Fix reasoning text rendering when restoring agent host sessions

src/vs/platform/agentHost/common/agentService.ts

@@ -340,6 +340,17 @@ export interface IAgentReasoningEvent extends IAgentProgressEventBase {
 	readonly content: string;
 }
 
+/**
+ * The set of events returned by {@link IAgent.getSessionMessages} when
+ * reconstructing a session's history. Reasoning is carried inline on
+ * {@link IAgentMessageEvent.reasoningText} rather than as a separate event.
+ */
+export type SessionHistoryEvent =
+	| IAgentMessageEvent
+	| IAgentToolStartEvent
+	| IAgentToolCompleteEvent
+	| IAgentSubagentStartedEvent;
+
 /** A steering message was consumed (sent to the model). */
 export interface IAgentSteeringConsumedEvent extends IAgentProgressEventBase {
 	readonly type: 'steering_consumed';
@@ -452,7 +463,7 @@ export interface IAgent {
 	setPendingMessages?(session: URI, steeringMessage: PendingMessage | undefined, queuedMessages: readonly PendingMessage[]): void;
 
 	/** Retrieve all session events/messages for reconstruction. */
-	getSessionMessages(session: URI): Promise<(IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent)[]>;
+	getSessionMessages(session: URI): Promise<SessionHistoryEvent[]>;
 
 	/** Dispose a session, freeing resources. */
 	disposeSession(session: URI): Promise<void>;

src/vs/platform/agentHost/node/agentService.ts

@@ -14,7 +14,7 @@ import { FileSystemProviderErrorCode, IFileService, toFileSystemProviderErrorCod
 import { InstantiationService } from '../../instantiation/common/instantiationService.js';
 import { ServiceCollection } from '../../instantiation/common/serviceCollection.js';
 import { ILogService } from '../../log/common/log.js';
-import { AgentProvider, AgentSession, IAgent, IAgentCreateSessionConfig, IAgentMessageEvent, IAgentResolveSessionConfigParams, IAgentService, IAgentSessionConfigCompletionsParams, IAgentSessionMetadata, IAgentSubagentStartedEvent, IAgentToolCompleteEvent, IAgentToolStartEvent, AuthenticateParams, AuthenticateResult } from '../common/agentService.js';
+import { AgentProvider, AgentSession, IAgent, IAgentCreateSessionConfig, IAgentResolveSessionConfigParams, IAgentService, IAgentSessionConfigCompletionsParams, IAgentSessionMetadata, IAgentSubagentStartedEvent, IAgentToolStartEvent, SessionHistoryEvent, AuthenticateParams, AuthenticateResult } from '../common/agentService.js';
 import { ISessionDataService } from '../common/sessionDataService.js';
 import { ActionType, ActionEnvelope, INotification, RootAction, SessionAction, TerminalAction, isSessionAction } from '../common/state/sessionActions.js';
 import type { CreateTerminalParams, ResolveSessionConfigResult, SessionConfigCompletionsResult } from '../common/state/protocol/commands.js';
@@ -690,7 +690,7 @@ export class AgentService extends Disposable implements IAgentService {
 	 * closes it.
 	 */
 	private _buildTurnsFromMessages(
-		messages: readonly (IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent)[],
+		messages: readonly SessionHistoryEvent[],
 	): Turn[] {
 		const turns: Turn[] = [];
 		// Track subagent metadata by parent tool call ID so we can inject
@@ -737,6 +737,16 @@ export class AgentService extends Disposable implements IAgentService {
 					currentTurn = startTurn(msg.messageId, '');
 				}
 
+				// Reasoning is bundled onto the assistant message and
+				// logically precedes its content/tool calls.
+				if (msg.reasoningText) {
+					currentTurn.responseParts.push({
+						kind: ResponsePartKind.Reasoning,
+						id: generateUuid(),
+						content: msg.reasoningText,
+					});
+				}
+
 				if (msg.content) {
 					currentTurn.responseParts.push({
 						kind: ResponsePartKind.Markdown,
@@ -821,7 +831,7 @@ export class AgentService extends Disposable implements IAgentService {
 	 * tool calls.
 	 */
 	private _buildSubagentTurns(
-		parentMessages: readonly (IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent)[],
+		parentMessages: readonly SessionHistoryEvent[],
 		parentToolCallId: string,
 		childSessionUri: string,
 	): Turn[] {
@@ -903,12 +913,21 @@ export class AgentService extends Disposable implements IAgentService {
 					kind: ResponsePartKind.ToolCall,
 					toolCall: tc,
 				});
-			} else if (msg.type === 'message' && msg.role === 'assistant' && msg.content) {
-				responseParts.push({
-					kind: ResponsePartKind.Markdown,
-					id: generateUuid(),
-					content: msg.content,
-				});
+			} else if (msg.type === 'message' && msg.role === 'assistant') {
+				if (msg.reasoningText) {
+					responseParts.push({
+						kind: ResponsePartKind.Reasoning,
+						id: generateUuid(),
+						content: msg.reasoningText,
+					});
+				}
+				if (msg.content) {
+					responseParts.push({
+						kind: ResponsePartKind.Markdown,
+						id: generateUuid(),
+						content: msg.content,
+					});
+				}
 			}
 		}
 

src/vs/platform/agentHost/node/copilot/copilotAgent.ts

@@ -23,7 +23,7 @@ import { IFileService } from '../../../files/common/files.js';
 import { IInstantiationService } from '../../../instantiation/common/instantiation.js';
 import { ILogService } from '../../../log/common/log.js';
 import { IAgentPluginManager, ISyncedCustomization } from '../../common/agentPluginManager.js';
-import { AgentSession, IAgent, IAgentAttachment, IAgentCreateSessionConfig, IAgentCreateSessionResult, IAgentDescriptor, IAgentDeltaEvent, IAgentMessageEvent, IAgentModelInfo, IAgentProgressEvent, IAgentResolveSessionConfigParams, IAgentSessionConfigCompletionsParams, IAgentSessionMetadata, IAgentSessionProjectInfo, IAgentSubagentStartedEvent, IAgentToolCompleteEvent, IAgentToolStartEvent } from '../../common/agentService.js';
+import { AgentSession, IAgent, IAgentAttachment, IAgentCreateSessionConfig, IAgentCreateSessionResult, IAgentDescriptor, IAgentDeltaEvent, IAgentMessageEvent, IAgentModelInfo, IAgentProgressEvent, IAgentResolveSessionConfigParams, IAgentSessionConfigCompletionsParams, IAgentSessionMetadata, IAgentSessionProjectInfo, SessionHistoryEvent } from '../../common/agentService.js';
 import { AutoApproveLevel, ISchemaProperty, createSchema, platformSessionSchema, schemaProperty } from '../../common/agentHostSchema.js';
 import { SessionConfigKey } from '../../common/sessionConfigKeys.js';
 import { ISessionDataService, SESSION_DB_FILENAME } from '../../common/sessionDataService.js';
@@ -121,7 +121,7 @@ function buildWorktreeAnnouncementText(branchName: string): string {
 	) + '\n\n';
 }
 
-type AgentMessageOrEvent = IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent;
+type AgentMessageOrEvent = SessionHistoryEvent;
 
 /**
  * Returns a copy of `messages` where `announcement` has been prepended to
@@ -727,7 +727,7 @@ export class CopilotAgent extends Disposable implements IAgent {
 		// No SDK-level enqueue is needed.
 	}
 
-	async getSessionMessages(session: URI): Promise<(IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent)[]> {
+	async getSessionMessages(session: URI): Promise<SessionHistoryEvent[]> {
 		const sessionId = AgentSession.id(session);
 		const entry = this._sessions.get(sessionId) ?? await this._resumeSession(sessionId).catch(err => {
 			this._logService.warn(`[Copilot:${sessionId}] Failed to resume session for message lookup`, err);

src/vs/platform/agentHost/node/copilot/copilotAgentSession.ts

@@ -17,7 +17,7 @@ import { INativeEnvironmentService } from '../../../environment/common/environme
 import { IFileService } from '../../../files/common/files.js';
 import { IInstantiationService } from '../../../instantiation/common/instantiation.js';
 import { ILogService } from '../../../log/common/log.js';
-import { IAgentAttachment, IAgentMessageEvent, IAgentProgressEvent, IAgentSubagentStartedEvent, IAgentToolCompleteEvent, IAgentToolStartEvent } from '../../common/agentService.js';
+import { IAgentAttachment, IAgentProgressEvent, SessionHistoryEvent } from '../../common/agentService.js';
 import { stripRedundantCdPrefix } from '../../common/commandLineHelpers.js';
 import { ISessionDatabase, ISessionDataService } from '../../common/sessionDataService.js';
 import type { FileEdit, ToolDefinition } from '../../common/state/protocol/state.js';
@@ -330,7 +330,7 @@ export class CopilotAgentSession extends Disposable {
 		}
 	}
 
-	async getMessages(): Promise<(IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent)[]> {
+	async getMessages(): Promise<SessionHistoryEvent[]> {
 		const events = await this._wrapper.session.getMessages();
 		let db: ISessionDatabase | undefined;
 		try {

src/vs/platform/agentHost/node/copilot/mapSessionEvents.ts

@@ -4,7 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 
 import { URI } from '../../../../base/common/uri.js';
-import { IAgentMessageEvent, IAgentSubagentStartedEvent, IAgentToolCompleteEvent, IAgentToolStartEvent } from '../../common/agentService.js';
+import { SessionHistoryEvent } from '../../common/agentService.js';
 import { stripRedundantCdPrefix } from '../../common/commandLineHelpers.js';
 import { IFileEditRecord, ISessionDatabase } from '../../common/sessionDataService.js';
 import { ToolResultContentType, type ToolResultContent } from '../../common/state/sessionState.js';
@@ -90,8 +90,8 @@ export async function mapSessionEvents(
 	db: ISessionDatabase | undefined,
 	events: readonly ISessionEvent[],
 	workingDirectory?: URI,
-): Promise<(IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent)[]> {
-	const result: (IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent)[] = [];
+): Promise<SessionHistoryEvent[]> {
+	const result: SessionHistoryEvent[] = [];
 	const toolInfoByCallId = new Map<string, { toolName: string; parameters: Record<string, unknown> | undefined; rewrittenArgs?: string }>();
 
 	// Collect all tool call IDs for edit tools so we can batch-query the database

src/vs/platform/agentHost/test/node/agentService.test.ts

@@ -375,6 +375,40 @@ suite('AgentService (node dispatcher)', () => {
 			assert.strictEqual(tc.confirmed, ToolCallConfirmationReason.NotNeeded);
 		});
 
+		test('interleaves reasoning, markdown, and tool calls in stream order on resume', async () => {
+			service.registerProvider(copilotAgent);
+			const { session } = await copilotAgent.createSession();
+			const sessions = await copilotAgent.listSessions();
+			const sessionResource = sessions[0].session;
+
+			copilotAgent.sessionMessages = [
+				{ type: 'message', session, role: 'user', messageId: 'u-1', content: 'Hello', toolRequests: [] },
+				{ type: 'message', session, role: 'assistant', messageId: 'a-1', content: 'Reply A', reasoningText: 'Thinking A', toolRequests: [{ toolCallId: 'tc-1', name: 'shell' }] },
+				{ type: 'tool_start', session, toolCallId: 'tc-1', toolName: 'shell', displayName: 'Shell', invocationMessage: 'Running...' },
+				{ type: 'tool_complete', session, toolCallId: 'tc-1', result: { success: true, pastTenseMessage: 'Ran', content: [{ type: ToolResultContentType.Text, text: 'ok' }] } },
+				{ type: 'message', session, role: 'assistant', messageId: 'a-2', content: 'Reply B', reasoningText: 'Thinking B', toolRequests: [] },
+			];
+
+			await service.restoreSession(sessionResource);
+
+			const state = service.stateManager.getSessionState(sessionResource.toString());
+			assert.ok(state);
+			const turn = state!.turns[0];
+			const summary = turn.responseParts.map(p => {
+				if (p.kind === ResponsePartKind.Reasoning) { return ['reasoning', p.content]; }
+				if (p.kind === ResponsePartKind.Markdown) { return ['markdown', p.content]; }
+				if (p.kind === ResponsePartKind.ToolCall) { return ['toolCall', p.toolCall.toolCallId]; }
+				return ['other'];
+			});
+			assert.deepStrictEqual(summary, [
+				['reasoning', 'Thinking A'],
+				['markdown', 'Reply A'],
+				['toolCall', 'tc-1'],
+				['reasoning', 'Thinking B'],
+				['markdown', 'Reply B'],
+			]);
+		});
+
 		test('flushes interrupted turns', async () => {
 			service.registerProvider(copilotAgent);
 			const { session } = await copilotAgent.createSession();

src/vs/platform/agentHost/test/node/mockAgent.ts

@@ -9,7 +9,7 @@ import { observableValue } from '../../../../base/common/observable.js';
 import type { IAuthorizationProtectedResourceMetadata } from '../../../../base/common/oauth.js';
 import { URI } from '../../../../base/common/uri.js';
 import { type ISyncedCustomization } from '../../common/agentPluginManager.js';
-import { AgentSession, type AgentProvider, type IAgent, type IAgentAttachment, type IAgentCreateSessionConfig, type IAgentCreateSessionResult, type IAgentDescriptor, type IAgentMessageEvent, type IAgentModelInfo, type IAgentProgressEvent, type IAgentResolveSessionConfigParams, type IAgentSessionConfigCompletionsParams, type IAgentSessionMetadata, type IAgentSubagentStartedEvent, type IAgentToolCompleteEvent, type IAgentToolStartEvent } from '../../common/agentService.js';
+import { AgentSession, type AgentProvider, type IAgent, type IAgentAttachment, type IAgentCreateSessionConfig, type IAgentCreateSessionResult, type IAgentDescriptor, type IAgentModelInfo, type IAgentProgressEvent, type IAgentResolveSessionConfigParams, type IAgentSessionConfigCompletionsParams, type IAgentSessionMetadata, type SessionHistoryEvent } from '../../common/agentService.js';
 import { ProtectedResourceMetadata, type ModelSelection } from '../../common/state/protocol/state.js';
 import type { ResolveSessionConfigResult, SessionConfigCompletionsResult } from '../../common/state/protocol/commands.js';
 import { CustomizationStatus, ToolResultContentType, type CustomizationRef, type PendingMessage, type ToolCallResult } from '../../common/state/sessionState.js';
@@ -48,7 +48,7 @@ export class MockAgent implements IAgent {
 	customizations: CustomizationRef[] = [];
 
 	/** Configurable return value for getSessionMessages. */
-	sessionMessages: (IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent)[] = [];
+	sessionMessages: SessionHistoryEvent[] = [];
 
 	/** Optional overrides applied to session metadata from listSessions. */
 	sessionMetadataOverrides: Partial<Omit<IAgentSessionMetadata, 'session'>> = {};
@@ -100,7 +100,7 @@ export class MockAgent implements IAgent {
 		this.setPendingMessagesCalls.push({ session, steeringMessage, queuedMessages });
 	}
 
-	async getSessionMessages(_session: URI): Promise<(IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent | IAgentSubagentStartedEvent)[]> {
+	async getSessionMessages(_session: URI): Promise<SessionHistoryEvent[]> {
 		return this.sessionMessages;
 	}
 
@@ -190,7 +190,7 @@ export class ScriptedMockAgent implements IAgent {
 	 * Message history for the pre-existing session: a single user→assistant
 	 * turn with a tool call.
 	 */
-	private readonly _preExistingMessages: (IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent)[] = [
+	private readonly _preExistingMessages: SessionHistoryEvent[] = [
 		{ type: 'message', role: 'user', session: PRE_EXISTING_SESSION_URI, messageId: 'h-msg-1', content: 'What files are here?' },
 		{ type: 'tool_start', session: PRE_EXISTING_SESSION_URI, toolCallId: 'h-tc-1', toolName: 'list_files', displayName: 'List Files', invocationMessage: 'Listing files...' },
 		{ type: 'tool_complete', session: PRE_EXISTING_SESSION_URI, toolCallId: 'h-tc-1', result: { pastTenseMessage: 'Listed files', content: [{ type: ToolResultContentType.Text, text: 'file1.ts\nfile2.ts' }], success: true } satisfies ToolCallResult },
@@ -611,7 +611,7 @@ export class ScriptedMockAgent implements IAgent {
 		}
 	}
 
-	async getSessionMessages(session: URI): Promise<(IAgentMessageEvent | IAgentToolStartEvent | IAgentToolCompleteEvent)[]> {
+	async getSessionMessages(session: URI): Promise<SessionHistoryEvent[]> {
 		if (session.toString() === PRE_EXISTING_SESSION_URI.toString()) {
 			return this._preExistingMessages;
 		}