diff --git a/README.md b/README.md
index aab74ef..a99082c 100644
--- a/README.md
+++ b/README.md
@@ -77,13 +77,23 @@ docker run --rm \
 
 - `-c, --config <configure-file>`：指定配置文件（默认 `./.lintmdrc`）
 - `-f, --fix`：自动修复可修复问题
-- `-t, --threads [thread-count]`：设置并发线程数；传 `auto` 时按文件大小自适应，默认取 CPU 核数
+- `-t, --threads [thread-count]`：设置并发线程数；默认 `auto` 按文件大小使用保守上限，传数字可显式覆盖自动限制
 - `-s, --suppress-warnings`：忽略 warning 对退出码的影响（便于 CI 渐进接入）
 - `-i, --stdin`：从标准输入读取 Markdown 内容
 - `--max-file-size <size>`：跳过超过指定大小的 Markdown 文件（如 `5mb`、`500kb`、`1gb`），并向 stderr 输出警告
 - `-d, --dev`：开发调试模式
 - `-v, --version`：查看版本
 
+未指定 `--threads` 时，程序等同于使用 `--threads auto`。`auto` 根据过滤后最大文件的大小设置 worker 上限：
+
+| 过滤后最大文件大小 | worker 上限 |
+| --- | ---: |
+| 小于 `1 MiB` | 4 |
+| 大于或等于 `1 MiB`，且小于 `5 MiB` | 2 |
+| 大于或等于 `5 MiB` | 1 |
+
+在 `auto` 模式下，实际 worker 数不会超过 CPU 并行度、文件数量和上述文件大小上限。显式传入 `--threads N` 会覆盖自动限制，但仍不会超过文件数量。
+
 ## 配置示例（`.lintmdrc`）
 
 ```json
diff --git a/__tests__/batch-lint.spec.ts b/__tests__/batch-lint.spec.ts
index 802dc7b..8835760 100644
--- a/__tests__/batch-lint.spec.ts
+++ b/__tests__/batch-lint.spec.ts
@@ -407,18 +407,18 @@ describe("resolveAdaptiveConcurrency", () => {
       });
     });
 
-    test("small files (< 1 MiB) use cpuLimit clamped to fileCount", async () => {
-      const files = await Promise.all([
-        writeSizedFile("a.md", 1024),
-        writeSizedFile("b.md", 2048),
-        writeSizedFile("c.md", 4096),
-      ]);
+    test("small files (< 1 MiB) cap concurrency at 4", async () => {
+      const files = await Promise.all(
+        Array.from({ length: 8 }, (_, index) =>
+          writeSizedFile(`small-${index}.md`, 4096)
+        )
+      );
       const cpuLimit = availableParallelism();
       const statSpy = jest.spyOn(require("fs/promises"), "stat");
 
       try {
         expect(await resolveAdaptiveConcurrency("auto", files, 4096)).toEqual({
-          concurrency: Math.min(cpuLimit, files.length),
+          concurrency: Math.min(cpuLimit, 4, files.length),
           maxFileSize: 4096,
           requestedConcurrency: cpuLimit,
         });
diff --git a/__tests__/configure.spec.ts b/__tests__/configure.spec.ts
index f3cf998..3ce7567 100644
--- a/__tests__/configure.spec.ts
+++ b/__tests__/configure.spec.ts
@@ -1,5 +1,5 @@
 import { mkdtempSync, rmSync, writeFileSync } from "fs";
-import { cpus, tmpdir } from "os";
+import { tmpdir } from "os";
 import * as path from "path";
 import { CliError } from "../src/cli/cli-error";
 import {
@@ -268,10 +268,10 @@ describe("configuration validation", () => {
     });
   });
 
-  test("uses the CPU count when threads are not specified", () => {
-    expect(getThreadCount(undefined)).toBe(cpus().length);
-    expect(getThreadCount(false)).toBe(cpus().length);
-    expect(getThreadCount(true)).toBe(cpus().length);
+  test("uses auto when threads are not specified", () => {
+    expect(getThreadCount(undefined)).toBe("auto");
+    expect(getThreadCount(false)).toBe("auto");
+    expect(getThreadCount(true)).toBe("auto");
   });
 
   test("accepts positive integer thread counts", () => {
diff --git a/__tests__/run-file-lint.spec.ts b/__tests__/run-file-lint.spec.ts
index 63cb16c..4c46d28 100644
--- a/__tests__/run-file-lint.spec.ts
+++ b/__tests__/run-file-lint.spec.ts
@@ -174,6 +174,37 @@ describe("runFileLint", () => {
     expect(outcome).toEqual({ exitCode: 0 });
   });
 
+  test("reports the small-file auto cap in development mode", async () => {
+    const fileStats = Array.from({ length: 8 }, (_, index) => ({
+      path: `small-${index}.md`,
+      size: 512 * 1024,
+    }));
+    mockLoadMdFiles.mockResolvedValue(fileStats.map(({ path }) => path));
+    mockStatFiles.mockResolvedValue(fileStats);
+    mockResolveAdaptiveConcurrency.mockResolvedValue({
+      concurrency: 4,
+      maxFileSize: 512 * 1024,
+      requestedConcurrency: 16,
+    });
+
+    await runFileLint(
+      makeOptions({
+        isDev: true,
+        threadCount: "auto",
+      })
+    );
+
+    expect(console.log).toHaveBeenCalledWith(
+      "[lint-md] Adaptive concurrency: requested auto, effective 4, max file 0.50 MiB"
+    );
+    expect(mockBatchLint).toHaveBeenCalledWith(
+      4,
+      fileStats.map(({ path }) => path),
+      false,
+      {}
+    );
+  });
+
   test("reports rule failures to stderr and returns failure without timing", async () => {
     const failedResult: BatchLintItem = {
       path: "failed.md",
diff --git a/src/cli/run-lint.ts b/src/cli/run-lint.ts
index ef836d7..73e8633 100644
--- a/src/cli/run-lint.ts
+++ b/src/cli/run-lint.ts
@@ -184,8 +184,7 @@ export const runFileLint = async ({
 
   if (isDev && concurrencyDecision.maxFileSize !== null) {
     const { maxFileSize, requestedConcurrency } = concurrencyDecision;
-    const adaptiveApplied = maxFileSize >= 1024 * 1024;
-    if (adaptiveApplied && effectiveThreads < requestedConcurrency) {
+    if (effectiveThreads < requestedConcurrency) {
       const maxMiB = (maxFileSize / (1024 * 1024)).toFixed(2);
       console.log(
         `[lint-md] Adaptive concurrency: requested auto, effective ${effectiveThreads}, max file ${maxMiB} MiB`
diff --git a/src/lint-md.ts b/src/lint-md.ts
index 215dced..3c612be 100644
--- a/src/lint-md.ts
+++ b/src/lint-md.ts
@@ -46,7 +46,7 @@ export const createProgram = (): Command => {
     .option("-d, --dev", "open dev mode（开启开发者模式）")
     .option(
       "-t, --threads [thread-count]",
-      'Number of worker threads, or "auto" to cap concurrency for large files. Default: CPU count.（执行 Lint / Fix 的线程数，传 "auto" 时根据文件大小自适应）'
+      'Worker threads, or "auto" for conservative file-size limits. Default: auto.（执行 Lint / Fix 的线程数。默认 auto；传数字可显式覆盖自动限制）'
     )
     .option(
       "-s, --suppress-warnings",
diff --git a/src/utils/adaptive-concurrency.ts b/src/utils/adaptive-concurrency.ts
index 3680d0c..b380da0 100644
--- a/src/utils/adaptive-concurrency.ts
+++ b/src/utils/adaptive-concurrency.ts
@@ -3,6 +3,7 @@ import type { ThreadCount } from "../types";
 
 const ONE_MIB = 1024 * 1024;
 const FIVE_MIB = 5 * ONE_MIB;
+const ADAPTIVE_SMALL_CAP = 4;
 const ADAPTIVE_MEDIUM_CAP = 2;
 const ADAPTIVE_LARGE_FILE_THRESHOLD = ONE_MIB;
 const ADAPTIVE_HUGE_FILE_THRESHOLD = FIVE_MIB;
@@ -37,7 +38,7 @@ export const resolveAdaptiveConcurrency = async (
     };
   }
 
-  let limit = requestedConcurrency;
+  let limit = Math.min(requestedConcurrency, ADAPTIVE_SMALL_CAP);
   if (maxFileSize >= ADAPTIVE_HUGE_FILE_THRESHOLD) {
     limit = 1;
   } else if (maxFileSize >= ADAPTIVE_LARGE_FILE_THRESHOLD) {
diff --git a/src/utils/configure.ts b/src/utils/configure.ts
index fcd298f..d2d194e 100644
--- a/src/utils/configure.ts
+++ b/src/utils/configure.ts
@@ -1,5 +1,4 @@
 import * as fs from "fs";
-import { availableParallelism } from "os";
 import * as path from "path";
 import { CliError } from "../cli/cli-error";
 import type { CLIConfig, ThreadCount } from "../types";
@@ -129,7 +128,7 @@ export const getThreadCount = (
   }
 
   if (typeof threadCount !== "number" && typeof threadCount !== "string") {
-    return availableParallelism();
+    return "auto";
   }
 
   if (typeof threadCount === "string" && !/^[1-9]\d*$/.test(threadCount)) {