Forward-compat invariants

Nika follows real semver toward 1.0. Features ship incrementally across 1.x minors over years. The 1.0 architecture must accommodate later minors (memory subsystem, agent-v2; WASM plugins, observability), and features we haven’t imagined without breaking changes to the public API.

Status: LOCKED at v0.80. Every crate admitted to the Diamond workspace must comply before passing Gate 12. Patterns distilled from Rust (2015→), Tokio (0.1→1.x), Deno, Cargo, Serde (9+ years stable), Axum/Tower, Bevy.Canonical source: docs/architecture/forward-compat-invariants.md.

The 8 patterns

FCI-001: Kernel traits upfront, implementations deferred

Define traits in nika-kernel at v0.90 for subsystems that ship later. Default methods return Err(Unsupported) until real implementations land.

#[trait_variant::make(MemoryStoreDyn: Send)]
pub trait MemoryStore: Send + Sync + 'static {
    async fn put(&self, frame: MemoryFrame) -> Result<MemoryFrameRef>;
    async fn query(&self, q: &MemoryQuery) -> Result<Vec<MemoryFrame>>;
    async fn forget(&self, _r: &MemoryFrameRef) -> Result<()> {
        Err(NikaError::Unsupported("forget"))
    }
}

Locked traits (v0.90): MemoryStore, EmbeddingProvider, ToolExecutor, WasmPluginHost, MetricsExporter + TracerProvider + AuditSink + EventSink + BillingSink, Sandbox. Stubs in src/plugin/, src/infra/.Impact: v0.95 lights up the memory subsystem by adding nika-memory-oxigraph that implements MemoryStore. Zero v0.90 code modification.

FCI-002: `#[non_exhaustive]` on every public struct, enum, error variant

Every public type carries #[non_exhaustive] so adding a field or variant is always additive. new() constructors provide ergonomic construction without exposing struct literal syntax.

#[non_exhaustive]
pub struct InferRequest {
    pub provider: ProviderId,
    pub model: ModelId,
    pub messages: Vec<Message>,
    // adding fields here never breaks downstream
}

impl InferRequest {
    pub fn new(provider: ProviderId, model: ModelId) -> Self { /* ... */ }
}

Invariant #19: every #[non_exhaustive] struct ships a new() constructor.

FCI-003: Single version marker per on-disk artifact

A workflow pins its contract with one header line · nika: v1 (a single version marker · the language name as key, the contract version as value · supersedes the older K8s apiVersion: and schema: nika/workflow@X forms). Parsers reject any other value (v1.0 · v2 …) with a clear error.

illustration · the envelope pin

nika: v1
workflow:
  id: my-flow
tasks: { ... }

v1 is the only value for the lifetime of the v1 contract: minor additions are additive and never change it. There is no nika: v2 — ever: pre-1.0 grammar changes happen inside v1 (the pre-1.0 stability contract) · after engine 1.0.0 the grammar is additive only. Non-workflow artifacts (package manifests, events) carry their own schema version field.

FCI-004: Extension namespaces (`nika.*` core + `x-*` community)

Public schemas reserve two namespaces: nika.* for core additions (authoritative, versioned) and x-* for community overlays (no stability guarantee, no collision with core). Scope note · in the workflow language both are RESERVED, not current: unknown top-level fields are rejected at v0.1, and the tool namespace set is closed at nika: / mcp: (engine-specific tools route through mcp:). The live x-* surface today is the pck registry.

illustration · RESERVED future extension fields (rejected today)

nika: v1
workflow:
  id: my-flow
nika.cache: { ttl: 3600 }      # nika.* core extension · a future additive minor
x-my-org-metric: 42             # x-* community overlay · reserved · NOT valid v0.1

FCI-005: Reserved error code ranges

Two error surfaces, one rule each. The WORKFLOW-VISIBLE surface is the spec’s NIKA-<NAMESPACE>-<NNN> taxonomy ( registered codes across namespaces · reference/error-codes). Namespaces own 001-099 ranges, codes are never repurposed. The ENGINE-INTERNAL registry (nika_error::codes · NIKA-1000+ blocks per L1 effect crate) is diagnostics machinery: reserved per crate, never reassigned, and never leaked into workflow-visible errors.

FCI-006: Sealed traits for core, open traits for extension

Core kernel traits (e.g., Provider, EventSink, Sandbox) are sealed via a private supertrait: only Nika-owned crates can implement them. Extension traits (e.g., MemoryStore) are open for community crates to implement.

mod sealed { pub trait Sealed {} }
pub trait Provider: sealed::Sealed + Send + Sync { /* ... */ }

Enforced by ADR-014.

FCI-007: Feature flags with stable defaults

New capability = new feature flag, OFF by default. After 1-2 minor cycles of stability, flips to default-ON. Never remove a default feature (breaking change): deprecate and redirect.

[features]
default = ["rustls-tls"]
rustls-tls = ["dep:rustls"]
native-tls = ["dep:native-tls"]   # alternative, off by default

FCI-008: Public API discipline via CI

Three tools on every CI run:

cargo public-api: detects API surface changes per crate.
cargo semver-checks: verifies SemVer compatibility.
cargo deny: enforces license + advisories + layer bans.

Gate 12 fails on any unintentional public API drift. .public-api.json snapshots are checked into the repo per crate.

The 5 locked decisions (v0.80)

ID	Decision	Impact
FCI-009	`EventKind` = scoped sub-enums (Pattern B), ~22 categories	Emit site is compile-time checked; sub-enums evolve independently
FCI-010	`InferRequest` / `InferResponse` fields reserved: `memory`, `budget`, `baggage`, `trust_level`, `trace_id`	v0.95 adds memory without touching kernel
FCI-011	`schema:` field mandatory day-1	Every on-disk artifact versioned from v0.90 forward
FCI-012	`Provider` trait shape frozen (4 methods, 1 sealed supertrait)	Cross-provider parity testable (shadow zone 2 + 7)
FCI-013	Telemetry = 5 sibling sinks (Event, Metrics, Trace, Audit, Billing)	OTel GenAI semconv bridged via typed `GenAiAttrs` (Q13 rev.3)

The 10 rules

Every crate admission answers “yes” to each. cargo public-api + cargo semver-checks catch violations before merge.

All public types carry #[non_exhaustive].
All public structs with #[non_exhaustive] ship a new() constructor.
All async trait methods use trait_variant::make for dynamic dispatch.
All core traits seal via private supertrait.
All error types expose a NIKA-XXX code via NikaError::code().
All on-disk schemas carry schema: "nika/<kind>@<major>".
All DTO fields reserve forward-compat extension slots (see FCI-010).
All feature flags default to a stable subset: new caps ship OFF.
All public API changes emit a cargo public-api diff in the PR.
All breaking changes require ADR Accepted before merge.

Layer registry

Six layers + L0.5, mechanical sort test, security axes.

L0 foundation decisions

Q1-Q13 locked 2026-04-16: proc macros, kernel prelude, transform crate, memory sinks.

12-gate admission

How a crate earns a seat at the workspace: SPEC through ATOMIC commit.

ADR index

35 decision records (30 Accepted + 5 Proposed).

Foundations

Decisions

Forward-compat invariants

The 8 patterns

The 5 locked decisions (v0.80)

The 10 rules

See also

Layer registry

L0 foundation decisions

12-gate admission

ADR index

​The 8 patterns

​The 5 locked decisions (v0.80)

​The 10 rules

​See also

Layer registry

L0 foundation decisions

12-gate admission

ADR index

The 8 patterns

The 5 locked decisions (v0.80)

The 10 rules

See also