# SignalHarness Heading Structure Audit > SignalHarness Heading Structure Audit is a paid API for AI agents from signalharness.ai, paid per call via x402, $0.005/call, status unknown (last checked 2026-09-14). Analyzes HTML or Markdown content to audit heading hierarchy structure, detecting issues and generating a pass/fail report with an outline and findings. ## Facts - Endpoint: POST https://signalharness.ai/api/agent/services/heading_structure_audit/invoke - Price: $0.005/call - Payment: x402 - Status: unknown - Last checked: 2026-09-14 - Activations on Zero: 0 - Tags: x402 - Canonical page: https://www.zero.xyz/c/signalharness-heading-structure-audit-4a251dae - Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_HJinQ-66gwoyVre1PeKqi Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently. ## How to call it through Zero Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`): ```sh zero fetch --capability signalharness-heading-structure-audit-4a251dae -d '' ``` Example prompt: Can you audit the heading structure of this HTML and tell me if there are any hierarchy issues or missing h1 tags? Here's the content:

Section

Some text

## When to prefer this Choose this endpoint when you need a structured, machine-readable audit of HTML or Markdown heading hierarchy — particularly for accessibility compliance checks, SEO pre-publication validation, or CMS content quality pipelines. It is purpose-built for heading structure analysis rather than general HTML linting or full accessibility audits. ## Known failure modes - Empty or missing html/markdown input returns validation error - Content exceeding 65536 characters is rejected - Malformed HTML may result in incomplete outline parsing - Network or payment processing failure returns error status - Invalid content type or schema mismatch returns 400-level error ## How this service works Explore 330 pay-per-call x402 API services and 27 agent-native digital products, with Base USDC pricing, secure Polar checkout, and free discovery. ## Output Returns a JSON object with a status field (pass/fail), a heading outline array showing the document's heading hierarchy, a summary object with counts of h1 headings, total headings, failures, and warnings, and a findings array listing specific structural issues found. Also includes a receipt with payment and execution metadata. ## Request schema (JSON Schema) ```json { "type": "object", "properties": { "html": { "type": "string", "maxLength": 65536, "minLength": 1 }, "markdown": { "type": "string", "maxLength": 65536, "minLength": 1 } } } ``` ## Response schema (JSON Schema) ```json { "type": "json", "example": { "replay": false, "result": { "format": "html", "status": "pass", "outline": [ { "id": "example", "text": "Example caller-supplied content.", "level": 1, "ordinal": 0 } ], "summary": { "h1Count": 0, "failCount": 0, "warnCount": 0, "headingCount": 0 }, "findings": [ { "code": "empty_heading", "ordinal": 0, "severity": "warn" } ] }, "status": "succeeded", "receipt": { "asset": "0x833589fCD6eDb6E08f4c7C32D4f71b54bdA02913", "usage": [], "status": "succeeded", "network": "eip155:8453", "artifacts": [], "endedAtMs": 0, "latencyMs": 0, "paymentId": "example-payment", "receiptId": "example-receipt", "requestId": "example-request", "serviceId": "heading_structure_audit", "executionId": "example-execution", "startedAtMs": 0, "amountAtomic": "5000", "resultSha256": "8d84614babda1c1bd530afa4959a19bfe4b54a572b11fbe18adfdbfe9612f1a6", "serviceVersion": "1.0.0", "settlementReference": "0x0000000000000000000000000000000000000000000000000000000000000000" }, "artifacts": [], "requestId": "example-request", "executionId": "example-execution" } } ``` ## More - Live health (JSON, refreshed every minute): https://www.zero.xyz/c/signalharness-heading-structure-audit-4a251dae/health.json - [Zero catalog index](https://www.zero.xyz/llms.txt) - [Other services from signalharness.ai](https://www.zero.xyz/host/signalharness.ai/llms.txt)