[github-actions]

What breaks

The nightly test_types_next job (TypeScript next) has failed on every nightly run for the past 7 days (2026-10-07 → 2026-10-13). Building the declaration files for @mui/internal-core-docs fails:

src/Demo/sandbox/CodeSandbox.ts - error TS5088: The inferred type of 'createMaterialTemplate' references a type with a cyclic structure which cannot be trivially serialized. A type annotation is necessary.
src/Demo/sandbox/StackBlitz.ts  - error TS5088: (same)

Last occurrence: https://app.circleci.com/pipelines/github/mui/material-ui/183506/workflows/aa5a8ba7-bd11-4d63-926b-e0f54da3f21c

Why it started failing

This was not caused by a change in our code. The nightly job installs typescript@next, which is a moving channel. createMaterialTemplate returns an object literal whose replaceContent() method does return this, so the inferred return type refers to itself. Stable TypeScript quietly emits replaceContent(...): /*elided*/ any in the .d.ts. typescript@7.1.0-dev now reports TS5088 and asks for an explicit annotation. I reproduced this with a minimal file: typescript@7.1.0-dev.20261012.1 errors, and the older compiler emits /*elided*/ any.

The fix

Add an explicit MaterialTemplate return type to createMaterialTemplate in both the CodeSandbox and StackBlitz helpers. replaceContent() returns MaterialTemplate, so the emitted declarations get a real type instead of any, and chaining .createMaterialTemplate(...).replaceContent(...).openSandbox() in the docs keeps working. I updated both copies of the helpers (src/Demo/sandbox/ and src/sandbox/) because they have identical code and are both part of the src/** declaration build. Runtime behavior does not change.

Verification

• Compiled the four changed files with typescript@next (7.1.0-dev.20261012.1) in a standalone setup. TS5088 is gone, there are no new type errors, and the emitted .d.ts contains the MaterialTemplate interface. The only errors left were about process missing, which comes from that setup not loading node types.
• I did not run the full monorepo pnpm lerna run build / typescript:ci locally. CI on this PR covers stable TS. The nightly test_types_next run will confirm the fix with TS next.

🤖 Generated with Claude Code

cc @mui/infra

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Opened by a sandboxed agent. Unverified beyond any local check noted above; CI on this PR is the real test.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-49453--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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Check out the code infra dashboard for more information about this PR.

[ZeeshanTamboli]

Resolved in #49497