Model failed-turn recovery as continuation turns

types/channels-chat/actions.ts

@@ -5,7 +5,7 @@
  */
 
 import { ActionType } from '../common/actions.js';
-import type { StringOrMarkdown, ErrorInfo, FileEdit, UsageInfo, URI } from '../common/state.js';
+import type { StringOrMarkdown, FileEdit, UsageInfo, URI } from '../common/state.js';
 import type { McpAuthRequirement } from '../channels-session/state.js';
 import type {
   Message,
@@ -20,6 +20,8 @@ import type {
   ToolCallRiskAssessment,
   ToolInput,
   Turn,
+  TurnError,
+  TurnMessage,
 } from './state.js';
 import {
   ToolCallConfirmationReason,
@@ -56,9 +58,12 @@ interface ToolCallActionBase {
 // ─── Chat Actions ───────────────────────────────────────────────────────────
 
 /**
- * A new message has been sent to the agent, and a new turn starts.
+ * A new turn starts from a user message or continuation.
  *
- * A client is only allowed to send {@link MessageKind.User} messages.
+ * A client may start a turn with a {@link MessageKind.User} message, or
+ * continue the latest failed turn with a {@link MessageKind.Continuation}
+ * message. A continuation starts a new turn and leaves the failed turn
+ * unchanged in history.
  *
  * @category Chat Actions
  * @version 1
@@ -70,8 +75,8 @@ export interface ChatTurnStartedAction {
   turnId: string;
   /** ISO 8601 timestamp when this turn started. */
   startedAt: string;
-  /** The new message */
-  message: Message;
+  /** The message that initiates the turn. */
+  message: TurnMessage;
   /** If this turn was auto-started from a queued message, the ID of that message */
   queuedMessageId?: string;
   /**
@@ -488,8 +493,8 @@ export interface ChatErrorAction {
    * data.
    */
   duration: number;
-  /** Error details */
-  error: ErrorInfo;
+  /** Error details and optional continuation eligibility. */
+  error: TurnError;
   /**
    * Additional provider-specific metadata for this action.
    *

types/channels-chat/reducer.ts

@@ -16,6 +16,9 @@ import type {
   PendingMessage,
   ConfirmationOption,
   ToolCallContributor,
+  ContinuationMessage,
+  TurnError,
+  TurnMessage,
 } from './state.js';
 import {
   TurnState,
@@ -25,13 +28,18 @@ import {
   ToolCallContributorKind,
   ResponsePartKind,
   PendingMessageKind,
+  MessageKind,
 } from './state.js';
 import { SessionStatus } from '../channels-session/state.js';
 import type { ChatAction } from '../action-origin.generated.js';
 import { softAssertNever } from '../common/reducer-helpers.js';
 
 // ─── Helpers ─────────────────────────────────────────────────────────────────
 
+function isContinuationMessage(message: TurnMessage): message is ContinuationMessage {
+  return message.origin.kind === MessageKind.Continuation;
+}
+
 /** Extracts the common base fields shared by all tool call lifecycle states. */
 function tcBase(tc: ToolCallState) {
   return {
@@ -170,7 +178,7 @@ function endTurn(
   turnState: TurnState,
   duration: number,
   terminalStatus?: SessionStatus.Error,
-  error?: { errorType: string; message: string; stack?: string },
+  error?: TurnError,
 ): ChatState {
   if (!state.activeTurn || state.activeTurn.id !== turnId) {
     return state;
@@ -343,6 +351,21 @@ export function chatReducer(state: ChatState, action: ChatAction, log?: (msg: st
     // ── Turn Lifecycle ────────────────────────────────────────────────────
 
     case ActionType.ChatTurnStarted: {
+      if (isContinuationMessage(action.message)) {
+        const previousTurn = state.turns[state.turns.length - 1];
+        if (
+          state.activeTurn
+          || !previousTurn
+          || previousTurn.state !== TurnState.Error
+          || previousTurn.error?.continuation !== true
+          || state.turns.some(turn => turn.id === action.turnId)
+          || action.message.text.length > 0
+          || action.message.attachments !== undefined
+          || action.queuedMessageId !== undefined
+        ) {
+          return state;
+        }
+      }
       let next: ChatState = {
         ...state,
         activeTurn: {

types/channels-chat/state.ts

@@ -522,6 +522,20 @@ export const enum TurnState {
   Error = 'error',
 }
 
+/**
+ * Error details for a failed turn.
+ *
+ * When {@link continuation} is present, a client may start a new adjacent turn
+ * with a {@link ContinuationMessage}. The continuation proceeds from the
+ * failed turn without adding user input or changing the failed turn.
+ *
+ * @category Turn Types
+ */
+export interface TurnError extends ErrorInfo {
+  /** Whether the latest failed turn may be continued. */
+  continuation?: true;
+}
+
 /**
  * Discriminant for {@link MessageAttachment} variants.
  *
@@ -553,7 +567,7 @@ export interface Turn {
   /** Turn duration in milliseconds. */
   duration?: number;
   /** The message that initiated the turn */
-  message: Message;
+  message: TurnMessage;
   /**
    * All response content in stream order: text, tool calls, reasoning, and content refs.
    *
@@ -566,7 +580,7 @@ export interface Turn {
   /** How the turn ended */
   state: TurnState;
   /** Error details if state is `'error'` */
-  error?: ErrorInfo;
+  error?: TurnError;
 }
 
 /**
@@ -580,7 +594,7 @@ export interface ActiveTurn {
   /** ISO 8601 timestamp when this turn started. */
   startedAt: string;
   /** The message that initiated the turn */
-  message: Message;
+  message: TurnMessage;
   /**
    * All response content in stream order: text, tool calls, reasoning, and content refs.
    *
@@ -592,7 +606,8 @@ export interface ActiveTurn {
 }
 
 /**
- * Discriminant for {@link MessageOrigin} — identifies who produced a message.
+ * Discriminant for {@link MessageOrigin} — identifies a message's origin or
+ * continuation provenance.
  *
  * @category Turn Types
  */
@@ -611,23 +626,50 @@ export enum MessageKind {
   Tool = 'tool',
   /** A system-generated notification rather than a direct user message. */
   SystemNotification = 'systemNotification',
+  /**
+   * Starts a new turn that continues a preceding failed turn without adding
+   * another user message.
+   */
+  Continuation = 'continuation',
 }
 
 /**
- * Identifies the origin of a {@link Message} — who produced it. For the message
- * that initiates a turn ({@link Turn.message}), this is also the origin of the
- * turn; for steering or queued messages it is just the origin of that message.
+ * Identifies the actor that produced an ordinary {@link Message}.
  *
  * @category Turn Types
  */
-export interface MessageOrigin {
+export interface ActorMessageOrigin {
   /** The kind of actor that produced the message. */
-  kind: MessageKind;
+  kind: MessageKind.User
+    | MessageKind.Agent
+    | MessageKind.Tool
+    | MessageKind.SystemNotification;
+}
+
+/**
+ * Identifies a message that starts a new turn as a continuation of the
+ * immediately preceding failed turn.
+ *
+ * Continuation messages carry no new user input: their text is empty and they
+ * have no attachments. The preceding turn remains unchanged in history.
+ *
+ * @category Turn Types
+ */
+export interface ContinuationMessageOrigin {
+  /** Discriminant */
+  kind: MessageKind.Continuation;
 }
 
 /**
- * A message that initiates or steers a turn. Messages can originate from the
- * user, the agent, a tool, or be system-generated (see {@link MessageOrigin}).
+ * Identifies the origin or provenance of a {@link TurnMessage}.
+ *
+ * @category Turn Types
+ */
+export type MessageOrigin = ActorMessageOrigin | ContinuationMessageOrigin;
+
+/**
+ * An ordinary message that initiates or steers a turn. Messages can originate
+ * from the user, the agent, a tool, or be system-generated.
  *
  * Attachments MAY be referenced inside {@link Message.text} via their
  * {@link MessageAttachmentBase.range} field. Attachments without a range are
@@ -640,7 +682,7 @@ export interface Message {
   /** Message text */
   text: string;
   /** The origin of the message */
-  origin: MessageOrigin;
+  origin: ActorMessageOrigin;
   /** File/selection attachments */
   attachments?: MessageAttachment[];
   /**
@@ -671,6 +713,34 @@ export interface Message {
   _meta?: Record<string, unknown>;
 }
 
+/**
+ * A message that starts a new turn to continue a preceding failed turn without
+ * adding user input.
+ *
+ * @category Turn Types
+ */
+export interface ContinuationMessage {
+  /** Continuations add no message text. */
+  text: '';
+  /** Continuation provenance. */
+  origin: ContinuationMessageOrigin;
+  /** Continuations cannot carry attachments. */
+  attachments?: never;
+  /** Optional model override for the continuation attempt. */
+  model?: ModelSelection;
+  /** Optional custom-agent override for the continuation attempt. */
+  agent?: AgentSelection;
+  /** Additional provider-specific metadata for this continuation. */
+  _meta?: Record<string, unknown>;
+}
+
+/**
+ * A message that starts a turn.
+ *
+ * @category Turn Types
+ */
+export type TurnMessage = Message | ContinuationMessage;
+
 /**
  * Common fields shared by all {@link MessageAttachment} variants.
  *

types/test-cases/reducers/263-chat-continuation-starts-new-turn-after-error.json

@@ -0,0 +1,95 @@
+{
+  "description": "chat/turnStarted starts a new continuation turn after the latest failed turn without rewriting history",
+  "reducer": "chat",
+  "initial": {
+    "turns": [
+      {
+        "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 1000,
+        "message": {
+          "text": "Fix the bug",
+          "origin": {
+            "kind": "user"
+          }
+        },
+        "responseParts": [
+          {
+            "kind": "markdown",
+            "id": "markdown-1",
+            "content": "Partial response"
+          }
+        ],
+        "usage": null,
+        "state": "error",
+        "error": {
+          "errorType": "runtime",
+          "message": "Something broke",
+          "continuation": true
+        }
+      }
+    ],
+    "resource": "copilot:/test-session",
+    "title": "Test Session",
+    "status": 2,
+    "modifiedAt": "1970-01-01T00:00:02.000Z"
+  },
+  "actions": [
+    {
+      "type": "chat/turnStarted",
+      "turnId": "turn-2",
+      "startedAt": "1970-01-01T00:00:03.000Z",
+      "message": {
+        "text": "",
+        "origin": {
+          "kind": "continuation"
+        }
+      }
+    }
+  ],
+  "expected": {
+    "turns": [
+      {
+        "id": "turn-1",
+        "startedAt": "1970-01-01T00:00:01.000Z",
+        "duration": 1000,
+        "message": {
+          "text": "Fix the bug",
+          "origin": {
+            "kind": "user"
+          }
+        },
+        "responseParts": [
+          {
+            "kind": "markdown",
+            "id": "markdown-1",
+            "content": "Partial response"
+          }
+        ],
+        "usage": null,
+        "state": "error",
+        "error": {
+          "errorType": "runtime",
+          "message": "Something broke",
+          "continuation": true
+        }
+      }
+    ],
+    "activeTurn": {
+      "id": "turn-2",
+      "startedAt": "1970-01-01T00:00:03.000Z",
+      "message": {
+        "text": "",
+        "origin": {
+          "kind": "continuation"
+        }
+      },
+      "responseParts": [],
+      "usage": null
+    },
+    "resource": "copilot:/test-session",
+    "title": "Test Session",
+    "status": 8,
+    "modifiedAt": "1970-01-01T00:00:09.999Z"
+  }
+}