docs: describe rule-based accounts in sandbox

[ls-bolt]

This PR has been claimed. The active PR is now #892.

Summary

Documents rule-based accounts in sandbox. Stacked on #835, which adds the endpoint and the sweep schemas.

No new paths, and no schema changes. Sandbox reuses the same surface: creation goes through POST /internal-accounts, and POST /sandbox/internal-accounts/{accountId}/fund already exists to stand in for a settled deposit. What was missing is a statement of which half of the flow is real, so an integrator knows what a green sandbox run has actually proved.

What changed

Two description blocks, +18 lines total:

• POST /internal-accounts — notes that sandbox platforms can create rule-based accounts, that the account number is generated locally rather than issued by a partner bank, and that funding via the sandbox endpoint stands in for a deposit.
• POST /sandbox/internal-accounts/{accountId}/fund — a "Funding a rule-based account" section: funding triggers the sweep exactly as a settled deposit does, and funding below the corridor minimum rehearses the BELOW_MINIMUM path that returns the deposit and delivers SWEEP.FAILED.

That second one is the point of the change. Below-minimum is the behaviour an integrator is most likely to hit without meaning to, and the one whose consequence — the deposit going back to the payer rather than onward — is least guessable from the happy path.

Edited at source, not in the bundle

openapi.yaml and mintlify/openapi.yaml are bundler output — make build regenerates them from the openapi/ directory. The two description blocks therefore live in their source files:

• openapi/paths/sandbox/sandbox_internal_accounts_{accountId}_fund.yaml
• openapi/paths/internal_accounts.yaml

with both bundles rebuilt. An earlier revision of this PR edited the bundle directly, which would have been silently reverted by the next make build — worth knowing if you review the diff and wonder why three files carry the same prose.

The boundary this draws

Sandbox simulates the API contract and the routing decision, not the banking rails. The quote, the corridor bounds, the destination checks and the webhook are all real; the settlement legs behind them are not, so timings and failure modes that originate at a partner bank cannot be reproduced here.

This is a deliberate boundary rather than a shortfall. The alternative — registering real partner-bank entities for fictional sandbox customers — is precisely what the platform eligibility rules exist to prevent. Everything above the rails genuinely runs: the same rule lookup, balance read, corridor bounds, destination and rail validation, quote path and webhook that production uses.

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2 Skipped Deployments

Project	Deployment	Actions	Updated
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[lightspark-bot]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• docs: describe rule-based accounts in sandbox #889 👈 (View in Graphite)
• feat: add rule-based internal accounts and the sweep failure webhook #835
• main

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[github-actions]

✱ Stainless preview builds for grid

This PR will update the grid SDKs with the following commit messages.

cli

chore(internal): regenerate SDK with no functional changes

go

docs(api): document rule-based funding for sandbox internal account

kotlin

docs(api): add rule-based account funding docs to sandbox internal accounts

openapi

docs(api): update internal accounts create and sandbox fund endpoint descriptions

php

docs(api): add rule-based account funding documentation to sandbox internal accounts

python

docs(api): document rule-based account funding in sandbox internal_accounts

ruby

docs(api): expand fund method documentation in sandbox internal_accounts

typescript

docs(api): update sandbox internal_accounts fund method documentation

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Last updated: 2026-09-02 00:21:26 UTC