},
+				"dependencies": {
+					"threshold": ["diff_color"]
+				}
+			},
+			"dependentRequired": {
+				"type": "string",
+				"description": "Legitimate property named dependentRequired"
+			},
+			"dependentSchemas": {
+				"type": "object",
+				"properties": {
+					"nested": {"type": "string"}
+				},
+				"dependentRequired": {
+					"nested": ["other"]
+				}
+			},
+			"dependencies": {
+				"type": "string",
+				"description": "Legitimate property named dependencies"
+			}
+		},
+		"dependentRequired": {
+			"baseline_url": ["candidate_url"]
+		},
+		"dependentSchemas": {
+			"baseline_url": {
+				"required": ["candidate_url"]
+			}
+		},
+		"dependencies": {
+			"baseline_url": ["candidate_url"]
+		}
+	}`
+
+	cleaners := map[string]func(string) string{
+		"Antigravity":         CleanJSONSchemaForAntigravity,
+		"AntigravityTool":     func(s string) string { return CleanJSONSchemaForAntigravityTool(s, false) },
+		"AntigravityResponse": CleanJSONSchemaForAntigravityResponse,
+		"Gemini":              CleanJSONSchemaForGemini,
+		"GeminiJSONSchema":    CleanJSONSchemaForGeminiJSONSchema,
+	}
+
+	for name, clean := range cleaners {
+		t.Run(name, func(t *testing.T) {
+			got := clean(input)
+			parsed := gjson.Parse(got)
+
+			for _, kw := range []string{"dependentRequired", "dependentSchemas", "dependencies"} {
+				if parsed.Get(kw).Exists() {
+					t.Errorf("[%s] root %q was not removed: %s", name, kw, got)
+				}
+				if parsed.Get("properties.options." + kw).Exists() {
+					t.Errorf("[%s] nested properties.options.%s was not removed: %s", name, kw, got)
+				}
+			}
+
+			if parsed.Get("properties.dependentSchemas.dependentRequired").Exists() {
+				t.Errorf("[%s] properties.dependentSchemas.dependentRequired was not removed: %s", name, got)
+			}
+
+			for _, prop := range []string{"dependentRequired", "dependentSchemas", "dependencies"} {
+				if !parsed.Get("properties." + prop).Exists() {
+					t.Errorf("[%s] legitimate property %q was removed: %s", name, prop, got)
+				}
+			}
+		})
+	}
+}