diff --git a/src/Internal/Tools/Exceptions.elm b/src/Internal/Tools/Exceptions.elm
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+++ b/src/Internal/Tools/Exceptions.elm
@@ -0,0 +1,217 @@
+module Internal.Values.Exceptions exposing (Error(..), ClientError(..), ServerError(..), errorCatches, errorToString)
+
+{-| This module contains all potential errors that may be passed around in the SDK.
+-}
+
+import Dict
+import Http
+import Internal.Tools.DecodeExtra exposing (opField)
+import Internal.Config.ErrorStrings as ES
+import Json.Decode as D
+import Json.Encode as E
+
+
+type Error
+    = InternetException Http.Error
+    | SDKException ClientError
+    | ServerException ServerError
+    | UnsupportedVersion
+
+
+{-| Errors that this SDK might return if it doesn't like its own input, if it
+notices some internal inconsistencies or if it cannot interpret the server's
+input.
+
+  - `ServerReturnsBadJSON` The homeserver sent JSON that does not parse.
+  - `ServerDoesntFollowJSONSpec` The homeserver sent data that lacks keys
+    required by spec.
+  - `ServerDoesntReturnBaseURL` The homeserver does not clarify its API
+    endpoints.
+  - `CouldntGetTimestamp` The Elm core somehow failed to get the current
+    Unix timestamp.
+  - `InvalidInputAccordingToSpec` The function contains invalid values.
+
+-}
+type ClientError
+    = ServerReturnsBadJSON String
+    | CouldntGetTimestamp
+    | InsufficientCredentials String
+    | NotSupportedYet String
+
+
+{-| Potential error codes that the server may return. If the error is not a
+default one described in the Matrix Spec, it will be a `CustomServerError`
+and provide with the custom string.
+-}
+type
+    ServerError
+    -- COMMON ERROR CODES
+    -- These error codes can be returned by any API endpoint.
+    -- See https://spec.matrix.org/v1.5/client-server-api/#common-error-codes
+    = M_FORBIDDEN { error : Maybe String }
+    | M_UNKNOWN_TOKEN { error : Maybe String, soft_logout : Maybe Bool }
+    | M_MISSING_TOKEN { error : Maybe String, soft_logout : Maybe Bool }
+    | M_BAD_JSON { error : Maybe String }
+    | M_NOT_JSON { error : Maybe String }
+    | M_NOT_FOUND { error : Maybe String }
+    | M_LIMIT_EXCEEDED { error : Maybe String, retryAfterMs : Maybe Int }
+    | M_UNKNOWN { error : Maybe String }
+      -- OTHER ERROR CODES
+      -- These error codes are specific to certain endpoints.
+      -- See https://spec.matrix.org/v1.4/client-server-api/#other-error-codes
+    | M_UNRECOGNIZED { error : Maybe String }
+    | M_UNAUTHORIZED { error : Maybe String }
+    | M_USER_DEACTIVATED { error : Maybe String }
+    | M_USER_IN_USE { error : Maybe String }
+    | M_INVALID_USERNAME { error : Maybe String }
+    | M_ROOM_IN_USE { error : Maybe String }
+    | M_INVALID_ROOM_STATE { error : Maybe String }
+    | M_THREEPID_IN_USE { error : Maybe String }
+    | M_THREEPID_NOT_FOUND { error : Maybe String }
+    | M_THREEPID_AUTH_FAILED { error : Maybe String }
+    | M_THREEPID_DENIED { error : Maybe String }
+    | M_SERVER_NOT_TRUSTED { error : Maybe String }
+    | M_UNSUPPORTED_ROOM_VERSION { error : Maybe String }
+    | M_INCOMPATIBLE_ROOM_VERSION
+        { error : Maybe String
+        , room_version : Maybe String
+        }
+    | M_BAD_STATE { error : Maybe String }
+    | M_GUEST_ACCESS_FORBIDDEN { error : Maybe String }
+    | M_CAPTCHA_NEEDED { error : Maybe String }
+    | M_CAPTCHA_INVALID { error : Maybe String }
+    | M_MISSING_PARAM { error : Maybe String }
+    | M_INVALID_PARAM { error : Maybe String }
+    | M_TOO_LARGE { error : Maybe String }
+    | M_EXCLUSIVE { error : Maybe String }
+    | M_RESOURCE_LIMIT_EXCEEDED
+        { error : Maybe String
+        , adminContact : String
+        }
+    | M_CANNOT_LEAVE_SERVER_NOTICE_ROOM { error : Maybe String }
+      -- Error codes that help understand the server's output
+    | RequiresUserInteractiveAuthentication
+        { completed : List String
+        , flows : List (List String)
+        , params : Dict.Dict String (Dict.Dict String String)
+        , session : Maybe String
+        }
+      -- CUSTOM ERROR CODES
+      -- These can be defined by custom homeserver implementations
+    | CustomServerError
+        { errcode : String
+        , fullError : E.Value
+        , statusCode : Int
+        }
+
+
+{-| Shortcut for the decoder of most errors defined in the Matrix spec
+-}
+standardErrorDescription : D.Decoder { error : Maybe String }
+standardErrorDescription =
+    D.map (\err -> { error = err }) (opField "error" D.string)
+
+
+{-| Dictionary of known errors that the homeserver may return.
+The key is the error type, while the value is a function that reads all required
+and optional fields from the response based on Matrix specifications.
+-}
+errorCatches : D.Decoder ServerError
+errorCatches =
+    D.oneOf
+        [ errorDecoder "M_FORBIDDEN" M_FORBIDDEN standardErrorDescription
+        , errorDecoder "M_UNKNOWN_TOKEN"
+            M_UNKNOWN_TOKEN
+            (D.map2
+                (\err slg -> { error = err, soft_logout = slg })
+                (opField "error" D.string)
+                (opField "soft_logout" D.bool)
+            )
+        , errorDecoder "M_MISSING_TOKEN"
+            M_MISSING_TOKEN
+            (D.map2
+                (\err slg -> { error = err, soft_logout = slg })
+                (opField "error" D.string)
+                (opField "soft_logout" D.bool)
+            )
+        , errorDecoder "M_BAD_JSON" M_BAD_JSON standardErrorDescription
+        , errorDecoder "M_NOT_JSON" M_NOT_JSON standardErrorDescription
+        , errorDecoder "M_NOT_FOUND" M_NOT_FOUND standardErrorDescription
+        , errorDecoder "M_LIMIT_EXCEEDED"
+            M_LIMIT_EXCEEDED
+            (D.map2
+                (\err rams -> { error = err, retryAfterMs = rams })
+                (opField "error" D.string)
+                (opField "retry_after_ms" D.int)
+            )
+        , errorDecoder "M_UNKNOWN" M_UNKNOWN standardErrorDescription
+        , errorDecoder "M_UNRECOGNIZED" M_UNRECOGNIZED standardErrorDescription
+        , errorDecoder "M_UNAUTHORIZED" M_UNAUTHORIZED standardErrorDescription
+        , errorDecoder "M_USER_DEACTIVATED" M_USER_DEACTIVATED standardErrorDescription
+        , errorDecoder "M_USER_IN_USE" M_USER_IN_USE standardErrorDescription
+        , errorDecoder "M_INVALID_USERNAME" M_INVALID_USERNAME standardErrorDescription
+        , errorDecoder "M_ROOM_IN_USE" M_ROOM_IN_USE standardErrorDescription
+        , errorDecoder "M_INVALID_ROOM_STATE" M_INVALID_ROOM_STATE standardErrorDescription
+        , errorDecoder "M_THREEPID_IN_USE" M_THREEPID_IN_USE standardErrorDescription
+        , errorDecoder "M_THREEPID_NOT_FOUND" M_THREEPID_NOT_FOUND standardErrorDescription
+        , errorDecoder "M_THREEPID_AUTH_FAILED" M_THREEPID_AUTH_FAILED standardErrorDescription
+        , errorDecoder "M_THREEPID_DENIED" M_THREEPID_DENIED standardErrorDescription
+        , errorDecoder "M_SERVER_NOT_TRUSTED" M_SERVER_NOT_TRUSTED standardErrorDescription
+        , errorDecoder "M_UNSUPPORTED_ROOM_VERSION" M_UNSUPPORTED_ROOM_VERSION standardErrorDescription
+        , errorDecoder "M_INCOMPATIBLE_ROOM_VERSION"
+            M_INCOMPATIBLE_ROOM_VERSION
+            (D.map2
+                (\err rv -> { error = err, room_version = rv })
+                (opField "error" D.string)
+                (opField "room_version" D.string)
+            )
+        , errorDecoder "M_BAD_STATE" M_BAD_STATE standardErrorDescription
+        , errorDecoder "M_GUEST_ACCESS_FORBIDDEN" M_GUEST_ACCESS_FORBIDDEN standardErrorDescription
+        , errorDecoder "M_CAPTCHA_NEEDED" M_CAPTCHA_NEEDED standardErrorDescription
+        , errorDecoder "M_CAPTCHA_INVALID" M_CAPTCHA_INVALID standardErrorDescription
+        , errorDecoder "M_MISSING_PARAM" M_MISSING_PARAM standardErrorDescription
+        , errorDecoder "M_INVALID_PARAM" M_INVALID_PARAM standardErrorDescription
+        , errorDecoder "M_TOO_LARGE" M_TOO_LARGE standardErrorDescription
+        , errorDecoder "M_EXCLUSIVE" M_EXCLUSIVE standardErrorDescription
+        , errorDecoder "M_RESOURCE_LIMIT_EXCEEDED"
+            M_RESOURCE_LIMIT_EXCEEDED
+            (D.map2
+                (\err ac -> { error = err, adminContact = ac })
+                (opField "error" D.string)
+                (D.field "admin_contact" D.string)
+            )
+        , errorDecoder "M_CANNOT_LEAVE_SERVER_NOTICE_ROOM" M_CANNOT_LEAVE_SERVER_NOTICE_ROOM standardErrorDescription
+        ]
+
+
+errorDecoder : String -> (a -> ServerError) -> D.Decoder a -> D.Decoder ServerError
+errorDecoder name code decoder =
+    D.field "errcode" D.string
+        |> D.andThen
+            (\errcode ->
+                if errcode == name then
+                    D.map code decoder
+
+                else
+                    D.fail "Not the right errcode"
+            )
+
+
+errorToString : Error -> String
+errorToString e =
+    case e of
+        UnsupportedVersion ->
+            ES.unsupportedVersion
+
+        SDKException (ServerReturnsBadJSON s) ->
+            ES.serverReturnsBadJSON s
+
+        SDKException CouldntGetTimestamp ->
+            ES.couldNotGetTimestamp
+
+        ServerException (M_FORBIDDEN data) ->
+            ES.serverSaysForbidden data.error
+
+        -- ServerError (M_UNKNOWN_TOKEN data) ->
+        _ ->
+            "ERROR NEEDS STRING"