fix(contract): only reserve then in the handler errors map

[dinwwwh]

The handler errors map now reserves only then, instead of every key in RECURSIVE_CLIENT_UNWRAP_KEYS as #2113 did. then is the only key the runtime reads on its own (promise resolution), so it is the only one that caused a real failure. The others are only reached by explicitly converting or serializing the map. This narrows the behavior change from #2113.

Fixes

• errors.toString(), errors.valueOf(), errors.toJSON(), errors.bind(), errors.call() and errors.apply() build errors with those codes again.
• await errors and returning errors from an async handler still settle instead of hanging.
• Adding a key to the client's unwrap set no longer silently takes that name away from error codes.

Testing

• The regression test now checks only symbols and then. The Object.prototype lookup test uses toString() again and fails if toString is reserved.
• Contract and server tests (687), eslint, and the @orpc/contract type-check pass.

[pullfrog]

✅ No new issues found.

Reviewed changes — narrowed error-map key reservation plus its regression tests.

• Proxy guard narrowed to then — createORPCErrorConstructorMap now returns the raw value only for symbols and the literal key then, dropping the RECURSIVE_CLIENT_UNWRAP_KEYS dependency. The #2113 hang fix is preserved (the map is still non-thenable), while toString/valueOf/toJSON/bind/call/apply build error constructors again.
• Tests realigned — the symbol/then test keeps its await assertion; the Object.prototype test switches back to toString() and asserts code === 'toString', which is exactly the assertion that fails if toString is reserved again.

The one behavior deliberately reintroduced from #2113 — String(errors) can throw and JSON.stringify(errors) can again emit a synthetic toJSON error — is a conscious tradeoff argued in the PR body and is only reachable through explicit conversion/serialization. No internal caller or test relies on it.

Verified locally: pnpm vitest run packages/contract/src/error-factory.test.ts → 24 passed.

  ^{｜ View workflow run ｜ Using DeepSeek Flash (free via Pullfrog for OSS) ｜ 𝕏}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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commit: 7b377b8

[codspeed]

Merging this PR will not alter performance

✅ 30 untouched benchmarks

---

_{Comparing dinwwwh:claude/orpc-error-constructors-unwrap-10db1f (7b377b8) with main (36b8bd9)¹}

Footnotes

1. No successful run was found on main (63b2053) during the generation of this report, so 36b8bd9 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩