Add schema-level prompt customization

[michaeldistel]

Problem

All schemas use the same generic prompt template. Some use cases need custom instructions:

• Medical data: HIPAA compliance reminders
• Legal docs: Exact quote preservation
• Financial: Decimal precision requirements
• Custom domains: Specific terminology or formats

Currently no way to customize prompts per schema.

Proposed Solution

Add optional prompt customization to schema metadata:

{
  "fields": { ... },
  "metadata": {
    "name": "Medical Record Extraction",
    "promptInstructions": "Follow HIPAA guidelines. Preserve exact medical terminology.",
    "promptExamples": [
      {
        "input": "...",
        "output": { ... }
      }
    ]
  }
}

Implementation

Update Schema type:

interface SchemaMetadata {
  name?: string;
  version?: string;
  description?: string;
  promptInstructions?: string;  // custom instructions
  promptExamples?: Array<{      // custom examples
    input: string;
    output: ExtractionResponse;
  }>;
}

Update prompt builder to include custom content:

export function buildSystemPrompt(schema: Schema): string {
  let prompt = basePrompt;
  
  // Add custom instructions
  if (schema.metadata?.promptInstructions) {
    prompt += ;
  }
  
  // Add custom examples
  if (schema.metadata?.promptExamples) {
    prompt += formatExamples(schema.metadata.promptExamples);
  }
  
  return prompt;
}

Acceptance Criteria

• Schema metadata supports custom prompt instructions
• Schema metadata supports custom examples
• Prompt builder incorporates custom content
• Tests verify customization works
• Documentation with examples
• Existing schemas work without changes (backward compatible)

Related

• Parent: Improve LLM prompt engineering and clarity #30
• Related: Add few-shot examples to prompts #32 (few-shot examples)

Priority

Medium - Useful but not blocking core functionality