diff --git a/internal/runtime/executor/antigravity_executor.go b/internal/runtime/executor/antigravity_executor.go
index 64d1995178..a7e564bd47 100644
--- a/internal/runtime/executor/antigravity_executor.go
+++ b/internal/runtime/executor/antigravity_executor.go
@@ -1318,6 +1318,12 @@ func (e *AntigravityExecutor) buildRequest(ctx context.Context, auth *cliproxyau
 		payload, _ = sjson.DeleteBytes(payload, "request.generationConfig.maxOutputTokens")
 	}
 
+	// Sanitize request.contents to remove any invalid entries that contain
+	// request-level metadata fields (safetySettings, model, systemInstruction, etc.)
+	// This prevents "Invalid JSON payload" errors from the Gemini/Antigravity API
+	// when malformed history entries are accidentally included in contents.
+	payload = sanitizeRequestContents(payload)
+
 	httpReq, errReq := http.NewRequestWithContext(ctx, http.MethodPost, requestURL.String(), bytes.NewReader(payload))
 	if errReq != nil {
 		return nil, errReq
@@ -1594,3 +1600,69 @@ func generateProjectID() string {
 	randomPart := strings.ToLower(uuid.NewString())[:5]
 	return adj + "-" + noun + "-" + randomPart
 }
+
+// sanitizeRequestContents removes invalid entries from request.contents that contain
+// request-level metadata fields instead of proper message content.
+// Valid content entries should only have: role, parts
+// Invalid entries may contain: safetySettings, model, userAgent, requestType, requestId,
+// sessionId, systemInstruction, toolConfig, generationConfig, etc.
+func sanitizeRequestContents(payload []byte) []byte {
+	contentsPath := "request.contents"
+	contentsResult := gjson.GetBytes(payload, contentsPath)
+	if !contentsResult.Exists() || !contentsResult.IsArray() {
+		return payload
+	}
+
+	invalidFieldsSet := map[string]bool{
+		"safetySettings":    true,
+		"model":             true,
+		"userAgent":         true,
+		"requestType":       true,
+		"requestId":         true,
+		"sessionId":         true,
+		"systemInstruction": true,
+		"toolConfig":        true,
+		"generationConfig":  true,
+		"project":           true,
+		"request":           true,
+		"contents":          true,
+	}
+
+	validContents := make([]gjson.Result, 0)
+	contentsResult.ForEach(func(_, content gjson.Result) bool {
+		if !content.IsObject() {
+			return true
+		}
+
+		hasInvalidField := false
+		content.ForEach(func(key, _ gjson.Result) bool {
+			if invalidFieldsSet[key.String()] {
+				hasInvalidField = true
+				log.Warnf("sanitizeRequestContents: dropping invalid content entry with field %q", key.String())
+				return false
+			}
+			return true
+		})
+
+		if !hasInvalidField {
+			validContents = append(validContents, content)
+		}
+		return true
+	})
+
+	if len(validContents) == len(contentsResult.Array()) {
+		return payload
+	}
+
+	newContentsJSON := "["
+	for i, content := range validContents {
+		if i > 0 {
+			newContentsJSON += ","
+		}
+		newContentsJSON += content.Raw
+	}
+	newContentsJSON += "]"
+
+	result, _ := sjson.SetRawBytes(payload, contentsPath, []byte(newContentsJSON))
+	return result
+}
diff --git a/internal/runtime/executor/antigravity_executor_test.go b/internal/runtime/executor/antigravity_executor_test.go
new file mode 100644
index 0000000000..87bb4974f7
--- /dev/null
+++ b/internal/runtime/executor/antigravity_executor_test.go
@@ -0,0 +1,140 @@
+package executor
+
+import (
+	"testing"
+
+	"github.com/tidwall/gjson"
+)
+
+func TestSanitizeRequestContents(t *testing.T) {
+	tests := []struct {
+		name           string
+		input          string
+		expectedCount  int
+		shouldModify   bool
+	}{
+		{
+			name: "valid contents unchanged",
+			input: `{
+				"request": {
+					"contents": [
+						{"role": "user", "parts": [{"text": "hello"}]},
+						{"role": "model", "parts": [{"text": "hi"}]}
+					]
+				}
+			}`,
+			expectedCount: 2,
+			shouldModify:  false,
+		},
+		{
+			name: "removes entry with safetySettings",
+			input: `{
+				"request": {
+					"contents": [
+						{"role": "user", "parts": [{"text": "hello"}]},
+						{"safetySettings": [], "model": "test"}
+					]
+				}
+			}`,
+			expectedCount: 1,
+			shouldModify:  true,
+		},
+		{
+			name: "removes entry with model field",
+			input: `{
+				"request": {
+					"contents": [
+						{"role": "user", "parts": [{"text": "hello"}]},
+						{"model": "gemini-pro", "userAgent": "test"}
+					]
+				}
+			}`,
+			expectedCount: 1,
+			shouldModify:  true,
+		},
+		{
+			name: "removes entry with systemInstruction",
+			input: `{
+				"request": {
+					"contents": [
+						{"systemInstruction": {}, "toolConfig": {}}
+					]
+				}
+			}`,
+			expectedCount: 0,
+			shouldModify:  true,
+		},
+		{
+			name: "removes entry with request metadata fields",
+			input: `{
+				"request": {
+					"contents": [
+						{"role": "user", "parts": [{"text": "hello"}]},
+						{"requestId": "123", "requestType": "agent", "sessionId": "456"}
+					]
+				}
+			}`,
+			expectedCount: 1,
+			shouldModify:  true,
+		},
+		{
+			name: "keeps function call/response entries",
+			input: `{
+				"request": {
+					"contents": [
+						{"role": "user", "parts": [{"text": "hello"}]},
+						{"role": "model", "parts": [{"functionCall": {"name": "test", "args": {}}}]},
+						{"role": "function", "parts": [{"functionResponse": {"name": "test", "response": {}}}]}
+					]
+				}
+			}`,
+			expectedCount: 3,
+			shouldModify:  false,
+		},
+		{
+			name:          "handles empty contents",
+			input:         `{"request": {"contents": []}}`,
+			expectedCount: 0,
+			shouldModify:  false,
+		},
+		{
+			name:          "handles missing contents",
+			input:         `{"request": {}}`,
+			expectedCount: -1,
+			shouldModify:  false,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := sanitizeRequestContents([]byte(tt.input))
+			
+			contentsResult := gjson.GetBytes(result, "request.contents")
+			
+			if tt.expectedCount == -1 {
+				if contentsResult.Exists() {
+					t.Errorf("expected no contents field, but got one")
+				}
+				return
+			}
+
+			if !contentsResult.IsArray() {
+				t.Fatalf("expected contents to be an array")
+			}
+
+			actualCount := len(contentsResult.Array())
+			if actualCount != tt.expectedCount {
+				t.Errorf("expected %d contents, got %d", tt.expectedCount, actualCount)
+			}
+
+			for i, content := range contentsResult.Array() {
+				invalidFields := []string{"safetySettings", "model", "userAgent", "requestType", "requestId", "sessionId", "systemInstruction", "toolConfig", "generationConfig", "project", "request", "contents"}
+				for _, field := range invalidFields {
+					if content.Get(field).Exists() {
+						t.Errorf("content[%d] should not have field %q", i, field)
+					}
+				}
+			}
+		})
+	}
+}