docs: Add ADR 0042 - API standards for creating resources

docs/architecture/adr/0042-api-standards-for-creating-resources.md

@@ -0,0 +1,74 @@
+---
+adr: "0042"
+status: Proposed
+date: 2026-10-05
+tags: [server, server-sdk]
+---
+
+# 0042 - API standards for creating resources
+
+<AdrTable frontMatter={frontMatter}></AdrTable>
+
+## Notation
+
+This ADR uses [RFC 2119](https://www.rfc-editor.org/info/rfc2119/) keywords (`MUST`, `MUST NOT`,
+`SHOULD`, `SHOULD NOT`, `MAY`) deliberately.
+
+## Context and problem statement
+
+[ADR-0036](./0036-service-to-service-api-standards.md) requires service-to-service APIs to conform
+to a documented set of standards, and ADR-0039 builds those standards on IETF RFCs. This ADR sets
+how a resource is created: the method, the path, the request, and what a successful response
+returns. Payload shape and errors are set by ADR-0040.
+
+## Considered options
+
+- **`POST` to the collection, returning the new resource.**
+- **`PUT` to a client-chosen ID** as the primary way to create.
+- **`POST` returning only a `Location` header** and no body.
+
+## Decision outcome
+
+Chosen option: **`POST` to the collection, returning the new resource**, because the caller usually
+needs the server-generated ID and the stored representation, and getting both in one response saves
+a second call.
+
+- The method `MUST` be `POST`, and the path `SHOULD` be the collection, `/api/v1/{resource plural}`.
+- When the service generates the ID, the request `MUST` omit `id`.
+- APIs that support create-or-update `MUST` do so through `PUT` (see ADR-0044).
+- On success, APIs `SHOULD` return `201` but `MAY` return `202` or `204`.
+
+| Status           | Meaning                                  | Body                                       |
+| ---------------- | ---------------------------------------- | ------------------------------------------ |
+| `201 Created`    | The resource was created.                | The latest representation of the resource. |
+| `202 Accepted`   | The resource is scheduled to be created. | The job scheduled to create it (ADR-0045). |
+| `204 No Content` | The resource was created.                | Nothing.                                   |
+
+```http
+POST /api/v1/users
+Content-Type: application/json
+```
+
+```json
+{
+  "data": {
+    "attributes": {
+      "firstName": "Bob",
+      "lastName": "Smith"
+    },
+    "type": "user"
+  }
+}
+```
+
+Creating many resources at once is a bulk create (see ADR-0046).
+
+### Positive consequences
+
+- One round trip returns everything a caller needs about the new resource.
+- Creation looks the same on every API.
+
+### Negative consequences
+
+- `201` responses carry the full representation, which is wasted when the caller does not need it.
+  `204` remains available for that case.