diff --git a/snapshots/output/diagnostics/index.ts b/snapshots/output/diagnostics/index.ts
index a13960dd..72da397e 100644
--- a/snapshots/output/diagnostics/index.ts
+++ b/snapshots/output/diagnostics/index.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition diagnostics 0.0.1 `index.ts`/
 
 /** @deprecated This class is deprecated */
diff --git a/snapshots/output/enclosing-ranges-ts/index.ts b/snapshots/output/enclosing-ranges-ts/index.ts
index 430c1451..803f49cb 100644
--- a/snapshots/output/enclosing-ranges-ts/index.ts
+++ b/snapshots/output/enclosing-ranges-ts/index.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition enclosing-ranges-ts 1.0.0 `index.ts`/
 
 // format-options: showRanges
diff --git a/snapshots/output/enclosing-ranges/range.js b/snapshots/output/enclosing-ranges/range.js
index 4684c559..c42206e1 100644
--- a/snapshots/output/enclosing-ranges/range.js
+++ b/snapshots/output/enclosing-ranges/range.js
@@ -1,3 +1,4 @@
+// language JavaScript
 // < definition enclosing-ranges 0.0.1 `range.js`/
 
 // format-options: showRanges
diff --git a/snapshots/output/invalid-package-json/packages/a/src/a.ts b/snapshots/output/invalid-package-json/packages/a/src/a.ts
index 78323324..c21d0223 100644
--- a/snapshots/output/invalid-package-json/packages/a/src/a.ts
+++ b/snapshots/output/invalid-package-json/packages/a/src/a.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition @example/a HEAD src/`a.ts`/
 
 export function a(): string {
diff --git a/snapshots/output/multi-project/packages/a/src/index.ts b/snapshots/output/multi-project/packages/a/src/index.ts
index 9597c70a..f5ace936 100644
--- a/snapshots/output/multi-project/packages/a/src/index.ts
+++ b/snapshots/output/multi-project/packages/a/src/index.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition @example/a 1.0.0 src/`index.ts`/
 //documentation ```ts
 //            > module "index.ts"
diff --git a/snapshots/output/multi-project/packages/b/src/b.ts b/snapshots/output/multi-project/packages/b/src/b.ts
index 430946cc..e992caf3 100644
--- a/snapshots/output/multi-project/packages/b/src/b.ts
+++ b/snapshots/output/multi-project/packages/b/src/b.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition @example/b 1.0.0 src/`b.ts`/
 
 import { a } from '@example/a/src'
diff --git a/snapshots/output/pnpm-workspaces/packages/a/src/a.ts b/snapshots/output/pnpm-workspaces/packages/a/src/a.ts
index 9183bd3d..b04e75fd 100644
--- a/snapshots/output/pnpm-workspaces/packages/a/src/a.ts
+++ b/snapshots/output/pnpm-workspaces/packages/a/src/a.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition @example/a 1.0.0 src/`a.ts`/
 
 export function a(): string {
diff --git a/snapshots/output/pnpm-workspaces/packages/b/src/b.ts b/snapshots/output/pnpm-workspaces/packages/b/src/b.ts
index 6a975141..b5efc02a 100644
--- a/snapshots/output/pnpm-workspaces/packages/b/src/b.ts
+++ b/snapshots/output/pnpm-workspaces/packages/b/src/b.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition @example/b 1.0.0 src/`b.ts`/
 
 import { a } from '@example/a'
diff --git a/snapshots/output/pure-js/src/main.js b/snapshots/output/pure-js/src/main.js
index 614be4fa..5d853161 100644
--- a/snapshots/output/pure-js/src/main.js
+++ b/snapshots/output/pure-js/src/main.js
@@ -1,3 +1,4 @@
+// language JavaScript
 // < definition pure-js 1.0.0 src/`main.js`/
 
 function fib(n) {
diff --git a/snapshots/output/react/src/LoaderInput.tsx b/snapshots/output/react/src/LoaderInput.tsx
index 09d95a30..957ddbb1 100644
--- a/snapshots/output/react/src/LoaderInput.tsx
+++ b/snapshots/output/react/src/LoaderInput.tsx
@@ -1,3 +1,4 @@
+// language TypeScriptReact
 // < definition react-example 1.0.0 src/`LoaderInput.tsx`/
 
 import React from 'react'
diff --git a/snapshots/output/react/src/MyTSXElement.tsx b/snapshots/output/react/src/MyTSXElement.tsx
index ec63c2cf..7e8fb4a7 100644
--- a/snapshots/output/react/src/MyTSXElement.tsx
+++ b/snapshots/output/react/src/MyTSXElement.tsx
@@ -1,3 +1,4 @@
+// language TypeScriptReact
 // < definition react-example 1.0.0 src/`MyTSXElement.tsx`/
 
 import React from 'react'
diff --git a/snapshots/output/react/src/UseMyTSXElement.tsx b/snapshots/output/react/src/UseMyTSXElement.tsx
index c3690187..f6970da1 100644
--- a/snapshots/output/react/src/UseMyTSXElement.tsx
+++ b/snapshots/output/react/src/UseMyTSXElement.tsx
@@ -1,3 +1,4 @@
+// language TypeScriptReact
 // < definition react-example 1.0.0 src/`UseMyTSXElement.tsx`/
 
 import React from "react";
diff --git a/snapshots/output/syntax/src/ClassWithPrivate.ts b/snapshots/output/syntax/src/ClassWithPrivate.ts
index 66e5c025..4701245d 100644
--- a/snapshots/output/syntax/src/ClassWithPrivate.ts
+++ b/snapshots/output/syntax/src/ClassWithPrivate.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`ClassWithPrivate.ts`/
 
 export class ClassWithPrivate {
diff --git a/snapshots/output/syntax/src/accessors.ts b/snapshots/output/syntax/src/accessors.ts
index d41606c1..bb46bf5f 100644
--- a/snapshots/output/syntax/src/accessors.ts
+++ b/snapshots/output/syntax/src/accessors.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`accessors.ts`/
 
 class C {
diff --git a/snapshots/output/syntax/src/class.ts b/snapshots/output/syntax/src/class.ts
index 0ac9ef3f..8530826a 100644
--- a/snapshots/output/syntax/src/class.ts
+++ b/snapshots/output/syntax/src/class.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`class.ts`/
 
 export class Class {
diff --git a/snapshots/output/syntax/src/conflicting-const-interface.ts b/snapshots/output/syntax/src/conflicting-const-interface.ts
index acfa5b2d..735b776c 100644
--- a/snapshots/output/syntax/src/conflicting-const-interface.ts
+++ b/snapshots/output/syntax/src/conflicting-const-interface.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`conflicting-const-interface.ts`/
 
 export const ConflictingConst = 42
diff --git a/snapshots/output/syntax/src/constructor.ts b/snapshots/output/syntax/src/constructor.ts
index 0f69f95d..bfb730f1 100644
--- a/snapshots/output/syntax/src/constructor.ts
+++ b/snapshots/output/syntax/src/constructor.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`constructor.ts`/
 
 namespace Yay {
diff --git a/snapshots/output/syntax/src/decorators.ts b/snapshots/output/syntax/src/decorators.ts
index 0abc6d30..8aa71b40 100644
--- a/snapshots/output/syntax/src/decorators.ts
+++ b/snapshots/output/syntax/src/decorators.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`decorators.ts`/
 
 import { Configuration } from './reusable-types'
diff --git a/snapshots/output/syntax/src/destructuring.ts b/snapshots/output/syntax/src/destructuring.ts
index 617bdaaf..11c9d709 100644
--- a/snapshots/output/syntax/src/destructuring.ts
+++ b/snapshots/output/syntax/src/destructuring.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`destructuring.ts`/
 
 interface Props {
diff --git a/snapshots/output/syntax/src/enum.ts b/snapshots/output/syntax/src/enum.ts
index 24d52ed9..c4cc4d7d 100644
--- a/snapshots/output/syntax/src/enum.ts
+++ b/snapshots/output/syntax/src/enum.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`enum.ts`/
 
 export enum Enum {
diff --git a/snapshots/output/syntax/src/function.ts b/snapshots/output/syntax/src/function.ts
index bd2835d7..e1ea57fa 100644
--- a/snapshots/output/syntax/src/function.ts
+++ b/snapshots/output/syntax/src/function.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`function.ts`/
 
 export function newFunction(): void {}
diff --git a/snapshots/output/syntax/src/import.ts b/snapshots/output/syntax/src/import.ts
index 40664fbc..93024918 100644
--- a/snapshots/output/syntax/src/import.ts
+++ b/snapshots/output/syntax/src/import.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`import.ts`/
 
 import * as namespace from './namespace'
diff --git a/snapshots/output/syntax/src/inheritance.ts b/snapshots/output/syntax/src/inheritance.ts
index 596614ec..f67e636b 100644
--- a/snapshots/output/syntax/src/inheritance.ts
+++ b/snapshots/output/syntax/src/inheritance.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`inheritance.ts`/
 
 import { Superinterface } from './reusable-types'
diff --git a/snapshots/output/syntax/src/interface.ts b/snapshots/output/syntax/src/interface.ts
index 3ea6368b..04b69fa5 100644
--- a/snapshots/output/syntax/src/interface.ts
+++ b/snapshots/output/syntax/src/interface.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`interface.ts`/
 
 export interface Interface {
diff --git a/snapshots/output/syntax/src/issue-45.d.ts b/snapshots/output/syntax/src/issue-45.d.ts
index 20fe2bbb..7354b111 100644
--- a/snapshots/output/syntax/src/issue-45.d.ts
+++ b/snapshots/output/syntax/src/issue-45.d.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`issue-45.d.ts`/
 
 export namespace example {
diff --git a/snapshots/output/syntax/src/local.ts b/snapshots/output/syntax/src/local.ts
index 0d688856..ea9aa8be 100644
--- a/snapshots/output/syntax/src/local.ts
+++ b/snapshots/output/syntax/src/local.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`local.ts`/
 
 export function local(): string {
diff --git a/snapshots/output/syntax/src/module.d.ts b/snapshots/output/syntax/src/module.d.ts
index 5e455a8e..31dd0dc7 100644
--- a/snapshots/output/syntax/src/module.d.ts
+++ b/snapshots/output/syntax/src/module.d.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`module.d.ts`/
 
 declare module 'a:b' {
diff --git a/snapshots/output/syntax/src/namespace.ts b/snapshots/output/syntax/src/namespace.ts
index 5633f648..d0d9cb42 100644
--- a/snapshots/output/syntax/src/namespace.ts
+++ b/snapshots/output/syntax/src/namespace.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`namespace.ts`/
 
 export declare namespace a {
diff --git a/snapshots/output/syntax/src/object-literals-arrow-function.ts b/snapshots/output/syntax/src/object-literals-arrow-function.ts
index 57a67dd0..0f77d535 100644
--- a/snapshots/output/syntax/src/object-literals-arrow-function.ts
+++ b/snapshots/output/syntax/src/object-literals-arrow-function.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`object-literals-arrow-function.ts`/
 
 import { Option } from './reusable-types'
diff --git a/snapshots/output/syntax/src/object-literals-call-signatures.ts b/snapshots/output/syntax/src/object-literals-call-signatures.ts
index ba1b8b3e..d3b5a099 100644
--- a/snapshots/output/syntax/src/object-literals-call-signatures.ts
+++ b/snapshots/output/syntax/src/object-literals-call-signatures.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`object-literals-call-signatures.ts`/
 
 import {
diff --git a/snapshots/output/syntax/src/object-literals-nested.ts b/snapshots/output/syntax/src/object-literals-nested.ts
index d7cca547..608f8bb7 100644
--- a/snapshots/output/syntax/src/object-literals-nested.ts
+++ b/snapshots/output/syntax/src/object-literals-nested.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`object-literals-nested.ts`/
 
 import { Option } from './reusable-types'
diff --git a/snapshots/output/syntax/src/object-literals.ts b/snapshots/output/syntax/src/object-literals.ts
index f7df0b2c..0aed7922 100644
--- a/snapshots/output/syntax/src/object-literals.ts
+++ b/snapshots/output/syntax/src/object-literals.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`object-literals.ts`/
 
 import { Configuration } from './reusable-types'
diff --git a/snapshots/output/syntax/src/overload.d.ts b/snapshots/output/syntax/src/overload.d.ts
index e57eedd9..a9749c98 100644
--- a/snapshots/output/syntax/src/overload.d.ts
+++ b/snapshots/output/syntax/src/overload.d.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`overload.d.ts`/
 
 export interface Overloader {
diff --git a/snapshots/output/syntax/src/property-assignment-reference.ts b/snapshots/output/syntax/src/property-assignment-reference.ts
index 06cfa23c..812a3ae1 100644
--- a/snapshots/output/syntax/src/property-assignment-reference.ts
+++ b/snapshots/output/syntax/src/property-assignment-reference.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`property-assignment-reference.ts`/
 
 import {
diff --git a/snapshots/output/syntax/src/property-assignment.ts b/snapshots/output/syntax/src/property-assignment.ts
index ce5a8531..26656820 100644
--- a/snapshots/output/syntax/src/property-assignment.ts
+++ b/snapshots/output/syntax/src/property-assignment.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`property-assignment.ts`/
 
 export function propertyAssignment() {
diff --git a/snapshots/output/syntax/src/reusable-types.ts b/snapshots/output/syntax/src/reusable-types.ts
index 38515b5c..d6884129 100644
--- a/snapshots/output/syntax/src/reusable-types.ts
+++ b/snapshots/output/syntax/src/reusable-types.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`reusable-types.ts`/
 
 // Reusable types for other snapshot tests
diff --git a/snapshots/output/syntax/src/string-literals.ts b/snapshots/output/syntax/src/string-literals.ts
index 90b385e9..165f6827 100644
--- a/snapshots/output/syntax/src/string-literals.ts
+++ b/snapshots/output/syntax/src/string-literals.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`string-literals.ts`/
 
 interface SomeInterface {
diff --git a/snapshots/output/syntax/src/structural-type.ts b/snapshots/output/syntax/src/structural-type.ts
index 54ac91c3..f1d2d440 100644
--- a/snapshots/output/syntax/src/structural-type.ts
+++ b/snapshots/output/syntax/src/structural-type.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`structural-type.ts`/
 
 export function foo(): Promise<{ member: number }> {
diff --git a/snapshots/output/syntax/src/symbol-kinds.ts b/snapshots/output/syntax/src/symbol-kinds.ts
index 5bfbd942..61c1716f 100644
--- a/snapshots/output/syntax/src/symbol-kinds.ts
+++ b/snapshots/output/syntax/src/symbol-kinds.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`symbol-kinds.ts`/
 //kind File
 
diff --git a/snapshots/output/syntax/src/type-alias.ts b/snapshots/output/syntax/src/type-alias.ts
index 0f23f64a..458fc140 100644
--- a/snapshots/output/syntax/src/type-alias.ts
+++ b/snapshots/output/syntax/src/type-alias.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`type-alias.ts`/
 
 type S = string
diff --git a/snapshots/output/syntax/src/type-parameter.ts b/snapshots/output/syntax/src/type-parameter.ts
index 99c7c5dd..e9e014ae 100644
--- a/snapshots/output/syntax/src/type-parameter.ts
+++ b/snapshots/output/syntax/src/type-parameter.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`type-parameter.ts`/
 
 export function typeParameter<A, B>(parameter: A, parameter2: B): [A, B] {
diff --git a/snapshots/output/syntax/src/typings.ts b/snapshots/output/syntax/src/typings.ts
index 37cc4198..2e37cb69 100644
--- a/snapshots/output/syntax/src/typings.ts
+++ b/snapshots/output/syntax/src/typings.ts
@@ -1,3 +1,4 @@
+// language TypeScript
 // < definition syntax 1.0.0 src/`typings.ts`/
 
 export function process() {
diff --git a/src/ProjectIndexer.test.ts b/src/ProjectIndexer.test.ts
index cfe61c1f..93848ba3 100644
--- a/src/ProjectIndexer.test.ts
+++ b/src/ProjectIndexer.test.ts
@@ -1,7 +1,7 @@
 import { test } from 'uvu'
 import * as assert from 'uvu/assert'
 
-import { prettyMilliseconds } from './ProjectIndexer'
+import { languageForFileName, prettyMilliseconds } from './ProjectIndexer'
 
 function minute(x: number): number {
   return x * 60 * 1000
@@ -24,4 +24,17 @@ test('prettyMilliseconds', () => {
   assert.is(prettyMilliseconds(minute(5) + second(8) + 999), '5m 8s 999ms')
 })
 
+test('languageForFileName', () => {
+  assert.is(languageForFileName('index.ts'), 'TypeScript')
+  assert.is(languageForFileName('index.mts'), 'TypeScript')
+  assert.is(languageForFileName('index.cts'), 'TypeScript')
+  assert.is(languageForFileName('index.tsx'), 'TypeScriptReact')
+  assert.is(languageForFileName('index.js'), 'JavaScript')
+  assert.is(languageForFileName('index.mjs'), 'JavaScript')
+  assert.is(languageForFileName('index.cjs'), 'JavaScript')
+  assert.is(languageForFileName('index.jsx'), 'JavaScriptReact')
+  assert.is(languageForFileName('package.json'), 'JSON')
+  assert.is(languageForFileName('Component.svelte'), '')
+})
+
 test.run()
diff --git a/src/ProjectIndexer.ts b/src/ProjectIndexer.ts
index b31b6455..d73ebb2f 100644
--- a/src/ProjectIndexer.ts
+++ b/src/ProjectIndexer.ts
@@ -130,6 +130,7 @@ export class ProjectIndexer {
         }
       }
       const document = new scip.scip.Document({
+        language: languageForFileName(sourceFile.fileName),
         relative_path: path.relative(this.options.cwd, sourceFile.fileName),
         occurrences: [],
       })
@@ -171,6 +172,22 @@ export class ProjectIndexer {
   }
 }
 
+export function languageForFileName(fileName: string): string {
+  // Document.language uses the exact names from SCIP's Language enum, not the
+  // lowercase language identifiers used by editors.
+  const extension = path.extname(fileName).toLowerCase()
+  if (extension === '.tsx') return 'TypeScriptReact'
+  if (extension === '.ts' || extension === '.mts' || extension === '.cts') {
+    return 'TypeScript'
+  }
+  if (extension === '.jsx') return 'JavaScriptReact'
+  if (extension === '.js' || extension === '.mjs' || extension === '.cjs') {
+    return 'JavaScript'
+  }
+  if (extension === '.json') return 'JSON'
+  return ''
+}
+
 export function prettyMilliseconds(milliseconds: number): string {
   let ms = Math.floor(milliseconds)
   let result = ''
diff --git a/src/SnapshotTesting.ts b/src/SnapshotTesting.ts
index 89e5c300..da810e8e 100644
--- a/src/SnapshotTesting.ts
+++ b/src/SnapshotTesting.ts
@@ -61,7 +61,7 @@ export function formatSnapshot(
   doc: scip.Document,
   externalSymbols: scip.SymbolInformation[] = []
 ): string {
-  const out: string[] = []
+  const out: string[] = [`${commentSyntax} language ${doc.language}\n`]
   const symbolTable = getSymbolTable(doc)
 
   const externalSymbolTable: Map<string, scip.SymbolInformation> = new Map()
diff --git a/src/main.test.ts b/src/main.test.ts
index 23b8b263..d0d9a887 100644
--- a/src/main.test.ts
+++ b/src/main.test.ts
@@ -75,6 +75,10 @@ for (const snapshotDirectory of snapshotDirectories) {
       throw new Error('empty LSIF index')
     }
     for (const document of index.documents) {
+      assert.ok(
+        document.language,
+        `${document.relative_path} should have a SCIP document language`
+      )
       if (document.relative_path === 'src/symbol-kinds.ts') {
         assert.equal(
           document.symbols