Cap execute_playwright_code output and fix snapshot tip

The tool returned executed-code output verbatim and unbounded, so a single
whole-page read (innerText/ariaSnapshot on body) could dump tens to hundreds
of KB into the model context. The code param also recommended
page._snapshotForAI(), which is a large full-page dump and is not reliably
available in the execution runtime.

- Cap each returned field at 25k chars with a truncation marker that nudges
  toward targeted selectors. Small structured results keep their shape.
- Replace the _snapshotForAI() tip with guidance to scope reads to a selector
  or region.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dprevoznik

src/lib/mcp/tools/playwright.ts

@@ -2,6 +2,18 @@ import type { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
 import { z } from "zod";
 import { createKernelClient } from "@/lib/mcp/kernel-client";
 
+// Cap per-field output so a single call can't flood the model's context. Whole-page
+// reads (innerText/ariaSnapshot on body) routinely run tens to hundreds of KB.
+const MAX_FIELD_CHARS = 25_000;
+
+function cap(value: unknown): unknown {
+  if (value === undefined || value === null) return value;
+  const text = typeof value === "string" ? value : JSON.stringify(value);
+  if (text.length <= MAX_FIELD_CHARS) return value;
+  const dropped = text.length - MAX_FIELD_CHARS;
+  return `${text.slice(0, MAX_FIELD_CHARS)}\n\n[output truncated: showing ${MAX_FIELD_CHARS} of ${text.length} chars, ${dropped} dropped. Return a targeted selector instead of a whole-page read.]`;
+}
+
 export function registerPlaywrightTool(server: McpServer) {
   // execute_playwright_code -- Run Playwright/TypeScript code against a browser
   server.tool(
@@ -11,7 +23,7 @@ export function registerPlaywrightTool(server: McpServer) {
       code: z
         .string()
         .describe(
-          'Playwright/TypeScript code with a `page` object in scope. Example: "await page.goto(\\"https://example.com\\"); return await page.title();" Tip: Use `await page._snapshotForAI()` for a comprehensive page state snapshot.',
+          'Playwright/TypeScript code with a `page` object in scope. Example: "await page.goto(\\"https://example.com\\"); return await page.title();" Return only what you need: prefer a targeted selector (e.g. `await page.locator(SELECTOR).innerText()`) over reading a whole page. Whole-page dumps like `innerText()`/`ariaSnapshot()` on `body` return large output that is truncated. To understand an unfamiliar page, scope the snapshot to a region (e.g. `await page.locator("main").ariaSnapshot()`).',
         ),
       session_id: z
         .string()
@@ -44,10 +56,10 @@ export function registerPlaywrightTool(server: McpServer) {
               text: JSON.stringify(
                 {
                   success: response.success,
-                  result: response.result,
-                  error: response.error,
-                  stdout: response.stdout,
-                  stderr: response.stderr,
+                  result: cap(response.result),
+                  error: cap(response.error),
+                  stdout: cap(response.stdout),
+                  stderr: cap(response.stderr),
                 },
                 null,
                 2,