[BUG] Dia browser: screenshot/zoom fail with "Failed to capture screenshot via CDP" (distinct from #16239)

Bug Description

computer tool's screenshot/zoom actions (chrome.debugger/CDP-based) fail on every page, every tab, every session in Dia browser, with:

Error capturing screenshot: Failed to capture screenshot via CDP

This is a distinct error string from Cannot access a chrome-extension:// URL of different extension, reported in #16239 and its duplicates (#29790, #45221). I searched that thread and the wider web/GitHub for this exact string and found no other reports, so I'm filing separately even though the failure category (CDP/debugger tools fail, accessibility-tree tools work) looks similar.

I have not yet tested whether computer screenshot works in stock Google Chrome on the same machine — flagging that gap rather than guessing. If a maintainer wants that control test, or the extension's service-worker console log (chrome://extensions → inspect) captured at the moment of failure, I can gather either.

Environment

• OS: macOS 27.0 (Build 26A5378n)
• Browser: Dia (The Browser Company, Chromium-based), v1.40.1
• Claude in Chrome extension: v1.0.81
• Claude Code: v2.1.214
• Not tested: stock Google Chrome on the same machine (see above)

What Works vs. What Fails

| Tool | API used (presumed) | Status |
|---|---|---|
| tabs_context_mcp | Chrome tabs API | ✅ Works |
| navigate | Chrome tabs API | ✅ Works |
| read_page | Accessibility tree | ✅ Works |
| get_page_text | Accessibility tree | ✅ Works |
| computer (screenshot, zoom) | chrome.debugger (CDP) | ❌ Fails every time |

Steps to Reproduce

1. Connect Claude Code to Dia via the Claude in Chrome extension.
2. Navigate to any URL, including trivial static pages (https://example.com).
3. Call computer with action: "screenshot".

Actual behavior: fails 100% of the time with Error capturing screenshot: Failed to capture screenshot via CDP.

Expected behavior: should succeed on a normal https:// page in Dia, matching the accessibility-tree tools which work reliably in the same environment — or, if Dia isn't a supported target for CDP-based tools, a clear error/docs note saying so instead of a generic CDP failure.

Troubleshooting Already Tried

None of the following resolved it:

• Disabling one suspected conflicting extension.
• Full Chrome/Dia restart (Cmd+Q, reopen) — new tab group created each time, error persists identically.
• Disabling all other extensions except Claude in Chrome (this is the workaround that resolves #16239 for other reporters — did not help here).
• Uninstalling and reinstalling the Claude in Chrome extension from scratch.
• Focusing/foregrounding the Dia window before capture (rules out a hidden-pane/occlusion cause seen in other threads).
• Stopping a separate local automation daemon, OpenCLI (also declares the debugger Chrome permission), via its own CLI — confirmed stopped, retested, no change. Restarted it afterward — no change either direction.
• Retried after each step above with fresh tabs/tab groups — same error every time.

I recall this already failing before the most recent Dia/extension update (2026-07-17) and the macOS 27.0 upgrade (2026-07-14, confirmed via system_profiler SPInstallHistoryDataType) — not independently verified beyond my own recollection, but noting it since it argues against either update being the trigger.

Additional Context

Related Dia-support threads, all closed (not_planned/duplicate) without a fix landing, none of which cover this specific CDP-capture error:

• #19268 — Dia doesn't fully implement chrome.tabGroups
• #34830 — Dia browser support request
• #36410 — Dia browser detection/native-messaging-host path
• #16239 (open) — same failure category, different error string, fixes there didn't apply here