feat: support config file (#2661)

Utilizes the Yargs default. We may need to change this to resolve this
ourself, but for now we can explicitly allow this.

docs/configuration.md

@@ -226,6 +226,11 @@ The Chrome DevTools MCP server supports the following configuration option:
   - **Type:** array
   - **Default:** `OS temp directory`
 
+- **`--config`**
+  Path to JSON configuration file.
+  - **Type:** string
+  - **Default:** `false`
+
 <!-- END AUTO GENERATED OPTIONS -->
 
 Pass them via the `args` property in the JSON configuration. For example:

src/config/mcp-options.ts

@@ -7,6 +7,7 @@
 import type {YargsOptions} from '../third_party/index.js';
 import {yargs, hideBin} from '../third_party/index.js';
 import os from 'node:os';
+import {readFileSync} from 'node:fs';
 
 export const DEFAULT_FILESYSTEM_ROOT = [os.tmpdir()];
 
@@ -259,6 +260,10 @@ export const mcpOptions = {
     describe:
       'A directory that filesystem tools are allowed to access. May be specified more than once.',
   },
+  config: {
+    type: 'string',
+    describe: 'Path to JSON configuration file.',
+  },
 } satisfies Record<string, YargsOptions>;
 
 export type ParsedArguments = ReturnType<typeof parseArguments>;
@@ -433,6 +438,32 @@ export function parser(
     ]);
 
   return yargsInstance
+    .config('config', 'Path to JSON configuration file', configPath => {
+      try {
+        const parsed = JSON.parse(readFileSync(configPath, 'utf-8'));
+        if (
+          typeof parsed !== 'object' ||
+          parsed === null ||
+          Array.isArray(parsed)
+        ) {
+          throw new Error('Config must be a JSON object');
+        }
+
+        return yargs()
+          .parserConfiguration({
+            'strip-aliased': true,
+            'camel-case-expansion': false,
+          })
+          .options(options)
+          .config(parsed)
+          .strict()
+          .fail(false)
+          .exitProcess(false)
+          .parseSync([]);
+      } catch (err) {
+        throw new Error(`Invalid JSON config file: ${(err as Error).message}`);
+      }
+    })
     .wrap(Math.min(120, yargsInstance.terminalWidth()))
     .help()
     .version(version);

src/telemetry/flag_usage_metrics.json

@@ -445,5 +445,9 @@
   {
     "name": "source_maps",
     "flagType": "boolean"
+  },
+  {
+    "name": "config_present",
+    "flagType": "boolean"
   }
 ]

tests/cli.test.ts

@@ -6,6 +6,9 @@
 
 import assert from 'node:assert';
 import {describe, it} from 'node:test';
+import fs from 'node:fs';
+import os from 'node:os';
+import path from 'node:path';
 
 import {
   DEFAULT_FILESYSTEM_ROOT,
@@ -19,6 +22,21 @@ function parseArguments(argv: string[], env: NodeJS.ProcessEnv = {}) {
     .parseSync();
 }
 
+function createTempFile(content: string, fileName: string) {
+  const filePath = path.join(os.tmpdir(), fileName);
+  fs.writeFileSync(filePath, content);
+  return {
+    path: filePath,
+    [Symbol.dispose]() {
+      try {
+        fs.unlinkSync(filePath);
+      } catch {
+        // ignore
+      }
+    },
+  };
+}
+
 describe('cli args parsing', () => {
   const defaultArgs = {
     categoryInput: true,
@@ -430,4 +448,69 @@ describe('cli args parsing', () => {
     const explicitTrueArgs = parseArguments(['--source-maps=true']);
     assert.strictEqual(explicitTrueArgs.sourceMaps, true);
   });
+
+  it('parses config option', async () => {
+    using testConfig = createTempFile(
+      JSON.stringify({
+        headless: true,
+        categoryInput: false,
+        blockedUrlPattern: ['https://example.com/*'],
+      }),
+      'cd4a.test.config.json',
+    );
+    const args = parseArguments(['--config', testConfig.path]);
+    assert.strictEqual(args.config, testConfig.path);
+    assert.strictEqual(args.headless, true);
+    assert.strictEqual(args.categoryInput, false);
+    assert.deepStrictEqual(args.blockedUrlPattern, ['https://example.com/*']);
+  });
+
+  it('parses config option mixed with cli arguments', async () => {
+    using testConfig = createTempFile(
+      JSON.stringify({
+        headless: true,
+        categoryInput: false,
+      }),
+      'cd4a.test.config.mixed.json',
+    );
+    const args = parseArguments([
+      '--config',
+      testConfig.path,
+      '--headless=false',
+      '--category-network=false',
+    ]);
+    assert.strictEqual(args.config, testConfig.path);
+    assert.strictEqual(args.headless, false);
+    assert.strictEqual(args.categoryInput, false);
+    assert.strictEqual(args.categoryNetwork, false);
+    assert.strictEqual(args.categoryMemory, true);
+  });
+
+  it('parses config should not allow no prefix', async () => {
+    using testConfig = createTempFile(
+      JSON.stringify({
+        headless: true,
+        'no-category-memory': true,
+      }),
+      'cd4a.test.config.mixed.json',
+    );
+    assert.throws(
+      () => parseArguments(['--config', testConfig.path]),
+      /Invalid JSON config file: Unknown argument: no-category-memory/,
+    );
+  });
+
+  it('parses config should not allow dashed property', async () => {
+    using testConfig = createTempFile(
+      JSON.stringify({
+        headless: true,
+        'category-memory': false,
+      }),
+      'cd4a.test.config.mixed.json',
+    );
+    assert.throws(
+      () => parseArguments(['--config', testConfig.path]),
+      /Invalid JSON config file: Unknown argument: category-memory/,
+    );
+  });
 });