"High-key chalky sky is capped by two defaults — texture AND tonemapping must both change"

You want a bright, warm, near-white sky (high-key look) but the viewport stays stubbornly mid-grey or tinted blue no matter how bright you push the sky texture or tint.

Two independent defaults each cap the brightness:

1. The stock sky material bakes blue into the sky and ambient. SkyBox2D.Tint is a multiply — it can only darken toward the baked color, never lift to warm-white. A brighter or warmer tint alone can't overcome a blue base texture.

2. The default tonemapping mode (Legacy / filmic curve) crushes highlights. Even with a genuinely near-white sky texture, the filmic rolloff re-caps the sky to roughly mid-grey. This is the opposite of the high-key look — it's designed for cinematic contrast, not bright diffuse scenes.

Both layers must move:

1. Replace the sky texture. Bake a flat pale-warm equirectangular PNG (a plain vertical gradient tiles seamlessly — no clouds or sun disc means haze-free). Create a new material:

shader "shaders/sky.shader"
SkyTexture "materials/sky/warm_sky.png"
g_flBrightnessExposureBias "0.000"

Point SkyBox2D.SkyMaterial at it — this is settable at runtime (sky.SkyMaterial = Material.Load(path)) so the scene file stays untouched.

2. Switch tonemapping to ReinhardJodie. This gives a gentle highlight rolloff that stays bright and chalky-soft:

Tonemapping.Mode = Tonemapping.TonemappingMode.ReinhardJodie;

The enum is nested under Sandbox.Tonemapping.TonemappingMode with members: ACES, AgX, HableFilmic, Linear, ReinhardJodie. The old Legacy value in scene JSON is a legacy serialization string not present in the current enum.

3. Warm the EnvmapProbe tint. The baked cubemap does NOT auto-rebake to the new sky material at runtime — without this, the shadow side re-casts cool. Set EnvmapProbe.TintColor to a warm, slightly dim color.

4. Zero out fog to complete the bright look.

The high-key look requires the entire luminance pipeline to agree. The sky texture sets the source brightness and hue, tonemapping controls how highlights map to display range, and the envmap probe controls indirect lighting color. Each layer has a default that independently caps or tints the result — you need all three aligned to get a genuinely bright, warm scene.