Add September 2026 changelog

[jlin27]

Summary

Adds fern/docs/changelog/2026-09-30.mdx, covering September 2026. Drafted from the 404 PRs merged in the monorepo between 2026-09-01 and 2026-09-30.

Headline items:

• Arc (5042) is generally available across the Swap, Gasless, and Cross-Chain APIs — announcement, a New Chains entry, and a New Features entry covering its dual native/ERC-20 USDC interfaces.
• New liquidity sources on nine chains, with Arc carrying the bulk as a launch chain.
• New LayerZero OFT routes in Cross-Chain for USDS, PENDLE, USDai/sUSDai, and USDe/sUSDe.
• Six bug fixes, mostly /cross-chain/status correctness.
• Two breaking changes: dedicated authorization error codes on /cross-chain/quotes, and RWA gating for the Reserve DTF tokens and PAXGy.

Every endpoint, parameter, field, error code, and bridge provider ID named in the entry was checked against fern/openapi.json, fern/openapi-cross-chain.json, or an existing docs page before being written. Liquidity-source names and their chains were resolved from the monorepo rather than inferred from PR titles, and each was confirmed to add a real sampler rather than only indexing coverage for something already routable. One source added and reverted inside the month is excluded; a same-named but distinct source that was not reverted is kept.

Release Highlights

The homepage table now holds September and August only; the three July rows were removed. Column padding re-aligned, nothing else on the page touched.

Date	Update	API
September 2026	Arc (5042) is now supported across all three APIs	Swap API, Gasless API, Cross-Chain API
September 2026	Arc's native USDC and ERC-20 USDC quote as separate assets	Swap API, Gasless API
August 2026	ERC-7597 permits now supported for gasless approvals	Gasless API
August 2026	Across V4 no longer serves Solana or HyperCore routes	Cross-Chain API

Both September rows link to #2026-09-30-… anchors. The Arc row points at the New Chains section rather than Announcements, matching how the July row avoided the emoji header.

Stale docs fixed

None needed. September retired no chain, endpoint, or parameter, so nothing published went stale. The pages this sweep normally touches were checked and are already current:

• introduction/supported-chains.mdx — Arc was added to the Swap/Gasless table, the Cross-Chain matrix, and kept in sync with its Gasless rollout by three separate human PRs during September. No edit needed.
• 0x-swap-api/additional-topics/chain-specific-native-tokens.mdx — already documents Arc's USDC-as-gas model in full. The changelog links to it.
• The AllowanceHolder hardfork lists in introduction/faq.mdx, core-concepts/contracts.mdx, and core-concepts/0x-cheat-sheet.mdx — all three agree with each other. See the note below.

Overlap with open work: PR #283 edits introduction/welcome.mdx and introduction/faq.mdx. It does not touch index.mdx, the changelog directory, or supported-chains.mdx, so there is no conflict with this PR. I left both of its files alone.

Notes for review

1. A server-side pricing change was left out. One change this month improves the quotes most integrators receive, but it is configured by us rather than expressed through any request parameter or response field, so it fails the internal-persistence test. It is a bigger practical win than several entries that did make the cut — worth a second opinion on whether it deserves an Announcements line written at the level of outcome rather than mechanism.

2. Response-field additions on an API with no published spec. Several additive response fields shipped on an API that this repo publishes no OpenAPI spec for, so verify-before-documenting excluded them. They look like they were meant to be public; they may need docs pages rather than a changelog line.

3. The RWA breaking change rests on an account-level grant, not a request parameter, which normally makes a change internal. I included it because the behaviour is plainly integrator-visible — a request that used to succeed now fails — and because prior entries document this same class for other tokenized assets. Flagging it rather than deciding it silently.

4. Arc is absent from the AllowanceHolder hardfork chain lists on all three pages that carry them. Adding it needs Arc's AllowanceHolder address, and I could not confirm it from a source I trust: the one per-chain mapping I found is scoped to a single service and disagrees with the published lists on two chains already in them. I left the lists unchanged rather than publish an address I could not verify. Two other supported chains are missing from the same lists, so this gap predates September and is worth one pass by someone who knows the authoritative source.

5. Some changes in this window were excluded because they have no presence in the published OpenAPI specs and are not public surface; the editorial policy for those calls is documented internally.

6. Process doc. CHANGELOG_PROCESS.md is present and matches the routine's instructions, with one gap: it does not mention refreshing the homepage Release Highlights table, which the routine prompt requires. Since that document is the durable copy of the policy, it should gain that step.

7. Collection method differed from the documented one. The gh pr list --search snippet in the process doc cannot run in this environment — it is GraphQL-backed and GraphQL is blocked, as is the search API. PRs were collected instead by paginating the REST pulls endpoint sorted by updated_at and filtering on merged_at, which is exact rather than best-effort: 404 merged PRs, first merge 2026-09-01, last 2026-09-30, with a plausible weekday/weekend distribution across all 30 days. The process doc's snippet is worth updating so the next run does not start from a command that fails.

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