fix(drive): emit watch events as single-line NDJSON in json mode

src/handwritten/commands/drive/watch.ts

@@ -3,7 +3,7 @@ import chokidar from "chokidar"
 import { watch as fsWatch } from "node:fs"
 import { relative, resolve } from "node:path"
 import { loadRealtimeAuthHeaders } from "../../auth/load-sdk-client.js"
-import { render } from "../../output/render.js"
+import { render, shouldOutputJson } from "../../output/render.js"
 import { createDriveRealtimeSource } from "./realtime.js"
 import { DRIVE_DIR, ensureDriveRealtimeState, readDriveState, writeDriveRealtimeState } from "./state.js"
 import { runDriveSyncOnce, type DriveSyncSummary } from "./sync.js"
@@ -45,7 +45,18 @@ export async function runDriveWatch(root: string, options: DriveWatchOptions = {
   const runSync = options.runSync ?? runDriveSyncOnce
   const debounceMs = options.debounceMs ?? 500
   const remoteDebounceMs = options.remoteDebounceMs ?? 2000
-  const emit = options.onEvent ?? ((event) => render({ kind: "drive_watch", display: { shape: "object" } }, event))
+  // Watch is a long-lived event stream: json mode must emit NDJSON (one event
+  // per line) so consumers can parse line-by-line instead of reassembling
+  // pretty-printed multi-line JSON, which breaks past any buffer limit.
+  const emit =
+    options.onEvent ??
+    ((event: unknown) => {
+      if (shouldOutputJson()) {
+        process.stdout.write(`${JSON.stringify(event)}\n`)
+        return
+      }
+      render({ kind: "drive_watch", display: { shape: "object" } }, event)
+    })
   let debounceTimer: NodeJS.Timeout | undefined
   let debounceDeadlineMs: number | undefined
   let retryTimer: NodeJS.Timeout | undefined

src/handwritten/output/render.ts

@@ -104,7 +104,7 @@ function drillDataPath(data: unknown, dataPath: string | undefined): unknown {
   return value === undefined ? data : value
 }
 
-function shouldOutputJson(): boolean {
+export function shouldOutputJson(): boolean {
   // Explicit overrides win over TTY detection so users can force either
   // mode in CI logs, screenshots, captured output, or AI-driven tooling.
   if (process.env.WSPC_OUTPUT === "json") return true

test/handwritten/drive/watch.test.ts

@@ -91,6 +91,34 @@ describe("drive watch", () => {
     vi.useRealTimers()
   })
 
+  it("emits single-line NDJSON events in json output mode", async () => {
+    const source = fakeSource()
+    const runSync = vi.fn(async () =>
+      syncSummary({} as never) as ReturnType<typeof syncSummary> & { paths: unknown[] },
+    )
+    const writes: string[] = []
+    const writeSpy = vi.spyOn(process.stdout, "write").mockImplementation(((chunk: unknown) => {
+      writes.push(String(chunk))
+      return true
+    }) as never)
+    vi.stubEnv("WSPC_OUTPUT", "json")
+
+    try {
+      await runDriveWatch("/tmp/root", { source, runSync, readState, once: true })
+    } finally {
+      writeSpy.mockRestore()
+      vi.unstubAllEnvs()
+    }
+
+    expect(writes.length).toBeGreaterThan(0)
+    for (const chunk of writes) {
+      expect(chunk.endsWith("\n")).toBe(true)
+      const line = chunk.slice(0, -1)
+      expect(line).not.toContain("\n")
+      expect(() => JSON.parse(line)).not.toThrow()
+    }
+  })
+
   it("runs one sync on startup", async () => {
     const source = fakeSource()
     const onEvent = vi.fn()