fix: render a tool result's structuredContent in the Tools screen (#1908)

[cliffhall]

Closes#1908

Problem

A tool that declares an outputSchema returns its real payload in structuredContent; the content[] blocks usually only summarize it ("Found 2 items."). v1 rendered that payload as its own inspectable JSON section. v2's Tools screen dropped it entirely — no section, no label, no hint it was ever returned.

ToolResultPanel only ever walked result.content; result.structuredContent reached the panel intact (the core client preserves it — it is what validateToolOutput checks) and was simply never rendered.

Fix

A new StructuredOutputPanel group renders a collapsible "Structured Output" section: the payload pretty-printed as syntax-highlighted, copyable JSON in a bordered box that mirrors the existing "Resource Links" box. ToolResultPanel renders it below the content blocks, and deliberately also:

• when content is empty (previously "No results yet" — which was wrong when a payload existed), and
• below the alert on an isError result,

so the payload is never silently dropped.

The section is bounded (scrolls within 400px) so a large payload can't push the rest of the panel out of view, and the heading is h4 under the panel's h3 "Results" title so the heading order never skips a level.

Scope is the web Tools screen: the CLI and TUI already print the whole result object, structuredContent included.

Proof of functionality

Real app, prod build, connected to the new structured-output-http.json test server, running list_items.

Before — the structuredContent payload is gone; only the text summary survives:

After — the payload gets its own collapsible, copyable section:

Testing

• New StructuredOutputPanel unit tests (payload rendering, default-expanded, collapsed, toggle, empty object) + 4 Storybook stories with play functions.
• New ToolResultPanel cases: section present alongside content, absent without structuredContent, replacing the empty state, below an error alert, and alongside a Resource Links box.
• npm run ci green (validate → coverage ≥90 gate → build gate → smokes → Storybook).
• Manual smoke against the new showcase server (screenshots below).

Try it

New showcase config test-servers/configs/structured-output-http.json (documented in the README): list_items (nested payload — the issue's repro shape), get_temp (flat), echo (none). Connect with the default legacy era and run list_items.

[Copilot]

Pull request overview

Adds structured tool output rendering to the web Tools screen.

Changes:

• Adds a collapsible, copyable JSON output panel.
• Integrates it across normal, empty, error, and resource-link results.
• Adds tests, stories, documentation, and a showcase server.

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
test-servers/src/test-server-fixtures.ts	Adds nested structured-output fixture.
test-servers/src/preset-registry.ts	Registers the new fixture preset.
test-servers/configs/structured-output-http.json	Adds manual showcase configuration.
README.md	Documents the showcase workflow.
ToolResultPanel.tsx	Renders structured output in result states.
ToolResultPanel.test.tsx	Tests result-panel integration.
ToolResultPanel.stories.tsx	Adds integrated structured-output story.
StructuredOutputPanel.tsx	Implements the collapsible JSON panel.
StructuredOutputPanel.test.tsx	Tests rendering and disclosure behavior.
StructuredOutputPanel.stories.tsx	Adds structured-output variants.

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[cliffhall]

Review round 1 — both comments addressed (8049b4f)

Mirroring the inline replies here, since they get collapsed as outdated once the fix is pushed.

1. Large story had no play assertion (StructuredOutputPanel.stories.tsx) — fixed. The play function now measures the real geometry: waits for the last row, grabs the .mantine-ScrollArea-viewport, and asserts clientHeight <= 400 (the cap holds) andscrollHeight > clientHeight (the overflow is scrollable, not clipped). Dropping mah or applying it to the wrong element fails the first assertion.

2. Nothing exercised the list_items fixture/preset (test-server-fixtures.ts) — fixed, and taken one step further than suggested. Instead of re-declaring the tool inline, the new integration test loads the shipped showcase config through loadConfig/resolveConfig, so a single test covers all three failure modes: a broken structured-output-http.json, a typo'd list_items registration in the preset registry, and a fixture that stops returning the nested payload. It connects over streamable HTTP, calls the tool, and asserts both the Found 2 items. summary block and the exact nested structuredContent.

Local gate green: validate, the ≥90 coverage gate, verify:build-gate, all smokes, and the Storybook play-function suite.

[Copilot]

Pull request overview

Copilot reviewed 11 out of 11 changed files in this pull request and generated no new comments.

Suppressed comments (1)

clients/web/src/test/integration/mcp/inspectorClient.test.ts:1236

• This test is nested inside Tool Methods, whose beforeEach already creates and connects a stdio InspectorClient (inspectorClient.test.ts:849-861). Reassigning client here loses that live client's reference, so the stdio child process is never disconnected; the outer afterEach only sees the replacement HTTP client. Disconnect the inherited client before replacing it, as the nearby server-switching test does at line 1190.

 client = new InspectorClient(

[cliffhall]

Review round 2 — no changes needed

Copilot generated no new comments; the one suppressed comment doesn't hold up, so nothing changed in this round.

Suppressed: "nested inside Tool Methods, whose beforeEach already creates and connects a stdio client … the stdio child process is never disconnected" — not applicable. describe("Structured output showcase config (#1908)") is a sibling of Tool Methods, not a child: Tool Methods closes at line 1218 and the new block opens at 1220 at the same nesting level (both direct children of describe("InspectorClient")). A vitest beforeEach only runs for its own describe's descendants, so no stdio client is ever created for this test and there is none to leak — the test creates the one HTTP client it uses, and the outer afterEach disconnects it and stops the server.

The line-1190 pattern it points at is the right guard for the case it describes — a test insideTool Methods that swaps servers — which is exactly why that one disconnects first and this one doesn't need to.

For the record the test does pass in isolation and in the full integration project, with no hanging-process warning from vitest.