diff --git a/src/runtime.ts b/src/runtime.ts
new file mode 100644
index 0000000..e203fc1
--- /dev/null
+++ b/src/runtime.ts
@@ -0,0 +1,150 @@
+/**
+ * Runtime auto-detection and lifecycle management for the `aasm` sidecar
+ * (F115 / AAASM-1205).
+ *
+ * The `initAssembly()` exported here is intentionally NOT re-exported from
+ * `@agent-assembly/sdk` at the top level: the existing gateway-based
+ * `initAssembly(config)` keeps its meaning. Opt in to the runtime-managed
+ * flow with `import { initAssembly } from "@agent-assembly/sdk/runtime"`.
+ */
+
+import { type ChildProcess, spawn } from "node:child_process";
+import { existsSync, openSync } from "node:fs";
+import { createRequire } from "node:module";
+import { createConnection } from "node:net";
+import { arch, homedir, platform } from "node:os";
+import { delimiter as PATH_DELIM, dirname, join } from "node:path";
+import { cwd, env } from "node:process";
+
+export const BINARY_NAME = "aasm";
+export const DEFAULT_PORT = 7878;
+export const DEFAULT_RUNTIME_HOST = "127.0.0.1";
+
+export const USER_LOCAL_BIN: string = join(homedir(), ".local", "bin");
+export const DOCKER_BASE_BIN = "/usr/local/bin";
+export const RUNTIME_LOG_FILENAME = ".aasm-runtime.log";
+
+/** npm sub-package name for the bundled platform binary (esbuild pattern). */
+export const RUNTIME_SUBPACKAGE: string = `runtime-${platform()}-${arch()}`;
+
+export const INSTALL_HINT: string = [
+  "agent-assembly runtime not found.",
+  "  Install with: pnpm add agent-assembly",
+  "  Or manually:  brew install agent-assembly/tap/aasm",
+  "               curl -fsSL https://get.agent-assembly.io | sh",
+].join("\n");
+
+/**
+ * Path to the platform-specific `aasm` binary bundled as an optional npm
+ * dependency, or `null` when the consumer hasn't installed that platform's
+ * `@agent-assembly/runtime-{platform}-{arch}` sub-package.
+ *
+ * Uses `createRequire(<cwd>/package.json)` — the same pattern used by
+ * `src/native/client.ts` and the framework-detection modules — so this
+ * module compiles cleanly under both the ESM and CJS build targets
+ * (`import.meta.url` is ESM-only and breaks `tsconfig.cjs.json`).
+ */
+function bundledRuntimeBinaryPath(): string | null {
+  try {
+    const requireFromCwd = createRequire(`${cwd()}/package.json`);
+    const pkgJson = requireFromCwd.resolve(`@agent-assembly/${RUNTIME_SUBPACKAGE}/package.json`);
+    return join(dirname(pkgJson), "bin", BINARY_NAME);
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Locate the `aasm` binary across the 4 supported install paths.
+ *
+ * Search order: `$PATH` (Homebrew, cargo install) → `~/.local/bin/aasm`
+ * (curl installer) → `node_modules/@agent-assembly/runtime-{platform}-{arch}/bin/aasm`
+ * (npm optionalDependency) → `/usr/local/bin/aasm` (Docker base image).
+ * Returns the first existing match, or `null` when none exist.
+ */
+export function findAasmBinary(): string | null {
+  for (const dir of (env.PATH ?? "").split(PATH_DELIM)) {
+    if (!dir) continue;
+    const candidate = join(dir, BINARY_NAME);
+    if (existsSync(candidate)) return candidate;
+  }
+  const userLocal = join(USER_LOCAL_BIN, BINARY_NAME);
+  if (existsSync(userLocal)) return userLocal;
+  const bundled = bundledRuntimeBinaryPath();
+  if (bundled !== null && existsSync(bundled)) return bundled;
+  const docker = join(DOCKER_BASE_BIN, BINARY_NAME);
+  if (existsSync(docker)) return docker;
+  return null;
+}
+
+/**
+ * Resolve to `true` iff a local TCP listener accepts a connect on
+ * `host:port` within 100 ms. Any socket error (refused, timeout,
+ * unreachable) resolves to `false` and is treated as no sidecar.
+ */
+export function isRunning(
+  port: number = DEFAULT_PORT,
+  host: string = DEFAULT_RUNTIME_HOST,
+): Promise<boolean> {
+  return new Promise((resolveResult) => {
+    const socket = createConnection({ port, host, timeout: 100 });
+    const settle = (value: boolean): void => {
+      socket.removeAllListeners();
+      socket.destroy();
+      resolveResult(value);
+    };
+    socket.once("connect", () => settle(true));
+    socket.once("timeout", () => settle(false));
+    socket.once("error", () => settle(false));
+  });
+}
+
+/**
+ * Spawn `aasm serve --port <port>` as a detached background subprocess.
+ *
+ * Stdout/stderr are appended to `<logDir>/.aasm-runtime.log` (default
+ * `process.cwd()`) so the sidecar outlives the parent. `detached: true`
+ * + `child.unref()` releases the event loop so the Node process can
+ * exit independently of the sidecar.
+ */
+export function startRuntime(
+  binaryPath: string,
+  port: number = DEFAULT_PORT,
+  logDir: string = cwd(),
+): ChildProcess {
+  const logPath = join(logDir, RUNTIME_LOG_FILENAME);
+  const fd = openSync(logPath, "a");
+  const child = spawn(binaryPath, ["serve", "--port", String(port)], {
+    detached: true,
+    stdio: ["ignore", fd, fd],
+  });
+  child.unref();
+  return child;
+}
+
+/**
+ * Ensure the local `aasm` sidecar is running, starting it if necessary.
+ *
+ * Lifecycle per F115 / AAASM-1205:
+ *  1. Probe `host:port` via {@link isRunning}; return early if already up.
+ *  2. Resolve the binary via {@link findAasmBinary}.
+ *  3. Spawn the sidecar via {@link startRuntime}.
+ *
+ * `agentId` is accepted to keep the ticket-specified signature stable;
+ * actual register-and-connect is performed by the existing gateway-aware
+ * `@agent-assembly/sdk` `initAssembly` once the sidecar is reachable.
+ *
+ * Throws `Error` with {@link INSTALL_HINT} when no binary is found.
+ */
+export async function initAssembly(
+  agentId?: string,
+  port: number = DEFAULT_PORT,
+): Promise<void> {
+  void agentId; // not consumed at the lifecycle layer; see jsdoc
+  if (await isRunning(port)) return;
+  const binary = findAasmBinary();
+  if (binary === null) {
+    throw new Error(INSTALL_HINT);
+  }
+  startRuntime(binary, port);
+}
diff --git a/tests/runtime.test.ts b/tests/runtime.test.ts
new file mode 100644
index 0000000..31bc53e
--- /dev/null
+++ b/tests/runtime.test.ts
@@ -0,0 +1,123 @@
+// Unit tests for src/runtime.ts (AAASM-1228 / F115).
+//
+// Covers the four scenarios from the AAASM-1230 AC checklist:
+//   - binary-in-PATH
+//   - binary-bundled (under node_modules/@agent-assembly/runtime-{platform}-{arch})
+//   - binary-not-found
+//   - already-running (initAssembly skips spawn when sidecar reachable)
+
+import { chmodSync, mkdirSync, mkdtempSync, realpathSync, rmSync, writeFileSync } from "node:fs";
+import { createServer } from "node:net";
+import { arch, platform, tmpdir } from "node:os";
+import { join } from "node:path";
+import { describe, expect, it } from "vitest";
+
+import { BINARY_NAME, INSTALL_HINT, findAasmBinary, initAssembly } from "../src/runtime.js";
+
+function makeFakeAasm(dir: string): string {
+  const path = join(dir, BINARY_NAME);
+  writeFileSync(path, "#!/bin/sh\nexit 0\n");
+  chmodSync(path, 0o755);
+  return path;
+}
+
+function makeBundledRuntimePackage(root: string): string {
+  const pkgDir = join(root, "node_modules", "@agent-assembly", `runtime-${platform()}-${arch()}`);
+  mkdirSync(join(pkgDir, "bin"), { recursive: true });
+  writeFileSync(join(pkgDir, "package.json"), JSON.stringify({ name: `@agent-assembly/runtime-${platform()}-${arch()}` }));
+  writeFileSync(join(root, "package.json"), JSON.stringify({ name: "test-bundled-runtime" }));
+  return makeFakeAasm(join(pkgDir, "bin"));
+}
+
+describe("runtime — F115 lifecycle", () => {
+  it("findAasmBinary returns the resolved path when binary is on $PATH", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "aasm-path-"));
+    const originalPath = process.env.PATH;
+    const originalHome = process.env.HOME;
+    try {
+      const fake = makeFakeAasm(tmp);
+      process.env.PATH = tmp;
+      process.env.HOME = "/var/empty-aasm-no-home";
+
+      expect(findAasmBinary()).toBe(fake);
+    } finally {
+      process.env.PATH = originalPath;
+      process.env.HOME = originalHome;
+      rmSync(tmp, { recursive: true, force: true });
+    }
+  });
+
+  it("findAasmBinary returns the bundled-runtime path when the npm optional sub-package is installed", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "aasm-bundled-"));
+    const originalCwd = process.cwd();
+    const originalPath = process.env.PATH;
+    const originalHome = process.env.HOME;
+    try {
+      const fake = makeBundledRuntimePackage(tmp);
+      process.chdir(tmp);
+      process.env.PATH = join(tmp, "no-such-path");
+      process.env.HOME = join(tmp, "no-such-home");
+
+      // createRequire.resolve canonicalises symlinks (on macOS, /var/folders
+      // → /private/var/folders); compare against the realpath form.
+      expect(findAasmBinary()).toBe(realpathSync(fake));
+    } finally {
+      process.chdir(originalCwd);
+      process.env.PATH = originalPath;
+      process.env.HOME = originalHome;
+      rmSync(tmp, { recursive: true, force: true });
+    }
+  });
+
+  it("initAssembly throws Error with INSTALL_HINT when binary not found", async () => {
+    const tmp = mkdtempSync(join(tmpdir(), "aasm-missing-"));
+    const originalCwd = process.cwd();
+    const originalPath = process.env.PATH;
+    const originalHome = process.env.HOME;
+    try {
+      // Empty cwd (no node_modules), empty PATH/HOME → every search path misses.
+      writeFileSync(join(tmp, "package.json"), JSON.stringify({ name: "test-missing" }));
+      process.chdir(tmp);
+      process.env.PATH = join(tmp, "no-such-path");
+      process.env.HOME = join(tmp, "no-such-home");
+
+      await expect(initAssembly()).rejects.toThrowError(INSTALL_HINT);
+      await expect(initAssembly()).rejects.toThrowError(/agent-assembly runtime not found/);
+    } finally {
+      process.chdir(originalCwd);
+      process.env.PATH = originalPath;
+      process.env.HOME = originalHome;
+      rmSync(tmp, { recursive: true, force: true });
+    }
+  });
+
+  it("initAssembly is idempotent when a sidecar is already running on the target port", async () => {
+    // Bind an ephemeral port and pass it explicitly; this avoids a fixed-port
+    // collision with whatever might be on 7878 on the host machine.
+    const server = createServer();
+    await new Promise<void>((resolve) => server.listen(0, "127.0.0.1", resolve));
+    const address = server.address();
+    if (typeof address === "string" || address === null) {
+      throw new Error("expected AddressInfo from createServer().address()");
+    }
+    const port = address.port;
+
+    const originalPath = process.env.PATH;
+    const originalHome = process.env.HOME;
+    try {
+      // Pre-load every search path with a guaranteed miss; the orchestrator
+      // should still succeed because is_running short-circuits ahead of
+      // findAasmBinary.
+      process.env.PATH = "/var/empty-aasm-no-path";
+      process.env.HOME = "/var/empty-aasm-no-home";
+
+      await expect(initAssembly(undefined, port)).resolves.toBeUndefined();
+    } finally {
+      process.env.PATH = originalPath;
+      process.env.HOME = originalHome;
+      await new Promise<void>((resolve, reject) =>
+        server.close((err) => (err ? reject(err) : resolve()))
+      );
+    }
+  });
+});