Context

I'm building microsoft/apm-action — a GitHub Action that deploys agent primitives (instructions, prompts, skills, agents) into CI environments via steps: frontmatter in GH AW workflows.

After apm install, the Copilot agent prints a deployment summary table listing all installed primitives. In the step summary, only 1 out of 7 rows is visible — the rest is silently truncated.

Observed behavior

In run 22749832020, the Copilot agent's full output includes a 7-row table of deployed primitives. The step summary shows only row 1, ending with ... — which inside the code fence looks like part of the table content, not a truncation indicator.

Three things contribute to this:

1. $GITHUB_STEP_SUMMARY env var is a dangling reference inside the sandbox

The env var is inherited from the parent runner process, but the file it points to (e.g. /home/runner/_layout/_work/_temp/step_summary_<guid>) doesn't exist inside the awf sandbox's isolated filesystem. The agent tries cat >> "$GITHUB_STEP_SUMMARY", it fails silently, then the agent detects the failure and pivots to printing the output directly — consuming ~200 chars of error recovery text like:

"The GITHUB_STEP_SUMMARY environment variable points to a file that doesn't exist or isn't accessible in this environment. Let me output the summary directly instead..."

This recovery text has zero value for the user but consumes ~40% of the truncation budget.

2. Per-text-block truncation at 500 chars with no visible indicator

In log_parser_shared.cjs, generateCopilotCliStyleSummary() (~line 1270):

const maxTextLength = 500;
let displayText = text;
if (displayText.length > maxTextLength) {
  displayText = displayText.substring(0, maxTextLength) + "...";
}

The ... appended after truncation is indistinguishable from content when rendered inside a code fence. There's no ⚠️ banner or distinguishable marker. The existing SIZE_LIMIT_WARNING only fires when the overall 1MB step summary limit is reached — not for individual text-block truncation.

3. Failed bash command shows ✓ instead of ✗

The cat >> "$GITHUB_STEP_SUMMARY" command fails (the file doesn't exist), but the step summary shows ✓ next to it. This is because scanForToolErrors() in parse_copilot_log.cjs only detects errors via [ERROR] log markers. Bash commands that fail without producing an [ERROR]-prefixed line default to is_error: false.

Impact

For actions that install or configure resources and report results via agent text output, the 500-char limit means structured output (tables, lists) with preamble text gets cut off before the useful content. The dangling $GITHUB_STEP_SUMMARY env var makes this worse by causing the agent to waste the budget on error recovery prose.

Workaround

We've worked around this in microsoft/apm-action#4 by outputting a compact one-liner first:

APM: 5 primitives deployed (2 prompts, 1 agent, 2 skills) + AGENTS.md

This ~70-char headline always survives the 500-char truncation. The detailed file listing follows as bonus info.

Environment

• GH AW version: v0.53.4
• Engine: Copilot CLI
• Run: https://github.com/microsoft/apm-action/actions/runs/22749832020