feat(coding-agent): improved Python startup performance with prelude caching and shared gateway mode

- Added Python prelude caching to improve startup performance by caching helper functions and module discovery results.
- Added OMP_DEBUG_STARTUP environment variable for detailed startup stage tracing across initialization pipeline.
- Changed Python kernel initialization to require shared gateway mode, removing support for local kernel gateway startup.
- Improved error handling in shared gateway acquisition with automatic retries on 5xx status codes.
- Added repro-stuck.ts script for reproducing and debugging rare CLI hang issues with Chrome DevTools integration.
- Added trace-loader.ts preload script for tracing module resolution during startup diagnostics.

packages/coding-agent/src/capability/index.ts

@@ -8,6 +8,12 @@
  */
 import * as os from "node:os";
 import * as path from "node:path";
+
+/** Conditional startup debug prints (stderr) when OMP_DEBUG_STARTUP is set */
+const debugStartup = process.env.OMP_DEBUG_STARTUP
+	? (stage: string) => process.stderr.write(`[startup] ${stage}\n`)
+	: () => {};
+
 import type { Settings } from "../config/settings";
 import { clearCache as clearFsCache, cacheStats as fsCacheStats, invalidate as invalidateFs } from "./fs";
 import type {
@@ -109,9 +115,12 @@ async function loadImpl<T>(
 	const results = await Promise.all(
 		providers.map(async provider => {
 			try {
+				debugStartup(`capability:${capability.id}:${provider.id}:start`);
 				const result = await provider.load(ctx);
+				debugStartup(`capability:${capability.id}:${provider.id}:done`);
 				return { provider, result };
 			} catch (error) {
+				debugStartup(`capability:${capability.id}:${provider.id}:error`);
 				return { provider, error };
 			}
 		}),

packages/coding-agent/src/ipy/executor.ts

@@ -1,4 +1,6 @@
-import { logger } from "@oh-my-pi/pi-utils";
+import * as path from "node:path";
+import { isEnoent, logger } from "@oh-my-pi/pi-utils";
+import { getAgentDir } from "../config";
 import { OutputSink } from "../session/streaming-output";
 import { time } from "../utils/timings";
 import { shutdownSharedGateway } from "./gateway-coordinator";
@@ -10,6 +12,12 @@ import {
 	type PreludeHelper,
 	PythonKernel,
 } from "./kernel";
+import { discoverPythonModules } from "./modules";
+import { PYTHON_PRELUDE } from "./prelude";
+
+const debugStartup = process.env.OMP_DEBUG_STARTUP
+	? (stage: string) => process.stderr.write(`[startup] ${stage}\n`)
+	: () => {};
 
 const IDLE_TIMEOUT_MS = 5 * 60 * 1000; // 5 minutes
 const MAX_KERNEL_SESSIONS = 4;
@@ -86,6 +94,72 @@ const kernelSessions = new Map<string, KernelSession>();
 let cachedPreludeDocs: PreludeHelper[] | null = null;
 let cleanupTimer: NodeJS.Timeout | null = null;
 
+interface PreludeCacheSource {
+	path: string;
+	hash: string;
+}
+
+interface PreludeCachePayload {
+	helpers: PreludeHelper[];
+	sources: PreludeCacheSource[];
+}
+
+interface PreludeCacheState {
+	cacheKey: string;
+	cachePath: string;
+	sources: PreludeCacheSource[];
+}
+
+const PRELUDE_CACHE_DIR = "pycache";
+
+function hashPreludeContent(content: string): string {
+	return Bun.hash(content).toString(16);
+}
+
+async function buildPreludeCacheState(cwd: string): Promise<PreludeCacheState> {
+	const modules = await discoverPythonModules({ cwd });
+	const moduleSources = modules
+		.map(module => ({ path: module.path, hash: hashPreludeContent(module.content) }))
+		.sort((a, b) => a.path.localeCompare(b.path));
+	const sources: PreludeCacheSource[] = [
+		{ path: "omp:prelude", hash: hashPreludeContent(PYTHON_PRELUDE) },
+		...moduleSources,
+	];
+	const composite = sources.map(source => `${source.path}:${source.hash}`).join("|");
+	const cacheKey = Bun.hash(composite).toString(16);
+	const cachePath = path.join(getAgentDir(), PRELUDE_CACHE_DIR, `${cacheKey}.json`);
+	return { cacheKey, cachePath, sources };
+}
+
+async function readPreludeCache(state: PreludeCacheState): Promise<PreludeHelper[] | null> {
+	let raw: string;
+	try {
+		raw = await Bun.file(state.cachePath).text();
+	} catch (err) {
+		if (isEnoent(err)) return null;
+		logger.warn("Failed to read Python prelude cache", { path: state.cachePath, error: String(err) });
+		return null;
+	}
+	try {
+		const parsed = JSON.parse(raw) as PreludeCachePayload | PreludeHelper[];
+		const helpers = Array.isArray(parsed) ? parsed : parsed.helpers;
+		if (!Array.isArray(helpers) || helpers.length === 0) return null;
+		return helpers;
+	} catch (err) {
+		logger.warn("Failed to parse Python prelude cache", { path: state.cachePath, error: String(err) });
+		return null;
+	}
+}
+
+async function writePreludeCache(state: PreludeCacheState, helpers: PreludeHelper[]): Promise<void> {
+	const payload: PreludeCachePayload = { helpers, sources: state.sources };
+	try {
+		await Bun.write(state.cachePath, JSON.stringify(payload));
+	} catch (err) {
+		logger.warn("Failed to write Python prelude cache", { path: state.cachePath, error: String(err) });
+	}
+}
+
 function startCleanupTimer(): void {
 	if (cleanupTimer) return;
 	cleanupTimer = setInterval(() => {
@@ -153,19 +227,34 @@ export async function warmPythonEnvironment(
 	useSharedGateway?: boolean,
 	sessionFile?: string,
 ): Promise<{ ok: boolean; reason?: string; docs: PreludeHelper[] }> {
+	let cacheState: PreludeCacheState | null = null;
 	try {
+		debugStartup("warmPython:ensureKernel:start");
 		await ensureKernelAvailable(cwd);
+		debugStartup("warmPython:ensureKernel:done");
 		time("warmPython:ensureKernelAvailable");
 	} catch (err: unknown) {
 		const reason = err instanceof Error ? err.message : String(err);
 		cachedPreludeDocs = [];
 		return { ok: false, reason, docs: [] };
 	}
+	try {
+		cacheState = await buildPreludeCacheState(cwd);
+		const cached = await readPreludeCache(cacheState);
+		if (cached) {
+			cachedPreludeDocs = cached;
+			return { ok: true, docs: cached };
+		}
+	} catch (err) {
+		logger.warn("Failed to resolve Python prelude cache", { error: String(err) });
+		cacheState = null;
+	}
 	if (cachedPreludeDocs && cachedPreludeDocs.length > 0) {
 		return { ok: true, docs: cachedPreludeDocs };
 	}
 	const resolvedSessionId = sessionId ?? `session:${cwd}`;
 	try {
+		debugStartup("warmPython:withKernelSession:start");
 		const docs = await withKernelSession(
 			resolvedSessionId,
 			cwd,
@@ -173,8 +262,13 @@ export async function warmPythonEnvironment(
 			useSharedGateway,
 			sessionFile,
 		);
+		debugStartup("warmPython:withKernelSession:done");
 		time("warmPython:withKernelSession");
 		cachedPreludeDocs = docs;
+		if (docs.length > 0) {
+			const state = cacheState ?? (await buildPreludeCacheState(cwd));
+			await writePreludeCache(state, docs);
+		}
 		return { ok: true, docs };
 	} catch (err: unknown) {
 		const reason = err instanceof Error ? err.message : String(err);
@@ -226,6 +320,7 @@ async function createKernelSession(
 	artifactsDir?: string,
 	isRetry?: boolean,
 ): Promise<KernelSession> {
+	debugStartup("kernel:createSession:entry");
 	const env: Record<string, string> | undefined =
 		sessionFile || artifactsDir
 			? {
@@ -236,7 +331,9 @@ async function createKernelSession(
 
 	let kernel: PythonKernel;
 	try {
+		debugStartup("kernel:PythonKernel.start:start");
 		kernel = await PythonKernel.start({ cwd, useSharedGateway, env });
+		debugStartup("kernel:PythonKernel.start:done");
 		time("createKernelSession:PythonKernel.start");
 	} catch (err) {
 		if (!isRetry && isResourceExhaustionError(err)) {
@@ -314,6 +411,7 @@ async function withKernelSession<T>(
 	sessionFile?: string,
 	artifactsDir?: string,
 ): Promise<T> {
+	debugStartup("kernel:withSession:entry");
 	let session = kernelSessions.get(sessionId);
 	if (!session) {
 		// Evict oldest session if at capacity
@@ -321,17 +419,21 @@ async function withKernelSession<T>(
 			await evictOldestSession();
 		}
 		session = await createKernelSession(sessionId, cwd, useSharedGateway, sessionFile, artifactsDir);
+		debugStartup("kernel:withSession:created");
 		kernelSessions.set(sessionId, session);
 		startCleanupTimer();
 	}
 
 	const run = async (): Promise<T> => {
+		debugStartup("kernel:withSession:run");
 		session!.lastUsedAt = Date.now();
 		if (session!.dead || !session!.kernel.isAlive()) {
 			await restartKernelSession(session!, cwd, useSharedGateway, sessionFile, artifactsDir);
 		}
 		try {
+			debugStartup("kernel:withSession:handler:start");
 			const result = await handler(session!.kernel);
+			debugStartup("kernel:withSession:handler:done");
 			session!.restartCount = 0;
 			return result;
 		} catch (err) {

packages/coding-agent/src/ipy/kernel.ts

@@ -1,24 +1,32 @@
-import { createServer } from "node:net";
-import * as path from "node:path";
-import { logger, ptree } from "@oh-my-pi/pi-utils";
+import { logger } from "@oh-my-pi/pi-utils";
 import { $ } from "bun";
 import { nanoid } from "nanoid";
 import { Settings } from "../config/settings";
-import { getOrCreateSnapshot } from "../utils/shell-snapshot";
 import { time } from "../utils/timings";
 import { htmlToBasicMarkdown } from "../web/scrapers/types";
-import { acquireSharedGateway, releaseSharedGateway } from "./gateway-coordinator";
+import { acquireSharedGateway, releaseSharedGateway, shutdownSharedGateway } from "./gateway-coordinator";
 import { loadPythonModules } from "./modules";
 import { PYTHON_PRELUDE } from "./prelude";
 import { filterEnv, resolvePythonRuntime } from "./runtime";
 
 const TEXT_ENCODER = new TextEncoder();
 const TEXT_DECODER = new TextDecoder();
-const GATEWAY_STARTUP_TIMEOUT_MS = 30000;
-const GATEWAY_STARTUP_ATTEMPTS = 3;
 const TRACE_IPC = process.env.OMP_PYTHON_IPC_TRACE === "1";
 const PRELUDE_INTROSPECTION_SNIPPET = "import json\nprint(json.dumps(__omp_prelude_docs__()))";
 
+const debugStartup = process.env.OMP_DEBUG_STARTUP
+	? (stage: string) => process.stderr.write(`[startup] ${stage}\n`)
+	: () => {};
+
+class SharedGatewayCreateError extends Error {
+	readonly status: number;
+
+	constructor(status: number, message: string) {
+		super(message);
+		this.status = status;
+	}
+}
+
 interface ExternalGatewayConfig {
 	url: string;
 	token?: string;
@@ -178,31 +186,6 @@ async function checkExternalGatewayAvailability(config: ExternalGatewayConfig):
 	}
 }
 
-async function allocatePort(): Promise<number> {
-	const { promise, resolve, reject } = Promise.withResolvers<number>();
-	const server = createServer();
-	server.unref();
-	server.on("error", reject);
-	server.listen(0, "127.0.0.1", () => {
-		const address = server.address();
-		if (address && typeof address === "object") {
-			const port = address.port;
-			server.close((err: Error | null | undefined) => {
-				if (err) {
-					reject(err);
-				} else {
-					resolve(port);
-				}
-			});
-		} else {
-			server.close();
-			reject(new Error("Failed to allocate port"));
-		}
-	});
-
-	return promise;
-}
-
 function normalizeDisplayText(text: string): string {
 	return text.endsWith("\n") ? text : `${text}\n`;
 }
@@ -287,7 +270,6 @@ export function serializeWebSocketMessage(msg: JupyterMessage): ArrayBuffer {
 export class PythonKernel {
 	readonly id: string;
 	readonly kernelId: string;
-	readonly gatewayProcess: ptree.ChildProcess | null;
 	readonly gatewayUrl: string;
 	readonly sessionId: string;
 	readonly username: string;
@@ -304,7 +286,6 @@ export class PythonKernel {
 	private constructor(
 		id: string,
 		kernelId: string,
-		gatewayProcess: ptree.ChildProcess | null,
 		gatewayUrl: string,
 		sessionId: string,
 		username: string,
@@ -313,18 +294,11 @@ export class PythonKernel {
 	) {
 		this.id = id;
 		this.kernelId = kernelId;
-		this.gatewayProcess = gatewayProcess;
 		this.gatewayUrl = gatewayUrl;
 		this.sessionId = sessionId;
 		this.username = username;
 		this.isSharedGateway = isSharedGateway;
 		this.#authToken = authToken;
-
-		if (this.gatewayProcess) {
-			this.gatewayProcess.exited.then(() => {
-				this.#alive = false;
-			});
-		}
 	}
 
 	#authHeaders(): Record<string, string> {
@@ -333,7 +307,9 @@ export class PythonKernel {
 	}
 
 	static async start(options: KernelStartOptions): Promise<PythonKernel> {
+		debugStartup("PythonKernel.start:entry");
 		const availability = await checkPythonKernelAvailability(options.cwd);
+		debugStartup("PythonKernel.start:availCheck");
 		time("PythonKernel.start:availabilityCheck");
 		if (!availability.ok) {
 			throw new Error(availability.reason ?? "Python kernel unavailable");
@@ -344,24 +320,40 @@ export class PythonKernel {
 			return PythonKernel.startWithExternalGateway(externalConfig, options.cwd, options.env);
 		}
 
-		// Try shared gateway first (unless explicitly disabled)
-		if (options.useSharedGateway !== false) {
+		if (options.useSharedGateway === false) {
+			throw new Error("Shared Python gateway required; local gateways are disabled");
+		}
+
+		for (let attempt = 0; attempt < 2; attempt += 1) {
 			try {
+				debugStartup("PythonKernel.start:acquireShared:start");
 				const sharedResult = await acquireSharedGateway(options.cwd);
+				debugStartup("PythonKernel.start:acquireShared:done");
 				time("PythonKernel.start:acquireSharedGateway");
-				if (sharedResult) {
-					const kernel = await PythonKernel.startWithSharedGateway(sharedResult.url, options.cwd, options.env);
-					time("PythonKernel.start:startWithSharedGateway");
-					return kernel;
+				if (!sharedResult) {
+					throw new Error("Shared Python gateway unavailable");
 				}
+				debugStartup("PythonKernel.start:startShared:start");
+				const kernel = await PythonKernel.startWithSharedGateway(sharedResult.url, options.cwd, options.env);
+				debugStartup("PythonKernel.start:startShared:done");
+				time("PythonKernel.start:startWithSharedGateway");
+				return kernel;
 			} catch (err) {
-				logger.warn("Failed to acquire shared gateway, falling back to local", {
+				debugStartup("PythonKernel.start:sharedFailed");
+				if (attempt === 0 && err instanceof SharedGatewayCreateError && err.status >= 500) {
+					logger.warn("Shared gateway kernel creation failed, retrying", {
+						status: err.status,
+					});
+					continue;
+				}
+				logger.warn("Failed to acquire shared gateway", {
 					error: err instanceof Error ? err.message : String(err),
 				});
+				throw err;
 			}
 		}
 
-		return PythonKernel.startWithLocalGateway(options);
+		throw new Error("Shared Python gateway unavailable after retry");
 	}
 
 	private static async startWithExternalGateway(
@@ -387,7 +379,7 @@ export class PythonKernel {
 		const kernelInfo = (await createResponse.json()) as { id: string };
 		const kernelId = kernelInfo.id;
 
-		const kernel = new PythonKernel(nanoid(), kernelId, null, config.url, nanoid(), "omp", false, config.token);
+		const kernel = new PythonKernel(nanoid(), kernelId, config.url, nanoid(), "omp", false, config.token);
 
 		try {
 			await kernel.connectWebSocket();
@@ -409,34 +401,54 @@ export class PythonKernel {
 		cwd: string,
 		env?: Record<string, string | undefined>,
 	): Promise<PythonKernel> {
+		debugStartup("sharedGateway:fetch:start");
 		const createResponse = await fetch(`${gatewayUrl}/api/kernels`, {
 			method: "POST",
 			headers: { "Content-Type": "application/json" },
 			body: JSON.stringify({ name: "python3" }),
 		});
+		debugStartup("sharedGateway:fetch:done");
 		time("startWithSharedGateway:createKernel");
 
 		if (!createResponse.ok) {
-			await releaseSharedGateway();
-			throw new Error(`Failed to create kernel on shared gateway: ${await createResponse.text()}`);
+			debugStartup(`sharedGateway:fetch:notOk:${createResponse.status}`);
+			await shutdownSharedGateway();
+			debugStartup("sharedGateway:fetch:shutdown");
+			const text = await createResponse.text();
+			debugStartup("sharedGateway:fetch:textRead");
+			throw new SharedGatewayCreateError(
+				createResponse.status,
+				`Failed to create kernel on shared gateway: ${text}`,
+			);
 		}
 
+		debugStartup("sharedGateway:json:start");
 		const kernelInfo = (await createResponse.json()) as { id: string };
+		debugStartup("sharedGateway:json:done");
 		const kernelId = kernelInfo.id;
 
-		const kernel = new PythonKernel(nanoid(), kernelId, null, gatewayUrl, nanoid(), "omp", true);
+		const kernel = new PythonKernel(nanoid(), kernelId, gatewayUrl, nanoid(), "omp", true);
+		debugStartup("sharedGateway:kernelCreated");
 
 		try {
+			debugStartup("sharedGateway:connectWS:start");
 			await kernel.connectWebSocket();
+			debugStartup("sharedGateway:connectWS:done");
 			time("startWithSharedGateway:connectWS");
+			debugStartup("sharedGateway:initEnv:start");
 			await kernel.initializeKernelEnvironment(cwd, env);
+			debugStartup("sharedGateway:initEnv:done");
 			time("startWithSharedGateway:initEnv");
+			debugStartup("sharedGateway:prelude:start");
 			const preludeResult = await kernel.execute(PYTHON_PRELUDE, { silent: true, storeHistory: false });
+			debugStartup("sharedGateway:prelude:done");
 			time("startWithSharedGateway:prelude");
 			if (preludeResult.cancelled || preludeResult.status === "error") {
 				throw new Error("Failed to initialize Python kernel prelude");
 			}
+			debugStartup("sharedGateway:loadModules:start");
 			await loadPythonModules(kernel, { cwd });
+			debugStartup("sharedGateway:loadModules:done");
 			time("startWithSharedGateway:loadModules");
 			return kernel;
 		} catch (err: unknown) {
@@ -445,120 +457,6 @@ export class PythonKernel {
 		}
 	}
 
-	private static async startWithLocalGateway(options: KernelStartOptions): Promise<PythonKernel> {
-		const settings = await Settings.init();
-		const { shell, env } = settings.getShellConfig();
-		const filteredEnv = filterEnv(env);
-		const runtime = resolvePythonRuntime(options.cwd, filteredEnv);
-		const snapshotPath = await getOrCreateSnapshot(shell, env).catch((err: unknown) => {
-			logger.warn("Failed to resolve shell snapshot for Python kernel", {
-				error: err instanceof Error ? err.message : String(err),
-			});
-			return null;
-		});
-
-		const kernelEnv: Record<string, string | undefined> = {
-			...runtime.env,
-			...options.env,
-			PYTHONUNBUFFERED: "1",
-			OMP_SHELL_SNAPSHOT: snapshotPath ?? undefined,
-		};
-
-		const pythonPathParts = [options.cwd, kernelEnv.PYTHONPATH].filter(Boolean).join(path.delimiter);
-		if (pythonPathParts) {
-			kernelEnv.PYTHONPATH = pythonPathParts;
-		}
-
-		let gatewayProcess: ptree.ChildProcess | null = null;
-		let gatewayUrl: string | null = null;
-		let lastError: string | null = null;
-
-		for (let attempt = 0; attempt < GATEWAY_STARTUP_ATTEMPTS; attempt += 1) {
-			const gatewayPort = await allocatePort();
-			const candidateUrl = `http://127.0.0.1:${gatewayPort}`;
-			const candidateProcess = ptree.spawn(
-				[
-					runtime.pythonPath,
-					"-m",
-					"kernel_gateway",
-					"--KernelGatewayApp.ip=127.0.0.1",
-					`--KernelGatewayApp.port=${gatewayPort}`,
-					"--KernelGatewayApp.port_retries=0",
-					"--KernelGatewayApp.allow_origin=*",
-					"--JupyterApp.answer_yes=true",
-				],
-				{
-					cwd: options.cwd,
-					env: kernelEnv,
-				},
-			);
-
-			let exited = false;
-			candidateProcess.exited
-				.then(() => {
-					exited = true;
-				})
-				.catch(() => {
-					exited = true;
-				});
-
-			const startTime = Date.now();
-			while (Date.now() - startTime < GATEWAY_STARTUP_TIMEOUT_MS) {
-				if (exited) break;
-				try {
-					const response = await fetch(`${candidateUrl}/api/kernelspecs`);
-					if (response.ok) {
-						gatewayProcess = candidateProcess;
-						gatewayUrl = candidateUrl;
-						break;
-					}
-				} catch {
-					// Gateway not ready yet
-				}
-				await Bun.sleep(100);
-			}
-
-			if (gatewayProcess && gatewayUrl) break;
-
-			candidateProcess.kill();
-			lastError = exited ? "Kernel gateway process exited during startup" : "Kernel gateway failed to start";
-		}
-
-		if (!gatewayProcess || !gatewayUrl) {
-			throw new Error(lastError ?? "Kernel gateway failed to start");
-		}
-
-		const createResponse = await fetch(`${gatewayUrl}/api/kernels`, {
-			method: "POST",
-			headers: { "Content-Type": "application/json" },
-			body: JSON.stringify({ name: "python3" }),
-		});
-
-		if (!createResponse.ok) {
-			gatewayProcess.kill();
-			throw new Error(`Failed to create kernel: ${await createResponse.text()}`);
-		}
-
-		const kernelInfo = (await createResponse.json()) as { id: string };
-		const kernelId = kernelInfo.id;
-
-		const kernel = new PythonKernel(nanoid(), kernelId, gatewayProcess, gatewayUrl, nanoid(), "omp", false);
-
-		try {
-			await kernel.connectWebSocket();
-			await kernel.initializeKernelEnvironment(options.cwd, options.env);
-			const preludeResult = await kernel.execute(PYTHON_PRELUDE, { silent: true, storeHistory: false });
-			if (preludeResult.cancelled || preludeResult.status === "error") {
-				throw new Error("Failed to initialize Python kernel prelude");
-			}
-			await loadPythonModules(kernel, { cwd: options.cwd });
-			return kernel;
-		} catch (err: unknown) {
-			await kernel.shutdown();
-			throw err;
-		}
-	}
-
 	private async connectWebSocket(): Promise<void> {
 		const wsBase = this.gatewayUrl.replace(/^http/, "ws");
 		let wsUrl = `${wsBase}/api/kernels/${this.kernelId}/channels`;
@@ -949,14 +847,6 @@ export class PythonKernel {
 
 		if (this.isSharedGateway) {
 			await releaseSharedGateway();
-		} else if (this.gatewayProcess) {
-			try {
-				this.gatewayProcess.kill();
-			} catch (err: unknown) {
-				logger.warn("Failed to terminate gateway process", {
-					error: err instanceof Error ? err.message : String(err),
-				});
-			}
 		}
 	}
 

packages/coding-agent/src/main.ts

@@ -42,6 +42,11 @@ import { resolvePromptInput } from "./system-prompt";
 import { getChangelogPath, getNewEntries, parseChangelog } from "./utils/changelog";
 import { printTimings, time } from "./utils/timings";
 
+/** Conditional startup debug prints (stderr) when OMP_DEBUG_STARTUP is set */
+const debugStartup = process.env.OMP_DEBUG_STARTUP
+	? (stage: string) => process.stderr.write(`[startup] ${stage}\n`)
+	: () => {};
+
 async function checkForNewVersion(currentVersion: string): Promise<string | undefined> {
 	try {
 		const response = await fetch("https://registry.npmjs.org/@oh-my-pi/pi-coding-agent/latest");
@@ -477,10 +482,12 @@ async function buildSessionOptions(
 
 export async function main(args: string[]) {
 	time("start");
+	debugStartup("main:entry");
 
 	// Initialize theme early with defaults (CLI commands need symbols)
 	// Will be re-initialized with user preferences later
 	await initTheme();
+	debugStartup("main:initTheme");
 
 	// Handle plugin subcommand before regular parsing
 	const pluginCmd = parsePluginArgs(args);
@@ -582,15 +589,18 @@ export async function main(args: string[]) {
 	}
 
 	const parsed = parseArgs(args);
+	debugStartup("main:parseArgs");
 	time("parseArgs");
 	await maybeAutoChdir(parsed);
 
 	// Run migrations (pass cwd for project-local migrations)
 	const { migratedAuthProviders: migratedProviders, deprecationWarnings } = await runMigrations(process.cwd());
+	debugStartup("main:runMigrations");
 
 	// Create AuthStorage and ModelRegistry upfront
 	const authStorage = await discoverAuthStorage();
 	const modelRegistry = discoverModels(authStorage);
+	debugStartup("main:discoverModels");
 	time("discoverModels");
 
 	if (parsed.version) {
@@ -629,6 +639,7 @@ export async function main(args: string[]) {
 
 	const cwd = process.cwd();
 	await Settings.init({ cwd });
+	debugStartup("main:Settings.init");
 	time("Settings.init");
 	const pipedInput = await readPipedInput();
 	let { initialMessage, initialImages } = await prepareInitialMessage(parsed, settings.get("images.autoResize"));
@@ -657,6 +668,7 @@ export async function main(args: string[]) {
 	}
 
 	await initTheme(settings.get("theme"), isInteractive, settings.get("symbolPreset"), settings.get("colorBlindMode"));
+	debugStartup("main:initTheme2");
 	time("initTheme");
 
 	// Show deprecation warnings in interactive mode
@@ -676,6 +688,7 @@ export async function main(args: string[]) {
 
 	// Create session manager based on CLI flags
 	let sessionManager = await createSessionManager(parsed, cwd);
+	debugStartup("main:createSessionManager");
 	time("createSessionManager");
 
 	// Handle --resume: show session picker
@@ -696,6 +709,7 @@ export async function main(args: string[]) {
 	}
 
 	const sessionOptions = await buildSessionOptions(parsed, scopedModels, sessionManager, modelRegistry);
+	debugStartup("main:buildSessionOptions");
 	sessionOptions.authStorage = authStorage;
 	sessionOptions.modelRegistry = modelRegistry;
 	sessionOptions.hasUI = isInteractive;
@@ -712,6 +726,7 @@ export async function main(args: string[]) {
 	time("buildSessionOptions");
 	const { session, setToolUIContext, modelFallbackMessage, lspServers, mcpManager } =
 		await createAgentSession(sessionOptions);
+	debugStartup("main:createAgentSession");
 	time("createAgentSession");
 
 	// Re-parse CLI args with extension flags and apply values
@@ -729,6 +744,7 @@ export async function main(args: string[]) {
 		}
 	}
 	time("applyExtensionFlags");
+	debugStartup("main:applyExtensionFlags");
 
 	if (!isInteractive && !session.model) {
 		writeStderr(chalk.red("No models available."));
@@ -769,6 +785,7 @@ export async function main(args: string[]) {
 		}
 
 		printTimings();
+		debugStartup("main:runInteractiveMode:start");
 		await runInteractiveMode(
 			session,
 			VERSION,

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

@@ -53,6 +53,11 @@ import { getEditorTheme, getMarkdownTheme, onThemeChange, theme } from "./theme/
 import type { CompactionQueuedMessage, InteractiveModeContext, TodoItem } from "./types";
 import { UiHelpers } from "./utils/ui-helpers";
 
+/** Conditional startup debug prints (stderr) when OMP_DEBUG_STARTUP is set */
+const debugStartup = process.env.OMP_DEBUG_STARTUP
+	? (stage: string) => process.stderr.write(`[startup] ${stage}\n`)
+	: () => {};
+
 const TODO_FILE_NAME = "todos.json";
 
 /** Options for creating an InteractiveMode instance (for future API use) */
@@ -261,14 +266,18 @@ export class InteractiveMode implements InteractiveModeContext {
 
 	async init(): Promise<void> {
 		if (this.isInitialized) return;
+		debugStartup("InteractiveMode.init:entry");
 
 		this.keybindings = await KeybindingsManager.create();
+		debugStartup("InteractiveMode.init:keybindings");
 
 		// Register session manager flush for signal handlers (SIGINT, SIGTERM, SIGHUP)
 		this.cleanupUnsubscribe = postmortem.register("session-manager-flush", () => this.sessionManager.flush());
+		debugStartup("InteractiveMode.init:cleanupRegistered");
 
 		// Load and convert file commands to SlashCommand format (async)
 		const fileCommands = await loadSlashCommands({ cwd: process.cwd() });
+		debugStartup("InteractiveMode.init:slashCommands");
 		this.fileSlashCommands = new Set(fileCommands.map(cmd => cmd.name));
 		const fileSlashCommands: SlashCommand[] = fileCommands.map(cmd => ({
 			name: cmd.name,
@@ -291,6 +300,7 @@ export class InteractiveMode implements InteractiveModeContext {
 			name: s.name,
 			timeAgo: s.timeAgo,
 		}));
+		debugStartup("InteractiveMode.init:recentSessions");
 
 		// Convert LSP servers to welcome format
 		const lspServerInfo =
@@ -304,7 +314,9 @@ export class InteractiveMode implements InteractiveModeContext {
 
 		if (!startupQuiet) {
 			// Add welcome header
+			debugStartup("InteractiveMode.init:welcomeComponent:start");
 			const welcome = new WelcomeComponent(this.version, modelName, providerName, recentSessions, lspServerInfo);
+			debugStartup("InteractiveMode.init:welcomeComponent:created");
 
 			// Setup UI layout
 			this.ui.addChild(new Spacer(1));

packages/coding-agent/src/sdk.ts

@@ -111,6 +111,11 @@ import { wrapToolsWithMetaNotice } from "./tools/output-meta";
 import { EventBus } from "./utils/event-bus";
 import { time } from "./utils/timings";
 
+/** Conditional startup debug prints (stderr) when OMP_DEBUG_STARTUP is set */
+const debugStartup = process.env.OMP_DEBUG_STARTUP
+	? (stage: string) => process.stderr.write(`[startup] ${stage}\n`)
+	: () => {};
+
 // Types
 export interface CreateAgentSessionOptions {
 	/** Working directory for project-local discovery. Default: process.cwd() */
@@ -568,6 +573,7 @@ function createCustomToolsExtension(tools: CustomTool[]): ExtensionFactory {
  * ```
  */
 export async function createAgentSession(options: CreateAgentSessionOptions = {}): Promise<CreateAgentSessionResult> {
+	debugStartup("sdk:createAgentSession:entry");
 	const cwd = options.cwd ?? process.cwd();
 	const agentDir = options.agentDir ?? getDefaultAgentDir();
 	const eventBus = options.eventBus ?? new EventBus();
@@ -681,6 +687,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		skillWarnings = discovered.warnings;
 	}
 	time("discoverSkills");
+	debugStartup("sdk:discoverSkills");
 
 	// Discover rules
 	const ttsrSettings = settingsInstance.getGroup("ttsr");
@@ -703,6 +710,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 	time("filterRulebookRules");
 
 	const contextFiles = options.contextFiles ?? (await discoverContextFiles(cwd, agentDir));
+	debugStartup("sdk:discoverContextFiles");
 	time("discoverContextFiles");
 
 	let agent: Agent;
@@ -766,11 +774,14 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 	toolSession.getArtifactsDir = getArtifactsDir;
 	toolSession.agentOutputManager = new AgentOutputManager(getArtifactsDir);
 
+	debugStartup("sdk:createTools:start");
 	// Create and wrap tools with meta notice formatting
 	const rawBuiltinTools = await createTools(toolSession, options.toolNames);
 	const builtinTools = wrapToolsWithMetaNotice(rawBuiltinTools);
+	debugStartup("sdk:createTools");
 	time("createAllTools");
 
+	debugStartup("sdk:discoverMCP:start");
 	// Discover MCP tools from .mcp.json files
 	let mcpManager: MCPManager | undefined;
 	const enableMCP = options.enableMCP ?? true;
@@ -791,6 +802,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			cacheStorage: settingsInstance.getStorage(),
 		});
 		time("discoverAndLoadMCPTools");
+		debugStartup("sdk:discoverAndLoadMCPTools");
 		mcpManager = mcpResult.manager;
 		toolSession.mcpManager = mcpManager;
 
@@ -810,6 +822,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		}
 	}
 
+	debugStartup("sdk:geminiImageTools:start");
 	// Add Gemini image tools if GEMINI_API_KEY (or GOOGLE_API_KEY) is available
 	const geminiImageTools = await getGeminiImageTools();
 	if (geminiImageTools.length > 0) {
@@ -837,6 +850,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		inlineExtensions.push(createCustomToolsExtension(customTools));
 	}
 
+	debugStartup("sdk:loadExtensions:start");
 	// Load extensions (discovers from standard locations + configured paths)
 	let extensionsResult: LoadExtensionsResult;
 	if (options.disableExtensionDiscovery) {
@@ -861,6 +875,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 			(settingsInstance.get("disabledExtensions") as string[]) ?? [],
 		);
 		time("discoverAndLoadExtensions");
+		debugStartup("sdk:discoverAndLoadExtensions");
 		for (const { path, error } of extensionsResult.errors) {
 			logger.error("Failed to load extension", { path, error });
 		}
@@ -1023,6 +1038,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 	}
 
 	const systemPrompt = await rebuildSystemPrompt(initialToolNames, toolRegistry);
+	debugStartup("sdk:buildSystemPrompt:done");
 	time("buildSystemPrompt");
 
 	const promptTemplates = options.promptTemplates ?? (await discoverPromptTemplates(cwd, agentDir));
@@ -1109,6 +1125,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		cursorExecHandlers,
 	});
 	cursorEventEmitter = event => agent.emitExternalEvent(event);
+	debugStartup("sdk:createAgent");
 	time("createAgent");
 
 	// Restore messages if session has existing data
@@ -1139,12 +1156,14 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		rebuildSystemPrompt,
 		ttsrManager,
 	});
+	debugStartup("sdk:createAgentSession");
 	time("createAgentSession");
 
 	// Warm up LSP servers (connects to detected servers)
 	let lspServers: CreateAgentSessionResult["lspServers"];
 	if (enableLsp && settingsInstance.get("lsp.diagnosticsOnWrite")) {
 		try {
+			debugStartup("sdk:warmupLspServers:start");
 			const result = await warmupLspServers(cwd, {
 				onConnecting: serverNames => {
 					if (options.hasUI && serverNames.length > 0) {
@@ -1152,6 +1171,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 					}
 				},
 			});
+			debugStartup("sdk:warmupLspServers:done");
 			lspServers = result.servers;
 			time("warmupLspServers");
 		} catch (error) {
@@ -1159,6 +1179,7 @@ export async function createAgentSession(options: CreateAgentSessionOptions = {}
 		}
 	}
 
+	debugStartup("sdk:return");
 	return {
 		session,
 		extensionsResult,