Surface extension build-script output when azd x build fails

[Copilot]

Extension creation/build failures were reporting a generic build error while dropping the actionable build-script output from the user-visible task summary. This made it unclear what failed and whether the extension state was recoverable.

• Failure-path output visibility

  • Keep task-list failure text concise (Build failed...) using subprocessErrorTail(...) for a short inline hint.
  • Persist full captured CombinedOutput() on build-script failure and print it after task-list rendering under a dedicated Build output: section.
  • Avoid embedding raw multi-line subprocess output in DetailedError description (task list only renders the description field).

• Behavior alignment with existing subprocess patterns

  • Reuse the existing subprocessErrorTail pattern already used by extension init subprocess handling, so users get a compact summary + full output details.

• Targeted test coverage

  • Add focused tests for the new output writer to verify:
    • output is emitted when non-empty
    • no output is emitted for whitespace-only content

if result, err := cmd.CombinedOutput(); err != nil {
    buildFailureOutput = string(result)

    return ux.Error, common.NewDetailedError(
        "Build failed"+subprocessErrorTail(result),
        fmt.Errorf("failed to build artifacts: %w", err),
    )
}

// deferred after task-list completion
writeBuildFailureOutput(os.Stderr, buildFailureOutput)

Original prompt

I noticed a small bug when creating an extension. When it failed (on the build step) the output appears to be thrown away, so (from a user perspective) you don't know what broke, or what you can do to fix it.

Can you take a look and see if that's deliberate, or if it's just a bug. If it's deliberate, I think we'll want to change it since it's at a very awkward spot, and it's unclear if you can retry or if it's busted completely.

The user has attached the following file paths as relevant context:

• .github/copilot-instructions.md
• AGENTS.md

Created from VS Code.

[richardpark-msft]

@copilot Are there are other stages in the extension workflow that have this same issue?

[Copilot]

@copilot Are there are other stages in the extension workflow that have this same issue?

Yes — I found at least one more in the extension workflow: azd x pack’s Building extension task (cli/azd/extensions/microsoft.azd.extensions/internal/cmd/pack.go). It puts CombinedOutput() into the wrapped error ("Command output: ..."), but TaskList renders only DetailedError.Description(), so that output is effectively dropped.

Also worth noting: init.go’s runSubprocess appends subprocessErrorTail(...) to the inner error (not the description), so the inline tail can be hidden there for the same reason.

[richardpark-msft]

@copilot Elaborate on 'the tail'

[Copilot]

@copilot Elaborate on 'the tail'

By “the tail” I mean the short suffix returned by subprocessErrorTail(...) (in init.go) and appended to the task description/error text.

It extracts one compact line from captured subprocess output:

• prefers the first ERROR: / Error: line
• otherwise uses the last non-empty line
• strips ANSI escape codes
• returns it as ": <message>"

So instead of dumping full multiline output, the task line becomes something like:
Build failed: failed to compile plugin

The full raw output should be printed separately (e.g., under Build output:).