diff --git a/src/observability/logger/root.test.ts b/src/observability/logger/root.test.ts
new file mode 100644
index 00000000..eb4b432b
--- /dev/null
+++ b/src/observability/logger/root.test.ts
@@ -0,0 +1,51 @@
+import { describe, expect, it, vi, afterEach } from "vitest";
+
+describe("createRootLogger", () => {
+  afterEach(async () => {
+    const mod = await import("./root.js");
+    mod.__resetRootLoggerForTests();
+    vi.resetModules();
+  });
+
+  it("returns a Logger with console transport in development", async () => {
+    process.env["NODE_ENV"] = "development";
+    const { createRootLogger, __resetRootLoggerForTests } = await import("./root.js");
+    __resetRootLoggerForTests();
+    const logger = createRootLogger();
+    expect(logger).toBeDefined();
+    expect(typeof logger.info).toBe("function");
+    expect(typeof logger.child).toBe("function");
+  });
+
+  it("returns a Logger with JSON-lines transport in production", async () => {
+    process.env["NODE_ENV"] = "production";
+    process.env["LOGS_ENDPOINT"] = "https://logs.example/ingest";
+    const { createRootLogger, __resetRootLoggerForTests } = await import("./root.js");
+    __resetRootLoggerForTests();
+    const logger = createRootLogger();
+    expect(logger).toBeDefined();
+    expect(typeof logger.info).toBe("function");
+  });
+
+  it("caches the logger instance (returns same object on repeated calls)", async () => {
+    process.env["NODE_ENV"] = "development";
+    const { createRootLogger, __resetRootLoggerForTests } = await import("./root.js");
+    __resetRootLoggerForTests();
+    const a = createRootLogger();
+    const b = createRootLogger();
+    expect(a).toBe(b);
+  });
+
+  it("child() produces a Logger with merged context", async () => {
+    process.env["NODE_ENV"] = "development";
+    const spy = vi.spyOn(console, "info").mockImplementation(() => {});
+    const { createRootLogger, __resetRootLoggerForTests } = await import("./root.js");
+    __resetRootLoggerForTests();
+    const logger = createRootLogger();
+    const child = logger.child({ traceId: "test-123" });
+    child.info("hello");
+    expect(spy).toHaveBeenCalledTimes(1);
+    expect(spy.mock.calls[0]?.[0]).toContain("test-123");
+    spy.mockRestore();
+  });
+});
diff --git a/src/observability/logger/root.ts b/src/observability/logger/root.ts
new file mode 100644
index 00000000..8e6b9d00
--- /dev/null
+++ b/src/observability/logger/root.ts
@@ -0,0 +1,39 @@
+import type { Logger, LogTransport } from "./types.js";
+import { LoggerImpl } from "./logger-impl.js";
+import { ConsoleTransport } from "./console-transport.js";
+import { JsonLinesHttpTransport } from "./json-lines-transport.js";
+
+let cached: Logger | undefined;
+
+/**
+ * Creates or returns the cached root logger. In development, uses
+ * ConsoleTransport. In other envs, uses JsonLinesHttpTransport if
+ * LOGS_ENDPOINT is set, otherwise falls back to console.
+ */
+export function createRootLogger(): Logger {
+  if (cached) return cached;
+
+  const env = process.env["NODE_ENV"] ?? "development";
+  const logsEndpoint = process.env["LOGS_ENDPOINT"];
+
+  let transport: LogTransport;
+
+  if (env === "development" || !logsEndpoint) {
+    transport = new ConsoleTransport();
+  } else {
+    transport = new JsonLinesHttpTransport({
+      endpoint: logsEndpoint,
+      batchSize: 50,
+      flushIntervalMs: 5000,
+      maxBufferSize: 500,
+    });
+  }
+
+  cached = new LoggerImpl(transport);
+  return cached;
+}
+
+/** Test-only: clears the cached root logger. */
+export function __resetRootLoggerForTests(): void {
+  cached = undefined;
+}