feat(tool-youtube-transcript): YouTube transcription tool

New standard tool extension backed by a self-hosted transcriber service
(http://100.102.55.49:41090, Tailscale, no API key). One tool
youtube_transcript — fetches transcripts for YouTube videos. Returns
completed (full text + timestamped segments), queued/processing (position
+ ETA + .youtube_subtitles_pending retry convention), or failed (error).

Pure core: validateUrl + format* functions + truncateOutput. Injected
edge: TranscriptClient (injectable fetchFn, AbortSignal.any for
cancellation). concurrencySafe true, capabilities network. 30 tests.

Verified: tsc EXIT 0, 1152 vitest, biome clean (327 files). Boot smoke
clean.

1 files changed, 118 insertions, 0 deletions

diff --git a/packages/tool-youtube-transcript/src/extension.test.ts b/packages/tool-youtube-transcript/src/extension.test.ts
new file mode 100644
index 0000000..70cf227
--- /dev/null
+++ b/packages/tool-youtube-transcript/src/extension.test.ts
@@ -0,0 +1,118 @@
+import { createLogger, type HostAPI, type ToolExecuteContext } from "@dispatch/kernel";
+import { afterEach, describe, expect, it, vi } from "vitest";
+import { activate, extension, manifest } from "./extension.js";
+
+function stubCtx(overrides?: Partial<ToolExecuteContext>): ToolExecuteContext {
+	return {
+		toolCallId: "test-call-1",
+		onOutput: () => {},
+		signal: new AbortController().signal,
+		log: createLogger(
+			{ extensionId: "test" },
+			{ emit: () => {} },
+			{ now: () => 0, newId: () => "id" },
+		),
+		...overrides,
+	};
+}
+
+function makeFakeHost(): { host: HostAPI; defineTool: ReturnType<typeof vi.fn> } {
+	const defineTool = vi.fn();
+	const host = {
+		defineTool,
+		logger: {
+			debug: vi.fn(),
+			info: vi.fn(),
+			warn: vi.fn(),
+			error: vi.fn(),
+			span: vi.fn(() => ({ end: vi.fn() })),
+		},
+	} as unknown as HostAPI;
+	return { host, defineTool };
+}
+
+const ORIG_FETCH = globalThis.fetch;
+const ORIG_ENV = process.env.YOUTUBE_TRANSCRIBER_URL;
+
+function restoreEnv(): void {
+	if (ORIG_ENV === undefined) {
+		delete process.env.YOUTUBE_TRANSCRIBER_URL;
+	} else {
+		process.env.YOUTUBE_TRANSCRIBER_URL = ORIG_ENV;
+	}
+}
+
+afterEach(() => {
+	globalThis.fetch = ORIG_FETCH;
+	restoreEnv();
+});
+
+function stubFetchCapture(): { calls: Array<{ url: string }> } {
+	const calls: Array<{ url: string }> = [];
+	globalThis.fetch = vi.fn(async (input: string | URL | Request) => {
+		calls.push({ url: String(input) });
+		return new Response(
+			JSON.stringify({
+				status: "completed",
+				video_id: "v",
+				full_text: "",
+				segments: [],
+			}),
+			{ status: 200, headers: { "Content-Type": "application/json" } },
+		);
+	}) as unknown as typeof globalThis.fetch;
+	return { calls };
+}
+
+describe("tool-youtube-transcript activation", () => {
+	it("registers the 'youtube_transcript' tool (defineTool called)", () => {
+		const { host, defineTool } = makeFakeHost();
+		activate(host);
+		expect(defineTool).toHaveBeenCalledTimes(1);
+		const registered = defineTool.mock.calls[0]?.[0];
+		if (!registered) throw new Error("no tool registered");
+		expect(registered.name).toBe("youtube_transcript");
+		expect(registered.concurrencySafe).toBe(true);
+	});
+
+	it("uses YOUTUBE_TRANSCRIBER_URL from env", async () => {
+		process.env.YOUTUBE_TRANSCRIBER_URL = "http://env-transcriber.local";
+		const { calls } = stubFetchCapture();
+		const { host, defineTool } = makeFakeHost();
+		activate(host);
+
+		const tool = defineTool.mock.calls[0]?.[0];
+		if (!tool) throw new Error("no tool registered");
+		await tool.execute({ url: "https://youtu.be/vid1" }, stubCtx());
+		expect(calls.length).toBeGreaterThan(0);
+		expect(calls[0]?.url).toContain("http://env-transcriber.local/api/transcript?url=");
+	});
+
+	it("uses default base URL when env unset", async () => {
+		delete process.env.YOUTUBE_TRANSCRIBER_URL;
+		const { calls } = stubFetchCapture();
+		const { host, defineTool } = makeFakeHost();
+		activate(host);
+
+		const tool = defineTool.mock.calls[0]?.[0];
+		if (!tool) throw new Error("no tool registered");
+		await tool.execute({ url: "https://youtu.be/vid1" }, stubCtx());
+		expect(calls.length).toBeGreaterThan(0);
+		expect(calls[0]?.url).toContain("100.102.55.49:41090/api/transcript?url=");
+	});
+});
+
+describe("tool-youtube-transcript manifest", () => {
+	it("declares network capability + youtube_transcript contribution", () => {
+		expect(manifest.id).toBe("tool-youtube-transcript");
+		expect(manifest.capabilities).toEqual({ network: true });
+		expect(manifest.contributes).toEqual({ tools: ["youtube_transcript"] });
+		expect(manifest.trust).toBe("bundled");
+		expect(manifest.activation).toBe("eager");
+	});
+
+	it("extension bundles the manifest + activate", () => {
+		expect(extension.manifest).toBe(manifest);
+		expect(typeof extension.activate).toBe("function");
+	});
+});