feat: add generation field to Task for ordering, long-polling, and optimistic concurrency

docs/specification.md

@@ -221,6 +221,8 @@ Retrieves the current state (including status, artifacts, and optionally history
 
 See [History Length Semantics](#324-history-length-semantics) for details about `historyLength`.
 
+See [Task Generation Semantics](#327-task-generation-semantics) for details about `currentGeneration` and long-polling.
+
 **Outputs:**
 
 - [`Task`](#411-task): Current state and artifacts of the requested task
@@ -453,6 +455,12 @@ The `return_immediately` field has no effect:
 - for streaming operations, which always return updates in real-time.
 - on configured push notification configurations, which operates independently of execution mode.
 
+**Optimistic Concurrency with `ifGenerationMatch`:**
+
+The `ifGenerationMatch` field enables optimistic concurrency control when sending a follow-up message to an existing task. If set, the server **MUST** compare the provided value against the task's current `generation` at the time the message is processed. If the values do not match, the server **MUST** reject the request with [`TaskGenerationMismatchError`](#332-error-handling) (HTTP `409 Conflict` / gRPC `ABORTED`). If the task referenced by the message does not yet exist (i.e., a new task will be created), this field **MUST** be ignored.
+
+See [Task Generation Semantics](#327-task-generation-semantics) for the full description of the `generation` field.
+
 #### 3.2.3. Stream Response
 
 <span id="323-stream-response"></span>
@@ -487,6 +495,51 @@ A key-value map for passing horizontally applicable context or parameters with c
 
 As service parameter names MAY need to co-exist with other parameters defined by the underlying transport protocol or infrastructure, all service parameters defined by this specification will be prefixed with `a2a-`.
 
+#### 3.2.7. Task Generation Semantics
+
+<span id="327-task-generation-semantics"></span>
+
+The `generation` field on [`Task`](#411-task) is a monotonically increasing integer maintained by the server. It is initialised to `0` when a task is created and **MUST** be incremented by the server on every state-changing mutation:
+
+- A `TaskState` transition (any change to `Task.status.state`).
+- An Artifact addition or update (any change to `Task.artifacts`).
+
+The `generation` value is included in [`TaskStatusUpdateEvent`](#421-taskstatusupdateevent) and [`TaskArtifactUpdateEvent`](#422-taskartifactupdateevent) to reflect the task's generation **after** the mutation that produced the event.
+
+**Event Ordering and Missed-Event Detection:**
+
+A client that tracks the last seen `generation` can detect missed events by observing gaps in the sequence. If a client receives an event with `generation = N+2` after previously seeing `generation = N`, it knows at least one event was not delivered and **SHOULD** re-fetch the full task state via [Get Task](#313-get-task) to reconcile.
+
+A correctly-implemented server **MUST NOT** emit two events for the same task with the same `generation` value. If a client observes two consecutive events carrying identical `generation` values (including two events both carrying `0`), it **MUST** conclude that the server does not implement the `generation` field and **MUST NOT** use `generation` for ordering, long-polling, or precondition checks for the remainder of that interaction. The client **SHOULD** fall back to stream ordering or timestamps for sequencing.
+
+**Long-Polling:**
+
+The optional `currentGeneration` field on [`GetTaskRequest`](#313-get-task) enables efficient long-polling:
+
+- If `currentGeneration` is **not set**, the server **MUST** return the task's current state immediately (standard behaviour).
+- If `currentGeneration` is **set** and the task's current `generation` is **already greater** than the supplied value, the server **MUST** return the current state immediately.
+- If `currentGeneration` is **set** and the task's current `generation` is **equal to or less than** the supplied value, the server **SHOULD** hold the request open and return only once the task's `generation` advances beyond the supplied value. The server **MAY** return earlier (for example, on an implementation-defined timeout) with the current task state.
+
+This allows clients to avoid repeated unconditional polling while still working correctly when a connection times out or when the server does not support the hold-open behaviour.
+
+**HTTP/REST long-poll example:**
+
+```http
+GET /tasks/task-abc?currentGeneration=3 HTTP/1.1
+Host: agent.example.com
+A2A-Version: 1.0
+```
+
+The server returns when the task's `generation` exceeds `3`, or on timeout.
+
+**Optimistic Concurrency:**
+
+The `ifGenerationMatch` field in [`SendMessageConfiguration`](#322-sendmessageconfiguration) provides an optimistic concurrency guard for multi-turn interactions. See that section for details.
+
+**Backwards Compatibility:**
+
+The `generation` field was introduced in version **1.1** of this specification. The field defaults to `0` in the Protocol Buffer encoding (proto3 default for `int64`). Clients that do not read or send `generation` continue to interoperate correctly with servers that support it. Servers that have not yet implemented `generation` will return `0` on all events and responses; clients will detect this via the duplicate-generation rule above and gracefully fall back.
+
 ### 3.3. Operation Semantics
 
 #### 3.3.1. Idempotency
@@ -561,6 +614,7 @@ Protocol bindings **MUST** map these elements to their native error representati
 | `ExtendedAgentCardNotConfiguredError` | The agent does not have an extended agent card configured when one is required for the requested operation.                                                       |
 | `ExtensionSupportRequiredError`       | Server requested use of an extension marked as `required: true` in the Agent Card but the client did not declare support for it in the request.                   |
 | `VersionNotSupportedError`            | The A2A protocol version specified in the request (via `A2A-Version` service parameter) is not supported by the agent.                                            |
+| `TaskGenerationMismatchError`         | The `ifGenerationMatch` value supplied in `SendMessageConfiguration` does not match the task's current `generation`. The client should re-fetch the task and retry if appropriate. |
 
 #### 3.3.3. Asynchronous Processing
 
@@ -656,6 +710,14 @@ The A2A protocol provides three complementary mechanisms for clients to receive
 - Higher latency, potential for unnecessary requests
 - Best for: Simple integrations, infrequent updates, clients behind restrictive firewalls
 
+**Long-Polling (Get Task with `currentGeneration`):**
+
+- Client calls Get Task with a `currentGeneration` value; the server holds the request until the task's generation advances
+- Eliminates unnecessary round-trips while retaining the simplicity of polling
+- Requires server support; falls back to normal polling if the server returns immediately regardless
+- Best for: Clients that cannot use streaming or push notifications but want lower latency than plain polling
+- See [Task Generation Semantics](#327-task-generation-semantics) for details
+
 **Streaming:**
 
 - Real-time delivery of events as they occur
@@ -682,6 +744,8 @@ The A2A protocol provides three complementary mechanisms for clients to receive
 
 All implementations MUST deliver events in the order they were generated. Events MUST NOT be reordered during transmission, regardless of protocol binding.
 
+Each [`TaskStatusUpdateEvent`](#421-taskstatusupdateevent) and [`TaskArtifactUpdateEvent`](#422-taskartifactupdateevent) carries the task's `generation` value after the mutation that produced it. Clients **SHOULD** use this value to verify that no events were missed (a gap in generation values indicates a missed event) and to establish a total ordering of events that is independent of wall-clock time. See [Task Generation Semantics](#327-task-generation-semantics) for details.
+
 **Multiple Streams Per Task:**
 
 An agent MAY serve multiple concurrent streams to one or more clients for the same task. This allows multiple clients (or the same client with multiple connections) to independently subscribe to and receive updates about a task's progress.
@@ -1188,6 +1252,7 @@ All A2A-specific errors defined in [Section 3.3.2](#332-error-handling) **MUST**
 | `ExtendedAgentCardNotConfiguredError` | `-32007`      | `FAILED_PRECONDITION` | `400 Bad Request`           |
 | `ExtensionSupportRequiredError`       | `-32008`      | `FAILED_PRECONDITION` | `400 Bad Request`           |
 | `VersionNotSupportedError`            | `-32009`      | `FAILED_PRECONDITION` | `400 Bad Request`           |
+| `TaskGenerationMismatchError`         | `-32010`      | `ABORTED`             | `409 Conflict`              |
 
 **Custom Binding Requirements:**
 
@@ -2859,12 +2924,13 @@ Query parameter names **MUST** use `camelCase` to match the JSON serialization o
 
 **Example Mappings:**
 
-| Protocol Buffer Field | Query Parameter Name | Example Usage       |
-| --------------------- | -------------------- | ------------------- |
-| `context_id`          | `contextId`          | `?contextId=uuid`   |
-| `page_size`           | `pageSize`           | `?pageSize=50`      |
-| `page_token`          | `pageToken`          | `?pageToken=cursor` |
-| `task_id`             | `taskId`             | `?taskId=uuid`      |
+| Protocol Buffer Field | Query Parameter Name | Example Usage                    |
+| --------------------- | -------------------- | -------------------------------- |
+| `context_id`          | `contextId`          | `?contextId=uuid`                |
+| `page_size`           | `pageSize`           | `?pageSize=50`                   |
+| `page_token`          | `pageToken`          | `?pageToken=cursor`              |
+| `task_id`             | `taskId`             | `?taskId=uuid`                   |
+| `current_generation`  | `currentGeneration`  | `?currentGeneration=5`           |
 
 **Usage Examples:**
 
@@ -2880,6 +2946,12 @@ Get task with history:
 GET /tasks/{id}?historyLength=10
 ```
 
+Get task with long-polling (wait until generation advances beyond 5):
+
+```http
+GET /tasks/{id}?currentGeneration=5
+```
+
 **Field Type Handling:**
 
 - **Strings**: Passed directly as query parameter values
@@ -2944,6 +3016,37 @@ Content-Type: application/a2a+json
 
 Extension fields like `taskId` and `timestamp` provide additional context to help diagnose the error.
 
+**Generation Mismatch Error Example (`ifGenerationMatch`):**
+
+When a `SendMessage` request includes `configuration.ifGenerationMatch` and the task's current generation does not match, the server **MUST** return HTTP `409 Conflict`:
+
+```http
+HTTP/1.1 409 Conflict
+Content-Type: application/a2a+json
+
+{
+  "error": {
+    "code": 409,
+    "status": "ABORTED",
+    "message": "Task generation mismatch: expected 3 but current generation is 5",
+    "details": [
+      {
+        "@type": "type.googleapis.com/google.rpc.ErrorInfo",
+        "reason": "TASK_GENERATION_MISMATCH",
+        "domain": "a2a-protocol.org",
+        "metadata": {
+          "taskId": "task-abc",
+          "expectedGeneration": "3",
+          "currentGeneration": "5"
+        }
+      }
+    ]
+  }
+}
+```
+
+The client **SHOULD** re-fetch the task via [Get Task](#313-get-task) to inspect the current state before deciding whether to retry the send.
+
 ### 11.7. Streaming
 
 <span id="stream-response"></span>

specification/a2a.proto

@@ -158,6 +158,12 @@ message SendMessageConfiguration {
   // terminal (`COMPLETED`, `FAILED`, `CANCELED`, `REJECTED`) or interrupted
   // (`INPUT_REQUIRED`, `AUTH_REQUIRED`) state before returning.
   bool return_immediately = 4;
+  // Optional. If set, the server MUST reject the request with an appropriate
+  // error (HTTP 409 Conflict / gRPC ABORTED) if the task's
+  // current generation does not equal this value at the time the message is
+  // processed. Use this to implement optimistic concurrency control and
+  // prevent lost-update races when multiple clients interact with the same task.
+  optional int64 if_generation_match = 5;
 }
 
 // `Task` is the core unit of action for A2A. It has a current status
@@ -181,6 +187,11 @@ message Task {
   // protolint:enable REPEATED_FIELD_NAMES_PLURALIZED
   // A key/value object to store custom metadata about a task.
   google.protobuf.Struct metadata = 6;
+  // Monotonically increasing counter incremented by the server on every
+  // state-changing mutation to this task (TaskState transition or
+  // Artifact addition/update). Starts at 0 when the task is created.
+  // May be used for event ordering, long-polling, and optimistic concurrency.
+  int64 generation = 7;
 }
 
 // Defines the possible lifecycle states of a `Task`.
@@ -302,6 +313,10 @@ message TaskStatusUpdateEvent {
   TaskStatus status = 3 [(google.api.field_behavior) = REQUIRED];
   // Optional. Metadata associated with the task update.
   google.protobuf.Struct metadata = 4;
+  // The task's generation after this update was applied. Clients can use this
+  // to detect missed events (a gap in generation values) and to correlate
+  // events with a specific task snapshot.
+  int64 generation = 5;
 }
 
 // A task delta where an artifact has been generated.
@@ -319,6 +334,10 @@ message TaskArtifactUpdateEvent {
   bool last_chunk = 5;
   // Optional. Metadata associated with the artifact update.
   google.protobuf.Struct metadata = 6;
+  // The task's generation after this artifact update was applied. Clients can
+  // use this to detect missed events (a gap in generation values) and to
+  // correlate events with a specific task snapshot.
+  int64 generation = 7;
 }
 
 // Defines authentication details, used for push notifications.
@@ -661,6 +680,12 @@ message GetTaskRequest {
   // a request to not include any messages. The server MUST NOT return more
   // messages than the provided value, but MAY apply a lower limit.
   optional int32 history_length = 3;
+  // Optional. If set, the server SHOULD NOT return until the task's generation
+  // is strictly greater than this value, enabling efficient long-polling.
+  // The server MAY return earlier (e.g. on timeout) with the current task state.
+  // If the task's current generation already exceeds this value the server
+  // MUST return immediately.
+  optional int64 current_generation = 4;
 }
 
 // Parameters for listing tasks with optional filtering criteria.