Likely to cause new Flow errors:

• Fixed unsound subtyping of instances against indexer in interfaces (example).
• Flow will no longer emit method-unbinding errors and instead perform stricter this-type checking (example).

IDE:

• Improve the primary type displayed by hover
• Show a single type in type-at-pos and hover
• Truncate oversized hover types and their reference lists

Library Definitions:

• React.ReactElement now matches latest type in @types/react. Added missing React.JSX.Element that makes the type in @types/react .

Likely to cause new Flow errors:

• Flow will now restrict the allowed syntax forms of .call/apply/bind calls. Only expressions of the form <expr>.m.call(<expr>, ...), <expr>.m.apply(<expr>, ...) and <expr>.m.bind(<expr>, ...) are allowed.
• [untyped] section of flowconfig will use the glob system. See https://flow.org/en/docs/config/untyped/

Notable bug fixes:

• Fixed #9407.
• Support instanceof refinement of constructor objects.
• Operations like Partial no longer strip readonly-ness of dictionaries. (example)

Likely to cause new Flow errors:

• More subtyping errors will print the types that are incompatible. Some errors might have new locations.
• A unique symbol annotation is now an error unless it is the type of a const variable, of a readonly object type or interface property, or of a static readonly class field.
• Flow will error on legacy variance syntax in library definition files

New Features:

• We have released flow-parser and flow-eslint packages that are drop-in replacement for hermes-parser and hermes-eslint. Babel docs and ESLint docs have been updated.
• Object properties, class fields, methods, and the members of an interface or declare class imported from another module, can be keyed by a unique symbol.

Notable bug fixes:

• Prevent a race that can leave server hanging
• Support TypeScript constructor objects in Flow class positions.
• keyof over a symbol index signature is now symbol rather than string.
• A class can extend a value with a construct signature.
• x !== undefined no longer stops refining after a call that passes undefined as an argument: (e.g. try-Flow)

Library Definitions:

• Well-known symbols on Symbol are typed as unique symbol, and Symbol.metadata is now available.

Breaking changes:

• [include] and [ignore] sections in flowconfig should use globs instead. Read https://flow.org/en/docs/config/include/ and https://flow.org/en/docs/config/ignore/ for more information.

New Features:

• this type guards can now be used on non-static methods of regular classes, not just declare class and interface methods. The method body is checked for consistency against the guard type.

Notable bug fixes:

• Prevent cached IDE results from blocking rechecks.

Library Definitions:

• Allow symbols to be used as WeakMap keys

New Features:

• In an object type, a bracketed key written as a string or number literal, or as the name of a value, is now a computed key naming a single property rather than an index signature, so several such keys can coexist. A key written as a type-only name is unchanged, and the labeled form [label: K] keeps index-signature semantics. Breaking: a key headed by a value binding is read as a computed key. Thus a class or enum key such as {[C]: V} is now an error, and a qualified type exposed through a class/namespace merge, as in {[C.K]: V}, must use the labeled form {[key: C.K]: V} to remain an index signature.
• In an interface or a declare class body, a bracketed key written as a string or number literal, or as the name of a value, is now a computed key naming a single property rather than an index signature, so several such keys can coexist, and static puts the property on the static side. A key written as a type-only name is unchanged, and the labeled form [label: K] keeps index-signature semantics. Breaking: a key headed by a value binding is read as a computed key, so a class or enum key such as interface I {[C]: V} is now an error and must use [key: C] to remain an index signature.
• A computed method key [expr](): T that resolves to a single literal string or safe-integer number is now accepted in interface, declare class, and object type bodies (previously a parse error outside .d.ts files) and reads as a named method.
• A bracketed access on an array with a string-literal key (for example arr['map'] or arr[Symbol.iterator]) now reads the corresponding named member instead of erroring that the key is not an array index, and a string key holding a safe integer (arr['0']) indexes the array like the equivalent numeric key.

Notable bug fixes:

• keyof any now evaluates to any as expected. Spurious errors related to the bug will no longer fire.
• Fix a server crash ("thread 'flow-tokio-runtime' has overflowed its stack") when rechecking a file with an attached LSP client

Parser:

• Parse component and hook type annotations as the declaration of declare export default

Library Definitions:

• Add ES2026 Map methods

Breaking changes:

• The following flags are removed since they no longer change anything
  • experimental.channel_mode
  • experimental.channel_mode.windows
  • experimental.long_lived_workers
  • experimental.long_lived_workers.windows
  • gc.worker.custom_major_ratio
  • gc.worker.custom_minor_max_size
  • gc.worker.custom_minor_ratio
  • gc.worker.major_heap_increment
  • gc.worker.minor_heap_size
  • gc.worker.space_overhead
  • gc.worker.window_size
  • sharedmemory.hash_table_pow
  • sharedmemory.heap_size

Likely to cause new Flow errors:

• General subtyping errors will print the types that are incompatible. Some errors might have new locations.

No behavioral changes in this release

New Features:

• .flowconfig's [include] and [ignore] sections can now be configured using globs, in addition to the current regex based system. Right now, glob items must be specified with glob: prefix. Globs do not support <PROJECT_ROOT>, since they are assumed to start from the project root.
• Class fields and methods whose computed key resolves to a literal string or number are now supported and type-checked as named members.

No behavioral changes in this release

Notable bug fixes:

• Fixed #9448
• Fixed a crash in pattern matching. (example)
• Refining a union by instanceof no longer leaks any from members that cannot match the guard (e.g. Foo<string> | Bar<number> refined by instanceof Foo now yields Foo<string> instead of Foo<string> | Foo<any>). This may surface real errors that were previously masked.