fix(gateway): skip restart when config.patch has no actual changes (#58502)

config.patch unconditionally writes the config file and sends SIGUSR1
even when diffConfigPaths detects zero changed paths. This causes a
full gateway restart (~10s downtime, all SSE/WebSocket connections
dropped) on every control-plane config.patch call, even when the
config is identical — e.g. a model hot-apply that doesn't change any
gateway.* paths.

Fix: when changedPaths is empty, return early with `noop: true`
without writing the file or scheduling SIGUSR1. The validated config
is still returned so the caller knows the current state.

This lets control-plane clients safely call config.patch for
idempotent updates without triggering unnecessary restarts.

src/gateway/server-methods/config.ts

@@ -476,6 +476,28 @@ export const configHandlers: GatewayRequestHandlers = {
     }
     const changedPaths = diffConfigPaths(snapshot.config, validated.config);
     const actor = resolveControlPlaneActor(client);
+
+    // No-op: if the validated config is identical to the current config,
+    // skip the file write and SIGUSR1 restart entirely. This avoids a full
+    // gateway restart (and the resulting connection drop) when a control-plane
+    // client re-sends the same config (e.g. hot-apply with no actual changes).
+    if (changedPaths.length === 0) {
+      context?.logGateway?.info(
+        `config.patch noop ${formatControlPlaneActor(actor)} (no changed paths)`,
+      );
+      respond(
+        true,
+        {
+          ok: true,
+          noop: true,
+          path: createConfigIO().configPath,
+          config: redactConfigObject(validated.config, schemaPatch.uiHints),
+        },
+        undefined,
+      );
+      return;
+    }
+
     context?.logGateway?.info(
       `config.patch write ${formatControlPlaneActor(actor)} changedPaths=${summarizeChangedPaths(changedPaths)} restartReason=config.patch`,
     );

src/gateway/server.config-patch.test.ts

@@ -252,6 +252,30 @@ describe("gateway config methods", () => {
     expect(res.error?.message).toBe("config schema path not found");
   });
 
+  it("returns noop for config.patch when config is unchanged", async () => {
+    const current = await rpcReq<{
+      config?: Record<string, unknown>;
+      hash?: string;
+    }>(requireWs(), "config.get", {});
+    expect(current.ok).toBe(true);
+
+    // Patch with the same config — no actual changes
+    const res = await rpcReq<{
+      ok?: boolean;
+      noop?: boolean;
+      config?: Record<string, unknown>;
+    }>(requireWs(), "config.patch", {
+      raw: JSON.stringify(current.payload?.config ?? {}),
+      baseHash: current.payload?.hash,
+    });
+
+    expect(res.ok).toBe(true);
+    expect(res.payload?.noop).toBe(true);
+    // Config hash should not change (no file write)
+    const after = await rpcReq<{ hash?: string }>(requireWs(), "config.get", {});
+    expect(after.payload?.hash).toBe(current.payload?.hash);
+  });
+
   it("rejects config.patch when raw is null", async () => {
     const res = await rpcReq<{ ok?: boolean }>(requireWs(), "config.patch", {
       raw: "null",