Head to head · Spend transactions · October 2026 research run
Brex vs Mercury API
Mercury API scores 63.8 (B) on agent readiness against Brex's 60.7 (C), and leads in 2 of 7 scored categories. Brex leads on schema & documentation. Both do spend transactions.
Which one, for what
Brex C
Good for A finance team already on Brex that wants an agent to read expenses and transactions, issue and lock cards, set spend limits, upload receipts and pay vendors.
Ahead on
- Schema & documentation, 80 against 72
Watch for
The Expenses API update endpoint accepts only memo, so an outside agent can't set a category or custom field on an expense through it
Good for A company that banks with Mercury and wants an agent to read balances and transactions, set notes and categories, upload receipts, issue virtual cards with spend limits and queue payments for human approval.
Ahead on
- Security & auth, 82 against 72
- Payments & pricing, 30 against 25
Also in its favour
- No incidents deducted, where Brex loses 3 points for them
Watch for
No rate limits with numbers and no 429 handling were found in the reviewed documentation
Score by category
| Category | Weight this run | Brex | Mercury API | Edge |
|---|---|---|---|---|
| Reliability | 16%20 | 60 | 59 | Brex +1 |
| Performance | 10%pending | pending | pending | not scored in this run |
| Schema & documentation | 13%16.2 | 80 | 72 | Brex +8 |
| Agent ergonomics | 13%16.2 | 70 | 68 | Brex +2 |
| Security & auth | 14%17.5 | 72 | 82 | Mercury API +10 |
| Payments & pricing | 10%12.5 | 25 | 30 | Mercury API +5 |
| Task success | 10%pending | pending | pending | not scored in this run |
| Maintenance & community | 7%8.8 | 66 | 64 | Brex +2 |
| Transparency & trust | 7%8.8 | 67 | 63 | Brex +4 |
| Negative events | ≤15 | -3 | 0 | |
| Total | 60.7 · C | 63.8 · B |
Facts side by side
| Fact | Brex | Mercury API |
|---|---|---|
| Kind | HTTP API | HTTP API |
| Vendor | Brex LLC | Mercury Technologies, Inc. |
| Hosted endpoint | https://api.brex.com | https://api.mercury.com/api/v1 |
| Transports | HTTP | HTTP |
| Auth | OAuth or key | OAuth or key |
| Pricing | Freemium | Freemium |
| x402 | no | no |
| Licence | Proprietary service under the Brex Platform Agreement and the Brex Access Agreement | Proprietary service under Mercury's Terms of Use. The Mercury CLI on GitHub is Apache-2.0 |
| Tools exposed | 43 | 35 |
| Read-only variant documented | yes | yes |
| llms.txt | yes | yes |
| Last release | 2026-10-01 | 2026-08-12 |
| Terms last updated | no date given | no date given |
| Privacy policy last updated | no date given | no date given |
| Customer content may train models | not found in the text | not found in the text |
| Terms restrict automated access | not found in the text | yes |
| Terms restrict benchmarking | yes | yes |
| Terms or service can change without notice | not found in the text | not found in the text |
| Arbitration or class-action waiver | yes | yes |
Verdicts
Brex
User tokens carry per-resource scopes with read-only variants, every POST and PUT accepts an Idempotency-Key, and ten OpenAPI specs are public. The Expenses API changes only an expense's memo, there is no customer sandbox or official SDK, and the status page logs API errors lasting over four hours on 4 August 2026.
Mercury API
Custom tokens take per-endpoint scopes, write tokens need an IP allowlist, and payments can be queued for approval in the dashboard through POST /account/{accountId}/request-send-money. No rate limits, error catalogue or SLA were found in the reviewed documentation, the changelog carries no dates, and the MCP server is beta.
Before you call either
Brex
- Ask an account admin or card admin for a user token with only the scopes the task needs, and prefer the
.readonlyvariants. A token unused for 90 days expires. - Send a stored
Idempotency-Keyon every POST and PUT. Create transfer and Create card reject requests without one. - Only settled transactions are returned. Poll card and cash transactions with
posted_at_startand a lookback of at least one day. - Keep under 1,000 requests in 60 seconds per client and account, and back off exponentially with jitter on 429.
- Send only ASCII in free-text fields, and quote the
X-Brex-Trace-Idresponse header when reporting an error.
Mercury API
- Ask for a Custom token with only the scopes the task needs. Scopes can't be edited later, and a token unused for 45 days is deleted.
- Without a fixed IP address, send payments through
POST /account/{accountId}/request-send-money. Direct sends throughPOST /account/{accountId}/transactionsneed an allowlisted IP. - Send a stored
idempotencyKeywith every payment. The same recipient, account, amount and method within 24 hours returns 400 even with a new key. - Set
limiton list calls.GET /transactionsreturns up to 1,000 rows by default. - Test against
https://api-sandbox.mercury.com/api/v1/with a sandbox token. Webhooks aren't available in the sandbox.
Questions
Which is better for AI agents, Brex or Mercury API?
Mercury API scores 63.8 (B) on agent readiness against Brex's 60.7 (C), and leads in 2 of 7 scored categories. Brex leads on schema & documentation.
Do Brex and Mercury API need an API key?
Both take an API key or an OAuth sign-in.
Can an agent call Brex and Mercury API without installing anything?
Yes. Brex has a hosted endpoint at https://api.brex.com and Mercury API at https://api.mercury.com/api/v1.
Other comparisons with Brex or Mercury API
Machine-readable
- This page as Markdown
/compare/brex-vs-mercury.md· slim.min.md· JSON.json(or sendAccept: text/markdown) - Each listing in full
/api/v1/tools/brex.json·/api/v1/tools/mercury.json - From a terminal
anchor compare brex mercury(the CLI) - Over MCP
compare_tools {"a": "brex", "b": "mercury"}at/mcp, no key