diff --git a/lib/os/os.ww b/lib/os/os.ww
index 0b68988a..0b09306f 100644
--- a/lib/os/os.ww
+++ b/lib/os/os.ww
@@ -34,17 +34,23 @@ def SYS_UNLINK:      i64 = 87;
 def SYS_GETCWD:      i64 = 79;
 def SYS_GETDENTS64:  i64 = 217;
 
-// open(2) flags. Linux values, matching <fcntl.h>.
-def O_RDONLY: i32 = 0;
-def O_WRONLY: i32 = 1;
-def O_RDWR:   i32 = 2;
-def O_CREAT:  i32 = 64;		// 0x40
-def O_TRUNC:  i32 = 512;	// 0x200
+// open(2) flags. Linux values, matching <fcntl.h>. Hare names them
+// `fs::flag::RDONLY` etc; we use the same leaf names so callers say
+// `os.flag.RDONLY` and `os.flag.WRONLY | os.flag.CREATE`.
+export type flag = enum i32 {
+	RDONLY  = 0,
+	WRONLY  = 1,
+	RDWR    = 2,
+	CREATE  = 64,	// 0x40
+	TRUNC   = 512,	// 0x200
+};
 
-// lseek(2) whence.
-def SEEK_SET: i32 = 0;
-def SEEK_CUR: i32 = 1;
-def SEEK_END: i32 = 2;
+// lseek(2) whence. Hare names it `io::whence`.
+export type whence = enum i32 {
+	SET = 0,
+	CUR = 1,
+	END = 2,
+};
 
 export fn exit(code: i32) void = {
 	syscall1(SYS_EXIT, code: i64);
@@ -92,11 +98,11 @@ export fn trywrite(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 // `str` must ensure the bytes are followed by a 0 byte (literals are,
 // arena-copied paths usually are by construction). Returns -errno on
 // failure, fd otherwise. Higher-level callers prefer `tryopen`.
-export fn open(path: *u8, flags: i32, mode: i32) i32 = {
-	return syscall3(SYS_OPEN, path: i64, flags: i64, mode: i64): i32;
+export fn open(path: *u8, flags: flag, mode: i32) i32 = {
+	return syscall3(SYS_OPEN, path: i64, (flags as i32): i64, mode: i64): i32;
 };
 
-export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
+export fn tryopen(path: *u8, flags: flag, mode: i32) (i32 | oserror) = {
 	let fd: i32 = open(path, flags, mode);
 	if (fd < 0) { return fd: i64: oserror; };
 	return fd;
@@ -105,8 +111,8 @@ export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
 // lseek — set/inspect the fd's position. Returns the new offset or
 // a negative errno. We use this for fstat-free file-size discovery
 // (open ⇒ lseek to end ⇒ lseek back).
-export fn lseek(fd: i32, off: i64, whence: i32) i64 = {
-	return syscall3(SYS_LSEEK, fd: i64, off, whence: i64);
+export fn lseek(fd: i32, off: i64, w: whence) i64 = {
+	return syscall3(SYS_LSEEK, fd: i64, off, (w as i32): i64);
 };
 
 // oserror — the underlying errno from a failed syscall, as a
@@ -118,9 +124,9 @@ export type oserror = !i64;
 
 // filesize — byte length of an open fd via lseek-to-end-and-back.
 export fn filesize(fd: i32) (i64 | oserror) = {
-	let end: i64 = lseek(fd, 0i64, SEEK_END);
+	let end: i64 = lseek(fd, 0i64, whence.END);
 	if (end < 0) { return end: oserror; };
-	let r: i64 = lseek(fd, 0i64, SEEK_SET);
+	let r: i64 = lseek(fd, 0i64, whence.SET);
 	if (r < 0) { return r: oserror; };
 	return end;
 };
diff --git a/selfhost/cmd/w6a/main.combined.ww b/selfhost/cmd/w6a/main.combined.ww
index aec6ca50..a8dd469e 100644
--- a/selfhost/cmd/w6a/main.combined.ww
+++ b/selfhost/cmd/w6a/main.combined.ww
@@ -35,17 +35,23 @@ def SYS_UNLINK:      i64 = 87;
 def SYS_GETCWD:      i64 = 79;
 def SYS_GETDENTS64:  i64 = 217;
 
-// open(2) flags. Linux values, matching <fcntl.h>.
-def O_RDONLY: i32 = 0;
-def O_WRONLY: i32 = 1;
-def O_RDWR:   i32 = 2;
-def O_CREAT:  i32 = 64;		// 0x40
-def O_TRUNC:  i32 = 512;	// 0x200
+// open(2) flags. Linux values, matching <fcntl.h>. Hare names them
+// `fs::flag::RDONLY` etc; we use the same leaf names so callers say
+// `os.flag.RDONLY` and `os.flag.WRONLY | os.flag.CREATE`.
+export type flag = enum i32 {
+	RDONLY  = 0,
+	WRONLY  = 1,
+	RDWR    = 2,
+	CREATE  = 64,	// 0x40
+	TRUNC   = 512,	// 0x200
+};
 
-// lseek(2) whence.
-def SEEK_SET: i32 = 0;
-def SEEK_CUR: i32 = 1;
-def SEEK_END: i32 = 2;
+// lseek(2) whence. Hare names it `io::whence`.
+export type whence = enum i32 {
+	SET = 0,
+	CUR = 1,
+	END = 2,
+};
 
 export fn exit(code: i32) void = {
 	syscall1(SYS_EXIT, code: i64);
@@ -93,11 +99,11 @@ export fn trywrite(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 // `str` must ensure the bytes are followed by a 0 byte (literals are,
 // arena-copied paths usually are by construction). Returns -errno on
 // failure, fd otherwise. Higher-level callers prefer `tryopen`.
-export fn open(path: *u8, flags: i32, mode: i32) i32 = {
-	return syscall3(SYS_OPEN, path: i64, flags: i64, mode: i64): i32;
+export fn open(path: *u8, flags: flag, mode: i32) i32 = {
+	return syscall3(SYS_OPEN, path: i64, (flags as i32): i64, mode: i64): i32;
 };
 
-export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
+export fn tryopen(path: *u8, flags: flag, mode: i32) (i32 | oserror) = {
 	let fd: i32 = open(path, flags, mode);
 	if (fd < 0) { return fd: i64: oserror; };
 	return fd;
@@ -106,8 +112,8 @@ export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
 // lseek — set/inspect the fd's position. Returns the new offset or
 // a negative errno. We use this for fstat-free file-size discovery
 // (open ⇒ lseek to end ⇒ lseek back).
-export fn lseek(fd: i32, off: i64, whence: i32) i64 = {
-	return syscall3(SYS_LSEEK, fd: i64, off, whence: i64);
+export fn lseek(fd: i32, off: i64, w: whence) i64 = {
+	return syscall3(SYS_LSEEK, fd: i64, off, (w as i32): i64);
 };
 
 // oserror — the underlying errno from a failed syscall, as a
@@ -119,9 +125,9 @@ export type oserror = !i64;
 
 // filesize — byte length of an open fd via lseek-to-end-and-back.
 export fn filesize(fd: i32) (i64 | oserror) = {
-	let end: i64 = lseek(fd, 0i64, SEEK_END);
+	let end: i64 = lseek(fd, 0i64, whence.END);
 	if (end < 0) { return end: oserror; };
-	let r: i64 = lseek(fd, 0i64, SEEK_SET);
+	let r: i64 = lseek(fd, 0i64, whence.SET);
 	if (r < 0) { return r: oserror; };
 	return end;
 };
@@ -2189,7 +2195,7 @@ fn cstrlen(p: *u8) u64 = {
 
 // Slurp the whole file into a fresh buffer.
 fn slurp(path: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
@@ -2265,7 +2271,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	if (encode(&asm) != 0) { return 1; };
 
 	// Open output for write.
-	let fd: i32 = os.open(out, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 420i32);	// 0o644
+	let fd: i32 = os.open(out, os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC, 420i32);	// 0o644
 	if (fd < 0) {
 		os.write(2, "w6a: cannot open output\n".ptr, 23u64);
 		return 1;
diff --git a/selfhost/cmd/w6a/main.ww b/selfhost/cmd/w6a/main.ww
index d69c24ff..ee4b6df2 100644
--- a/selfhost/cmd/w6a/main.ww
+++ b/selfhost/cmd/w6a/main.ww
@@ -32,7 +32,7 @@ fn cstrlen(p: *u8) u64 = {
 
 // Slurp the whole file into a fresh buffer.
 fn slurp(path: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
@@ -108,7 +108,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	if (encode(&asm) != 0) { return 1; };
 
 	// Open output for write.
-	let fd: i32 = os.open(out, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 420i32);	// 0o644
+	let fd: i32 = os.open(out, os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC, 420i32);	// 0o644
 	if (fd < 0) {
 		os.write(2, "w6a: cannot open output\n".ptr, 23u64);
 		return 1;
diff --git a/selfhost/cmd/w6c/main.combined.ww b/selfhost/cmd/w6c/main.combined.ww
index 362a536f..5c120999 100644
--- a/selfhost/cmd/w6c/main.combined.ww
+++ b/selfhost/cmd/w6c/main.combined.ww
@@ -35,17 +35,23 @@ def SYS_UNLINK:      i64 = 87;
 def SYS_GETCWD:      i64 = 79;
 def SYS_GETDENTS64:  i64 = 217;
 
-// open(2) flags. Linux values, matching <fcntl.h>.
-def O_RDONLY: i32 = 0;
-def O_WRONLY: i32 = 1;
-def O_RDWR:   i32 = 2;
-def O_CREAT:  i32 = 64;		// 0x40
-def O_TRUNC:  i32 = 512;	// 0x200
+// open(2) flags. Linux values, matching <fcntl.h>. Hare names them
+// `fs::flag::RDONLY` etc; we use the same leaf names so callers say
+// `os.flag.RDONLY` and `os.flag.WRONLY | os.flag.CREATE`.
+export type flag = enum i32 {
+	RDONLY  = 0,
+	WRONLY  = 1,
+	RDWR    = 2,
+	CREATE  = 64,	// 0x40
+	TRUNC   = 512,	// 0x200
+};
 
-// lseek(2) whence.
-def SEEK_SET: i32 = 0;
-def SEEK_CUR: i32 = 1;
-def SEEK_END: i32 = 2;
+// lseek(2) whence. Hare names it `io::whence`.
+export type whence = enum i32 {
+	SET = 0,
+	CUR = 1,
+	END = 2,
+};
 
 export fn exit(code: i32) void = {
 	syscall1(SYS_EXIT, code: i64);
@@ -93,11 +99,11 @@ export fn trywrite(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 // `str` must ensure the bytes are followed by a 0 byte (literals are,
 // arena-copied paths usually are by construction). Returns -errno on
 // failure, fd otherwise. Higher-level callers prefer `tryopen`.
-export fn open(path: *u8, flags: i32, mode: i32) i32 = {
-	return syscall3(SYS_OPEN, path: i64, flags: i64, mode: i64): i32;
+export fn open(path: *u8, flags: flag, mode: i32) i32 = {
+	return syscall3(SYS_OPEN, path: i64, (flags as i32): i64, mode: i64): i32;
 };
 
-export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
+export fn tryopen(path: *u8, flags: flag, mode: i32) (i32 | oserror) = {
 	let fd: i32 = open(path, flags, mode);
 	if (fd < 0) { return fd: i64: oserror; };
 	return fd;
@@ -106,8 +112,8 @@ export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
 // lseek — set/inspect the fd's position. Returns the new offset or
 // a negative errno. We use this for fstat-free file-size discovery
 // (open ⇒ lseek to end ⇒ lseek back).
-export fn lseek(fd: i32, off: i64, whence: i32) i64 = {
-	return syscall3(SYS_LSEEK, fd: i64, off, whence: i64);
+export fn lseek(fd: i32, off: i64, w: whence) i64 = {
+	return syscall3(SYS_LSEEK, fd: i64, off, (w as i32): i64);
 };
 
 // oserror — the underlying errno from a failed syscall, as a
@@ -119,9 +125,9 @@ export type oserror = !i64;
 
 // filesize — byte length of an open fd via lseek-to-end-and-back.
 export fn filesize(fd: i32) (i64 | oserror) = {
-	let end: i64 = lseek(fd, 0i64, SEEK_END);
+	let end: i64 = lseek(fd, 0i64, whence.END);
 	if (end < 0) { return end: oserror; };
-	let r: i64 = lseek(fd, 0i64, SEEK_SET);
+	let r: i64 = lseek(fd, 0i64, whence.SET);
 	if (r < 0) { return r: oserror; };
 	return end;
 };
@@ -9265,7 +9271,7 @@ fn cstrlen(p: *u8) u64 = {
 };
 
 fn slurp(path: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
@@ -9332,7 +9338,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	// threading a file descriptor through the emit helpers.
 	if (out != nil) {
 		let ofd: i32 = os.open(out,
-		    os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 420i32);	// 0o644
+		    os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC, 420i32);	// 0o644
 		if (ofd < 0) {
 			os.write(2, "w6c: cannot open output\n".ptr, 23u64);
 			return 1;
diff --git a/selfhost/cmd/w6c/main.ww b/selfhost/cmd/w6c/main.ww
index a210687f..c2b83a2a 100644
--- a/selfhost/cmd/w6c/main.ww
+++ b/selfhost/cmd/w6c/main.ww
@@ -40,7 +40,7 @@ fn cstrlen(p: *u8) u64 = {
 };
 
 fn slurp(path: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
@@ -107,7 +107,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 	// threading a file descriptor through the emit helpers.
 	if (out != nil) {
 		let ofd: i32 = os.open(out,
-		    os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 420i32);	// 0o644
+		    os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC, 420i32);	// 0o644
 		if (ofd < 0) {
 			os.write(2, "w6c: cannot open output\n".ptr, 23u64);
 			return 1;
diff --git a/selfhost/cmd/w6l/dyn.ww b/selfhost/cmd/w6l/dyn.ww
index e223b222..17c7926b 100644
--- a/selfhost/cmd/w6l/dyn.ww
+++ b/selfhost/cmd/w6l/dyn.ww
@@ -124,7 +124,7 @@ fn dbasename(p: *u8) *u8 = {
 // ---- file slurp --------------------------------------------------------
 
 fn slurpso(path: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
diff --git a/selfhost/cmd/w6l/main.combined.ww b/selfhost/cmd/w6l/main.combined.ww
index 253705f8..e194abd1 100644
--- a/selfhost/cmd/w6l/main.combined.ww
+++ b/selfhost/cmd/w6l/main.combined.ww
@@ -35,17 +35,23 @@ def SYS_UNLINK:      i64 = 87;
 def SYS_GETCWD:      i64 = 79;
 def SYS_GETDENTS64:  i64 = 217;
 
-// open(2) flags. Linux values, matching <fcntl.h>.
-def O_RDONLY: i32 = 0;
-def O_WRONLY: i32 = 1;
-def O_RDWR:   i32 = 2;
-def O_CREAT:  i32 = 64;		// 0x40
-def O_TRUNC:  i32 = 512;	// 0x200
+// open(2) flags. Linux values, matching <fcntl.h>. Hare names them
+// `fs::flag::RDONLY` etc; we use the same leaf names so callers say
+// `os.flag.RDONLY` and `os.flag.WRONLY | os.flag.CREATE`.
+export type flag = enum i32 {
+	RDONLY  = 0,
+	WRONLY  = 1,
+	RDWR    = 2,
+	CREATE  = 64,	// 0x40
+	TRUNC   = 512,	// 0x200
+};
 
-// lseek(2) whence.
-def SEEK_SET: i32 = 0;
-def SEEK_CUR: i32 = 1;
-def SEEK_END: i32 = 2;
+// lseek(2) whence. Hare names it `io::whence`.
+export type whence = enum i32 {
+	SET = 0,
+	CUR = 1,
+	END = 2,
+};
 
 export fn exit(code: i32) void = {
 	syscall1(SYS_EXIT, code: i64);
@@ -93,11 +99,11 @@ export fn trywrite(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 // `str` must ensure the bytes are followed by a 0 byte (literals are,
 // arena-copied paths usually are by construction). Returns -errno on
 // failure, fd otherwise. Higher-level callers prefer `tryopen`.
-export fn open(path: *u8, flags: i32, mode: i32) i32 = {
-	return syscall3(SYS_OPEN, path: i64, flags: i64, mode: i64): i32;
+export fn open(path: *u8, flags: flag, mode: i32) i32 = {
+	return syscall3(SYS_OPEN, path: i64, (flags as i32): i64, mode: i64): i32;
 };
 
-export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
+export fn tryopen(path: *u8, flags: flag, mode: i32) (i32 | oserror) = {
 	let fd: i32 = open(path, flags, mode);
 	if (fd < 0) { return fd: i64: oserror; };
 	return fd;
@@ -106,8 +112,8 @@ export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
 // lseek — set/inspect the fd's position. Returns the new offset or
 // a negative errno. We use this for fstat-free file-size discovery
 // (open ⇒ lseek to end ⇒ lseek back).
-export fn lseek(fd: i32, off: i64, whence: i32) i64 = {
-	return syscall3(SYS_LSEEK, fd: i64, off, whence: i64);
+export fn lseek(fd: i32, off: i64, w: whence) i64 = {
+	return syscall3(SYS_LSEEK, fd: i64, off, (w as i32): i64);
 };
 
 // oserror — the underlying errno from a failed syscall, as a
@@ -119,9 +125,9 @@ export type oserror = !i64;
 
 // filesize — byte length of an open fd via lseek-to-end-and-back.
 export fn filesize(fd: i32) (i64 | oserror) = {
-	let end: i64 = lseek(fd, 0i64, SEEK_END);
+	let end: i64 = lseek(fd, 0i64, whence.END);
 	if (end < 0) { return end: oserror; };
-	let r: i64 = lseek(fd, 0i64, SEEK_SET);
+	let r: i64 = lseek(fd, 0i64, whence.SET);
 	if (r < 0) { return r: oserror; };
 	return end;
 };
@@ -505,7 +511,7 @@ def RELA_ADDEND:   u64 = 16u64;
 // ---- file slurp --------------------------------------------------------
 
 fn slurp(path: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
@@ -1050,7 +1056,7 @@ fn dbasename(p: *u8) *u8 = {
 // ---- file slurp --------------------------------------------------------
 
 fn slurpso(path: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
@@ -2393,7 +2399,7 @@ fn buildpathv(dst: *u8, dir: *u8, name: *u8, v: u64) u64 = {
 
 // islinkable: read first 8 bytes; require !<arch>\n or \x7fELF.
 fn islinkable(path: *u8) bool = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return false; };
 	let mp: *u8 = os.alloc(8u64): *u8;
 	let n: i64 = os.read(fd, mp, 8u64);
@@ -2462,7 +2468,7 @@ fn resolvelib(a: *arena, name: *u8, libdirs: **u8, nlibdirs: i32) *u8 = {
 
 // Read first 20 bytes; return 1 for ET_DYN .so, 0 for ar/.o.
 fn isso(path: *u8) i32 = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return 0; };
 	let mp: *u8 = os.alloc(20u64): *u8;
 	let n: i64 = os.read(fd, mp, 20u64);
@@ -2606,7 +2612,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 		return 1;
 	};
 
-	let flags: i32 = os.O_WRONLY | os.O_CREAT | os.O_TRUNC;
+	let flags: os.flag = os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC;
 	let fd: i32 = os.open(outpath, flags, 493i32);	// 0o755
 	if (fd < 0) {
 		os.write(2, "w6l: cannot open output\n".ptr, 23u64);
diff --git a/selfhost/cmd/w6l/main.ww b/selfhost/cmd/w6l/main.ww
index 57ac898d..db103b05 100644
--- a/selfhost/cmd/w6l/main.ww
+++ b/selfhost/cmd/w6l/main.ww
@@ -99,7 +99,7 @@ fn buildpathv(dst: *u8, dir: *u8, name: *u8, v: u64) u64 = {
 
 // islinkable: read first 8 bytes; require !<arch>\n or \x7fELF.
 fn islinkable(path: *u8) bool = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return false; };
 	let mp: *u8 = os.alloc(8u64): *u8;
 	let n: i64 = os.read(fd, mp, 8u64);
@@ -168,7 +168,7 @@ fn resolvelib(a: *arena, name: *u8, libdirs: **u8, nlibdirs: i32) *u8 = {
 
 // Read first 20 bytes; return 1 for ET_DYN .so, 0 for ar/.o.
 fn isso(path: *u8) i32 = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return 0; };
 	let mp: *u8 = os.alloc(20u64): *u8;
 	let n: i64 = os.read(fd, mp, 20u64);
@@ -312,7 +312,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 		return 1;
 	};
 
-	let flags: i32 = os.O_WRONLY | os.O_CREAT | os.O_TRUNC;
+	let flags: os.flag = os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC;
 	let fd: i32 = os.open(outpath, flags, 493i32);	// 0o755
 	if (fd < 0) {
 		os.write(2, "w6l: cannot open output\n".ptr, 23u64);
diff --git a/selfhost/cmd/w6l/obj.ww b/selfhost/cmd/w6l/obj.ww
index 624072e5..f4a13a81 100644
--- a/selfhost/cmd/w6l/obj.ww
+++ b/selfhost/cmd/w6l/obj.ww
@@ -76,7 +76,7 @@ def RELA_ADDEND:   u64 = 16u64;
 // ---- file slurp --------------------------------------------------------
 
 fn slurp(path: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(path, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(path, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
diff --git a/selfhost/cmd/ww/main.combined.ww b/selfhost/cmd/ww/main.combined.ww
index 84edc88b..c6f25415 100644
--- a/selfhost/cmd/ww/main.combined.ww
+++ b/selfhost/cmd/ww/main.combined.ww
@@ -35,17 +35,23 @@ def SYS_UNLINK:      i64 = 87;
 def SYS_GETCWD:      i64 = 79;
 def SYS_GETDENTS64:  i64 = 217;
 
-// open(2) flags. Linux values, matching <fcntl.h>.
-def O_RDONLY: i32 = 0;
-def O_WRONLY: i32 = 1;
-def O_RDWR:   i32 = 2;
-def O_CREAT:  i32 = 64;		// 0x40
-def O_TRUNC:  i32 = 512;	// 0x200
+// open(2) flags. Linux values, matching <fcntl.h>. Hare names them
+// `fs::flag::RDONLY` etc; we use the same leaf names so callers say
+// `os.flag.RDONLY` and `os.flag.WRONLY | os.flag.CREATE`.
+export type flag = enum i32 {
+	RDONLY  = 0,
+	WRONLY  = 1,
+	RDWR    = 2,
+	CREATE  = 64,	// 0x40
+	TRUNC   = 512,	// 0x200
+};
 
-// lseek(2) whence.
-def SEEK_SET: i32 = 0;
-def SEEK_CUR: i32 = 1;
-def SEEK_END: i32 = 2;
+// lseek(2) whence. Hare names it `io::whence`.
+export type whence = enum i32 {
+	SET = 0,
+	CUR = 1,
+	END = 2,
+};
 
 export fn exit(code: i32) void = {
 	syscall1(SYS_EXIT, code: i64);
@@ -93,11 +99,11 @@ export fn trywrite(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 // `str` must ensure the bytes are followed by a 0 byte (literals are,
 // arena-copied paths usually are by construction). Returns -errno on
 // failure, fd otherwise. Higher-level callers prefer `tryopen`.
-export fn open(path: *u8, flags: i32, mode: i32) i32 = {
-	return syscall3(SYS_OPEN, path: i64, flags: i64, mode: i64): i32;
+export fn open(path: *u8, flags: flag, mode: i32) i32 = {
+	return syscall3(SYS_OPEN, path: i64, (flags as i32): i64, mode: i64): i32;
 };
 
-export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
+export fn tryopen(path: *u8, flags: flag, mode: i32) (i32 | oserror) = {
 	let fd: i32 = open(path, flags, mode);
 	if (fd < 0) { return fd: i64: oserror; };
 	return fd;
@@ -106,8 +112,8 @@ export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
 // lseek — set/inspect the fd's position. Returns the new offset or
 // a negative errno. We use this for fstat-free file-size discovery
 // (open ⇒ lseek to end ⇒ lseek back).
-export fn lseek(fd: i32, off: i64, whence: i32) i64 = {
-	return syscall3(SYS_LSEEK, fd: i64, off, whence: i64);
+export fn lseek(fd: i32, off: i64, w: whence) i64 = {
+	return syscall3(SYS_LSEEK, fd: i64, off, (w as i32): i64);
 };
 
 // oserror — the underlying errno from a failed syscall, as a
@@ -119,9 +125,9 @@ export type oserror = !i64;
 
 // filesize — byte length of an open fd via lseek-to-end-and-back.
 export fn filesize(fd: i32) (i64 | oserror) = {
-	let end: i64 = lseek(fd, 0i64, SEEK_END);
+	let end: i64 = lseek(fd, 0i64, whence.END);
 	if (end < 0) { return end: oserror; };
-	let r: i64 = lseek(fd, 0i64, SEEK_SET);
+	let r: i64 = lseek(fd, 0i64, whence.SET);
 	if (r < 0) { return r: oserror; };
 	return end;
 };
@@ -607,7 +613,7 @@ fn locateimport(a: *arena, dirs: *u8, name: *u8, namelen: u64) *u8 = {
 // ---- file slurp -------------------------------------------------------
 
 fn slurp(pathcs: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(pathcs, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(pathcs, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
@@ -862,7 +868,7 @@ fn buildone(selfdir: *u8, src: *u8, out: *u8, incs: *u8, lf: *lflags) i32 = {
 	};
 
 	// Step 1: expand `use`s into the combined file.
-	let cf: i32 = os.open(combined, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 420i32);	// 0o644
+	let cf: i32 = os.open(combined, os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC, 420i32);	// 0o644
 	if (cf < 0) {
 		os.write(2, "ww: cannot open combined\n".ptr, 25u64);
 		return 1;
@@ -1402,7 +1408,7 @@ fn runsingletest(selfdir: *u8, src: *u8) i32 = {
 };
 
 fn rundirtests(selfdir: *u8, dir: *u8) i32 = {
-	let fd: i32 = os.open(dir, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(dir, os.flag.RDONLY, 0i32);
 	if (fd < 0) {
 		os.write(2, "ww test: cannot open directory\n".ptr, 31u64);
 		return 1;
diff --git a/selfhost/cmd/ww/main.ww b/selfhost/cmd/ww/main.ww
index b5e6a3e0..ae0077ac 100644
--- a/selfhost/cmd/ww/main.ww
+++ b/selfhost/cmd/ww/main.ww
@@ -281,7 +281,7 @@ fn locateimport(a: *arena, dirs: *u8, name: *u8, namelen: u64) *u8 = {
 // ---- file slurp -------------------------------------------------------
 
 fn slurp(pathcs: *u8) (*u8, u64) = {
-	let fd: i32 = os.open(pathcs, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(pathcs, os.flag.RDONLY, 0i32);
 	if (fd < 0) { return nil, 0u64; };
 	let szr: (i64 | os.oserror) = os.filesize(fd);
 	let n: i64 = 0i64;
@@ -536,7 +536,7 @@ fn buildone(selfdir: *u8, src: *u8, out: *u8, incs: *u8, lf: *lflags) i32 = {
 	};
 
 	// Step 1: expand `use`s into the combined file.
-	let cf: i32 = os.open(combined, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 420i32);	// 0o644
+	let cf: i32 = os.open(combined, os.flag.WRONLY | os.flag.CREATE | os.flag.TRUNC, 420i32);	// 0o644
 	if (cf < 0) {
 		os.write(2, "ww: cannot open combined\n".ptr, 25u64);
 		return 1;
@@ -1076,7 +1076,7 @@ fn runsingletest(selfdir: *u8, src: *u8) i32 = {
 };
 
 fn rundirtests(selfdir: *u8, dir: *u8) i32 = {
-	let fd: i32 = os.open(dir, os.O_RDONLY, 0i32);
+	let fd: i32 = os.open(dir, os.flag.RDONLY, 0i32);
 	if (fd < 0) {
 		os.write(2, "ww test: cannot open directory\n".ptr, 31u64);
 		return 1;
diff --git a/selfhost/cmd/wwdump/main.combined.ww b/selfhost/cmd/wwdump/main.combined.ww
index e4450fdb..4a8bb209 100644
--- a/selfhost/cmd/wwdump/main.combined.ww
+++ b/selfhost/cmd/wwdump/main.combined.ww
@@ -35,17 +35,23 @@ def SYS_UNLINK:      i64 = 87;
 def SYS_GETCWD:      i64 = 79;
 def SYS_GETDENTS64:  i64 = 217;
 
-// open(2) flags. Linux values, matching <fcntl.h>.
-def O_RDONLY: i32 = 0;
-def O_WRONLY: i32 = 1;
-def O_RDWR:   i32 = 2;
-def O_CREAT:  i32 = 64;		// 0x40
-def O_TRUNC:  i32 = 512;	// 0x200
+// open(2) flags. Linux values, matching <fcntl.h>. Hare names them
+// `fs::flag::RDONLY` etc; we use the same leaf names so callers say
+// `os.flag.RDONLY` and `os.flag.WRONLY | os.flag.CREATE`.
+export type flag = enum i32 {
+	RDONLY  = 0,
+	WRONLY  = 1,
+	RDWR    = 2,
+	CREATE  = 64,	// 0x40
+	TRUNC   = 512,	// 0x200
+};
 
-// lseek(2) whence.
-def SEEK_SET: i32 = 0;
-def SEEK_CUR: i32 = 1;
-def SEEK_END: i32 = 2;
+// lseek(2) whence. Hare names it `io::whence`.
+export type whence = enum i32 {
+	SET = 0,
+	CUR = 1,
+	END = 2,
+};
 
 export fn exit(code: i32) void = {
 	syscall1(SYS_EXIT, code: i64);
@@ -93,11 +99,11 @@ export fn trywrite(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 // `str` must ensure the bytes are followed by a 0 byte (literals are,
 // arena-copied paths usually are by construction). Returns -errno on
 // failure, fd otherwise. Higher-level callers prefer `tryopen`.
-export fn open(path: *u8, flags: i32, mode: i32) i32 = {
-	return syscall3(SYS_OPEN, path: i64, flags: i64, mode: i64): i32;
+export fn open(path: *u8, flags: flag, mode: i32) i32 = {
+	return syscall3(SYS_OPEN, path: i64, (flags as i32): i64, mode: i64): i32;
 };
 
-export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
+export fn tryopen(path: *u8, flags: flag, mode: i32) (i32 | oserror) = {
 	let fd: i32 = open(path, flags, mode);
 	if (fd < 0) { return fd: i64: oserror; };
 	return fd;
@@ -106,8 +112,8 @@ export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
 // lseek — set/inspect the fd's position. Returns the new offset or
 // a negative errno. We use this for fstat-free file-size discovery
 // (open ⇒ lseek to end ⇒ lseek back).
-export fn lseek(fd: i32, off: i64, whence: i32) i64 = {
-	return syscall3(SYS_LSEEK, fd: i64, off, whence: i64);
+export fn lseek(fd: i32, off: i64, w: whence) i64 = {
+	return syscall3(SYS_LSEEK, fd: i64, off, (w as i32): i64);
 };
 
 // oserror — the underlying errno from a failed syscall, as a
@@ -119,9 +125,9 @@ export type oserror = !i64;
 
 // filesize — byte length of an open fd via lseek-to-end-and-back.
 export fn filesize(fd: i32) (i64 | oserror) = {
-	let end: i64 = lseek(fd, 0i64, SEEK_END);
+	let end: i64 = lseek(fd, 0i64, whence.END);
 	if (end < 0) { return end: oserror; };
-	let r: i64 = lseek(fd, 0i64, SEEK_SET);
+	let r: i64 = lseek(fd, 0i64, whence.SET);
 	if (r < 0) { return r: oserror; };
 	return end;
 };
@@ -9299,7 +9305,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 		return 2;
 	};
 
-	let fdorerr: (i32 | os.oserror) = os.tryopen(path, os.O_RDONLY, 0i32);
+	let fdorerr: (i32 | os.oserror) = os.tryopen(path, os.flag.RDONLY, 0i32);
 	let fd: i32 = -1;
 	match (fdorerr) {
 		case let v: i32 => fd = v;
diff --git a/selfhost/cmd/wwdump/main.ww b/selfhost/cmd/wwdump/main.ww
index 22fa2685..43db3de7 100644
--- a/selfhost/cmd/wwdump/main.ww
+++ b/selfhost/cmd/wwdump/main.ww
@@ -74,7 +74,7 @@ export fn main(argc: i32, argv: **u8) i32 = {
 		return 2;
 	};
 
-	let fdorerr: (i32 | os.oserror) = os.tryopen(path, os.O_RDONLY, 0i32);
+	let fdorerr: (i32 | os.oserror) = os.tryopen(path, os.flag.RDONLY, 0i32);
 	let fd: i32 = -1;
 	match (fdorerr) {
 		case let v: i32 => fd = v;
diff --git a/selfhost/test/smoke.combined.ww b/selfhost/test/smoke.combined.ww
index 96819efb..8487549e 100644
--- a/selfhost/test/smoke.combined.ww
+++ b/selfhost/test/smoke.combined.ww
@@ -35,17 +35,23 @@ def SYS_UNLINK:      i64 = 87;
 def SYS_GETCWD:      i64 = 79;
 def SYS_GETDENTS64:  i64 = 217;
 
-// open(2) flags. Linux values, matching <fcntl.h>.
-def O_RDONLY: i32 = 0;
-def O_WRONLY: i32 = 1;
-def O_RDWR:   i32 = 2;
-def O_CREAT:  i32 = 64;		// 0x40
-def O_TRUNC:  i32 = 512;	// 0x200
+// open(2) flags. Linux values, matching <fcntl.h>. Hare names them
+// `fs::flag::RDONLY` etc; we use the same leaf names so callers say
+// `os.flag.RDONLY` and `os.flag.WRONLY | os.flag.CREATE`.
+export type flag = enum i32 {
+	RDONLY  = 0,
+	WRONLY  = 1,
+	RDWR    = 2,
+	CREATE  = 64,	// 0x40
+	TRUNC   = 512,	// 0x200
+};
 
-// lseek(2) whence.
-def SEEK_SET: i32 = 0;
-def SEEK_CUR: i32 = 1;
-def SEEK_END: i32 = 2;
+// lseek(2) whence. Hare names it `io::whence`.
+export type whence = enum i32 {
+	SET = 0,
+	CUR = 1,
+	END = 2,
+};
 
 export fn exit(code: i32) void = {
 	syscall1(SYS_EXIT, code: i64);
@@ -93,11 +99,11 @@ export fn trywrite(fd: i32, buf: *u8, n: u64) (i64 | oserror) = {
 // `str` must ensure the bytes are followed by a 0 byte (literals are,
 // arena-copied paths usually are by construction). Returns -errno on
 // failure, fd otherwise. Higher-level callers prefer `tryopen`.
-export fn open(path: *u8, flags: i32, mode: i32) i32 = {
-	return syscall3(SYS_OPEN, path: i64, flags: i64, mode: i64): i32;
+export fn open(path: *u8, flags: flag, mode: i32) i32 = {
+	return syscall3(SYS_OPEN, path: i64, (flags as i32): i64, mode: i64): i32;
 };
 
-export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
+export fn tryopen(path: *u8, flags: flag, mode: i32) (i32 | oserror) = {
 	let fd: i32 = open(path, flags, mode);
 	if (fd < 0) { return fd: i64: oserror; };
 	return fd;
@@ -106,8 +112,8 @@ export fn tryopen(path: *u8, flags: i32, mode: i32) (i32 | oserror) = {
 // lseek — set/inspect the fd's position. Returns the new offset or
 // a negative errno. We use this for fstat-free file-size discovery
 // (open ⇒ lseek to end ⇒ lseek back).
-export fn lseek(fd: i32, off: i64, whence: i32) i64 = {
-	return syscall3(SYS_LSEEK, fd: i64, off, whence: i64);
+export fn lseek(fd: i32, off: i64, w: whence) i64 = {
+	return syscall3(SYS_LSEEK, fd: i64, off, (w as i32): i64);
 };
 
 // oserror — the underlying errno from a failed syscall, as a
@@ -119,9 +125,9 @@ export type oserror = !i64;
 
 // filesize — byte length of an open fd via lseek-to-end-and-back.
 export fn filesize(fd: i32) (i64 | oserror) = {
-	let end: i64 = lseek(fd, 0i64, SEEK_END);
+	let end: i64 = lseek(fd, 0i64, whence.END);
 	if (end < 0) { return end: oserror; };
-	let r: i64 = lseek(fd, 0i64, SEEK_SET);
+	let r: i64 = lseek(fd, 0i64, whence.SET);
 	if (r < 0) { return r: oserror; };
 	return end;
 };
@@ -576,8 +582,11 @@ export fn main() i32 = {
 	// Use raw os.open here (returns i32 with -errno) for the same
 	// reason as os.read below: probe 6 in 990_selfhost compiles
 	// smoke.ww standalone (no `use` expansion), so cross-module type
-	// references like `os.oserror` don't resolve at that step.
-	let fd: i32 = os.open(path.ptr, os.O_RDONLY, 0i32);
+	// references like `os.oserror` and `os.flag` don't resolve at
+	// that step. RDONLY is 0; passing the literal keeps the call
+	// site standalone-compilable to byte-identical asm on both
+	// compilers.
+	let fd: i32 = os.open(path.ptr, 0, 0i32);
 	if (fd < 0) { return 21; };
 	let rbuf: [128]u8;
 	// Use raw os.read here (single syscall, plain i64) instead of
diff --git a/selfhost/test/smoke.ww b/selfhost/test/smoke.ww
index 68b1561b..1c8ddb4c 100644
--- a/selfhost/test/smoke.ww
+++ b/selfhost/test/smoke.ww
@@ -155,8 +155,11 @@ export fn main() i32 = {
 	// Use raw os.open here (returns i32 with -errno) for the same
 	// reason as os.read below: probe 6 in 990_selfhost compiles
 	// smoke.ww standalone (no `use` expansion), so cross-module type
-	// references like `os.oserror` don't resolve at that step.
-	let fd: i32 = os.open(path.ptr, os.O_RDONLY, 0i32);
+	// references like `os.oserror` and `os.flag` don't resolve at
+	// that step. RDONLY is 0; passing the literal keeps the call
+	// site standalone-compilable to byte-identical asm on both
+	// compilers.
+	let fd: i32 = os.open(path.ptr, 0, 0i32);
 	if (fd < 0) { return 21; };
 	let rbuf: [128]u8;
 	// Use raw os.read here (single syscall, plain i64) instead of
diff --git a/test/wcc/700_e2e.c b/test/wcc/700_e2e.c
index 89ede8ac..bec27bc2 100644
--- a/test/wcc/700_e2e.c
+++ b/test/wcc/700_e2e.c
@@ -1100,13 +1100,13 @@ static const struct row rows[] = {
 	  "  let m: mode = mode.R | mode.W;\n"
 	  "  return m as i32;\n"
 	  "};", 3 },
-	/* enum: pkg-qualified access — `os.whence.CUR` resolves through
+	/* enum: pkg-qualified access — `pkg.dir.SOUTH` resolves through
 	 * SK_USE and folds to the member literal. */
-	{ "// MODULE: os\n"
-	  "type whence = enum { SET, CUR, END };\n"
+	{ "// MODULE: pkg\n"
+	  "type dir = enum { NORTH, SOUTH, EAST, WEST };\n"
 	  "// MODULE: main\n"
-	  "use os;\n"
-	  "fn main() i32 = { return os.whence.CUR as i32; };", 1 },
+	  "use pkg;\n"
+	  "fn main() i32 = { return pkg.dir.SOUTH as i32; };", 1 },
 	{ NULL, 0 }
 };