diff --git a/package-lock.json b/package-lock.json
index 79eb7b36f..7f83b1cea 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -9220,6 +9220,7 @@
       "name": "@gsd/pi-coding-agent",
       "version": "2.51.0",
       "dependencies": {
+        "@gsd/rpc-client": "2.51.0",
         "@mariozechner/jiti": "^2.6.2",
         "@silvia-odwyer/photon-node": "^0.3.4",
         "chalk": "^5.5.0",
diff --git a/packages/pi-coding-agent/package.json b/packages/pi-coding-agent/package.json
index 7d3cb624e..37ddf8fe6 100644
--- a/packages/pi-coding-agent/package.json
+++ b/packages/pi-coding-agent/package.json
@@ -20,6 +20,7 @@
     "copy-assets": "node scripts/copy-assets.cjs"
   },
   "dependencies": {
+    "@gsd/rpc-client": "2.51.0",
     "@mariozechner/jiti": "^2.6.2",
     "@silvia-odwyer/photon-node": "^0.3.4",
     "chalk": "^5.5.0",
diff --git a/packages/pi-coding-agent/src/modes/rpc/jsonl.ts b/packages/pi-coding-agent/src/modes/rpc/jsonl.ts
index 5392defef..42cbbe93a 100644
--- a/packages/pi-coding-agent/src/modes/rpc/jsonl.ts
+++ b/packages/pi-coding-agent/src/modes/rpc/jsonl.ts
@@ -1,64 +1 @@
-import type { Readable } from "node:stream";
-import { StringDecoder } from "node:string_decoder";
-
-/**
- * Serialize a single strict JSONL record.
- *
- * Framing is LF-only. Payload strings may contain other Unicode separators such as
- * U+2028 and U+2029. Clients must split records on `\n` only.
- */
-export function serializeJsonLine(value: unknown): string {
-	return `${JSON.stringify(value)}\n`;
-}
-
-/**
- * Attach an LF-only JSONL reader to a stream.
- *
- * This intentionally does not use Node readline. Readline splits on additional
- * Unicode separators that are valid inside JSON strings and therefore does not
- * implement strict JSONL framing.
- */
-export function attachJsonlLineReader(stream: Readable, onLine: (line: string) => void): () => void {
-	const decoder = new StringDecoder("utf8");
-	let buffer = "";
-
-	const emitLine = (line: string) => {
-		onLine(line.endsWith("\r") ? line.slice(0, -1) : line);
-	};
-
-	const onData = (chunk: string | Buffer) => {
-		buffer += typeof chunk === "string" ? chunk : decoder.write(chunk);
-
-		while (true) {
-			const newlineIndex = buffer.indexOf("\n");
-			if (newlineIndex === -1) {
-				return;
-			}
-
-			emitLine(buffer.slice(0, newlineIndex));
-			buffer = buffer.slice(newlineIndex + 1);
-		}
-	};
-
-	const onEnd = () => {
-		buffer += decoder.end();
-		if (buffer.length > 0) {
-			emitLine(buffer);
-			buffer = "";
-		}
-	};
-
-	const onError = (_err: Error) => {
-		// Stream errors are non-fatal for JSONL reading
-	};
-
-	stream.on("data", onData);
-	stream.on("end", onEnd);
-	stream.on("error", onError);
-
-	return () => {
-		stream.off("data", onData);
-		stream.off("end", onEnd);
-		stream.off("error", onError);
-	};
-}
+export { serializeJsonLine, attachJsonlLineReader } from '@gsd/rpc-client';
diff --git a/packages/rpc-client/README.md b/packages/rpc-client/README.md
new file mode 100644
index 000000000..500322f67
--- /dev/null
+++ b/packages/rpc-client/README.md
@@ -0,0 +1,125 @@
+# @gsd/rpc-client
+
+Standalone RPC client SDK for GSD. Spawn the agent process, perform a v2 protocol handshake, send commands, and consume typed events via an async generator — all in a few lines of TypeScript.
+
+Zero internal dependencies. Ships its own inlined types.
+
+## Installation
+
+```bash
+npm install @gsd/rpc-client
+```
+
+## Quick Start
+
+```typescript
+import { RpcClient } from '@gsd/rpc-client';
+
+const client = new RpcClient({ cwd: process.cwd() });
+await client.start();
+const { sessionId } = await client.init({ clientId: 'my-app' });
+console.log(`Session: ${sessionId}`);
+
+await client.prompt('Create a hello world script');
+for await (const event of client.events()) {
+  if (event.type === 'execution_complete') break;
+  console.log(event.type);
+}
+await client.shutdown();
+```
+
+## API
+
+### Constructor
+
+```typescript
+const client = new RpcClient(options?: RpcClientOptions);
+```
+
+| Option     | Type                     | Description                              |
+|------------|--------------------------|------------------------------------------|
+| `cliPath`  | `string`                 | Path to the CLI entry point              |
+| `cwd`      | `string`                 | Working directory for the agent          |
+| `env`      | `Record<string, string>` | Environment variables                    |
+| `provider` | `string`                 | AI provider (e.g. `"anthropic"`)         |
+| `model`    | `string`                 | Model ID (e.g. `"claude-sonnet"`)        |
+| `args`     | `string[]`               | Additional CLI arguments                 |
+
+### Lifecycle
+
+| Method        | Description                                    |
+|---------------|------------------------------------------------|
+| `start()`     | Spawn the agent process                        |
+| `init(opts?)` | v2 handshake — returns `sessionId`, capabilities |
+| `shutdown()`  | Graceful shutdown                              |
+| `stop()`      | Force-kill the process                         |
+
+### Commands
+
+| Method                         | Description                            |
+|--------------------------------|----------------------------------------|
+| `prompt(message, images?)`     | Send a prompt                          |
+| `steer(message, images?)`      | Interrupt with a steering message      |
+| `followUp(message, images?)`   | Queue a follow-up message              |
+| `abort()`                      | Abort current operation                |
+| `subscribe(events)`            | Subscribe to event types (`["*"]` for all) |
+
+### Events
+
+```typescript
+// Async generator — recommended
+for await (const event of client.events()) {
+  console.log(event.type);
+}
+
+// Callback-based
+const unsubscribe = client.onEvent((event) => {
+  console.log(event.type);
+});
+```
+
+### Helpers
+
+| Method                                | Description                              |
+|---------------------------------------|------------------------------------------|
+| `waitForIdle(timeout?)`               | Wait for `agent_end` event               |
+| `collectEvents(timeout?)`             | Collect events until idle                |
+| `promptAndWait(message, images?, t?)` | Send prompt and collect events           |
+
+### Session & Model
+
+| Method                           | Description                       |
+|----------------------------------|-----------------------------------|
+| `getState()`                     | Get session state                 |
+| `setModel(provider, modelId)`    | Set model                         |
+| `cycleModel()`                   | Cycle to next model               |
+| `getAvailableModels()`           | List available models             |
+| `setThinkingLevel(level)`        | Set thinking level                |
+| `cycleThinkingLevel()`           | Cycle thinking level              |
+| `compact(instructions?)`         | Compact session context           |
+| `getSessionStats()`              | Get session statistics            |
+| `bash(command)`                  | Execute a bash command            |
+| `newSession(parent?)`            | Start a new session               |
+| `sendUIResponse(id, response)`   | Respond to extension UI requests  |
+
+## Type Exports
+
+All protocol types are exported from the package root:
+
+```typescript
+import type {
+  RpcCommand,
+  RpcResponse,
+  RpcInitResult,
+  RpcExecutionCompleteEvent,
+  RpcCostUpdateEvent,
+  RpcV2Event,
+  SessionStats,
+  SdkAgentEvent,
+  RpcClientOptions,
+} from '@gsd/rpc-client';
+```
+
+## License
+
+MIT
diff --git a/packages/rpc-client/examples/basic-usage.ts b/packages/rpc-client/examples/basic-usage.ts
new file mode 100644
index 000000000..bd821aa86
--- /dev/null
+++ b/packages/rpc-client/examples/basic-usage.ts
@@ -0,0 +1,13 @@
+import { RpcClient } from '@gsd/rpc-client';
+
+const client = new RpcClient({ cwd: process.cwd() });
+await client.start();
+const { sessionId } = await client.init({ clientId: 'my-app' });
+console.log(`Session: ${sessionId}`);
+
+await client.prompt('Create a hello world script');
+for await (const event of client.events()) {
+  if (event.type === 'execution_complete') break;
+  console.log(event.type);
+}
+await client.shutdown();
diff --git a/packages/rpc-client/package.json b/packages/rpc-client/package.json
index 50461c856..e6e51e4a9 100644
--- a/packages/rpc-client/package.json
+++ b/packages/rpc-client/package.json
@@ -14,6 +14,10 @@
   "files": [
     "dist"
   ],
+  "scripts": {
+    "build": "tsc -p tsconfig.json",
+    "test": "node --test dist/rpc-client.test.js"
+  },
   "engines": {
     "node": ">=22.0.0"
   }
diff --git a/packages/rpc-client/src/index.ts b/packages/rpc-client/src/index.ts
new file mode 100644
index 000000000..329e36560
--- /dev/null
+++ b/packages/rpc-client/src/index.ts
@@ -0,0 +1,10 @@
+/**
+ * @gsd/rpc-client — standalone RPC client SDK for GSD.
+ *
+ * Re-exports all types, JSONL utilities, and the RpcClient class.
+ */
+
+export * from "./rpc-types.js";
+export { serializeJsonLine, attachJsonlLineReader } from "./jsonl.js";
+export { RpcClient } from "./rpc-client.js";
+export type { RpcClientOptions, RpcEventListener, SdkAgentEvent } from "./rpc-client.js";
diff --git a/packages/rpc-client/src/jsonl.ts b/packages/rpc-client/src/jsonl.ts
new file mode 100644
index 000000000..5392defef
--- /dev/null
+++ b/packages/rpc-client/src/jsonl.ts
@@ -0,0 +1,64 @@
+import type { Readable } from "node:stream";
+import { StringDecoder } from "node:string_decoder";
+
+/**
+ * Serialize a single strict JSONL record.
+ *
+ * Framing is LF-only. Payload strings may contain other Unicode separators such as
+ * U+2028 and U+2029. Clients must split records on `\n` only.
+ */
+export function serializeJsonLine(value: unknown): string {
+	return `${JSON.stringify(value)}\n`;
+}
+
+/**
+ * Attach an LF-only JSONL reader to a stream.
+ *
+ * This intentionally does not use Node readline. Readline splits on additional
+ * Unicode separators that are valid inside JSON strings and therefore does not
+ * implement strict JSONL framing.
+ */
+export function attachJsonlLineReader(stream: Readable, onLine: (line: string) => void): () => void {
+	const decoder = new StringDecoder("utf8");
+	let buffer = "";
+
+	const emitLine = (line: string) => {
+		onLine(line.endsWith("\r") ? line.slice(0, -1) : line);
+	};
+
+	const onData = (chunk: string | Buffer) => {
+		buffer += typeof chunk === "string" ? chunk : decoder.write(chunk);
+
+		while (true) {
+			const newlineIndex = buffer.indexOf("\n");
+			if (newlineIndex === -1) {
+				return;
+			}
+
+			emitLine(buffer.slice(0, newlineIndex));
+			buffer = buffer.slice(newlineIndex + 1);
+		}
+	};
+
+	const onEnd = () => {
+		buffer += decoder.end();
+		if (buffer.length > 0) {
+			emitLine(buffer);
+			buffer = "";
+		}
+	};
+
+	const onError = (_err: Error) => {
+		// Stream errors are non-fatal for JSONL reading
+	};
+
+	stream.on("data", onData);
+	stream.on("end", onEnd);
+	stream.on("error", onError);
+
+	return () => {
+		stream.off("data", onData);
+		stream.off("end", onEnd);
+		stream.off("error", onError);
+	};
+}
diff --git a/packages/rpc-client/src/rpc-client.test.ts b/packages/rpc-client/src/rpc-client.test.ts
new file mode 100644
index 000000000..9fcb7874f
--- /dev/null
+++ b/packages/rpc-client/src/rpc-client.test.ts
@@ -0,0 +1,568 @@
+import { describe, it, beforeEach, afterEach } from "node:test";
+import assert from "node:assert/strict";
+import { PassThrough } from "node:stream";
+import { serializeJsonLine, attachJsonlLineReader } from "./jsonl.js";
+import type {
+	RpcInitResult,
+	RpcExecutionCompleteEvent,
+	RpcCostUpdateEvent,
+	RpcProtocolVersion,
+	SessionStats,
+	RpcV2Event,
+} from "./rpc-types.js";
+import { RpcClient } from "./rpc-client.js";
+import type { SdkAgentEvent } from "./rpc-client.js";
+
+// ============================================================================
+// JSONL Tests
+// ============================================================================
+
+describe("serializeJsonLine", () => {
+	it("produces valid JSON terminated with LF", () => {
+		const result = serializeJsonLine({ type: "test", value: 42 });
+		assert.ok(result.endsWith("\n"), "must end with LF");
+		const parsed = JSON.parse(result.trim());
+		assert.equal(parsed.type, "test");
+		assert.equal(parsed.value, 42);
+	});
+
+	it("serializes strings with special characters", () => {
+		const result = serializeJsonLine({ msg: "hello\nworld" });
+		assert.ok(result.endsWith("\n"));
+		// The embedded \n must be escaped inside the JSON — only the trailing LF is the framing delimiter
+		const lines = result.split("\n");
+		// Should be exactly 2 parts: the JSON line and the empty string after trailing LF
+		assert.equal(lines.length, 2);
+		assert.equal(lines[1], "");
+		const parsed = JSON.parse(lines[0]);
+		assert.equal(parsed.msg, "hello\nworld");
+	});
+
+	it("handles empty objects", () => {
+		const result = serializeJsonLine({});
+		assert.equal(result, "{}\n");
+	});
+});
+
+describe("attachJsonlLineReader", () => {
+	it("splits on LF correctly", async () => {
+		const stream = new PassThrough();
+		const lines: string[] = [];
+
+		attachJsonlLineReader(stream, (line) => lines.push(line));
+
+		stream.write('{"a":1}\n{"b":2}\n');
+		stream.end();
+
+		// Let microtask queue flush
+		await new Promise((r) => setTimeout(r, 10));
+
+		assert.equal(lines.length, 2);
+		assert.equal(JSON.parse(lines[0]).a, 1);
+		assert.equal(JSON.parse(lines[1]).b, 2);
+	});
+
+	it("handles chunked data across boundaries", async () => {
+		const stream = new PassThrough();
+		const lines: string[] = [];
+
+		attachJsonlLineReader(stream, (line) => lines.push(line));
+
+		// Write in fragments that split mid-line
+		stream.write('{"type":"hel');
+		stream.write('lo"}\n{"type":"w');
+		stream.write('orld"}\n');
+		stream.end();
+
+		await new Promise((r) => setTimeout(r, 10));
+
+		assert.equal(lines.length, 2);
+		assert.equal(JSON.parse(lines[0]).type, "hello");
+		assert.equal(JSON.parse(lines[1]).type, "world");
+	});
+
+	it("emits trailing data on stream end", async () => {
+		const stream = new PassThrough();
+		const lines: string[] = [];
+
+		attachJsonlLineReader(stream, (line) => lines.push(line));
+
+		stream.write('{"final":true}');
+		stream.end();
+
+		await new Promise((r) => setTimeout(r, 10));
+
+		assert.equal(lines.length, 1);
+		assert.equal(JSON.parse(lines[0]).final, true);
+	});
+
+	it("returns a detach function that stops reading", async () => {
+		const stream = new PassThrough();
+		const lines: string[] = [];
+
+		const detach = attachJsonlLineReader(stream, (line) => lines.push(line));
+
+		stream.write('{"a":1}\n');
+		await new Promise((r) => setTimeout(r, 10));
+		assert.equal(lines.length, 1);
+
+		detach();
+
+		stream.write('{"b":2}\n');
+		stream.end();
+		await new Promise((r) => setTimeout(r, 10));
+
+		// Should still be 1 — detach removed listeners
+		assert.equal(lines.length, 1);
+	});
+
+	it("strips CR from CRLF line endings", async () => {
+		const stream = new PassThrough();
+		const lines: string[] = [];
+
+		attachJsonlLineReader(stream, (line) => lines.push(line));
+
+		stream.write('{"v":1}\r\n');
+		stream.end();
+
+		await new Promise((r) => setTimeout(r, 10));
+
+		assert.equal(lines.length, 1);
+		assert.equal(JSON.parse(lines[0]).v, 1);
+	});
+});
+
+// ============================================================================
+// Type Shape Tests
+// ============================================================================
+
+describe("type shapes", () => {
+	it("RpcInitResult has protocolVersion, sessionId, capabilities", () => {
+		const init: RpcInitResult = {
+			protocolVersion: 2,
+			sessionId: "sess_123",
+			capabilities: {
+				events: ["execution_complete", "cost_update"],
+				commands: ["prompt", "steer"],
+			},
+		};
+		assert.equal(init.protocolVersion, 2);
+		assert.equal(init.sessionId, "sess_123");
+		assert.ok(Array.isArray(init.capabilities.events));
+		assert.ok(Array.isArray(init.capabilities.commands));
+	});
+
+	it("RpcExecutionCompleteEvent has required fields", () => {
+		const event: RpcExecutionCompleteEvent = {
+			type: "execution_complete",
+			runId: "run_abc",
+			status: "completed",
+			stats: {
+				sessionFile: "/tmp/session.json",
+				sessionId: "sess_123",
+				userMessages: 5,
+				assistantMessages: 5,
+				toolCalls: 3,
+				toolResults: 3,
+				totalMessages: 10,
+				tokens: { input: 1000, output: 500, cacheRead: 200, cacheWrite: 100, total: 1800 },
+				cost: 0.05,
+			},
+		};
+		assert.equal(event.type, "execution_complete");
+		assert.equal(event.runId, "run_abc");
+		assert.equal(event.status, "completed");
+		assert.ok(event.stats);
+		assert.equal(event.stats.sessionId, "sess_123");
+	});
+
+	it("RpcCostUpdateEvent has required fields", () => {
+		const event: RpcCostUpdateEvent = {
+			type: "cost_update",
+			runId: "run_abc",
+			turnCost: 0.01,
+			cumulativeCost: 0.05,
+			tokens: { input: 500, output: 200, cacheRead: 100, cacheWrite: 50 },
+		};
+		assert.equal(event.type, "cost_update");
+		assert.equal(event.runId, "run_abc");
+		assert.equal(event.turnCost, 0.01);
+		assert.equal(event.cumulativeCost, 0.05);
+		assert.ok(event.tokens);
+	});
+
+	it("SessionStats has all expected fields", () => {
+		const stats: SessionStats = {
+			sessionFile: "/tmp/session.json",
+			sessionId: "s1",
+			userMessages: 10,
+			assistantMessages: 10,
+			toolCalls: 5,
+			toolResults: 5,
+			totalMessages: 20,
+			tokens: { input: 2000, output: 1000, cacheRead: 500, cacheWrite: 200, total: 3700 },
+			cost: 0.10,
+		};
+		assert.equal(stats.sessionId, "s1");
+		assert.equal(stats.userMessages, 10);
+		assert.equal(stats.tokens.total, 3700);
+		assert.equal(stats.cost, 0.10);
+	});
+
+	it("RpcProtocolVersion accepts 1 and 2", () => {
+		const v1: RpcProtocolVersion = 1;
+		const v2: RpcProtocolVersion = 2;
+		assert.equal(v1, 1);
+		assert.equal(v2, 2);
+	});
+
+	it("RpcV2Event discriminated union covers both event types", () => {
+		const events: RpcV2Event[] = [
+			{
+				type: "execution_complete",
+				runId: "r1",
+				status: "completed",
+				stats: {
+					sessionFile: undefined,
+					sessionId: "s1",
+					userMessages: 1,
+					assistantMessages: 1,
+					toolCalls: 0,
+					toolResults: 0,
+					totalMessages: 2,
+					tokens: { input: 100, output: 50, cacheRead: 0, cacheWrite: 0, total: 150 },
+					cost: 0.001,
+				},
+			},
+			{
+				type: "cost_update",
+				runId: "r1",
+				turnCost: 0.001,
+				cumulativeCost: 0.001,
+				tokens: { input: 100, output: 50, cacheRead: 0, cacheWrite: 0 },
+			},
+		];
+		assert.equal(events.length, 2);
+		assert.equal(events[0].type, "execution_complete");
+		assert.equal(events[1].type, "cost_update");
+	});
+});
+
+// ============================================================================
+// RpcClient Construction Tests
+// ============================================================================
+
+describe("RpcClient construction", () => {
+	it("creates with default options", () => {
+		const client = new RpcClient();
+		assert.ok(client);
+	});
+
+	it("creates with custom options", () => {
+		const client = new RpcClient({
+			cliPath: "/usr/local/bin/gsd",
+			cwd: "/tmp",
+			env: { NODE_ENV: "test" },
+			provider: "anthropic",
+			model: "claude-sonnet",
+			args: ["--verbose"],
+		});
+		assert.ok(client);
+	});
+});
+
+// ============================================================================
+// events() Generator Tests
+// ============================================================================
+
+describe("events() async generator", () => {
+	it("yields events from a mock stream in order", async () => {
+		const client = new RpcClient();
+
+		// Reach into the client to set up a mock process with a PassThrough stdout
+		const mockStdout = new PassThrough();
+		const mockStderr = new PassThrough();
+		const mockStdin = new PassThrough();
+
+		// Simulate a started process by setting internal state
+		// We use Object.assign to set private fields for testing
+		const clientAny = client as any;
+		clientAny.process = {
+			stdout: mockStdout,
+			stderr: mockStderr,
+			stdin: mockStdin,
+			exitCode: null,
+			kill: () => {},
+			on: (event: string, handler: (...args: any[]) => void) => {
+				if (event === "exit") {
+					// Store exit handler so we can trigger it
+					clientAny._testExitHandler = handler;
+				}
+			},
+			removeListener: () => {},
+		};
+
+		// Attach the JSONL reader like start() does
+		clientAny.stopReadingStdout = attachJsonlLineReader(mockStdout, (line: string) => {
+			clientAny.handleLine(line);
+		});
+
+		// Collect events from the generator
+		const received: SdkAgentEvent[] = [];
+		const genPromise = (async () => {
+			for await (const event of client.events()) {
+				received.push(event);
+				if (event.type === "done") break;
+			}
+		})();
+
+		// Simulate server sending events
+		await new Promise((r) => setTimeout(r, 20));
+		mockStdout.write(serializeJsonLine({ type: "agent_start", runId: "r1" }));
+		await new Promise((r) => setTimeout(r, 20));
+		mockStdout.write(serializeJsonLine({ type: "token", text: "hello" }));
+		await new Promise((r) => setTimeout(r, 20));
+		mockStdout.write(serializeJsonLine({ type: "done" }));
+
+		await genPromise;
+
+		assert.equal(received.length, 3);
+		assert.equal(received[0].type, "agent_start");
+		assert.equal(received[1].type, "token");
+		assert.equal(received[2].type, "done");
+	});
+
+	it("terminates when process exits", async () => {
+		const client = new RpcClient();
+		const mockStdout = new PassThrough();
+		const mockStderr = new PassThrough();
+		const mockStdin = new PassThrough();
+
+		const exitHandlers: Array<() => void> = [];
+		const clientAny = client as any;
+		clientAny.process = {
+			stdout: mockStdout,
+			stderr: mockStderr,
+			stdin: mockStdin,
+			exitCode: null,
+			kill: () => {},
+			on: (event: string, handler: () => void) => {
+				if (event === "exit") exitHandlers.push(handler);
+			},
+			removeListener: (event: string, handler: () => void) => {
+				const idx = exitHandlers.indexOf(handler);
+				if (idx !== -1) exitHandlers.splice(idx, 1);
+			},
+		};
+
+		clientAny.stopReadingStdout = attachJsonlLineReader(mockStdout, (line: string) => {
+			clientAny.handleLine(line);
+		});
+
+		const received: SdkAgentEvent[] = [];
+		const genPromise = (async () => {
+			for await (const event of client.events()) {
+				received.push(event);
+			}
+		})();
+
+		// Send one event, then simulate process exit
+		await new Promise((r) => setTimeout(r, 20));
+		mockStdout.write(serializeJsonLine({ type: "agent_start" }));
+		await new Promise((r) => setTimeout(r, 20));
+
+		// Fire exit handlers
+		for (const h of exitHandlers) h();
+
+		await genPromise;
+
+		assert.equal(received.length, 1);
+		assert.equal(received[0].type, "agent_start");
+	});
+
+	it("throws if client not started", async () => {
+		const client = new RpcClient();
+		await assert.rejects(async () => {
+			// eslint-disable-next-line @typescript-eslint/no-unused-vars
+			for await (const _event of client.events()) {
+				// should not reach
+			}
+		}, /Client not started/);
+	});
+});
+
+// ============================================================================
+// sendUIResponse Serialization Test
+// ============================================================================
+
+describe("sendUIResponse serialization", () => {
+	it("writes correct JSONL to stdin", () => {
+		const client = new RpcClient();
+		const chunks: string[] = [];
+		const mockStdin = {
+			write: (data: string) => {
+				chunks.push(data);
+				return true;
+			},
+		};
+
+		const clientAny = client as any;
+		clientAny.process = { stdin: mockStdin };
+
+		client.sendUIResponse("ui_1", { value: "hello" });
+
+		assert.equal(chunks.length, 1);
+		const parsed = JSON.parse(chunks[0].trim());
+		assert.equal(parsed.type, "extension_ui_response");
+		assert.equal(parsed.id, "ui_1");
+		assert.equal(parsed.value, "hello");
+	});
+
+	it("serializes confirmed response", () => {
+		const client = new RpcClient();
+		const chunks: string[] = [];
+		const mockStdin = {
+			write: (data: string) => {
+				chunks.push(data);
+				return true;
+			},
+		};
+		const clientAny = client as any;
+		clientAny.process = { stdin: mockStdin };
+
+		client.sendUIResponse("ui_2", { confirmed: true });
+
+		const parsed = JSON.parse(chunks[0].trim());
+		assert.equal(parsed.confirmed, true);
+		assert.equal(parsed.id, "ui_2");
+	});
+
+	it("serializes cancelled response", () => {
+		const client = new RpcClient();
+		const chunks: string[] = [];
+		const mockStdin = {
+			write: (data: string) => {
+				chunks.push(data);
+				return true;
+			},
+		};
+		const clientAny = client as any;
+		clientAny.process = { stdin: mockStdin };
+
+		client.sendUIResponse("ui_3", { cancelled: true });
+
+		const parsed = JSON.parse(chunks[0].trim());
+		assert.equal(parsed.cancelled, true);
+	});
+});
+
+// ============================================================================
+// init/shutdown/subscribe Serialization Tests
+// ============================================================================
+
+describe("v2 command serialization", () => {
+	// Helper: capture what the client sends to stdin
+	function createMockClient(): { client: RpcClient; sent: any[]; respondNext: (data?: any) => void } {
+		const client = new RpcClient();
+		const sent: any[] = [];
+		let respondFn: ((data: any) => void) | null = null;
+
+		const clientAny = client as any;
+		clientAny.process = {
+			stdin: {
+				write: (data: string) => {
+					const parsed = JSON.parse(data.trim());
+					sent.push(parsed);
+					// Auto-respond with success after a tick
+					if (respondFn) {
+						setTimeout(() => respondFn!(parsed), 5);
+					}
+					return true;
+				},
+			},
+			stderr: new PassThrough(),
+			exitCode: null,
+			kill: () => {},
+			on: () => {},
+			removeListener: () => {},
+		};
+
+		const respondNext = (overrides: any = {}) => {
+			respondFn = (parsed) => {
+				const response = {
+					type: "response",
+					id: parsed.id,
+					command: parsed.type,
+					success: true,
+					data: {},
+					...overrides,
+				};
+				clientAny.handleLine(JSON.stringify(response));
+			};
+		};
+
+		return { client, sent, respondNext };
+	}
+
+	it("init sends correct v2 init command", async () => {
+		const { client, sent, respondNext } = createMockClient();
+		respondNext({ data: { protocolVersion: 2, sessionId: "s1", capabilities: { events: [], commands: [] } } });
+
+		const result = await client.init({ clientId: "test-app" });
+
+		assert.equal(sent.length, 1);
+		assert.equal(sent[0].type, "init");
+		assert.equal(sent[0].protocolVersion, 2);
+		assert.equal(sent[0].clientId, "test-app");
+		assert.equal(result.protocolVersion, 2);
+		assert.equal(result.sessionId, "s1");
+	});
+
+	it("shutdown sends shutdown command", async () => {
+		const { client, sent, respondNext } = createMockClient();
+
+		// Override the process exit wait
+		const clientAny = client as any;
+		const originalProcess = clientAny.process;
+		const exitHandlers: Array<(code: number) => void> = [];
+		clientAny.process = {
+			...originalProcess,
+			on: (event: string, handler: (code: number) => void) => {
+				if (event === "exit") exitHandlers.push(handler);
+			},
+		};
+
+		respondNext();
+
+		// Call shutdown and simulate process exit
+		const shutdownPromise = client.shutdown();
+		await new Promise((r) => setTimeout(r, 20));
+		for (const h of exitHandlers) h(0);
+
+		await shutdownPromise;
+
+		assert.equal(sent.length, 1);
+		assert.equal(sent[0].type, "shutdown");
+	});
+
+	it("subscribe sends subscribe command with event list", async () => {
+		const { client, sent, respondNext } = createMockClient();
+		respondNext();
+
+		await client.subscribe(["execution_complete", "cost_update"]);
+
+		assert.equal(sent.length, 1);
+		assert.equal(sent[0].type, "subscribe");
+		assert.deepEqual(sent[0].events, ["execution_complete", "cost_update"]);
+	});
+
+	it("subscribe with wildcard", async () => {
+		const { client, sent, respondNext } = createMockClient();
+		respondNext();
+
+		await client.subscribe(["*"]);
+
+		assert.equal(sent[0].events.length, 1);
+		assert.equal(sent[0].events[0], "*");
+	});
+});
diff --git a/packages/rpc-client/src/rpc-client.ts b/packages/rpc-client/src/rpc-client.ts
new file mode 100644
index 000000000..4d5edc53c
--- /dev/null
+++ b/packages/rpc-client/src/rpc-client.ts
@@ -0,0 +1,666 @@
+/**
+ * RPC Client for programmatic access to the coding agent.
+ *
+ * Spawns the agent in RPC mode and provides a typed API for all operations.
+ * This is a standalone SDK client — all types are inlined with zero internal
+ * package dependencies.
+ */
+
+import { type ChildProcess, spawn } from "node:child_process";
+import { attachJsonlLineReader, serializeJsonLine } from "./jsonl.js";
+import type {
+	BashResult,
+	CompactionResult,
+	ImageContent,
+	ModelInfo,
+	RpcCommand,
+	RpcInitResult,
+	RpcResponse,
+	RpcSessionState,
+	RpcSlashCommand,
+	ThinkingLevel,
+	SessionStats,
+} from "./rpc-types.js";
+
+// ============================================================================
+// Types
+// ============================================================================
+
+/** Distributive Omit that works with union types */
+type DistributiveOmit<T, K extends keyof T> = T extends unknown ? Omit<T, K> : never;
+
+/** RpcCommand without the id field (for internal send) */
+type RpcCommandBody = DistributiveOmit<RpcCommand, "id">;
+
+/** Agent event — a loosely-typed record from the server. The `type` field is always present. */
+export interface SdkAgentEvent {
+	type: string;
+	[key: string]: unknown;
+}
+
+export interface RpcClientOptions {
+	/** Path to the CLI entry point (default: searches for dist/cli.js) */
+	cliPath?: string;
+	/** Working directory for the agent */
+	cwd?: string;
+	/** Environment variables */
+	env?: Record<string, string>;
+	/** Provider to use */
+	provider?: string;
+	/** Model ID to use */
+	model?: string;
+	/** Additional CLI arguments */
+	args?: string[];
+}
+
+export type RpcEventListener = (event: SdkAgentEvent) => void;
+
+// ============================================================================
+// RPC Client
+// ============================================================================
+
+export class RpcClient {
+	private process: ChildProcess | null = null;
+	private stopReadingStdout: (() => void) | null = null;
+	private _stderrHandler?: (data: Buffer) => void;
+	private eventListeners: RpcEventListener[] = [];
+	private pendingRequests: Map<string, { resolve: (response: RpcResponse) => void; reject: (error: Error) => void }> =
+		new Map();
+	private requestId = 0;
+	private stderr = "";
+	private _stopped = false;
+
+	constructor(private options: RpcClientOptions = {}) {}
+
+	/**
+	 * Start the RPC agent process.
+	 */
+	async start(): Promise<void> {
+		if (this.process) {
+			throw new Error("Client already started");
+		}
+
+		this._stopped = false;
+
+		const cliPath = this.options.cliPath ?? "dist/cli.js";
+		const args = ["--mode", "rpc"];
+
+		if (this.options.provider) {
+			args.push("--provider", this.options.provider);
+		}
+		if (this.options.model) {
+			args.push("--model", this.options.model);
+		}
+		if (this.options.args) {
+			args.push(...this.options.args);
+		}
+
+		this.process = spawn("node", [cliPath, ...args], {
+			cwd: this.options.cwd,
+			env: { ...process.env, ...this.options.env },
+			stdio: ["pipe", "pipe", "pipe"],
+		});
+
+		// Collect stderr for debugging
+		this._stderrHandler = (data: Buffer) => {
+			this.stderr += data.toString();
+		};
+		this.process.stderr?.on("data", this._stderrHandler);
+
+		// Set up strict JSONL reader for stdout.
+		this.stopReadingStdout = attachJsonlLineReader(this.process.stdout!, (line) => {
+			this.handleLine(line);
+		});
+
+		// Detect unexpected subprocess exit and reject all pending requests
+		this.process.on("exit", (code, signal) => {
+			if (this.pendingRequests.size > 0) {
+				const reason = signal ? `signal ${signal}` : `code ${code}`;
+				const error = new Error(`Agent process exited unexpectedly (${reason}). Stderr: ${this.stderr}`);
+				for (const [id, pending] of this.pendingRequests) {
+					this.pendingRequests.delete(id);
+					pending.reject(error);
+				}
+			}
+		});
+
+		// Wait a moment for process to initialize
+		await new Promise((resolve) => setTimeout(resolve, 100));
+
+		if (this.process.exitCode !== null) {
+			throw new Error(`Agent process exited immediately with code ${this.process.exitCode}. Stderr: ${this.stderr}`);
+		}
+	}
+
+	/**
+	 * Stop the RPC agent process.
+	 */
+	async stop(): Promise<void> {
+		if (!this.process) return;
+
+		this._stopped = true;
+
+		this.stopReadingStdout?.();
+		this.stopReadingStdout = null;
+		if (this._stderrHandler) {
+			this.process.stderr?.removeListener("data", this._stderrHandler);
+			this._stderrHandler = undefined;
+		}
+		this.process.kill("SIGTERM");
+
+		// Wait for process to exit
+		await new Promise<void>((resolve) => {
+			const timeout = setTimeout(() => {
+				this.process?.kill("SIGKILL");
+				resolve();
+			}, 1000);
+
+			this.process?.on("exit", () => {
+				clearTimeout(timeout);
+				resolve();
+			});
+		});
+
+		this.process = null;
+		this.pendingRequests.clear();
+	}
+
+	/**
+	 * Subscribe to agent events via callback.
+	 */
+	onEvent(listener: RpcEventListener): () => void {
+		this.eventListeners.push(listener);
+		return () => {
+			const index = this.eventListeners.indexOf(listener);
+			if (index !== -1) {
+				this.eventListeners.splice(index, 1);
+			}
+		};
+	}
+
+	/**
+	 * Async generator that yields agent events as they arrive.
+	 *
+	 * Usage:
+	 * ```ts
+	 * for await (const event of client.events()) {
+	 *   console.log(event.type, event);
+	 * }
+	 * ```
+	 *
+	 * The generator terminates when:
+	 * - `stop()` is called
+	 * - The agent process exits
+	 * - The consumer breaks out of the loop
+	 */
+	async *events(): AsyncGenerator<SdkAgentEvent, void, undefined> {
+		if (!this.process) {
+			throw new Error("Client not started — call start() before events()");
+		}
+
+		if (this._stopped) {
+			return;
+		}
+
+		const buffer: SdkAgentEvent[] = [];
+		let resolve: ((value: void) => void) | null = null;
+		let done = false;
+
+		// When a new event arrives, either push to buffer or wake up the awaiting generator
+		const listener = (event: SdkAgentEvent) => {
+			buffer.push(event);
+			if (resolve) {
+				const r = resolve;
+				resolve = null;
+				r();
+			}
+		};
+
+		// When the process exits, signal the generator to stop
+		const onExit = () => {
+			done = true;
+			if (resolve) {
+				const r = resolve;
+				resolve = null;
+				r();
+			}
+		};
+
+		const unsubscribe = this.onEvent(listener);
+		this.process.on("exit", onExit);
+
+		try {
+			while (!done && !this._stopped) {
+				// Drain buffer first
+				while (buffer.length > 0) {
+					yield buffer.shift()!;
+				}
+
+				// If done after draining, break
+				if (done || this._stopped) {
+					break;
+				}
+
+				// Wait for next event or process exit
+				await new Promise<void>((r) => {
+					resolve = r;
+				});
+			}
+
+			// Drain any remaining events that arrived with the exit signal
+			while (buffer.length > 0) {
+				yield buffer.shift()!;
+			}
+		} finally {
+			unsubscribe();
+			this.process?.removeListener("exit", onExit);
+		}
+	}
+
+	/**
+	 * Get collected stderr output (useful for debugging).
+	 */
+	getStderr(): string {
+		return this.stderr;
+	}
+
+	// =========================================================================
+	// Command Methods
+	// =========================================================================
+
+	/**
+	 * Send a prompt to the agent.
+	 * Returns immediately after sending; use onEvent() or events() to receive streaming events.
+	 * Use waitForIdle() to wait for completion.
+	 */
+	async prompt(message: string, images?: ImageContent[]): Promise<void> {
+		await this.send({ type: "prompt", message, images });
+	}
+
+	/**
+	 * Queue a steering message to interrupt the agent mid-run.
+	 */
+	async steer(message: string, images?: ImageContent[]): Promise<void> {
+		await this.send({ type: "steer", message, images });
+	}
+
+	/**
+	 * Queue a follow-up message to be processed after the agent finishes.
+	 */
+	async followUp(message: string, images?: ImageContent[]): Promise<void> {
+		await this.send({ type: "follow_up", message, images });
+	}
+
+	/**
+	 * Abort current operation.
+	 */
+	async abort(): Promise<void> {
+		await this.send({ type: "abort" });
+	}
+
+	/**
+	 * Start a new session, optionally with parent tracking.
+	 * @param parentSession - Optional parent session path for lineage tracking
+	 * @returns Object with `cancelled: true` if an extension cancelled the new session
+	 */
+	async newSession(parentSession?: string): Promise<{ cancelled: boolean }> {
+		const response = await this.send({ type: "new_session", parentSession });
+		return this.getData(response);
+	}
+
+	/**
+	 * Get current session state.
+	 */
+	async getState(): Promise<RpcSessionState> {
+		const response = await this.send({ type: "get_state" });
+		return this.getData(response);
+	}
+
+	/**
+	 * Set model by provider and ID.
+	 */
+	async setModel(provider: string, modelId: string): Promise<{ provider: string; id: string }> {
+		const response = await this.send({ type: "set_model", provider, modelId });
+		return this.getData(response);
+	}
+
+	/**
+	 * Cycle to next model.
+	 */
+	async cycleModel(): Promise<{
+		model: { provider: string; id: string };
+		thinkingLevel: ThinkingLevel;
+		isScoped: boolean;
+	} | null> {
+		const response = await this.send({ type: "cycle_model" });
+		return this.getData(response);
+	}
+
+	/**
+	 * Get list of available models.
+	 */
+	async getAvailableModels(): Promise<ModelInfo[]> {
+		const response = await this.send({ type: "get_available_models" });
+		return this.getData<{ models: ModelInfo[] }>(response).models;
+	}
+
+	/**
+	 * Set thinking level.
+	 */
+	async setThinkingLevel(level: ThinkingLevel): Promise<void> {
+		await this.send({ type: "set_thinking_level", level });
+	}
+
+	/**
+	 * Cycle thinking level.
+	 */
+	async cycleThinkingLevel(): Promise<{ level: ThinkingLevel } | null> {
+		const response = await this.send({ type: "cycle_thinking_level" });
+		return this.getData(response);
+	}
+
+	/**
+	 * Set steering mode.
+	 */
+	async setSteeringMode(mode: "all" | "one-at-a-time"): Promise<void> {
+		await this.send({ type: "set_steering_mode", mode });
+	}
+
+	/**
+	 * Set follow-up mode.
+	 */
+	async setFollowUpMode(mode: "all" | "one-at-a-time"): Promise<void> {
+		await this.send({ type: "set_follow_up_mode", mode });
+	}
+
+	/**
+	 * Compact session context.
+	 */
+	async compact(customInstructions?: string): Promise<CompactionResult> {
+		const response = await this.send({ type: "compact", customInstructions });
+		return this.getData(response);
+	}
+
+	/**
+	 * Set auto-compaction enabled/disabled.
+	 */
+	async setAutoCompaction(enabled: boolean): Promise<void> {
+		await this.send({ type: "set_auto_compaction", enabled });
+	}
+
+	/**
+	 * Set auto-retry enabled/disabled.
+	 */
+	async setAutoRetry(enabled: boolean): Promise<void> {
+		await this.send({ type: "set_auto_retry", enabled });
+	}
+
+	/**
+	 * Abort in-progress retry.
+	 */
+	async abortRetry(): Promise<void> {
+		await this.send({ type: "abort_retry" });
+	}
+
+	/**
+	 * Execute a bash command.
+	 */
+	async bash(command: string): Promise<BashResult> {
+		const response = await this.send({ type: "bash", command });
+		return this.getData(response);
+	}
+
+	/**
+	 * Abort running bash command.
+	 */
+	async abortBash(): Promise<void> {
+		await this.send({ type: "abort_bash" });
+	}
+
+	/**
+	 * Get session statistics.
+	 */
+	async getSessionStats(): Promise<SessionStats> {
+		const response = await this.send({ type: "get_session_stats" });
+		return this.getData(response);
+	}
+
+	/**
+	 * Export session to HTML.
+	 */
+	async exportHtml(outputPath?: string): Promise<{ path: string }> {
+		const response = await this.send({ type: "export_html", outputPath });
+		return this.getData(response);
+	}
+
+	/**
+	 * Switch to a different session file.
+	 * @returns Object with `cancelled: true` if an extension cancelled the switch
+	 */
+	async switchSession(sessionPath: string): Promise<{ cancelled: boolean }> {
+		const response = await this.send({ type: "switch_session", sessionPath });
+		return this.getData(response);
+	}
+
+	/**
+	 * Fork from a specific message.
+	 * @returns Object with `text` (the message text) and `cancelled` (if extension cancelled)
+	 */
+	async fork(entryId: string): Promise<{ text: string; cancelled: boolean }> {
+		const response = await this.send({ type: "fork", entryId });
+		return this.getData(response);
+	}
+
+	/**
+	 * Get messages available for forking.
+	 */
+	async getForkMessages(): Promise<Array<{ entryId: string; text: string }>> {
+		const response = await this.send({ type: "get_fork_messages" });
+		return this.getData<{ messages: Array<{ entryId: string; text: string }> }>(response).messages;
+	}
+
+	/**
+	 * Get text of last assistant message.
+	 */
+	async getLastAssistantText(): Promise<string | null> {
+		const response = await this.send({ type: "get_last_assistant_text" });
+		return this.getData<{ text: string | null }>(response).text;
+	}
+
+	/**
+	 * Set the session display name.
+	 */
+	async setSessionName(name: string): Promise<void> {
+		await this.send({ type: "set_session_name", name });
+	}
+
+	/**
+	 * Get all messages in the session.
+	 * Messages are returned as opaque objects — the internal structure may vary.
+	 */
+	async getMessages(): Promise<unknown[]> {
+		const response = await this.send({ type: "get_messages" });
+		return this.getData<{ messages: unknown[] }>(response).messages;
+	}
+
+	/**
+	 * Get available commands (extension commands, prompt templates, skills).
+	 */
+	async getCommands(): Promise<RpcSlashCommand[]> {
+		const response = await this.send({ type: "get_commands" });
+		return this.getData<{ commands: RpcSlashCommand[] }>(response).commands;
+	}
+
+	/**
+	 * Send a UI response to a pending extension_ui_request.
+	 * Fire-and-forget — no request/response correlation.
+	 */
+	sendUIResponse(id: string, response: { value?: string; values?: string[]; confirmed?: boolean; cancelled?: boolean }): void {
+		if (!this.process?.stdin) {
+			throw new Error("Client not started");
+		}
+		this.process.stdin.write(serializeJsonLine({
+			type: "extension_ui_response",
+			id,
+			...response,
+		}));
+	}
+
+	/**
+	 * Initialize a v2 protocol session. Must be sent as the first command.
+	 * Returns the negotiated protocol version, session ID, and server capabilities.
+	 */
+	async init(options?: { clientId?: string }): Promise<RpcInitResult> {
+		const response = await this.send({ type: "init", protocolVersion: 2, clientId: options?.clientId });
+		return this.getData<RpcInitResult>(response);
+	}
+
+	/**
+	 * Request a graceful shutdown of the agent process.
+	 * Waits for the response before the process exits.
+	 */
+	async shutdown(): Promise<void> {
+		await this.send({ type: "shutdown" });
+		// Wait for process to exit after shutdown acknowledgment
+		if (this.process) {
+			await new Promise<void>((resolve) => {
+				const timeout = setTimeout(() => {
+					this.process?.kill("SIGKILL");
+					resolve();
+				}, 5000);
+				this.process?.on("exit", () => {
+					clearTimeout(timeout);
+					resolve();
+				});
+			});
+		}
+	}
+
+	/**
+	 * Subscribe to specific event types (v2 only).
+	 * Pass ["*"] to receive all events, or a list of event type strings to filter.
+	 */
+	async subscribe(events: string[]): Promise<void> {
+		await this.send({ type: "subscribe", events });
+	}
+
+	// =========================================================================
+	// Helpers
+	// =========================================================================
+
+	/**
+	 * Wait for agent to become idle (no streaming).
+	 * Resolves when agent_end event is received.
+	 */
+	waitForIdle(timeout = 60000): Promise<void> {
+		return new Promise((resolve, reject) => {
+			const timer = setTimeout(() => {
+				unsubscribe();
+				reject(new Error(`Timeout waiting for agent to become idle. Stderr: ${this.stderr}`));
+			}, timeout);
+
+			const unsubscribe = this.onEvent((event) => {
+				if (event.type === "agent_end") {
+					clearTimeout(timer);
+					unsubscribe();
+					resolve();
+				}
+			});
+		});
+	}
+
+	/**
+	 * Collect events until agent becomes idle.
+	 */
+	collectEvents(timeout = 60000): Promise<SdkAgentEvent[]> {
+		return new Promise((resolve, reject) => {
+			const events: SdkAgentEvent[] = [];
+			const timer = setTimeout(() => {
+				unsubscribe();
+				reject(new Error(`Timeout collecting events. Stderr: ${this.stderr}`));
+			}, timeout);
+
+			const unsubscribe = this.onEvent((event) => {
+				events.push(event);
+				if (event.type === "agent_end") {
+					clearTimeout(timer);
+					unsubscribe();
+					resolve(events);
+				}
+			});
+		});
+	}
+
+	/**
+	 * Send prompt and wait for completion, returning all events.
+	 */
+	async promptAndWait(message: string, images?: ImageContent[], timeout = 60000): Promise<SdkAgentEvent[]> {
+		const eventsPromise = this.collectEvents(timeout);
+		await this.prompt(message, images);
+		return eventsPromise;
+	}
+
+	// =========================================================================
+	// Internal
+	// =========================================================================
+
+	private handleLine(line: string): void {
+		try {
+			const data = JSON.parse(line);
+
+			// Check if it's a response to a pending request
+			if (data.type === "response" && data.id && this.pendingRequests.has(data.id)) {
+				const pending = this.pendingRequests.get(data.id)!;
+				this.pendingRequests.delete(data.id);
+				pending.resolve(data as RpcResponse);
+				return;
+			}
+
+			// Otherwise it's an event — dispatch to listeners
+			for (const listener of this.eventListeners) {
+				listener(data as SdkAgentEvent);
+			}
+		} catch {
+			// Ignore non-JSON lines
+		}
+	}
+
+	private async send(command: RpcCommandBody): Promise<RpcResponse> {
+		if (!this.process?.stdin) {
+			throw new Error("Client not started");
+		}
+
+		const id = `req_${++this.requestId}`;
+		const fullCommand = { ...command, id } as RpcCommand;
+
+		return new Promise((resolve, reject) => {
+			const timeout = setTimeout(() => {
+				this.pendingRequests.delete(id);
+				reject(new Error(`Timeout waiting for response to ${command.type}. Stderr: ${this.stderr}`));
+			}, 30000);
+
+			this.pendingRequests.set(id, {
+				resolve: (response) => {
+					clearTimeout(timeout);
+					resolve(response);
+				},
+				reject: (error) => {
+					clearTimeout(timeout);
+					reject(error);
+				},
+			});
+
+			this.process!.stdin!.write(serializeJsonLine(fullCommand));
+		});
+	}
+
+	private getData<T>(response: RpcResponse): T {
+		if (!response.success) {
+			const errorResponse = response as Extract<RpcResponse, { success: false }>;
+			throw new Error(errorResponse.error);
+		}
+		// Type assertion: we trust response.data matches T based on the command sent.
+		const successResponse = response as Extract<RpcResponse, { success: true; data: unknown }>;
+		return successResponse.data as T;
+	}
+}
diff --git a/packages/rpc-client/src/rpc-types.ts b/packages/rpc-client/src/rpc-types.ts
new file mode 100644
index 000000000..be8bca73b
--- /dev/null
+++ b/packages/rpc-client/src/rpc-types.ts
@@ -0,0 +1,399 @@
+/**
+ * RPC protocol types for headless operation.
+ *
+ * Commands are sent as JSON lines on stdin.
+ * Responses and events are emitted as JSON lines on stdout.
+ *
+ * This file is self-contained — all types that were previously imported from
+ * internal packages are inlined so that this package has zero internal
+ * dependencies.
+ */
+
+// ============================================================================
+// Inlined types (originally from internal packages)
+// ============================================================================
+
+/** Thinking budget level (inlined from agent-core) */
+export type ThinkingLevel = "off" | "minimal" | "low" | "medium" | "high" | "xhigh";
+
+/** Image attachment (inlined from pi-ai) */
+export interface ImageContent {
+	type: "image";
+	data: string; // base64 encoded image data
+	mimeType: string; // e.g., "image/jpeg", "image/png"
+}
+
+/** Model descriptor — opaque for SDK consumers */
+export interface ModelInfo {
+	provider: string;
+	id: string;
+	contextWindow?: number;
+	reasoning?: boolean;
+	[key: string]: unknown;
+}
+
+/** Session statistics (from agent-session.ts) */
+export interface SessionStats {
+	sessionFile: string | undefined;
+	sessionId: string;
+	userMessages: number;
+	assistantMessages: number;
+	toolCalls: number;
+	toolResults: number;
+	totalMessages: number;
+	tokens: {
+		input: number;
+		output: number;
+		cacheRead: number;
+		cacheWrite: number;
+		total: number;
+	};
+	cost: number;
+}
+
+/** Bash command result (from bash-executor.ts) */
+export interface BashResult {
+	/** Combined stdout + stderr output (sanitized, possibly truncated) */
+	output: string;
+	/** Process exit code (undefined if killed/cancelled) */
+	exitCode: number | undefined;
+	/** Whether the command was cancelled via signal */
+	cancelled: boolean;
+	/** Whether the output was truncated */
+	truncated: boolean;
+	/** Path to temp file containing full output (if output exceeded truncation threshold) */
+	fullOutputPath?: string;
+}
+
+/** Compaction result (from compaction.ts) */
+export interface CompactionResult<T = unknown> {
+	summary: string;
+	firstKeptEntryId: string;
+	tokensBefore: number;
+	/** Extension-specific data (e.g., ArtifactIndex, version markers for structured compaction) */
+	details?: T;
+}
+
+// ============================================================================
+// RPC Protocol Versioning
+// ============================================================================
+
+/** Supported protocol versions. v1 is the implicit default; v2 requires an init handshake. */
+export type RpcProtocolVersion = 1 | 2;
+
+// ============================================================================
+// RPC Commands (stdin)
+// ============================================================================
+
+export type RpcCommand =
+	// Prompting
+	| { id?: string; type: "prompt"; message: string; images?: ImageContent[]; streamingBehavior?: "steer" | "followUp" }
+	| { id?: string; type: "steer"; message: string; images?: ImageContent[] }
+	| { id?: string; type: "follow_up"; message: string; images?: ImageContent[] }
+	| { id?: string; type: "abort" }
+	| { id?: string; type: "new_session"; parentSession?: string }
+
+	// State
+	| { id?: string; type: "get_state" }
+
+	// Model
+	| { id?: string; type: "set_model"; provider: string; modelId: string }
+	| { id?: string; type: "cycle_model" }
+	| { id?: string; type: "get_available_models" }
+
+	// Thinking
+	| { id?: string; type: "set_thinking_level"; level: ThinkingLevel }
+	| { id?: string; type: "cycle_thinking_level" }
+
+	// Queue modes
+	| { id?: string; type: "set_steering_mode"; mode: "all" | "one-at-a-time" }
+	| { id?: string; type: "set_follow_up_mode"; mode: "all" | "one-at-a-time" }
+
+	// Compaction
+	| { id?: string; type: "compact"; customInstructions?: string }
+	| { id?: string; type: "set_auto_compaction"; enabled: boolean }
+
+	// Retry
+	| { id?: string; type: "set_auto_retry"; enabled: boolean }
+	| { id?: string; type: "abort_retry" }
+
+	// Bash
+	| { id?: string; type: "bash"; command: string }
+	| { id?: string; type: "abort_bash" }
+
+	// Session
+	| { id?: string; type: "get_session_stats" }
+	| { id?: string; type: "export_html"; outputPath?: string }
+	| { id?: string; type: "switch_session"; sessionPath: string }
+	| { id?: string; type: "fork"; entryId: string }
+	| { id?: string; type: "get_fork_messages" }
+	| { id?: string; type: "get_last_assistant_text" }
+	| { id?: string; type: "set_session_name"; name: string }
+
+	// Messages
+	| { id?: string; type: "get_messages" }
+
+	// Commands (available for invocation via prompt)
+	| { id?: string; type: "get_commands" }
+
+	// Bridge-hosted native terminal
+	| { id?: string; type: "terminal_input"; data: string }
+	| { id?: string; type: "terminal_resize"; cols: number; rows: number }
+	| { id?: string; type: "terminal_redraw" }
+
+	// v2 Protocol
+	| { id?: string; type: "init"; protocolVersion: 2; clientId?: string }
+	| { id?: string; type: "shutdown"; graceful?: boolean }
+	| { id?: string; type: "subscribe"; events: string[] };
+
+// ============================================================================
+// RPC Slash Command (for get_commands response)
+// ============================================================================
+
+/** A command available for invocation via prompt */
+export interface RpcSlashCommand {
+	/** Command name (without leading slash) */
+	name: string;
+	/** Human-readable description */
+	description?: string;
+	/** What kind of command this is */
+	source: "extension" | "prompt" | "skill";
+	/** Where the command was loaded from (undefined for extensions) */
+	location?: "user" | "project" | "path";
+	/** File path to the command source */
+	path?: string;
+}
+
+// ============================================================================
+// RPC State
+// ============================================================================
+
+export interface RpcSessionState {
+	model?: ModelInfo;
+	thinkingLevel: ThinkingLevel;
+	isStreaming: boolean;
+	isCompacting: boolean;
+	steeringMode: "all" | "one-at-a-time";
+	followUpMode: "all" | "one-at-a-time";
+	sessionFile?: string;
+	sessionId: string;
+	sessionName?: string;
+	autoCompactionEnabled: boolean;
+	autoRetryEnabled: boolean;
+	retryInProgress: boolean;
+	retryAttempt: number;
+	messageCount: number;
+	pendingMessageCount: number;
+	/** Whether extension loading has completed. Commands from `get_commands` may be incomplete until true. */
+	extensionsReady: boolean;
+}
+
+// ============================================================================
+// RPC Responses (stdout)
+// ============================================================================
+
+// Success responses with data
+export type RpcResponse =
+	// Prompting (async - events follow)
+	| { id?: string; type: "response"; command: "prompt"; success: true; runId?: string }
+	| { id?: string; type: "response"; command: "steer"; success: true; runId?: string }
+	| { id?: string; type: "response"; command: "follow_up"; success: true; runId?: string }
+	| { id?: string; type: "response"; command: "abort"; success: true }
+	| { id?: string; type: "response"; command: "new_session"; success: true; data: { cancelled: boolean } }
+
+	// State
+	| { id?: string; type: "response"; command: "get_state"; success: true; data: RpcSessionState }
+
+	// Model
+	| {
+			id?: string;
+			type: "response";
+			command: "set_model";
+			success: true;
+			data: ModelInfo;
+	  }
+	| {
+			id?: string;
+			type: "response";
+			command: "cycle_model";
+			success: true;
+			data: { model: ModelInfo; thinkingLevel: ThinkingLevel; isScoped: boolean } | null;
+	  }
+	| {
+			id?: string;
+			type: "response";
+			command: "get_available_models";
+			success: true;
+			data: { models: ModelInfo[] };
+	  }
+
+	// Thinking
+	| { id?: string; type: "response"; command: "set_thinking_level"; success: true }
+	| {
+			id?: string;
+			type: "response";
+			command: "cycle_thinking_level";
+			success: true;
+			data: { level: ThinkingLevel } | null;
+	  }
+
+	// Queue modes
+	| { id?: string; type: "response"; command: "set_steering_mode"; success: true }
+	| { id?: string; type: "response"; command: "set_follow_up_mode"; success: true }
+
+	// Compaction
+	| { id?: string; type: "response"; command: "compact"; success: true; data: CompactionResult }
+	| { id?: string; type: "response"; command: "set_auto_compaction"; success: true }
+
+	// Retry
+	| { id?: string; type: "response"; command: "set_auto_retry"; success: true }
+	| { id?: string; type: "response"; command: "abort_retry"; success: true }
+
+	// Bash
+	| { id?: string; type: "response"; command: "bash"; success: true; data: BashResult }
+	| { id?: string; type: "response"; command: "abort_bash"; success: true }
+
+	// Session
+	| { id?: string; type: "response"; command: "get_session_stats"; success: true; data: SessionStats }
+	| { id?: string; type: "response"; command: "export_html"; success: true; data: { path: string } }
+	| { id?: string; type: "response"; command: "switch_session"; success: true; data: { cancelled: boolean } }
+	| { id?: string; type: "response"; command: "fork"; success: true; data: { text: string; cancelled: boolean } }
+	| {
+			id?: string;
+			type: "response";
+			command: "get_fork_messages";
+			success: true;
+			data: { messages: Array<{ entryId: string; text: string }> };
+	  }
+	| {
+			id?: string;
+			type: "response";
+			command: "get_last_assistant_text";
+			success: true;
+			data: { text: string | null };
+	  }
+	| { id?: string; type: "response"; command: "set_session_name"; success: true }
+
+	// Messages — AgentMessage is opaque for SDK consumers
+	| { id?: string; type: "response"; command: "get_messages"; success: true; data: { messages: unknown[] } }
+
+	// Commands
+	| {
+			id?: string;
+			type: "response";
+			command: "get_commands";
+			success: true;
+			data: { commands: RpcSlashCommand[] };
+	  }
+
+	// Bridge-hosted native terminal
+	| { id?: string; type: "response"; command: "terminal_input"; success: true }
+	| { id?: string; type: "response"; command: "terminal_resize"; success: true }
+	| { id?: string; type: "response"; command: "terminal_redraw"; success: true }
+
+	// v2 Protocol
+	| { id?: string; type: "response"; command: "init"; success: true; data: RpcInitResult }
+	| { id?: string; type: "response"; command: "shutdown"; success: true }
+	| { id?: string; type: "response"; command: "subscribe"; success: true }
+
+	// Error response (any command can fail)
+	| { id?: string; type: "response"; command: string; success: false; error: string };
+
+// ============================================================================
+// v2 Protocol Types
+// ============================================================================
+
+/** Result of the init handshake (v2 only) */
+export interface RpcInitResult {
+	protocolVersion: 2;
+	sessionId: string;
+	capabilities: {
+		events: string[];
+		commands: string[];
+	};
+}
+
+/** v2 execution_complete event — emitted when a prompt/steer/follow_up finishes */
+export interface RpcExecutionCompleteEvent {
+	type: "execution_complete";
+	runId: string;
+	status: "completed" | "error" | "cancelled";
+	reason?: string;
+	stats: SessionStats;
+}
+
+/** v2 cost_update event — emitted per-turn with running cost data */
+export interface RpcCostUpdateEvent {
+	type: "cost_update";
+	runId: string;
+	turnCost: number;
+	cumulativeCost: number;
+	tokens: {
+		input: number;
+		output: number;
+		cacheRead: number;
+		cacheWrite: number;
+	};
+}
+
+/** Discriminated union of all v2-only event types */
+export type RpcV2Event = RpcExecutionCompleteEvent | RpcCostUpdateEvent;
+
+// ============================================================================
+// Extension UI Events (stdout)
+// ============================================================================
+
+/** Emitted when an extension needs user input */
+export type RpcExtensionUIRequest =
+	| { type: "extension_ui_request"; id: string; method: "select"; title: string; options: string[]; timeout?: number; allowMultiple?: boolean }
+	| { type: "extension_ui_request"; id: string; method: "confirm"; title: string; message: string; timeout?: number }
+	| {
+			type: "extension_ui_request";
+			id: string;
+			method: "input";
+			title: string;
+			placeholder?: string;
+			timeout?: number;
+	  }
+	| { type: "extension_ui_request"; id: string; method: "editor"; title: string; prefill?: string }
+	| {
+			type: "extension_ui_request";
+			id: string;
+			method: "notify";
+			message: string;
+			notifyType?: "info" | "warning" | "error";
+	  }
+	| {
+			type: "extension_ui_request";
+			id: string;
+			method: "setStatus";
+			statusKey: string;
+			statusText: string | undefined;
+	  }
+	| {
+			type: "extension_ui_request";
+			id: string;
+			method: "setWidget";
+			widgetKey: string;
+			widgetLines: string[] | undefined;
+			widgetPlacement?: "aboveEditor" | "belowEditor";
+	  }
+	| { type: "extension_ui_request"; id: string; method: "setTitle"; title: string }
+	| { type: "extension_ui_request"; id: string; method: "set_editor_text"; text: string };
+
+// ============================================================================
+// Extension UI Commands (stdin)
+// ============================================================================
+
+/** Response to an extension UI request */
+export type RpcExtensionUIResponse =
+	| { type: "extension_ui_response"; id: string; value: string }
+	| { type: "extension_ui_response"; id: string; values: string[] }
+	| { type: "extension_ui_response"; id: string; confirmed: boolean }
+	| { type: "extension_ui_response"; id: string; cancelled: true };
+
+// ============================================================================
+// Helper type for extracting command types
+// ============================================================================
+
+export type RpcCommandType = RpcCommand["type"];
diff --git a/packages/rpc-client/tsconfig.examples.json b/packages/rpc-client/tsconfig.examples.json
new file mode 100644
index 000000000..819d2d06c
--- /dev/null
+++ b/packages/rpc-client/tsconfig.examples.json
@@ -0,0 +1,17 @@
+{
+  "compilerOptions": {
+    "target": "ES2024",
+    "module": "Node16",
+    "lib": ["ES2024"],
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "moduleResolution": "Node16",
+    "noEmit": true,
+    "types": ["node"],
+    "paths": {
+      "@gsd/rpc-client": ["./src/index.ts"]
+    }
+  },
+  "include": ["examples/**/*.ts"]
+}
diff --git a/packages/rpc-client/tsconfig.json b/packages/rpc-client/tsconfig.json
new file mode 100644
index 000000000..779b48aca
--- /dev/null
+++ b/packages/rpc-client/tsconfig.json
@@ -0,0 +1,24 @@
+{
+  "compilerOptions": {
+    "target": "ES2024",
+    "module": "Node16",
+    "lib": ["ES2024"],
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "declaration": true,
+    "declarationMap": true,
+    "sourceMap": true,
+    "inlineSources": true,
+    "inlineSourceMap": false,
+    "moduleResolution": "Node16",
+    "resolveJsonModule": true,
+    "allowImportingTsExtensions": false,
+    "types": ["node"],
+    "outDir": "./dist",
+    "rootDir": "./src"
+  },
+  "include": ["src/**/*.ts"],
+  "exclude": ["node_modules", "dist", "**/*.d.ts", "src/**/*.d.ts"]
+}