🐛 Bug-fix PR

📌 Summary

Closes #2203

Fixed tags not being properly stored in the database for MCP servers/gateways. Tags were being validated and converted to list of dictionaries [{"id": "tag", "label": "Tag"}] by the Pydantic schema, but the database model type annotation incorrectly specified List[str], causing tags to be stored as empty lists [] instead of the expected format.

🔁 Reproduction Steps

1. Navigate to MCP Servers tab in the UI
2. Create or update an MCP server/gateway with tags (e.g., "dev", "git", "mcp")
3. Check the database: SELECT tags FROM gateways;
4. Bug:
   • Tags column shows [] (empty list) instead of [{"id": "dev", "label": "dev"}, ...]
   • Tags do not display in the UI

🐞 Root Cause

1. The database model in mcpgateway/db.py had an incorrect type annotation:

tags: Mapped[List[str]] = mapped_column(JSON, default=list, nullable=False)

2. The validate_tags_field() function is used as a Pydantic field validator in both GatewayCreate and GatewayUpdate schemas. This function:

   1. Takes input: Optional[List[str]] - raw tag strings like ["dev", "git", "mcp"]
   2. Normalizes: Converts to lowercase, removes duplicates, validates format
   3. Returns: List[Dict[str, str]] - structured format like [{"id": "dev", "label": "dev"}, {"id": "git", "label": "git"}]

3. The Problem:

   1. Pydantic validator returns: List[Dict[str, str]] → [{"id": "dev", "label": "dev"}]
   2. Database model expects: List[str] → ["dev", "git"]
   3. SQLAlchemy sees the type mismatch and fails to serialize the dictionaries properly
   4. Result: Empty list [] stored in database

💡 Fix Description

1. Database Model Type Fix: mcpgateway/db.py

Updated the Gateway model's tags type annotation to match what validate_tags_field() actually returns:

# Before - incorrect type annotation
tags: Mapped[List[str]] = mapped_column(JSON, default=list, nullable=False)

# After - correct type annotation matching validate_tags_field() output
tags: Mapped[List[Dict[str, str]]] = mapped_column(JSON, default=list, nullable=False)

This aligns the database model with:
1. The output of validate_tags_field()
2. The format used by virtual servers and tools (which were working correctly)
3. The UI's expectation of [{"id": "tag", "label": "Tag"}] format

2. Service Layer Cleanup mcpgateway/services/gateway_service.py
   Only call the tag conversion in convert_gateway_to_read() method in the tags are List[str], otherwise the fucntion call is not needed. Previously, this method was calling validate_tags_field() again on already-validated tags from the database, which was redundant. Now it passes tags through directly with backward compatibility:

# Before - was re-validating already validated tags
if gateway.tags:
    gateway_dict["tags"] = validate_tags_field(gateway.tags)
else:
    gateway_dict["tags"] = []

# After - pass through directly with backward compatibility check
if gateway.tags:
    # Check if tags are old format (List[str]) or new format (List[Dict[str, str]])
    if isinstance(gateway.tags[0], str):
        # Convert old format to new format for backward compatibility
        gateway_dict["tags"] = validate_tags_field(gateway.tags)
    else:
        # Already in correct format, pass through
        gateway_dict["tags"] = gateway.tags
else:
    gateway_dict["tags"] = []

🧪 Verification

📐 MCP Compliance (if relevant)

• Matches current MCP spec
• No breaking change to MCP clients

✅ Checklist

• Code formatted (make black isort pre-commit)
• No secrets/credentials committed