How to force valid JSON output (no markdown)

Extract data from the text below.
Return ONLY valid JSON (no markdown, no comments).

Schema:
{
  "people": [
    {
      "name": "string",
      "role": "string|null",
      "email": "string|null"
    }
  ],
  "dates": ["YYYY-MM-DD"]
}

Rules:
- If missing, use null (do not invent).
- If a date is ambiguous, omit it.

Text:
"""[paste here]"""

Before finalizing, validate that the output is valid JSON and matches the schema.

You will receive JSON input. Convert it to the output schema.
Return ONLY valid JSON.

Input JSON:
"""[paste JSON]"""

Output schema:
{
  "items": [
    { "id": "string", "title": "string", "tags": ["string"] }
  ]
}

Rules:
- Preserve meaning; do not invent new items.
- If an input field doesn't map, ignore it.
- Ensure tags are lowercase.

Validate JSON before output.