fix(cli): reject file-only options with --stdin

__tests__/cli.spec.ts

@@ -176,4 +176,84 @@ describe("cli tests", () => {
     expect(runFileLint).not.toHaveBeenCalled();
     expect(process.exitCode).toBe(1);
   });
+
+  test("rejects --threads combined with --stdin before validating its value", async () => {
+    const runFileLint = jest.fn().mockResolvedValue({ exitCode: 0 });
+    const runStdinLint = jest.fn().mockReturnValue({ exitCode: 0 });
+    jest.doMock("../src/cli/run-lint", () => ({
+      runFileLint,
+      runStdinLint,
+    }));
+    const mockError = jest.spyOn(console, "error").mockImplementation();
+    const { runCli } = require("../src/lint-md");
+
+    process.exitCode = undefined;
+    runCli(["node", "lint-md", "--stdin", "--threads", "abc"]);
+    await new Promise<void>((resolve) => setImmediate(resolve));
+
+    expect(mockError).toHaveBeenCalledWith(
+      expect.stringContaining(
+        "[lint-md] --threads cannot be used with --stdin."
+      )
+    );
+    expect(mockError).not.toHaveBeenCalledWith(
+      expect.stringContaining("INVALID_THREADS")
+    );
+    expect(runStdinLint).not.toHaveBeenCalled();
+    expect(process.exitCode).toBe(1);
+  });
+
+  test("rejects --max-file-size combined with --stdin", async () => {
+    const runFileLint = jest.fn().mockResolvedValue({ exitCode: 0 });
+    const runStdinLint = jest.fn().mockReturnValue({ exitCode: 0 });
+    jest.doMock("../src/cli/run-lint", () => ({
+      runFileLint,
+      runStdinLint,
+    }));
+    const mockError = jest.spyOn(console, "error").mockImplementation();
+    const { runCli } = require("../src/lint-md");
+
+    process.exitCode = undefined;
+    runCli(["node", "lint-md", "--stdin", "--max-file-size", "5mb"]);
+    await new Promise<void>((resolve) => setImmediate(resolve));
+
+    expect(mockError).toHaveBeenCalledWith(
+      expect.stringContaining(
+        "[lint-md] --max-file-size cannot be used with --stdin."
+      )
+    );
+    expect(runStdinLint).not.toHaveBeenCalled();
+    expect(process.exitCode).toBe(1);
+  });
+
+  test("aggregates all file-only options rejected with --stdin", async () => {
+    const runFileLint = jest.fn().mockResolvedValue({ exitCode: 0 });
+    const runStdinLint = jest.fn().mockReturnValue({ exitCode: 0 });
+    jest.doMock("../src/cli/run-lint", () => ({
+      runFileLint,
+      runStdinLint,
+    }));
+    const mockError = jest.spyOn(console, "error").mockImplementation();
+    const { runCli } = require("../src/lint-md");
+
+    process.exitCode = undefined;
+    runCli([
+      "node",
+      "lint-md",
+      "--stdin",
+      "--threads",
+      "4",
+      "--max-file-size",
+      "5mb",
+    ]);
+    await new Promise<void>((resolve) => setImmediate(resolve));
+
+    expect(mockError).toHaveBeenCalledWith(
+      expect.stringContaining(
+        "[lint-md] The following options cannot be used with --stdin:\n--threads\n--max-file-size"
+      )
+    );
+    expect(runStdinLint).not.toHaveBeenCalled();
+    expect(process.exitCode).toBe(1);
+  });
 });

__tests__/threads-validation.spec.ts

@@ -7,7 +7,7 @@ const TSX = path.resolve(__dirname, "../node_modules/tsx/dist/cli.mjs");
 const CLI = path.resolve(__dirname, "../src/lint-md.ts");
 
 describe("--threads validation across CLI paths", () => {
-  test("stdin + --threads abc → exit 1 + stderr", () => {
+  test("stdin + --threads abc → rejected as a conflicting option, exit 1", () => {
     try {
       execFileSync(
         process.execPath,
@@ -21,7 +21,9 @@ describe("--threads validation across CLI paths", () => {
       throw new Error("should have thrown");
     } catch (e: any) {
       expect(e.status).toBe(1);
-      expect(e.stderr).toContain("--threads must be a positive integer");
+      expect(e.stderr).toContain(
+        "[lint-md] --threads cannot be used with --stdin."
+      );
     }
   });
 
@@ -42,17 +44,24 @@ describe("--threads validation across CLI paths", () => {
     }
   });
 
-  test("stdin + --threads auto → does not exit 1 (numeric validation skipped)", () => {
-    const result = execFileSync(
-      process.execPath,
-      [TSX, CLI, "--stdin", "--threads", "auto"],
-      {
-        input: "# title\n",
-        encoding: "utf8",
-        stdio: ["pipe", "pipe", "pipe"],
-      }
-    );
-    expect(result).toContain("Done in");
+  test("stdin + --threads auto → rejected as a conflicting option, exit 1", () => {
+    try {
+      execFileSync(
+        process.execPath,
+        [TSX, CLI, "--stdin", "--threads", "auto"],
+        {
+          input: "# title\n",
+          encoding: "utf8",
+          stdio: ["pipe", "pipe", "pipe"],
+        }
+      );
+      throw new Error("should have thrown");
+    } catch (e: any) {
+      expect(e.status).toBe(1);
+      expect(e.stderr).toContain(
+        "[lint-md] --threads cannot be used with --stdin."
+      );
+    }
   });
 
   test("files + --threads auto → exit 0 on a small markdown file", () => {

src/lint-md.ts

@@ -92,6 +92,31 @@ export const createProgram = (): Command => {
         );
       }
 
+      // --threads and --max-file-size only affect file linting. Reject them
+      // here instead of silently accepting options that would do nothing.
+      const conflictingOptions = [
+        [threads !== undefined, "--threads"],
+        [maxFileSize !== undefined, "--max-file-size"],
+      ]
+        .filter(([present]) => present)
+        .map(([, name]) => name);
+
+      if (stdin && conflictingOptions.length === 1) {
+        throw new CliError(
+          "CONFLICTING_INPUT",
+          `[lint-md] ${conflictingOptions[0]} cannot be used with --stdin.`
+        );
+      }
+
+      if (stdin && conflictingOptions.length > 1) {
+        throw new CliError(
+          "CONFLICTING_INPUT",
+          `[lint-md] The following options cannot be used with --stdin:\n${conflictingOptions.join(
+            "\n"
+          )}`
+        );
+      }
+
       if (isDev) {
         console.log(`dev -- version: ${version}, ${new Date().toString()}`);
       }