Add copy-to-output-directory MSBuild skill (IfDifferent / SkipUnchangedFilesOnCopyAlways) by YuliiaKovalova · Pull Request #883 · dotnet/skills

YuliiaKovalova · 2026-07-15T16:13:33Z

Summary

Adds a focused dotnet-msbuild skill — copy-to-output-directory — documenting how to choose a CopyToOutputDirectory / CopyToPublishDirectory mode, including the IfDifferent mode and the $(SkipUnchangedFilesOnCopyAlways) property introduced in MSBuild 17.13 / .NET SDK 9.0.2xx by dotnet/msbuild#11052.

No existing skill covered this feature — it was only mentioned tangentially in incremental-build and msbuild-antipatterns.

What the skill covers

The four modes (Never / PreserveNewest / Always / IfDifferent) with a comparison table.
Why Always is a recurring per-build performance hit, and the destination-mutation scenario it was historically used for.
IfDifferent semantics: copies when source and destination differ in either direction (newer or older, or size differs), via the Copy task's SkipUnchangedFiles="true" (timestamp + size comparison).
The $(SkipUnchangedFilesOnCopyAlways) property for a bulk, non-invasive opt-in on legacy Always items.
How the modes flow through the common targets, transitive copy through ProjectReference, and the MSBuild 17.13 / .NET SDK 9.0.2xx+ version requirement.

Changes

New: plugins/dotnet-msbuild/skills/copy-to-output-directory/SKILL.md
Registered the skill for routing in the msbuild, msbuild-code-review, and build-perf agent files.

Validation

markdownlint-cli2 passes with 0 errors on the new file.

Copilot

Pull request overview

Adds a new dotnet-msbuild skill, copy-to-output-directory, to document MSBuild’s copy-to-output/publish metadata modes (including the newer IfDifferent mode and $(SkipUnchangedFilesOnCopyAlways)), and wires the skill into existing agent routing so it can be recommended during troubleshooting, code review, and build performance work.

Changes:

Added a new skill doc: copy-to-output-directory (mode comparison, IfDifferent semantics, and version gating guidance).
Updated msbuild, msbuild-code-review, and build-perf agents to reference the new skill for routing and specialization.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
plugins/dotnet-msbuild/skills/copy-to-output-directory/SKILL.md	New skill documentation covering `CopyToOutputDirectory`/`CopyToPublishDirectory` mode selection and MSBuild 17.13+ features.
plugins/dotnet-msbuild/agents/msbuild.agent.md	Adds routing and skill list entry for the new `copy-to-output-directory` skill.
plugins/dotnet-msbuild/agents/msbuild-code-review.agent.md	Adds the new skill to the companion skills reference list.
plugins/dotnet-msbuild/agents/build-perf.agent.md	Adds the new skill to the performance-focused specialized skills reference list.

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Adds a focused dotnet-msbuild skill documenting how to choose a CopyToOutputDirectory / CopyToPublishDirectory mode, including the `IfDifferent` mode and the `$(SkipUnchangedFilesOnCopyAlways)` property introduced in MSBuild 17.13 / .NET SDK 9.0.2xx (dotnet/msbuild#11052). Covers mode semantics (Never/PreserveNewest/Always/IfDifferent), why `Always` is a per-build perf hit, the SkipUnchangedFiles timestamp+size comparison, transitive copy through ProjectReference, and version requirements. Registers the skill in the msbuild, msbuild-code-review, and build-perf agent routing lists. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 881d26ad-87b7-48e3-8ec6-043163944437

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Copilot

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated no new comments.

github-actions · 2026-07-15T16:56:54Z

Skill Validation Results

Model: claude-opus-4.6 | Judge: claude-opus-4.6

🔍 Full Results - additional metrics and failure investigation steps

▶ Sessions Visualisation -- interactive replay of all evaluation sessions
📊 Session Analytics (preview) -- aggregated metrics across evaluation sessions

github-actions · 2026-07-15T17:45:20Z

✅ Evaluation passed for 63ffc97. cc @ViktorHofer @JanKrivanek @dotnet/skills-msbuild-reviewers — please review.

Evangelink

Reviewed in depth — approving. Verified every technical claim against the live dotnet/msbuild Microsoft.Common.CurrentVersion.targets:

Target names _CopyOutOfDateSourceItemsToOutputDirectoryAlways, _CopyDifferingSourceItemsToOutputDirectory, and _CopyToOutputDirectoryTransitiveItems — exact match.
$(SkipUnchangedFilesOnCopyAlways) defaults to false and is passed as SkipUnchangedFiles="$(SkipUnchangedFilesOnCopyAlways)" on the Always target — correct.
IfDifferent target uses SkipUnchangedFiles="true" (timestamp + size heuristic) — correct, and the doc's careful "not a content hash" wording is accurate.
FileWrites registration, transitive copy via ProjectReference, and ClickOnce inclusion of IfDifferent alongside Always/PreserveNewest — all confirmed.
Version claim (MSBuild 17.13 / SDK 9.0.2xx / dotnet/msbuild#11052, merged 2024-12-05) — accurate.

Non-blocking notes for consideration:

Skill-menu budget headroom (heads-up). Using the validator's exact RenderedSkillMenuCost formula, this brings dotnet-msbuild to 14,953 / 15,000 chars — only ~47 chars of headroom. It passes check, but given the recent restore-15k-skill-budget work, any later skill or a small description tweak will tip the plugin over and silently truncate the alphabetically-last skill's description. Might be worth reclaiming a few chars from an over-long sibling description (e.g. property-patterns ~975, target-authoring ~929) at some point.
Description routing pattern. 16/18 sibling skills use the USE FOR: / DO NOT USE FOR: pattern for routing. This one uses a plain sentence — reasonable given the budget pressure above, just flagging it as an intentional trade-off.
Minor wording. "GetCopyToOutputDirectoryItems ... into three copy targets, which run from CopyFilesToOutputDirectory" — the three copy targets are actually invoked via _CopySourceItemsToOutputDirectory (a dependency of CopyFilesToOutputDirectory). Harmless simplification.

Nice, well-sourced addition. 👍

- Soften the IfDifferent description to avoid overclaiming: it skips when "unchanged per MSBuild's heuristic" (timestamp + size, not a content hash), rather than "identical"/full Always equivalence. - Correct the target-flow wording: the three copy targets run as dependencies of _CopySourceItemsToOutputDirectory (invoked by CopyFilesToOutputDirectory). - Restore the USE FOR / DO NOT USE FOR routing pattern for the skill's frontmatter description, matching sibling skills. - Reclaim skill-menu budget headroom (per maintainer note) by tightening the verbose property-patterns and target-authoring descriptions; dotnet-msbuild rendered menu is now ~14,897/15,000 chars. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 881d26ad-87b7-48e3-8ec6-043163944437

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 881d26ad-87b7-48e3-8ec6-043163944437

Copilot

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

Evangelink

Re-reviewed after the follow-up commits — all three points from my prior review are addressed, and I re-verified the changes:

Budget headroom reclaimed. Trimming property-patterns and target-authoring descriptions offsets the now-richer copy-to-output-directory description. Recomputed with the validator's exact RenderedSkillMenuCost formula: dotnet-msbuild is now 14,901 / 15,000 chars (~99 headroom, up from ~47). Fits with a bit more room.
USE FOR / DO NOT USE FOR restored on the new skill's description — matches sibling convention.
Target-flow wording corrected and accurate. Verified against the common targets: CopyFilesToOutputDirectory → _CopySourceItemsToOutputDirectory → the three copy targets (_CopyOutOfDateSourceItemsToOutputDirectory, _CopyOutOfDateSourceItemsToOutputDirectoryAlways, _CopyDifferingSourceItemsToOutputDirectory).
IfDifferent description softened to correctly frame SkipUnchangedFiles as a timestamp+size heuristic (not a content hash) — accurate.
"an SQLite" article fix — good catch.

All previously-verified technical claims still hold. LGTM. 👍

github-actions · 2026-07-16T09:05:05Z

👋 @YuliiaKovalova — this PR has 1 unresolved review thread(s). When you're ready, please address the feedback and push an update; the triage bot will pick up the next state automatically. (Add the no-stale label to silence further pings.)

Evangelink

Approving — no changes since my last review (head 7b047f2, already verified). Technical claims check out against MSBuild source, skill-menu budget fits (14,901/15,000), and all earlier feedback was addressed. LGTM. 👍

github-actions · 2026-07-16T10:18:11Z

Skill Validation Results

Skill	Scenario	Quality	Skills Loaded	Overfit	Verdict
property-patterns	Diagnose shared build property issues	5.0/5 → 5.0/5	✅ property-patterns; tools: skill / ✅ property-patterns; tools: glob, skill	✅ 0.18	❌ [1]
property-patterns	Diagnose multi-level property hierarchy bugs	4.3/5 → 5.0/5 🟢	✅ property-patterns; tools: skill / ✅ property-patterns; tools: skill, bash	✅ 0.18	✅ [2]
property-patterns	Fix shared property configuration	5.0/5 → 5.0/5	✅ property-patterns; tools: glob, skill / ✅ property-patterns; tools: skill	✅ 0.18	❌
target-authoring	Diagnose custom target build regression	5.0/5 → 5.0/5	✅ target-authoring; tools: skill / ✅ target-authoring; tools: skill, bash	✅ 0.18	❌ [3]
target-authoring	Diagnose broken SDK target chain across files	3.0/5 → 3.0/5	✅ target-authoring; tools: skill	✅ 0.18	✅ [4]
target-authoring	Fix custom target anti-patterns	4.0/5 → 4.7/5 🟢	✅ target-authoring; tools: bash, skill, glob / ⚠️ NOT ACTIVATED	✅ 0.18	❌ [5]

[1] (Isolated) Quality unchanged but weighted score is -18.1% due to: judgment, tokens (108549 → 227575), time (66.4s → 83.7s)
[2] ⚠️ High run-to-run variance (CV=101%) — consider re-running with --runs 5
[3] ⚠️ High run-to-run variance (CV=143%) — consider re-running with --runs 5. (Isolated) Quality unchanged but weighted score is -11.9% due to: judgment, tokens (94587 → 118464)
[4] ⚠️ High run-to-run variance (CV=1080%) — consider re-running with --runs 5
[5] ⚠️ High run-to-run variance (CV=320%) — consider re-running with --runs 5. (Plugin) Quality unchanged but weighted score is -6.7% due to: tokens (91323 → 290219), time (41.1s → 80.8s), tool calls (9 → 15)

Model: claude-opus-4.6 | Judge: claude-opus-4.6

🔍 Full Results - additional metrics and failure investigation steps

To investigate failures, paste this to your AI coding agent:

For PR 883 in dotnet/skills, download eval artifacts with gh run download 29489283964 --repo dotnet/skills --pattern "skill-validator-results-*" --dir ./eval-results, then fetch https://raw.githubusercontent.com/dotnet/skills/7b047f26a74f59b806f66b31231fb9cf741bbb7e/eng/skill-validator/src/docs/InvestigatingResults.md and follow it to analyze the results.json files. Diagnose each failure, suggest fixes to the eval.yaml and skill content, and tell me what to fix first.

Copilot AI review requested due to automatic review settings July 15, 2026 16:13

YuliiaKovalova requested review from a team, JanKrivanek and ViktorHofer as code owners July 15, 2026 16:13

Copilot started reviewing on behalf of YuliiaKovalova July 15, 2026 16:14 View session

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Copilot AI review requested due to automatic review settings July 15, 2026 16:25

YuliiaKovalova force-pushed the ykovalova/copy-to-output-directory-skill branch from 2875d0e to a608f8e Compare July 15, 2026 16:25

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Comment thread plugins/dotnet-msbuild/skills/copy-to-output-directory/SKILL.md Outdated

Potential fix for pull request finding

63ffc97

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings July 15, 2026 16:40

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github-actions Bot added the pr-state/ready-for-eval PR is mergeable and awaiting evaluation label Jul 15, 2026

github-actions Bot added waiting-on-review PR state label and removed pr-state/ready-for-eval PR is mergeable and awaiting evaluation labels Jul 15, 2026

Evangelink approved these changes Jul 16, 2026

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Copilot AI review requested due to automatic review settings July 16, 2026 08:35

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Comment thread plugins/dotnet-msbuild/skills/copy-to-output-directory/SKILL.md Outdated

Fix article: 'an SQLite' per review

7b047f2

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com> Copilot-Session: 881d26ad-87b7-48e3-8ec6-043163944437

Copilot AI review requested due to automatic review settings July 16, 2026 08:40

Copilot started reviewing on behalf of YuliiaKovalova July 16, 2026 08:40 View session

Copilot AI reviewed Jul 16, 2026

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Evangelink approved these changes Jul 16, 2026

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YuliiaKovalova enabled auto-merge (squash) July 16, 2026 08:49

github-actions Bot added waiting-on-author PR state label and removed waiting-on-review PR state label labels Jul 16, 2026

github-actions Bot added pr-state/ready-for-eval PR is mergeable and awaiting evaluation and removed waiting-on-author PR state label labels Jul 16, 2026

Evangelink approved these changes Jul 16, 2026

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YuliiaKovalova merged commit 9c7436b into dotnet:main Jul 16, 2026
37 checks passed

github-actions Bot added a commit that referenced this pull request Jul 16, 2026

Update PR token usage data (PR #883)

fad232d

github-actions Bot mentioned this pull request Jul 31, 2026

Weekly plugin version sync #978

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YuliiaKovalova commented Jul 15, 2026

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