optimize-web

SKILL.md

Performance Notes

Take your time to do this thoroughly.

Quality is more important than speed.

Running C# in the Editor

Every step below that reads or writes a Player Setting runs inside a live Editor through the Unity
CLI. The unity-cli skill owns getting you there — installing the CLI, confirming a connected
Editor, adding the project's com.unity.pipeline package, telling a genuinely absent Editor apart
from one stuck in Safe Mode, and discovering the Editor's command catalog. Follow it first; don't
re-derive any of it here.

Two things it can't know for you:

You need eval in particular, not just a reachable Editor. Confirm it appears in the catalog.
Its presence depends on the Pipeline package version, not on the CLI, so a healthy install can
still lack it — if it's missing, say so and stop.

Player Settings can be read from ProjectSettings/ProjectSettings.asset in a pinch, but do not
write them that way. The serialized names don't match the API names, several of these settings
are per-build-target, and a hand-edited value silently disagrees with what the build actually
uses. An unreachable Editor is a stop for the write steps.

Run C# with unity command eval --code '<snippet>'. unity command defaults to a 30 second
timeout.

Passing C# to eval

eval compiles a statement block, not a file. Two consequences, both compile errors rather than
warnings:

No using directives. The compiler reads using UnityEditor; as a resource-disposal
statement and rejects it (CS0210).

Types must be fully qualified. A bare PlayerSettings does not resolve (CS0246), and a bare
Object is ambiguous with object (CS0104).

Reading the settings this skill audits

One call returns the whole Pre-Flight picture. Verified against Unity 6000.5.7f1:

var target = UnityEditor.Build.NamedBuildTarget.WebGL;
var w = new System.Collections.Generic.List<string>();
w.Add($"activeBuildTarget={UnityEditor.EditorUserBuildSettings.activeBuildTarget}");
w.Add($"compressionFormat={UnityEditor.PlayerSettings.WebGL.compressionFormat}");
w.Add($"decompressionFallback={UnityEditor.PlayerSettings.WebGL.decompressionFallback}");
w.Add($"stripEngineCode={UnityEditor.PlayerSettings.stripEngineCode}");
w.Add($"managedStrippingLevel={UnityEditor.PlayerSettings.GetManagedStrippingLevel(target)}");
w.Add($"il2cppCodeGeneration={UnityEditor.PlayerSettings.GetIl2CppCodeGeneration(target)}");
w.Add($"apiCompatibilityLevel={UnityEditor.PlayerSettings.GetApiCompatibilityLevel(target)}");
w.Add($"exceptionSupport={UnityEditor.PlayerSettings.WebGL.exceptionSupport}");
w.Add($"debugSymbolMode={UnityEditor.PlayerSettings.WebGL.debugSymbolMode}");
w.Add($"dataCaching={UnityEditor.PlayerSettings.WebGL.dataCaching}");
w.Add($"wasm2023={UnityEditor.PlayerSettings.WebGL.wasm2023}");
w.Add($"initialMemorySize={UnityEditor.PlayerSettings.WebGL.initialMemorySize}");
w.Add($"maximumMemorySize={UnityEditor.PlayerSettings.WebGL.maximumMemorySize}");
w.Add($"memoryGrowthMode={UnityEditor.PlayerSettings.WebGL.memoryGrowthMode}");
w.Add($"targetFrameRate={UnityEngine.Application.targetFrameRate}");
w.Add($"vSyncCount={UnityEngine.QualitySettings.vSyncCount}");
return string.Join("\n", w);

Three API names to get right, because the obvious spellings do not exist and fail to compile:

Setting
Correct form
Does NOT exist

Managed stripping level
PlayerSettings.GetManagedStrippingLevel(NamedBuildTarget.WebGL)
PlayerSettings.managedStrippingLevel

Wasm code optimization
UnityEditor.WebGL.UserBuildSettings.codeOptimization
PlayerSettings.WebGL.codeOptimization, PlayerSettings.WebGL.optimizationLevel

IL2CPP code generation
PlayerSettings.GetIl2CppCodeGeneration(NamedBuildTarget.WebGL)
a bare property

UserBuildSettings lives in the WebGL build-support module, so it only resolves when that module is
installed. Read it in a separate call from the rest, and treat a resolution failure as "the Web
module isn't installed" rather than as a bad snippet.

codeOptimization is the one setting here that does not live in the project file. It persists to
Library/EditorUserBuildSettings.asset, and Library/ is gitignored by every standard Unity
.gitignore, so this value is per-machine and does not travel: teammates and CI do not inherit it.
Two consequences. Read it back through the API and do not look for it in
ProjectSettings/ProjectSettings.asset — it is absent there even on a successful apply, so its
absence is not a failure. And if the release build runs in CI, apply it there as a build step rather
than assuming the repository carries it.

Applying the settings

Most of the writes in this skill are a single batch, and
resources/WebOptimizer.cs already is that batch. It declares a class
with a [MenuItem], so it is a project file, not eval input — a class declaration cannot be
flattened into a statement block. Save it under Assets/Editor/, let Unity compile, then invoke it
in one line:

UnityEditor.EditorApplication.ExecuteMenuItem("Tools/Apply Web Release Settings");

Keep its using directives; they are correct in a file. For one-off changes — a single quality
level, a frame-rate flip — an inline eval statement is fine.

Verify from disk, not from the objects you just wrote

Applying a setting and reading it back in the same session proves nothing. Player Settings are
in-memory objects until something saves them, so every read-back returns the value you just
assigned whether or not it ever reached
ProjectSettings/ProjectSettings.asset. A run that skips the save reports success, and when the
Editor session ends the whole change is gone. This was observed, not theorised: a run applied
everything, read back Brotli / High / None, said it was done, and the file on disk never changed.

So after any write:

Save. UnityEditor.AssetDatabase.SaveAssets(). WebOptimizer.cs now does this itself; an
inline eval write has to do it explicitly.

Read the values back and report them. Not "applied successfully" but the actual values, so the
user can see what is stored. WebOptimizer.cs logs all nine.

For anything per-build-target, name the target you read. Several of these settings exist once
per target, so a value can be correct for one target and unset for the one being built.

The same trap exists on the file-editing route in reverse: a hand-edited
ProjectSettings.asset reads back fine while the running Editor and the build still use the old
value. Verifying after a save and a reimport is what catches both.

Do not try to reproduce this in batch mode. A batch Editor invoked with -quit saves settings on
exit, so an unsaved write persists anyway and the run looks correct. Both a saving and a non-saving
version of the script pass under -quit. The bug only appears in a live Editor session, which is
where it was found. Concluding from a green batch run that the save is unnecessary is the wrong
conclusion from a test that cannot see the defect.

0. Pre-Flight

Confirm Web build target: Read, with the Pre-Flight snippet above, EditorUserBuildSettings.activeBuildTarget — must be WebGL; if not, warn the user.

Read compression and stripping settings: Read compressionFormat, decompressionFallback, stripEngineCode and the managed stripping level with the Pre-Flight snippet above. Note the stripping level is PlayerSettings.GetManagedStrippingLevel(NamedBuildTarget.WebGL) — there is no PlayerSettings.managedStrippingLevel property.

Read exception and optimization settings: Read PlayerSettings.WebGL.exceptionSupport from the Pre-Flight snippet above. For the wasm code optimization level use UnityEditor.WebGL.UserBuildSettings.codeOptimization — the PlayerSettings.WebGL.codeOptimization and optimizationLevel spellings do not exist and will not compile.

Read frame rate settings: Read, with the Pre-Flight snippet above, Application.targetFrameRate and QualitySettings.vSyncCount.

Read additional player settings: Read, with the Pre-Flight snippet above, PlayerSettings.WebGL.dataCaching, PlayerSettings.WebGL.debugSymbolMode, PlayerSettings.WebGL.maximumMemorySize, and PlayerSettings.GetApiCompatibilityLevel.

Proceed only after compression, stripping, frame rate, and player settings are confirmed.

1. Assess Current State

Check Build Report: Instruct the user to open Window > General > Build Report after a build and identify the largest asset and code size contributors.

Verify server configuration: Ask the user to confirm whether the hosting server sends Content-Encoding: br (Brotli) or Content-Encoding: gzip headers, and whether Content-Type: application/wasm is set for .wasm files.

Check frame rate config: Confirm, with the Pre-Flight snippet above, Application.targetFrameRate — should be -1 for Web (let the browser drive).

Check memory settings: Read, with the Pre-Flight snippet above, PlayerSettings.WebGL.initialMemorySize and PlayerSettings.WebGL.memoryGrowthMode.

Report findings before making recommendations.

2. Understand Request

User Says
Default Interpretation

"build too large" / "download too slow"
Strip Engine Code on; Managed Stripping High; Disk Size + LTO; Brotli

"Decompression Fallback" / "slow startup"
Decompression Fallback off; fix server to send Content-Encoding

"stutter in Chrome" / "stutter in Safari"
Profile in browser DevTools; Safari caps at 60 fps

"excessive battery in browser"
OnDemandRendering on static screens; targetFrameRate = -1

"exceptions too large"
None for release; Wasm 2023 exceptions if browser baseline allows

"set up CDN"
Addressables remote groups + Brotli/Gzip on CDN

"WebAssembly 2023"
Enable when browser baseline supports it — smaller and faster

"memory growth slow"
Tune Initial Memory Size to peak estimate; use Geometric growth mode

"KTX" / "Basis Universal" / "texture formats unknown GPU"
KTX2 with Basis Universal; ETC1S for size, UASTC for quality

"strip unused code" / "remove unused packages"
Web Stripping Tool + remove unused packages + shader stripping

"quality settings for web"
Quality Level to Very Low or Low; lower quality = faster load

"shader variants too many"
Graphics settings: auto lightmap/fog modes; strip instancing + BRG variants; audit Always Included Shaders

"video not playing" / "audio issues"
Video: URL-only or StreamingAssets; Audio: no AudioEffects on Web, use Mono, compress

"profiler symbols" / "can't read Wasm stacks"
Embed profiling symbols via build processor or emscriptenArgs

"iOS crashes" / "Safari memory"
iOS memory limits; set Initial Memory Size high rather than growing; Gigacage 2GB limit pre-iOS 18

3. Web Build Optimization Workflow

IMPORTANT: One-click optimization script

Always offer to generate this script for the user. Unity's official web optimization docs provide a single editor menu script that applies all recommended release settings at once. Place in Assets/Editor/WebOptimizer.cs — see resources/WebOptimizer.cs for the template.

Adapt the script to the user's project needs (e.g. keep exceptions if they use try/catch, switch Brotli to Gzip for HTTP hosting). This script is the single most impactful action for a new web project — it prevents settings from being missed.

Player Settings audit

Verify and set these values through eval:

Setting
Release recommendation

Compression Format
Brotli (HTTPS hosting); Gzip for HTTP

Decompression Fallback
Off when server is correctly configured

Strip Engine Code
On

Managed Stripping Level
High (release) / Medium (dev)

Code Optimization
Disk Size with LTO (release) / Build Times (dev)

WebAssembly Language Features
2023 if browser baseline allows

Enable Exceptions
None (smallest); Explicitly Thrown Only if try/catch required

Initial Memory Size
Tune to peak estimate; too small causes expensive growth

Memory Growth Mode
Geometric

API Compatibility Level
.NET Standard 2.1 — smaller than .NET Framework

IL2CPP Code Generation
Optimize Size — smaller Wasm at slight runtime cost

Debug Symbols
Off for release; on for development builds only

Data Caching
On — caches asset data in browser IndexedDB for faster repeat loads

Strip Unused Mesh Components
On — removes unused vertex attributes

Maximum Memory Size
2048 MB default; up to 4096 for complex 3D (Firefox and Chrome < 119 have issues above 2048)

vSyncCount
0 (browser handles pacing)

targetFrameRate
-1 (use requestAnimationFrame)

Compression and server configuration

Compression
Use when
Notes

Brotli
HTTPS or localhost
Best ratio; browsers accept only over secure contexts

Gzip
HTTP delivery, legacy CDNs
Universal

None
Local dev / file://
Largest payload; do not ship

Configure the server to:

Serve .br files with Content-Encoding: br.

Serve .gz files with Content-Encoding: gzip.

Set Content-Type: application/wasm for .wasm, application/javascript for .js.

Enable HTTP/2 or HTTP/3 to parallelize chunk fetches.

If the host cannot inject Content-Encoding: set Decompression Fallback = On as a fallback only — it adds ~150 KB JS and slows startup.

Exception handling

Setting
Build size
Use

None
Smallest
Release builds where uncaught exceptions are acceptable

Explicitly Thrown Only
Modest
Default for projects that catch exceptions

Full
Largest, slowest
Rarely needed; avoid for release

Wasm 2023 introduces a cheaper exception model; switching from Explicitly Thrown Only (legacy) to Wasm exceptions reduces both size and cost when browser targets support it.

Remove unused resources

Three categories to audit for build size reduction:

1. Unused packages — Check Packages/manifest.json and the Package Manager In Project and Built-in views. Remove or disable packages the project does not use. The Input System package is a significant size contributor if unused.

2. Shader stripping — Configure in Edit > Project Settings > Graphics:

Setting
Recommendation

Lightmap Modes
Automatic (strips unused lightmap shader variants)

Fog Modes
Automatic (strips unused fog shader variants)

Instancing Variants
Strip Unused

Batch Renderer Group Variants
Strip All (if BRGs are not used)

Always Included Shaders
Audit and remove any shaders the project does not reference

Test after stripping — ensure no referenced shaders were removed.

3. Web Stripping Tool (com.unity.web.stripping-tool) — Analyzes the WebAssembly binary and identifies unused Unity engine submodules (e.g. 3D graphics in a 2D-only game). Install via Package Manager, profile the build, then configure which submodules to exclude. Can yield substantial size reductions beyond what Managed Stripping Level achieves alone.

Quality settings for Web

Lower quality levels reduce load time and improve runtime performance. Set via Edit > Project Settings > Quality:

Use Very Low or Low as the default Web quality level.

Set it with eval: UnityEngine.QualitySettings.SetQualityLevel(0, true); where 0 = Very Low.

Consider creating a Web-specific quality level that disables features unnecessary in-browser (real-time shadows, post-processing effects, high particle counts).

Frame rate on Web

Set it with eval: UnityEngine.Application.targetFrameRate = -1; — let the browser use requestAnimationFrame.

Note: Safari caps at 60 fps in WebGL; high-refresh targets do not apply.

Use OnDemandRendering.renderFrameInterval to drop to 5–10 fps on static/idle screens to save battery.

KTX / Basis Universal textures

KTX2 with Basis Universal supercompression ships a single texture file that transcodes at load time to the optimal GPU format for the browser's device (BC7 on desktop, ASTC on mobile, ETC2 on older Android). This avoids shipping separate texture variants for each GPU family — critical for Web where the target hardware is unknown.

Topic
Guidance

Package
Install com.unity.cloud.ktx (KtxUnity) via Package Manager

When to use
Runtime-loaded textures via Addressables or asset bundles served to unknown GPU targets

When NOT to use
Textures baked into the player build — Unity already selects the correct format at build time

Supercompression
Use ETC1S for smallest size (lossy, good for diffuse/albedo); UASTC for higher quality (near-lossless, better for normals/UI)

Encoding
Encode offline with toktx or basisu CLI; do not encode at runtime

Linear data
Set --assign_oetf linear when encoding normal maps, masks, or data textures to avoid incorrect sRGB conversion

Mip maps
Generate mips at encode time (--genmipmap) — browser-side mip generation is expensive

Loading
Use KtxTexture.LoadFromStreamingAssets or load bytes via UnityWebRequest and call KtxTexture.LoadFromBytes

Memory
Transcoded textures are standard GPU textures; memory cost equals the target format, not the KTX2 file size

Orientation
Always include --lower_left_maps_to_s0t0 to match Unity's UV convention

toktx CLI examples: See resources/toktx-examples.sh for commands covering albedo (ETC1S), normals/detail (UASTC), ICC profile errors, and linear data.

Streaming on Web

Use Addressables with remote groups hosted on a CDN with Brotli / Gzip.

Avoid bundling the entire game into the initial download; stream levels on demand.

Target < 30 MB initial download for "instant play"; level data follows.

For streamed textures targeting mixed GPU hardware, prefer KTX2 bundles over per-platform variants — one bundle serves all browsers.

Profiling Web builds

Tool
Use
Notes

Chrome DevTools > Performance
CPU flamegraph; main-thread analysis
Default first stop for WebGL hitches; inspect Wasm call stacks

Chrome DevTools > Memory
Heap snapshot; allocation timeline
Find JS/Wasm memory leaks; compare snapshots before/after scene load

Firefox Profiler
Cross-platform; shareable URLs; native + Wasm view
Better Wasm symbolication than Chrome in some cases; shareable profile URLs for team review

Safari Web Inspector
iOS Safari and macOS Safari debugging
Required for Safari-specific issues; WebGL/Wasm runtime differs from Chromium

Unity Profiler over WebSocket
Connect to a development build; standard markers
Use for Unity-side markers (GC, rendering, scripts); does not capture browser-side overhead

Symptom → tool quick reference:

Symptom
First-line tool
Second-line tool

WebGL hitch / stutter
Chrome DevTools > Performance
Firefox Profiler

Memory climbing over time
Chrome DevTools > Memory
Unity Memory Profiler (WebSocket)

Slow initial load
Chrome DevTools > Network
Build Report Inspector

Safari-only rendering issue
Safari Web Inspector
Compare with Chrome DevTools

Embedding profiling symbols — browser profilers show mangled Wasm function names by default. To get readable C# method names in Chrome/Firefox flamegraphs, either enable Player Settings > Publishing > Debug Symbols for dev builds, or add a build processor:

using UnityEditor;
using UnityEditor.Build;
using UnityEditor.Build.Reporting;

public class WebProfilingBuildProcessor : IPreprocessBuildWithReport
{
    public int callbackOrder => 0;
    public void OnPreprocessBuild(BuildReport report)
    {
        PlayerSettings.SetAdditionalIl2CppArgs("--compiler-flags=--profiling-funcs");
    }
}

Emscripten built-in profilers — enable one at a time via PlayerSettings.WebGL.emscriptenArgs:

Flag
What it shows

--cpuprofiler
CPU profiler overlay in browser

--memoryprofiler
Visual memory map (white=allocated unused, pink=stack, blue=dynamic, green=fragmented)

--threadprofiler
Thread activity profiler

GPU debugging — No Frame Debugger support on Web. Use Spector.js as a browser-based alternative — it captures draw calls and WebGL state.

Firefox about:memory — type about:memory as a URL in Firefox, click Measure to see per-tab breakdown: WASM code size, WASM heap, .data file, web audio. Watch for WASM heap > 300 MB (crash risk, especially on iOS Safari).

Editor Play Mode does not represent browser runtime; always measure in browser. Chrome and Safari GC and JIT behavior differ — test both.

Web memory directives

Disable Read/Write Enabled on textures and meshes — it duplicates data into the WASM heap.

Reduce .data file size by moving assets to Addressables or AssetBundles.

Use compressed texture formats (KTX2/Basis) to reduce both download and decoded memory cost.

iOS Safari memory limits

iOS < 18: WebContent process limit ~1.5 GB. WASM memory (Gigacage) capped at 2 GB. Typed arrays share this pool. On iPhone X (iOS 16) heap growth caps at ~512 MB, but setting Initial Memory Size to 512 MB–1.5 GB upfront works.

iOS 18+: Limits largely lifted; iPhone 11 can allocate ~4 GB.

On iOS, set Initial Memory Size to the target peak rather than relying on growth — Safari handles large upfront allocations better than incremental growth.

WASM heap > 300 MB risks crashes on older iOS; target < 200 MB for broad compatibility.

Video and audio on Web

Video: Playback only works from a URL (server with CORS enabled) or from StreamingAssets. On iOS the server must support HTTP range requests for streaming. Use browser-compatible formats (MP4/H.264).

Audio: AudioEffects (mixer effects) require compute shaders — not available on WebGL. Mixers and MixerGroups work for volume control only. Set audio to Mono to improve loading. If about:memory shows web audio > 100 MB, audio is likely uncompressed — switch to Vorbis.

Canvas and DPI

If the canvas is scaled up it takes the new resolution. Use devicePixelRatio in the web template to offset DPI scaling and avoid rendering at unnecessarily high resolution.

4. Validation

Re-read the Player Settings with the Pre-Flight snippet (compression, stripping, exceptions, targetFrameRate).

Rebuild the player and compare Build Report file sizes with baseline.

Verify in at least Chrome and Safari (GC and JIT behavior differ).

Max 3 iterations before asking the user for feedback.

5. Troubleshooting

Build still large after enabling Strip Engine Code

Is Managed Stripping Level set to Medium or Low? → Set to High for release.

Are plug-ins using reflection to access engine modules that would otherwise be stripped? → Add a link.xml to preserve needed symbols.

Is Exceptions set to Full? → Full adds the largest code overhead; switch to None or Explicitly Thrown Only.

Brotli not working — Decompression Fallback required

Is the server sending Content-Encoding: br? → Without this header the browser won't decompress; the fallback JS decompressor is then needed.

Is the build hosted over HTTP (not HTTPS)? → Brotli requires a secure context; degrade to Gzip for HTTP hosting.

Stutter in Safari but not Chrome

Does the project set Application.targetFrameRate = 60? → On Safari WebGL this conflicts with browser pacing; set to -1.

Are there shaders that behave differently on Safari's WebGL implementation? → Test on device; Safari's WebGL/Wasm runtime differs from Chromium — some GLSL constructs are handled differently.

Memory growth slow path triggered

Is Initial Memory Size too small for the project's peak? → Wasm memory growth requires a full buffer copy; set Initial Memory Size to a realistic peak estimate.

Is Memory Growth Mode set to Linear? → Switch to Geometric for saner growth curve.

Frame rate set to 60 but browser runs erratically

Is Application.targetFrameRate = 60 set in code? → On Web this conflicts with requestAnimationFrame browser pacing. Set to -1.

Is vSyncCount non-zero? → Set to 0; the browser handles pacing.

Firefox cache rejecting large files

Firefox limits individual cache entries via browser.cache.disk.max_entry_size. If the build exceeds this (default ~50 MB), assets won't cache. Solution: use Addressables to split into bundles < 51 MB, or instruct users to increase the setting in about:config.

Local dev server setup

For testing builds locally with proper MIME types:

# Python (HTTP)
python -m http.server 55553 -d path/to/build

# Node.js (install serve-handler)
npx serve path/to/build -l 3001

For Brotli testing, use HTTPS — Brotli requires a secure context. Generate a self-signed cert with OpenSSL for local testing.

6. Completion

Summarize: initial download size delta, settings changed (compression, stripping, exceptions, targetFrameRate), server configuration confirmed.

List follow-up actions: CDN setup for Addressables remote groups, Safari testing, Wasm 2023 feature set upgrade when browser baseline allows.

See also

These point at Unity tooling rather than other skills, because the topics they cover are not in
this plugin:

Addressables package — remote groups served over a CDN, when the download budget needs content
moved out of the initial payload.

Unity Profiler, connected to the browser — the cross-platform profiling methodology. Section 3
covers the Web-specific part of attaching it.

Shader variant stripping (Graphics settings → Shader Stripping, and ShaderVariantCollection)
— variant count feeds directly into Wasm size, so it is worth checking when stripping alone hasn't
moved the number.

Project Settings → Player — the same flags this skill reads, if the user would rather see them
in the inspector than have them reported.

Mobile browser battery behaviour follows the same frame-rate and quality-level guidance in
Sections 3 and 4; there is no separate mobile path here.