Handle gateway RPC socket causes in Effect

ts/EFFECT_NATIVE_REWRITE_AUDIT.md

@@ -134,6 +134,10 @@ Notes:
   `Stream.filterMapEffect`, `Result`, and `Stream.runHead`, dropping the
   remaining `while (` count from 3 to 2. The two remaining production `while`
   hits are synchronous parsing/CLI traversal loops, not async polling loops.
+- The gateway RPC WebSocket cause-handling slice removed the Promise `.catch`
+  around the socket program by sandboxing the Effect and handling the resulting
+  `Cause` in the Effect pipeline before the Fastify fire-and-forget
+  `runPromise` boundary.
 - `Record<string, any>` and `throwLibrarianServiceError` are now clean in
   `ts/packages`.
 
@@ -962,6 +966,25 @@ Notes:
   - `cd ts && bun run test`
   - `git diff --check`
 
+### 2026-06-02: Gateway RPC WebSocket Cause Handling Slice
+
+- Status: migrated and root-verified.
+- Completed:
+  - `ts/packages/flow/src/gateway/server.ts` now handles RPC WebSocket program
+    defects and interruptions inside the Effect pipeline with `Effect.sandbox`,
+    `Effect.catch`, and `Cause.pretty`.
+  - The previous Promise `.catch(...)` around `Effect.runPromise(...)`, plus the
+    nested `Effect.runPromise` used only for logging and socket close, is removed.
+  - The outer `Effect.runPromise` remains as the Fastify WebSocket host boundary.
+- Verification:
+  - `cd ts && bun run check:tsgo`
+  - `bun run --cwd ts/packages/flow build`
+  - `bun run --cwd ts/packages/flow test`
+  - `cd ts && bun run check`
+  - `cd ts && bun run build`
+  - `cd ts && bun run test`
+  - `git diff --check`
+
 ## Subagent Findings To Preserve
 
 - MCP/workbench:

ts/packages/flow/src/gateway/server.ts

@@ -10,7 +10,7 @@
 import Fastify, { type FastifyReply } from "fastify";
 import websocketPlugin from "@fastify/websocket";
 import { NodeRuntime } from "@effect/platform-node";
-import { Clock, Config, Effect, Exit, Layer, ManagedRuntime, Random, Scope } from "effect";
+import { Cause, Clock, Config, Effect, Exit, Layer, ManagedRuntime, Random, Scope } from "effect";
 import * as O from "effect/Option";
 import * as RpcSerialization from "effect/unstable/rpc/RpcSerialization";
 import * as EffectSocket from "effect/unstable/socket/Socket";
@@ -208,19 +208,23 @@ export function createGateway(config: GatewayConfig) {
           }),
         );
 
-        void Effect.runPromise(program.pipe(Scope.provide(rpcScope))).catch((error) => {
-          void Effect.runPromise(
-            Effect.logError("[Gateway] RPC WebSocket error", { error: toTgError(error).message }).pipe(
-              Effect.flatMap(() =>
-                Effect.sync(() => {
-                  if (socket.readyState === 1) {
-                    socket.close(1011, "Internal server error");
-                  }
-                }),
-              ),
+        void Effect.runPromise(
+          program.pipe(
+            Scope.provide(rpcScope),
+            Effect.sandbox,
+            Effect.catch((cause) =>
+              Effect.logError("[Gateway] RPC WebSocket error", { error: Cause.pretty(cause) }).pipe(
+                Effect.flatMap(() =>
+                  Effect.sync(() => {
+                    if (socket.readyState === 1) {
+                      socket.close(1011, "Internal server error");
+                    }
+                  }),
+                ),
+              )
             ),
-          );
-        });
+          ),
+        );
       });
 
       // Metrics endpoint — returns Prometheus metrics from prom-client