[Bug]: preview_evaluate forbids WebMCP write tools even when the user explicitly asks for the change
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Assessment
- Difficulty
- 2/5
- Estimated time
- 1-3 hours
- Newbie friendliness
- 74/100
- Issue type
- Bug
- Clarity
- Clearly specified
- Activity status
- Active
- Tech stack
- typescript
- Domain
- ai-infra-agents
Research direction
Start by reading the preview_evaluate description string in apps/server/src/mcp/toolkits/preview/tools.ts, the file named in the report, and compare it with the sibling preview_act description. Then read the eval case webmcp-charts-saved-to-site-not-inline in evals/mcp-tool-use/cases/visual-preview.ndjson to see which behaviour the existing wording protects. Done means the description still forbids using page tools to display results while allowing writes the user explicitly asked for, and the related eval case still passes.
Written by the indexing model from the issue text.
Description
Before submitting
- I searched existing issues and did not find a duplicate.
- I included enough detail to reproduce or investigate the problem.
Area
apps/server
Steps to reproduce
- Open a page that exposes WebMCP tools via
navigator.modelContextin a preview tab owned by the agent (e.g. a notes app that registerslist_notes,create_note,delete_note). - Ask the agent to list the page's tools: it calls
navigator.modelContext.getTools()throughpreview_evaluateand lists them correctly. - Explicitly ask the agent to use one of the write tools (e.g. "create a note titled Groceries").
Expected behavior
When the user explicitly asks for a change, the agent can call the page's write tools through preview_evaluate, the same way it can already drive the same change by clicking through the page UI with preview_act. Pages can already mark their read-only tools with annotations.readOnlyHint, so the agent can tell read tools from write tools.
Actual behavior
The agent refuses. The preview_evaluate description says:
Page tools reached here: read data only, never create, save, or display.
The agent takes this as an absolute rule, so the write tools are unusable even when the user asks for the change directly. The only remaining path is to reproduce the same change by clicking through the UI with preview_act, which is slower and more fragile than calling the tool the page exposes for that purpose.
The sentence was added in #24850 (apps/server/src/mcp/toolkits/preview/[tools.ts](https://github.com/tmp/apps/server/src/mcp/toolkits/preview/tools.ts)). The related eval case webmcp-charts-saved-to-site-not-inline (evals/mcp-tool-use/cases/[visual-preview.ndjson](https://github.com/tmp/evals/mcp-tool-use/cases/visual-preview.ndjson)) covers a narrower failure: the agent saved charts to the site's dashboard when the user wanted inline visuals in T3 Code. "display" covers that case. "create, save" also blocks writes the user explicitly requested.
A possible narrower wording: page tools are for reading unless the user asked for that change on the site; never use them to show results.
Impact
Major degradation or frequent failure
Version or commit
0.1.4-nightly.20261028.3822
Environment
macOS 27.0.1, T3 Code desktop (Nightly), Claude provider (claude-opus-5-5).
Logs or stack traces
Screenshots, recordings, or supporting files
No response
Workaround
Ask the agent to make the change through the page UI with preview_act (click and type, confirming before save).
- Dominant language
- TypeScript
- Stars
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- Avg merge
- 3h 21m
- Merged PRs (30d)
- 297
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