Getting Structured Output from AI: JSON, Tables & Formatting

One of the most practical prompt engineering skills is getting AI to output data in a specific, parseable format. Whether you need JSON for an API, a table for a report, or structured text for processing, here's how to get it right every time.

The Core Principle

AI models are text generators. They'll output any format — but only if you show them exactly what you want. The key is: specify the format AND provide an example.

JSON Output

Basic Approach

Extract the following information from this text and return it 
as a JSON object with these exact keys: 
"name", "email", "company", "role"

Text: "Hi, I'm Sarah Chen from Acme Corp. I'm the VP of Engineering. 
Reach me at sarah@acme.com"

Better: Include Schema

Return a JSON object matching this exact schema:
{
  "name": "string (full name)",
  "email": "string (email address)",
  "company": "string (company name)",
  "role": "string (job title)",
  "seniority": "junior | mid | senior | executive"
}

Do NOT include any text outside the JSON. 
Start with { and end with }.

Best: Provide an Example

Example input: "John Doe, CTO at TechCo, john@techco.io"
Example output: {"name": "John Doe", "email": "john@techco.io", 
"company": "TechCo", "role": "CTO", "seniority": "executive"}

Now process: "Sarah Chen, VP Engineering, Acme Corp, sarah@acme.com"

Table Output

Markdown Tables

Create a comparison table with these exact columns:
| Feature | Tool A | Tool B | Tool C |

Include rows for: pricing, free tier, API access, support level.
Use ✅, ❌, or specific values in each cell.

CSV Format

Output the data in CSV format with headers.
Use double quotes around fields containing commas.
Do not include any explanation — just the raw CSV data.

Headers: name,email,company,role,location

Consistent Formatting Tricks

1. The "Only" Trick

"Return ONLY the JSON. No explanations, no markdown code fences, no additional text."

2. The Delimiter Trick

"Place the structured data between these delimiters:

[your JSON here]

"

3. The Start Trigger

"Begin your response with { " — this forces JSON mode in most models.

4. The Negative Example

"Do NOT format it like this: 'The answer is: [data]' DO format it like this: [data]"

Handling Lists and Arrays

Numbered Lists

List exactly 5 action items. Format each as:
1. [ACTION VERB] [specific task] — [expected outcome]

Bullet Points with Structure

For each finding, use this format:
- **Finding**: [what you found]
  - **Impact**: [who/what is affected]
  - **Priority**: HIGH / MEDIUM / LOW
  - **Action**: [recommended next step]

Platform-Specific Features

OpenAI JSON Mode

In the API, set response_format: { type: "json_object" }. This forces valid JSON output. You still need to describe the schema in your prompt.

Claude XML Tags

Claude responds well to XML-style structure:

Return your analysis in this format:
<analysis>
  <summary>Brief overview</summary>
  <findings>
    <finding severity="high">Description</finding>
  </findings>
  <recommendation>What to do next</recommendation>
</analysis>

Structured Output APIs

Both OpenAI and Anthropic now support structured output schemas in their APIs. If you're building an application, use these instead of prompt-based formatting.

Common Failures and Fixes

Problem	Solution
AI adds explanation before/after data	"Return ONLY the [format]. No other text."
JSON has trailing commas	Provide a valid example; use JSON mode API
Table columns misalign	Specify exact column names and provide one example row
Inconsistent key names	Use camelCase OR snake_case in your example — pick one
Missing fields	"If a field is unknown, use null (not empty string)"
Extra fields added	"Include ONLY these keys: ..."

Pro Tips

1. Test with edge cases: What happens with empty input? Special characters? Very long text?
2. Validate programmatically: Always validate AI output with a schema validator (Zod, JSON Schema, Pydantic)
3. Retry on failure: If the format is wrong, send it back: "This JSON is invalid because X. Fix it."
4. Version your schemas: when you change the format, update your examples too

Structured output transforms AI from a conversation partner into a data processing tool. Master it, and you can automate workflows that would otherwise require manual formatting.