Stop returning structuredContent in MCP tool call results

[rhennigan]

Summary

Some MCP clients discard a tool result's content (text and images) entirely when structuredContent is present, using it as a replacement for the model-visible content. That is not what we want. The MCP Apps viewers only need the notebookUrl, which already travels out-of-band via _meta (with the in-content <internal><url> marker as the fallback for hosts that drop _meta), so structuredContent is unnecessary here and actively harmful.

This PR stops emitting structuredContent from tool call results end-to-end, keeping _meta exactly as-is, and removes the resulting dead code.

Changes

Stop sending structuredContent

• Kernel/StartMCPServer.wl — evaluateTool no longer forwards structuredContent into the tool result. The _meta forwarding (e.g. notebookUrl for MCP Apps) is unchanged.
• Kernel/UIResources.wl — dropped the now-orphaned StructuredContent key from makeNotebookUIResult; its only consumer was the forwarding above. It now carries notebookUrl in _meta only.

Remove the resulting dead code in the viewers

Since makeNotebookUIResult was the sole producer of the key and the server no longer emits it, params.structuredContent / result.structuredContent can never reach a viewer — the reads were unreachable.

• Assets/Apps/evaluator-viewer.html, notebook-viewer.html, wolframalpha-viewer.html — removed the dead structured parameter and its unreachable check from findNotebookUrl/handleToolResult and all call sites. The live _meta → per-item _meta → <internal><url> marker recovery chain is unchanged.

Tests, comments, docs

• Tests/MCPApps.wlt — the makeNotebookUIResult test now asserts StructuredContent is not produced.
• Tests/EvaluatorSessions.wlt — removed the moot AppendSessionInfo-PreservesStructuredContent test (PreservesMeta already covers generic sibling-key preservation).
• Updated tool comments and docs/mcp-apps.md to reflect _meta-only delivery.

Test plan

• Tests/MCPApps.wlt — 94/94 pass
• Tests/EvaluatorSessions.wlt — 34/34 pass
• Tests/NotebookViewer.wlt — 24/24 pass
• Tests/MCPAppsTest.wlt — 19/19 pass
• Tests/StartMCPServer.wlt — 51/51 non-subprocess tests pass. The 34 failures are pre-existing/environmental: this sandbox can't spawn a wolframscript subprocess (StartProcess::pnfd → MCPTestServerNotRunning), so the server never starts and evaluateTool is never reached. Should be re-run in an environment that can spawn subprocesses.
• CodeInspector clean on all four changed .wl files (only pre-existing, unrelated FixMeComment warnings).
• node --check on all three extracted viewer scripts — syntax valid.

Notes

Related upstream context (hosts dropping _meta/structuredContent): ext-apps#696. No internal tracking issue.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR updates AgentTools’ MCP tool-call result pipeline and MCP Apps viewers to stop emitting structuredContent, because some MCP clients treat it as a replacement for content and therefore drop model-visible text/images. Notebook UI delivery is preserved via _meta.notebookUrl (plus the existing <internal><url>…</url></internal> marker fallback when _meta is dropped).

Changes:

• Remove structuredContent forwarding from Kernel/StartMCPServer.wl tool results while preserving _meta forwarding.
• Stop producing the StructuredContent key in makeNotebookUIResult and delete now-dead viewer logic that read params/result.structuredContent.
• Update tests and documentation to assert/describe _meta-only delivery for notebook URLs.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
Tests/MCPApps.wlt	Updates expectation to assert StructuredContent is not produced by makeNotebookUIResult.
Tests/EvaluatorSessions.wlt	Removes now-moot structured-content preservation test.
Kernel/UIResources.wl	Drops StructuredContent from makeNotebookUIResult; updates comments to _meta-only rationale.
Kernel/Tools/WolframLanguageEvaluator.wl	Updates UI-result comment to reflect _meta-only delivery.
Kernel/Tools/WolframAlpha.wl	Updates UI-result comment to reflect _meta-only delivery.
Kernel/StartMCPServer.wl	Stops forwarding structuredContent into MCP tool call results; continues forwarding _meta.
docs/mcp-apps.md	Documents _meta-only delivery and explains why structuredContent is intentionally not sent.
Assets/Apps/wolframalpha-viewer.html	Removes dead structuredContent parameter plumbing; keeps _meta + marker recovery.
Assets/Apps/notebook-viewer.html	Removes dead structuredContent reads; keeps _meta + marker recovery.
Assets/Apps/evaluator-viewer.html	Removes dead structuredContent reads; keeps _meta + marker recovery.

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