diff --git a/package.json b/package.json
index 4366e644..c1e41b2e 100644
--- a/package.json
+++ b/package.json
@@ -43,9 +43,9 @@
     "bench": "tsx bin/benchmark.ts",
     "scenario": "tsx bin/scenario.ts",
     "eval:structural-extract": "tsx bin/structuralExtractEval.ts",
-    "pretest:jest": "npm run build:info",
-    "test:jest": "jest",
-    "test:jest:watch": "jest --watch",
+    "pretest:jest": "npm run build:info && node bin/copyTreeSitterWasm.mjs",
+    "test:jest": "cross-env NODE_OPTIONS=--experimental-vm-modules jest",
+    "test:jest:watch": "cross-env NODE_OPTIONS=--experimental-vm-modules jest --watch",
     "release": "release-it",
     "release:dry-run": "release-it --ci --dry-run --no-git --no-github.release --no-github.publish --no-npm.publish",
     "wiki:clone": "tsx bin/wiki.ts clone",
@@ -73,6 +73,7 @@
     "@types/yargs": "^17.0.33",
     "@typescript-eslint/eslint-plugin": "^7.13.1",
     "@typescript-eslint/parser": "^7.13.1",
+    "cross-env": "^10.1.0",
     "eslint": "^8.54.0",
     "ink-testing-library": "4.0.0",
     "jest": "^30.0.5",
diff --git a/src/lib/parsers/default/__evals__/fixtures.ts b/src/lib/parsers/default/__evals__/fixtures.ts
index a3687fc6..b9de4079 100644
--- a/src/lib/parsers/default/__evals__/fixtures.ts
+++ b/src/lib/parsers/default/__evals__/fixtures.ts
@@ -75,6 +75,33 @@ export const evalFixtures: readonly EvalFixture[] = [
       ].join('\n')),
     ],
   },
+  {
+    name: 'ts-arrow-fn-export',
+    // Marquee tree-sitter advantage case (#933 phase 1.1 + 2).
+    //
+    // The regex parser sees `export const handler = (...)` and
+    // classifies it as a `const` declaration, surfacing the symbol
+    // as `const handler` in the summary. Tree-sitter inspects the
+    // declarator's value, recognizes the `arrow_function` node,
+    // and classifies the binding as a function — so the same diff
+    // surfaces as `handler()` instead.
+    //
+    // Both parsers fire the fast path (so the LLM-call-saved metric
+    // is identical in mock-mode), but the qualitative output
+    // differs. Run the eval with the tree-sitter wasms present vs.
+    // absent (e.g. delete `dist/tree-sitter/` and re-run) to see the
+    // diff in the per-fixture JSON.
+    description: 'TS module replaces a function declaration with an arrow-function const — regex outputs `const handler`, tree-sitter outputs `handler()`.',
+    diffs: [
+      buildDiff('src/api.ts', [
+        '@@ -1,5 +1,5 @@',
+        ' import { Request, Response } from "./types"',
+        ' ',
+        '-export function handler(req: Request): Response { return process(req) }',
+        '+export const handler = (req: Request): Response => process(req)',
+      ].join('\n')),
+    ],
+  },
   {
     name: 'python-class-method-add',
     description: 'Python module replaces a legacy helper with a class + factory.',
diff --git a/src/lib/parsers/default/__tree_sitter__/runtime.ts b/src/lib/parsers/default/__tree_sitter__/runtime.ts
index 1c427cae..472af6cf 100644
--- a/src/lib/parsers/default/__tree_sitter__/runtime.ts
+++ b/src/lib/parsers/default/__tree_sitter__/runtime.ts
@@ -123,7 +123,17 @@ export type TreeSitterRuntime = {
 }
 
 async function ensureRuntime(): Promise<TreeSitterRuntime | undefined> {
-  if (initPromise) return initPromise
+  // If an init is already in flight OR completed successfully, reuse it.
+  // We deliberately do NOT cache `undefined` results: a transient failure
+  // (e.g. jest tearing down its environment mid-dynamic-import in a
+  // sibling test file) shouldn't poison the cache for the whole process.
+  // Each call that finds no successful cached runtime tries again,
+  // paying the init cost at most one extra time per failure.
+  if (initPromise) {
+    const cached = await initPromise
+    if (cached) return cached
+    // Fall through and retry — the previous attempt surrendered.
+  }
   initPromise = (async () => {
     const locations = resolveWasmLocations()
     if (!locations) return undefined
diff --git a/src/lib/parsers/default/__tree_sitter__/tsTreeSitterParser.test.ts b/src/lib/parsers/default/__tree_sitter__/tsTreeSitterParser.test.ts
index 9e08c123..2a44015c 100644
--- a/src/lib/parsers/default/__tree_sitter__/tsTreeSitterParser.test.ts
+++ b/src/lib/parsers/default/__tree_sitter__/tsTreeSitterParser.test.ts
@@ -2,45 +2,40 @@ import { existsSync } from 'node:fs'
 import { join } from 'node:path'
 import type { FileDiff } from '../../../types'
 import { treeSitterTsParser } from './tsTreeSitterParser'
-import { _resetTreeSitterRuntimeForTesting } from './runtime'
 
 function fileDiff(file: string, diff: string): FileDiff {
   return { file, diff, summary: '', tokenCount: Math.ceil(diff.length / 4) }
 }
 
-// Tree-sitter integration tests require TWO things to actually exercise
-// the .wasm code path:
-//   1. The .wasm files copied into `dist/tree-sitter/` by the postbuild
-//      script (only present after `npm run build`).
-//   2. Jest running with `NODE_OPTIONS=--experimental-vm-modules` so the
-//      `web-tree-sitter` package (`"type": "module"`) can be loaded via
-//      the dynamic-import shim in `runtime.ts`. Without the flag, Jest
-//      throws "A dynamic import callback was invoked without
-//      --experimental-vm-modules" and the runtime surrenders.
+// Tree-sitter integration tests need the .wasm files copied to
+// `dist/tree-sitter/` to actually exercise the code path. The
+// `pretest:jest` npm script handles that automatically by running the
+// postbuild copy step. ESM dynamic import (required to load
+// `web-tree-sitter`, which is `"type": "module"`) works out of the box
+// because `test:jest` runs with `NODE_OPTIONS=--experimental-vm-modules`
+// (#933 phase 2).
 //
-// When either is missing, the suite skips itself — the production code
-// path (regex fallback) is still tested via the registry-level tests in
-// `structuralParserRegistry.test.ts`, and end-to-end tree-sitter
-// validation runs via the eval harness CLI (#934), which executes under
-// vanilla Node with full ESM support and is the right tool for
-// integration verification anyway.
-//
-// Opt in locally with:
-//   NODE_OPTIONS=--experimental-vm-modules COCO_TEST_TREE_SITTER=1 npx jest ...
+// The wasm-available check stays as a defensive guard: if someone
+// invokes `npx jest` directly without running `pretest:jest` first, the
+// wasms won't be there. Skipping in that case keeps the runner honest
+// about what was actually exercised, and the production code path
+// (regex fallback) is still tested via the registry-level tests in
+// `structuralParserRegistry.test.ts`.
 const wasmDir = join(__dirname, '..', '..', '..', '..', '..', 'dist', 'tree-sitter')
 const wasmAvailable = existsSync(join(wasmDir, 'web-tree-sitter.wasm')) &&
   existsSync(join(wasmDir, 'tree-sitter-typescript.wasm')) &&
   existsSync(join(wasmDir, 'tree-sitter-tsx.wasm'))
-const esmEnabled = process.env.COCO_TEST_TREE_SITTER === '1'
 
-const describeWithWasm = (wasmAvailable && esmEnabled) ? describe : describe.skip
+const describeWithWasm = wasmAvailable ? describe : describe.skip
 
 describeWithWasm('treeSitterTsParser (.wasm-backed)', () => {
-  beforeEach(() => {
-    // Each test gets a fresh runtime so we exercise the init path
-    // at least once and don't rely on inter-test cache state.
-    _resetTreeSitterRuntimeForTesting()
-  })
+  // Note: deliberately NO `beforeEach` reset of the runtime. The runtime
+  // is designed as process-lifetime (one init + one parser cache for the
+  // whole run), and resetting between tests has caused flakes under
+  // jest + ESM dynamic imports — a previous test file's teardown could
+  // race the init promise. The init failure path is now retry-friendly
+  // (see `ensureRuntime` in runtime.ts), and we trust the first test in
+  // this suite to exercise the init path cleanly.
 
   it('returns undefined for non-TS / non-JS file paths', async () => {
     expect(await treeSitterTsParser.summarize(fileDiff('README.md', '+x'))).toBeUndefined()
diff --git a/yarn.lock b/yarn.lock
index 7afe9df8..e7f4673e 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -486,6 +486,11 @@
   dependencies:
     tslib "^2.4.0"
 
+"@epic-web/invariant@^1.0.0":
+  version "1.0.0"
+  resolved "https://registry.yarnpkg.com/@epic-web/invariant/-/invariant-1.0.0.tgz#1073e5dee6dd540410784990eb73e4acd25c9813"
+  integrity sha512-lrTPqgvfFQtR/eY/qkIzp98OGdNJu0m5ji3q/nJI8v3SXkRKEnWiOxMmbvcSoAIzv/cGiuvRy57k4suKQSAdwA==
+
 "@esbuild/aix-ppc64@0.27.3":
   version "0.27.3"
   resolved "https://registry.yarnpkg.com/@esbuild/aix-ppc64/-/aix-ppc64-0.27.3.tgz#815b39267f9bffd3407ea6c376ac32946e24f8d2"
@@ -2455,6 +2460,14 @@ create-require@^1.1.0:
   resolved "https://registry.npmjs.org/create-require/-/create-require-1.1.1.tgz"
   integrity sha512-dcKFX3jn0MpIaXjisoRvexIJVEKzaq7z2rZKxf+MSr9TkdmHmsU4m2lcLojrj/FHl8mk5VxMmYA+ftRkP/3oKQ==
 
+cross-env@^10.1.0:
+  version "10.1.0"
+  resolved "https://registry.yarnpkg.com/cross-env/-/cross-env-10.1.0.tgz#cfd2a6200df9ed75bfb9cb3d7ce609c13ea21783"
+  integrity sha512-GsYosgnACZTADcmEyJctkJIoqAhHjttw7RsFrVoJNXbsWWqaq6Ym+7kZjq6mS45O0jij6vtiReppKQEtqWy6Dw==
+  dependencies:
+    "@epic-web/invariant" "^1.0.0"
+    cross-spawn "^7.0.6"
+
 cross-spawn@7.0.6, cross-spawn@^7.0.2, cross-spawn@^7.0.3, cross-spawn@^7.0.6:
   version "7.0.6"
   resolved "https://registry.yarnpkg.com/cross-spawn/-/cross-spawn-7.0.6.tgz#8a58fe78f00dcd70c370451759dfbfaf03e8ee9f"