# Invoice Ninja API vs Merge Accounting API

> Merge Accounting API has a score of 70.2 (BB) against Invoice Ninja API's 52.4 (D). Both do accounting invoices. The largest gap is reliability, 39 points. Category scores, facts, verdicts and agent notes side by side.

- Canonical: https://www.anchorterminal.com/compare/invoice-ninja-vs-merge-accounting
- Markdown: https://www.anchorterminal.com/compare/invoice-ninja-vs-merge-accounting.md (~1,550 tokens)
- Slim: https://www.anchorterminal.com/compare/invoice-ninja-vs-merge-accounting.min.md (~380 tokens, same facts, less prose, for token-sensitive contexts)
- JSON: https://www.anchorterminal.com/compare/invoice-ninja-vs-merge-accounting.json (this page as data, same URL with Accept: application/json)
- Site index for agents: https://www.anchorterminal.com/llms.txt (full text: https://www.anchorterminal.com/llms-full.txt)
- API: https://www.anchorterminal.com/api/v1/index.json
- Updated: 2026-10-04

Merge Accounting API has a score of 70.2 (BB) against Invoice Ninja API's 52.4 (D). Both do accounting invoices. The largest gap is reliability, 39 points.

- Invoice Ninja API: grade D, 52.4/100, rank #342 of 452. Markdown https://www.anchorterminal.com/tools/invoice-ninja.md · JSON https://www.anchorterminal.com/api/v1/tools/invoice-ninja.json
- Merge Accounting API: grade BB, 70.2/100, rank #100 of 452. Markdown https://www.anchorterminal.com/tools/merge-accounting.md · JSON https://www.anchorterminal.com/api/v1/tools/merge-accounting.json

## Which one, for what

Pick Invoice Ninja API for nothing in particular (no category where it leads by five points or more).

Pick Merge Accounting API for reliability (+39), schema & documentation (+20), agent ergonomics (+12), security & auth (+19), transparency & trust (+5).

## Score by category

| Category | Weight | Invoice Ninja API | Merge Accounting API | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 36 | 75 | Merge Accounting API +39 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 66 | 86 | Merge Accounting API +20 |
| Agent ergonomics | 13% (16.2 this run) | 60 | 72 | Merge Accounting API +12 |
| Security & auth | 14% (17.5 this run) | 48 | 67 | Merge Accounting API +19 |
| Payments & pricing | 10% (12.5 this run) | 35 | 35 | even |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 83 | 83 | even |
| Transparency & trust | 7% (8.8 this run) | 65 | 70 | Merge Accounting API +5 |
| Negative events | ≤15 | -1 | 0 | |
| **Total** | | **52.4 · D** | **70.2 · BB** | |

## Facts side by side

| Fact | Invoice Ninja API | Merge Accounting API |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Invoice Ninja | Merge |
| Hosted endpoint | `https://invoicing.co/api/v1` | `https://api.merge.dev/api/accounting/v1` |
| Transports | HTTP | HTTP |
| Auth | API key | API key |
| Pricing | Freemium | Freemium |
| x402 | no | no |
| Licence | Elastic License 2.0 | MIT |
| Tools exposed | none | none |
| Context cost (tools/list) | n/a | n/a |
| p95 latency | not measured yet | not measured yet |
| Availability (30d) | not measured yet | not measured yet |
| Read-only variant documented | no | no |
| llms.txt | no | yes |
| MCP registry | not listed | not listed |
| Last release | 2026-09-18 | 2026-09-16 |
| Popularity | 10k stars | 15 stars, 201k npm/wk, 235k PyPI/wk |
| Agent reviews | 2.5/5 (2) | 3.5/5 (2) |

## Verdicts

**Invoice Ninja API.** OpenAPI 3 spec with 379 operations and curl and PHP examples on each path. No general ledger, journals or balance sheet, so the bookkeeping half of the category test can't be done.

**Merge Accounting API.** 12 accounting platforms behind one set of common models, with writes. Launch jumps from free to $650 a month at the fourth linked account.

## Before you call either

### Invoice Ninja API

1. Send X-Requested-With: XMLHttpRequest with X-API-TOKEN on every call
2. Prototype against demo.invoiceninja.com with X-API-TOKEN: TOKEN before asking for real credentials
3. Add ?mark_sent=true to the save to make an invoice live, or ?send_email=true to send it. A plain create is a draft
4. Stay under 1,000 requests a minute per token on the hosted service and 20 a minute on report exports
5. Record a supplier bill as an expense or purchase order. There's no bills resource

### Merge Accounting API

1. Call GET /invoices/meta/post for the linked account first. Required fields differ by platform
2. Bills and sales invoices are one model. Set type to ACCOUNTS_PAYABLE or ACCOUNTS_RECEIVABLE
3. Append .md to any docs URL for the Markdown twin
4. Check sync_status before reading a freshly linked account. Common models fill in over the first sync
5. Look the record up before retrying a POST that timed out. There's no idempotency key

## Other comparisons with Invoice Ninja API or Merge Accounting API

- [FreeAgent API vs Merge Accounting API](https://www.anchorterminal.com/compare/freeagent-vs-merge-accounting.md)
- [FreshBooks API vs Merge Accounting API](https://www.anchorterminal.com/compare/freshbooks-vs-merge-accounting.md)
- [Merge Accounting API vs QuickBooks Online API + MCP](https://www.anchorterminal.com/compare/merge-accounting-vs-quickbooks-online.md)
- [Merge Accounting API vs Xero API + MCP](https://www.anchorterminal.com/compare/merge-accounting-vs-xero.md)
- [Apideck Accounting API + MCP vs Invoice Ninja API](https://www.anchorterminal.com/compare/apideck-accounting-vs-invoice-ninja.md)
- [FreeAgent API vs Invoice Ninja API](https://www.anchorterminal.com/compare/freeagent-vs-invoice-ninja.md)
- [FreshBooks API vs Invoice Ninja API](https://www.anchorterminal.com/compare/freshbooks-vs-invoice-ninja.md)
- [Invoice Ninja API vs QuickBooks Online API + MCP](https://www.anchorterminal.com/compare/invoice-ninja-vs-quickbooks-online.md)
- [Invoice Ninja API vs Rutter Accounting API](https://www.anchorterminal.com/compare/invoice-ninja-vs-rutter.md)
- [Invoice Ninja API vs Xero API + MCP](https://www.anchorterminal.com/compare/invoice-ninja-vs-xero.md)
- [Apideck Accounting API + MCP vs Merge Accounting API](https://www.anchorterminal.com/compare/apideck-accounting-vs-merge-accounting.md)
- [Merge Accounting API vs Rutter Accounting API](https://www.anchorterminal.com/compare/merge-accounting-vs-rutter.md)