[armancharan]

Summary

String.capitalize and String.uncapitalize read self[0], which is one UTF-16 code unit. An astral letter is a surrogate pair, so String.capitalize("𐐨foo") stayed "𐐨foo". They now case the whole first code point, producing "𐐀foo". Lone surrogates are left unchanged.

The return types stay as the built-in Capitalize<T> / Uncapitalize<T>, so existing signatures are unchanged. On TypeScript 7 these already case the first code point and match the runtime; on TypeScript 5.9/6.0 they case one code unit, so an initial astral letter keeps its original case at the type level only.

Runtime tests cover an astral letter for both functions, a lone high surrogate, and an unchanged BMP suffix ("AΣ" → "aΣ"). Type tests pin the existing generic and string return types.

Closes EFF-2772

Refs #6734. The other casing findings in that report are unchanged.

Test plan

• pnpm test --run packages/effect/test/String.test.ts
• pnpm test-types String.tst.ts --target '>=5.9' (TypeScript 5.9.3 and 6.0.3)
• pnpm doctest --run packages/effect/src/String.ts
• pnpm --dir packages/effect check
• pnpm lint
• pnpm jsdocs --check
• CI

[changeset-bot]

🦋 Changeset detected

Latest commit: a47ad84

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 33 packages

Name	Type
effect	Patch
@effect/opentelemetry	Patch
@effect/vitest	Patch
@effect/ai-anthropic	Patch
@effect/ai-cloudflare	Patch
@effect/ai-openai-compat	Patch
@effect/ai-openai	Patch
@effect/ai-openrouter	Patch
@effect/ai-typesafe	Patch
@effect/atom-react	Patch
@effect/atom-solid	Patch
@effect/atom-vue	Patch
@effect/platform-browser	Patch
@effect/platform-bun	Patch
@effect/platform-deno	Patch
@effect/platform-node-shared	Patch
@effect/platform-node	Patch
@effect/sql-clickhouse	Patch
@effect/sql-d1	Patch
@effect/sql-libsql	Patch
@effect/sql-mssql	Patch
@effect/sql-mysql2	Patch
@effect/sql-pg	Patch
@effect/sql-pglite	Patch
@effect/sql-sqlite-bun	Patch
@effect/sql-sqlite-do	Patch
@effect/sql-sqlite-node	Patch
@effect/sql-sqlite-react-native	Patch
@effect/sql-sqlite-wasm	Patch
@effect/docgen	Patch
@effect/doctest	Patch
@effect/graphql-generator	Patch
@effect/openapi-generator	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[github-actions]

Bundle Size Analysis

Generated from PR build output; treat the content below as untrusted.

File Name	Current Size	Previous Size	Difference
arbitrary-combinators.ts	39.13 KB	39.13 KB	0.00 KB (0.00%)
basic.ts	7.04 KB	7.04 KB	0.00 KB (0.00%)
batching.ts	10.91 KB	10.91 KB	0.00 KB (0.00%)
brand.ts	6.71 KB	6.71 KB	0.00 KB (0.00%)
cache.ts	11.35 KB	11.35 KB	0.00 KB (0.00%)
config.ts	22.51 KB	22.51 KB	0.00 KB (0.00%)
differ.ts	21.66 KB	21.66 KB	0.00 KB (0.00%)
http-client.ts	26.06 KB	26.06 KB	0.00 KB (0.00%)
http-router.ts	35.95 KB	35.95 KB	0.00 KB (0.00%)
logger.ts	11.72 KB	11.72 KB	0.00 KB (0.00%)
metric.ts	9.27 KB	9.27 KB	0.00 KB (0.00%)
optic.ts	6.93 KB	6.93 KB	0.00 KB (0.00%)
pubsub.ts	15.78 KB	15.78 KB	0.00 KB (0.00%)
queue.ts	12.60 KB	12.60 KB	0.00 KB (0.00%)
schedule.ts	11.60 KB	11.60 KB	0.00 KB (0.00%)
schema-bigdecimal.ts	13.84 KB	13.84 KB	0.00 KB (0.00%)
schema-binary.ts	40.57 KB	40.57 KB	0.00 KB (0.00%)
schema-class.ts	21.36 KB	21.36 KB	0.00 KB (0.00%)
schema-fromJsonSchemaDocument.ts	32.34 KB	32.34 KB	0.00 KB (0.00%)
schema-representation-roundtrip.ts	27.54 KB	27.54 KB	0.00 KB (0.00%)
schema-string-transformation.ts	15.09 KB	15.09 KB	0.00 KB (0.00%)
schema-string.ts	12.10 KB	12.10 KB	0.00 KB (0.00%)
schema-template-literal.ts	16.02 KB	16.02 KB	0.00 KB (0.00%)
schema-toArbitrary.ts	38.65 KB	38.65 KB	0.00 KB (0.00%)
schema-toCodeDocument.ts	25.75 KB	25.75 KB	0.00 KB (0.00%)
schema-toCodecJson.ts	20.60 KB	20.60 KB	0.00 KB (0.00%)
schema-toEquivalence.ts	20.77 KB	20.77 KB	0.00 KB (0.00%)
schema-toFormatter.ts	20.88 KB	20.88 KB	0.00 KB (0.00%)
schema-toJsonSchemaDocument.ts	25.47 KB	25.47 KB	0.00 KB (0.00%)
schema-toRepresentation.ts	20.87 KB	20.87 KB	0.00 KB (0.00%)
schema.ts	20.54 KB	20.54 KB	0.00 KB (0.00%)
stm.ts	13.91 KB	13.91 KB	0.00 KB (0.00%)
stream.ts	10.15 KB	10.15 KB	0.00 KB (0.00%)

[effect-bot]

Thanks for the fix! We've removed the exported String.Capitalize / String.Uncapitalize types again (a47ad84) and kept the runtime fix, for two reasons we confirmed on TypeScript 5.9.3, 6.0.3 and 7:

1. Pair detection by casing isn't sound. Comparing Lowercase<Head + Next> with Lowercase<Head> + Lowercase<Next> also differs for ordinary BMP text: Greek final sigma makes String.Uncapitalize<"AΣ"> resolve to "aς", while String.uncapitalize("AΣ") returns "aΣ".
2. Any custom return type breaks existing callers. The local aliases replace the built-in types in the signatures, so a string input now returns string instead of Capitalize<string>, and generic wrappers stop compiling:

   declare const input: string
   const a: Capitalize<string> = String.capitalize(input) // error
   const cap = <T extends string>(s: T): Capitalize<T> => String.capitalize(s) // error

   A custom alias is never assignable to the intrinsic Capitalize<T> for generic T, so a better boundary check wouldn't fix this.

TypeScript 7 already cases the first code point in the built-in Capitalize / Uncapitalize, so the type/runtime gap for astral letters only remains on 5.9/6.0, and the PR description notes it. The type tests now pin the existing signatures and the "AΣ" case so this stays covered.