diff --git a/PATCHES.md b/PATCHES.md
index 158e2c1..449b33f 100644
--- a/PATCHES.md
+++ b/PATCHES.md
@@ -205,6 +205,36 @@ rebuild whenever a guest patch changes. Built locally, not in CI: the host frame
      into a never-accepted backlog. A failed pre-relay connection is also closed explicitly.
    Marked `[Nucleic vendored patch]`.
 
+14. **Non-blocking, non-spinning guest I/O plane (`IOPair.swift`, `OSFile+Splice.swift`,
+   `VsockProxy.swift`) — the root cause of the "control plane unresponsive / all sessions stall"
+   wedge.** Every exec's stdio relay (`IOPair`) and every control-plane relay connection
+   (`VsockProxy`) share ONE thread: `ProcessSupervisor.default`'s epoll poller. Three defects let
+   a single slow peer freeze that thread — and with it every session's stdio AND the whole
+   container's control plane at once (probes then read "accepts connections but never answers";
+   before the manager-side recovery rework the host restarted the container over this, SIGKILLing
+   every session):
+   - **`IOPair` blocking write:** only *registered* fds get `O_NONBLOCK` (set by `Epoll.add`), and
+     the relay registers only its READ fd — the write fd (e.g. the vsock socket carrying an exec's
+     stdout to the host) stayed blocking. One slow-drained stream parked the poller thread in
+     `write(2)`. The relay now sets the write fd non-blocking up front and implements real
+     backpressure: a full destination stashes the remainder in a `pending` backlog, registers the
+     write fd for EPOLLOUT, and suspends reads until the flush completes (throttling the producing
+     process via its own pipe, not the shared thread). The old close-on-short-write path — dead
+     while the fd was blocking, live and stdio-dropping once non-blocking — is subsumed by the
+     backlog; genuine write errors still close the pair.
+   - **`OSFile.splice` busy-spin:** when the destination was full (EAGAIN) and the source idle,
+     the outer loop had no exit — it spun the poller thread at 100% until the peer drained (or
+     forever if it never did). The flush leg's EAGAIN branch now returns partial progress; the
+     un-flushed bytes stay in the transfer pipe and the destination's EPOLLOUT edge resumes the
+     flush (both `VsockProxy` handlers already pump both directions on both events).
+   - **`VsockProxy` registration race:** the client fd's epoll handler can fire — and splice
+     toward the server fd — before the second registration makes that fd non-blocking; a full
+     destination made that a genuinely blocking splice on the poller thread. Both fds are now set
+     non-blocking before either is registered.
+   All three are guest-side and INERT until the initfs image is rebuilt (`make vminit-image`, tag
+   `0.34.0-nucleic4`) and `ContainerEngine.vminitReference` is bumped after runtime validation.
+   Marked `[Nucleic vendored patch]`.
+
 ## Re-vendoring a newer upstream commit
 
 1. `git clone` upstream (or copy `.build/checkouts/containerization` after bumping the URL pin
@@ -222,9 +252,11 @@ rebuild whenever a guest patch changes. Built locally, not in CI: the host frame
    `Cgroup2Manager.swift` / `ManagedContainer.swift` / `ManagedProcess.swift`, patch #10
    (`Socket.acceptStream` transient-error tolerance + `isTransientAcceptError`), patch #11 (the
    `UnixSocketRelay` per-connection containment + fail-fast closes), patch #12 (the `VsockProxy`
-   cleanup/`try!`/listener hardening in `vminitd/`), and patch #13 (the prompt-free
+   cleanup/`try!`/listener hardening in `vminitd/`), patch #13 (the prompt-free
    `KeychainQuery` reads: `withoutInteractiveUI` + the `errSecInteractionNotAllowed` handling +
-   the `save` duplicate retry). After re-applying any
+   the `save` duplicate retry), and patch #14 (the non-blocking/non-spinning guest I/O plane:
+   `IOPair` backpressure, the `OSFile.splice` EAGAIN return, and the `VsockProxy` pre-registration
+   non-blocking fds — all in `vminitd/`). After re-applying any
    `vminitd/` patch, rebuild + publish the custom init image
    with `make vminit-image` + `make vminit-image-push`, and bump `ContainerEngine.vminitReference`.
 5. Update the commit hash above and in the root `Package.swift` comment.
diff --git a/vminitd/Package.resolved b/vminitd/Package.resolved
index e37bc3a..c1dc9ac 100644
--- a/vminitd/Package.resolved
+++ b/vminitd/Package.resolved
@@ -1,249 +1,258 @@
 {
-  "originHash" : "6ccceb47b6a402e9ac07d23204ec7f4792823b22b96275cd67f8531787a60c04",
-  "pins" : [
+  "originHash": "264b211a5ea74fa24ced86faade5901700722c925484a26379cc4a6b40083c6c",
+  "pins": [
     {
-      "identity" : "async-http-client",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/swift-server/async-http-client.git",
-      "state" : {
-        "revision" : "4b99975677236d13f0754339864e5360142ff5a1",
-        "version" : "1.30.3"
+      "identity": "async-http-client",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/swift-server/async-http-client.git",
+      "state": {
+        "revision": "4603a8036d921ea999fadb742931546c341f4bd7",
+        "version": "1.35.0"
       }
     },
     {
-      "identity" : "grpc-swift-2",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/grpc/grpc-swift-2.git",
-      "state" : {
-        "revision" : "f28854bc760a116e053fdfc4a48a9428c34625c0",
-        "version" : "2.3.0"
+      "identity": "grpc-swift-2",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/grpc/grpc-swift-2.git",
+      "state": {
+        "revision": "28cdd63ef88583ddc67d7bb179eab46fab465ce9",
+        "version": "2.4.2"
       }
     },
     {
-      "identity" : "grpc-swift-nio-transport",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/grpc/grpc-swift-nio-transport.git",
-      "state" : {
-        "revision" : "f37e0c2d293cea668b11e10e1fb1c24cb40781ff",
-        "version" : "2.4.4"
+      "identity": "grpc-swift-nio-transport",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/grpc/grpc-swift-nio-transport.git",
+      "state": {
+        "revision": "2ca31f06658ed288a2560e23ad649acbb3d6b3a3",
+        "version": "2.9.0"
       }
     },
     {
-      "identity" : "grpc-swift-protobuf",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/grpc/grpc-swift-protobuf.git",
-      "state" : {
-        "revision" : "19153231a03c2fda1f4ea60da1b92a2cb9c011d8",
-        "version" : "2.2.0"
+      "identity": "grpc-swift-protobuf",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/grpc/grpc-swift-protobuf.git",
+      "state": {
+        "revision": "176c5a434fd76f6f479848d1a8f7d44967534168",
+        "version": "2.4.1"
       }
     },
     {
-      "identity" : "swift-algorithms",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-algorithms.git",
-      "state" : {
-        "revision" : "87e50f483c54e6efd60e885f7f5aa946cee68023",
-        "version" : "1.2.1"
+      "identity": "swift-algorithms",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-algorithms.git",
+      "state": {
+        "revision": "87e50f483c54e6efd60e885f7f5aa946cee68023",
+        "version": "1.2.1"
       }
     },
     {
-      "identity" : "swift-argument-parser",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-argument-parser.git",
-      "state" : {
-        "revision" : "c5d11a805e765f52ba34ec7284bd4fcd6ba68615",
-        "version" : "1.7.0"
+      "identity": "swift-argument-parser",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-argument-parser.git",
+      "state": {
+        "revision": "6a52f3251125d74daf04fcbd5e6f08a75d074382",
+        "version": "1.8.2"
       }
     },
     {
-      "identity" : "swift-asn1",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-asn1.git",
-      "state" : {
-        "revision" : "a54383ada6cecde007d374f58f864e29370ba5c3",
-        "version" : "1.3.2"
+      "identity": "swift-asn1",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-asn1.git",
+      "state": {
+        "revision": "a9a5efd40eaf558a2bcd48d64b1d1646be686008",
+        "version": "1.7.1"
       }
     },
     {
-      "identity" : "swift-async-algorithms",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-async-algorithms.git",
-      "state" : {
-        "revision" : "042e1c4d9d19748c9c228f8d4ebc97bb1e339b0b",
-        "version" : "1.0.4"
+      "identity": "swift-async-algorithms",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-async-algorithms.git",
+      "state": {
+        "revision": "3da39bbc4e687d4192af7c9cf4eab805745a0b9c",
+        "version": "1.1.5"
       }
     },
     {
-      "identity" : "swift-atomics",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-atomics.git",
-      "state" : {
-        "revision" : "cd142fd2f64be2100422d658e7411e39489da985",
-        "version" : "1.2.0"
+      "identity": "swift-atomics",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-atomics.git",
+      "state": {
+        "revision": "0442cb5a3f98ab802acb777929fdb446bda11a34",
+        "version": "1.3.1"
       }
     },
     {
-      "identity" : "swift-certificates",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-certificates.git",
-      "state" : {
-        "revision" : "f4cd9e78a1ec209b27e426a5f5c693675f95e75a",
-        "version" : "1.15.0"
+      "identity": "swift-certificates",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-certificates.git",
+      "state": {
+        "revision": "89fbc3714264cce8db8e4ec51b64e01c3e28c6c5",
+        "version": "1.19.3"
       }
     },
     {
-      "identity" : "swift-collections",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-collections.git",
-      "state" : {
-        "revision" : "c1805596154bb3a265fd91b8ac0c4433b4348fb0",
-        "version" : "1.2.0"
+      "identity": "swift-collections",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-collections.git",
+      "state": {
+        "revision": "a0cb0954ecb21e4e31b0070e6ed5674e8556685a",
+        "version": "1.6.0"
       }
     },
     {
-      "identity" : "swift-crypto",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-crypto.git",
-      "state" : {
-        "revision" : "e8d6eba1fef23ae5b359c46b03f7d94be2f41fed",
-        "version" : "3.12.3"
+      "identity": "swift-configuration",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-configuration.git",
+      "state": {
+        "revision": "be76c4ad929eb6c4bcaf3351799f2adf9e6848a9",
+        "version": "1.2.0"
       }
     },
     {
-      "identity" : "swift-distributed-tracing",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-distributed-tracing.git",
-      "state" : {
-        "revision" : "dc4030184203ffafbb2ec614352487235d747fe0",
-        "version" : "1.4.1"
+      "identity": "swift-crypto",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-crypto.git",
+      "state": {
+        "revision": "95ba0316a9b733e92bb6b071255ff46263bbe7dc",
+        "version": "3.15.1"
       }
     },
     {
-      "identity" : "swift-http-structured-headers",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-http-structured-headers.git",
-      "state" : {
-        "revision" : "db6eea3692638a65e2124990155cd220c2915903",
-        "version" : "1.3.0"
+      "identity": "swift-distributed-tracing",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-distributed-tracing.git",
+      "state": {
+        "revision": "dc4030184203ffafbb2ec614352487235d747fe0",
+        "version": "1.4.1"
       }
     },
     {
-      "identity" : "swift-http-types",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-http-types.git",
-      "state" : {
-        "revision" : "a0a57e949a8903563aba4615869310c0ebf14c03",
-        "version" : "1.4.0"
+      "identity": "swift-http-structured-headers",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-http-structured-headers.git",
+      "state": {
+        "revision": "933538faa42c432d385f02e07df0ace7c5ecfc47",
+        "version": "1.7.0"
       }
     },
     {
-      "identity" : "swift-log",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-log.git",
-      "state" : {
-        "revision" : "bbd81b6725ae874c69e9b8c8804d462356b55523",
-        "version" : "1.10.1"
+      "identity": "swift-http-types",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-http-types.git",
+      "state": {
+        "revision": "db774a277f60063a32d854f2980299caf06da041",
+        "version": "1.6.0"
       }
     },
     {
-      "identity" : "swift-nio",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-nio.git",
-      "state" : {
-        "revision" : "4e8f4b1c9adaa59315c523540c1ff2b38adc20a9",
-        "version" : "2.87.0"
+      "identity": "swift-log",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-log.git",
+      "state": {
+        "revision": "a878e7f8f46cfc0e1125e565b5c08e7d5272dc9a",
+        "version": "1.14.0"
       }
     },
     {
-      "identity" : "swift-nio-extras",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-nio-extras.git",
-      "state" : {
-        "revision" : "145db1962f4f33a4ea07a32e751d5217602eea29",
-        "version" : "1.28.0"
+      "identity": "swift-nio",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-nio.git",
+      "state": {
+        "revision": "cd3e1152083706d77b223fb29110e590efcc70c0",
+        "version": "2.101.2"
       }
     },
     {
-      "identity" : "swift-nio-http2",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-nio-http2.git",
-      "state" : {
-        "revision" : "5e9e99ec96c53bc2c18ddd10c1e25a3cd97c55e5",
-        "version" : "1.38.0"
+      "identity": "swift-nio-extras",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-nio-extras.git",
+      "state": {
+        "revision": "88a51340f59cf181ebde888bd1b749296b3ec029",
+        "version": "1.34.3"
       }
     },
     {
-      "identity" : "swift-nio-ssl",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-nio-ssl.git",
-      "state" : {
-        "revision" : "173cc69a058623525a58ae6710e2f5727c663793",
-        "version" : "2.36.0"
+      "identity": "swift-nio-http2",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-nio-http2.git",
+      "state": {
+        "revision": "61d1b44f6e4e118792be1cff88ee2bc0267c6f9a",
+        "version": "1.44.0"
       }
     },
     {
-      "identity" : "swift-nio-transport-services",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-nio-transport-services.git",
-      "state" : {
-        "revision" : "cd1e89816d345d2523b11c55654570acd5cd4c56",
-        "version" : "1.24.0"
+      "identity": "swift-nio-ssl",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-nio-ssl.git",
+      "state": {
+        "revision": "407d82d5b6cc00e1c3fb83a81b1539b70c788c5e",
+        "version": "2.37.1"
       }
     },
     {
-      "identity" : "swift-numerics",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-numerics.git",
-      "state" : {
-        "revision" : "e0ec0f5f3af6f3e4d5e7a19d2af26b481acb6ba8",
-        "version" : "1.0.3"
+      "identity": "swift-nio-transport-services",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-nio-transport-services.git",
+      "state": {
+        "revision": "67787bb645a5e67d2edcdfbe48a216cc549222d5",
+        "version": "1.28.0"
       }
     },
     {
-      "identity" : "swift-protobuf",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-protobuf.git",
-      "state" : {
-        "revision" : "86970144a0b86068c81ff48ee29b3f97cae0b879",
-        "version" : "1.36.0"
+      "identity": "swift-numerics",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-numerics.git",
+      "state": {
+        "revision": "0c0290ff6b24942dadb83a929ffaaa1481df04a2",
+        "version": "1.1.1"
       }
     },
     {
-      "identity" : "swift-service-context",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-service-context.git",
-      "state" : {
-        "revision" : "d0997351b0c7779017f88e7a93bc30a1878d7f29",
-        "version" : "1.3.0"
+      "identity": "swift-protobuf",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-protobuf.git",
+      "state": {
+        "revision": "55d7a1cc5666b85c13464aea1c4b4a90feccb4c8",
+        "version": "1.38.1"
       }
     },
     {
-      "identity" : "swift-service-lifecycle",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/swift-server/swift-service-lifecycle.git",
-      "state" : {
-        "revision" : "e7187309187695115033536e8fc9b2eb87fd956d",
-        "version" : "2.8.0"
+      "identity": "swift-service-context",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-service-context.git",
+      "state": {
+        "revision": "d0997351b0c7779017f88e7a93bc30a1878d7f29",
+        "version": "1.3.0"
       }
     },
     {
-      "identity" : "swift-system",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/apple/swift-system.git",
-      "state" : {
-        "revision" : "7c6ad0fc39d0763e0b699210e4124afd5041c5df",
-        "version" : "1.6.4"
+      "identity": "swift-service-lifecycle",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/swift-server/swift-service-lifecycle.git",
+      "state": {
+        "revision": "9829955b385e5bb88128b73f1b8389e9b9c3191a",
+        "version": "2.11.0"
       }
     },
     {
-      "identity" : "zstd",
-      "kind" : "remoteSourceControl",
-      "location" : "https://github.com/facebook/zstd.git",
-      "state" : {
-        "revision" : "f8745da6ff1ad1e7bab384bd1f9d742439278e99",
-        "version" : "1.5.7"
+      "identity": "swift-system",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/apple/swift-system.git",
+      "state": {
+        "revision": "b5544ba79a70a0cb3563e75bf26dc198d6b40ed3",
+        "version": "1.7.4"
+      }
+    },
+    {
+      "identity": "zstd",
+      "kind": "remoteSourceControl",
+      "location": "https://github.com/facebook/zstd.git",
+      "state": {
+        "revision": "f8745da6ff1ad1e7bab384bd1f9d742439278e99",
+        "version": "1.5.7"
       }
     }
   ],
-  "version" : 3
+  "version": 3
 }
diff --git a/vminitd/Sources/VminitdCore/IOPair.swift b/vminitd/Sources/VminitdCore/IOPair.swift
index 7d9df42..372554d 100644
--- a/vminitd/Sources/VminitdCore/IOPair.swift
+++ b/vminitd/Sources/VminitdCore/IOPair.swift
@@ -33,6 +33,16 @@ final class IOPair: Sendable {
         let buffer: UnsafeMutableBufferPointer<UInt8>
         var closed: Bool
         var registeredFd: Int32?
+        // [Nucleic vendored patch] Backpressure state: bytes read from `from` that `to` couldn't
+        // take yet (its buffer was full), plus whether `to` is currently registered for EPOLLOUT
+        // to flush them. While `pending` is non-empty the relay reads nothing more — the source
+        // pipe backs up and throttles the *producing process* instead of this thread. The write
+        // fd used to be BLOCKING (only registered fds get O_NONBLOCK, and it never was), so one
+        // slow-drained stream parked the shared ProcessSupervisor poller thread — freezing every
+        // exec's stdio and every control-plane relay in the container at once.
+        var pending: [UInt8]
+        var pendingOffset: Int
+        var writeFdRegistered: Bool
 
         func drain() {
             let readFrom = OSFile(fd: from.fileDescriptor)
@@ -66,10 +76,27 @@ final class IOPair: Sendable {
                 return
             }
 
-            // Try and drain IO first.
-            self.drain()
+            // [Nucleic vendored patch] Flush what we can IN ORDER: the pending backlog first,
+            // then (only if it fully flushed) a best-effort drain of the source. Draining with
+            // unsent pending bytes would reorder the stream.
+            let writeTo = OSFile(fd: to.fileDescriptor)
+            while pendingOffset < pending.count {
+                let offset = pendingOffset
+                let result = pending.withUnsafeMutableBufferPointer { buf in
+                    writeTo.write(
+                        UnsafeMutableBufferPointer(
+                            start: buf.baseAddress!.advanced(by: offset),
+                            count: buf.count - offset))
+                }
+                if result.wrote > 0 { pendingOffset += result.wrote }
+                if result.action != .success { break }
+            }
+            if pendingOffset >= pending.count {
+                // Try and drain IO first.
+                self.drain()
+            }
 
-            // Remove the fd from our global epoll instance first.
+            // Remove the fds from our global epoll instance first.
             if let fd = self.registeredFd {
                 do {
                     try ProcessSupervisor.default.unregisterFd(fd)
@@ -78,6 +105,15 @@ final class IOPair: Sendable {
                 }
                 self.registeredFd = nil
             }
+            // [Nucleic vendored patch] The write fd may be registered for backpressure flushing.
+            if self.writeFdRegistered {
+                do {
+                    try ProcessSupervisor.default.unregisterFd(to.fileDescriptor)
+                } catch {
+                    logger?.error("failed to delete write fd from epoll \(to.fileDescriptor): \(error)")
+                }
+                self.writeFdRegistered = false
+            }
 
             do {
                 try self.from.close()
@@ -108,7 +144,10 @@ final class IOPair: Sendable {
                 to: writeTo,
                 buffer: buffer,
                 closed: false,
-                registeredFd: nil
+                registeredFd: nil,
+                pending: [],
+                pendingOffset: 0,
+                writeFdRegistered: false
             ))
         self.reason = reason
         self.logger = logger
@@ -122,8 +161,16 @@ final class IOPair: Sendable {
             return (io.from.fileDescriptor, io.to.fileDescriptor)
         }
 
-        let readFrom = OSFile(fd: readFromFd)
-        let writeTo = OSFile(fd: writeToFd)
+        // [Nucleic vendored patch] The write fd must be non-blocking BEFORE the first relay write.
+        // `Epoll.add` only sets O_NONBLOCK on fds it registers, and the write fd is registered only
+        // on demand (EPOLLOUT backpressure below) — so without this, the very first full-buffer
+        // write blocked the shared poller thread.
+        let flags = fcntl(writeToFd, F_GETFL)
+        if flags == -1 || fcntl(writeToFd, F_SETFL, flags | O_NONBLOCK) == -1 {
+            self.logger?.error(
+                "failed to set relay write fd non-blocking",
+                metadata: ["fd": "\(writeToFd)", "errno": "\(errno)"])
+        }
 
         try ProcessSupervisor.default.registerFd(readFromFd, mask: .input) { mask in
             self.io.withLock { io in
@@ -139,42 +186,129 @@ final class IOPair: Sendable {
                     return
                 }
 
-                // Loop so we drain fully.
-                while true {
-                    let r = readFrom.read(io.buffer)
-                    if r.read > 0 {
-                        let view = UnsafeMutableBufferPointer(
-                            start: io.buffer.baseAddress,
-                            count: r.read
-                        )
+                self.pump(&io, mask: mask, ignoreHup: ignoreHup)
+            }
+        }
+    }
 
-                        let w = writeTo.write(view)
-                        if w.wrote != r.read {
-                            self.logger?.error("stopping relay: short write for stdio")
-                            io.close(logger: self.logger)
-                            return
-                        }
-                    }
+    /// [Nucleic vendored patch] One relay pass, non-blocking end to end: flush any pending
+    /// backlog toward `to`, then (only once it's empty) drain `from`. On a full destination the
+    /// remainder is stashed in `pending` and the write fd registered for EPOLLOUT, whose edge
+    /// re-enters this pump — so backpressure suspends the relay instead of blocking or spinning
+    /// the shared poller thread. Must be called with the `io` lock held.
+    private func pump(_ io: inout IO, mask: Epoll.Mask, ignoreHup: Bool) {
+        let readFrom = OSFile(fd: io.from.fileDescriptor)
+        let writeTo = OSFile(fd: io.to.fileDescriptor)
 
-                    switch r.action {
-                    case .error(let errno):
-                        self.logger?.error("failed with errno \(errno) while reading for fd \(readFromFd)")
-                        fallthrough
-                    case .eof:
-                        self.logger?.debug("closing relay for \(readFromFd)")
-                        io.close(logger: self.logger)
-                        return
+        // Flush the pending backlog first; reads stay suspended until it clears.
+        while io.pendingOffset < io.pending.count {
+            let offset = io.pendingOffset
+            let result = io.pending.withUnsafeMutableBufferPointer { buf in
+                writeTo.write(
+                    UnsafeMutableBufferPointer(
+                        start: buf.baseAddress!.advanced(by: offset),
+                        count: buf.count - offset))
+            }
+            if result.wrote > 0 { io.pendingOffset += result.wrote }
+            switch result.action {
+            case .success:
+                continue
+            case .again:
+                self.ensureWriteRegistered(&io)
+                return
+            default:
+                self.logger?.error("stopping relay: write failed during backlog flush")
+                io.close(logger: self.logger)
+                return
+            }
+        }
+        if !io.pending.isEmpty {
+            io.pending = []
+            io.pendingOffset = 0
+            self.unregisterWrite(&io)
+        }
+
+        // Loop so we drain fully (edge-triggered epoll requires reading until EAGAIN).
+        while true {
+            let r = readFrom.read(io.buffer)
+            if r.read > 0 {
+                let view = UnsafeMutableBufferPointer(
+                    start: io.buffer.baseAddress,
+                    count: r.read
+                )
+
+                let w = writeTo.write(view)
+                if w.wrote != r.read {
+                    switch w.action {
                     case .again:
-                        if mask.isHangup && !ignoreHup {
-                            self.logger?.error("received EPOLLHUP and EAGAIN exiting")
-                            self.close()
-                        }
+                        // Destination full: stash the remainder and suspend reads until its
+                        // EPOLLOUT edge flushes it. (A later `read` re-reports EOF if this
+                        // chunk was the stream's last, so no EOF is lost by returning here.)
+                        io.pending = Array(
+                            UnsafeBufferPointer(
+                                start: io.buffer.baseAddress!.advanced(by: w.wrote),
+                                count: r.read - w.wrote))
+                        io.pendingOffset = 0
+                        self.ensureWriteRegistered(&io)
                         return
                     default:
-                        break
+                        self.logger?.error("stopping relay: short write for stdio")
+                        io.close(logger: self.logger)
+                        return
                     }
                 }
             }
+
+            switch r.action {
+            case .error(let errno):
+                self.logger?.error("failed with errno \(errno) while reading for fd \(io.from.fileDescriptor)")
+                fallthrough
+            case .eof:
+                self.logger?.debug("closing relay for \(io.from.fileDescriptor)")
+                io.close(logger: self.logger)
+                return
+            case .again:
+                if mask.isHangup && !ignoreHup {
+                    self.logger?.error("received EPOLLHUP and EAGAIN exiting")
+                    io.close(logger: self.logger)
+                }
+                return
+            default:
+                break
+            }
+        }
+    }
+
+    /// [Nucleic vendored patch] Register the write fd for EPOLLOUT so the pending backlog is
+    /// flushed when the destination drains. Registration failure closes the pair — without the
+    /// flush wakeup the relay would hang with data stranded. Must be called with the lock held.
+    private func ensureWriteRegistered(_ io: inout IO) {
+        guard !io.writeFdRegistered else { return }
+        let writeToFd = io.to.fileDescriptor
+        do {
+            try ProcessSupervisor.default.registerFd(writeToFd, mask: .output) { _ in
+                self.io.withLock { io in
+                    guard !io.closed else { return }
+                    // An empty mask: HUP/EOF handling rides the read fd's own events.
+                    self.pump(&io, mask: [], ignoreHup: true)
+                }
+            }
+            io.writeFdRegistered = true
+        } catch {
+            self.logger?.error("failed to register relay write fd for backpressure: \(error)")
+            io.close(logger: self.logger)
+        }
+    }
+
+    /// [Nucleic vendored patch] Drop the EPOLLOUT registration once the backlog has flushed.
+    /// Must be called with the lock held.
+    private func unregisterWrite(_ io: inout IO) {
+        guard io.writeFdRegistered else { return }
+        io.writeFdRegistered = false
+        do {
+            try ProcessSupervisor.default.unregisterFd(io.to.fileDescriptor)
+        } catch {
+            self.logger?.error("failed to unregister relay write fd: \(error)")
         }
     }
 
diff --git a/vminitd/Sources/VminitdCore/OSFile+Splice.swift b/vminitd/Sources/VminitdCore/OSFile+Splice.swift
index 7101b43..9a6f652 100644
--- a/vminitd/Sources/VminitdCore/OSFile+Splice.swift
+++ b/vminitd/Sources/VminitdCore/OSFile+Splice.swift
@@ -89,7 +89,17 @@ extension OSFile {
                     if errno != EAGAIN && errno != EIO {
                         throw POSIXError(.init(rawValue: errno)!)
                     }
-                    break
+                    // [Nucleic vendored patch] Destination full: RETURN, don't `break`. Breaking
+                    // sent the outer `while true` straight back here — with the source idle and the
+                    // destination still full, neither leg could progress and this spun the caller's
+                    // thread at 100% until the peer drained. The caller is an epoll handler on
+                    // vminitd's SINGLE ProcessSupervisor poller thread, so the spin froze every
+                    // exec's stdio and every control-plane relay in the container at once (the
+                    // all-sessions "produced no output within 60s" stall / dead control plane).
+                    // The un-flushed bytes stay in the transfer pipe (`from.offset > to.offset`
+                    // persists in the SpliceFiles); the destination fd is registered for EPOLLOUT,
+                    // whose edge re-enters transferData and resumes the flush.
+                    return (from.offset - fromOffset, to.offset - toOffset, .success)
                 }
                 to.offset += bytesWrote
                 if bytesWrote == 0 {
diff --git a/vminitd/Sources/VminitdCore/VsockProxy.swift b/vminitd/Sources/VminitdCore/VsockProxy.swift
index e490a04..3e6059c 100644
--- a/vminitd/Sources/VminitdCore/VsockProxy.swift
+++ b/vminitd/Sources/VminitdCore/VsockProxy.swift
@@ -244,6 +244,22 @@ extension VsockProxy {
 
                 try relayTo.connect()
 
+                // [Nucleic vendored patch] BOTH fds must be non-blocking BEFORE either is
+                // registered. `Epoll.add` sets O_NONBLOCK only at registration time, and the first
+                // (client) registration's handler can fire — and splice toward the server fd —
+                // before the second (server) registration has made that fd non-blocking. A full
+                // destination then turned the splice into a genuinely BLOCKING call on vminitd's
+                // single ProcessSupervisor poller thread, freezing every exec's stdio and every
+                // control-plane relay in the container until the peer drained.
+                for fd in [conn.fileDescriptor, relayTo.fileDescriptor] {
+                    let flags = fcntl(fd, F_GETFL)
+                    if flags == -1 || fcntl(fd, F_SETFL, flags | O_NONBLOCK) == -1 {
+                        self.log?.error(
+                            "failed to set proxy fd non-blocking",
+                            metadata: ["fd": "\(fd)", "errno": "\(errno)"])
+                    }
+                }
+
                 // `clientFile` isn't used concurrently.
                 nonisolated(unsafe) var clientFile = OSFile.SpliceFile(fd: conn.fileDescriptor)
                 nonisolated(unsafe) var eofFromClient = false