diff --git a/.changeset/type-alias-declarations.md b/.changeset/type-alias-declarations.md
new file mode 100644
index 0000000..7e1c717
--- /dev/null
+++ b/.changeset/type-alias-declarations.md
@@ -0,0 +1,6 @@
+---
+'@vortexlabs/kirari': patch
+'@vortexlabs/kirari-react': patch
+---
+
+Public object types such as `Token`, `Rule`, `Language`, `RenderOptions`, `Decoration`, `TokenPatch`, `TokenStream`, and `StreamedTokens` are now type aliases instead of interfaces. Their members are unchanged, but they can no longer be extended through declaration merging (module augmentation). Types that extend another type, such as `HighlightOptions` and `CodeProps`, remain interfaces.
diff --git a/docs/decorations.md b/docs/decorations.md
index 35c7193..d96a46e 100644
--- a/docs/decorations.md
+++ b/docs/decorations.md
@@ -19,7 +19,7 @@ The range decorates `answer`. Supply your own CSS for `search-match`, `active-li
 ## API
 
 ```ts
-interface Decoration {
+type Decoration = {
   readonly start: number
   readonly end: number
   readonly className: string
diff --git a/packages/benchmarks/benchmark.ts b/packages/benchmarks/benchmark.ts
index 7fec9d4..8f7b5f2 100644
--- a/packages/benchmarks/benchmark.ts
+++ b/packages/benchmarks/benchmark.ts
@@ -9,7 +9,7 @@ import { corpus } from '../kirari/test/corpus/fixtures.js'
 import { bundleEntry, highlight, languageNames } from './adapters.js'
 import type { LanguageName } from './adapters.js'
 
-export interface Input {
+export type Input = {
   id: string
   language: LanguageName
   source: string
diff --git a/packages/kirari/scripts/themes.ts b/packages/kirari/scripts/themes.ts
index 7f4870a..76d5f5f 100644
--- a/packages/kirari/scripts/themes.ts
+++ b/packages/kirari/scripts/themes.ts
@@ -4,7 +4,7 @@ import { tokenTypes, type TokenType } from '../src/types.js'
 
 type Colors = Partial<Record<TokenType, string>> & { background: string; text: string; keyword: string; function: string; string: string; number: string; comment: string }
 
-interface Theme {
+type Theme = {
   id: string
   label: string
   appearance: 'light' | 'dark'
diff --git a/packages/kirari/src/decorations.ts b/packages/kirari/src/decorations.ts
index 8369282..4fe998c 100644
--- a/packages/kirari/src/decorations.ts
+++ b/packages/kirari/src/decorations.ts
@@ -1,7 +1,7 @@
 import { escape, render, tokenClasses } from './render.js'
 import type { RenderOptions, Token } from './types.js'
 
-export interface Decoration {
+export type Decoration = {
   readonly start: number
   readonly end: number
   readonly className: string
diff --git a/packages/kirari/src/grammars/definitions/state.ts b/packages/kirari/src/grammars/definitions/state.ts
index 5ef23b0..75546f8 100644
--- a/packages/kirari/src/grammars/definitions/state.ts
+++ b/packages/kirari/src/grammars/definitions/state.ts
@@ -9,7 +9,7 @@ const backReference = /^(def|\*|&|\.|:|\?\.|->|\?->|::|>+)$/
  * Significant-token state before a point in Python source, tracked so a stream can tell when
  * `classifyDefinitions` results before that point no longer depend on later tokens.
  */
-export interface DefinitionState {
+export type DefinitionState = {
   /** Open brackets, as in `matchBrackets`, with the value before each opening. */
   brackets: { close: string; before: string }[]
   /** Bracket depths of `name:` annotation scans that have not reached `=` or a stop token. */
diff --git a/packages/kirari/src/grammars/functions.ts b/packages/kirari/src/grammars/functions.ts
index d2feae4..892320e 100644
--- a/packages/kirari/src/grammars/functions.ts
+++ b/packages/kirari/src/grammars/functions.ts
@@ -5,10 +5,10 @@ import { lastLineBreak } from '../scanner/lexical.js'
 type Result = { function: boolean; pending: boolean }
 type Group = { end: number; complete: boolean }
 /** Source-wide lookahead data; share only between scanners of the same immutable source. */
-export interface FunctionLookupCache {
+export type FunctionLookupCache = {
   lastLine?: number
 }
-export interface LookupOptions {
+export type LookupOptions = {
   /** A colon after the name may introduce a property value or a typed binding. */
   readonly colon: boolean
   /** The name may take type parameters, as after `function` or in a method position. */
diff --git a/packages/kirari/src/grammars/markup/types.ts b/packages/kirari/src/grammars/markup/types.ts
index 5b4a484..816b861 100644
--- a/packages/kirari/src/grammars/markup/types.ts
+++ b/packages/kirari/src/grammars/markup/types.ts
@@ -49,7 +49,7 @@ export type MarkupState = {
 }
 
 /** Where a JSX scan starts, and where to report root-level newlines it can resume from. */
-export interface MarkupResume {
+export type MarkupResume = {
   /** Restart point in the full source; omission starts scanning at offset zero. */
   readonly from?: ResumePoint | undefined
   /** Reports safe restart points with indexes in this scan's returned token array. */
diff --git a/packages/kirari/src/registry.ts b/packages/kirari/src/registry.ts
index 82a575c..74eb47f 100644
--- a/packages/kirari/src/registry.ts
+++ b/packages/kirari/src/registry.ts
@@ -36,7 +36,7 @@ const aliases: Readonly<Record<string, readonly string[]>> = {
   yaml: ['yml'],
 }
 
-export interface LanguageRegistry {
+export type LanguageRegistry = {
   resolve(input: string): Language | undefined
 }
 
diff --git a/packages/kirari/src/resumable.ts b/packages/kirari/src/resumable.ts
index 9699e2e..10869e3 100644
--- a/packages/kirari/src/resumable.ts
+++ b/packages/kirari/src/resumable.ts
@@ -2,7 +2,7 @@ import type { ScanContext } from './tokenize.js'
 import type { Language, Token } from './types.js'
 
 /** A root-level point where scanning can restart with the same results as a full scan. */
-export interface ResumePoint {
+export type ResumePoint = {
   readonly offset: number
   readonly previous: Token | undefined
   /** Script scan state at the point, for grammars that embed JavaScript. */
diff --git a/packages/kirari/src/stream.ts b/packages/kirari/src/stream.ts
index d033107..fecc9ce 100644
--- a/packages/kirari/src/stream.ts
+++ b/packages/kirari/src/stream.ts
@@ -9,14 +9,14 @@ import { createResumableStream } from './stream/resumable.js'
 import { isSignificant } from './types.js'
 import type { Language, Token } from './types.js'
 
-export interface TokenPatch {
+export type TokenPatch = {
   /** Replace previous tokens from this UTF-16 source offset onward. */
   readonly from: number
   readonly tokens: readonly Token[]
   readonly length: number
 }
 
-export interface TokenStream {
+export type TokenStream = {
   readonly mode: 'checkpoint' | 'full'
   write(chunk: string): TokenPatch
   end(chunk?: string): TokenPatch
diff --git a/packages/kirari/src/tokenize.ts b/packages/kirari/src/tokenize.ts
index 7d30dbe..f56dbc5 100644
--- a/packages/kirari/src/tokenize.ts
+++ b/packages/kirari/src/tokenize.ts
@@ -22,7 +22,7 @@ import { canStartRegex, escapedIdentifierEnd, nextVisibleChar, regexEnd, templat
 
 export type Frame = { kind: 'template'; opening: boolean } | { kind: 'expression'; depth: number }
 /** Lexical state for JavaScript-like languages, carried between scanner steps. */
-export interface ScanContext {
+export type ScanContext = {
   parentheses: { control: boolean; braces: number }[]
   braces?: number
   afterControl?: boolean
diff --git a/packages/kirari/src/types.ts b/packages/kirari/src/types.ts
index 7a466d9..5ef08b4 100644
--- a/packages/kirari/src/types.ts
+++ b/packages/kirari/src/types.ts
@@ -12,7 +12,7 @@ export type TokenType = (typeof tokenTypes)[number]
 export const isSignificant = (type: string): boolean => type !== 'text' && type !== 'comment'
 
 /** An unchanged source slice with UTF-16 offsets into the original source. */
-export interface Token {
+export type Token = {
   /** Category used for the `kirari-${type}` CSS class; custom names are supported. */
   readonly type: string
   /** Exact source text between `start` and `end`. */
@@ -24,7 +24,7 @@ export interface Token {
 }
 
 /** A lexical rule tried at the current source offset; the first matching rule wins. */
-export interface Rule {
+export type Rule = {
   /** Category assigned to the matched source text. */
   readonly type: string
   /** Cloned with `g`/`y` replaced by sticky `y`; supplied `lastIndex` is untouched. Empty matches throw. */
@@ -32,7 +32,7 @@ export interface Rule {
 }
 
 /** Scanner rules and optional structural processing for a language. */
-export interface Language {
+export type Language = {
   /** Language identifier used by diagnostics and grammar-specific scanning. */
   readonly name: string
   /** Ordered lexical rules used by the default scanner. */
@@ -58,7 +58,7 @@ export interface Language {
 }
 
 /** Extra CSS classes added alongside the renderer's standard token classes. */
-export interface RenderOptions {
+export type RenderOptions = {
   /** Additional classes by token category, including custom categories. */
   readonly classes?: Readonly<Record<string, string>>
   /** Additional classes per token, combined with `classes`; `undefined` adds nothing. */
diff --git a/packages/kirari/test/corpus/fixtures.ts b/packages/kirari/test/corpus/fixtures.ts
index fab8d91..f618999 100644
--- a/packages/kirari/test/corpus/fixtures.ts
+++ b/packages/kirari/test/corpus/fixtures.ts
@@ -1,6 +1,6 @@
 export type CorpusLanguage = 'javascript' | 'typescript' | 'tsx' | 'python' | 'json' | 'shell'
 
-export interface CorpusAssertion {
+export type CorpusAssertion = {
   text: string
   type: string
   occurrence?: number
@@ -10,7 +10,7 @@ export interface CorpusGap extends CorpusAssertion {
   reason: string
 }
 
-export interface CorpusFixture {
+export type CorpusFixture = {
   id: string
   language: CorpusLanguage
   source: string
diff --git a/packages/react/src/stream.ts b/packages/react/src/stream.ts
index 2c59f19..d27be1b 100644
--- a/packages/react/src/stream.ts
+++ b/packages/react/src/stream.ts
@@ -5,7 +5,7 @@ import type { Language, Token } from '@vortexlabs/kirari/core'
 import { createTokenStream } from '@vortexlabs/kirari/stream'
 import type { TokenPatch, TokenStream } from '@vortexlabs/kirari/stream'
 
-export interface StreamedTokens {
+export type StreamedTokens = {
   readonly tokens: readonly Token[]
   write(chunk: string): void
   end(chunk?: string): void
diff --git a/packages/react/src/types.ts b/packages/react/src/types.ts
index 093e797..9133756 100644
--- a/packages/react/src/types.ts
+++ b/packages/react/src/types.ts
@@ -6,7 +6,7 @@ import type { Decoration } from '@vortexlabs/kirari/decorations'
 export type LineRange = number | readonly [first: number, last: number]
 
 /** A rendered source line; CRLF, CR, and LF separators sit outside its wrapper. */
-export interface Line {
+export type Line = {
   /** One-based line number. */
   readonly number: number
   /** Inclusive UTF-16 offset into the complete source. */