diff --git a/lang/src/compiler/entry/compiler_stageb.hako b/lang/src/compiler/entry/compiler_stageb.hako
index f54796dd..5106018b 100644
--- a/lang/src/compiler/entry/compiler_stageb.hako
+++ b/lang/src/compiler/entry/compiler_stageb.hako
@@ -65,8 +65,11 @@ static box StageBArgsBox {
       local inline = env.get("HAKO_SOURCE_FILE_CONTENT")
       if inline != null { src = "" + inline }
     }
-    // Skip env.local.get check - Stage-3 keyword conflict
-    // Original: if src == null { src = env.local.get("HAKO_SOURCE") }
+    // Phase 73: Fallback to HAKO_SRC env variable (unified I/O path)
+    // This eliminates the need for large argv strings (solves "Argument list too long")
+    if src == null {
+      src = env.get("HAKO_SRC")
+    }
     if src == null { src = "return 0" }
 
     {
diff --git a/tools/selfhost/selfhost_build.sh b/tools/selfhost/selfhost_build.sh
index a4782b7a..0ab3e359 100644
--- a/tools/selfhost/selfhost_build.sh
+++ b/tools/selfhost/selfhost_build.sh
@@ -93,10 +93,11 @@ else
   apply_selfhost_env
   stageb_cmd_desc="compiler.hako --stage-b --stage3"
   (
+    export HAKO_SRC="$SRC_CONTENT"
     cd "$ROOT" && \
       "$BIN" --backend vm \
         "$ROOT/lang/src/compiler/entry/compiler.hako" -- \
-        --stage-b --stage3 --source "$SRC_CONTENT"
+        --stage-b --stage3
   ) > "$RAW" 2>&1 || stageb_rc=$?
 fi