revert: fix rpc stdout backpressure

This reverts commit d0d1d8edca.

packages/coding-agent/CHANGELOG.md

@@ -4,7 +4,6 @@
 
 ### Fixed
 
-- Fixed RPC mode to respect stdout backpressure while streaming events, avoiding `ENOBUFS` crashes when clients drain stdout slowly ([#4897](https://github.com/earendil-works/pi/issues/4897)).
 - Fixed managed npm extension updates to avoid package managers installing or resolving pi host packages as peer dependencies ([#4907](https://github.com/earendil-works/pi/issues/4907)).
 
 ## [0.75.5] - 2026-05-23

packages/coding-agent/docs/sdk.md

@@ -81,7 +81,7 @@ interface AgentSession {
   followUp(text: string): Promise<void>;
 
   // Subscribe to events (returns unsubscribe function)
-  subscribe(listener: (event: AgentSessionEvent) => void | Promise<void>): () => void;
+  subscribe(listener: (event: AgentSessionEvent) => void): () => void;
 
   // Session info
   sessionFile: string | undefined;
@@ -191,7 +191,7 @@ interface PromptOptions {
   images?: ImageContent[];
   streamingBehavior?: "steer" | "followUp";
   source?: InputSource;
-  preflightResult?: (success: boolean) => void | Promise<void>;
+  preflightResult?: (success: boolean) => void;
 }
 ```
 

packages/coding-agent/src/core/agent-session.ts

@@ -147,7 +147,7 @@ export type AgentSessionEvent =
 	| { type: "auto_retry_end"; success: boolean; attempt: number; finalError?: string };
 
 /** Listener function for agent session events */
-export type AgentSessionEventListener = (event: AgentSessionEvent) => void | Promise<void>;
+export type AgentSessionEventListener = (event: AgentSessionEvent) => void;
 
 // ============================================================================
 // Types
@@ -202,7 +202,7 @@ export interface PromptOptions {
 	/** Source of input for extension input event handlers. Defaults to "interactive". */
 	source?: InputSource;
 	/** Internal hook used by RPC mode to observe prompt preflight acceptance or rejection. */
-	preflightResult?: (success: boolean) => void | Promise<void>;
+	preflightResult?: (success: boolean) => void;
 }
 
 /** Result from cycleModel() */
@@ -448,18 +448,14 @@ export class AgentSession {
 	// =========================================================================
 
 	/** Emit an event to all listeners */
-	private async _emit(event: AgentSessionEvent): Promise<void> {
+	private _emit(event: AgentSessionEvent): void {
 		for (const l of this._eventListeners) {
-			await l(event);
+			l(event);
 		}
 	}
 
-	private _emitDetached(event: AgentSessionEvent): void {
+	private _emitQueueUpdate(): void {
-		void this._emit(event);
+		this._emit({
-	}
-
-	private async _emitQueueUpdate(): Promise<void> {
-		await this._emit({
 			type: "queue_update",
 			steering: [...this._steeringMessages],
 			followUp: [...this._followUpMessages],
@@ -481,13 +477,13 @@ export class AgentSession {
 				const steeringIndex = this._steeringMessages.indexOf(messageText);
 				if (steeringIndex !== -1) {
 					this._steeringMessages.splice(steeringIndex, 1);
-					await this._emitQueueUpdate();
+					this._emitQueueUpdate();
 				} else {
 					// Check follow-up queue
 					const followUpIndex = this._followUpMessages.indexOf(messageText);
 					if (followUpIndex !== -1) {
 						this._followUpMessages.splice(followUpIndex, 1);
-						await this._emitQueueUpdate();
+						this._emitQueueUpdate();
 					}
 				}
 			}
@@ -497,9 +493,7 @@ export class AgentSession {
 		await this._emitExtensionEvent(event);
 
 		// Notify all listeners
-		await this._emit(
+		this._emit(event.type === "agent_end" ? { ...event, willRetry: this._willRetryAfterAgentEnd(event) } : event);
-			event.type === "agent_end" ? { ...event, willRetry: this._willRetryAfterAgentEnd(event) } : event,
-		);
 
 		// Handle session persistence
 		if (event.type === "message_end") {
@@ -534,7 +528,7 @@ export class AgentSession {
 				// Reset retry counter immediately on successful assistant response
 				// This prevents accumulation across multiple LLM calls within a turn
 				if (assistantMsg.stopReason !== "error" && this._retryAttempt > 0) {
-					await this._emit({
+					this._emit({
 						type: "auto_retry_end",
 						success: true,
 						attempt: this._retryAttempt,
@@ -944,7 +938,7 @@ export class AgentSession {
 		}
 
 		if (msg.stopReason === "error" && this._retryAttempt > 0) {
-			await this._emit({
+			this._emit({
 				type: "auto_retry_end",
 				success: false,
 				attempt: this._retryAttempt,
@@ -977,7 +971,7 @@ export class AgentSession {
 				const handled = await this._tryExecuteExtensionCommand(text);
 				if (handled) {
 					// Extension command executed, no prompt to send
-					await preflightResult?.(true);
+					preflightResult?.(true);
 					return;
 				}
 			}
@@ -992,7 +986,7 @@ export class AgentSession {
 					options?.source ?? "interactive",
 				);
 				if (inputResult.action === "handled") {
-					await preflightResult?.(true);
+					preflightResult?.(true);
 					return;
 				}
 				if (inputResult.action === "transform") {
@@ -1020,7 +1014,7 @@ export class AgentSession {
 				} else {
 					await this._queueSteer(expandedText, currentImages);
 				}
-				await preflightResult?.(true);
+				preflightResult?.(true);
 				return;
 			}
 
@@ -1105,7 +1099,7 @@ export class AgentSession {
 				this.agent.state.systemPrompt = this._baseSystemPrompt;
 			}
 		} catch (error) {
-			await preflightResult?.(false);
+			preflightResult?.(false);
 			throw error;
 		}
 
@@ -1113,7 +1107,7 @@ export class AgentSession {
 			return;
 		}
 
-		await preflightResult?.(true);
+		preflightResult?.(true);
 		await this._runAgentPrompt(messages);
 	}
 
@@ -1223,7 +1217,7 @@ export class AgentSession {
 	 */
 	private async _queueSteer(text: string, images?: ImageContent[]): Promise<void> {
 		this._steeringMessages.push(text);
-		await this._emitQueueUpdate();
+		this._emitQueueUpdate();
 		const content: (TextContent | ImageContent)[] = [{ type: "text", text }];
 		if (images) {
 			content.push(...images);
@@ -1240,7 +1234,7 @@ export class AgentSession {
 	 */
 	private async _queueFollowUp(text: string, images?: ImageContent[]): Promise<void> {
 		this._followUpMessages.push(text);
-		await this._emitQueueUpdate();
+		this._emitQueueUpdate();
 		const content: (TextContent | ImageContent)[] = [{ type: "text", text }];
 		if (images) {
 			content.push(...images);
@@ -1309,8 +1303,8 @@ export class AgentSession {
 				message.display,
 				message.details,
 			);
-			await this._emit({ type: "message_start", message: appMessage });
+			this._emit({ type: "message_start", message: appMessage });
-			await this._emit({ type: "message_end", message: appMessage });
+			this._emit({ type: "message_end", message: appMessage });
 		}
 	}
 
@@ -1365,7 +1359,7 @@ export class AgentSession {
 		this._steeringMessages = [];
 		this._followUpMessages = [];
 		this.agent.clearAllQueues();
-		void this._emitQueueUpdate();
+		this._emitQueueUpdate();
 		return { steering, followUp };
 	}
 
@@ -1528,7 +1522,7 @@ export class AgentSession {
 			if (this.supportsThinking() || effectiveLevel !== "off") {
 				this.settingsManager.setDefaultThinkingLevel(effectiveLevel);
 			}
-			this._emitDetached({ type: "thinking_level_changed", level: effectiveLevel });
+			this._emit({ type: "thinking_level_changed", level: effectiveLevel });
 			void this._extensionRunner.emit({
 				type: "thinking_level_select",
 				level: effectiveLevel,
@@ -1618,7 +1612,7 @@ export class AgentSession {
 		this._disconnectFromAgent();
 		await this.abort();
 		this._compactionAbortController = new AbortController();
-		await this._emit({ type: "compaction_start", reason: "manual" });
+		this._emit({ type: "compaction_start", reason: "manual" });
 
 		try {
 			if (!this.model) {
@@ -1719,7 +1713,7 @@ export class AgentSession {
 				tokensBefore,
 				details,
 			};
-			await this._emit({
+			this._emit({
 				type: "compaction_end",
 				reason: "manual",
 				result: compactionResult,
@@ -1730,7 +1724,7 @@ export class AgentSession {
 		} catch (error) {
 			const message = error instanceof Error ? error.message : String(error);
 			const aborted = message === "Compaction cancelled" || (error instanceof Error && error.name === "AbortError");
-			await this._emit({
+			this._emit({
 				type: "compaction_end",
 				reason: "manual",
 				result: undefined,
@@ -1800,7 +1794,7 @@ export class AgentSession {
 		// Case 1: Overflow - LLM returned context overflow error
 		if (sameModel && isContextOverflow(assistantMessage, contextWindow)) {
 			if (this._overflowRecoveryAttempted) {
-				await this._emit({
+				this._emit({
 					type: "compaction_end",
 					reason: "overflow",
 					result: undefined,
@@ -1857,12 +1851,12 @@ export class AgentSession {
 	private async _runAutoCompaction(reason: "overflow" | "threshold", willRetry: boolean): Promise<boolean> {
 		const settings = this.settingsManager.getCompactionSettings();
 
-		await this._emit({ type: "compaction_start", reason });
+		this._emit({ type: "compaction_start", reason });
 		this._autoCompactionAbortController = new AbortController();
 
 		try {
 			if (!this.model) {
-				await this._emit({
+				this._emit({
 					type: "compaction_end",
 					reason,
 					result: undefined,
@@ -1877,7 +1871,7 @@ export class AgentSession {
 			if (this.agent.streamFn === streamSimple) {
 				const authResult = await this._modelRegistry.getApiKeyAndHeaders(this.model);
 				if (!authResult.ok || !authResult.apiKey) {
-					await this._emit({
+					this._emit({
 						type: "compaction_end",
 						reason,
 						result: undefined,
@@ -1896,7 +1890,7 @@ export class AgentSession {
 
 			const preparation = prepareCompaction(pathEntries, settings);
 			if (!preparation) {
-				await this._emit({
+				this._emit({
 					type: "compaction_end",
 					reason,
 					result: undefined,
@@ -1919,7 +1913,7 @@ export class AgentSession {
 				})) as SessionBeforeCompactResult | undefined;
 
 				if (extensionResult?.cancel) {
-					await this._emit({
+					this._emit({
 						type: "compaction_end",
 						reason,
 						result: undefined,
@@ -1965,7 +1959,7 @@ export class AgentSession {
 			}
 
 			if (this._autoCompactionAbortController.signal.aborted) {
-				await this._emit({
+				this._emit({
 					type: "compaction_end",
 					reason,
 					result: undefined,
@@ -1999,7 +1993,7 @@ export class AgentSession {
 				tokensBefore,
 				details,
 			};
-			await this._emit({ type: "compaction_end", reason, result, aborted: false, willRetry });
+			this._emit({ type: "compaction_end", reason, result, aborted: false, willRetry });
 
 			if (willRetry) {
 				const messages = this.agent.state.messages;
@@ -2015,7 +2009,7 @@ export class AgentSession {
 			return this.agent.hasQueuedMessages();
 		} catch (error) {
 			const errorMessage = error instanceof Error ? error.message : "compaction failed";
-			await this._emit({
+			this._emit({
 				type: "compaction_end",
 				reason,
 				result: undefined,
@@ -2466,7 +2460,7 @@ export class AgentSession {
 
 		const delayMs = settings.baseDelayMs * 2 ** (this._retryAttempt - 1);
 
-		await this._emit({
+		this._emit({
 			type: "auto_retry_start",
 			attempt: this._retryAttempt,
 			maxAttempts: settings.maxRetries,
@@ -2488,7 +2482,7 @@ export class AgentSession {
 			// Aborted during sleep - emit end event so UI can clean up
 			const attempt = this._retryAttempt;
 			this._retryAttempt = 0;
-			await this._emit({
+			this._emit({
 				type: "auto_retry_end",
 				success: false,
 				attempt,
@@ -2642,7 +2636,7 @@ export class AgentSession {
 	 */
 	setSessionName(name: string): void {
 		this.sessionManager.appendSessionInfo(name);
-		this._emitDetached({ type: "session_info_changed", name: this.sessionManager.getSessionName() });
+		this._emit({ type: "session_info_changed", name: this.sessionManager.getSessionName() });
 	}
 
 	// =========================================================================

packages/coding-agent/src/core/output-guard.ts

@@ -1,5 +1,3 @@
-import { once } from "node:events";
-
 interface StdoutTakeoverState {
 	rawStdoutWrite: (chunk: string, callback?: (error?: Error | null) => void) => boolean;
 	rawStderrWrite: (chunk: string, callback?: (error?: Error | null) => void) => boolean;
@@ -48,11 +46,12 @@ export function isStdoutTakenOver(): boolean {
 	return stdoutTakeoverState !== undefined;
 }
 
-export async function writeRawStdout(text: string): Promise<void> {
+export function writeRawStdout(text: string): void {
-	const canContinue = stdoutTakeoverState ? stdoutTakeoverState.rawStdoutWrite(text) : process.stdout.write(text);
+	if (stdoutTakeoverState) {
-	if (!canContinue) {
+		stdoutTakeoverState.rawStdoutWrite(text);
-		await once(process.stdout, "drain");
+		return;
 	}
+	process.stdout.write(text);
 }
 
 export async function flushRawStdout(): Promise<void> {

packages/coding-agent/src/modes/print-mode.ts

@@ -100,9 +100,9 @@ export async function runPrintMode(runtimeHost: AgentSessionRuntime, options: Pr
 		});
 
 		unsubscribe?.();
-		unsubscribe = session.subscribe(async (event) => {
+		unsubscribe = session.subscribe((event) => {
 			if (mode === "json") {
-				await writeRawStdout(`${JSON.stringify(event)}\n`);
+				writeRawStdout(`${JSON.stringify(event)}\n`);
 			}
 		});
 	};
@@ -111,7 +111,7 @@ export async function runPrintMode(runtimeHost: AgentSessionRuntime, options: Pr
 		if (mode === "json") {
 			const header = session.sessionManager.getHeader();
 			if (header) {
-				await writeRawStdout(`${JSON.stringify(header)}\n`);
+				writeRawStdout(`${JSON.stringify(header)}\n`);
 			}
 		}
 
@@ -137,7 +137,7 @@ export async function runPrintMode(runtimeHost: AgentSessionRuntime, options: Pr
 				} else {
 					for (const content of assistantMsg.content) {
 						if (content.type === "text") {
-							await writeRawStdout(`${content.text}\n`);
+							writeRawStdout(`${content.text}\n`);
 						}
 					}
 				}

packages/coding-agent/src/modes/rpc/rpc-mode.ts

@@ -50,14 +50,8 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 	let session = runtimeHost.session;
 	let unsubscribe: (() => void) | undefined;
 
-	const output = async (obj: RpcResponse | RpcExtensionUIRequest | object): Promise<void> => {
+	const output = (obj: RpcResponse | RpcExtensionUIRequest | object) => {
-		await writeRawStdout(serializeJsonLine(obj));
+		writeRawStdout(serializeJsonLine(obj));
-	};
-
-	const outputDetached = (obj: RpcResponse | RpcExtensionUIRequest | object): void => {
-		void output(obj).catch((err: unknown) => {
-			process.stderr.write(`RPC output failed: ${err instanceof Error ? err.message : String(err)}\n`);
-		});
 	};
 
 	const success = <T extends RpcCommand["type"]>(
@@ -87,30 +81,28 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 	const signalCleanupHandlers: Array<() => void> = [];
 
 	/** Helper for dialog methods with signal/timeout support */
-	async function createDialogPromise<T>(
+	function createDialogPromise<T>(
 		opts: ExtensionUIDialogOptions | undefined,
 		defaultValue: T,
 		request: Record<string, unknown>,
 		parseResponse: (response: RpcExtensionUIResponse) => T,
 	): Promise<T> {
-		if (opts?.signal?.aborted) return defaultValue;
+		if (opts?.signal?.aborted) return Promise.resolve(defaultValue);
 
 		const id = crypto.randomUUID();
-		let cleanup = () => {};
+		return new Promise((resolve, reject) => {
-		const responsePromise = new Promise<T>((resolve, reject) => {
 			let timeoutId: ReturnType<typeof setTimeout> | undefined;
 
-			const onAbort = () => {
+			const cleanup = () => {
-				cleanup();
-				resolve(defaultValue);
-			};
-
-			cleanup = () => {
 				if (timeoutId) clearTimeout(timeoutId);
 				opts?.signal?.removeEventListener("abort", onAbort);
 				pendingExtensionRequests.delete(id);
 			};
 
+			const onAbort = () => {
+				cleanup();
+				resolve(defaultValue);
+			};
 			opts?.signal?.addEventListener("abort", onAbort, { once: true });
 
 			if (opts?.timeout) {
@@ -125,20 +117,10 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 					cleanup();
 					resolve(parseResponse(response));
 				},
-				reject: (error) => {
+				reject,
-					cleanup();
-					reject(error);
-				},
 			});
+			output({ type: "extension_ui_request", id, ...request } as RpcExtensionUIRequest);
 		});
-
-		try {
-			await output({ type: "extension_ui_request", id, ...request } as RpcExtensionUIRequest);
-		} catch (err) {
-			cleanup();
-			throw err;
-		}
-		return await responsePromise;
 	}
 
 	/**
@@ -162,7 +144,7 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 
 		notify(message: string, type?: "info" | "warning" | "error"): void {
 			// Fire and forget - no response needed
-			outputDetached({
+			output({
 				type: "extension_ui_request",
 				id: crypto.randomUUID(),
 				method: "notify",
@@ -178,7 +160,7 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 
 		setStatus(key: string, text: string | undefined): void {
 			// Fire and forget - no response needed
-			outputDetached({
+			output({
 				type: "extension_ui_request",
 				id: crypto.randomUUID(),
 				method: "setStatus",
@@ -206,7 +188,7 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 		setWidget(key: string, content: unknown, options?: ExtensionWidgetOptions): void {
 			// Only support string arrays in RPC mode - factory functions are ignored
 			if (content === undefined || Array.isArray(content)) {
-				outputDetached({
+				output({
 					type: "extension_ui_request",
 					id: crypto.randomUUID(),
 					method: "setWidget",
@@ -228,7 +210,7 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 
 		setTitle(title: string): void {
 			// Fire and forget - host can implement terminal title control
-			outputDetached({
+			output({
 				type: "extension_ui_request",
 				id: crypto.randomUUID(),
 				method: "setTitle",
@@ -248,7 +230,7 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 
 		setEditorText(text: string): void {
 			// Fire and forget - host can implement editor control
-			outputDetached({
+			output({
 				type: "extension_ui_request",
 				id: crypto.randomUUID(),
 				method: "set_editor_text",
@@ -264,14 +246,9 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 
 		async editor(title: string, prefill?: string): Promise<string | undefined> {
 			const id = crypto.randomUUID();
-			let cleanup = () => {};
+			return new Promise((resolve, reject) => {
-			const responsePromise = new Promise<string | undefined>((resolve, reject) => {
-				cleanup = () => {
-					pendingExtensionRequests.delete(id);
-				};
 				pendingExtensionRequests.set(id, {
 					resolve: (response: RpcExtensionUIResponse) => {
-						cleanup();
 						if ("cancelled" in response && response.cancelled) {
 							resolve(undefined);
 						} else if ("value" in response) {
@@ -280,25 +257,10 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 							resolve(undefined);
 						}
 					},
-					reject: (error) => {
+					reject,
-						cleanup();
-						reject(error);
-					},
 				});
+				output({ type: "extension_ui_request", id, method: "editor", title, prefill } as RpcExtensionUIRequest);
 			});
-			try {
-				await output({
-					type: "extension_ui_request",
-					id,
-					method: "editor",
-					title,
-					prefill,
-				} as RpcExtensionUIRequest);
-			} catch (err) {
-				cleanup();
-				throw err;
-			}
-			return await responsePromise;
 		},
 
 		addAutocompleteProvider(): void {
@@ -376,18 +338,13 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 				shutdownRequested = true;
 			},
 			onError: (err) => {
-				outputDetached({
+				output({ type: "extension_error", extensionPath: err.extensionPath, event: err.event, error: err.error });
-					type: "extension_error",
-					extensionPath: err.extensionPath,
-					event: err.event,
-					error: err.error,
-				});
 			},
 		});
 
 		unsubscribe?.();
-		unsubscribe = session.subscribe(async (event) => {
+		unsubscribe = session.subscribe((event) => {
-			await output(event);
+			output(event);
 		});
 	};
 
@@ -428,17 +385,16 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 						images: command.images,
 						streamingBehavior: command.streamingBehavior,
 						source: "rpc",
-						preflightResult: async (didSucceed) => {
+						preflightResult: (didSucceed) => {
 							if (didSucceed) {
-								await output(success(id, "prompt"));
 								preflightSucceeded = true;
+								output(success(id, "prompt"));
 							}
 						},
 					})
-					.catch((err: unknown) => {
+					.catch((e) => {
 						if (!preflightSucceeded) {
-							const message = err instanceof Error ? err.message : String(err);
+							output(error(id, "prompt", e.message));
-							outputDetached(error(id, "prompt", message));
 						}
 					});
 				return undefined;
@@ -734,7 +690,7 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 		try {
 			parsed = JSON.parse(line);
 		} catch (parseError: unknown) {
-			await output(
+			output(
 				error(
 					undefined,
 					"parse",
@@ -764,11 +720,11 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 		try {
 			const response = await handleCommand(command);
 			if (response) {
-				await output(response);
+				output(response);
 			}
 			await checkShutdownRequested();
 		} catch (commandError: unknown) {
-			await output(
+			output(
 				error(
 					command.id,
 					command.type,
@@ -785,9 +741,7 @@ export async function runRpcMode(runtimeHost: AgentSessionRuntime): Promise<neve
 
 	detachInput = (() => {
 		const detachJsonl = attachJsonlLineReader(process.stdin, (line) => {
-			void handleInputLine(line).catch((err: unknown) => {
+			void handleInputLine(line);
-				process.stderr.write(`RPC command handling failed: ${err instanceof Error ? err.message : String(err)}\n`);
-			});
 		});
 		return () => {
 			detachJsonl();