feat(mcp): save preview snapshots to disk on request

[jakeleventhal]

Problem

preview_snapshot returns a PNG to the agent, but the user never sees it: the image only lives inside the tool result. When the agent wants to show the user what the page looked like, it has nothing on disk to point at, unlike preview_recording_stop, which already writes a file.

Fix

• preview_snapshot gains an opt-in save input. When set, the server writes the PNG it already holds in McpHttpServer to <stateDir>/browser-artifacts (the same directory the desktop uses for recordings) and adds screenshotPath to the structured and text results. Files are named browser-screenshot-<host>-<time>.png, matching the desktop's own naming.
• The save flag is stripped before the request reaches the browser, so there is no new automation op and no desktop change.
• A failed write returns an error result tagged PreviewScreenshotSaveError instead of silently succeeding without a path. The failure text now names the tag (Preview snapshot failed: PreviewScreenshotSaveError.) because agents usually only see the text content.
• The preview_snapshot and preview_recording_stop descriptions now tell the agent to embed the returned path with markdown image syntax so the user sees the capture inline.

packages/contracts gets a PreviewAutomationSnapshotInput schema and apps/server config gains a derived browserArtifactsDir.

Verification

Unit tests in McpHttpServer.test.ts cover: the file lands in the artifacts dir with the expected name and bytes, screenshotPath appears in both result forms, the browser request arrives without save, no path is returned when save is omitted, and an unwritable artifacts path yields the tagged error result.

End to end on the desktop dev build (vp run dev:desktop), a Claude Sonnet 5 thread was asked to open example.com, snapshot with save enabled, and show the result, without being told the markdown syntax. The agent embedded the saved screenshot on its own from the tool description hint, then recorded a scroll and embedded the recording the same way. With the artifacts path replaced by a plain file, the same request produced an error result and the server trace logged PreviewScreenshotSaveError.

Agent reply with the saved path and the inline screenshot:

The PNG written to browser-artifacts by the new save path:

preview_recording_stop embed following the updated description:

One observation from testing that is not caused by this change: while the dev Electron window was fully occluded by another app, snapshot requests timed out on the desktop side before reaching the save code. Bringing the window forward resolved it.

Note: in a remote setup the screenshot path is on the server machine, where the agent runs, while the recording path comes from the desktop hosting the browser.

Built with Claude Fable 5.1 in Claude Code.

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Note

Low Risk
Opt-in filesystem writes under state dir with explicit errors on failure; no change to browser automation protocol when save is not used.

Overview
Adds an opt-in save flag on the MCP preview_snapshot tool so agents can persist the PNG the server already receives and surface it to users via a filesystem path.

When save=true, the MCP HTTP server writes the decoded screenshot under browserArtifactsDir (<stateDir>/browser-artifacts, matching desktop recordings), names files browser-screenshot-<host-slug>-<time>.png, and returns screenshotPath in structured metadata and JSON text (still returning the inline image). The save parameter is stripped in preview handlers so the collaborative browser automation path is unchanged.

Failed disk writes fail the tool with PreviewScreenshotSaveError; snapshot failure text now includes the error tag (e.g. Preview snapshot failed: PreviewAutomationExecutionError.). Contracts gain PreviewAutomationSnapshotInput; tool descriptions nudge agents to embed saved paths with markdown image syntax. Tests cover save success, omitted save, and unwritable artifact paths.

^{Reviewed by Cursor Bugbot for commit 2065ed796e8237c006b37bfb2ae4bc7a8b48f0f2. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Add save option to preview_snapshot MCP tool to write PNG screenshots to disk

• The preview_snapshot MCP tool now accepts a save boolean in PreviewAutomationSnapshotInput. When true, the server writes the screenshot PNG to ServerConfig.browserArtifactsDir using a filename derived from the site slug, timestamp, and a random UUID, and returns screenshotPath in both structured and text metadata.
• The save and includeImage options are stripped before the request reaches the browser automation broker; save is handled at the MCP layer and includeImage remains a response-only concern.
• The tool's annotations changed from read-only to readOnlyHint=false and idempotentHint=false, reflecting that it now writes to disk.
• Failed preview snapshots now include the derived error tag (e.g. PreviewAutomationExecutionError, AiError) in MCP text content, not just a generic failure message.
• Risk: PreviewScreenshotSaveError is a new tagged error type in McpHttpServer.ts; directory-creation or file-write failures during save produce this tag in both text and structured content. Concurrent saves at the same millisecond rely on UUID suffixes for uniqueness — if UUID collision occurs, the second write overwrites the first file.

^{Macroscope summarized edcce5c.}

Summary by CodeRabbit

• New Features

  • Preview snapshots can now be saved as PNG files and return the saved screenshot path.
  • Snapshot requests support options to include an image in the response and/or save it to disk.
  • Saved screenshots use unique filenames to prevent collisions.
  • Snapshot and recording results provide guidance for embedding returned files using Markdown images.

• Bug Fixes

  • Improved error reporting when saving preview screenshots fails, including the affected path and error type.

[macroscopeapp]

Approvability

Verdict: Not approved

Macroscope's review found this PR not approvable — This PR adds opt-in persistent screenshot artifacts and changes the MCP snapshot tool’s side-effect metadata, making it a user-facing behavior and product-default change rather than a purely local adjustment. The supplied unresolved Medium finding also concerns retry/idempotence behavior for the new filesystem side effect.

Not approved because:

• 1 blocking correctness issue found at or above your repo's Minimum Blocking Severity

^{Adjust the Minimum Blocking Severity for this repo — including turning it Off — in Settings. You can add or adjust custom eligibility rules. Learn more.}

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📝 Walkthrough

Walkthrough

The preview snapshot MCP tool now accepts save, writes screenshots to the configured browser artifacts directory, returns screenshotPath, and reports tagged save failures. Shared contracts, server path derivation, request handling, tool descriptions, and tests were updated.

Changes

Preview screenshot persistence

Layer / File(s)	Summary
Snapshot contract and artifact paths packages/contracts/src/previewAutomation.ts, apps/server/src/config.ts	The shared snapshot input supports includeImage and save. Server-derived paths now include browserArtifactsDir.
Server screenshot save flow apps/server/src/mcp/McpHttpServer.ts	The server saves PNG screenshots with timestamped, hostname-based, UUID-suffixed filenames, returns screenshotPath, encodes response text through a schema, and reports tagged failures.
Tool wiring and validation apps/server/src/mcp/toolkits/preview/tools.ts, apps/server/src/mcp/toolkits/preview/handlers.ts, apps/server/src/mcp/McpHttpServer.test.ts	The tool uses the shared contract, removes save before browser execution, updates artifact instructions, and tests saved files, omitted images, save failures, concurrent saves, and tool metadata.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: ⚪ Minimal · up to edcce

The snapshot tool can optionally save PNG artifacts and return their paths, with failure handling and coverage for saving, request filtering, and concurrent filenames. No current merge-blocking risk remains.

Suggested reviewers: juliusmarminge

Sequence Diagram(s)

sequenceDiagram
  participant MCPClient
  participant PreviewSnapshotTool
  participant McpHttpServer
  participant FileSystem
  MCPClient->>PreviewSnapshotTool: preview_snapshot(save=true)
  PreviewSnapshotTool->>McpHttpServer: snapshot request without save
  McpHttpServer->>FileSystem: create browser-artifacts directory and write PNG
  FileSystem-->>McpHttpServer: screenshotPath
  McpHttpServer-->>MCPClient: snapshot response with screenshotPath

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🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: saving MCP preview snapshots to disk on request.
Description check	✅ Passed	The description clearly explains the problem, implementation, behavior, error handling, verification, and scope. It does not use the template headings exactly and omits the checklist, but it is otherw…
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 6…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@apps/server/src/mcp/McpHttpServer.ts`:
- Line 140: The screenshot filename generated in the save flow must remain
unique for concurrent requests sharing the same URL and millisecond timestamp.
Update the filename logic around screenshotSiteSlug, millis, and
FileSystem.writeFile to add a collision-resistant suffix or use exclusive
creation with retry, then add a concurrent test using one URL and a fixed clock
that verifies distinct paths and preservation of both file contents.

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• apps/server/src/config.ts
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• apps/server/src/mcp/toolkits/preview/handlers.ts
• apps/server/src/mcp/toolkits/preview/tools.ts
• packages/contracts/src/previewAutomation.ts

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[juliusmarminge]

Closing as part of the open-PR backlog sweep (wave 2).

Reason: Superseded by newer Theo snapshot-save work in #10501
Related: #10501

Reopen if this is still wanted and you’re willing to rebase onto current main.