docs: Announce the registered live dev folder in the build log

skills/actor-runtime/SKILL.md

@@ -86,7 +86,8 @@ build, so changing them needs an `apify push`.
 ## Iterate without rebuilding (dev folder)
 
 After that first `apify push`, the runtime registers the pushed directory as the Actor's **dev
-folder** and bind-mounts it into every later run. Edit locally, recompile locally (`tsc`, or the
+folder** (the build log's `Registered live dev folder` line confirms it) and bind-mounts it into every
+later run. Edit locally, recompile locally (`tsc`, or the
 language equivalent), and `apify call` again - no `apify push`, no rebuild:
 
 ```sh

src/services/builds.ts

@@ -12,6 +12,7 @@ import { resolveDockerfileLocation } from './dockerfile-location.js';
 import { resolveInputSchemaLocation } from './input-schema-location.js';
 import { resolveActorMemorySettings } from './actor-memory.js';
 import { appendLog, appendRuntimeLog, flushLog, markLogTerminal } from './logs.js';
+import { registeredDevFolderBuildLine } from './dev-folder.js';
 import { isTerminalJobStatus, transitionJobStatus } from './job-status.js';
 
 /**
@@ -255,6 +256,12 @@ export async function runBuildInBackground(
 			},
 			(chunk) => appendLog(record.id, chunk),
 		);
+		// Re-read: `apify push` registers the folder right before starting the build, and the console can
+		// change it mid-build. No working directory means no mount - the run log explains that instead.
+		const { localDevFolder } = (await getRegistries().actors.get(actor.id)) ?? actor;
+		if (localDevFolder && outcome.imageWorkingDirectory) {
+			appendRuntimeLog(record.id, registeredDevFolderBuildLine(localDevFolder, outcome.imageWorkingDirectory));
+		}
 		// Flush before writing the terminal status, not after (mirrors the same fix in
 		// `services/runs.ts`'s `runInBackground`): by the time `driver.startBuild` resolves every `onLog`
 		// call has already happened, so flushing here guarantees the persisted log is complete before a

src/services/dev-folder.ts

@@ -131,6 +131,17 @@ export function devFolderStatus(actor: ActorRecord): DevFolderStatus {
 	return { localDevFolder: actor.localDevFolder ?? null };
 }
 
+/** Printed at the end of a successful build, so whoever pushed learns their folder is live without the
+ * CLI having to know anything about the runtime. */
+export function registeredDevFolderBuildLine(localDevFolder: string, imageWorkingDirectory: string): string {
+	return (
+		`Registered live dev folder: ${localDevFolder}. Runs mount it over this build's working directory ` +
+		`${imageWorkingDirectory}, so local edits apply on the next \`apify call\` without another push. A compiled ` +
+		'Actor (e.g. TypeScript) needs its local build first. Use `apify call --no-dev-folder` to run from the ' +
+		'built image alone.'
+	);
+}
+
 /** Printed when a registered dev folder cannot be mounted because the run's build has no working
  * directory (an image with no `WORKDIR`, or `/`). The run is unaffected; only the silence is. */
 export function unknownWorkingDirectoryLine(localDevFolder: string): string {

test/integration/job-lifecycle.test.ts

@@ -1022,3 +1022,48 @@ describe('startRun (public entry point) still creates a normal READY record the
 		await waitForRunFinish(record.id, 5);
 	});
 });
+
+describe('build log: the registered live dev folder', () => {
+	let server: TestServerHandle;
+
+	afterEach(async () => {
+		await server.close();
+	});
+
+	async function buildWith(localDevFolder: string | undefined, imageWorkingDirectory: string | undefined) {
+		server = await startTestServer(fixedBuildOutcomeDriver({ imageId: 'x', imageWorkingDirectory }));
+		const seeded = await seedActor(server, 'dev-folder-build-log-actor');
+		// Registered after the caller's snapshot, like `apify push` does right before starting the build.
+		await getRegistries().actors.update(seeded.id, (current) =>
+			current ? { ...current, localDevFolder } : current,
+		);
+		const record: BuildRecord = {
+			id: generateId(),
+			userId: seeded.userId,
+			actorId: seeded.id,
+			versionNumber: '0.0',
+			buildNumber: '0.0.1',
+			tag: 'latest',
+			status: 'READY',
+			startedAt: new Date().toISOString(),
+		};
+		await getRegistries().builds.set(record.id, record);
+		await runBuildInBackground(server.driver, seeded, VERSION, record, { tag: 'latest', useCache: true });
+		return getFullLog(record.id);
+	}
+
+	it('a successful build announces the registered folder as a runtime line', async () => {
+		const log = await buildWith('/home/me/actor', '/usr/src/app');
+		const line = log.split('\n').find((l) => l.includes('Registered live dev folder'));
+		expect(line).toContain('[actor-runtime]');
+		expect(line).toContain('/home/me/actor');
+		expect(line).toContain('/usr/src/app');
+		expect(line).toContain('apify call --no-dev-folder');
+	});
+
+	it('says nothing without a registration, or without a working directory to mount over', async () => {
+		expect(await buildWith(undefined, '/usr/src/app')).not.toContain('Registered live dev folder');
+		await server.close();
+		expect(await buildWith('/home/me/actor', undefined)).not.toContain('Registered live dev folder');
+	});
+});