diff --git a/AGENTS.md b/AGENTS.md
index 0802b5eb..2a6a1943 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -152,7 +152,7 @@ The client manages a singleton connection to LaunchDarkly and the associated tel
 | Export | Description |
 |---|---|
 | `initClient(options?)` | Auto-discovers and initializes `@launchdarkly/node-server-sdk` (optional peer dep, loaded via dynamic import). Optional — the first AI API call triggers lazy init when `LD_SDK_KEY` is set. Accepts optional overrides for SDK key, base URIs, service name, environment, and OTLP endpoint. Returns `Promise<LDClientInterface>`. On every successful path, including the already-initialized path, flushes `$ld:ai:sdk:info` for any LaunchDarkly AI packages that have not yet reported. |
-| `initClient(client)` | **BYOC overload** — accepts a pre-initialized `LDClientInterface` (e.g. from `@launchdarkly/vercel-server-sdk`). Stores it directly without calling the node SDK. Flushes pending `$ld:ai:sdk:info` events. |
+| `initClient(client, options?)` | **BYOC overload** — accepts a pre-initialized `LDClientInterface` (e.g. from `@launchdarkly/vercel-server-sdk`). Stores it directly without calling the node SDK. Flushes pending `$ld:ai:sdk:info` events. The optional second argument is the same options bag as the other overload. Idempotent like that overload: once a client is set, a second call returns it and ignores both a different client and the new telemetry options — `shutdown()` first to swap clients. |
 | `getClient()` | Returns the initialized `LDClientInterface`. Throws if initialization has not completed. |
 | `shutdown()` | Flushes all pending events and telemetry, then closes the client. Must be called before the process exits. Clears sdk-info reporting so a later client reports again. |
 | `waitForTelemetry()` | Waits for the OTel provider to be ready. Useful when spans must not be dropped at startup. |
diff --git a/packages/client/agents.md b/packages/client/agents.md
index 2d561e57..002b98ed 100644
--- a/packages/client/agents.md
+++ b/packages/client/agents.md
@@ -473,7 +473,7 @@ When `enabled` is `false`, `config` is always `null`. When `enabled` is `true` b
 
 - **Lazy initialization.** Importing the package does not initialize the LD client. The first API call that needs LaunchDarkly (`extractVariation`, graph resolution, etc.) calls `initClient()` internally, provided `LD_SDK_KEY` is set.
 - **Explicit initialization — Node SDK path.** `initClient(options?)` dynamically imports `@launchdarkly/node-server-sdk` at runtime (optional peer dep). If the package is not installed it throws with a clear message.
-- **Explicit initialization — BYOC path.** `initClient(client, options?)` accepts any pre-initialized object that satisfies `LDClientInterface` — this is the path for Vercel, Cloudflare, or other edge runtimes whose SDK has different init semantics. No `@launchdarkly/node-server-sdk` is required. The optional second argument is the same options bag as the other overload, passed whole to telemetry setup (`otlpEndpoint`, `serviceName` and `environment` mean the same thing).
+- **Explicit initialization — BYOC path.** `initClient(client, options?)` accepts any pre-initialized object that satisfies `LDClientInterface` — this is the path for Vercel, Cloudflare, or other edge runtimes whose SDK has different init semantics. No `@launchdarkly/node-server-sdk` is required. The optional second argument is the same options bag as the other overload, passed whole to telemetry setup (`otlpEndpoint`, `serviceName` and `environment` mean the same thing). Once a client is set, a repeat call — with the same client or a different one — returns the existing client and ignores the new telemetry options; call `shutdown()` first to swap clients.
 - **Options are ignored once the client singleton exists.** The skill store is not an option: `setSkillStore(store)` (experimental entry point) sets it independently of `initClient`, on every call, so a client that initialized lazily can be given one later. A nullish argument never clears a configured store; only `shutdown()` does, and it clears skills state unconditionally, because that state can exist without a client. A throw from that clearing is logged and does not fail `shutdown()`. Any options key `initClient` does not read, a `skillStore` key included, is ignored with a generic warning that names the key but not the feature or its setter.
 - **Return value.** `initClient()` returns `Promise<LDClientInterface>`. Callers that don't need the instance may discard the return value — this is a non-breaking change from the previous `Promise<void>` signature.
 - `getClient()` throws if `initClient()` has not resolved — any code that calls `getClient()` directly must ensure initialization has occurred.
diff --git a/packages/client/src/__tests__/lifecycle.test.ts b/packages/client/src/__tests__/lifecycle.test.ts
index 0ce59834..888977a5 100644
--- a/packages/client/src/__tests__/lifecycle.test.ts
+++ b/packages/client/src/__tests__/lifecycle.test.ts
@@ -419,6 +419,55 @@ describe('lifecycle', () => {
       await expect(initClient()).rejects.toThrow('timeout');
     });
 
+    it('does not re-run telemetry setup on a repeat BYOC call', async () => {
+      // The repeat call's options cannot take effect: OTel's globals are
+      // already held. Both the early return and setupTelemetry's one-provider
+      // guard keep the first call's configuration.
+      const { initClient } = await import('../lifecycle.js');
+      const byocClient = makeMockClient();
+      await initClient(byocClient, { serviceName: 'first', otlpEndpoint: 'https://first.test' });
+      await initClient(byocClient, { serviceName: 'second', otlpEndpoint: 'https://second.test' });
+      await initClient(byocClient);
+
+      expect(mockOTLPTraceExporter).toHaveBeenCalledOnce();
+      expect(mockOTLPTraceExporter).toHaveBeenCalledWith(
+        expect.objectContaining({ url: 'https://first.test/v1/traces' }),
+      );
+      expect(mockResourceFromAttributes).toHaveBeenCalledOnce();
+      expect(mockResourceFromAttributes).toHaveBeenCalledWith(expect.objectContaining({ 'service.name': 'first' }));
+    });
+
+    it('returns the first client on a repeat BYOC call rather than swapping it', async () => {
+      const { initClient, getClient } = await import('../lifecycle.js');
+      const first = makeMockClient();
+      const second = makeMockClient();
+      expect(await initClient(first)).toBe(first);
+      expect(await initClient(second)).toBe(first);
+      expect(getClient()).toBe(first);
+    });
+
+    it('swaps in a new BYOC client after shutdown', async () => {
+      const { initClient, getClient, shutdown } = await import('../lifecycle.js');
+      const first = makeMockClient();
+      const second = { ...makeMockClient(), close: vi.fn().mockResolvedValue(undefined) };
+      await initClient(first);
+      await shutdown();
+      expect(await initClient(second)).toBe(second);
+      expect(getClient()).toBe(second);
+    });
+
+    it('flushes registered AI package information on a repeat BYOC call', async () => {
+      const client = makeMockClient();
+      const { initClient } = await import('../lifecycle.js');
+      await initClient(client);
+      client.track.mockClear();
+      registerAiSdkPackage('@launchdarkly/ai-openai-agents', '0.1.1');
+      await initClient(client);
+
+      expect(client.track).toHaveBeenCalledOnce();
+      expect(client.track.mock.calls[0][2].aiSdkName).toBe('@launchdarkly/ai-openai-agents');
+    });
+
     it('flushes registered AI package information on the BYOC path', async () => {
       const client = makeMockClient();
       registerAiSdkPackage('@launchdarkly/ai-server', '0.1.1');
@@ -777,7 +826,7 @@ describe('lifecycle', () => {
     it('builds one provider across repeat BYOC calls', async () => {
       // A second provider would be refused registration yet replace the handle
       // shutdown flushes, so the live provider would leak and never flush.
-      const byocClient = { ...makeMockClient(), variation: vi.fn() };
+      const byocClient = makeMockClient();
       const { initClient, shutdown } = await import('../lifecycle.js');
       await initClient(byocClient);
       await initClient(byocClient);
diff --git a/packages/client/src/lifecycle.ts b/packages/client/src/lifecycle.ts
index 61b99637..eec0a7ae 100644
--- a/packages/client/src/lifecycle.ts
+++ b/packages/client/src/lifecycle.ts
@@ -111,8 +111,9 @@ async function setupTelemetry(options: InitBaseClientOptions, sdkKey: string): P
 
   // One provider at a time. A second would be refused the global registration,
   // so it would receive no spans while replacing the handle `shutdownTelemetry`
-  // flushes, leaking the live one. A repeat BYOC call reuses the existing
-  // provider, as OTel itself would; a failed init has already torn its own down.
+  // flushes, leaking the live one. A BYOC call made while an options-path init
+  // is still in flight reuses that attempt's provider, as OTel itself would; a
+  // failed init has already torn its own down.
   // Checked after the imports, since nothing between here and the assignment
   // below awaits.
   if (tracerProvider) return null;
@@ -366,6 +367,11 @@ function warnOnUnknownOptions(options: unknown): void {
  *
  * Idempotent: later calls return the existing client and ignore their options.
  *
+ * That idempotency covers overload 2 too: once a client is set, passing a
+ * *different* pre-initialized client does not swap it, and the second call's
+ * telemetry options are ignored rather than re-running telemetry setup. Call
+ * `shutdown()` first to hand the SDK a new client.
+ *
  * A call that rejects caches neither a client nor the failure: a later call
  * retries initialization (once `LD_SDK_KEY` is available, say) rather than
  * replaying the same rejection.
@@ -389,6 +395,16 @@ export async function initClient(
 
   warnOnUnknownOptions(isLDClient(optionsOrClient) ? clientOptions : optionsOrClient);
 
+  // Ahead of *both* init paths, so "a second call returns the existing client
+  // and every other option is ignored" holds for BYOC as well, as it does in
+  // the Python SDK's `_resolve_client`. Below the BYOC branch, a repeat
+  // `initClient(client)` re-ran telemetry setup with the new options, which
+  // could not take effect, and swapped the stored client.
+  if (singleton.client) {
+    flushAiSdkInfo(singleton.client);
+    return singleton.client;
+  }
+
   if (isLDClient(optionsOrClient)) {
     // Pre-initialized client path (edge / custom runtimes).
     // Still run telemetry setup (with the caller's options) so OTel traces work
@@ -401,10 +417,6 @@ export async function initClient(
     return optionsOrClient;
   }
 
-  if (singleton.client) {
-    flushAiSdkInfo(singleton.client);
-    return singleton.client;
-  }
   const pending = singleton.initPromise ?? startInit(singleton, optionsOrClient);
   let client: LDClientInterface;
   try {