> ## Documentation Index
> Fetch the complete documentation index at: https://agenticadvertisingorg-snap-format-preview-links.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# L3 — Protocol semantics

> Protocol-semantics layer of the AdCP stack. Lifecycle state machines, idempotency, error catalog, async-task contract, conformance test surface, webhook emission. Where most of an SDK's value lives.

L3 enforces what AdCP *means* on the agent side. The wire shape is well-formed (L0); the caller is authentic (L1) and authorized (L2); now: is the request legal given the current state of the world?

For agents, L3 is the bulk of the protocol surface — the [3–4 person-month from-scratch build](/docs/building/cross-cutting/sdk-stack#why-sdks-matter-more-in-adcp-than-in-eg-http) lives almost entirely here. For callers, L3 is consumer-side: weeks of handler glue, classifying error codes and handling state transitions rather than enforcing them.

## What an SDK at L3 must provide

If you're picking an SDK or porting one to a new language, this is the L3 build target:

* **Lifecycle state-machine graphs** for all spec-defined resources, with a transition-assertion primitive that emits the spec-correct error code (`NOT_CANCELLABLE` / `INVALID_STATE` / etc.).
* **Idempotency cache** with cross-payload conflict detection and the no-payload-echo invariant on `IDEMPOTENCY_CONFLICT` envelopes.
* **Async-task store + dispatcher** — tools opt into async; the SDK returns `task_id`, accepts polling, and emits the terminal artifact.
* **Webhook emitter** — signed, retried, idempotent.
* **The conformance test surface** (`comply_test_controller`), wired to drive state deterministically when the resolved account is in sandbox or mock mode (and rejected otherwise).
* **Per-resource persistence primitives** that handle the spec's echo contracts.
* **Server-construction entry point** that ties all of the above together with sane defaults.

For the cumulative cross-layer story (what L0+L1+L2+L3 buys you), see the [SDK stack reference](/docs/building/cross-cutting/sdk-stack#l3--protocol-semantics). For what changed at L3 between 2.5 and 3.0, see [What changed at L3 in 3.0](/docs/building/cross-cutting/version-adaptation#what-changed-at-l3-in-3-0).

## Pages in this layer

* **[Task lifecycle](/docs/building/by-layer/L3/task-lifecycle)** — status values, transitions, polling.
* **[Async operations](/docs/building/by-layer/L3/async-operations)** — sync, async, and interactive task handling.
* **[Webhooks](/docs/building/by-layer/L3/webhooks)** — push notifications, signing, retry, idempotency.
* **[Error handling](/docs/building/by-layer/L3/error-handling)** — error categories, codes, recovery classification.
* **[`comply_test_controller`](/docs/building/by-layer/L3/comply-test-controller)** — sandbox-only conformance test surface.