Test failure

Postmortem bundle delivered after a renderFailed notification: phase, error type / message / top stack frame, a bounded stack trace, and explicit fallback fields for partial screenshot, pending effects, animation state, and a redacted snapshot summary.

At a glance

Kind	test/failure
Schema version	1
Modules	:data-render-core (published) · :data-render-connector
Render mode	failed render
Cost	low
Token usage	~195 tok per query (test/failure ~680 chars). See token usage.
Transport	inline
Availability	fetch-only after renderFailed
Platforms	Android · Desktop · shared

What it answers

• Why did the most recent render of this preview fail?
• What phase failed (compose, capture, encode, setup)?
• What error type / message / top stack frame, and a bounded stack trace?
• What was the renderer’s last visible state — pending effects, running animations, frame-clock state — at the moment of failure?

What it does NOT answer

• It does not include a partial screenshot in schema v1 (partialScreenshotAvailable: false); the fallback fields are placeholders for a future schema bump.
• It does not capture state object values — lastSnapshotSummary.redaction is “state values are not captured in schema v1”.
• It is not a live debugger — read the stack and the phase, then iterate; you will not get a step-debugger session out of this.

Use cases

• Triage a CI failure: fetch test/failure for every red preview to cluster failures by error type / phase.
• Distinguish a renderer bug from a consumer bug — a NPE in the consumer’s LaunchedEffect is theirs; a Robolectric sandbox boot failure is the renderer’s.
• Drive a “show me the failure” panel in the VS Code extension when a preview can’t render.

Payload shape

TestFailureDataProduct in :data-render-core.

// test/failure
{
  "status": "failed",
  "phase": "compose",
  "error": {
    "type": "NullPointerException",
    "message": "Cannot invoke \"...\" because \"...\" is null",
    "topFrame": "HomeScreen.kt:42",
    "stackTrace": [
      "com.example.HomeScreenKt.HomeScreen(HomeScreen.kt:42)",
      "..."
    ]
  },
  "partialScreenshot": null,
  "partialScreenshotAvailable": false,
  "pendingEffects": [],
  "runningAnimations": [],
  "frameClockState": { "status": "unknown" },
  "lastSnapshotSummary": {
    "stateObjects": null,
    "valuesCaptured": false,
    "redaction": "state values are not captured in schema v1"
  }
}

Enabling

Always available for the Render-pipeline extension. After a renderFailed notification, call:

{ "method": "tools/call", "params": { "name": "get_preview_data",
  "arguments": { "uri": "compose-preview://<id>/_module/com.example.Foo",
                 "kind": "test/failure" } } }

The kind is fetch-only — it does not ride attachments.

Companion products

• Render trace — render/trace, render/composeAiTrace for successful-render timing context.
• Layout inspector — layout/inspector from a previous successful render to compare against the failure point.