diff --git a/bunfig.toml b/bunfig.toml
index 00cbc1231e..318845b44a 100644
--- a/bunfig.toml
+++ b/bunfig.toml
@@ -5,6 +5,7 @@
 # so a bare `bun test` — or `bun test tests/` (a substring filter that also matches
 # devlog/opencode-cursor/tests/) — drags them in and reports hundreds of spurious failures.
 # `root` pins discovery to ./tests so every invocation stays on the real suite.
+# File-level `--parallel` has no bunfig key; `scripts/test.ts` passes it for `bun run test`.
 # The npm script already uses `bun test ./tests/`; this makes a bare `bun test` behave the same.
 [test]
 root = "tests"
diff --git a/scripts/test.ts b/scripts/test.ts
index 5297a17722..41d141b7fd 100644
--- a/scripts/test.ts
+++ b/scripts/test.ts
@@ -59,6 +59,120 @@ export function createIsolatedTestEnvironment(
   };
 }
 
+function hasCliFlag(requested: string[], name: string): boolean {
+  const delimiterIndex = requested.indexOf("--");
+  const wrapperArgs = delimiterIndex === -1 ? requested : requested.slice(0, delimiterIndex);
+  return wrapperArgs.some(arg => arg === name || arg.startsWith(`${name}=`));
+}
+
+// Bun 1.4.0 builds `bun test` options from its test, runtime, transpiler, and base tables.
+// Only required values consume the next argument. Optional values such as `--parallel=2`
+// must stay attached so a bare option cannot hide the positional filter that follows it.
+const BUN_TEST_OPTIONS_REQUIRING_VALUES = new Set([
+  // Test options.
+  "--timeout",
+  "--rerun-each",
+  "--retry",
+  "--seed",
+  "--coverage-reporter",
+  "--coverage-dir",
+  "-t",
+  "--test-name-pattern",
+  "--grep",
+  "--reporter",
+  "--reporter-outfile",
+  "--max-concurrency",
+  "--path-ignore-patterns",
+  "--parallel-delay",
+  "--shard",
+  "--timings",
+  // Runtime options accepted by `bun test`.
+  "--watch-kill-signal",
+  "-r",
+  "--preload",
+  "--require",
+  "--import",
+  "--cpu-prof-name",
+  "--cpu-prof-dir",
+  "--cpu-prof-interval",
+  "--heap-prof-name",
+  "--heap-prof-dir",
+  "--heap-prof-interval",
+  "--install",
+  "-e",
+  "--eval",
+  "-p",
+  "--print",
+  "--port",
+  "--origin",
+  "--conditions",
+  "--fetch-preconnect",
+  "--max-http-header-size",
+  "--dns-result-order",
+  "--redirect-warnings",
+  "--disable-warning",
+  "--title",
+  "--unhandled-rejections",
+  "--console-depth",
+  "--user-agent",
+  "--cron-title",
+  "--cron-period",
+  "--trace-event-categories",
+  "--trace-event-file-pattern",
+  "--stack-trace-limit",
+  // Transpiler and base options accepted by `bun test`.
+  "--main-fields",
+  "--extension-order",
+  "--tsconfig-override",
+  "-d",
+  "--define",
+  "--drop",
+  "--feature",
+  "-l",
+  "--loader",
+  "--jsx-factory",
+  "--jsx-fragment",
+  "--jsx-import-source",
+  "--jsx-runtime",
+  "--env-file",
+  "--cwd",
+  "-c",
+  "--config",
+]);
+
+/** True for a filter-less `bun run test`. `--timeout` / `--dots` / `--parallel=N` still count. */
+function isFullSuiteRun(requested: string[]): boolean {
+  const delimiterIndex = requested.indexOf("--");
+  const wrapperArgs = delimiterIndex === -1 ? requested : requested.slice(0, delimiterIndex);
+  const passedThrough = delimiterIndex === -1 ? [] : requested.slice(delimiterIndex + 1);
+  if (passedThrough.length > 0) return false;
+
+  for (let index = 0; index < wrapperArgs.length; index++) {
+    const arg = wrapperArgs[index];
+    if (arg === "-" || !arg.startsWith("-")) return false;
+    if (!arg.includes("=") && BUN_TEST_OPTIONS_REQUIRING_VALUES.has(arg)) index++;
+  }
+  return true;
+}
+
+/**
+ * Default `bun test` argv for this repo.
+ *
+ * `--isolate` keeps a fresh global per file, and is the substring the exclusive-run pgrep
+ * matches. `--parallel` is what makes the suite finishable: with isolate alone Bun re-evaluates
+ * the module graph once per file on a single core, so past ~900 files the run stops looking slow
+ * and starts looking hung — measured here at 1 h 29 m with zero output, ~57 % CPU and 8.5 MB RSS,
+ * against ~110-190 s for the identical suite with `--parallel`. A caller-supplied `--parallel=N`
+ * is left alone.
+ */
+export function resolveBunTestArgs(requested: string[]): string[] {
+  const args = ["--isolate"];
+  if (!hasCliFlag(requested, "--parallel")) args.push("--parallel");
+  args.push(...requested);
+  if (isFullSuiteRun(requested)) args.push("./tests/");
+  return args;
+}
+
 /**
  * Other `bun test` runners already on this machine.
  *
@@ -141,7 +255,7 @@ if (import.meta.main) {
     await waitForExclusiveRun(process.pid);
     const startedAt = Date.now();
     const child = Bun.spawnSync(
-      [process.execPath, "test", "--isolate", ...(requestedTests.length > 0 ? requestedTests : ["./tests/"])],
+      [process.execPath, "test", ...resolveBunTestArgs(requestedTests)],
       {
         env: isolated.env,
         stdin: "inherit",
@@ -152,7 +266,7 @@ if (import.meta.main) {
     const elapsedSeconds = Math.round((Date.now() - startedAt) / 1000);
     if (requestedTests.length === 0 && elapsedSeconds > 600) {
       console.warn(
-        `[test] the suite took ${elapsedSeconds}s; it normally runs in about 210s on an idle machine. `
+        `[test] the suite took ${elapsedSeconds}s; with --parallel it should finish in a few minutes on an idle machine. `
         + "Check for another test runner, a busy CPU, or a test that started polling something real.",
       );
     }
diff --git a/tests/test-runner.test.ts b/tests/test-runner.test.ts
index ef48654f15..408d5dcf89 100644
--- a/tests/test-runner.test.ts
+++ b/tests/test-runner.test.ts
@@ -1,7 +1,8 @@
 import { describe, expect, test } from "bun:test";
-import { existsSync } from "node:fs";
+import { existsSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
 import { isAbsolute, join } from "node:path";
-import { createIsolatedTestEnvironment } from "../scripts/test";
+import { createIsolatedTestEnvironment, resolveBunTestArgs } from "../scripts/test";
 import {
   decodeWindowsIdentityPowerShellOutputForTests,
   windowsIdentityPowerShellCommandForTests,
@@ -68,3 +69,96 @@ describe("test runner isolation", () => {
     },
   );
 });
+
+/**
+ * Without `--parallel`, `--isolate` re-evaluates the module graph once per file on a single
+ * core. Past ~900 files that stops reading as slow and starts reading as hung: measured at
+ * 1 h 29 m with zero output, ~57 % CPU and 8.5 MB RSS, against ~110-190 s for the identical
+ * suite with the flag. These pin the argv so the flag cannot be dropped again silently.
+ */
+describe("bun test argv", () => {
+  test("a filter-less run gets isolate, parallel and the suite path", () => {
+    expect(resolveBunTestArgs([])).toEqual(["--isolate", "--parallel", "./tests/"]);
+  });
+
+  test("a file filter keeps isolate and parallel but no suite path", () => {
+    expect(resolveBunTestArgs(["tests/foo.test.ts"]))
+      .toEqual(["--isolate", "--parallel", "tests/foo.test.ts"]);
+    expect(resolveBunTestArgs(["-"]))
+      .toEqual(["--isolate", "--parallel", "-"]);
+  });
+
+  test("a caller-supplied concurrency is left alone", () => {
+    expect(resolveBunTestArgs(["--parallel=2"]))
+      .toEqual(["--isolate", "--parallel=2", "./tests/"]);
+    expect(resolveBunTestArgs(["--parallel"]))
+      .toEqual(["--isolate", "--parallel", "./tests/"]);
+    expect(resolveBunTestArgs(["--parallel", "tests/foo.test.ts"]))
+      .toEqual(["--isolate", "--parallel", "tests/foo.test.ts"]);
+    expect(resolveBunTestArgs(["--parallel=2", "tests/foo.test.ts"]))
+      .toEqual(["--isolate", "--parallel=2", "tests/foo.test.ts"]);
+  });
+
+  test("option-only arguments still count as a full suite run", () => {
+    expect(resolveBunTestArgs(["--timeout=30000"]))
+      .toEqual(["--isolate", "--parallel", "--timeout=30000", "./tests/"]);
+    expect(resolveBunTestArgs(["--timeout", "30000"]))
+      .toEqual(["--isolate", "--parallel", "--timeout", "30000", "./tests/"]);
+    expect(resolveBunTestArgs(["--timeout", "30000", "tests/foo.test.ts"]))
+      .toEqual(["--isolate", "--parallel", "--timeout", "30000", "tests/foo.test.ts"]);
+    expect(resolveBunTestArgs(["--timings", ".bun-test-timings/current.json"]))
+      .toEqual([
+        "--isolate",
+        "--parallel",
+        "--timings",
+        ".bun-test-timings/current.json",
+        "./tests/",
+      ]);
+    for (const configFlag of ["-c", "--config"]) {
+      expect(resolveBunTestArgs([configFlag, "ci.bunfig.toml"]))
+        .toEqual(["--isolate", "--parallel", configFlag, "ci.bunfig.toml", "./tests/"]);
+    }
+    expect(resolveBunTestArgs(["-t", "serial test"])).toEqual([
+      "--isolate",
+      "--parallel",
+      "-t",
+      "serial test",
+      "./tests/",
+    ]);
+  });
+
+  test("arguments after the delimiter are passed through instead of parsed as wrapper flags", () => {
+    expect(resolveBunTestArgs(["--", "--parallel=2"]))
+      .toEqual(["--isolate", "--parallel", "--", "--parallel=2"]);
+  });
+
+  test("the wrapper passes parallel execution through to bun", () => {
+    const fixtureRoot = mkdtempSync(join(tmpdir(), "opencodex-test-runner-"));
+    const fixturePath = join(fixtureRoot, "parallel-smoke.test.ts");
+    const markerPath = join(fixtureRoot, "executed.marker");
+    writeFileSync(
+      fixturePath,
+      `import { test } from "bun:test"; import { writeFileSync } from "node:fs"; test("smoke", () => writeFileSync(${JSON.stringify(markerPath)}, "executed"));\n`,
+    );
+    try {
+      const result = Bun.spawnSync([
+        process.execPath,
+        join(import.meta.dir, "../scripts/test.ts"),
+        fixturePath,
+      ], {
+        cwd: join(import.meta.dir, ".."),
+        env: { ...process.env, OCX_TEST_NO_QUEUE: "1" },
+        stdout: "pipe",
+        stderr: "pipe",
+      });
+
+      const output = new TextDecoder().decode(result.stdout)
+        + new TextDecoder().decode(result.stderr);
+      expect(result.exitCode).toBe(0);
+      expect(output).toContain("PARALLEL");
+      expect(existsSync(markerPath)).toBe(true);
+    } finally {
+      rmSync(fixtureRoot, { recursive: true, force: true });
+    }
+  });
+});