feat: add turn cancellation UI and main process wiring

Adds cancel-turn IPC channel, SidecarClient.cancelTurn(), TurnCancelledError,
EryxAppService.cancelSessionTurn(), finalizeCancelledTurn(), stop button in
ChatPane, and cancelled run status throughout the run timeline.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/main/EryxAppService.ts

@@ -65,6 +65,7 @@ import {
 import { prepareChatMessageContent } from '@shared/utils/chatMessage';
 import {
   appendRunActivityEvent,
+  cancelSessionRunRecord,
   completeSessionRunRecord,
   createSessionRunRecord,
   failSessionRunRecord,
@@ -99,6 +100,7 @@ import {
   SIDECAR_STOPPED_BEFORE_COMPLETION_MESSAGE,
   SidecarClient,
 } from '@main/sidecar/sidecarProcess';
+import { TurnCancelledError } from '@main/sidecar/turnCancelledError';
 import { GitService } from '@main/git/gitService';
 import {
   buildRunTurnToolingConfig as buildSessionToolingConfig,
@@ -621,6 +623,12 @@ export class EryxAppService extends EventEmitter<AppServiceEvents> {
       this.finalizeTurn(workspace, session.id, requestId, responseMessages);
       await this.persistAndBroadcast(workspace);
     } catch (error) {
+      if (error instanceof TurnCancelledError) {
+        this.finalizeCancelledTurn(workspace, session, requestId);
+        await this.persistAndBroadcast(workspace);
+        return;
+      }
+
       const failedAt = nowIso();
       session.status = 'error';
       session.lastError = error instanceof Error ? error.message : String(error);
@@ -643,6 +651,21 @@ export class EryxAppService extends EventEmitter<AppServiceEvents> {
     }
   }
 
+  async cancelSessionTurn(sessionId: string): Promise<void> {
+    const workspace = await this.loadWorkspace();
+    const session = this.requireSession(workspace, sessionId);
+    if (session.status !== 'running') {
+      return;
+    }
+
+    const runningRun = session.runs.find((run) => run.status === 'running');
+    if (!runningRun) {
+      return;
+    }
+
+    await this.sidecar.cancelTurn(runningRun.requestId);
+  }
+
   async resolveSessionApproval(
     sessionId: string,
     approvalId: string,
@@ -1101,6 +1124,36 @@ export class EryxAppService extends EventEmitter<AppServiceEvents> {
     }
   }
 
+  private finalizeCancelledTurn(
+    workspace: WorkspaceState,
+    session: SessionRecord,
+    requestId: string,
+  ): void {
+    for (const message of session.messages) {
+      if (message.pending) {
+        message.pending = false;
+      }
+    }
+
+    this.rejectPendingApprovals(session, nowIso(), 'The turn was cancelled.');
+
+    const cancelledAt = nowIso();
+    session.status = 'idle';
+    session.lastError = undefined;
+    session.updatedAt = cancelledAt;
+    const cancelledRun = this.updateSessionRun(session, requestId, (run) =>
+      cancelSessionRunRecord(run, cancelledAt));
+    this.emitSessionEvent({
+      sessionId: session.id,
+      kind: 'status',
+      occurredAt: cancelledAt,
+      status: 'idle',
+    });
+    if (cancelledRun) {
+      this.emitRunUpdated(session.id, cancelledAt, cancelledRun);
+    }
+  }
+
   private async handleApprovalRequested(
     workspace: WorkspaceState,
     sessionId: string,

src/main/ipc/registerIpcHandlers.ts

@@ -2,6 +2,7 @@ import { BrowserWindow, ipcMain } from 'electron';
 
 import { ipcChannels } from '@shared/contracts/channels';
 import type {
+  CancelSessionTurnInput,
   CreateSessionInput,
   ResolveProjectDiscoveredToolingInput,
   ResolveWorkspaceDiscoveredToolingInput,
@@ -100,6 +101,9 @@ export function registerIpcHandlers(window: BrowserWindow, service: EryxAppServi
   ipcMain.handle(ipcChannels.sendSessionMessage, (_event, input: SendSessionMessageInput) =>
     service.sendSessionMessage(input.sessionId, input.content),
   );
+  ipcMain.handle(ipcChannels.cancelSessionTurn, (_event, input: CancelSessionTurnInput) =>
+    service.cancelSessionTurn(input.sessionId),
+  );
   ipcMain.handle(ipcChannels.resolveSessionApproval, (_event, input: ResolveSessionApprovalInput) =>
     service.resolveSessionApproval(input.sessionId, input.approvalId, input.decision),
   );

src/main/sidecar/sidecarProcess.ts

@@ -4,6 +4,7 @@ import { spawn, type ChildProcessWithoutNullStreams } from 'node:child_process';
 import type {
   AgentActivityEvent,
   ApprovalRequestedEvent,
+  CancelTurnCommand,
   SidecarCommand,
   SidecarCapabilities,
   SidecarEvent,
@@ -19,29 +20,54 @@ import {
   shouldHandleRunTurnEvent,
   type RunTurnPendingCommand,
 } from '@main/sidecar/runTurnPending';
+import { TurnCancelledError } from '@main/sidecar/turnCancelledError';
 import { resolveSidecarProcess } from '@main/sidecar/sidecarRuntime';
 
 type PendingCommand =
-  | {
+  | ({
+      processId: number;
       kind: 'capabilities';
       resolve: (capabilities: SidecarCapabilities) => void;
       reject: (error: Error) => void;
-    }
-  | {
+    })
+  | ({
+      processId: number;
       kind: 'validate-pattern';
       resolve: (issues: ValidatePatternCommand['pattern'] extends never ? never : unknown) => void;
       reject: (error: Error) => void;
-    }
-  | {
+    })
+  | ({
+      processId: number;
       kind: 'resolve-approval';
       resolve: () => void;
       reject: (error: Error) => void;
-    }
-  | RunTurnPendingCommand;
+    })
+  | ({
+      processId: number;
+      kind: 'cancel-turn';
+      resolve: () => void;
+      reject: (error: Error) => void;
+    })
+  | ({
+      processId: number;
+    } & RunTurnPendingCommand);
+
+type ManagedSidecarProcess = {
+  id: number;
+  child: ChildProcessWithoutNullStreams;
+  stdoutBuffer: string;
+  exitExpected: boolean;
+  terminated: boolean;
+  closed: Promise<void>;
+  resolveClosed: () => void;
+};
+
+export const SIDECAR_STOPPED_BEFORE_COMPLETION_MESSAGE =
+  'The .NET sidecar was stopped before the command completed.';
 
 export class SidecarClient {
-  private process?: ChildProcessWithoutNullStreams;
-  private stdoutBuffer = '';
+  private processState?: ManagedSidecarProcess;
+  private nextProcessId = 0;
   private readonly pending = new Map<string, PendingCommand>();
 
   async describeCapabilities(): Promise<SidecarCapabilities> {
@@ -79,18 +105,39 @@ export class SidecarClient {
     });
   }
 
+  async cancelTurn(targetRequestId: string): Promise<void> {
+    return this.dispatch<void>({
+      type: 'cancel-turn',
+      requestId: `cancel-${Date.now()}`,
+      targetRequestId,
+    } satisfies CancelTurnCommand);
+  }
+
   async dispose(): Promise<void> {
-    if (!this.process) {
+    const state = this.processState;
+    if (!state) {
       return;
     }
 
-    this.process.kill();
-    this.process = undefined;
+    state.exitExpected = true;
+    if (!state.child.killed && state.child.exitCode === null) {
+      state.child.kill();
+    }
+    await state.closed;
   }
 
-  private async ensureProcess(): Promise<ChildProcessWithoutNullStreams> {
-    if (this.process && !this.process.killed) {
-      return this.process;
+  private async ensureProcess(): Promise<ManagedSidecarProcess> {
+    if (
+      this.processState &&
+      !this.processState.exitExpected &&
+      !this.processState.terminated &&
+      this.processState.child.exitCode === null
+    ) {
+      return this.processState;
+    }
+
+    if (this.processState) {
+      await this.processState.closed;
     }
 
     const sidecar = resolveSidecarProcess({
@@ -105,12 +152,29 @@ export class SidecarClient {
       stdio: 'pipe',
       windowsHide: true,
     });
-    this.process = childProcess;
+    let resolveClosed!: () => void;
+    const state: ManagedSidecarProcess = {
+      id: this.nextProcessId + 1,
+      child: childProcess,
+      stdoutBuffer: '',
+      exitExpected: false,
+      terminated: false,
+      closed: new Promise<void>((resolve) => {
+        resolveClosed = resolve;
+      }),
+      resolveClosed,
+    };
+    this.nextProcessId = state.id;
+    this.processState = state;
 
     childProcess.stdout.setEncoding('utf8');
     childProcess.stdout.on('data', (chunk: string) => {
-      this.stdoutBuffer += chunk;
-      this.flushStdoutBuffer();
+      if (state.terminated) {
+        return;
+      }
+
+      state.stdoutBuffer += chunk;
+      this.flushStdoutBuffer(state);
     });
 
     childProcess.stderr.setEncoding('utf8');
@@ -118,17 +182,14 @@ export class SidecarClient {
       console.error('[aryx sidecar]', chunk.trim());
     });
 
-    childProcess.on('exit', (code) => {
-      const error = new Error(`The .NET sidecar exited unexpectedly with code ${code ?? 'unknown'}.`);
-      for (const pending of this.pending.values()) {
-        pending.reject(error);
-      }
-      this.pending.clear();
-      this.process = undefined;
-      this.stdoutBuffer = '';
+    childProcess.on('close', (code) => {
+      const error = state.exitExpected
+        ? new Error(SIDECAR_STOPPED_BEFORE_COMPLETION_MESSAGE)
+        : new Error(`The .NET sidecar exited unexpectedly with code ${code ?? 'unknown'}.`);
+      this.handleProcessClosed(state, error);
     });
 
-    return childProcess;
+    return state;
   }
 
   private async dispatch<TResult>(
@@ -137,11 +198,12 @@ export class SidecarClient {
     onActivity?: (event: AgentActivityEvent) => void | Promise<void>,
     onApproval?: (event: ApprovalRequestedEvent) => void | Promise<void>,
   ): Promise<TResult> {
-    const process = await this.ensureProcess();
+    const state = await this.ensureProcess();
 
     return new Promise<TResult>((resolve, reject) => {
       if (command.type === 'run-turn') {
         this.pending.set(command.requestId, {
+          processId: state.id,
           kind: 'run-turn',
           resolve: resolve as (messages: ChatMessageRecord[]) => void,
           reject,
@@ -152,46 +214,56 @@ export class SidecarClient {
         });
       } else if (command.type === 'validate-pattern') {
         this.pending.set(command.requestId, {
+          processId: state.id,
           kind: 'validate-pattern',
           resolve: resolve as (issues: unknown) => void,
           reject,
         });
       } else if (command.type === 'resolve-approval') {
         this.pending.set(command.requestId, {
+          processId: state.id,
           kind: 'resolve-approval',
           resolve: resolve as () => void,
           reject,
         });
+      } else if (command.type === 'cancel-turn') {
+        this.pending.set(command.requestId, {
+          processId: state.id,
+          kind: 'cancel-turn',
+          resolve: resolve as () => void,
+          reject,
+        });
       } else {
         this.pending.set(command.requestId, {
+          processId: state.id,
           kind: 'capabilities',
           resolve: resolve as (capabilities: SidecarCapabilities) => void,
           reject,
         });
       }
 
-      process.stdin.write(`${JSON.stringify(command)}\n`);
+      state.child.stdin.write(`${JSON.stringify(command)}\n`);
     });
   }
 
-  private flushStdoutBuffer(): void {
-    let newlineIndex = this.stdoutBuffer.indexOf('\n');
+  private flushStdoutBuffer(state: ManagedSidecarProcess): void {
+    let newlineIndex = state.stdoutBuffer.indexOf('\n');
 
     while (newlineIndex >= 0) {
-      const rawLine = this.stdoutBuffer.slice(0, newlineIndex).trim();
-      this.stdoutBuffer = this.stdoutBuffer.slice(newlineIndex + 1);
+      const rawLine = state.stdoutBuffer.slice(0, newlineIndex).trim();
+      state.stdoutBuffer = state.stdoutBuffer.slice(newlineIndex + 1);
 
       if (rawLine) {
-        this.handleEvent(JSON.parse(rawLine) as SidecarEvent);
+        this.handleEvent(state.id, JSON.parse(rawLine) as SidecarEvent);
       }
 
-      newlineIndex = this.stdoutBuffer.indexOf('\n');
+      newlineIndex = state.stdoutBuffer.indexOf('\n');
     }
   }
 
-  private handleEvent(event: SidecarEvent): void {
+  private handleEvent(processId: number, event: SidecarEvent): void {
     const pending = this.pending.get(event.requestId);
-    if (!pending) {
+    if (!pending || pending.processId !== processId) {
       return;
     }
 
@@ -226,7 +298,11 @@ export class SidecarClient {
       case 'turn-complete':
         if (pending.kind === 'run-turn') {
           if (shouldHandleRunTurnEvent(pending)) {
-            pending.resolve(event.messages);
+            if (event.cancelled) {
+              markRunTurnPendingErrored(pending, new TurnCancelledError());
+            } else {
+              pending.resolve(event.messages);
+            }
           }
           this.pending.delete(event.requestId);
         }
@@ -240,7 +316,7 @@ export class SidecarClient {
         this.pending.delete(event.requestId);
         return;
       case 'command-complete':
-        if (pending.kind === 'resolve-approval') {
+        if (pending.kind === 'resolve-approval' || pending.kind === 'cancel-turn') {
           pending.resolve();
           this.pending.delete(event.requestId);
         } else if (pending.kind !== 'run-turn' || pending.errored) {
@@ -262,4 +338,27 @@ export class SidecarClient {
       }
     });
   }
+
+  private handleProcessClosed(state: ManagedSidecarProcess, error: Error): void {
+    if (state.terminated) {
+      return;
+    }
+
+    state.terminated = true;
+    if (this.processState === state) {
+      this.processState = undefined;
+    }
+
+    state.stdoutBuffer = '';
+    state.resolveClosed();
+
+    for (const [requestId, pending] of this.pending.entries()) {
+      if (pending.processId !== state.id) {
+        continue;
+      }
+
+      pending.reject(error);
+      this.pending.delete(requestId);
+    }
+  }
 }

src/main/sidecar/turnCancelledError.ts

@@ -0,0 +1,6 @@
+export class TurnCancelledError extends Error {
+  constructor() {
+    super('The turn was cancelled.');
+    this.name = 'TurnCancelledError';
+  }
+}

src/preload/index.ts

@@ -32,6 +32,7 @@ const api: ElectronApi = {
   setSessionPinned: (input) => ipcRenderer.invoke(ipcChannels.setSessionPinned, input),
   setSessionArchived: (input) => ipcRenderer.invoke(ipcChannels.setSessionArchived, input),
   sendSessionMessage: (input) => ipcRenderer.invoke(ipcChannels.sendSessionMessage, input),
+  cancelSessionTurn: (input) => ipcRenderer.invoke(ipcChannels.cancelSessionTurn, input),
   resolveSessionApproval: (input) => ipcRenderer.invoke(ipcChannels.resolveSessionApproval, input),
   updateScratchpadSessionConfig: (input) =>
     ipcRenderer.invoke(ipcChannels.updateScratchpadSessionConfig, input),

src/renderer/App.tsx

@@ -246,6 +246,7 @@ export default function App() {
     content = (
         <ChatPane
           onSend={(c) => api.sendSessionMessage({ sessionId: selectedSession.id, content: c })}
+          onCancelTurn={() => { void api.cancelSessionTurn({ sessionId: selectedSession.id }); }}
           onResolveApproval={(approvalId, decision) =>
             api.resolveSessionApproval({ sessionId: selectedSession.id, approvalId, decision })
           }

src/renderer/components/ChatPane.tsx

@@ -1,5 +1,5 @@
 import { useEffect, useMemo, useRef, useState } from 'react';
-import { AlertCircle, ArrowUp, Bot, Circle, GitBranch, Loader2, ShieldAlert, User } from 'lucide-react';
+import { AlertCircle, ArrowUp, Bot, Circle, GitBranch, Loader2, ShieldAlert, Square, User } from 'lucide-react';
 
 import { MarkdownContent } from '@renderer/components/MarkdownContent';
 import { MarkdownComposer, type MarkdownComposerHandle } from '@renderer/components/MarkdownComposer';
@@ -34,6 +34,7 @@ interface ChatPaneProps {
   toolingSettings: WorkspaceToolingSettings;
   runtimeTools?: ReadonlyArray<RuntimeToolDefinition>;
   onSend: (content: string) => Promise<void>;
+  onCancelTurn?: () => void;
   onResolveApproval?: (approvalId: string, decision: ApprovalDecision) => Promise<unknown>;
   onUpdateScratchpadConfig?: (config: {
     model: string;
@@ -51,6 +52,7 @@ export function ChatPane({
   toolingSettings,
   runtimeTools,
   onSend,
+  onCancelTurn,
   onResolveApproval,
   onUpdateScratchpadConfig,
   onUpdateSessionTooling,
@@ -432,16 +434,25 @@ export function ChatPane({
             >
               <button
                 className={`absolute bottom-2 right-2 flex size-8 items-center justify-center rounded-lg transition ${
-                  canSubmitInput
-                    ? 'bg-indigo-600 text-white hover:bg-indigo-500'
-                    : 'bg-zinc-800 text-zinc-600'
+                  isSessionBusy
+                    ? 'bg-red-600/80 text-white hover:bg-red-500'
+                    : canSubmitInput
+                      ? 'bg-indigo-600 text-white hover:bg-indigo-500'
+                      : 'bg-zinc-800 text-zinc-600'
                 }`}
-                disabled={!canSubmitInput}
-                onClick={() => composerRef.current?.submit()}
+                disabled={!canSubmitInput && !isSessionBusy}
+                onClick={() => {
+                  if (isSessionBusy) {
+                    onCancelTurn?.();
+                  } else {
+                    composerRef.current?.submit();
+                  }
+                }}
                 type="button"
+                aria-label={isSessionBusy ? 'Stop generating' : 'Send message'}
               >
                 {isSessionBusy ? (
-                  <Loader2 className="size-4 animate-spin" />
+                  <Square className="size-3.5" fill="currentColor" />
                 ) : (
                   <ArrowUp className="size-4" />
                 )}

src/renderer/components/RunTimeline.tsx

@@ -43,6 +43,7 @@ const modeAccent: Record<OrchestrationMode, { dot: string; ring: string; text: s
 const runStatusStyles: Record<SessionRunRecord['status'], { icon: ReactNode; className: string }> = {
   running:   { icon: <CircleDot className="size-3" />, className: 'text-blue-400' },
   completed: { icon: <CheckCircle2 className="size-3" />, className: 'text-emerald-400' },
+  cancelled: { icon: <XCircle className="size-3" />, className: 'text-zinc-400' },
   error:     { icon: <XCircle className="size-3" />, className: 'text-red-400' },
 };
 
@@ -65,6 +66,8 @@ function EventIcon({ kind, status }: { kind: RunTimelineEventRecord['kind']; sta
       return <MessageSquare className={`${base} ${status === 'running' ? 'text-blue-400 animate-pulse' : status === 'error' ? 'text-red-400' : 'text-emerald-400'}`} />;
     case 'run-completed':
       return <CheckCircle2 className={`${base} text-emerald-400`} />;
+    case 'run-cancelled':
+      return <XCircle className={`${base} text-zinc-400`} />;
     case 'run-failed':
       return <AlertTriangle className={`${base} text-red-400`} />;
   }

src/renderer/lib/runTimelineFormatting.ts

@@ -59,6 +59,8 @@ export function formatRunStatusLabel(status: SessionRunStatus): string {
       return 'Running';
     case 'completed':
       return 'Completed';
+    case 'cancelled':
+      return 'Cancelled';
     case 'error':
       return 'Failed';
   }
@@ -91,6 +93,8 @@ export function formatEventLabel(event: RunTimelineEventRecord): string {
       return event.agentName ?? 'Response';
     case 'run-completed':
       return 'Completed';
+    case 'run-cancelled':
+      return 'Cancelled';
     case 'run-failed':
       return 'Failed';
   }
@@ -144,11 +148,12 @@ const eventKindOrder: Record<RunTimelineEventKind, number> = {
   'approval': 4,
   'message': 5,
   'run-completed': 6,
+  'run-cancelled': 6,
   'run-failed': 6,
 };
 
 export function isTerminalEvent(kind: RunTimelineEventKind): boolean {
-  return kind === 'run-completed' || kind === 'run-failed' || kind === 'run-started';
+  return kind === 'run-completed' || kind === 'run-cancelled' || kind === 'run-failed' || kind === 'run-started';
 }
 
 export function eventSortKey(event: RunTimelineEventRecord): number {

src/shared/contracts/channels.ts

@@ -24,6 +24,7 @@ export const ipcChannels = {
   setSessionPinned: 'sessions:set-pinned',
   setSessionArchived: 'sessions:set-archived',
   sendSessionMessage: 'sessions:send-message',
+  cancelSessionTurn: 'sessions:cancel-turn',
   resolveSessionApproval: 'sessions:resolve-approval',
   querySessions: 'sessions:query',
   updateScratchpadSessionConfig: 'sessions:update-scratchpad-config',

src/shared/contracts/ipc.ts

@@ -26,6 +26,10 @@ export interface SendSessionMessageInput {
   content: string;
 }
 
+export interface CancelSessionTurnInput {
+  sessionId: string;
+}
+
 export interface ResolveSessionApprovalInput {
   sessionId: string;
   approvalId: string;
@@ -120,6 +124,7 @@ export interface ElectronApi {
   setSessionPinned(input: SetSessionPinnedInput): Promise<WorkspaceState>;
   setSessionArchived(input: SetSessionArchivedInput): Promise<WorkspaceState>;
   sendSessionMessage(input: SendSessionMessageInput): Promise<void>;
+  cancelSessionTurn(input: CancelSessionTurnInput): Promise<void>;
   resolveSessionApproval(input: ResolveSessionApprovalInput): Promise<WorkspaceState>;
   updateScratchpadSessionConfig(input: UpdateScratchpadSessionConfigInput): Promise<WorkspaceState>;
   querySessions(input: QuerySessionsInput): Promise<SessionQueryResult[]>;

src/shared/contracts/sidecar.ts

@@ -79,6 +79,12 @@ export interface RunTurnCommand {
   tooling?: RunTurnToolingConfig;
 }
 
+export interface CancelTurnCommand {
+  type: 'cancel-turn';
+  requestId: string;
+  targetRequestId: string;
+}
+
 export interface ResolveApprovalCommand {
   type: 'resolve-approval';
   requestId: string;
@@ -90,6 +96,7 @@ export type SidecarCommand =
   | DescribeCapabilitiesCommand
   | ValidatePatternCommand
   | RunTurnCommand
+  | CancelTurnCommand
   | ResolveApprovalCommand;
 
 export interface RunTurnLocalMcpServerConfig {
@@ -157,6 +164,7 @@ export interface TurnCompleteEvent {
   requestId: string;
   sessionId: string;
   messages: ChatMessageRecord[];
+  cancelled?: boolean;
 }
 
 export type AgentActivityType = 'thinking' | 'tool-calling' | 'handoff' | 'completed';

src/shared/domain/runTimeline.ts

@@ -7,7 +7,7 @@ import type { PatternDefinition, ReasoningEffort } from '@shared/domain/pattern'
 import type { ProjectRecord } from '@shared/domain/project';
 import { createId } from '@shared/utils/ids';
 
-export type SessionRunStatus = 'running' | 'completed' | 'error';
+export type SessionRunStatus = 'running' | 'completed' | 'cancelled' | 'error';
 export type SessionRunWorkspaceKind = 'project' | 'scratchpad';
 export type RunTimelineEventKind =
   | 'run-started'
@@ -17,6 +17,7 @@ export type RunTimelineEventKind =
   | 'approval'
   | 'message'
   | 'run-completed'
+  | 'run-cancelled'
   | 'run-failed';
 export type RunTimelineEventStatus = 'running' | 'completed' | 'error';
 
@@ -318,7 +319,7 @@ export function normalizeSessionRunRecords(
         triggerMessageId,
         startedAt,
         completedAt: normalizeOptionalString(run.completedAt),
-        status: run.status === 'error' ? 'error' : run.status === 'running' ? 'running' : 'completed',
+        status: run.status === 'error' ? 'error' : run.status === 'running' ? 'running' : run.status === 'cancelled' ? 'cancelled' : 'completed',
         agents: run.agents.flatMap((agent) => {
           const normalized = normalizeRunTimelineAgent(agent);
           return normalized ? [normalized] : [];
@@ -516,6 +517,24 @@ export function completeSessionRunRecord(
   });
 }
 
+export function cancelSessionRunRecord(
+  run: SessionRunRecord,
+  cancelledAt: string,
+): SessionRunRecord {
+  const settledRun = settleOpenMessageEvents(run, 'completed', cancelledAt);
+  const cancelledRun: SessionRunRecord = {
+    ...settledRun,
+    status: 'cancelled',
+    completedAt: cancelledAt,
+  };
+
+  return appendRunTimelineEvent(cancelledRun, {
+    kind: 'run-cancelled',
+    occurredAt: cancelledAt,
+    status: 'completed',
+  });
+}
+
 export function failSessionRunRecord(
   run: SessionRunRecord,
   failedAt: string,