diff --git a/specification/error-processing-flow.md b/specification/error-processing-flow.md
new file mode 100644
index 00000000..241ab081
--- /dev/null
+++ b/specification/error-processing-flow.md
@@ -0,0 +1,186 @@
+## General errors
+
+```mermaid
+stateDiagram-v2
+    [*] --> A1
+    A1 --> A2
+    A2 --> A3
+    A3 --> A4
+    A4 --> A5
+    A5 --> A6
+    A6 --> A7 
+    A7 --> A8
+    A8 --> A9
+    A9 --> A10
+    A10 --> A11
+    A11 --> A12
+    A12 --> A13
+    A13 --> A14
+    A14 --> A15
+    A15 --> A16
+    A16 --> A17
+    A17 --> A18
+    A18 --> A19
+    A19 --> A20
+    A20 --> A21
+    A21 --> A22
+    A22 --> A23
+    A23 --> A24
+    A24 --> A25
+
+    A1 --> 400_SEND_BAD_REQUEST: Required header not present (code - invalid)
+    A2 --> 401_SEND_UNAUTHORIZED: Access Token is invalid/expired (code - security)
+    A3 --> 401_SEND_UNAUTHORIZED2: Access Token not provided (code - unknown)
+    A4 --> 403_SEND_FORBIDDEN: Access Token is valid but BaRS is not enabled (code - forbidden)
+    A5 --> 429_SEND_TOO_MANY_REQUESTS: when rate limiting is applied (code - throttled)
+    A6 --> 404_PROXY_NOT_FOUND: Endpoint on the API does not exist (code - not-found)
+    A7 --> 405_SEND_METHOD_NOT_ALLOWED: using the wrong http verb (code - not-supported)
+    A8 --> 406_SEND_NOT_ACCEPTABLE: The requested resource is capable of generating only content not acceptable according to the Accept headers sent in the request (code - processing)
+    A9 --> 500_PROXY_SERVER_ERROR: Unexpected error (code - exception)
+    A10 --> 400_PROXY_BAD_REQUEST: A structural issue in the content such as the Target identifier is malformed (code - structure)
+    A11 --> 404_PROXY_NOT_FOUND2: The target endpoint does not exist in the Endpoint Catalogue (code - not-found)
+    A12 --> 404_PROXY_NOT_FOUND3: The Target Identifier found two endpoints (code - multiple-matches)
+    A13 --> 500_PROXY_SERVER_ERROR2: A sub-service encountered an unhandled exception (code - exception)
+    A14 --> 503_PROXY_UNAVAILABLE: A sub-service was unavailble (code - transient)
+    A15 --> 408_REC_TIMEOUT: The connection to the Receiver timed out (code - timeout)
+    A16 --> 500_REC_SERVER_ERROR: 500 response received but no operation outcome (code - exception)
+    A17 --> 503_REC_SERVICE_UNAVAILABLE: 503 response received but no operation outcome (code - transient)
+    A18 --> 400_REC_NOT_FOUND: The payload was invalid, or malformed (code - invalid)
+    A19 --> 401_REC_UNAUTHORIZED: Access Control (code - security)
+    A20 --> 403_REC_FORBIDDEN: TLS-MA failure (code - forbidden)
+    A21 --> 403_REC_FORBIDDEN2: TLS-MA failure (code - security)
+    A22 --> 500_REC_SERVER_ERROR2: The start params in a slot request were two wide resulting in a large number of results (code - too-costly)
+    A23 --> 500_REC_SERVER_ERROR3: unhandled exception (code - exception)
+    A24 --> 500_REC_SERVER_ERROR4: internal data storage issue (code - no-store)
+    A25 --> 501_REC_NOT_IMPLEMENTED: The receiver has not yet implemented that endpoint or functionality (code - not-supported)
+```
+
+```mermaid
+stateDiagram-v2
+    [*] --> A1
+    A1 --> A2
+    A2 --> A3
+    A3 --> A4
+    A4 --> A5
+    A5 --> A6
+    A6 --> A7
+    A7 --> A8
+    A8 --> A9
+    A9 --> A10
+    A10 --> A11
+    A11 --> A12
+
+    A1 --> 401_SEND_UNAUTHORIZED: token invalid/expired
+    A2 --> 400_SEND_BAD_REQUEST: missing required param
+    A3 --> 405_SEND_METHOD_NOT_ALLOWED: wrong http verb
+    A4 --> 429_SEND_TOO_MANY_REQUESTS: rate limiting applied
+    A5 --> 406_SEND_NOT_ACCEPTABLE: requested resource not acceptable
+    A6 --> 500_PROXY_SERVER_ERROR/SERVER_ERROR : unexpected exception in proxy
+    A7 --> 400_REC_BAD_REQUEST: invalid param
+    A8 --> 400_REC_BAD_REQUEST2: feature not yet supported
+    A9 --> 406_REC_NOT_ACCEPTABLE: requested resource not acceptable
+    A10 --> 409_REC_TIMEOUT: receiver timed out (proxy triggering 504)
+    A11 --> 500_REC_SERVER_ERROR: unexpected exception in receiver 
+    A12 --> 501_REC_NOT_IMPLEMENTED: receiver not yet implemented endpoint
+```
+
+## General errors implemented so far
+
+```mermaid
+stateDiagram-v2
+    [*] --> A1
+    A1 --> A2
+    A2 --> A3
+    A3 --> A4
+    A4 --> A5
+    A5 --> A6
+    A6 --> A7
+    A7 --> A8
+
+
+    A1 --> 401_REC_UNAUTHORIZED: proxy to receiver unauthorized, TLS-MA failure?
+    A2 --> 403_REC_FORBIDDEN: TLS-MA faliure
+    A3 --> 406_SEND_NOT_ACCEPTABLE: requested resource not acceptable
+    A4 --> 409_REC_CONFLICT: no longer valid, should be REC_TIMEOUT receiver timed out (proxy triggering 504)
+    A5 --> 422_REC_UNPROCESSABLE_ENTITY: The start time range requested is too wide (/Slot specific)
+    A6 --> 500_REC_SERVER_ERROR: unexpected exception in receiver 
+    A7 --> 501_REC_NOT_IMPLEMENTED : receiver not yet implemented endpoint
+    A8 --> 503_REC_SERVICE_UNAVAILABLE: The service(s) is unavailable but is able to respond as such (endpoint specific?)
+```
+
+## Endpoint specific errors
+
+### /Appointment/{id}
+```mermaid
+graph LR
+    A[GET /Appointment/id] -->B{ID parameter correct format?}
+    B --> |false| C[400 REC_BAD_REQUEST]
+    B --> |true| D{GET request?}
+    D --> |false| E[405 REC_METHOD_NOT_ALLOWED]
+    D --> |true| F{Resource exist?}
+    F --> |false| G[403 REC_NOT_FOUND]
+    F --> |true| H[200 OK]
+```
+### /Appointment
+```mermaid
+graph LR
+    A[GET /Appointment] -->B{required headers present?}
+    B --> |false| C[400 REC_BAD_REQUEST]
+    B --> |true| D{GET request?}
+    D --> |false| E[405 REC_METHOD_NOT_ALLOWED]
+    D --> |true| F[200 OK]
+```
+### /MessageDefinition
+```mermaid
+graph LR
+    A[GET /MessageDefinition] -->B{required headers present?}
+    B --> |false| C[400 REC_BAD_REQUEST]
+    B --> |true| D{GET request?}
+    D --> |false| E[405 REC_METHOD_NOT_ALLOWED]
+    D --> |true| F[200 OK]
+```
+### /metadata
+```mermaid
+graph LR
+    A[GET /metadata] -->B{required headers present?}
+    B --> |false| C[400 REC_BAD_REQUEST]
+    B --> |true| D{GET request?}
+    D --> |false| E[405 REC_METHOD_NOT_ALLOWED]
+    D --> |true| F[200 OK]
+```
+### /$process-message
+```mermaid
+graph LR
+    A[POST /$process-message] -->B{required headers present?}
+    B --> |false| C[400 REC_BAD_REQUEST]
+    B --> |true| D{POST request?}
+    D --> |false| E[405 REC_METHOD_NOT_ALLOWED]
+    D --> |true| F[200 OK]
+```
+### /ServiceRequest
+```mermaid
+graph LR
+    A[GET /ServiceRequest] -->B{required headers present?}
+    B --> |false| C[400 REC_BAD_REQUEST]
+    B --> |true| D{GET request?}
+    D --> |false| E[405 REC_METHOD_NOT_ALLOWED]
+    D --> |true| F[200 OK]
+```
+### /ServiceRequest/{id}
+```mermaid
+graph LR
+    A[GET /ServiceRequest/id] -->B{required headers present?}
+    B --> |false| C[400 REC_BAD_REQUEST]
+    B --> |true| D{GET request?}
+    D --> |false| E[405 REC_METHOD_NOT_ALLOWED]
+    D --> |true| F[200 OK]
+```
+### /Slot
+```mermaid
+graph LR
+    A[GET /Slot] -->B{required headers present?}
+    B --> |false| C[400 REC_BAD_REQUEST]
+    B --> |true| D{GET request?}
+    D --> |false| E[405 REC_METHOD_NOT_ALLOWED]
+    D --> |true| F[200 OK]
+```
\ No newline at end of file
diff --git a/tests/test_endpoints.py b/tests/test_endpoints.py
index f519aedc..029e8013 100644
--- a/tests/test_endpoints.py
+++ b/tests/test_endpoints.py
@@ -98,7 +98,7 @@ def test_invalid_nhsd_service_identifier(self, get_token_client_credentials):
     def test_endpoint_not_found(self, get_token_client_credentials):
         """
           test for /invalid..  to check it making a request to an unexisting endpoint
-          must return 500 server error.
+          must return 404 server error.
         """
         # Given
         token = get_token_client_credentials["access_token"]