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Improve Copilot guidance for compatibility and validation

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Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
76/100
Issue type
Documentation
Clarity
Clearly specified
Activity status
Active
Domain
developer-experience, documentation

Research direction

Start with .github/copilot-instructions.md and review its existing Task Scope and Completion and Running tests sections, then compare the repository and product-area guidance mentioned in the issue. Add repository-wide guidance covering compatibility characterization, fresh-worktree preflight and bounded fallback, and observable validation evidence. Done means the guidance is explicit without changing product code, build scripts, tests, CI, or product-specific instructions.

Written by the indexing model from the issue text.

Description

design-proposal

Summary

Add repository-wide Copilot guidance for three recurring agent-workflow failure modes: late compatibility analysis, fresh-worktree build prerequisite thrash, and false-green validation based only on command exit codes.

Motivation and goals

Recent agent-assisted work in this repository repeatedly encountered gaps that are not explicit in .github/copilot-instructions.md:

  • In #69237, replacing formatter-based SAN parsing with a typed decoder correctly fixed culture/OS-sensitive parsing, but malformed-input behavior, common-name fallback, and the security/compatibility policy were only fully characterized during review. Similar representation changes involving protected Blazor circuit payloads required repeated clarification of legacy, reconnect, pause/resume, and rolling-upgrade semantics.
  • Several fresh-worktree validations spent time reaching unrelated prerequisites before the changed target: missing repository SDK/bootstrap state, submodules, generated JavaScript assets, native dependencies, or wix.exe. The eventual useful validation often required a narrower faithful target, but the repository-wide guidance only names the canonical area wrapper.
  • A successful exit code can be false-green. Examples include filtered test commands selecting no intended tests, Pack succeeding without producing the required package, and builds stopping at unrelated prerequisites before the requested tests execute.

The goal is to make compatibility decisions earlier, reduce repeated build-command experimentation, and require evidence that validation exercised and produced the intended result.

In scope

  • Add guidance under Task Scope and Completion requiring compatibility characterization before replacing parsers, formatters, serializers, or protected/persisted payload representations. The characterization should cover valid, malformed, and legacy inputs; exceptions versus fallback; relevant OS/culture differences; rolling upgrades; and downstream security/identity effects.
  • Add a Running tests preflight for fresh worktrees: read product-area guidance and identify required SDK bootstrap, submodules, generated assets, and native tools before invoking an expensive area build.
  • Keep the canonical area build as the first choice, but define a bounded fallback when it fails before reaching the changed target because of an unrelated prerequisite: use the smallest faithful command permitted by product-area guidance and report the exact limitation.
  • Require observable validation evidence: the intended nonzero test set ran, required artifacts were produced, and execution reached the relevant assertion or behavior.

Out of scope

  • Changing product code, build scripts, test infrastructure, or CI.
  • Replacing product-specific AGENTS.md, README, or .github/instructions/*.instructions.md guidance.
  • Treating a narrow fallback as equivalent to the canonical area build or full suite.
  • Requiring every worktree to initialize every submodule or build every generated asset when the intended validation path does not need them.

Risks / unknowns

  • The fallback wording must not encourage agents to bypass canonical build scripts prematurely. It should apply only when the canonical path fails before reaching the changed project/test due to a demonstrably unrelated prerequisite.
  • Repository-wide wording could duplicate product-specific setup guidance. The proposed rule should require discovering and following that guidance rather than restating individual prerequisites.
  • Compatibility analysis can become unbounded. The rule should apply to representation/decoder changes where malformed, legacy, or platform-specific behavior can materially change, not to routine internal refactoring.

Examples

Possible guidance under Task Scope and Completion:

Before replacing a parser, formatter, serializer, or protected/persisted payload representation, characterize the existing behavior for valid, malformed, and legacy inputs. Include exception versus fallback behavior, platform or culture differences where relevant, rolling-upgrade behavior, and downstream security or identity effects. Resolve the intended compatibility policy before implementation or tests codify the new behavior.

Possible guidance under Running tests:

Before the first build or test in a fresh worktree, read the product-area guidance and identify required repository SDK bootstrap, submodules, generated assets, and native tools. Prepare the prerequisites needed by the intended validation path before invoking an expensive area build.

If the canonical area build fails before reaching the changed project or test because of an unrelated repository prerequisite, do not repeatedly retry it or describe the affected behavior as validated. Use the smallest faithful product or project command permitted by the product-area guidance, and report both the blocking prerequisite and the narrower validation boundary.

Verify the observable result of each validation command, not only its exit code. Confirm filtered test commands select the intended nonzero set of tests, pack or build commands produce the required artifacts, and test execution reaches the relevant assertion or behavior.

Dominant language
C#
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Forks
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Avg merge
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Merged PRs (30d)
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