fix(lsp): fall back to downloaded dart sdk when system dart cannot run

packages/lsp/src/server/dart.ts

@@ -111,13 +111,47 @@ async function setupDartDependencies(platformId: PlatformId): Promise<string | u
   }
 }
 
+const DART_PROBE_TIMEOUT_MS = 10_000
+
+/**
+ * Find a system Dart that actually runs. A version-manager shim (e.g. mise) can resolve
+ * on PATH while exiting non-zero because no SDK version is configured.
+ */
+export async function findSystemDart(root: string): Promise<string | undefined> {
+  // Bun.which defaults to the PATH captured at startup; follow the current environment
+  const systemDart = Bun.which('dart', { PATH: process.env.PATH ?? '' })
+  if (!systemDart) {
+    return undefined
+  }
+
+  try {
+    // Run in the project root: version managers resolve the SDK per directory
+    const proc = Bun.spawn([systemDart, '--version'], {
+      cwd: root,
+      stdout: 'ignore',
+      stderr: 'ignore',
+      timeout: DART_PROBE_TIMEOUT_MS,
+    })
+    const exitCode = await proc.exited
+    if (exitCode !== 0) {
+      log.warn({ systemDart, exitCode, signal: proc.signalCode }, 'System Dart failed --version, trying auto-download')
+      return undefined
+    }
+  }
+  catch (err) {
+    log.warn({ systemDart, err }, 'Failed to spawn system Dart for version check, trying auto-download')
+    return undefined
+  }
+
+  return systemDart
+}
+
 export const DartServer: LSPServerInfo = {
   id: 'dart',
   extensions: ['.dart'],
   root: nearestRoot(['pubspec.yaml', 'pubspec.lock']),
   async spawn(root) {
-    // Try system dart first
-    const systemDart = Bun.which('dart')
+    const systemDart = await findSystemDart(root)
     if (systemDart) {
       try {
         const proc = spawn(systemDart, ['language-server', '--client-id', 'dora.dart', '--client-version', '1.0'], {

packages/lsp/test/unit/server.test.ts

@@ -18,6 +18,7 @@ import {
   TypescriptServer,
   VueServer,
 } from '../../src/server'
+import { findSystemDart } from '../../src/server/dart'
 
 describe('LSP_SERVERS', () => {
   test('contains expected servers', () => {
@@ -398,6 +399,48 @@ describe('DartServer', () => {
       await fs.rm(tempDir, { recursive: true, force: true })
     }
   })
+
+  describe.skipIf(process.platform === 'win32')('findSystemDart', () => {
+    async function findSystemDartWith(dartScript: string, rootFiles: string[] = []): Promise<{ result: string | undefined, fakeDart: string }> {
+      const binDir = await fs.mkdtemp(path.join(os.tmpdir(), 'dart-bin-'))
+      const rootDir = await fs.mkdtemp(path.join(os.tmpdir(), 'dart-root-'))
+      const savedPath = process.env.PATH
+      try {
+        const fakeDart = path.join(binDir, 'dart')
+        await fs.writeFile(fakeDart, dartScript, { mode: 0o755 })
+        for (const file of rootFiles) {
+          await fs.writeFile(path.join(rootDir, file), '')
+        }
+        process.env.PATH = binDir
+        return { result: await findSystemDart(rootDir), fakeDart }
+      }
+      finally {
+        process.env.PATH = savedPath
+        await fs.rm(binDir, { recursive: true, force: true })
+        await fs.rm(rootDir, { recursive: true, force: true })
+      }
+    }
+
+    test('ignores a dart on PATH that fails --version', async () => {
+      const { result } = await findSystemDartWith('#!/bin/sh\necho "No version is set for shim: dart" >&2\nexit 1\n')
+      expect(result).toBeUndefined()
+    })
+
+    test('ignores a dart on PATH that cannot be spawned', async () => {
+      const { result } = await findSystemDartWith('\x7FELF not a real binary')
+      expect(result).toBeUndefined()
+    })
+
+    test('returns a dart on PATH that runs --version', async () => {
+      const { result, fakeDart } = await findSystemDartWith('#!/bin/sh\nexit 0\n')
+      expect(result).toBe(fakeDart)
+    })
+
+    test('probes --version in the project root', async () => {
+      const { result, fakeDart } = await findSystemDartWith('#!/bin/sh\n[ -f .tool-versions ] || exit 1\n', ['.tool-versions'])
+      expect(result).toBe(fakeDart)
+    })
+  })
 })
 
 describe('VueServer', () => {