diff --git a/src/system.c b/src/system.c
index a278b16..a98a999 100644
--- a/src/system.c
+++ b/src/system.c
@@ -4,6 +4,7 @@
 #include <assert.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stdarg.h>
 
 #include "system.h"
 #include "chunk_server.h"
@@ -98,6 +99,9 @@ typedef struct {
     // last "poll" call.
     void *context;
 
+    // Whether this socket is configured as non-blocking
+    bool is_nonblocking;
+
     // Address bound to this descriptor by the
     // "bind" call.
     DescriptorAddress address;
@@ -811,6 +815,7 @@ SOCKET mock_socket(int domain, int type, int protocol)
     desc->events = 0;
     desc->revents = 0;
     desc->context = NULL;
+    desc->is_nonblocking = false;
     desc->address = (DescriptorAddress) { .type=DESC_ADDR_VOID };
 
     current_process->num_desc++;
@@ -912,6 +917,11 @@ SOCKET mock_accept(SOCKET fd, void *addr, socklen_t *addr_len)
 
     DescriptorHandle peer_handle;
     if (!accept_queue_pop(&desc->accept_queue, &peer_handle)) {
+        // Socket would block - abort if not configured as non-blocking
+        if (!desc->is_nonblocking) {
+            fprintf(stderr, "SIMULATION ERROR: accept() would block on socket %d but socket is not configured as non-blocking\n", (int)fd);
+            abort();
+        }
         SET_SOCKET_ERROR(SOCKET_ERROR_WOULDBLOCK);  // Would block (no pending connections)
         return INVALID_SOCKET;
     }
@@ -937,6 +947,7 @@ SOCKET mock_accept(SOCKET fd, void *addr, socklen_t *addr_len)
     new_desc->events = 0;
     new_desc->revents = 0;
     new_desc->context = NULL;
+    new_desc->is_nonblocking = false;
     new_desc->address = (DescriptorAddress) { .type=DESC_ADDR_VOID };
     new_desc->connection_state = CONNECTION_ESTABLISHED;
     new_desc->connection_peer = peer_handle;
@@ -1063,6 +1074,11 @@ int mock_recv(SOCKET fd, void *dst, int len, int flags)
 
     // If no data available, would block
     if (bytes_read == 0) {
+        // Socket would block - abort if not configured as non-blocking
+        if (!desc->is_nonblocking) {
+            fprintf(stderr, "SIMULATION ERROR: recv() would block on socket %d but socket is not configured as non-blocking\n", (int)fd);
+            abort();
+        }
         SET_SOCKET_ERROR(SOCKET_ERROR_WOULDBLOCK);
         return -1;
     }
@@ -1106,6 +1122,11 @@ int mock_send(SOCKET fd, void *src, int len, int flags)
 
     // If queue is full, we would block
     if (bytes_written < len) {
+        // Socket would block - abort if not configured as non-blocking
+        if (!desc->is_nonblocking && bytes_written == 0) {
+            fprintf(stderr, "SIMULATION ERROR: send() would block on socket %d but socket is not configured as non-blocking\n", (int)fd);
+            abort();
+        }
         SET_SOCKET_ERROR(SOCKET_ERROR_WOULDBLOCK);
         return bytes_written > 0 ? bytes_written : -1;
     }
@@ -1451,6 +1472,37 @@ BOOL mock_QueryPerformanceFrequency(LARGE_INTEGER *lpFrequency)
     return TRUE;
 }
 
+int mock_ioctlsocket(SOCKET fd, long cmd, u_long *argp)
+{
+    if (fd == INVALID_SOCKET || (int)fd < 0 || (int)fd >= MAX_DESCRIPTORS) {
+        SET_SOCKET_ERROR(SOCKET_ERROR_BADF);
+        return -1;
+    }
+
+    int idx = (int) fd;
+    Descriptor *desc = &current_process->desc[idx];
+
+    if (desc->type == DESC_EMPTY) {
+        SET_SOCKET_ERROR(SOCKET_ERROR_BADF);
+        return -1;
+    }
+
+    if (desc->type != DESC_SOCKET &&
+        desc->type != DESC_LISTEN_SOCKET &&
+        desc->type != DESC_CONNECTION_SOCKET) {
+        SET_SOCKET_ERROR(SOCKET_ERROR_NOTSOCK);
+        return -1;
+    }
+
+    if (cmd == FIONBIO) {
+        desc->is_nonblocking = (*argp != 0);
+        return 0;
+    }
+
+    SET_SOCKET_ERROR(SOCKET_ERROR_INVAL);
+    return -1;
+}
+
 #else
 
 int mock_clock_gettime(clockid_t clockid, struct timespec *tp)
@@ -1630,6 +1682,60 @@ int mock_mkdir(char *path, mode_t mode)
     return mkdir(path, mode);
 }
 
+int mock_fcntl(int fd, int cmd, ...)
+{
+    if (fd < 0 || fd >= MAX_DESCRIPTORS) {
+        errno = EBADF;
+        return -1;
+    }
+
+    int idx = fd;
+    Descriptor *desc = &current_process->desc[idx];
+
+    if (desc->type == DESC_EMPTY) {
+        errno = EBADF;
+        return -1;
+    }
+
+    if (cmd == F_GETFL) {
+        // Return flags - only O_NONBLOCK is tracked
+        if (desc->type == DESC_FILE) {
+            // For files, forward to real fcntl
+            return fcntl(desc->real_fd, cmd);
+        }
+        return desc->is_nonblocking ? O_NONBLOCK : 0;
+    }
+
+    if (cmd == F_SETFL) {
+        if (desc->type == DESC_FILE) {
+            // For files, forward to real fcntl
+            va_list args;
+            va_start(args, cmd);
+            int flags = va_arg(args, int);
+            va_end(args);
+            return fcntl(desc->real_fd, cmd, flags);
+        }
+
+        if (desc->type != DESC_SOCKET &&
+            desc->type != DESC_LISTEN_SOCKET &&
+            desc->type != DESC_CONNECTION_SOCKET) {
+            errno = EBADF;
+            return -1;
+        }
+
+        va_list args;
+        va_start(args, cmd);
+        int flags = va_arg(args, int);
+        va_end(args);
+
+        desc->is_nonblocking = (flags & O_NONBLOCK) != 0;
+        return 0;
+    }
+
+    errno = EINVAL;
+    return -1;
+}
+
 #endif // !_WIN32
 
 #endif // BUILD_TEST
diff --git a/src/system.h b/src/system.h
index de22ea2..aeea168 100644
--- a/src/system.h
+++ b/src/system.h
@@ -56,6 +56,7 @@ int    mock_connect(SOCKET fd, void *addr, size_t addr_len);
 
 #ifdef _WIN32
 int      mock_closesocket(SOCKET fd);
+int      mock_ioctlsocket(SOCKET fd, long cmd, u_long *argp);
 HANDLE   mock_CreateFileW(WCHAR *lpFileName, DWORD dwDesiredAccess, DWORD dwShareMode, LPSECURITY_ATTRIBUTES lpSecurityAttributes, DWORD dwCreationDisposition, DWORD dwFlagsAndAttributes, HANDLE hTemplateFile);
 BOOL     mock_CloseHandle(HANDLE handle);
 BOOL     mock_LockFile(HANDLE hFile, DWORD dwFileOffsetLow, DWORD dwFileOffsetHigh, DWORD nNumberOfBytesToLockLow, DWORD nNumberOfBytesToLockHigh);
@@ -80,6 +81,7 @@ int     mock_fstat(int fd, struct stat *buf);
 int     mock_mkstemp(char *path);
 char*   mock_realpath(char *path, char *dst);
 int     mock_mkdir(char *path, mode_t mode);
+int     mock_fcntl(int fd, int cmd, ...);
 #endif
 
 // Common
@@ -100,6 +102,7 @@ int     mock_mkdir(char *path, mode_t mode);
 // Windows
 #define sys__mkdir           mock__mkdir
 #define sys_closesocket      mock_closesocket
+#define sys_ioctlsocket      mock_ioctlsocket
 #define sys_CreateFileW      mock_CreateFileW
 #define sys_CloseHandle      mock_CloseHandle
 #define sys_LockFile         mock_LockFile
@@ -124,6 +127,7 @@ int     mock_mkdir(char *path, mode_t mode);
 #define sys_mkstemp          mock_mkstemp
 #define sys_realpath         mock_realpath
 #define sys_clock_gettime    mock_clock_gettime
+#define sys_fcntl            mock_fcntl
 #define sys_getrandom        mock_getrandom
 
 #else
@@ -146,6 +150,7 @@ int     mock_mkdir(char *path, mode_t mode);
 // Windows
 #define sys__mkdir           _mkdir
 #define sys_closesocket      closesocket
+#define sys_ioctlsocket      ioctlsocket
 #define sys_CreateFileW      CreateFileW
 #define sys_CloseHandle      CloseHandle
 #define sys_LockFile         LockFile
@@ -170,6 +175,7 @@ int     mock_mkdir(char *path, mode_t mode);
 #define sys_mkstemp          mkstemp
 #define sys_realpath         realpath
 #define sys_clock_gettime    clock_gettime
+#define sys_fcntl            fcntl
 
 #endif
 #endif // SYSTEM_INCLUDED
diff --git a/src/tcp.c b/src/tcp.c
index 38a0f2d..0cef493 100644
--- a/src/tcp.c
+++ b/src/tcp.c
@@ -30,6 +30,21 @@ static SOCKET create_listen_socket(string addr, uint16_t port)
     if (fd == INVALID_SOCKET)
         return INVALID_SOCKET;
 
+    // Set socket as non-blocking
+#ifdef _WIN32
+    u_long mode = 1;
+    if (sys_ioctlsocket(fd, FIONBIO, &mode) != 0) {
+        CLOSE_SOCKET(fd);
+        return INVALID_SOCKET;
+    }
+#else
+    int flags = sys_fcntl(fd, F_GETFL, 0);
+    if (flags < 0 || sys_fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0) {
+        CLOSE_SOCKET(fd);
+        return INVALID_SOCKET;
+    }
+#endif
+
     char tmp[1<<10];
     if (addr.len >= (int) sizeof(tmp))
         return INVALID_SOCKET;
@@ -200,6 +215,20 @@ int tcp_translate_events(TCP *tcp, Event *events, void **contexts, struct pollfd
 
             SOCKET new_fd = sys_accept(tcp->listen_fd, NULL, NULL);
             if (new_fd != INVALID_SOCKET) {
+                // Set accepted socket as non-blocking
+#ifdef _WIN32
+                u_long mode = 1;
+                if (sys_ioctlsocket(new_fd, FIONBIO, &mode) != 0) {
+                    CLOSE_SOCKET(new_fd);
+                    continue;
+                }
+#else
+                int flags = sys_fcntl(new_fd, F_GETFL, 0);
+                if (flags < 0 || sys_fcntl(new_fd, F_SETFL, flags | O_NONBLOCK) < 0) {
+                    CLOSE_SOCKET(new_fd);
+                    continue;
+                }
+#endif
                 events[num_events++] = (Event) { EVENT_CONNECT, tcp->num_conns };
                 conn_init(&tcp->conns[tcp->num_conns++], new_fd, false);
             }
@@ -304,6 +333,21 @@ int tcp_connect(TCP *tcp, Address addr, int tag, ByteQueue **output)
     if (fd == INVALID_SOCKET)
         return -1;
 
+    // Set socket as non-blocking
+#ifdef _WIN32
+    u_long mode = 1;
+    if (sys_ioctlsocket(fd, FIONBIO, &mode) != 0) {
+        CLOSE_SOCKET(fd);
+        return -1;
+    }
+#else
+    int flags = sys_fcntl(fd, F_GETFL, 0);
+    if (flags < 0 || sys_fcntl(fd, F_SETFL, flags | O_NONBLOCK) < 0) {
+        CLOSE_SOCKET(fd);
+        return -1;
+    }
+#endif
+
     int ret;
     if (addr.is_ipv4) {
         struct sockaddr_in buf;