pr-description-skill

PR Description Skill -- Anchored, Concise, Validated PR Bodies

When to use

Trigger this skill on any of the following intents:

• "write a PR description"
• "draft a PR body"
• "open a PR" / "open this PR" / "let's open the PR"
• "fill in the PR template"
• "summarize this branch as a PR"
• "create the PR write-up"

Reusable for any PR against microsoft/apm. The output is one markdown file that the orchestrator pastes into gh pr create --body-file or surfaces to the maintainer.

Output charset rule (read this first)

The repo-wide encoding rule at .github/instructions/encoding.instructions.md constrains source files and CLI output to printable ASCII because Windows cp1252 terminals raise UnicodeEncodeError on anything else. PR comments are NOT source code and NOT CLI output -- they are rendered by GitHub's Primer engine, which expects UTF-8 GitHub-Flavored Markdown.

Two distinct rules therefore apply:

1. Source files in this bundle (SKILL.md, assets/*) MUST stay ASCII. They live in the repo and are subject to .github/instructions/encoding.instructions.md.
2. The PR body output the skill produces MUST be UTF-8 GitHub-Flavored Markdown. Use em dashes, smart punctuation, alerts, collapsibles, task lists, and Unicode where it improves readability. Mermaid diagram labels MAY use Unicode -- there is no constraint here. The output is consumed by GitHub's renderer, not by a Windows terminal.

A previous version of this skill incorrectly required ASCII in the PR body. That made the output unreadable: no alerts, no collapsibles for long evidence, no em dashes, no smart quotes. Reviewers had to scroll through hundreds of flat lines instead of scanning a body shaped by GFM features.

Concision targets (hard ceilings)

The skill aims for 150-220 lines for a typical PR body. 300+ lines is a smell, not a virtue. If your draft exceeds 250 lines, run a tightening pass: every sentence that does not change the reviewer's understanding must be cut.

Per-section ceilings (enforced by assets/section-rubric.md):

Long verbatim quote blocks, full file listings, and full validation transcripts SHOULD live inside <details> so the body stays scannable.

Core principles (with quoted anchors)

Each rule the skill enforces is backed by a verbatim quote from one of the two reference docs. If a rule below cannot be backed by a quote, it is downgraded to a "should" with the reason given.

1. Self-sufficient body. A reviewer must be able to read the PR body and form an opinion without opening any other doc, issue, or chat. Every WHY-claim cites the source doc inline; every named file is qualified with what changed in it; every diagram has a one-sentence legend.

   Anchor: Agent Skills, "agents pattern-match well against concrete structures".

2. Anchored: every WHY-claim cites its source. Every claim of the form "this violates X" or "this satisfies Y" is followed by a verbatim quoted phrase wrapped in a hyperlink to the source page. Reproduce quotes character-for-character; do not paraphrase inside link text.

   Anchor: PROSE, "Grounding outputs in deterministic tool execution transforms probabilistic generation into verifiable action.".

3. Cite-or-omit. If a WHY-claim cannot be backed by a verbatim quote, drop it or soften to a tradeoff statement. Never invent justification.

   Anchor: Agent Skills, "Add what the agent lacks, omit what it knows".

4. Visual aid where structure is non-trivial. Any change that touches more than one file or alters control flow SHOULD include at least one mermaid diagram. Add a second only when the relationships are non-trivial. Never add a third unless it earns its place. Each diagram MUST be preceded by a one-sentence legend.

   Anchor: Agent Skills, "agents pattern-match well against concrete structures".

5. Trade-offs explicit. Address every non-obvious decision (option chosen vs option rejected). For mechanical PRs this section may be 1-2 bullets. For cross-cutting changes, surface the rejected alternatives.

   Anchor: PROSE, "Favor small, chainable primitives over monolithic frameworks.".

6. Single artifact, no fluff. One markdown file. No marketing tone, no self-congratulation. TL;DR is at most four sentences.

   Anchor: Agent Skills, "When you find yourself covering every edge case, consider whether most are better handled by the agent's own judgment.".

GitHub-Flavored Markdown features the skill MUST use

The PR body is rendered by GitHub's Primer engine. Use the features that engine provides; do not flatten the output to plain text.

• Alerts for high-signal callouts: > [!NOTE], > [!TIP], > [!IMPORTANT], > [!WARNING], > [!CAUTION]. Reference: https://github.com/orgs/community/discussions/16925.

• Collapsible sections for long diffs, full validation output, or appendix material:

  <details><summary>Full audit output</summary>
   
  ...content...
  </details>

  Use <details open> only when the content answers the most likely first reviewer question.

• Task lists for "How to test" sections: - [ ] Apply label, observe X.

• Tables with alignment: | col | :---: | ---: | for matrices.

• Permalink references to specific lines in the diff: https://github.com/microsoft/apm/blob/<sha>/path#L12-L34.

Long verbatim quote blocks, full file listings, and full validation transcripts SHOULD live inside <details> so the body stays scannable.

Required body structure

The Trade-offs (7) and How to test (10) sections are non-skippable for any PR that changes more than docs.

Activation contract -- inputs the orchestrator MUST gather first

Before invoking this skill, the orchestrator MUST have collected all of the following. The skill MUST NOT invent facts not present in these inputs.

If any required input is missing, the orchestrator MUST stop and collect it. This is a Progressive Disclosure boundary: "Context arrives just-in-time, not just-in-case.". Do not load assets/pr-body-template.md until the table above is complete.

Execution checklist

Run these steps in order. Tick each before moving on.

1. Confirm every row of the activation contract is filled in. Defense-in-depth gate: before drafting the body, confirm the repo's lint contract is green (canonical commands and lifecycle binding live in .apm/instructions/linting.instructions.md). If lint is red, STOP, fix, re-run; a PR body claiming green CI while lint fails is a credibility tax we refuse to take on.
2. Read the diff in full. Identify per-file change summary, new files, deleted files, behavior changes at module boundaries.
3. Load assets/pr-body-template.md. This is the only point at which the template enters context. Progressive Disclosure in action: "store them in assets/ and reference them from SKILL.md so they only load when needed.".
4. Fill in the template top-to-bottom using only facts from the activation contract. Every WHY-claim gets a verbatim quoted anchor. If you cannot anchor a claim, drop it.
5. Generate 1-3 mermaid diagrams. Before drafting any block, load assets/mermaid-conventions.md to pick the right diagram type per intent (sequenceDiagram for execution flow, flowchart LR for pipeline / architecture, stateDiagram-v2 for state machines) and apply the boxing convention for NEW behavior. Add a one-sentence legend above each diagram.
6. Validate every mermaid block deterministically (see below). Do NOT save the draft until every block validates.
7. Load assets/section-rubric.md and run the self-check pass. Validation loop pattern from Agent Skills: "do the work, run a validator (a script, a reference checklist, or a self-check), fix any issues, and repeat until validation passes.".
8. Run the line-count check. If the body exceeds 250 lines, tighten until it fits 150-220.
9. Write the final body to a single file path provided by the orchestrator (default: .git/PR_BODY.md or session-state-relative). Return the path; do not paste the body inline unless explicitly asked.

Mandatory mermaid validation step

Run every mermaid block in the draft through mmdc and refuse to save until all pass.

# Extract mermaid blocks and validate each one.
# Requires: npx --yes -p @mermaid-js/mermaid-cli mmdc (one-shot, no global install needed)
awk '/^```mermaid/{n++; f=outdir"/diag"n".mmd"; getline; while($0 != "```") {print > f; getline}}' outdir=/tmp/mermaid-check pr-body-draft.md
for f in /tmp/mermaid-check/diag*.mmd; do
  npx --yes -p @mermaid-js/mermaid-cli mmdc -i "$f" -o "${f%.mmd}.svg" --quiet || { echo "INVALID: $f"; exit 1; }
done

If mmdc reports any error, fix the diagram and re-run. The skill MUST NOT save the draft until every mermaid block validates.

Diagram type and pitfalls reference

The full diagram-type-by-intent table, canonical templates, and the GitHub-renderer gotcha list (mmdc does NOT always catch GitHub rejections) live in assets/mermaid-conventions.md. Load it whenever a PR body needs a mermaid block.

Critical drift-known gotcha (the one most likely to bite, captured inline because it is not obvious from mmdc output):

• Square brackets in flowchart edge labels MUST be quoted. A -->|[EXEC] work| B parses on mmdc but is rejected by GitHub's renderer (Expecting 'TAGEND', ..., got 'SQS'). Quote the label: A -->|"[EXEC] work"| B. The same rule applies to parentheses, colons, slashes, and pipes in edge labels.

For everything else (semicolons in classDiagram links, note right of closing rules, round brackets in node labels, inline :::cssClass failing in classDiagram on GitHub), see assets/mermaid-conventions.md.

Output contract

• Exactly ONE markdown file is produced.
• The file is UTF-8 GitHub-Flavored Markdown. Em dashes, smart quotes, Unicode in mermaid labels, alerts, and collapsibles are all permitted and encouraged where they improve readability.
• Every mermaid block has been validated by mmdc and renders without error.
• The cite-or-omit rule applies absolutely.
• The TL;DR is at most four sentences.
• The body ends with the trailer: Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Anti-patterns flagged -- refuse these

• Posting unvalidated mermaid. A parser error renders as raw code on GitHub and signals carelessness. Validate every block before saving.
• Pasting commit messages as the body. Commit messages are inputs, not output.
• Marketing tone or self-congratulation ("this is a great improvement", "significantly enhances", "best-in-class"). Strip on sight.
• Diagrams without a legend, OR diagrams that fail mmdc.
• A TL;DR longer than four sentences.
• Skipping any required section because "the PR is small". A small PR can have a one-line Implementation per file, but the section header must still be present.
• Restating the diff line-by-line in Implementation. That is what the Files Changed tab is for.
• Quoting a doc out of context. The self-check pass must verify that the quoted phrase actually supports the claim.
• Forcing ASCII-only on the PR body. That rule applies to source files and CLI output, not to Primer-rendered markdown. See "Output charset rule" above.

Gotchas

• Do not restate the diff. Implementation is for intent, risk, and decisions -- not a textual re-rendering of the patch.
• Do not quote out of context. Re-read the surrounding paragraph of the source doc before pasting a quote.
• Verify the source URL still serves the quoted text. If the doc has been edited and the phrase no longer appears verbatim, drop the citation or find a new anchor.
• A doc-only PR still needs TL;DR, Problem, Validation, and How-to-test. "The PR is trivial" is not an exemption.
• Long evidence belongs in <details>. Reviewers should be able to read the whole body in a single screen-and-a-half scroll and expand evidence on demand.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com