5 files changed, 71 insertions, 2 deletions
diff --git a/docs/api/authentication.md b/docs/api/authentication.md
index 7e05443..135e91b 100644
--- a/docs/api/authentication.md
+++ b/docs/api/authentication.md
@@ -13,6 +13,23 @@ stateDiagram-v2
 
 Authentication associates each authenticated request with a login.
 
+To create logins, see [initial setup](./initial-setup.md) and [invitations](./invitations.md).
+
+
+## Names
+
+<!-- This prose is duplicated in channels-messages.md. If you change it here, consider changing it there, too. -->
+The service handles login names using two separate forms.
+
+The first form is as given in the request used to create the login. This form of the login name is used throughout the API, and the service will preserve the name as entered (other than applying normalization), so that users' preferences around capitalization and accent marks are preserved.
+
+The second form is a "canonical" form, used internally by the service to control uniqueness and match names to logins. The canonical form is both case-folded and normalized.
+
+The canonical form is not available to API clients, but its use has practical consequences:
+
+* Names that differ only by case or only by code point sequence are treated as the same name. If the name is in use, changing the capitalization or changing the sequence of combining marks will not allow the creation of a second "identical" login.
+* The login API accepts any name that canonicalizes to the form stored in the database, making login names effectively case-insensitive.
+
 
 ## Identity tokens
 
@@ -32,8 +49,6 @@ Unless the endpoint's documentation says otherwise, all endpoints require authen
 
 Authenticates the user using their login name and password. The login must exist before calling this endpoint.
 
-To create logins, see [initial setup](./initial-setup.md) and [invitations](./invitations.md).
-
 **This endpoint does not require an `identity` cookie.**
 
 ### Request
diff --git a/docs/api/channels-messages.md b/docs/api/channels-messages.md
index 1ff037d..9ef4e66 100644
--- a/docs/api/channels-messages.md
+++ b/docs/api/channels-messages.md
@@ -27,6 +27,18 @@ Messages allow logins to communicate with one another. Channels are the conversa
 Every channel has a unique name, chosen when the channel is created.
 
 
+## Names
+
+<!-- This prose is duplicated in authentication.md. If you change it here, consider changing it there, too. -->
+The service handles channel names using two separate forms.
+
+The first form is as given in the request used to create the channel. This form of the channel name is used throughout the API, and the service will preserve the name as entered (other than applying normalization), so that users' preferences around capitalization and accent marks are preserved.
+
+The second form is a "canonical" form, used internally by the service to control uniqueness and match names to channels. The canonical form is both case-folded and normalized.
+
+The canonical form is not available to API clients, but its use has practical consequences. Names that differ only by case or only by code point sequence are treated as the same name. If the name is in use, changing the capitalization or changing the sequence of combining marks will not allow the creation of a second "identical" channel.
+
+
 ## Expiry and purging
 
 Both channels and messages expire after a time. Messages expire 90 days after being sent. Channels expire 90 days after the last message sent to them, or after creation if no messages are sent in that time.
@@ -70,6 +82,8 @@ The response will have the following fields:
 | `id`   | string | A unique identifier for the channel. This can be used to associate the channel with events, or to make API calls targeting the channel. |
 | `name` | string | The channel's name. |
 
+The returned name may not be identical to the name requested, as the name will be converted to [normalization form C](http://www.unicode.org/reports/tr15/) automatically. The returned name will include this normalization; the service will use the normalized name elsewhere, and does not store the originally requested name.
+
 When completed, the service will emit a [channel created](events.md#channel-created) event with the channel's ID.
 
 ### Duplicate channel name
@@ -125,6 +139,8 @@ The response will have the following fields:
 | `id`      | string    | A unique identifier for the message. This can be used to associate the message with events, or to make API calls targeting the message. |
 | `body`    | string    | The message's body. |
 
+The returned message body may not be identical to the body as sent, as the body will be converted to [normalization form C](http://www.unicode.org/reports/tr15/) automatically. The returned body will include this normalization; the service will use the normalized body elsewhere, and does not store the originally submitted body.
+
 When completed, the service will emit a [message sent](events.md#message-sent) event with the message's ID.
 
 ### Invalid channel ID
diff --git a/docs/api/initial-setup.md b/docs/api/initial-setup.md
index 3c5a8a6..306d798 100644
--- a/docs/api/initial-setup.md
+++ b/docs/api/initial-setup.md
@@ -71,6 +71,10 @@ The response will include the following fields:
 | `id`        | string | A unique identifier for the newly-created login. This can be used to associate the login with other events, or to make API calls targeting the login. |
 | `name`      | string | The login's name. |
 
+The returned name may not be identical to the name requested, as the name will be converted to [normalization form C](http://www.unicode.org/reports/tr15/) automatically. The returned name will include this normalization; the service will use the normalized name elsewhere, and does not store the originally requested name.
+
+The provided password will also be converted to normalization form C. However, the normalized password is not returned to the client.
+
 The response will include a `Set-Cookie` header for the `identity` cookie, providing the client with a newly-minted identity token associated with the initial login created for this request. See the [authentication](./authentication) section for details on how this cookie may be used.
 
 The cookie will expire if it is not used regularly.
diff --git a/docs/api/invitations.md b/docs/api/invitations.md
index d3431d7..ddbef8a 100644
--- a/docs/api/invitations.md
+++ b/docs/api/invitations.md
@@ -150,6 +150,10 @@ The response will include the following fields:
 | `id`        | string | A unique identifier for the newly-created login. This can be used to associate the login with other events, or to make API calls targeting the login. |
 | `name`      | string | The login's name. |
 
+The returned name may not be identical to the name requested, as the name will be converted to [normalization form C](http://www.unicode.org/reports/tr15/) automatically. The returned name will include this normalization; the service will use the normalized name elsewhere, and does not store the originally requested name.
+
+The provided password will also be converted to normalization form C. However, the normalized password is not returned to the client.
+
 The response will include a `Set-Cookie` header for the `identity` cookie, providing the client with a newly-minted identity token associated with the login created for this request. See the [authentication](./authentication.md) section for details on how this cookie may be used.
 
 The cookie will expire if it is not used regularly.
diff --git a/docs/internal-server-errors.md b/docs/internal-server-errors.md
new file mode 100644
index 0000000..4f679b7
--- /dev/null
+++ b/docs/internal-server-errors.md
@@ -0,0 +1,30 @@
+# Internal Server Errors
+
+When `hi` encounters a problem that prevents a request from completing, it may report a `500 Internal Server Error` to clients, along with an error code. The actual error will be printed to standard error, with the error code. The following sections describe errors we've encountered, the likely operational consequences, and recommend approaches for addressing them.
+
+## database is locked
+
+The server attempted two write transactions at the same time, and encountered [sqlite's write locks](https://www.sqlite.org/rescode.html#busy). This is unfortunately unavoidable, but generally only occurs as a result of extremely bad luck, or very high load.
+
+This error will almost always resolve itself if clients re-try their requests; no further action is needed.
+
+This is a known issue. If you are encountering this consistently (or if you can trigger it on demand), let us know. We are aware of sqlite's features for mitigating this issue but have been unsuccessful in applying them; we're working on it, but patches _are_ welcome, if you have the opportunity.
+
+## stored canonical form […] does not match computed canonical form […] for name […]
+
+When `hi` applies the `migrations/20241019191531_canonical_names.sql` migration (from commit `3f9648eed48cd8b6cd35d0ae2ee5bbe25fa735ac`), this can leave existing names in a state where the stored canonical form is not the correct canonicalization of the stored display names of channels and logins. `hi` will abort requests when it encounters this situation, to avoid incorrect behaviours such as duplicate channels or duplicate logins.
+
+As channel and login names may be presented during client startup, this can render the service unusable until repaired. Treat this as an immediate outage if you see it.
+
+You can verify that login names are unique by running the following commands as the user the `hi` server runs as:
+
+* `sqlite3 .hi 'select display_name from login'`
+* `sqlite3 .hi 'select display_name from channel_name'`
+
+Substitute `.hi` with the path to your `hi` database if it differs from the default.
+
+If the names are unique, you can repair the database:
+
+* Stop the `hi` server.
+* Run `hi-recanonicalize`, as the same user the `hi` server runs as, with the same database options.
+* Start the `hi` server.