diff --git a/SPEC.md b/SPEC.md
index c6e10ce..5f50096 100644
--- a/SPEC.md
+++ b/SPEC.md
@@ -43,6 +43,10 @@ properties:
     type: string
     description: A simple text message.
 
+  stackTrace:
+    type: string
+    description: An optional stack trace that may be emitted in languages that support it.
+
   metadata:
     type: object
     additionalProperties:
@@ -55,6 +59,10 @@ properties:
     properties:
     description: |
       Additional JSON-serializable structured data.
+
+  cause:
+    # An optional nested failure structure.
+    $ref: '#'
 ```
 
 ### OperationInfo
@@ -141,17 +149,9 @@ The body may contain arbitrary data. Headers should specify content type and enc
   **Headers**:
 
   - `Content-Type: application/json`
-  - `Nexus-Operation-State: failed | canceled`
-
-  **Body**: A JSON-serialized [`Failure`](#failure) object.
+  - `Nexus-Operation-State: failed | canceled` (DEPRECATED)
 
-- `409 Conflict`: Operation was already started with a different request ID.
-
-  **Headers**:
-
-  - `Content-Type: application/json`
-
-  **Body**: A JSON-serialized [`Failure`](#failure) object.
+  **Body**: A JSON-serialized [`Failure`](#failure) object representing an [`OperationError`](#operation-error).
 
 ### Cancel Operation
 
@@ -178,33 +178,80 @@ The operation token received as a response to the Start Operation method must be
 
   **Body**: Empty.
 
-- `404 Not Found`: Operation token not recognized or references deleted.
+## Predefined Failure Types
 
-  **Headers**:
+### `OperationError`
 
-  - `Content-Type: application/json`
+An Operation Error represents a failed or canceled operation outcome. It may be returned responses to `StartOperation`
+requests, and in the body of async completion requests.
+
+Operation Error [`Failure`](#failure) representation is as follows:
 
-  **Body**: A JSON serialized [`Failure`](#failure) object.
+```json
+{
+  "metadata": {
+    "type": "nexus.OperationError",
+  },
+  "message": "<Optional error message>",
+  "stackTrace": "<Optional stack trace>",
+  "cause": { /* <Optional cause> */ },
+  "details": {
+    "state": "canceled | failed",
+    // Aribtrary details may be added here as needed.
+  },
+}
+```
+
+### `HandlerError`
+
+A HandlerError represents errors while handling a request. They include an error type as defined
+[below](#predefined-handler-errors). Each error type has predefined, overridable, retry semantics. Handler Errors can be
+returned responses to any of the methods defined above as well as completion callbacks.
+
+Handler Error [`Failure`](#failure) representation is as follows:
+
+```json
+{
+  "metadata": {
+    "type": "nexus.HandlerError",
+  },
+  "message": "<Optional error message>",
+  "stackTrace": "<Optional stack trace>",
+  "cause": { /* <Optional cause> */ },
+  "details": {
+    "type": "<predefined error type (e.g. INTERNAL)",
+    // "retryableOverride": Optional boolean.
+    // Aribtrary details may be added here as needed.
+  },
+}
+```
 
-## Predefined Handler Errors
+### Predefined Handler Errors
 
 For compatiblity of this HTTP spec with future transports, when a handler fails a request, it **should** use one of the
 following predefined error codes.
 
-| Name                 | Status Code | Description                                                                                                                                                               |
-| -------------------- | ----------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
-| `BAD_REQUEST`        | 400         | The handler cannot or will not process the request due to an apparent client error. Clients should not retry this request unless advised otherwise.                       |
-| `UNAUTHENTICATED`    | 401         | The client did not supply valid authentication credentials for this request. Clients should not retry this request unless advised otherwise.                              |
-|                      |             |                                                                                                                                                                           |
-| `UNAUTHORIZED`       | 403         | The caller does not have permission to execute the specified operation. Clients should not retry this request unless advised otherwise.                                   |
-|                      |             |                                                                                                                                                                           |
-| `NOT_FOUND`          | 404         | The requested resource could not be found but may be available in the future. Subsequent requests by the client are permissible but not advised.                          |
-|                      |             |                                                                                                                                                                           |
-| `RESOURCE_EXHAUSTED` | 429         | Some resource has been exhausted, perhaps a per-user quota, or perhaps the entire file system is out of space. Subsequent requests by the client are permissible.         |
-| `INTERNAL`           | 500         | An internal error occured. Subsequent requests by the client are permissible.                                                                                             |
-| `NOT_IMPLEMENTED`    | 501         | The handler either does not recognize the request method, or it lacks the ability to fulfill the request. Clients should not retry this request unless advised otherwise. |
-| `UNAVAILABLE`        | 503         | The service is currently unavailable. Subsequent requests by the client are permissible.                                                                                  |
-| `UPSTREAM_TIMEOUT`   | 520         | Used by gateways to report that a request to an upstream handler has timed out. Subsequent requests by the client are permissible.                                        |
+| Name                 | Status Code | Description                                                                                                                                                                                                                                                 |
+| -------------------- | ----------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| `BAD_REQUEST`        | 400         | The handler cannot or will not process the request due to an apparent client error. Clients should not retry this request unless advised otherwise.                                                                                                         |
+| `UNAUTHENTICATED`    | 401         | The client did not supply valid authentication credentials for this request. Clients should not retry this request unless advised otherwise.                                                                                                                |
+| `UNAUTHORIZED`       | 403         | The caller does not have permission to execute the specified operation. Clients should not retry this request unless advised otherwise.                                                                                                                     |
+| `NOT_FOUND`          | 404         | The requested resource could not be found but may be available in the future. Subsequent requests by the client are permissible but not advised.                                                                                                            |
+| `REQUEST_TIMEOUT`    | 408         | Returned by the server to when it has given up handling a request. The may occur by enforcing a client provided `Request-Timeout` or for any arbitrary reason such as enforcing some configurable limit. Subsequent requests by the client are permissible. |
+| `CONFLICT`           | 409         | The request could not be made due to a conflict. The may happen when trying to create an operation that has already been started. Clients should not retry this request unless advised otherwise.                                                           |
+| `RESOURCE_EXHAUSTED` | 429         | Some resource has been exhausted, perhaps a per-user quota, or perhaps the entire file system is out of space. Subsequent requests by the client are permissible.                                                                                           |
+| `INTERNAL`           | 500         | An internal error occured. Subsequent requests by the client are permissible.                                                                                                                                                                               |
+| `NOT_IMPLEMENTED`    | 501         | The handler either does not recognize the request method, or it lacks the ability to fulfill the request. Clients should not retry this request unless advised otherwise.                                                                                   |
+| `UNAVAILABLE`        | 503         | The service is currently unavailable. Subsequent requests by the client are permissible.                                                                                                                                                                    |
+| `UPSTREAM_TIMEOUT`   | 520         | Used by gateways to report that a request to an upstream handler has timed out. Subsequent requests by the client are permissible.                                                                                                                          |
+
+Client implementations should try to rehydrate a `HandlerError` from the serialized `Failure` object in the response
+body whenever a request fails with one of the status codes listed below. If the handler error type in the `Failure`
+object details doesn't match the response status code, the `Failure` object takes precendence.
+
+If the serialized `Failure` does not represent a `HandlerError`, clients should construct a wrapper `HandlerError`,
+setting the response `Failure` as the `cause` (if available) translating the response status code to the `HandlerError`
+type and status text to the message.
 
 ## General Purpose Headers
 
@@ -220,7 +267,7 @@ Links must contain a `type` parameter that expresses how they should be parsed.
 
 **Example**: `Nexus-Link: <myscheme://somepath?k=v>; type="com.example.MyResource"`
 
-### `Nexus-Request-Retryable`
+### `Nexus-Request-Retryable` (DEPRECATED)
 
 Handlers may specify the `Nexus-Request-Retryable` header with a value of `true` or `false` to explicitly instruct a
 caller whether or not to retry a request. Unless specified, retry behavior is determined by the