roslyn-bug-fix

name	roslyn-bug-fix
description	Drive a Roslyn compiler/IDE bug from a GitHub issue to an accepted fix using a repro-first, subagent-orchestrated pipeline (repro → baseline commit → root-cause → fix → evaluate → summary). Use when asked to fix a dotnet/roslyn issue, reproduce a Roslyn bug as a unit test, or run the bug-fix loop on an issue number.
user-invocable	true
argument-hint	<roslyn issue number> (e.g. 81679)

Roslyn Bug Fix

Orchestrate a single dotnet/roslyn bug fix. You are the orchestrator: you own setup, the journal, commits, and the fixer↔evaluator loop. You delegate the thinking-heavy steps to subagents (Task tool, general-purpose) so each works in a clean context with a focused, self-contained prompt.

Core principles, in priority order: correctness > readability > minimal diff. The fix is not done until a repro test goes red→green, all changed product code is covered by tests, and the broader suite shows no regressions.

Run in the local roslyn checkout (default Q:\repos\roslyn; confirm before assuming). Operate on one issue per skill invocation.

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Setup (orchestrator, once)

1. Resolve the issue. gh issue view <n> --repo dotnet/roslyn --json title,body,labels,comments. Determine area from labels: Area-Compilers → compiler, Area-IDE/Area-Analyzers → IDE.
2. Working branch. From an up-to-date base: git switch -c fix-issue-<n>-<slug>.
3. Journal. Create .agent/roslyn-bug-fix/issue-<n>/journal.md and keep it untracked by appending /.agent/roslyn-bug-fix/ to .git/info/exclude. Never git add the journal or the .agent dir.
4. Append a journal Setup entry (issue link, area, branch, base SHA).

Journal everything. After every step below, append a dated section: step name, actor (orchestrator / which subagent), what happened, key decisions, learnings, dead-ends. The journal is the durable record — write to it before moving on.

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Pipeline

1 — Read the issue (orchestrator)

Summarize the expected vs actual behavior and the minimal trigger into the journal. Extract any code snippet the reporter gave. Note: repro snippets may live behind 3rd-party links or msft-internal complogs — if the repro is not self-contained, reconstruct it from the description.

2 — Create a repro test — BLACK BOX (subagent)

Dispatch a general-purpose subagent with prompts/2-repro.md, filling {ISSUE_URL}, {ISSUE_NUMBER}, {AREA}, {EXPECTED_VS_ACTUAL}, {REPRO_SNIPPET}. The template enforces the black-box boundary (no product code). After it returns, verify the subagent did not touch product code (git diff --name-only) and journal the test's FQN and observed output.

3 — Simplify the repro (subagent)

Dispatch prompts/3-simplify.md (still black-box), filling {REPRO_FQN}, {CURRENT_TEST_SOURCE}. Journal the minimized repro.

4 — Baseline commit (orchestrator)

Stage only the test file(s) and commit the bug-demonstrating baseline: git commit -m "Add failing repro for #<n>: <title>". This snapshot encodes current (wrong) behavior so the fix's effect is reviewable as a diff. Record the SHA in the journal.

5 — Root-cause analysis (subagent)

Dispatch prompts/5-root-cause.md (this subagent now reads product code, analysis only), filling {ISSUE_URL}, {ISSUE_NUMBER}, {AREA}, {MINIMIZED_REPRO}, {EXPECTED_VS_ACTUAL}. Record the options and recommendation in the journal. Orchestrator confirms the recommended option (ask the user only if genuinely ambiguous or risky).

6 — Fix + update test (fixer subagent)

Dispatch prompts/6-fixer.md, filling {ISSUE_URL}, {ISSUE_NUMBER}, {AREA}, {CHOSEN_OPTION}, {ROOT_CAUSE_NOTES}, {REPRO_FQN}, and {PRIOR_FEEDBACK} (empty on the first pass; the evaluator's numbered list on later loops). Orchestrator commits progress: git commit -m "Fix #<n>: <short description>". Journal what changed and why.

7 — Evaluate (evaluator subagent + rubber-duck review)

Evaluation has two parts; run both each round and combine their findings.

7a — Mechanical evaluation. Dispatch prompts/7-evaluator.md to a fresh general-purpose evaluator (no fixer context beyond the diff), filling {ISSUE_URL}, {ISSUE_NUMBER}, {AREA}, {BASE_SHA}. The template judges the diff against correctness > readability > minimal diff: confirms the repro fails without the product change, checks every changed branch is test-covered, removes stale issue references (keeping WorkItem attributes), and runs the area's tests for regressions.

7b — Rubber-duck review. Also delegate the diff to a rubber-duck agent (Task tool, rubber-duck type) to catch logic errors, missed edge cases, and design flaws the author and the mechanical pass may have overlooked. Give it the issue, the chosen fix option, and the branch diff (git diff <base>..HEAD); ask for bugs/correctness/edge-case concerns only (it already ignores style/formatting). Fold any genuine issues it raises into the verdict.

Combine 7a + 7b: output exactly ACCEPTED (only if both passes are clean) or a single numbered list of concrete, actionable feedback. Append both the mechanical result and the rubber-duck findings to the journal.

8 — Loop (orchestrator)

If the verdict is not ACCEPTED, hand the numbered feedback back to the fixer (step 6) and re-evaluate (step 7). Keep iterating until ACCEPTED; journal each round. If the loop stalls (≈3 rounds with no progress), pause and surface the blocker to the user.

9 — Summary (orchestrator)

Once ACCEPTED, write a concise HTML summary to present: root cause, the fix and why it was chosen over alternatives, tests added/updated, files changed, validation run, and any follow-ups. Offer next steps (push branch, open a draft PR referencing Fixes #<n>). Confirm the push remote with the user before pushing, and always create the PR as a draft (gh pr create --draft).

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Subagent prompting notes

• The per-role prompt templates live in prompts/ (2-repro, 3-simplify, 5-root-cause, 6-fixer, 7-evaluator). Fill every {PLACEHOLDER} before dispatching — subagents are stateless and see only what you paste in.
• Enforce the black-box boundary in steps 2–3: the repro author must not look at product code, so the reproduction stays independent of the implementation.
• Keep the evaluator independent from the fixer (separate invocation, judge the diff, not the reasoning) so it can catch the fixer's blind spots.
• Commits are made by the orchestrator with explicit file paths — never git add -A (keeps the journal/.agent out of history). Do not add Co-authored-by trailers.

Quick reference (roslyn)

• Build: dotnet build Compilers.slnf · dotnet build Ide.slnf
• Run one test: dotnet test <test.csproj> --framework net10.0 --filter "FullyQualifiedName~<Name>"
• Compiler tests assert via comp.VerifyEmitDiagnostics(); prefer raw string literals and .Single().
• After .resx edits: dotnet msbuild <proj> /t:UpdateXlf. After Syntax.xml/BoundNodes.xml: dotnet run --file eng/generate-compiler-code.cs.
• Update PublicAPI.Unshipped.txt when public APIs change.