Update Skia Milestone Skill

Update Google Skia to a new Chrome milestone in SkiaSharp's mono/skia fork.

Key References

references/breaking-changes-checklist.md — How to analyze breaking changes between milestones
references/known-gotchas.md — Hard-won lessons and troubleshooting table
references/typical-changes.md — Files typically changed during an update
documentation/dev/dependencies.md — Dependency tracking and cgmanifest.json format
RELEASE_NOTES.md in upstream Skia — Official Skia release notes

Scripts

scripts/update-versions.ps1 — Phase 6: Updates all version files and runs verification
scripts/regenerate-bindings.ps1 — Phase 8: Regenerates bindings, reverts HarfBuzz, reports new functions

Overview

Updating Skia is the highest-risk operation in SkiaSharp. It touches:

The native C++ Skia library (upstream merge)
The custom C API shim layer (must be adapted to new C++ APIs)
Generated P/Invoke bindings
C# wrapper code
All platforms (macOS, Windows, Linux, iOS, Android, etc.)

Go slow. Research first. Build and test before any PR.

⚠️ Follow Every Phase In Order

This is an 11-phase workflow where each phase builds on the previous one. The phases exist because Skia updates touch four layers (C++ → C API → generated bindings → C# wrappers) and two repositories (mono/skia + mono/SkiaSharp). Skipping a phase doesn't just risk a build failure — it risks shipping broken binaries to customers who won't see the problem until runtime.

Each phase ends with a gate — a verification step that confirms the phase completed correctly. Re-read each phase's instructions before executing it, because the details are project-specific and easy to get wrong from memory.

What You're About to Do

A. Research (Phases 1–3)
   1. Discovery & Current State
   2. Breaking Change Analysis
   3. Validation

B. Branch & Merge (Phases 4–5)
   4. Branch Setup
   5. Upstream Merge

C. Update & Build (Phases 6–7)
   6. Update Version Files
   7. Fix C API Shim & Build Native

D. Regenerate & Verify (Phases 8–10)
   8. Regenerate Bindings
   9. Fix C# Wrappers
  10. Build & Test

E. Ship (Phase 11)
  11. Create PRs

Critical Rules

🛑 STOP AND ASK before: Merging PRs, Force pushing, Deleting branches, Any destructive git operations

🚫 BRANCH PROTECTION (MANDATORY)

Repository	Protected Branches	Action Required
mono/SkiaSharp (parent)	`main`	Create feature branch first
mono/skia (submodule)	`main`, `skiasharp`	Create feature branch first

❌ NEVER Do These

Shortcut	Why It's Wrong
Push directly to `skiasharp` or `main`	Bypasses PR review and CI
Skip breaking change analysis	Causes runtime crashes for customers
Use `externals-download` after C API changes	Causes `EntryPointNotFoundException`
Merge both PRs without updating submodule in between	Squash-merge orphans commits
Skip tests	Untested code = broken customers

A. Research (Phases 1–3)

Phase 1: Discovery & Current State

Identify current milestone:

bash

grep SK_MILESTONE externals/skia/include/core/SkMilestone.h
grep "^libSkiaSharp.*milestone" scripts/VERSIONS.txt
grep chrome_milestone cgmanifest.json

Identify target milestone from user request.
Check for existing PRs — Search both mono/SkiaSharp and mono/skia for open update PRs.
Verify upstream branches exist and fetch:
bash
```
cd externals/skia
git remote add upstream https://github.com/google/skia.git 2>/dev/null
git fetch upstream chrome/m{TARGET}
```
Note: When this phase is pre-computed by the automated workflow, you still need to add the upstream remote and fetch chrome/m{TARGET} — Phase 5 depends on it.

🛑 GATE: Confirm current milestone, target milestone, and that upstream branch exists.

Phase 2: Breaking Change Analysis

This is the most critical phase. Thorough analysis here prevents customer-facing breakage.

Read official release notes for EVERY milestone being skipped:
- Fetch https://raw.githubusercontent.com/google/skia/main/RELEASE_NOTES.md
- Document all changes for each milestone between current and target

Categorize changes by impact:

Category	Risk	Examples
Removed APIs	🔴 HIGH	Functions deleted, enums removed
Renamed/Moved APIs	🟡 MEDIUM	Namespace changes, header moves
New APIs	🟢 LOW	Additive changes, new factories
Behavior changes	🟡 MEDIUM	Default changes, semantic shifts
Graphite-only	⚪ SKIP	SkiaSharp uses Ganesh, not Graphite

Map each HIGH/MEDIUM change to C API files:

bash

cd externals/skia
# Check which C API files reference affected APIs
grep -r "GrMipmapped\|GrMipMapped" src/c/ include/c/
grep -r "refTypefaceOrDefault\|getTypefaceOrDefault" src/c/ include/c/

Run structural diff on include/ directory:

bash

git diff upstream/chrome/m{CURRENT}..upstream/chrome/m{TARGET} --stat -- include/
git diff upstream/chrome/m{CURRENT}..upstream/chrome/m{TARGET} -- include/core/ include/gpu/ganesh/

👉 See references/breaking-changes-checklist.md for the full analysis template, including verification steps for struct sizes, moved files, and diff-reading traps.

🛑 GATE: Include the breaking change analysis in the PR description body. Summarize the key findings (HIGH/MEDIUM risk changes and their C API impact) for the user.

Phase 3: Validation

The agent performing the breaking change analysis has blind spots — it may filter out relevant changes or miss moved headers. An independent validation catches these before they become runtime crashes.

Launch an explore agent with model: "claude-opus-4.6" using the prompt template from references/validation-prompt.md — substitute the milestone numbers and paste your breaking change analysis table. The default explore model (Haiku) is too weak for accurate header-level validation — use Opus for reliability.

🛑 GATE: Validation agent has run and confirmed analysis. If it found missed items, update the analysis and re-present to user before proceeding.

✅ Before proceeding to B (Branch & Merge):

Current and target milestones confirmed

Breaking change analysis complete

Validation passed

B. Branch & Merge (Phases 4–5)

Phase 4: Branch Setup

⚠️ This phase creates BOTH branches before making ANY changes. You may be on a workflow branch, a feature branch, or a detached HEAD — none of which is the right base. You MUST branch from origin/main (parent) and origin/skiasharp (submodule).

Parent repo (SkiaSharp):

Fetch the latest main:
bash
```
git fetch origin main
```

Create the feature branch from origin/main:

bash

git checkout -b skia-sync/m{TARGET} origin/main

If the branch already exists on the remote, check it out instead:

bash

git fetch origin skia-sync/m{TARGET} && git checkout skia-sync/m{TARGET}

Verify you are on the correct branch and it is based on origin/main:
bash
```
git log --oneline -1 origin/main
git log --oneline -1 HEAD
```
These should show the same commit (or HEAD should be ahead by only your own commits).

Submodule (mono/skia):

Enter the submodule:
bash
```
cd externals/skia
```

Align to the SHA that origin/main expects (the submodule tracks the skiasharp branch in mono/skia, NOT main):

bash

MAIN_SUB_SHA=$(git -C ../.. ls-tree origin/main -- externals/skia | awk '{print $3}')
git fetch origin skiasharp
git checkout "$MAIN_SUB_SHA"

Verify this SHA is on origin/skiasharp:

bash

git branch -r --contains "$MAIN_SUB_SHA" | grep 'origin/skiasharp'

Create the submodule feature branch:
bash
```
git checkout -b skia-sync/m{TARGET}
```

⚠️ Do NOT skip the SHA alignment step (step 5). If the submodule is at a different SHA than origin/main expects, the merge will produce phantom diffs — functions that already exist on main will appear as new or removed.

🛑 GATE: Both branches created. Verify:

bash

# In parent repo:
git rev-parse --abbrev-ref HEAD          # → skia-sync/m{TARGET}
git merge-base HEAD origin/main          # → should match origin/main tip
# In submodule:
git -C externals/skia rev-parse --abbrev-ref HEAD  # → skia-sync/m{TARGET}

Phase 5: Upstream Merge (mono/skia)

You should still be inside externals/skia from Phase 4.

Merge upstream — use --no-commit for manual conflict resolution:
bash
```
git merge --no-commit upstream/chrome/m{TARGET}
```

Resolve conflicts — each conflict must be resolved individually. Never use git merge -s ours or git read-tree --reset — this destroys git blame attribution.

⚠️ MANDATORY: Before resolving ANY conflict, check file history for fork-specific patches. Run git log --oneline skiasharp -- <conflicted-file> — if the log shows intentional fork patches, keep our version. See gotcha #15 for details.

File Category	Strategy
`BUILD.gn`	Combine both — keep upstream structure AND SkiaSharp's platform flags + `skiasharp_build` target
`DEPS`	Combine — keep our dependency pins, accept upstream structure
`RELEASE_NOTES.md`, `infra/`	Take upstream
C API (`include/c/`, `src/c/`)	Keep SkiaSharp — adapt includes/API calls in post-merge commits
Other upstream source (`src/`, `include/`)	Check history first — see gotcha #15

Commit the merge:

bash

git commit  # Creates proper two-parent merge

Verify our C API files survived the merge:

bash

ls src/c/*.cpp include/c/*.h  # All files should still exist

Source file verification — Check for added/deleted upstream files:
bash
```
git diff upstream/chrome/m{CURRENT}..upstream/chrome/m{TARGET} --diff-filter=AD --name-only -- src/ include/
```
Cross-reference against BUILD.gn — new source files may need to be added.

🛑 GATE: Merge complete, conflicts resolved. Verify:

bash

ls src/c/*.cpp include/c/*.h                    # C API files intact
git diff --check                                  # Zero conflict markers
git blame src/c/sk_canvas.cpp | head -20         # Attribution shows original commits, not just merge

✅ Before proceeding to C (Update & Build):

Parent branch is based on origin/main

Submodule is at the SHA referenced by the parent's origin/main submodule pointer

Upstream merge committed with proper two-parent history

C API files intact, zero conflict markers

C. Update & Build (Phases 6–7)

Phase 6: Update SkiaSharp Version Files

⚠️ This MUST be done before any native build. The build scripts verify version consistency — if VERSIONS.txt still says the old milestone, the build will fail.

Note: The script automatically resets SK_C_INCREMENT to 0 when the milestone changes. If you had a pending increment that must survive, capture it before running.

📋 This phase is handled by a script. The script updates VERSIONS.txt, cgmanifest.json, azure-templates-variables.yml, and verifies SK_C_INCREMENT — then runs the mandatory verification greps. It exits non-zero if any stale references remain.

In the SkiaSharp parent repo, run:

bash

cd ../..  # back to parent repo (Phase 5 ends inside externals/skia)
pwsh .agents/skills/update-skia/scripts/update-versions.ps1 -Current {CURRENT} -Target {TARGET}

The script handles all of these (so you don't have to do them manually):

scripts/VERSIONS.txt: milestone, increment→0, soname, assembly, file, ALL ~30 nuget lines
cgmanifest.json: commitHash, version, chrome_milestone, upstream_merge_commit
scripts/azure-templates-variables.yml: SKIASHARP_VERSION (must match VERSIONS.txt nuget version)
Verifies SK_C_INCREMENT is 0 in externals/skia/include/c/sk_types.h
Runs mandatory grep verification — fails if any stale references remain

🛑 GATE: Script exits with ✅. If it exits with ❌, fix the reported stale references and re-run until it passes.

Note: The SK_C_INCREMENT reset modifies a file in the submodule (externals/skia/). Don't commit it separately — it will be committed with Phase 7's C API fixes.

Phase 7: Fix C API Shim & Build Native

This is where most of the work happens. The C API (src/c/, include/c/) wraps Skia C++ and must be updated when the underlying C++ APIs change.

❌ NEVER use externals-download during a milestone update. It downloads pre-built binaries from the OLD milestone that don't contain your C API changes. Always build from source with externals-{platform}.

Restore tools and attempt to build to identify all compilation errors:
bash
```
dotnet tool restore
dotnet cake --target=externals-{platform} --arch={arch}
```
Replace {platform} with your OS (macos, linux, windows) and {arch} with your architecture (arm64, x64).

Fix each error following these patterns:

Error Type	Fix Pattern
Missing type	Add/update typedef in `sk_types.h`
Renamed function	Update call in `*.cpp`
Removed enum value	Remove from `sk_enums.cpp` + `sk_types.h`. Note this for Phase 9 — it needs `[Obsolete]` or documented removal
Changed signature	Update C wrapper function signature
New header required	Add `#include` in the relevant `.cpp`
Legacy flag breaks C API	Update C API to use replacement API (see gotcha #6). Do not just comment out the flag without a plan

Update sk_types.h for any new enums or type changes. Phase 6 reset SK_C_INCREMENT to 0. Only bump it if you add new C API functions in this milestone. The build enforces that SK_C_INCREMENT matches libSkiaSharp increment in VERSIONS.txt.
Build again — iterate until clean compilation.

🛑 GATE: Native library builds successfully on at least one platform.

✅ Before proceeding to D (Regenerate & Verify):

Version files updated (Phase 6 script passed)

Native library builds cleanly

D. Regenerate & Verify (Phases 8–10)

Phase 8: Regenerate Bindings

Prerequisite: Phase 7's native build must have completed at least once — it runs git-sync-deps, which fetches HarfBuzz and other headers the generator needs.

📋 This phase is handled by a script. The script runs the generator, IMMEDIATELY reverts HarfBuzz bindings (HarfBuzz updates are always separate), reports what changed, and lists any new functions that may need C# wrappers.

bash

pwsh .agents/skills/update-skia/scripts/regenerate-bindings.ps1

The script handles all of these (so you don't forget any):

Runs pwsh ./utils/generate.ps1
Reverts binding/HarfBuzzSharp/HarfBuzzApi.generated.cs (proactively, not reactively)
Reports the binding diff summary
Lists NEW generated functions that may need C# wrappers in Phase 9

After the script completes, build C# to verify compilation:

bash

dotnet build binding/SkiaSharp/SkiaSharp.csproj

🛑 GATE: Script prints ✅ Phase 8 complete. C# build succeeds with 0 errors.

Phase 9: Fix C# Wrappers

The C# build can pass with 0 errors while new C API functions remain invisible to users. New functions compile fine as unused internal static methods in the generated file, but without C# wrappers they're not part of the public API. This phase applies even when the build succeeds.

Review new generated bindings for unwrapped functions:
bash
```
git diff origin/main -- binding/SkiaSharp/SkiaApi.generated.cs | grep "^+.*internal static"
```
⚠️ The git diff origin/main may show additional changes beyond new functions (e.g. struct renames, type changes from Phase 7 shim work). These are expected and correct. Only investigate +internal static lines — ignore other diff noise.
Check whether each new function has a C# wrapper:
bash
```
# Example: if sk_foo_bar was added, check for a wrapper
grep -rn "sk_foo_bar" binding/SkiaSharp/*.cs | grep -v generated
```
New functions from our custom C API additions typically need wrappers. New functions from upstream changes are usually additive and can be deferred.

Fix files in binding/SkiaSharp/ based on the breaking change analysis:

File	When to Update
`Definitions.cs`	New enums, types, or constants
`EnumMappings.cs`	New enum values that need C#↔C mapping
`GRDefinitions.cs`	Graphics context changes (Ganesh)
`SKImage.cs`	SkImage factory changes
`SKTypeface.cs`	SkTypeface API changes
`SKFont.cs`	SkFont API changes
`SKCanvas.cs`	Canvas drawing API changes

Key rules:

Add new overloads, never modify existing signatures (ABI stability)
Use [Obsolete] for deprecated APIs with migration guidance
Return null from factory methods on failure (don't throw)

Phase 10: Build & Test

bash

# Rebuild native only if you touched C API files in Phase 9
# (Phase 7 already built — skip if no native changes since then)
dotnet cake --target=externals-{platform} --arch={arch}

# Build C#
dotnet build binding/SkiaSharp/SkiaSharp.csproj

Smoke tests (fast gate, ~100ms):
bash
```
dotnet test tests/SkiaSharp.Tests.Console/SkiaSharp.Tests.Console.csproj --filter "Category=Smoke"
```
Smoke tests verify basic native interop: version compatibility, object creation, drawing, image loading, fonts, codecs, effects, and more. If these fail, something fundamental is broken — go back and fix before wasting time on the full suite.

⚠️ If the version compatibility smoke test fails with "incompatible native library", you missed a version update — go back to Phase 6 and verify ALL version lines. Do NOT work around this with --no-incremental or by copying native libs manually.
Full test suite (required before any PR):
bash
```
dotnet test tests/SkiaSharp.Tests.Console/SkiaSharp.Tests.Console.csproj 2>&1 | tee /tmp/gh-aw/agent/test-output.txt
```
Wait for it to finish (takes 5–7 min). Then read the summary:
bash
```
tail -5 /tmp/gh-aw/agent/test-output.txt
```
The last line will look like: Passed! - Failed: 0, Passed: 5435, Skipped: 171, Total: 5606

This runs all test projects (core, Vulkan, Direct3D). Backend-specific tests self-skip when hardware isn't available. CI handles WASM/Android/iOS separately.
⚠️ These MUST be two separate commands. Do NOT combine them into a single pipeline like | tee ... | tail — the piped tail runs immediately and will show nothing useful while tests are still running. Capture with tee first, wait for completion, then tail the output file. After the run, inspect failures with:
bash
```
grep '^  Failed' /tmp/gh-aw/agent/test-output.txt
```

Smoke tests are just that — smoke. They verify the basics. The full suite MUST pass before the update can be considered complete. Do not create PRs with only smoke tests passing.

🛑 GATE: ALL tests pass (full suite, not just smoke). Do NOT skip failing tests. Do NOT proceed with failures.

✅ Before proceeding to E (Ship):

Bindings regenerated (Phase 8 script passed)

C# builds with 0 errors

ALL tests pass (full suite)

E. Ship (Phase 11)

Phase 11: Create PRs

Same-milestone bug-fix syncs: When CURRENT == TARGET, only cgmanifest.json's commitHash/upstream_merge_commit change. Use PR titles like [skia-sync] Merge upstream chrome/m{TARGET} bug fixes instead of milestone-bump titles.

PR 1: mono/skia (submodule)

Field	Value
Branch	`skia-sync/m{TARGET}`
Target	`skiasharp`
Title	`[skia-sync] Merge upstream chrome/m{TARGET}`

PR 2: mono/SkiaSharp (parent)

Field	Value
Branch	`skia-sync/m{TARGET}`
Target	`main`
Title	`[skia-sync] Update skia to milestone {TARGET}`

Submodule must point to the mono/skia PR branch.

Cross-link both PRs (same as native-dependency-update).

After creating BOTH PRs, update the earlier PR's description to include a link to the later one. Both PRs must reference each other.

Phase 11 Completion Checklist

Before proceeding to merge, verify ALL of these:

Merge Sequence (CRITICAL)

Merge mono/skia PR first → creates new squashed SHA on skiasharp
Fetch new SHA in SkiaSharp's submodule
Update submodule pointer, push to SkiaSharp PR branch
Only then merge SkiaSharp PR

Merge Checklist

Before proceeding past each step, verify:

mono/skia PR merged
Fetched skiasharp branch to get new squashed SHA
Updated SkiaSharp submodule to new SHA (cd externals/skia && git fetch origin && git checkout {new-sha})
Pushed submodule update to SkiaSharp PR branch
CI passes on updated SkiaSharp PR
SkiaSharp PR merged
Submodule points to a commit on skiasharp branch (not an orphaned branch commit)

❌ NEVER merge both PRs without updating the submodule in between. ❌ NEVER assume the submodule reference is correct after squash-merging mono/skia.

Reference Material

These files contain lookup information — consult them when you hit a problem or need context, not necessarily upfront:

references/known-gotchas.md — Hard-won lessons from past updates (DEF_STRUCT_MAP, emsdk, BUILD.gn flags, HarfBuzz, DEPS forks, etc.) and a troubleshooting table
references/typical-changes.md — Files typically changed in each repository during an update
references/breaking-changes-checklist.md — How to analyze breaking changes between milestones