diff --git a/.github/workflows/core-ci.yml b/.github/workflows/core-ci.yml
index 57daeefb..31ca91b2 100644
--- a/.github/workflows/core-ci.yml
+++ b/.github/workflows/core-ci.yml
@@ -50,7 +50,7 @@ jobs:
         if: steps.filter.outputs.code == 'true'
         uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0
         with:
-          bun-version: 1.3.14
+          bun-version: 1.4.0
 
       - name: Cache bun install
         if: steps.filter.outputs.code == 'true'
diff --git a/.github/workflows/docs-linkcheck.yml b/.github/workflows/docs-linkcheck.yml
index 3c135b45..d7f4cfcf 100644
--- a/.github/workflows/docs-linkcheck.yml
+++ b/.github/workflows/docs-linkcheck.yml
@@ -51,7 +51,7 @@ jobs:
         if: steps.filter.outputs.docs == 'true'
         uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0
         with:
-          bun-version: 1.3.14
+          bun-version: 1.4.0
 
       - name: Cache bun install
         if: steps.filter.outputs.docs == 'true'
diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index dcb37c0b..7bd4bff2 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -31,7 +31,7 @@ jobs:
       - name: Set up Bun
         uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0
         with:
-          bun-version: 1.3.14
+          bun-version: 1.4.0
 
       - name: Cache bun install
         uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
diff --git a/.github/workflows/security-deps.yml b/.github/workflows/security-deps.yml
index 4d505677..8b203abc 100644
--- a/.github/workflows/security-deps.yml
+++ b/.github/workflows/security-deps.yml
@@ -100,7 +100,7 @@ jobs:
         if: steps.filter.outputs.code == 'true'
         uses: oven-sh/setup-bun@0c5077e51419868618aeaa5fe8019c62421857d6 # v2.2.0
         with:
-          bun-version: 1.3.14
+          bun-version: 1.4.0
 
       - name: bun audit
         if: steps.filter.outputs.code == 'true'
diff --git a/package.json b/package.json
index 6bf6209a..89920da0 100644
--- a/package.json
+++ b/package.json
@@ -36,7 +36,7 @@
   "packageManager": "bun@1.3.14",
   "engines": {
     "node": ">=24.0.0",
-    "bun": "1.3.14"
+    "bun": "1.4.0"
   },
   "devDependencies": {
     "@ast-grep/cli": "^0.45.0",
diff --git a/packages/core/package.json b/packages/core/package.json
index ecb2f293..50eefa6f 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -23,7 +23,7 @@
     "strict.type-aware.eslint.config.mjs"
   ],
   "engines": {
-    "bun": ">=1.3.14"
+    "bun": ">=1.4.0"
   },
   "publishConfig": {
     "access": "public"
diff --git a/packages/core/tests/edit-autoformat.e2e.test.ts b/packages/core/tests/edit-autoformat.e2e.test.ts
index 6bb14440..16c74c64 100644
--- a/packages/core/tests/edit-autoformat.e2e.test.ts
+++ b/packages/core/tests/edit-autoformat.e2e.test.ts
@@ -39,4 +39,4 @@ test("a pre-format edit anchor survives the real write-guard auto-format", async
   } finally {
     await rm(dir, { recursive: true, force: true });
   }
-});
+}, 15_000);
diff --git a/packages/core/tests/script-tool.test.ts b/packages/core/tests/script-tool.test.ts
index bde59b6c..b190f8f0 100644
--- a/packages/core/tests/script-tool.test.ts
+++ b/packages/core/tests/script-tool.test.ts
@@ -394,8 +394,13 @@ test("the output drain is bounded when a backgrounded child holds the pipe open"
     // stdout pipe) then exits immediately, so `bun` exits fast while the orphan
     // keeps the pipe open. An unbounded drain (`Response(stdout).text()`) would
     // block the whole 5s waiting for EOF; the shared runner bounds it.
+    // Fire-and-forget async `Bun.spawn` here, NOT `Bun.spawnSync`: Bun 1.4 made
+    // `spawnSync` with inherited stdio wait for everything sharing that stdio
+    // (including this backgrounded job) before returning, which defeated the
+    // simulation — the script itself would block for the full 5s. `Bun.spawn`
+    // launches it and returns immediately, matching a real shell `&`.
     const code = [
-      `Bun.spawnSync(["sh", "-c", "sleep 5 &"], { stdout: "inherit" });`,
+      `Bun.spawn(["sh", "-c", "sleep 5 &"], { stdout: "inherit" });`,
       'console.log("SCRIPT_DONE");',
     ].join("\n");
     const events: ILoopEvent[] = [];