[Dreaming] Dreaming: curate learning atoms for week of 2026-07-08 #14368

Weekly learning-atom curation (2026-07-08 to 2026-07-15)

This PR encodes two small recurring lessons observed in human-authored PRs from the past week.

Change 1 — .github/instructions/tasks.instructions.md

What: Added a bullet clarifying that changing an existing parameter default value is a behavioral change requiring a ChangeWave.

Why: PR #14285 (Default XML tasks to prohibit DTDs) received three separate inline reviewer comments (on XmlPeek.cs, XmlPoke.cs, and XslTransformation.cs) all pointing out the same issue: flipping a task parameter default from ignore to prohibit changes behavior for existing builds and must be gated behind a ChangeWave. The existing guidance covered new parameters but not changing existing defaults.

Not already covered: Existing bullet says "New optional parameters must default to preserving existing behavior" — does not address changing an already-established default.

Change 2 — .github/instructions/tests.instructions.md

What: Added a sentence requiring a unit test for every new contract, guard, or boundary condition.

Why: Two separate PRs had reviewer comments about missing boundary tests:

• PR Add opt-in partial (stop-after-pass) project evaluation #14290 (Add opt-in partial evaluation): reviewer flagged that ItemDefinitions at the Properties stage was undocumented by tests, risking silent regressions.
• PR Preventing coordinator messages from getting filtered out by terminal logger #14356 (Preventing coordinator messages from being filtered): reviewer noted the new null-BEC + HIGH-importance logger path had no unit test.

Not already covered: Existing tests instructions cover helpers, platform conditions, and data-driven patterns — nothing about testing boundary/contract conditions.

network:
  allowed:
    - defaults
    - "awmgmcpg"

Generated by Dreaming (learning atoms curation) · 44.7 AIC · ⌖ 9.09 AIC · ⊞ 6.7K · ◷

To create a pull request with the changes:

# Download the artifact from the workflow run
gh run download 29411365525 -n agent -D /tmp/agent-29411365525

# Fetch the bundle into a temporary ref, then update the local branch
git fetch /tmp/agent-29411365525/aw-dreaming-learning-atoms-2026-07-15.bundle refs/heads/dreaming/learning-atoms-2026-07-15:refs/bundles/create-pr-dreaming-learning-atoms-2026-07-15-256762709b214f9c-40a697a0
git update-ref refs/heads/dreaming/learning-atoms-2026-07-15-256762709b214f9c refs/bundles/create-pr-dreaming-learning-atoms-2026-07-15-256762709b214f9c-40a697a0
git checkout dreaming/learning-atoms-2026-07-15-256762709b214f9c
# Ensure the working tree matches the updated branch
git reset --hard
# Remove the temporary bundle ref
git update-ref -d refs/bundles/create-pr-dreaming-learning-atoms-2026-07-15-256762709b214f9c-40a697a0

# Push the branch to origin
git push origin dreaming/learning-atoms-2026-07-15-256762709b214f9c

# Create the pull request
gh pr create --title '[Dreaming] Dreaming: curate learning atoms for week of 2026-07-08' --base main --head dreaming/learning-atoms-2026-07-15-256762709b214f9c --repo dotnet/msbuild