fix(mcp): preserve viewport screenshot size by default

[Nikhil (shadowfax92)]

Summary

The screenshot tool no longer silently shrinks an omitted size into 1024x768. Rust and TypeScript now capture the current CSS layout viewport with clip.scale = 1; explicit bounds keep the aspect ratio and never upscale.

- omitted size -> implicit 1024x768 box
+ omitted size -> current viewport, scale 1
+ partial size -> constrain only the supplied axis

Sizing decisions:

• CSS layout metrics define the captured region. Preserve CDP's native device pixel density rather than resampling Retina output: scale 1 returned 2880x1800 for a 1440x900 viewport at DPR 2. This retains rendered detail and the existing explicit-size behavior. A size box therefore caps CSS dimensions; encoded dimensions include device density. The tool descriptions and clip comments make that distinction explicit.
• Width and height are independently optional. Width-only constrains width; height-only constrains height; {} is uncapped. Supplied dimensions retain the 1..4096 validation. Removed Rust Default, default_width/default_height, and the TS default-size constant.
• fullPage retains its existing capture path and ignores size. The Rust SDK/run helper delegates to the same screenshot tool; its instructions now state the rule. The TS server agrees, and a repository search found no other documentation/skills advertising the old default.

The CDP viewport contract defines CSS/DIP geometry; the image dimensions below were measured from actual returned PNG bytes.

Evidence

Before: the existing screenshot-tool regression expected scale 1 for a 2048x1536 viewport and failed with scale 0.5. After: it passes with scale 1.

Built this worktree's browseros-claw-server-rs, connected it over MCP stdio to running BrowserOS neo (Chrome/155.0.8311.26), and called screenshot four ways in a task-owned background fixture tab. Explicit box: 1024x768; width-only: 1024; fullPage also passed width 1 to verify it is ignored. DPR 2 was emulated only on that target; the attached display was DPR 1.

Call	CSS viewport	DPR	Image pixels
omitted	1440x1800	1	1440x1800
size	1440x1800	1	614x768
width-only	1440x1800	1	1024x1280
fullPage	1440x1800	1	1440x2421
omitted	1440x1800	2	2880x3600
size	1440x1800	2	1229x1536
width-only	1440x1800	2	2048x2560
fullPage	1440x1800	2	2880x4842

All captures left innerWidth, innerHeight, DPR and scroll offsets unchanged. fullPage used size.width=1 and matched the unchanged direct-CDP full-page baseline. DPR 2 is emulated on this task-owned target; the attached display is DPR 1.

• cargo check -p browseros-mcp
• cargo clippy -p browseros-mcp --all-targets -- -D warnings
• cargo test -p browseros-mcp: 119 unit and 4 integration tests passed.
• cargo build -p claw-server
• bun run check: passed; existing non-fatal lint/Fallow warnings remain.
• bun run test: initial complete suite passed (2144 tests), including the 18-case real-browser MCP smoke suite. After integrating current main, the only local failure was an unchanged model-picker test exceeding its five-second timeout. That test and both screenshot regression files passed with bun test --timeout 30000 (66 tests); no unrelated source was changed.
• All CI test suites and quality checks passed on integrated head 76cb765639753cd582da0bfa2b8f630262ad473b. Rust checks, build, and all eight live captures were repeated after integration and passed.
• git diff --check
• One independent Codex review; vetted result recorded in the task artifacts.

Merge Danger

Door: two-way, reversible by reverting this change.
Blast Radius: Rust/TS MCP screenshots and the Rust SDK helper. Omitted/partial sizes can produce larger images; clients wanting the old cap can explicitly pass both dimensions. JPEG quality, formats, and full-page capture are unchanged.

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge; no actionable issues were found.

Summary

Rust and TypeScript screenshots now preserve the current CSS viewport by default.

• Screenshots keep their current size unless a caller adds a cap.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Screenshot request] --> B{fullPage?}
  B -->|Yes| C[Existing full-page capture; no clip]
  B -->|No| D[Read CSS layout viewport]
  D --> E[Start with scale 1]
  E --> F[Reduce scale for supplied size axes]
  F --> G[Capture unchanged viewport region]
  C --> H[Return inline image]
  G --> H

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_{Reviews (1) · Last reviewed commit: "fix(mcp): preserve viewport screenshot s..." · Reviewed by Greptile}

[github-actions]

✅ Tests passed: 2474/2477

Ran 11 of 16 suites (5 not affected by this change).

Suite	Passed	Failed	Skipped
✅ server-agent	216/216	0	0
✅ server-api	289/289	0	0
✅ server-tools	256/256	0	0
✅ server-browser	10/10	0	0
✅ server-integration	10/10	0	0
✅ server-lib	199/199	0	0
✅ server-root	38/41	0	3
✅ agent	372/372	0	0
⏩ claw-app	n/a	n/a	not affected
⏩ claw-onboard	n/a	n/a	not affected
⏩ app-onboard	n/a	n/a	not affected
⏩ build	n/a	n/a	not affected
⏩ release	n/a	n/a	not affected
✅ claw-server	932/932	0	0
✅ claw-server-quality	passed	0	0
✅ claw-mcp	152/152	0	0

passed = ran successfully but emits no JUnit counts (a lint/format gate).

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