Your system prompt has a compile time

TL;DR A system prompt has two planes: the runtime prose the model reads, and the compile-time scaffolding (comments, @@ notes, <$CONFIG>, and every $-command) that it never sees. Keeping them straight is what stops a compiler-only symbol from leaking into the shipped prompt as a dangling {placeholder}. Build the file and read the artifact to see exactly what the model receives.

Source code has a compile step, and everyone who writes it carries a quiet mental model of what survives it: comments do not run, #ifdef branches get chosen, macros expand. Prompts have the same split, but almost no one writes them with it in mind, so the file you edit and the text the model reads get treated as one thing. They are not. A TypeGlish .tg file is source; what a prompt-taking system receives is a build artifact. Most of the frustrating "the model is ignoring my config" bugs are really border bugs: something you meant for compile time ended up at runtime, or the reverse.

§1Two planes in one file

Every line you write lands on one of two planes. The runtime plane is what the model actually reads: the role prose, the MUST/NEVER rules, the declarations that render. The compile-time plane is everything that exists to help you author and the compiler check, and is gone before the model sees a token. That second plane is larger than most builders realize:

• Line comments (//) and block comments (/* … */) - documentation, never sent.
• Annotations (@@ why this rule exists) - authoring metadata for the line below, never compiled.
• The <$CONFIG> section - foldable scaffolding whose whole extent never reaches the model.
• Every $-command - $DEFINE, $IMPORT, $REQUIRE, $CONFIG. They are compiler directives, not content.
• The losing arms of a $IF / $SWITCH - the compiler evaluates the selection and only the winning branch's prose ships.

This is why a heavily commented, heavily configured TypeGlish prompt can still be lean for the model: you pay for annotation and structure on the compile-time plane, and the runtime plane stays exactly as long as the instructions require. The trouble starts only when a symbol from one plane shows up on the other.

§2The line that leaks

Here is the border bug in its natural habitat. A $DEFINE names a compile-time condition, vip, meaning "the tier input equals gold." That belongs on the compile-time plane. Then a normal instruction line points at it, dragging a compiler-only symbol into runtime prose.

# Role
You are a support agent for an online electronics retailer.

<$CONFIG>
  $REQUIRE variable tier: one of gold, standard
  $DEFINE vip AS @{tier} is equal to gold
</$CONFIG>

# Constraints
- MUST reply in at most 3 sentences.
- MUST offer priority handling to @{vip} customers.

support.tg:11:35  error  structure/define-in-prose  Compile-time condition in prose - "vip"
  is a $DEFINE, resolved by the compiler, so the model would read a dangling {vip}
  placeholder here. Use it in a $IF / $SWITCH ON condition or a when= guard; for a
  value the host fills at runtime, declare "$REQUIRE variable vip" instead.

✗ 1 file - 1 error, 0 warning, 0 info

Notice what the diagnostic understands that a spellchecker never could: it knows vip lives on the compile-time plane, it knows the instruction line lives on the runtime plane, and it knows the crossing is illegal. The fix is not to reword the sentence. It is to keep vip where it belongs, as a condition the compiler resolves.

§3What the model actually reads

Move the decision into a $IF and the symbol stays on the compile-time plane. The compiler evaluates the condition and emits only the branch that wins, so the runtime plane never contains vip at all, just the sentence the customer's tier earned.

# Role
@@ role: scope the agent so "helpful" has a domain
You are a support agent for an online electronics retailer.

<$CONFIG>
  $REQUIRE variable tier: one of gold, standard
  $DEFINE vip AS @{tier} is equal to gold
</$CONFIG>

# Constraints
@@ brevity: replies over three sentences get skimmed
- MUST reply in at most 3 sentences.
@@ vip_path: gold-tier customers pay for priority, so honor it
$IF @{vip}: Offer priority handling and a direct callback number.
$ELSE: Offer standard handling.

Build it, and the artifact is the proof. This is the runtime plane, verbatim, for a gold-tier customer: the config is gone, the notes are gone, the conditional has collapsed to its winning line.

✓ built .typeglish/dist/support.txt ← support.tg (d5e02163b91e, full)

# --- support.txt (what the model receives) ---
# Role
You are a support agent for an online electronics retailer.

# Constraints
- MUST reply in at most 3 sentences.
Offer priority handling and a direct callback number.

The score puts a number on the same split. It reports two planes: a runtime score for what the model reads, and a hygiene score for the source-only material. A prompt can be immaculate on one plane and a mess on the other, and reading them separately tells you which problem you have.

support.tg - A (100/100)  proven errors: none  tiers: base+z3
  planes  runtime 100 (what the model reads) · hygiene 100 (source only)
  facets  enforceability 100 x.26 · consistency 100 x.21 · structure 100 x.16 (hygiene) · annotation 100 x.16 (hygiene) · style 100 x.11 · security 100 x.11

The file you edit is not the prompt the model reads. Confusing the two is where the leaks come from.

§4Writing to the boundary on purpose

Once you can see the two planes, a few habits keep symbols on the right one and keep surprises out of production.

• Comment and annotate freely. Compile-time notes cost the model nothing, so the reason a rule exists belongs in an @@ note, not in the rule text where it becomes runtime tokens the model has to weigh.
• Keep decisions in conditions, not prose. A $DEFINE or a switch predicate is compile-time; the moment you want to reference it from an instruction, reach for a $IF or a when= guard, never a bare @{} in a sentence.
• Read the artifact before you ship. build writes the exact runtime plane to a file. If a config line, a comment, or a stray placeholder shows up there, you found a leak the cheap way.
• Deploy the artifact, not the source. The .tg is for humans and the compiler. Sending it to the model ships the whole compile-time plane as noise, which is the worst of both worlds.