diff --git a/electron/electron-env.d.ts b/electron/electron-env.d.ts
index 1b82992..b86c4ff 100644
--- a/electron/electron-env.d.ts
+++ b/electron/electron-env.d.ts
@@ -81,6 +81,12 @@ interface Window {
 			message?: string;
 			error?: string;
 		}>;
+		openRecordingStream: (fileName: string) => Promise<{ success: boolean; error?: string }>;
+		appendRecordingChunk: (
+			fileName: string,
+			chunk: ArrayBuffer,
+		) => Promise<{ success: boolean; error?: string }>;
+		closeRecordingStream: (fileName: string) => Promise<{ success: boolean; error?: string }>;
 		getRecordedVideoPath: () => Promise<{
 			success: boolean;
 			path?: string;
diff --git a/electron/ipc/handlers.ts b/electron/ipc/handlers.ts
index 669f1dd..d367728 100644
--- a/electron/ipc/handlers.ts
+++ b/electron/ipc/handlers.ts
@@ -40,7 +40,9 @@ import { RECORDINGS_DIR } from "../main";
 import { createCursorRecordingSession } from "../native-bridge/cursor/recording/factory";
 import { requestMacCursorAccessibilityAccess } from "../native-bridge/cursor/recording/macNativeCursorRecordingSession";
 import type { CursorRecordingSession } from "../native-bridge/cursor/recording/session";
+import { patchWebmDurationOnDisk } from "../recording/webm-duration";
 import { registerNativeBridgeHandlers } from "./nativeBridge";
+import { RecordingStreamRegistry, registerRecordingStreamHandlers } from "./recordingStream";
 
 const PROJECT_FILE_EXTENSION = "openscreen";
 const SHORTCUTS_FILE = path.join(app.getPath("userData"), "shortcuts.json");
@@ -265,6 +267,30 @@ function resolveRecordingOutputPath(fileName: string): string {
 	return path.join(RECORDINGS_DIR, parsedPath.base);
 }
 
+function isValidDurationMs(value: number | undefined): value is number {
+	return typeof value === "number" && Number.isFinite(value) && value > 0;
+}
+
+/**
+ * Finalize a single recording file: if it was streamed to disk, flush and close
+ * the stream; otherwise (a short recording, or the stream failed to open and the
+ * renderer fell back to in-memory buffering) write the buffered bytes. Returns
+ * whether the file was streamed, which the caller uses to decide whether the
+ * WebM duration needs patching on disk.
+ */
+async function finalizeRecordingFile(
+	registry: RecordingStreamRegistry,
+	fileName: string,
+	filePath: string,
+	videoData?: ArrayBuffer,
+): Promise<boolean> {
+	const streamed = await registry.finalize(fileName);
+	if (!streamed && videoData && videoData.byteLength > 0) {
+		await fs.writeFile(filePath, Buffer.from(videoData));
+	}
+	return streamed;
+}
+
 async function getApprovedProjectSession(
 	project: unknown,
 	projectFilePath?: string,
@@ -2141,6 +2167,12 @@ export function registerIpcHandlers(
 		},
 	);
 
+	// On-disk write streams for in-progress recordings, keyed by output file name.
+	// Chunks are appended as they arrive from ondataavailable so the renderer
+	// never buffers the full video in memory (the #616 fix).
+	const recordingStreams = new RecordingStreamRegistry();
+	registerRecordingStreamHandlers(ipcMain, recordingStreams, resolveRecordingOutputPath);
+
 	ipcMain.handle("store-recorded-session", async (_, payload: StoreRecordedSessionInput) => {
 		try {
 			return await storeRecordedSessionFiles(payload);
@@ -2161,12 +2193,37 @@ export function registerIpcHandlers(
 				: Date.now();
 		const cursorCaptureMode = normalizeCursorCaptureMode(payload.cursorCaptureMode);
 		const screenVideoPath = resolveRecordingOutputPath(payload.screen.fileName);
-		await fs.writeFile(screenVideoPath, Buffer.from(payload.screen.videoData));
+		const screenStreamed = await finalizeRecordingFile(
+			recordingStreams,
+			payload.screen.fileName,
+			screenVideoPath,
+			payload.screen.videoData,
+		);
 
 		let webcamVideoPath: string | undefined;
+		let webcamStreamed = false;
 		if (payload.webcam) {
 			webcamVideoPath = resolveRecordingOutputPath(payload.webcam.fileName);
-			await fs.writeFile(webcamVideoPath, Buffer.from(payload.webcam.videoData));
+			webcamStreamed = await finalizeRecordingFile(
+				recordingStreams,
+				payload.webcam.fileName,
+				webcamVideoPath,
+				payload.webcam.videoData,
+			);
+		}
+
+		// Streamed files lack the WebM Duration header (the renderer no longer holds
+		// the blob to patch). Patch on disk so the editor's seek bar and timeline
+		// work. Best-effort and independent per file, so the patches run together.
+		if (isValidDurationMs(payload.durationMs)) {
+			const patches: Promise<unknown>[] = [];
+			if (screenStreamed) {
+				patches.push(patchWebmDurationOnDisk(screenVideoPath, payload.durationMs));
+			}
+			if (webcamStreamed && webcamVideoPath) {
+				patches.push(patchWebmDurationOnDisk(webcamVideoPath, payload.durationMs));
+			}
+			await Promise.all(patches);
 		}
 
 		const session: RecordingSession = webcamVideoPath
diff --git a/electron/ipc/recordingStream.test.ts b/electron/ipc/recordingStream.test.ts
new file mode 100644
index 0000000..776fcf1
--- /dev/null
+++ b/electron/ipc/recordingStream.test.ts
@@ -0,0 +1,84 @@
+import { mkdtemp, readFile, rm, stat } from "node:fs/promises";
+import { tmpdir } from "node:os";
+import path from "node:path";
+import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { RecordingStreamRegistry } from "./recordingStream";
+
+describe("RecordingStreamRegistry", () => {
+	let dir: string;
+	const pathFor = (name: string) => path.join(dir, name);
+
+	beforeEach(async () => {
+		dir = await mkdtemp(path.join(tmpdir(), "openscreen-stream-"));
+	});
+
+	afterEach(async () => {
+		await rm(dir, { recursive: true, force: true });
+	});
+
+	it("streams chunks to disk in order and reports streamed on finalize", async () => {
+		const registry = new RecordingStreamRegistry();
+		await registry.open("rec.webm", pathFor("rec.webm"));
+		await registry.append("rec.webm", Buffer.from("hello "));
+		await registry.append("rec.webm", Buffer.from("world"));
+
+		const streamed = await registry.finalize("rec.webm");
+
+		expect(streamed).toBe(true);
+		expect(await readFile(pathFor("rec.webm"), "utf8")).toBe("hello world");
+		// A second finalize has nothing to close.
+		expect(await registry.finalize("rec.webm")).toBe(false);
+	});
+
+	it("reports not-streamed when no stream was opened", async () => {
+		const registry = new RecordingStreamRegistry();
+		expect(await registry.finalize("missing.webm")).toBe(false);
+		expect(registry.has("missing.webm")).toBe(false);
+	});
+
+	it("rejects open when the target path is not writable (open is awaited, not assumed)", async () => {
+		const registry = new RecordingStreamRegistry();
+		// Parent directory does not exist, so createWriteStream emits 'error' on open.
+		await expect(
+			registry.open("rec.webm", path.join(dir, "does-not-exist", "rec.webm")),
+		).rejects.toThrow();
+		// A failed open must not register a stream the renderer would treat as live.
+		expect(registry.has("rec.webm")).toBe(false);
+	});
+
+	it("rejects append when no stream is open", async () => {
+		const registry = new RecordingStreamRegistry();
+		await expect(registry.append("rec.webm", Buffer.from("x"))).rejects.toThrow(
+			/No active recording stream/,
+		);
+	});
+
+	it("discard closes the stream and removes the partial file", async () => {
+		const registry = new RecordingStreamRegistry();
+		await registry.open("rec.webm", pathFor("rec.webm"));
+		await registry.append("rec.webm", Buffer.from("partial"));
+
+		await registry.discard("rec.webm", pathFor("rec.webm"));
+
+		expect(registry.has("rec.webm")).toBe(false);
+		await expect(stat(pathFor("rec.webm"))).rejects.toThrow();
+		// Nothing left to finalize after a discard.
+		expect(await registry.finalize("rec.webm")).toBe(false);
+	});
+
+	it("discard tolerates a missing file", async () => {
+		const registry = new RecordingStreamRegistry();
+		await expect(registry.discard("never.webm", pathFor("never.webm"))).resolves.toBeUndefined();
+	});
+
+	it("opening the same file twice replaces the prior stream", async () => {
+		const registry = new RecordingStreamRegistry();
+		await registry.open("rec.webm", pathFor("rec.webm"));
+		await registry.append("rec.webm", Buffer.from("first"));
+		await registry.open("rec.webm", pathFor("rec.webm"));
+		await registry.append("rec.webm", Buffer.from("second"));
+		await registry.finalize("rec.webm");
+
+		expect(await readFile(pathFor("rec.webm"), "utf8")).toBe("second");
+	});
+});
diff --git a/electron/ipc/recordingStream.ts b/electron/ipc/recordingStream.ts
new file mode 100644
index 0000000..3dce5b9
--- /dev/null
+++ b/electron/ipc/recordingStream.ts
@@ -0,0 +1,147 @@
+import { createWriteStream, type WriteStream } from "node:fs";
+import { unlink } from "node:fs/promises";
+import type { IpcMain } from "electron";
+
+/**
+ * Owns the lifecycle of on-disk write streams for in-progress recordings, keyed
+ * by the recording's output file name. Browser MediaRecorder chunks are appended
+ * here as they arrive so a long recording never buffers the whole video in the
+ * renderer (the #616 fix).
+ *
+ * The file name is the key because it is the one value the renderer and main
+ * process already exchange and it is globally unique per recording, so there is
+ * no derived/offset key to keep in sync across the IPC boundary.
+ */
+export class RecordingStreamRegistry {
+	private readonly streams = new Map<string, WriteStream>();
+
+	/**
+	 * Open a write stream and resolve only once the OS confirms it is writable.
+	 * Resolving on the `open` event (rather than on `createWriteStream` returning)
+	 * means a bad path or permission error rejects here instead of surfacing as a
+	 * silent chunk drop later, so the renderer's fallback can take over.
+	 */
+	async open(fileName: string, filePath: string): Promise<void> {
+		await this.endStream(fileName);
+
+		const ws = createWriteStream(filePath, { flags: "w" });
+		await new Promise<void>((resolve, reject) => {
+			const onError = (error: Error) => reject(error);
+			ws.once("error", onError);
+			ws.once("open", () => {
+				ws.removeListener("error", onError);
+				resolve();
+			});
+		});
+		// Keep a listener for the stream's lifetime so a late error logs rather
+		// than crashing the main process with an unhandled 'error' event. Per-write
+		// failures still surface through the `append` callback below.
+		ws.on("error", (error) => {
+			console.error(`[recording-stream] ${fileName}:`, error);
+		});
+
+		this.streams.set(fileName, ws);
+	}
+
+	has(fileName: string): boolean {
+		return this.streams.has(fileName);
+	}
+
+	/** Append a chunk; rejects if no stream is open or the write fails. */
+	async append(fileName: string, chunk: Buffer): Promise<void> {
+		const ws = this.streams.get(fileName);
+		if (!ws) {
+			throw new Error(`No active recording stream for ${fileName}`);
+		}
+		await new Promise<void>((resolve, reject) => {
+			ws.write(chunk, (error) => (error ? reject(error) : resolve()));
+		});
+	}
+
+	/**
+	 * Flush and close the stream, keeping the file. Returns whether a stream was
+	 * open — i.e. whether the recording was streamed to disk (true) or needs its
+	 * in-memory buffer written by the caller (false).
+	 */
+	async finalize(fileName: string): Promise<boolean> {
+		const ws = this.streams.get(fileName);
+		if (!ws) {
+			return false;
+		}
+		this.streams.delete(fileName);
+		await new Promise<void>((resolve, reject) => {
+			ws.end((error?: Error | null) => (error ? reject(error) : resolve()));
+		});
+		return true;
+	}
+
+	/**
+	 * Close the stream (if any) and delete the partial file. Used when a streamed
+	 * recording is discarded or fails before a successful save, so cancelled runs
+	 * don't leak file descriptors or orphan partial recordings on disk.
+	 */
+	async discard(fileName: string, filePath: string): Promise<void> {
+		await this.endStream(fileName);
+		await unlink(filePath).catch(() => undefined);
+	}
+
+	private async endStream(fileName: string): Promise<void> {
+		const ws = this.streams.get(fileName);
+		if (!ws) {
+			return;
+		}
+		this.streams.delete(fileName);
+		await new Promise<void>((resolve) => ws.end(() => resolve()));
+	}
+}
+
+/**
+ * Register the streaming IPC handlers. Thin wrappers that translate the
+ * registry's throw-on-failure contract into the `{ success, error }` shape the
+ * renderer expects.
+ */
+export function registerRecordingStreamHandlers(
+	ipcMain: IpcMain,
+	registry: RecordingStreamRegistry,
+	resolveRecordingOutputPath: (fileName: string) => string,
+): void {
+	ipcMain.handle(
+		"open-recording-stream",
+		async (_, fileName: string): Promise<{ success: boolean; error?: string }> => {
+			try {
+				await registry.open(fileName, resolveRecordingOutputPath(fileName));
+				return { success: true };
+			} catch (error) {
+				return { success: false, error: String(error) };
+			}
+		},
+	);
+
+	ipcMain.handle(
+		"append-recording-chunk",
+		async (
+			_,
+			fileName: string,
+			chunk: ArrayBuffer,
+		): Promise<{ success: boolean; error?: string }> => {
+			try {
+				await registry.append(fileName, Buffer.from(chunk));
+				return { success: true };
+			} catch (error) {
+				return { success: false, error: String(error) };
+			}
+		},
+	);
+
+	ipcMain.handle(
+		"close-recording-stream",
+		async (_, fileName: string): Promise<{ success: boolean; error?: string }> => {
+			try {
+				await registry.discard(fileName, resolveRecordingOutputPath(fileName));
+				return { success: true };
+			} catch (error) {
+				return { success: false, error: String(error) };
+			}
+		},
+	);
+}
diff --git a/electron/preload.ts b/electron/preload.ts
index 933ce9d..4b69740 100644
--- a/electron/preload.ts
+++ b/electron/preload.ts
@@ -64,6 +64,15 @@ contextBridge.exposeInMainWorld("electronAPI", {
 	storeRecordedSession: (payload: StoreRecordedSessionInput) => {
 		return ipcRenderer.invoke("store-recorded-session", payload);
 	},
+	openRecordingStream: (fileName: string) => {
+		return ipcRenderer.invoke("open-recording-stream", fileName);
+	},
+	appendRecordingChunk: (fileName: string, chunk: ArrayBuffer) => {
+		return ipcRenderer.invoke("append-recording-chunk", fileName, chunk);
+	},
+	closeRecordingStream: (fileName: string) => {
+		return ipcRenderer.invoke("close-recording-stream", fileName);
+	},
 
 	getRecordedVideoPath: () => {
 		return ipcRenderer.invoke("get-recorded-video-path");
diff --git a/electron/recording/webm-duration.ts b/electron/recording/webm-duration.ts
new file mode 100644
index 0000000..5b2c197
--- /dev/null
+++ b/electron/recording/webm-duration.ts
@@ -0,0 +1,97 @@
+import fs from "node:fs/promises";
+import { fixParsedWebmDuration } from "@fix-webm-duration/fix";
+import { WebmFile } from "@fix-webm-duration/parser";
+
+export type DurationPatchResult =
+	| { patched: true }
+	| { patched: false; reason: "no-section" | "already-valid" | "io-error" | "internal" };
+
+/**
+ * Patch the WebM Duration header on a finalized recording file.
+ *
+ * Browser MediaRecorder writes WebM with no Duration EBML element. With the
+ * streaming-to-disk path the renderer never holds the blob, so the historical
+ * `fixWebmDuration(blob, durationMs)` call can't run. Patching on disk after
+ * `WriteStream.end()` produces an equivalent result: the editor's seek bar and
+ * timeline read a real duration instead of `N/A`.
+ *
+ * Atomic by design: writes the patched bytes to `<filePath>.duration-patch.tmp`
+ * and renames in place. If the process crashes mid-rewrite, the original file
+ * survives intact, so the user never loses their recording to a partial write.
+ *
+ * Best-effort by intent: any failure (read, parse, write) logs and returns a
+ * non-`patched` result rather than throwing. The file is still playable without
+ * the patch (decoders walk frames sequentially); the only cost is that the
+ * editor's seek bar and timeline break until it is patched.
+ *
+ * Memory: reads the whole file into a main-process Buffer, the same footprint
+ * as the pre-streaming renderer path, just on the side without V8's heap cap.
+ */
+export async function patchWebmDurationOnDisk(
+	filePath: string,
+	durationMs: number,
+): Promise<DurationPatchResult> {
+	try {
+		const fileBytes = await fs.readFile(filePath);
+		const webm = new WebmFile(new Uint8Array(fileBytes));
+
+		const patched = fixParsedWebmDuration(webm, durationMs, { logger: false });
+		if (!patched) {
+			// fixParsedWebmDuration returns false for: missing Segment, missing
+			// Info, or a Duration that is already valid. The first two mean a
+			// malformed (most likely truncated) file; the third is a no-op.
+			const reason = inferUnpatchedReason(webm);
+			if (reason === "no-section") {
+				console.warn(
+					`[webm-duration] no Segment/Info section in ${filePath}; file may be truncated`,
+				);
+			}
+			return { patched: false, reason };
+		}
+
+		if (!webm.source) {
+			console.error(`[webm-duration] patched but source missing for ${filePath}`);
+			return { patched: false, reason: "internal" };
+		}
+
+		const tmpPath = `${filePath}.duration-patch.tmp`;
+		const patchedBytes = Buffer.from(
+			webm.source.buffer,
+			webm.source.byteOffset,
+			webm.source.byteLength,
+		);
+		try {
+			await fs.writeFile(tmpPath, patchedBytes);
+			await fs.rename(tmpPath, filePath);
+			return { patched: true };
+		} catch (writeError) {
+			console.error(`[webm-duration] failed to write patched ${filePath}:`, writeError);
+			// Best-effort cleanup of the temp file; if unlink also fails, leave it.
+			// The original recording is untouched because the rename never ran.
+			await fs.unlink(tmpPath).catch(() => undefined);
+			return { patched: false, reason: "io-error" };
+		}
+	} catch (error) {
+		console.error(`[webm-duration] failed to patch ${filePath}:`, error);
+		return { patched: false, reason: "io-error" };
+	}
+}
+
+/**
+ * Distinguish "no Segment/Info section" (malformed/truncated file) from "Info
+ * present but Duration already valid" (patch unnecessary).
+ *
+ * The IDs are the length-descriptor-stripped form that @fix-webm-duration/parser
+ * uses as its lookup keys (Segment `0x8538067`, Info `0x549a966`), verified
+ * against the parser's `src/lib/sections.js` — not the canonical 4-byte EBML
+ * IDs (`0x18538067` / `0x1549A966`), which this parser's `getSectionById` would
+ * never match.
+ */
+function inferUnpatchedReason(webm: WebmFile): "no-section" | "already-valid" {
+	const segment = webm.getSectionById?.(0x8538067);
+	if (!segment) return "no-section";
+	const info = (
+		segment as unknown as { getSectionById?: (id: number) => unknown }
+	).getSectionById?.(0x549a966);
+	return info ? "already-valid" : "no-section";
+}
diff --git a/src/hooks/recorderHandle.test.ts b/src/hooks/recorderHandle.test.ts
new file mode 100644
index 0000000..3f16437
--- /dev/null
+++ b/src/hooks/recorderHandle.test.ts
@@ -0,0 +1,264 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { createRecorderHandle } from "./recorderHandle";
+
+type ElectronAPI = Window["electronAPI"];
+
+const tick = () => new Promise<void>((resolve) => setTimeout(resolve, 0));
+const decode = (buffer: ArrayBuffer) => new TextDecoder().decode(new Uint8Array(buffer));
+
+/** Minimal MediaRecorder stand-in the tests can drive directly. */
+class FakeMediaRecorder {
+	ondataavailable: ((event: BlobEvent) => void) | null = null;
+	onstop: (() => void) | null = null;
+	onerror: (() => void) | null = null;
+	state: "inactive" | "recording" = "inactive";
+
+	start(): void {
+		this.state = "recording";
+	}
+
+	stop(): void {
+		this.state = "inactive";
+		this.onstop?.();
+	}
+
+	emit(data: Blob): void {
+		this.ondataavailable?.({ data } as BlobEvent);
+	}
+}
+
+function stubElectronAPI(api: Partial<ElectronAPI>): void {
+	window.electronAPI = api as unknown as ElectronAPI;
+}
+
+function driver(handle: { recorder: MediaRecorder }): FakeMediaRecorder {
+	return handle.recorder as unknown as FakeMediaRecorder;
+}
+
+describe("createRecorderHandle", () => {
+	beforeEach(() => {
+		vi.stubGlobal("MediaRecorder", FakeMediaRecorder);
+	});
+
+	afterEach(() => {
+		vi.unstubAllGlobals();
+		window.electronAPI = undefined as unknown as ElectronAPI;
+	});
+
+	it("streams chunks to disk in arrival order and resolves an empty blob", async () => {
+		const appended: string[] = [];
+		const openRecordingStream = vi.fn(async () => ({ success: true }));
+		const appendRecordingChunk = vi.fn(async (_fileName: string, buffer: ArrayBuffer) => {
+			appended.push(decode(buffer));
+			return { success: true };
+		});
+		stubElectronAPI({ openRecordingStream, appendRecordingChunk });
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" }, "rec.webm");
+		const fake = driver(handle);
+
+		fake.emit(new Blob(["a"])); // arrives before open resolves -> buffered
+		await tick(); // open resolves -> buffered chunk flushes, mode becomes streaming
+		fake.emit(new Blob(["b"]));
+		fake.emit(new Blob(["c"]));
+		fake.stop();
+
+		const blob = await handle.recordedBlobPromise;
+		expect(openRecordingStream).toHaveBeenCalledWith("rec.webm");
+		expect(appended).toEqual(["a", "b", "c"]);
+		expect(blob.size).toBe(0);
+		expect(handle.isStreaming()).toBe(true);
+	});
+
+	it("falls back to a complete in-memory blob when the stream fails to open", async () => {
+		const openRecordingStream = vi.fn(async () => ({ success: false, error: "nope" }));
+		const appendRecordingChunk = vi.fn(async () => ({ success: true }));
+		stubElectronAPI({ openRecordingStream, appendRecordingChunk });
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" }, "rec.webm");
+		const fake = driver(handle);
+
+		fake.emit(new Blob(["a"]));
+		await tick(); // open resolves false -> buffering, keep everything in memory
+		fake.emit(new Blob(["bc"]));
+		fake.stop();
+
+		const blob = await handle.recordedBlobPromise;
+		expect(appendRecordingChunk).not.toHaveBeenCalled();
+		expect(handle.isStreaming()).toBe(false);
+		expect(blob.size).toBe(3);
+		expect(decode(await blob.arrayBuffer())).toBe("abc");
+	});
+
+	it("falls back to in-memory buffering when the open IPC call rejects", async () => {
+		const openRecordingStream = vi.fn(async () => {
+			throw new Error("ipc channel closed");
+		});
+		stubElectronAPI({
+			openRecordingStream,
+			appendRecordingChunk: vi.fn(async () => ({ success: true })),
+		});
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" }, "rec.webm");
+		const fake = driver(handle);
+
+		fake.emit(new Blob(["a"]));
+		await tick(); // open rejects -> treated as a failed open, keep buffering
+		fake.emit(new Blob(["b"]));
+		fake.stop();
+
+		const blob = await handle.recordedBlobPromise;
+		expect(handle.isStreaming()).toBe(false);
+		expect(blob.size).toBe(2);
+		expect(decode(await blob.arrayBuffer())).toBe("ab");
+	});
+
+	it("waits for in-flight chunk writes before stop resolves (no truncation)", async () => {
+		let releaseAppend: () => void = () => undefined;
+		const appendGate = new Promise<void>((resolve) => {
+			releaseAppend = resolve;
+		});
+		const appendRecordingChunk = vi.fn(async () => {
+			await appendGate;
+			return { success: true };
+		});
+		stubElectronAPI({
+			openRecordingStream: vi.fn(async () => ({ success: true })),
+			appendRecordingChunk,
+		});
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" }, "rec.webm");
+		const fake = driver(handle);
+
+		await tick(); // open resolves
+		fake.emit(new Blob(["a"])); // write blocks on the gate
+		fake.stop();
+
+		let resolved = false;
+		void handle.recordedBlobPromise.then(() => {
+			resolved = true;
+		});
+		await tick();
+		expect(resolved).toBe(false); // must not resolve while the write is in flight
+
+		releaseAppend();
+		await handle.recordedBlobPromise;
+		expect(resolved).toBe(true);
+		expect(appendRecordingChunk).toHaveBeenCalledTimes(1);
+	});
+
+	it("rejects when a chunk fails to write mid-stream", async () => {
+		stubElectronAPI({
+			openRecordingStream: vi.fn(async () => ({ success: true })),
+			appendRecordingChunk: vi.fn(async () => ({ success: false, error: "disk full" })),
+			closeRecordingStream: vi.fn(async () => ({ success: true })),
+		});
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" }, "rec.webm");
+		const fake = driver(handle);
+
+		await tick();
+		fake.emit(new Blob(["a"]));
+		fake.stop();
+
+		await expect(handle.recordedBlobPromise).rejects.toThrow(/disk full/);
+		expect(handle.isStreaming()).toBe(false);
+	});
+
+	it("treats a rejected append the same as a failed write", async () => {
+		stubElectronAPI({
+			openRecordingStream: vi.fn(async () => ({ success: true })),
+			appendRecordingChunk: vi.fn(async () => {
+				throw new Error("kernel said no");
+			}),
+			closeRecordingStream: vi.fn(async () => ({ success: true })),
+		});
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" }, "rec.webm");
+		const fake = driver(handle);
+
+		await tick();
+		fake.emit(new Blob(["a"]));
+		fake.stop();
+
+		await expect(handle.recordedBlobPromise).rejects.toThrow(/kernel said no/);
+		expect(handle.isStreaming()).toBe(false);
+	});
+
+	it("buffers in memory and never opens a stream when no file name is given", async () => {
+		const openRecordingStream = vi.fn(async () => ({ success: true }));
+		stubElectronAPI({
+			openRecordingStream,
+			appendRecordingChunk: vi.fn(async () => ({ success: true })),
+		});
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" });
+		const fake = driver(handle);
+
+		fake.emit(new Blob(["xy"]));
+		await tick();
+		fake.stop();
+
+		const blob = await handle.recordedBlobPromise;
+		expect(openRecordingStream).not.toHaveBeenCalled();
+		expect(handle.isStreaming()).toBe(false);
+		expect(blob.size).toBe(2);
+	});
+
+	it("buffers in memory when appendRecordingChunk is unavailable (version skew)", async () => {
+		const openRecordingStream = vi.fn(async () => ({ success: true }));
+		// appendRecordingChunk intentionally omitted to simulate renderer/main skew.
+		stubElectronAPI({ openRecordingStream });
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" }, "rec.webm");
+		const fake = driver(handle);
+
+		fake.emit(new Blob(["a"]));
+		await tick();
+		fake.emit(new Blob(["b"]));
+		fake.stop();
+
+		const blob = await handle.recordedBlobPromise;
+		// Never even attempts to open the stream when it can't append to it.
+		expect(openRecordingStream).not.toHaveBeenCalled();
+		expect(handle.isStreaming()).toBe(false);
+		expect(blob.size).toBe(2);
+	});
+
+	it("discard closes the disk stream for a streamed recording", async () => {
+		const closeRecordingStream = vi.fn(async () => ({ success: true }));
+		stubElectronAPI({
+			openRecordingStream: vi.fn(async () => ({ success: true })),
+			appendRecordingChunk: vi.fn(async () => ({ success: true })),
+			closeRecordingStream,
+		});
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" }, "rec.webm");
+		const fake = driver(handle);
+		await tick();
+		fake.emit(new Blob(["a"]));
+		fake.stop();
+		await handle.recordedBlobPromise;
+
+		await handle.discard();
+		expect(closeRecordingStream).toHaveBeenCalledWith("rec.webm");
+	});
+
+	it("discard is a no-op when the stream never opened", async () => {
+		const closeRecordingStream = vi.fn(async () => ({ success: true }));
+		stubElectronAPI({
+			openRecordingStream: vi.fn(async () => ({ success: false })),
+			appendRecordingChunk: vi.fn(async () => ({ success: true })),
+			closeRecordingStream,
+		});
+
+		const handle = createRecorderHandle({} as MediaStream, { mimeType: "video/webm" }, "rec.webm");
+		const fake = driver(handle);
+		await tick();
+		fake.stop();
+		await handle.recordedBlobPromise;
+
+		await handle.discard();
+		expect(closeRecordingStream).not.toHaveBeenCalled();
+	});
+});
diff --git a/src/hooks/recorderHandle.ts b/src/hooks/recorderHandle.ts
new file mode 100644
index 0000000..e98000e
--- /dev/null
+++ b/src/hooks/recorderHandle.ts
@@ -0,0 +1,160 @@
+const RECORDER_TIMESLICE_MS = 1000;
+
+export type RecorderHandle = {
+	recorder: MediaRecorder;
+	/**
+	 * Resolves once the recording has fully drained. For a streamed recording the
+	 * blob is empty (the bytes are already on disk); for an in-memory recording it
+	 * holds the full WebM. Rejects if a chunk failed to write to disk mid-stream,
+	 * so a truncated recording surfaces as an error instead of a silent partial save.
+	 */
+	recordedBlobPromise: Promise<Blob>;
+	/**
+	 * Whether the recording's bytes went to disk via the streaming path. Computed
+	 * at finalize time rather than construction, so a stream that fails to open is
+	 * correctly reported as not-streamed and its in-memory fallback is used.
+	 */
+	isStreaming: () => boolean;
+	/**
+	 * Close the disk stream (if one opened) and delete its partial file. Called
+	 * when a recording is discarded or fails before a successful save, so cancelled
+	 * runs don't leak the stream or orphan a partial file. No-op for in-memory
+	 * recorders.
+	 */
+	discard: () => Promise<void>;
+};
+
+/**
+ * Wrap a MediaRecorder, optionally streaming its chunks to disk.
+ *
+ * When `fileName` is given, chunks are written to disk in arrival order through
+ * the main process as they arrive, so a long recording never buffers the whole
+ * video in the renderer (the #616 fix). Until the disk stream confirms it is
+ * open, chunks are held in memory; if the open fails, that buffer becomes a
+ * complete in-memory fallback so nothing is lost. Native-capture webcam sidecars
+ * omit `fileName` and always buffer in memory, since their finalize path reads
+ * the blob directly to attach the webcam track.
+ */
+export function createRecorderHandle(
+	stream: MediaStream,
+	options: MediaRecorderOptions,
+	fileName?: string,
+): RecorderHandle {
+	const recorder = new MediaRecorder(stream, options);
+	const mimeType = options.mimeType || "video/webm";
+	const api = window.electronAPI;
+
+	// Chunks held in memory: everything before the stream opens, plus everything
+	// when not streaming at all. On a successful open these flush to disk and are
+	// dropped; on open failure they remain as the complete fallback recording.
+	const memoryChunks: Blob[] = [];
+	let mode: "pending" | "streaming" | "buffering" = fileName ? "pending" : "buffering";
+	let streamOpened = false;
+	let appendError: Error | null = null;
+
+	// Serialize chunk writes so they land on disk in arrival order, and so stop
+	// can await every in-flight write before the main process closes the stream
+	// (otherwise a late chunk arrives after close and truncates the recording).
+	let writeChain: Promise<void> = Promise.resolve();
+	const enqueueWrite = (chunk: Blob) => {
+		writeChain = writeChain.then(async () => {
+			if (appendError || !fileName || !api?.appendRecordingChunk) {
+				return;
+			}
+			// Capture both outcomes — a `{ success: false }` result and an outright
+			// rejection (channel/handler error) — into appendError, so writeChain
+			// never rejects and isStreaming() stays consistent after a failure.
+			try {
+				const buffer = await chunk.arrayBuffer();
+				const result = await api.appendRecordingChunk(fileName, buffer);
+				if (!result.success) {
+					appendError = new Error(result.error ?? "Failed to write recording chunk to disk");
+				}
+			} catch (error) {
+				appendError = error instanceof Error ? error : new Error(String(error));
+			}
+		});
+	};
+
+	// Require BOTH stream IPC methods before attempting to stream. If only
+	// openRecordingStream exists (renderer/main version skew), streaming would
+	// open but every append would silently no-op, saving an empty file — so in
+	// that case fall through to in-memory buffering instead.
+	const openPromise: Promise<{ success: boolean; error?: string }> =
+		fileName !== undefined &&
+		typeof api?.openRecordingStream === "function" &&
+		typeof api?.appendRecordingChunk === "function"
+			? api.openRecordingStream(fileName)
+			: Promise.resolve({ success: false });
+
+	void openPromise.then(
+		(result) => {
+			if (result.success) {
+				streamOpened = true;
+				mode = "streaming";
+				for (const chunk of memoryChunks) {
+					enqueueWrite(chunk);
+				}
+				memoryChunks.length = 0;
+			} else {
+				mode = "buffering";
+			}
+		},
+		() => {
+			// The IPC call itself rejected (channel or handler error). Treat it the
+			// same as a failed open: keep buffering in memory so nothing is lost.
+			mode = "buffering";
+		},
+	);
+
+	const recordedBlobPromise = new Promise<Blob>((resolve, reject) => {
+		recorder.ondataavailable = (event: BlobEvent) => {
+			if (!event.data || event.data.size === 0) {
+				return;
+			}
+			if (mode === "streaming") {
+				enqueueWrite(event.data);
+			} else {
+				// "pending" (stream not open yet) or "buffering" (not streaming).
+				memoryChunks.push(event.data);
+			}
+		};
+
+		recorder.onerror = () => {
+			reject(new Error("Recording failed"));
+		};
+
+		recorder.onstop = () => {
+			resolve(finalizeBlob());
+		};
+	});
+
+	async function finalizeBlob(): Promise<Blob> {
+		// Wait for the open attempt to settle so its flush (or fallback switch) has
+		// been applied, then for every queued write to land, so we never resolve
+		// while chunks are still in flight to the about-to-close disk stream.
+		await openPromise.catch(() => undefined);
+		await writeChain;
+		if (appendError) {
+			throw appendError;
+		}
+		if (mode === "streaming") {
+			return new Blob([], { type: mimeType });
+		}
+		return new Blob(memoryChunks, { type: mimeType });
+	}
+
+	async function discard(): Promise<void> {
+		if (streamOpened && fileName && api?.closeRecordingStream) {
+			await api.closeRecordingStream(fileName);
+		}
+	}
+
+	recorder.start(RECORDER_TIMESLICE_MS);
+	return {
+		recorder,
+		recordedBlobPromise,
+		isStreaming: () => mode === "streaming" && !appendError,
+		discard,
+	};
+}
diff --git a/src/hooks/useScreenRecorder.ts b/src/hooks/useScreenRecorder.ts
index 14e55b3..f5fb920 100644
--- a/src/hooks/useScreenRecorder.ts
+++ b/src/hooks/useScreenRecorder.ts
@@ -13,6 +13,7 @@ import {
 } from "@/lib/nativeWindowsRecording";
 import type { CursorCaptureMode, RecordedVideoAssetInput } from "@/lib/recordingSession";
 import { requestCameraAccess } from "@/lib/requestCameraAccess";
+import { createRecorderHandle, type RecorderHandle } from "./recorderHandle";
 
 const TARGET_FRAME_RATE = 60;
 const MIN_FRAME_RATE = 30;
@@ -34,7 +35,6 @@ const DEFAULT_HEIGHT = 1080;
 
 const CODEC_ALIGNMENT = 2;
 
-const RECORDER_TIMESLICE_MS = 1000;
 const BITS_PER_MEGABIT = 1_000_000;
 const CHROME_MEDIA_SOURCE = "desktop";
 const RECORDING_FILE_PREFIX = "recording-";
@@ -74,11 +74,6 @@ type UseScreenRecorderReturn = {
 	setCursorCaptureMode: (mode: CursorCaptureMode) => void;
 };
 
-type RecorderHandle = {
-	recorder: MediaRecorder;
-	recordedBlobPromise: Promise<Blob>;
-};
-
 type NativeWindowsRecordingHandle = {
 	recordingId: number;
 	finalizing: boolean;
@@ -92,28 +87,6 @@ type NativeMacRecordingHandle = {
 	paused: boolean;
 };
 
-function createRecorderHandle(stream: MediaStream, options: MediaRecorderOptions): RecorderHandle {
-	const recorder = new MediaRecorder(stream, options);
-	const chunks: Blob[] = [];
-	const mimeType = options.mimeType || "video/webm";
-	const recordedBlobPromise = new Promise<Blob>((resolve, reject) => {
-		recorder.ondataavailable = (event: BlobEvent) => {
-			if (event.data && event.data.size > 0) {
-				chunks.push(event.data);
-			}
-		};
-		recorder.onerror = () => {
-			reject(new Error("Recording failed"));
-		};
-		recorder.onstop = () => {
-			resolve(new Blob(chunks, { type: mimeType }));
-		};
-	});
-
-	recorder.start(RECORDER_TIMESLICE_MS);
-	return { recorder, recordedBlobPromise };
-}
-
 export function useScreenRecorder(): UseScreenRecorderReturn {
 	const t = useScopedT("editor");
 	const [recording, setRecording] = useState(false);
@@ -355,46 +328,65 @@ export function useScreenRecorder(): UseScreenRecorderReturn {
 			window.electronAPI?.setRecordingState(false);
 
 			void (async () => {
+				// Each disk stream must end up either saved or explicitly discarded.
+				// store-recorded-session finalizes the streams included in a successful
+				// save; the finally block discards everything else.
+				let storeSucceeded = false;
+				let webcamIncludedInSave = false;
 				try {
 					const screenBlob = await activeScreenRecorder.recordedBlobPromise;
 					if (discardRecordingId.current === activeRecordingId) {
 						window.electronAPI?.discardCursorTelemetry(activeRecordingId);
 						return;
 					}
-					if (screenBlob.size === 0) {
+					// When streaming succeeded the blob is empty — the data is already on disk.
+					if (!activeScreenRecorder.isStreaming() && screenBlob.size === 0) {
 						return;
 					}
 
-					const fixedScreenBlob = await fixWebmDuration(screenBlob, duration);
-					let fixedWebcamBlob: Blob | null = null;
-					if (activeWebcamRecorder) {
-						const webcamBlob = await activeWebcamRecorder.recordedBlobPromise.catch(() => null);
-						if (webcamBlob && webcamBlob.size > 0) {
-							fixedWebcamBlob = await fixWebmDuration(webcamBlob, duration);
-						}
-					}
-
 					const screenFileName = `${RECORDING_FILE_PREFIX}${activeRecordingId}${VIDEO_FILE_EXTENSION}`;
 					const webcamFileName = `${RECORDING_FILE_PREFIX}${activeRecordingId}${WEBCAM_FILE_SUFFIX}${VIDEO_FILE_EXTENSION}`;
+
+					// Only fix duration / convert to ArrayBuffer for in-memory data;
+					// streamed recordings are patched on disk by the main process.
+					let screenVideoData: ArrayBuffer = new ArrayBuffer(0);
+					if (!activeScreenRecorder.isStreaming() && screenBlob.size > 0) {
+						const fixedScreenBlob = await fixWebmDuration(screenBlob, duration);
+						screenVideoData = await fixedScreenBlob.arrayBuffer();
+					}
+
+					let webcamVideoData: ArrayBuffer | undefined;
+					if (activeWebcamRecorder) {
+						const webcamBlob = await activeWebcamRecorder.recordedBlobPromise.catch(() => null);
+						if (!activeWebcamRecorder.isStreaming() && webcamBlob && webcamBlob.size > 0) {
+							const fixedWebcamBlob = await fixWebmDuration(webcamBlob, duration);
+							webcamVideoData = await fixedWebcamBlob.arrayBuffer();
+						} else if (activeWebcamRecorder.isStreaming()) {
+							webcamVideoData = new ArrayBuffer(0);
+						}
+					}
+					webcamIncludedInSave = webcamVideoData !== undefined;
+
 					const result = await window.electronAPI.storeRecordedSession({
 						screen: {
-							videoData: await fixedScreenBlob.arrayBuffer(),
+							videoData: screenVideoData,
 							fileName: screenFileName,
 						},
-						webcam: fixedWebcamBlob
-							? {
-									videoData: await fixedWebcamBlob.arrayBuffer(),
-									fileName: webcamFileName,
-								}
-							: undefined,
+						webcam:
+							webcamVideoData !== undefined
+								? { videoData: webcamVideoData, fileName: webcamFileName }
+								: undefined,
 						createdAt: activeRecordingId,
 						cursorCaptureMode,
+						durationMs: duration,
 					});
 
 					if (!result.success) {
 						console.error("Failed to store recording session:", result.message);
 						return;
 					}
+					// store-recorded-session has flushed and closed the saved streams.
+					storeSucceeded = true;
 
 					if (result.session) {
 						await window.electronAPI.setCurrentRecordingSession(result.session);
@@ -406,6 +398,16 @@ export function useScreenRecorder(): UseScreenRecorderReturn {
 				} catch (error) {
 					console.error("Error saving recording:", error);
 				} finally {
+					// Discard any recorder whose data was not part of a successful save
+					// — a discarded run, a failed save, or a webcam whose disk write
+					// failed (so it was omitted while the screen still saved) — so no
+					// stream or partial file is left open or orphaned.
+					if (!storeSucceeded) {
+						await activeScreenRecorder.discard().catch(() => undefined);
+					}
+					if (activeWebcamRecorder && !(storeSucceeded && webcamIncludedInSave)) {
+						await activeWebcamRecorder.discard().catch(() => undefined);
+					}
 					if (finalizingRecordingId.current === activeRecordingId) {
 						finalizingRecordingId.current = null;
 					}
@@ -1336,13 +1338,17 @@ export function useScreenRecorder(): UseScreenRecorderReturn {
 
 			recordingId.current = Date.now();
 			const activeRecordingId = recordingId.current;
-			screenRecorder.current = createRecorderHandle(stream.current, {
-				mimeType,
-				videoBitsPerSecond,
-				...(hasAudio
-					? { audioBitsPerSecond: systemAudioTrack ? AUDIO_BITRATE_SYSTEM : AUDIO_BITRATE_VOICE }
-					: {}),
-			});
+			screenRecorder.current = createRecorderHandle(
+				stream.current,
+				{
+					mimeType,
+					videoBitsPerSecond,
+					...(hasAudio
+						? { audioBitsPerSecond: systemAudioTrack ? AUDIO_BITRATE_SYSTEM : AUDIO_BITRATE_VOICE }
+						: {}),
+				},
+				`${RECORDING_FILE_PREFIX}${activeRecordingId}${VIDEO_FILE_EXTENSION}`,
+			);
 			screenRecorder.current.recorder.addEventListener(
 				"error",
 				() => {
@@ -1352,10 +1358,11 @@ export function useScreenRecorder(): UseScreenRecorderReturn {
 			);
 
 			if (webcamStream.current) {
-				webcamRecorder.current = createRecorderHandle(webcamStream.current, {
-					mimeType,
-					videoBitsPerSecond: Math.min(videoBitsPerSecond, BITRATE_BASE),
-				});
+				webcamRecorder.current = createRecorderHandle(
+					webcamStream.current,
+					{ mimeType, videoBitsPerSecond: Math.min(videoBitsPerSecond, BITRATE_BASE) },
+					`${RECORDING_FILE_PREFIX}${activeRecordingId}${WEBCAM_FILE_SUFFIX}${VIDEO_FILE_EXTENSION}`,
+				);
 			}
 
 			accumulatedDurationMs.current = 0;
diff --git a/src/lib/recordingSession.ts b/src/lib/recordingSession.ts
index f5ebf9c..12a6afd 100644
--- a/src/lib/recordingSession.ts
+++ b/src/lib/recordingSession.ts
@@ -20,6 +20,14 @@ export interface StoreRecordedSessionInput {
 	webcam?: RecordedVideoAssetInput;
 	createdAt?: number;
 	cursorCaptureMode?: CursorCaptureMode;
+	/**
+	 * Recording wall-clock duration in milliseconds. Used by the main process
+	 * to patch the WebM Duration header on streamed recordings, since the
+	 * renderer no longer holds the bytes. Browser MediaRecorder writes WebM
+	 * with no/zero duration; without this patch, the editor's seek bar and
+	 * timeline break for any recording that took the streaming path.
+	 */
+	durationMs?: number;
 }
 
 export function normalizeCursorCaptureMode(value: unknown): CursorCaptureMode | undefined {
diff --git a/vitest.config.ts b/vitest.config.ts
index 9108f69..5a52a9b 100644
--- a/vitest.config.ts
+++ b/vitest.config.ts
@@ -5,7 +5,7 @@ export default defineConfig({
 	test: {
 		globals: true,
 		environment: "jsdom",
-		include: ["src/**/*.{test,spec}.{js,mjs,cjs,ts,mts,cts,jsx,tsx}"],
+		include: ["{src,electron}/**/*.{test,spec}.{js,mjs,cjs,ts,mts,cts,jsx,tsx}"],
 		exclude: ["src/**/*.browser.test.{ts,tsx}"],
 	},
 	resolve: {