diff --git a/pyproject.toml b/pyproject.toml
index 85b5984..fe32d59 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -38,6 +38,11 @@ dependencies = [
     "opentelemetry-sdk>=1.20.0",
     "time-machine>=2.10.0",
     "typing_extensions>=4.4.0",
+    # Required so httpx.Client(...) construction doesn't crash inside the
+    # fence-based replay sandbox, which injects ALL_PROXY=socks5h://... into
+    # the service env. httpx evaluates proxy config eagerly in Client.__init__
+    # and imports socksio to build a SOCKS5 transport at construction time.
+    "socksio>=1.0.0",
 ]
 
 [project.optional-dependencies]