diff --git a/packages/coding-agent/src/modes/rpc/rpc-client.ts b/packages/coding-agent/src/modes/rpc/rpc-client.ts
index 7596cb2c2..2b08b0f6b 100644
--- a/packages/coding-agent/src/modes/rpc/rpc-client.ts
+++ b/packages/coding-agent/src/modes/rpc/rpc-client.ts
@@ -123,23 +123,54 @@ export class RpcClient {
 			stdin: "pipe",
 		});
 
-		// Process lines in background
+		// Wait for the "ready" signal or process exit
+		const { promise: readyPromise, resolve: readyResolve, reject: readyReject } = Promise.withResolvers<void>();
+		let readySettled = false;
+
+		// Process lines in background, intercepting the ready signal
 		const lines = readJsonl(this.#process.stdout, this.#abortController.signal);
 		void (async () => {
 			for await (const line of lines) {
+				if (!readySettled && isRecord(line) && line.type === "ready") {
+					readySettled = true;
+					readyResolve();
+					continue;
+				}
 				this.#handleLine(line);
 			}
-		})().catch(() => {});
+			// Stream ended without ready signal — process exited
+			if (!readySettled) {
+				readySettled = true;
+				readyReject(new Error(`Agent process exited before ready. Stderr: ${this.#process?.peekStderr() ?? ""}`));
+			}
+		})().catch((err: Error) => {
+			if (!readySettled) {
+				readySettled = true;
+				readyReject(err);
+			}
+		});
 
-		// Wait a moment for process to initialize
-		await Bun.sleep(100);
+		// Also race against process exit (in case stdout closes before we read it)
+		void this.#process.exited.then((exitCode: number) => {
+			if (!readySettled) {
+				readySettled = true;
+				readyReject(
+					new Error(`Agent process exited with code ${exitCode}. Stderr: ${this.#process?.peekStderr() ?? ""}`),
+				);
+			}
+		});
 
-		const exitCode = await Promise.race([this.#process.exited, Bun.sleep(500).then(() => null)]);
-		if (exitCode !== null) {
-			throw new Error(
-				`Agent process exited immediately with code ${exitCode}. Stderr: ${this.#process.peekStderr()}`,
-			);
-		}
+		// Timeout to prevent hanging forever
+		void Bun.sleep(30000).then(() => {
+			if (!readySettled) {
+				readySettled = true;
+				readyReject(
+					new Error(`Timeout waiting for agent to become ready. Stderr: ${this.#process?.peekStderr() ?? ""}`),
+				);
+			}
+		});
+
+		await readyPromise;
 	}
 
 	/**
diff --git a/packages/coding-agent/src/modes/rpc/rpc-mode.ts b/packages/coding-agent/src/modes/rpc/rpc-mode.ts
index 4988913f3..66c75893a 100644
--- a/packages/coding-agent/src/modes/rpc/rpc-mode.ts
+++ b/packages/coding-agent/src/modes/rpc/rpc-mode.ts
@@ -36,6 +36,8 @@ export type {
  * Listens for JSON commands on stdin, outputs events and responses on stdout.
  */
 export async function runRpcMode(session: AgentSession): Promise<never> {
+	// Signal to RPC clients that the server is ready to accept commands
+	process.stdout.write(`${JSON.stringify({ type: "ready" })}\n`);
 	const output = (obj: RpcResponse | RpcExtensionUIRequest | object) => {
 		process.stdout.write(`${JSON.stringify(obj)}\n`);
 	};
diff --git a/packages/coding-agent/test/rpc-client.start.test.ts b/packages/coding-agent/test/rpc-client.start.test.ts
index 7009b93ef..f938564b1 100644
--- a/packages/coding-agent/test/rpc-client.start.test.ts
+++ b/packages/coding-agent/test/rpc-client.start.test.ts
@@ -12,6 +12,6 @@ describe("RpcClient.start", () => {
 			env: { PI_NO_TITLE: "1" },
 		});
 
-		await expect(client.start()).rejects.toThrow('Unknown provider "__missing_provider__"');
+		await expect(client.start()).rejects.toThrow(/Unknown provider.*__missing_provider__/);
 	});
 });