diff --git a/lib/fmt/fmttest.ww b/lib/fmt/fmttest.ww
index 08ccfd88..0eea56b1 100644
--- a/lib/fmt/fmttest.ww
+++ b/lib/fmt/fmttest.ww
@@ -10,20 +10,13 @@
 use fmt;
 use io;
 use memio;
-
-// Direct exit(2) binding rather than `use os;` — os exports
-// read/write/close, which collide with io.read/write/close under
-// the driver's flat-scope concat.
-@symbol("rt_syscall") fn syscall1ww(num: i64, a: i64) i64;
-fn doexit(code: i32) void = {
-	syscall1ww(60i64, code: i64);
-};
+use os;
 
 // signalled — bumped before each scenario so a failing exit code
 // pinpoints the offending case.
 let signalled: i32 = 0;
 
-fn fail() void = { doexit(signalled + 10); };
+fn fail() void = { os.exit(signalled + 10); };
 
 fn streq(a: str, b: str) bool = {
 	if (a.len != b.len) { return false; };
diff --git a/lib/log/logtest.ww b/lib/log/logtest.ww
index 6bd9efa7..e5caeede 100644
--- a/lib/log/logtest.ww
+++ b/lib/log/logtest.ww
@@ -13,21 +13,13 @@ use fmt;
 use io;
 use log;
 use memio;
-
-// Direct exit(2) binding rather than `use os;` — os exports
-// read/write/close, which collide with io.read/write/close under
-// the driver's flat-scope concat. Same workaround as bufiotest /
-// fmttest / memiotest.
-@symbol("rt_syscall") fn syscall1ww(num: i64, a: i64) i64;
-fn doexit(code: i32) void = {
-	syscall1ww(60i64, code: i64);
-};
+use os;
 
 // signalled — bumped before each scenario so a failing exit code
 // pinpoints the offending case.
 let signalled: i32 = 0;
 
-fn fail() void = { doexit(signalled + 10); };
+fn fail() void = { os.exit(signalled + 10); };
 
 fn streq(a: str, b: str) bool = {
 	if (a.len != b.len) { return false; };
diff --git a/lib/memio/memio.ww b/lib/memio/memio.ww
index 2ed5fb3e..753f81aa 100644
--- a/lib/memio/memio.ww
+++ b/lib/memio/memio.ww
@@ -28,9 +28,9 @@
 
 use io;
 
-// Direct rt_alloc/rt_free bindings rather than `use os;` because os
-// exports read/write/close, which collide with io.read/write/close
-// under the driver's flat-scope concatenation. Same pattern as
+// Direct rt_alloc/rt_free bindings — the runtime heap primitives
+// memio.dynamic / dynamicfrom need to own the backing buffer. lib/os
+// holds the same FFI shape but doesn't export it; same pattern as
 // rt/ensure.ww.
 @symbol("rt_alloc") fn rtalloc(n: u64) *void;
 @symbol("rt_free")  fn rtfree(p: *void, n: u64) void;
diff --git a/lib/memio/memiotest.ww b/lib/memio/memiotest.ww
index 34dbcab1..228fe003 100644
--- a/lib/memio/memiotest.ww
+++ b/lib/memio/memiotest.ww
@@ -8,20 +8,13 @@
 use bytes;
 use io;
 use memio;
-
-// Direct exit(2) binding rather than `use os;` — os exports
-// read/write/close, which collide with io.read/write/close under
-// the driver's flat-scope concat (task #7).
-@symbol("rt_syscall") fn syscall1ww(num: i64, a: i64) i64;
-fn doexit(code: i32) void = {
-	syscall1ww(60i64, code: i64);
-};
+use os;
 
 // signalled — bumped by main before each test so a failing exit code
 // pinpoints the offending case.
 let signalled: i32 = 0;
 
-fn fail() void = { doexit(signalled + 10); };
+fn fail() void = { os.exit(signalled + 10); };
 
 fn putstr(s: str, into: []u8, off: i32) i32 = {
 	let i: i32 = 0;