diff --git a/client/src/extension.ts b/client/src/extension.ts
index 666445c4..65c5ce91 100644
--- a/client/src/extension.ts
+++ b/client/src/extension.ts
@@ -153,6 +153,10 @@ async function handleError({ message, actions }: { message: string; actions: Act
 }
 
 export async function activate(context: vscode.ExtensionContext, launchOptions: LaunchOptions = defaultLaunchOptions) {
+  if (!isProjectMode()) {
+    vscode.window.showWarningMessage(USER_MESSAGE.SINGLE_FILE_MODE())
+  }
+
   // activate state
   initialiseState(context)
 
@@ -205,50 +209,64 @@ export async function activate(context: vscode.ExtensionContext, launchOptions:
   // While most other commands can be awaited directly, this is useful for stuff like file creation, which indirectely triggers an asynchronous job.
   registerCommand('flix.allJobsFinished', handlers.allJobsFinished(client, eventEmitter))
 
-  // watch for changes on the file system (delete, create, rename .flix files)
-  flixWatcher = vscode.workspace.createFileSystemWatcher(getFlixGlobPattern())
-  flixWatcher.onDidDelete((vsCodeUri: vscode.Uri) => {
-    const uri = vsCodeUriToUriString(vsCodeUri)
-    client.sendNotification(jobs.Request.apiRemUri, { uri })
-  })
-  flixWatcher.onDidCreate((vsCodeUri: vscode.Uri) => {
-    const uri = vsCodeUriToUriString(vsCodeUri)
-    client.sendNotification(jobs.Request.apiAddUri, { uri })
-  })
+  if (isProjectMode()) {
+    // In project mode, watch the file system for .flix/.fpkg/.jar/flix.toml changes.
 
-  // watch for changes on the file system (delete, create .fpkg files)
-  pkgWatcher = vscode.workspace.createFileSystemWatcher(getFpkgGlobPattern())
-  pkgWatcher.onDidDelete((vsCodeUri: vscode.Uri) => {
-    const uri = vsCodeUriToUriString(vsCodeUri)
-    client.sendNotification(jobs.Request.apiRemPkg, { uri })
-  })
-  pkgWatcher.onDidCreate((vsCodeUri: vscode.Uri) => {
-    const uri = vsCodeUriToUriString(vsCodeUri)
-    client.sendNotification(jobs.Request.apiAddPkg, { uri })
-  })
+    flixWatcher = vscode.workspace.createFileSystemWatcher(getFlixGlobPattern())
+    flixWatcher.onDidDelete((vsCodeUri: vscode.Uri) => {
+      const uri = vsCodeUriToUriString(vsCodeUri)
+      client.sendNotification(jobs.Request.apiRemUri, { uri })
+    })
+    flixWatcher.onDidCreate((vsCodeUri: vscode.Uri) => {
+      const uri = vsCodeUriToUriString(vsCodeUri)
+      client.sendNotification(jobs.Request.apiAddUri, { uri })
+    })
 
-  // watch for changes on the file system (delete, create .jar files)
-  pkgWatcher = vscode.workspace.createFileSystemWatcher(getJarGlobPattern())
-  pkgWatcher.onDidDelete((vsCodeUri: vscode.Uri) => {
-    const uri = vsCodeUriToUriString(vsCodeUri)
-    client.sendNotification(jobs.Request.apiRemJar, { uri })
-  })
-  pkgWatcher.onDidCreate((vsCodeUri: vscode.Uri) => {
-    const uri = vsCodeUriToUriString(vsCodeUri)
-    client.sendNotification(jobs.Request.apiAddJar, { uri })
-  })
+    pkgWatcher = vscode.workspace.createFileSystemWatcher(getFpkgGlobPattern())
+    pkgWatcher.onDidDelete((vsCodeUri: vscode.Uri) => {
+      const uri = vsCodeUriToUriString(vsCodeUri)
+      client.sendNotification(jobs.Request.apiRemPkg, { uri })
+    })
+    pkgWatcher.onDidCreate((vsCodeUri: vscode.Uri) => {
+      const uri = vsCodeUriToUriString(vsCodeUri)
+      client.sendNotification(jobs.Request.apiAddPkg, { uri })
+    })
 
-  // watch for changes to the flix.toml file
-  tomlWatcher = vscode.workspace.createFileSystemWatcher(getFlixTomlGlobPattern())
-  tomlWatcher.onDidChange(() => {
-    const { msg, option1, option2 } = USER_MESSAGE.ASK_RELOAD_TOML()
-    const doReload = vscode.window.showInformationMessage(msg, option1, option2)
-    doReload.then(res => {
-      if (res === 'Yes') {
-        makeHandleRestartClient(context, launchOptions)()
+    pkgWatcher = vscode.workspace.createFileSystemWatcher(getJarGlobPattern())
+    pkgWatcher.onDidDelete((vsCodeUri: vscode.Uri) => {
+      const uri = vsCodeUriToUriString(vsCodeUri)
+      client.sendNotification(jobs.Request.apiRemJar, { uri })
+    })
+    pkgWatcher.onDidCreate((vsCodeUri: vscode.Uri) => {
+      const uri = vsCodeUriToUriString(vsCodeUri)
+      client.sendNotification(jobs.Request.apiAddJar, { uri })
+    })
+
+    tomlWatcher = vscode.workspace.createFileSystemWatcher(getFlixTomlGlobPattern())
+    tomlWatcher.onDidChange(() => {
+      const { msg, option1, option2 } = USER_MESSAGE.ASK_RELOAD_TOML()
+      const doReload = vscode.window.showInformationMessage(msg, option1, option2)
+      doReload.then(res => {
+        if (res === 'Yes') {
+          makeHandleRestartClient(context, launchOptions)()
+        }
+      })
+    })
+  } else {
+    // In single-file mode there is no workspace folder to watch.
+    // Instead, track document open/close to add/remove .flix files from the compiler.
+    // Content changes are already handled by the LSP TextDocumentSync mechanism.
+    vscode.workspace.onDidOpenTextDocument(doc => {
+      if (doc.uri.path.endsWith('.flix')) {
+        client.sendNotification(jobs.Request.apiAddUri, { uri: vsCodeUriToUriString(doc.uri) })
       }
     })
-  })
+    vscode.workspace.onDidCloseTextDocument(doc => {
+      if (doc.uri.path.endsWith('.flix')) {
+        client.sendNotification(jobs.Request.apiRemUri, { uri: vsCodeUriToUriString(doc.uri) })
+      }
+    })
+  }
 
   vscode.window.onDidChangeActiveTextEditor(handlers.handleChangeEditor)
   vscode.workspace.onDidChangeConfiguration(() => {
@@ -274,9 +292,23 @@ async function startSession(
 
   const globalStoragePath = context.globalStorageUri.fsPath
   const workspaceFolders = vscode.workspace.workspaceFolders?.map(ws => ws.uri.fsPath)
-  const workspaceFiles = (await vscode.workspace.findFiles(getFlixGlobPattern())).map(vsCodeUriToUriString)
-  const workspacePkgs = (await vscode.workspace.findFiles(getFpkgGlobPattern())).map(vsCodeUriToUriString)
-  const workspaceJars = (await vscode.workspace.findFiles(getJarGlobPattern())).map(vsCodeUriToUriString)
+
+  // In project mode, discover files via workspace glob patterns.
+  // In single-file mode, use the currently open .flix documents instead.
+  let workspaceFiles: string[]
+  let workspacePkgs: string[]
+  let workspaceJars: string[]
+  if (isProjectMode()) {
+    workspaceFiles = (await vscode.workspace.findFiles(getFlixGlobPattern())).map(vsCodeUriToUriString)
+    workspacePkgs = (await vscode.workspace.findFiles(getFpkgGlobPattern())).map(vsCodeUriToUriString)
+    workspaceJars = (await vscode.workspace.findFiles(getJarGlobPattern())).map(vsCodeUriToUriString)
+  } else {
+    workspaceFiles = vscode.workspace.textDocuments
+      .filter(doc => doc.uri.path.endsWith('.flix'))
+      .map(doc => vsCodeUriToUriString(doc.uri))
+    workspacePkgs = []
+    workspaceJars = []
+  }
 
   // Wait until we're sure flix exists
   const flixFilename = await ensureFlixExists({
diff --git a/client/src/handlers/handlers.ts b/client/src/handlers/handlers.ts
index 28a6cfbb..2fdc534d 100644
--- a/client/src/handlers/handlers.ts
+++ b/client/src/handlers/handlers.ts
@@ -1,10 +1,11 @@
+import * as path from 'path'
 import * as vscode from 'vscode'
 import { LanguageClient } from 'vscode-languageclient/node'
 import { EventEmitter } from 'events'
 
 import * as jobs from '../engine/jobs'
 import ensureFlixExists from './../util/ensureFlixExists'
-import { LaunchOptions, defaultLaunchOptions, getFlixGlobPattern } from './../extension'
+import { LaunchOptions, defaultLaunchOptions, getFlixGlobPattern, isProjectMode } from './../extension'
 import { USER_MESSAGE } from '../util/userMessages'
 
 let flixTerminal: vscode.Terminal | null = null
@@ -57,10 +58,17 @@ async function launchRepl(context: vscode.ExtensionContext, launchOptions: Launc
   const { cmd, args } = await getJvmCmd(context, launchOptions)
   args.push('repl')
   args.push(...getExtraFlixArgs())
+
+  // In single-file mode, set cwd to the active file's directory so the REPL
+  // can find the file. In project mode, VS Code defaults to the workspace root.
+  const activeFilePath = vscode.window.activeTextEditor?.document.uri.fsPath
+  const cwd = !isProjectMode() && activeFilePath ? path.dirname(activeFilePath) : undefined
+
   flixTerminal = vscode.window.createTerminal({
     name: 'Flix REPL',
     shellPath: cmd,
     shellArgs: args,
+    cwd,
 
     // The terminal will not be kept alive when restarting VSCode.
     // This is necessary in the case where flix.jar has been removed while VSCode has been closed.
@@ -155,6 +163,11 @@ export function handleChangeEditor(editor: vscode.TextEditor | undefined) {
     return
   }
 
+  // In single-file mode every .flix file is valid — there is no project boundary.
+  if (!isProjectMode()) {
+    return
+  }
+
   const included = vscode.languages.match({ pattern: getFlixGlobPattern() }, editor.document)
   if (!included) {
     vscode.window.showWarningMessage(USER_MESSAGE.FILE_NOT_PART_OF_PROJECT())
diff --git a/client/src/util/userMessages.ts b/client/src/util/userMessages.ts
index 0cc3fe65..4db01cd4 100644
--- a/client/src/util/userMessages.ts
+++ b/client/src/util/userMessages.ts
@@ -86,4 +86,8 @@ export class USER_MESSAGE {
   static FILE_NOT_PART_OF_PROJECT() {
     return `Flix will only load source files from \`*.flix\`, \`src/**\`, and \`test/**\``
   }
+
+  static SINGLE_FILE_MODE() {
+    return 'Running in single-file mode. Open a folder for full project support.'
+  }
 }