# FCS/1 — Function Composition Schema (Execution-Only)

Status: Approved
Owner: Architecture
Version: 1.0.0
SoT: Yes
Last Updated: 2026-02-21
Linked Phase Pack: PH07
Tags: [execution, descriptor, deterministic]

**Document ID:** `FCS/1`
**Layer:** L2 — Function descriptor schema for deterministic execution

**Depends on (normative):**

* `DDS` — canonical FCS/1 v1-min TLV layout and validation rules
* `PCB1` — parameter block encoding rules (DDS §7.3)
* `ASL/1-CORE v0.4.x` — logical value model (`Artifact`, `Reference`)

---

## 0. Purpose & Scope

`FCS/1` defines the **execution-only** descriptor schema for composite and
custom functions. It explicitly excludes policy, intent, scope, authority, and
other governance metadata (those belong in `FCT/1`).

`FCS/1` descriptors are immutable and content-addressed; identical descriptors
MUST hash to identical CIDs.

---

## 1. Descriptor Model (logical)

An `FCS/1` descriptor captures the deterministic execution recipe:

```
FCSDescriptor {
  function_ptr     : CID   // FPS/1 primitive or nested FCS/1 descriptor
  parameter_block  : CID   // PCB1 parameter block
  arity            : uint32
}
```

* `function_ptr` MAY reference another `FCS/1` descriptor (composition), or a
  primitive function descriptor (e.g., `FPS/1`).
* `parameter_block` MUST reference a valid PCB1 payload.
* `arity` defines the expected number of input slots (must match PCB1).

---

## 2. Canonical Encoding (v1-min)

The canonical byte encoding for `FCS/1` descriptors is defined by DDS §7.2:

* COR/1 framing with strict TLV order.
* Required TLVs only: `function_ptr (0x30)`, `parameter_block (0x31)`,
  `arity (0x32)`.

No other TLVs are permitted in v1-min.

---

## 3. Validation Rules

Implementations MUST reject descriptors that violate any of the following:

1. TLVs are not in strict ascending order, or appear more than once.
2. `parameter_block` does not decode as a valid PCB1 envelope.
3. `arity` does not match PCB1 slot manifest length.
4. The descriptor graph (following nested `function_ptr` references) is cyclic.
5. Any legacy or unknown governance TLV appears.

Error mapping follows DDS (e.g., `ERR_FCS_TAG_ORDER`, `ERR_FCS_PARAMETER_FORMAT`,
`ERR_PCB_ARITY_MISMATCH`, `ERR_FCS_CYCLE_DETECTED`, `ERR_FCS_UNKNOWN_TAG`).

---

## 4. Composition Semantics

* Composition is **pure**: a descriptor that references another `FCS/1`
  descriptor inherits its deterministic execution recipe.
* The composed descriptor graph MUST be acyclic.
* Registries MUST admit only canonical, validated `FCS/1` descriptors.

---

## 5. Conformance

An implementation is **FCS/1–conformant** if it:

* Accepts only v1-min canonical encodings.
* Enforces the validation rules in §3.
* Treats descriptors as immutable and content-addressed.
* Rejects any descriptor with policy/intent metadata embedded.