diff --git a/nextcloud_mcp_server/models/base.py b/nextcloud_mcp_server/models/base.py
index d9303a7..40f091f 100644
--- a/nextcloud_mcp_server/models/base.py
+++ b/nextcloud_mcp_server/models/base.py
@@ -1,23 +1,39 @@
 """Base Pydantic models for common response patterns."""
 
-from datetime import datetime
+from datetime import datetime, timezone
 from typing import Any, Dict, Optional, Union
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_serializer
+
+
+def _utc_now() -> datetime:
+    """Generate UTC timestamp for responses."""
+    return datetime.now(timezone.utc)
 
 
 class BaseResponse(BaseModel):
     """Base response model for all MCP tool responses."""
 
-    model_config = {"json_encoders": {datetime: lambda v: v.isoformat()}}
-
     success: bool = Field(
         default=True, description="Whether the operation was successful"
     )
     timestamp: datetime = Field(
-        default_factory=datetime.now, description="Response timestamp"
+        default_factory=_utc_now, description="Response timestamp"
     )
 
+    @field_serializer("timestamp")
+    def serialize_timestamp(self, timestamp: datetime) -> str:
+        """Serialize timestamp to RFC3339 format for MCP compliance."""
+        if timestamp.tzinfo is None:
+            # If somehow we get a naive datetime, assume UTC
+            timestamp = timestamp.replace(tzinfo=timezone.utc)
+        # Use isoformat() which produces RFC3339 compliant format
+        # For UTC times, replace '+00:00' with 'Z' as preferred by many systems
+        iso_string = timestamp.isoformat()
+        if iso_string.endswith("+00:00"):
+            return iso_string[:-6] + "Z"
+        return iso_string
+
 
 class ErrorResponse(BaseResponse):
     """Response model for error cases."""
diff --git a/tests/test_models.py b/tests/test_models.py
new file mode 100644
index 0000000..304f916
--- /dev/null
+++ b/tests/test_models.py
@@ -0,0 +1,82 @@
+"""Unit tests for Pydantic models and serialization."""
+
+import json
+import re
+from datetime import datetime, timezone
+
+
+from nextcloud_mcp_server.models.base import BaseResponse, SuccessResponse
+
+
+def test_timestamp_format_validation():
+    """Test that timestamps in BaseResponse are RFC3339 compliant for MCP validation.
+
+    This test should initially fail, demonstrating the timestamp validation error
+    seen in MCP inspector. MCP expects RFC3339 format with timezone information.
+    """
+    # Create a response object
+    response = SuccessResponse(message="Test message")
+
+    # Serialize to JSON (mimics what MCP inspector sees)
+    json_str = response.model_dump_json()
+    data = json.loads(json_str)
+
+    timestamp_str = data["timestamp"]
+
+    # RFC3339 regex pattern (what MCP expects)
+    # Format: YYYY-MM-DDTHH:MM:SS[.ffffff][Z|±HH:MM]
+    rfc3339_pattern = (
+        r"^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d+)?(Z|[+-]\d{2}:\d{2})$"
+    )
+
+    # This assertion should FAIL with current implementation
+    assert re.match(rfc3339_pattern, timestamp_str), (
+        f"Timestamp '{timestamp_str}' is not RFC3339 compliant. "
+        f"MCP expects format like '2025-08-30T19:22:58.862377Z' or '2025-08-30T19:22:58.862377+00:00'"
+    )
+
+
+def test_base_response_timestamp_is_utc():
+    """Test that BaseResponse timestamps are in UTC timezone."""
+    response = BaseResponse()
+
+    # The timestamp should be timezone-aware and in UTC
+    assert response.timestamp.tzinfo is not None, (
+        "Timestamp should have timezone information"
+    )
+    assert response.timestamp.tzinfo == timezone.utc, (
+        "Timestamp should be in UTC timezone"
+    )
+
+
+def test_serialized_timestamp_ends_with_z_or_offset():
+    """Test that serialized timestamps have proper timezone suffix."""
+    response = BaseResponse()
+    json_str = response.model_dump_json()
+    data = json.loads(json_str)
+
+    timestamp_str = data["timestamp"]
+
+    # Should end with 'Z' (UTC) or timezone offset like '+00:00'
+    assert timestamp_str.endswith("Z") or re.search(
+        r"[+-]\d{2}:\d{2}$", timestamp_str
+    ), (
+        f"Timestamp '{timestamp_str}' should end with 'Z' or timezone offset like '+00:00'"
+    )
+
+
+def test_current_broken_format():
+    """Test showing the current broken timestamp format that causes MCP validation errors."""
+    # This demonstrates what the current code produces
+    current_naive_dt = datetime.now()
+    current_format = current_naive_dt.isoformat()
+
+    # Show that current format lacks timezone info
+    assert "Z" not in current_format
+    assert "+" not in current_format
+    assert "-" not in current_format[-6:]  # Check last 6 chars for timezone
+
+    print(f"Current broken format: {current_format}")
+    print(
+        "This format causes MCP validation errors because it lacks timezone information"
+    )