Inconsistent performance results

[umaar]

Describe the bug
A clear and concise description of what the bug is.

Expected behavior
The correct tools are used, and relevant performance metrics are returned.

Chrome version: 142
Coding agent version: Claude code
Model version: Sonnet 4

Chat log

> get performance of https://cwv-guide.vercel.app/

⏺ I'll get the performance metrics for https://cwv-guide.vercel.app/ using the Chrome DevTools MCP tool to analyze Core Web Vitals and other performance metrics.
  ⎿  Error: No such tool available: mcp__chrome-devtools__new_page

⏺ Let me check what MCP tools are available and then use Playwright to get the performance metrics:

⏺ Bash(claude mcp list)
  ⎿  Checking MCP server health...

     chrome-devtools: npx chrome-devtools-mcp@latest --channel=canary --headless=false --isolated=true - ✓ Connected

⏺ Bash(claude tool list)
  ⎿  I see you've sent a system reminder, but I don't have a specific task to work on. How can I help you today?

⏺ Let me use the correct Chrome DevTools MCP tool:
  ⎿  Error: No such tool available: mcp__chrome-devtools__new_page

⏺ Let me check the exact Chrome DevTools MCP tool names available:

⏺ Bash(npx chrome-devtools-mcp@latest --help)
  ⎿ Options:
      -u, --browserUrl      Connect to a running Chrome instance using port forwarding. For more details see: https://developer.chrome.com/docs/devtools/remote-debugging/local-
    server.  [string]
    … +16 lines (ctrl+o to expand)


⏺ Let me create a new page using the correct Chrome DevTools MCP tool naming:
  ⎿  Error: No such tool available: mcp__chrome-devtools__new_page

⏺ Let me check what tools are actually available by looking at the .mcp.json file:

⏺ Read(.mcp.json)
  ⎿  Read 14 lines (ctrl+o to expand)

⏺ Let me use a simple approach with curl to get basic performance metrics:

[umaar]

Also, once it works, and then when trying to get INP data from a button click:

...
⏺ Great! I can see the interaction happened - there's now "Time taken: 359.40ms"
  displayed on the page after clicking the button. Let me stop the performance
  trace to get the INP measurement.
  ⎿  # performance_stop_trace response
     The performance trace has been stopped.
     Here is a high level summary of the trace and the Insights that were found:
     … +7 lines (ctrl+o to expand)

⏺ The trace didn't capture INP metrics in this summary. Let me use JavaScript to
  check if there are any performance entries or INP data available:

⏺ chrome-devtools - evaluate_script (MCP)(function: "() => {\n  // Check for INP using
                                          the Performance Observer API\n  const
                                         entries =
                                         performance.getEntriesByType('event');\n
                                         const interactionEntries =
                                         entries.filter(entry => entry.name ===
                                         'pointerup' || entry.name === 'click');\n  \n
                                           // Also check for any navigation timing\n
                                         const navigation = performance.getEntriesByTy
                                         pe('navigation')[0];\n  \n  return {\n
                                         interactionEntries:
                                         interactionEntries.map(entry => ({\n
                                         name: entry.name,\n      startTime:
                                         entry.startTime,\n      duration:
                                         entry.duration,\n      processingStart:
                                         entry.processingStart,\n      processingEnd:
                                         entry.processingEnd\n    })),\n
                                         navigationTiming: navigation ? {\n
                                         loadEventEnd: navigation.loadEventEnd,\n
                                          domContentLoadedEventEnd:
                                         navigation.domContentLoadedEventEnd\n    } :
                                         null,\n    timeShownOnPage: \"359.40ms\"\n
                                         };\n}")
  ⎿  # evaluate_script response
     Script ran on page and returned:
     ```json
     … +9 lines (ctrl+o to expand)

⏺ Based on the interaction with the "Get colours" button, I can see that the page displays "Time taken: 359.40ms" after the click. However, the performance trace didn't capture
  specific INP metrics in the summary.

  The 359.40ms response time shown on the page likely represents the processing time for the color generation function, but this doesn't directly correspond to INP.

  For INP (Interaction to Next Paint), we'd need to measure the time from the user interaction to when the browser presents the visual update. Since INP measures responsiveness
  and the target is <200ms for good performance, a 359ms processing time suggests the interaction might have poor INP if that's the actual paint delay.

  To get more accurate INP measurements, you'd typically need:
  1. Real user monitoring tools
  2. Chrome DevTools with specific INP recording enabled
  3. Web Vitals JavaScript library implementation

  The current interaction appears to have suboptimal responsiveness based on the 359ms timing displayed, which exceeds the 200ms INP target.

It might the case that I'm being vague/ambiguous in my requests, however, more detailed tool descriptions could be helpful here!

[jackfranklin]

It definitely is far too keen to try to inject JS into the page to read values; we want to encourage it to always use the start & stop tracing here. I think I'll play with the tool descriptions to add more advice like this to them. Thanks for reporting!