Text-only preview snapshots capture and encode a full-frame PNG that is then discarded #24174

What happened

preview_snapshot captures a full-frame screenshot on every call, even when the caller asked for neither the inline image nor a saved file. Agents snapshot before nearly every browser interaction, almost always text-only, so this cost is paid constantly.

Diagnosis

The MCP handler strips includeImage/save before invoking the snapshot operation, so ServerBrowserPage.capture always runs Page.captureScreenshot + PNG encode, the base64 travels through the broker, and McpHttpServer base64-decodes it before dropping the bytes.

Note this changes one visible behavior: when no screenshot is captured, the result's screenshot field (and its dimensions) would be null instead of carrying dimensions of a discarded image; preview_status still reports viewport sizes. Adjacent: #21494 (text-only snapshots hanging on hidden tabs) — skipping the capture also removes that capture from text-only calls, but this does not fix hidden-tab captures when a screenshot is requested.

Steps to reproduce

1. Open a preview tab and call preview_snapshot with no includeImage and no save.
2. Count Page.captureScreenshot CDP calls (real Chromium, ServerBrowserPage.test.ts): one per text-only snapshot, though the resulting bytes are dropped.
   No wall-time figure is claimed; the cost is one full-frame capture and encode per snapshot.

Version

0.1.3-nightly.20261023 (upstream main 807319c)

Environment

Linux 7.2.9 (WSL2), Node 26.12.1, Chromium via the server browser

Evidence

Page.captureScreenshot calls per text-only preview_snapshot: 1 before, 0 after (branch; 39/39 preview tests pass)

Related issues

#21494 (text-only snapshots hanging on hidden tabs): adjacent, not the same defect

Fix applied or workaround

None applied. I can open a pull request for this once triage confirms the direction. A measured branch exists (perf/preview-snapshot-skip-screenshot on my fork, fork review PR bompus#17): thread includeScreenshot (includeImage || save) through the snapshot operation, skip captureViewport when false, and make the contract's screenshot field nullable; hosts that ignore the new input field keep returning a screenshot.

Filed by

Muse Wonder Full 1.0 (Muse Code) investigated and measured; Claude Sonnet 5.5 (Claude Code) filed it, via T3 Code