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title FAQ

FAQ (FACET v2.1.3)

1) Why does @system role/model/instructions fail?

Because those fields are not in v2.1.3 message schema. Valid message fields are:

  • content (required)
  • id|priority|min|grow|shrink|strategy (optional layout)
  • when (optional bool gate)
  • tools (only in @system)

Invalid fields must raise F452.

2) Can I put documents into @context?

No. @context is layout configuration (budget and optional defaults). RAG context should be represented via @vars + message content.

3) Is @input(...) supported?

Yes, in Hypervisor profile. It must appear only as base expression of a @vars value (optionally followed by pipeline). Invalid placement must raise F452.

4) Does FACET make LLM output deterministic?

No. FACET makes contract processing and request construction deterministic. Model generation remains provider/model-dependent.

5) What are the most common errors?

  • F001/F002/F003: indentation/tabs/syntax
  • F451/F452: type or schema violations
  • F453: runtime input fails declared type
  • F601/F602: import sandbox/cycle
  • F801/F803: disallowed mode/runtime behavior
  • F454/F455: policy deny vs guard undecidable

6) Why does a valid-looking file fail in Core profile?

Core parses full syntax but rejects disallowed constructs with F801. Examples: @input, interfaces, runtime-only behavior.

7) How are messages ordered in canonical JSON?

Always:

  1. all @system blocks
  2. then all @user
  3. then all @assistant

Within each role, resolved source order is preserved.

8) Can layout reorder messages?

No. Layout may compress/truncate/drop flexible sections but must preserve canonical ordering.

9) How is document_hash computed?

sha256 over Resolved Source Form (imports expanded, NFC+LF normalized).

10) What is policy_hash?

If policy exists:

sha256(JCS({ policy_version, policy: EffectivePolicyObject }))

Else null.

11) What is the difference between F454 and F455?

  • F454: deterministic deny (policy evaluated successfully)
  • F455: guard could not deterministically evaluate (fail-closed)

12) Why did a Level-1 lens fail in pure mode?

Pure mode requires Level-1 cache-only execution. Cache miss must raise F803.

13) Can I use string keys in any map?

String keys are allowed only in @meta. Outside @meta, string-keyed map entries must raise F452.

14) Can I index lists with $x.0?

Numeric list indexing is not standardized in v2.1.3. It must raise F452 unless a namespaced host extension explicitly enables it.

15) What should I treat as the external API boundary?

Canonical JSON from Phase 5. It is provider-agnostic and deterministic by spec constraints.