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+# MSC4528: `M_INCOMPATIBLE_SERVER` error code
+
+Following [MSC4291](https://github.com/matrix-org/matrix-spec-proposals/pull/4291) and
+[MSC4311](https://github.com/matrix-org/matrix-spec-proposals/pull/4311), the Server-Server API
+tells sending servers to convert certain federation errors into 5xx errors over the Client-Server
+API. [`PUT /_matrix/federation/v2/send_join/{roomId}/{eventId}`](https://spec.matrix.org/v1.19/server-server-api/#put_matrixfederationv2send_joinroomideventid)
+and [`PUT /_matrix/federation/v2/invite/{roomId}/{eventId}`](https://spec.matrix.org/v1.19/server-server-api/#put_matrixfederationv2inviteroomideventid)
+both say of their `400` response:
+
+> If `M_MISSING_PARAM` or `M_INVALID_PARAM` is returned and the request is associated
+> with a Client-Server API request, the Client-Server API request SHOULD fail
+> with a 5xx error rather than being passed through.
+
+These rejections occur when the events in the request fail the validation introduced by those MSCs,
+for example when an invite lacks a full-PDU `m.room.create` event in its stripped state. The
+rationale for a 5xx is that there is nothing the client can do differently to make the request
+succeed.
+
+This translation has three problems:
+
+1. No specific status code or `errcode` is given, so clients cannot detect this case and give a more
+   helpful error message.
+
+2. Many clients and client SDKs automatically retry requests which fail with a 5xx status code.
+   However, the failures in the MSCs are not transient as the two servers will remain incompatible
+   until one of them is updated, which will not happen within any retry window.
+
+3. There is no good 5xx error code to use. 500 response rates are often monitored and assumed to
+   mean something has gone wrong and needs fixing. 502/503 response are often interpreted by reverse
+   proxies and load balancers as the backend being down, and will stop sending traffic to said
+   backend. The other defined 5xx errors don't really fit and are rarely used.
+
+This proposal replaces the 5xx translation with a `400` response carrying a new standard error code,
+`M_INCOMPATIBLE_SERVER`, meaning the request failed because the local server and a remote server it
+needed to communicate with are incompatible with each other.
+
+The spec already uses `400` error codes to indicate final error responses that are not the fault of
+the sending client, such as `M_INCOMPATIBLE_ROOM_VERSION`.
+
+## Proposal
+
+A new standard error code is added to the Client-Server API:
+
+* `M_INCOMPATIBLE_SERVER`: The request could not be completed because it required communicating with
+  a remote server which is incompatible with the local server.
+
+Clients SHOULD NOT automatically retry a request which failed with `M_INCOMPATIBLE_SERVER` and
+SHOULD surface the failure to the user. Servers SHOULD use the human-readable `error` field to describe
+the incompatibility, which clients MAY show to the user.
+
+Future proposals SHOULD continue to strive to maintain backwards compatibility where feasible,
+rather than use `M_INCOMPATIBLE_SERVER`.
+
+The paragraph quoted above is replaced, on both the `/send_join` and `/invite` endpoints, with the
+following:
+
+> If `M_MISSING_PARAM` or `M_INVALID_PARAM` is returned and the request is associated with a
+> Client-Server API request, the Client-Server API request SHOULD fail with a
+> `400 M_INCOMPATIBLE_SERVER` standard Matrix error rather than the federation error being passed
+> through.
+
+For example, a client calling [`POST /_matrix/client/v3/rooms/{roomId}/invite`](https://spec.matrix.org/v1.19/client-server-api/#post_matrixclientv3roomsroomidinvite)
+where the invited user's server rejects the federation invite would receive:
+
+```json
+{
+  "errcode": "M_INCOMPATIBLE_SERVER",
+  "error": "example.org rejected the invite because the two servers are incompatible. One of them may need updating."
+}
+```
+
+Note that the message does not say which server is at fault. On receiving the federation error, the
+sending server cannot know which of the two implementations is outdated.
+
+While the [MSC4291](https://github.com/matrix-org/matrix-spec-proposals/pull/4291) and
+[MSC4311](https://github.com/matrix-org/matrix-spec-proposals/pull/4311) validation failures are the
+motivating cases, the error code is defined generically. Other endpoints and future proposals MAY
+use `M_INCOMPATIBLE_SERVER` wherever a request fails because a remote server could not interoperate
+with the local server, rather than because of a client error or a fault within the local server
+itself.
+
+## Potential issues
+
+* Strictly speaking, `400` is the wrong class for this failure. The client's request was well-formed
+  and the fault lies between the two servers. In practice, however, Matrix (and the lot of the web)
+  treats 4xx responses as permanent failures and 5xx responses as potentially transient ones. The
+  strict interpretation of the status ranges offers no third option for a final error that is
+  no-one's fault (which is the case here). Matrix already returns `400` for failures outside the
+  client's control, such as
+  [`M_INCOMPATIBLE_ROOM_VERSION`](https://spec.matrix.org/v1.19/client-server-api/#other-error-codes).
+
+* The receiving server may return `M_MISSING_PARAM` or `M_INVALID_PARAM` because of a bug rather
+  than version skew. The sending server cannot tell the difference, and does not need to. Either
+  way the two implementations failed to interoperate, and retrying will not help.
+
+* This proposal is backwards compatible as the current text is only a SHOULD and does not specify
+  the response body. Given the underspecified nature of the existing text, no client can be relying
+  on the current behaviour anyway.
+
+## Alternatives
+
+We could keep the 5xx translation but specify it fully, e.g. as `502 M_INCOMPATIBLE_SERVER`. In
+strict HTTP terms `502 Bad Gateway` is the closest fit, since the local server is acting as a
+gateway to a remote server which returned an unusable response. However, using a 502 here goes
+against the common assumptions made by clients and proxies about such errors.
+
+## Security considerations
+
+None foreseen.
+
+## Unstable prefix
+
+None needed.
+
+## Dependencies
+
+None.