fix(server): prevent preview_snapshot from bricking sessions with oversized images

[Exotic209093]

The preview_snapshot MCP tool embedded full-resolution base64 PNGs directly into tool history as inline image content blocks. When providers reject inline images, this permanently bricks the session because the oversized payload cannot be removed from history. Removed the inline image content block from snapshot results; structured metadata (dimensions, mime type) is still returned for reference while the client renders the actual image from its own copy.

Fixes #11295

Summary by CodeRabbit

• Bug Fixes
  • Snapshot previews no longer include inline image data, reducing failures caused by rejected images.
  • Snapshot results now provide textual JSON metadata and structured screenshot metadata.

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[chatgpt-codex-connector]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-13T19:12:38.053928Z	431dcc9	PR opened

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[macroscopeapp]

Approvability

Verdict: Approved at 431dcc9

Macroscope's review found this PR approvable — This is a small, self-contained regression fix that removes oversized inline PNG payloads from MCP history while preserving snapshot metadata and existing preview behavior. The focused test covers the changed response shape.

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[chatgpt-codex-connector]

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Reviewed commit: 431dcc93cc

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[chatgpt-codex-connector]

Return a bounded screenshot to image-capable providers

For providers/models that accept MCP image blocks, every preview_snapshot now returns only page metadata, so pages whose relevant state is visual-only (canvas output, layout defects, video frames, or other non-semantic content) can no longer be inspected by the agent even though PreviewSnapshotTool still promises a PNG screenshot. The client preview's copy is visible to the user, not to the provider consuming this MCP response; preserve a size-bounded/downscaled image for compatible providers, or omit it only for adapters that cannot accept one.

AGENTS.md reference: AGENTS.md:L69-L72

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📥 Commits

Reviewing files that changed from the base of the PR and between 77bca8b and 431dcc9.

📒 Files selected for processing (2)

• apps/server/src/mcp/McpHttpServer.test.ts
• apps/server/src/mcp/McpHttpServer.ts

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📝 Walkthrough

Walkthrough

The preview snapshot MCP tool no longer returns inline base64 PNG content. It returns textual metadata and structured screenshot metadata. The test now verifies that the response contains one text entry and no image entry.

Changes

Preview snapshot response

Layer / File(s)	Summary
Text-only snapshot result apps/server/src/mcp/McpHttpServer.ts, apps/server/src/mcp/McpHttpServer.test.ts	The snapshot result removes inline image content while retaining screenshot dimensions and MIME type. The test verifies one text entry and no image content.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: juliusmarminge

Merge Risk: ⚪ Minimal · up to 431dc

The snapshot response no longer embeds inline image data that can break provider sessions, while clients retain the metadata needed to render their own image copy.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description clearly explains what changed, why the change was needed, and references issue #11295. It omits the template headings and checklist, but it contains the core required information.
Title check	✅ Passed	The title clearly identifies the server fix and its purpose: preventing oversized images in preview_snapshot from bricking sessions.
Linked Issues check	✅ Passed	The implementation satisfies #11295. McpHttpServer.ts removes screenshot.data before creating CallToolResult. The result contains only text JSON and structured screenshot metadata with `mimeType…
Out of Scope Changes check	✅ Passed	The reviewed changes are limited to McpHttpServer.ts and its MCP regression test. They remove inline snapshot image data and verify the required metadata behavior. These changes directly implement #…
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 2…

✨ Finishing Touches 💡 2

⚔️ Resolve merge conflicts 💡

• Resolve merge conflict in branch fix/preview-snapshot-image-size

🛠️ Fix failing CI checks 💡

• Create stacked PR
• Commit on current branch

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