Clarify custom-chain bundler requirements

[defiQUG]

Summary

• clarify in the README that chain overrides resolve custom EVM chains for standard Engine operations, but do not by themselves enable hosted bundler/paymaster support
• expand the generic invalid-chain error message so operators using smart wallet / UserOp routes are not pointed only at chain overrides

Why

Engine supports custom chain overrides, but the UserOp routes still post to https://{chainId}.bundler.thirdweb.com. In practice that means a custom chain can be valid for normal Engine reads/writes/deployments while still failing for smart wallet flows if hosted bundler support is unavailable.

This change keeps the scope narrow and just makes that boundary explicit.

Validation

• local diff review only
• attempted to run the repo build/lint flow via Corepack/Yarn in this environment, but dependency install did not complete cleanly enough to provide working tsc / biome binaries before publish

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PR-Codex overview

This PR enhances error messaging and documentation regarding custom chains in the project. It clarifies the requirements for using smart wallets and UserOp routes with custom chains.

Detailed summary

• Updated the error message in the badChainError function to include information about bundler support requirements for smart wallet and UserOp routes.
• Added a new section in README.md about custom chains, explaining chain overrides and their dependencies on bundler support.

✨ Ask PR-Codex anything about this PR by commenting with /codex {your question}

Summary by CodeRabbit

• Documentation

  • Added guidance on custom chain behavior, including limitations for smart wallet and UserOp routes.
  • Clarified that custom chain configuration alone does not provide hosted bundler or paymaster support.

• Bug Fixes

  • Improved unsupported-chain error messages with clearer guidance about required bundler support.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: db73a29f-ce64-44ce-925a-de0a33931fd6

📥 Commits

Reviewing files that changed from the base of the PR and between 0b2cfe9 and 41eabb2.

📒 Files selected for processing (2)

• README.md
• src/server/middleware/error.ts

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Walkthrough

Custom chain guidance was added to the README and badChainError, clarifying that chain overrides support standard Engine operations while Smart Wallet and UserOp routes require bundler support.

Changes

Custom chain guidance

Layer / File(s)	Summary
Document custom chain routing requirements README.md, src/server/middleware/error.ts	README documentation and bad-chain error messaging explain chain override behavior and the bundler requirement for Smart Wallet and UserOp routes.

Estimated code review effort: 1 (Trivial) | ~3 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly matches the main change: clarifying custom-chain bundler requirements.
Description check	✅ Passed	The description is mostly complete and covers summary, rationale, and validation, though it omits the Linear link and explicit testing/output sections.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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