After Effects via ExtendScript (.jsx)

After Effects is scripted with ExtendScript against the AE scripting DOM (app, comp, layers, properties). A .jsx builds a comp deterministically and is fully parameterized — the right tool for repeatable, brand-locked motion (intros, titles, logo animations) where you want a CONFIG block instead of hand-keyframing.

There is no AE MCP. You write the script; the user runs it in AE and renders/previews to verify. Treat the render (a frame or clip the user shares) as the feedback loop — build, ask for a render, read it, fix, repeat. There is also no automated screenshot; favor a CONFIG block + an end-of-run alert() summary so the user can self-tune.

Run + verify model

• Run: AE → File > Scripts > Run Script File... → pick the .jsx. No project/comp setup needed if the script builds its own.
• Syntax-check locally before handing off: node rejects the .jsx extension, so cp script.jsx _check.js && node --check _check.js && rm _check.js. This catches parse errors; it does NOT validate AE DOM calls.
• Headless render (optional): aerender CLI can render a comp for an automated check, but it needs AE installed and is heavy — default to the user previewing.

ExtendScript is ES3 — write accordingly

• var only. No const/let, arrow functions, template literals, destructuring, or for...of. Use function declarations, for loops, and string concatenation. ===, try/catch, Array.push are fine; avoid Array.forEach/map (spotty).
• Wrap mutations in app.beginUndoGroup("...") / app.endUndoGroup(), and the whole body in try/catch ending with an alert() so failures surface.
• Build expression strings by concatenation and inject CONFIG numbers/strings at build time (expressions can't read your CONFIG). Escape injected strings ('"' + s.replace(/"/g,'\\"') + '"'). Newlines in expression source: \r.

Core principles

1. Anchor at the origin, then measure. For text layout set Anchor Point = [0,0] (baseline-left), then layer.sourceRectAtTime(0, false) → {left, top, width, height} relative to that anchor. Left-edge placement: position.x = desiredLeft - rect.left; baseline position.y = baselineY. Vertical-center off the tallest glyph: baselineY = compH/2 - (rect.top + rect.height/2). The same sourceRectAtTime convention holds in expressions, so build-time layout and expression-time tracking agree.
2. Prefer expressions over keyframes for anything derived. Typing-on, edge-tracking, blinking — all are one expression and need no rebuild when strings change.
3. CONFIG at the top. Every tunable (strings, fonts, colors as hex, sizes, timings) in one object. Colors: store hex, convert to 0–1 RGB. This is what makes the script reusable and brand-switchable.
4. Verify state before reading children. app.project always exists. For multi-page/precomp work, set the active item before reading its layers.

Property matchname cheatsheet

Use matchnames (stable across locales), not display names:

• Transform: ADBE Transform Group → ADBE Anchor Point, ADBE Position, ADBE Scale, ADBE Rotate Z (2D rotation), ADBE Opacity.
• Text source: layer.property("ADBE Text Properties").property("ADBE Text Document") — .value is a TextDocument; mutate it then setValue(doc). Put a typing expression on this property's .expression.
• Shape layer: ADBE Root Vectors Group → addProperty("ADBE Vector Group") → .property("ADBE Vectors Group") → addProperty("ADBE Vector Shape - Star") (or ...- Rect, ...- Ellipse) + addProperty("ADBE Vector Graphic - Fill"). Fill color via .property("ADBE Vector Fill Color").setValue([r,g,b,1]).
• Polystar: ADBE Vector Star Type (1 = star, 2 = polygon), ADBE Vector Star Points, ADBE Vector Star Outer Radius, ADBE Vector Star Inner Radius, and the misspelled ADBE Vector Star Outer Roundess / ADBE Vector Star Inner Roundess (wrap in try/catch). A 4-point star with inner radius ≈ 0.16×outer = a sparkle/glint.

TextDocument gotchas

• Set applyFill=true + fillColor=[r,g,b], applyStroke=false, justification (use ParagraphJustification.LEFT_JUSTIFY for left-anchored growth), then text, then setValue(doc).
• AE style names use spaces: "Semi Bold", "Extra Bold" — not "SemiBold".

Fonts: PostScript names + graceful fallback

TextDocument.font wants a PostScript name, not a family name. Resolve family→PS via app.fonts.getFontsByFamilyNameAndStyleName(family, style) or by scanning app.fonts.allFonts when that API exists; otherwise trust your first guess (AE substitutes a missing font rather than erroring). Pattern: resolvePS(family, style, guessList, fallbackPS) returning a PS name; fallbacks like Consolas (mono) / ArialMT (sans). See assets/helpers.jsx.

AE renders only SYSTEM-installed fonts. A brand's web font (e.g. one loaded via a Google Fonts @import in a webapp) is invisible to AE — it silently falls back (you'll see Arial-BoldMT in the build alert). To actually match an in-app wordmark, install the font on the OS first. Windows per-user install (no admin), then restart AE: copy the static .ttfs to %LOCALAPPDATA%\Microsoft\Windows\Fonts and add HKCU:\Software\Microsoft\Windows NT\CurrentVersion\Fonts values like "Inter Bold (TrueType)" = the full font path. Have the script's alert print the resolved PS name and warn if the family didn't resolve, so a silent substitution is caught.

Keyframes + easy-ease

Set values with prop.setValueAtTime(t, v), then per key: setInterpolationTypeAtKey(i, BEZIER, BEZIER) and setTemporalEaseAtKey(i, easeIn, easeOut) where each ease array has one new KeyframeEase(0, 33) per value dimension — 1 for opacity/rotation, 2 for Scale [x,y]. Exception: spatial properties (Position) take a SINGLE element regardless of dimensionality — passing 2 to a keyframed Position throws Unable to call "setTemporalEaseAtKey" ... Value array does not have 1 elements. Gate on prop.isSpatial (use 1) vs prop.value.length. Wrong length throws. easeAll(prop) in assets/helpers.jsx handles this.

Patterns

• Typing-on: Source Text expression n=(time<ts)?0:Math.floor((time-ts)/iv)+1; s.substr(0,n). Measure the full string first for layout, then apply the expression.
• Edge tracking: drive a follower's position.x off the typed layer's sourceRectAtTime so a closing bracket / caret tracks the growing text. See reference/audio-sync.md and assets/helpers.jsx (edgeTrackExpr).
• Blink: opacity expression gated to a window: (time<ts||time>te)?0:(Math.floor((time-ts)*hz*2)%2===0?100:0).
• Sparkle/glint: a polystar that scales 0→peak→0, slow-rotates, and flashes opacity, timed to a beat (e.g. a ding). makeSparkle + animSparkle in helpers.
• Audio sync: see reference/audio-sync.md — convert non-WAV with ffmpeg, extract onsets with silencedetect, import + layer.startTime, lock the typing interval to the recording.

Portability + render

• Resolve bundled assets relative to the script: (new File($.fileName)).parent.fsName + "/sfx/". Bundle SFX/images next to the .jsx.
• Transparent background = no solid layer (comps are transparent by default). Render with an alpha codec (ProRes 4444 / PNG sequence) and Audio Output On.

Committing AE scripts (secret scanners)

High-entropy asset filename literals (e.g. keystrokes-14.wav) inside a .jsx can trip a secret scanner's generic-api-key rule. Path-allowlist the script in your scanner config rather than renaming the asset.

Keep a canonical example

Maintain one CONFIG-driven reference script in your repo and point new work at it. A good intro/splash reference exercises the whole toolkit in one file: transparent 1920×1080 / 30fps, a two-tone-capable wordmark, expression typing + an edge-tracked bracket + a blinking caret, timed keystroke/enter/ding SFX locked to the typing, and a polystar sparkle on the ding. When the code and this skill disagree, the code wins — update the skill.