diff --git a/cmd/ww/main.c b/cmd/ww/main.c
index 5ad1aa2c..ace14165 100644
--- a/cmd/ww/main.c
+++ b/cmd/ww/main.c
@@ -43,15 +43,6 @@ toolpath(const char *envvar, const char *name)
 	return strdup(buf);
 }
 
-static int
-run(const char *cmd)
-{
-	int rc = system(cmd);
-	if (rc == -1) return -1;
-	if (WIFEXITED(rc)) return WEXITSTATUS(rc);
-	return 1;
-}
-
 static int
 run_argv(const char *prog, char *const argv[])
 {
@@ -1949,7 +1940,7 @@ build_one_sep_impl(const char *src, int entry_is_dir,
 		char asmf[SEP_ARTIFACT_MAX], obj[SEP_ARTIFACT_MAX];
 		char apath[SEP_ARTIFACT_MAX], unitnew[SEP_ARTIFACT_MAX];
 		char wwinew[SEP_ARTIFACT_MAX], asmnew[SEP_ARTIFACT_MAX];
-		char objnew[SEP_ARTIFACT_MAX], anew[SEP_ARTIFACT_MAX], cmd[8192];
+		char objnew[SEP_ARTIFACT_MAX], anew[SEP_ARTIFACT_MAX];
 		sep_fname(g, pi, scratch, ".unit.ww", unitf, sizeof unitf);
 		sep_fname(g, pi, scratch, ".wwi", wwi, sizeof wwi);
 		sep_fname(g, pi, scratch, ".s", asmf, sizeof asmf);
@@ -1994,25 +1985,30 @@ build_one_sep_impl(const char *src, int entry_is_dir,
 		 * LOCAL type (the root is never imported), which the
 		 * export-check rejects. Skip -I for the root; its `.wwi`
 		 * is never consumed. */
-		if (!needs_export) {
+		char *cargv[12];
+		int cpos = 0;
+		cargv[cpos++] = "w6c";
+		if (!needs_export && is_test && g->pkg[pi].root) {
 			/* #79: the root carries -T under `ww test`
 			 * so w6c synthesizes the test main. Deps never
 			 * get -T. */
-			if (is_test && g->pkg[pi].root)
-				snprintf(cmd, sizeof cmd,
-				    "%s -T --test-support-module %s -c -o %s %s",
-				    c6, test_support_module, cs, cu);
-			else
-				snprintf(cmd, sizeof cmd, "%s -c -o %s %s",
-				    c6, cs, cu);
-		} else
-			snprintf(cmd, sizeof cmd, "%s %s%s%s-c -I %s -o %s %s",
-			    c6, g->pkg[pi].test_support
-				? "--test-support-module " : "",
-			    g->pkg[pi].test_support ? test_support_module : "",
-			    g->pkg[pi].test_support ? " " : "",
-			    cw, cs, cu);
-		if (run(cmd) != 0) {
+			cargv[cpos++] = "-T";
+			cargv[cpos++] = "--test-support-module";
+			cargv[cpos++] = (char *)test_support_module;
+		} else if (needs_export && g->pkg[pi].test_support) {
+			cargv[cpos++] = "--test-support-module";
+			cargv[cpos++] = (char *)test_support_module;
+		}
+		cargv[cpos++] = "-c";
+		if (needs_export) {
+			cargv[cpos++] = "-I";
+			cargv[cpos++] = (char *)cw;
+		}
+		cargv[cpos++] = "-o";
+		cargv[cpos++] = (char *)cs;
+		cargv[cpos++] = (char *)cu;
+		cargv[cpos] = NULL;
+		if (run_argv(c6, cargv) != 0) {
 			fprintf(stderr, "ww: w6c failed for %s\n",
 			    g->pkg[pi].path[0] ? g->pkg[pi].path : "(root)");
 			g->pkg[pi].failed = 1;
@@ -2020,8 +2016,9 @@ build_one_sep_impl(const char *src, int entry_is_dir,
 			continue;
 		}
 		if (!emit_asm) {
-			snprintf(cmd, sizeof cmd, "%s -o %s %s", a6, co, cs);
-			if (run(cmd) != 0) {
+			char *aargv[] = {"w6a", "-o", (char *)co,
+			    (char *)cs, NULL};
+			if (run_argv(a6, aargv) != 0) {
 				fprintf(stderr, "ww: w6a failed for %s\n",
 				    g->pkg[pi].path[0] ? g->pkg[pi].path : "(root)");
 				g->pkg[pi].failed = 1;
diff --git a/selfhost/cmd/ww/main.ww b/selfhost/cmd/ww/main.ww
index 1f16ff22..a584c9ab 100644
--- a/selfhost/cmd/ww/main.ww
+++ b/selfhost/cmd/ww/main.ww
@@ -8,8 +8,8 @@
 //   ww version       →  print version
 //
 // Tool paths default to siblings of $0 so a fresh build runs out of
-// out/bin/. Env-var overrides (WW_W6C / WW_W6A / WW_W6L / WW_LIB) are
-// not yet supported in this port; the bootstrap doesn't need them.
+// out/bin/. WW_W6C / WW_W6A / WW_W6L select exact executable paths.
+// Source-library and runtime-library overrides are not yet supported.
 
 package main;
 
@@ -162,6 +162,16 @@ fn owncstr(s: str) *u8 = {
 	return b.ptr;
 };
 
+fn toolpath(selfdir: *u8, envvar: str, name: str) *u8 = {
+	match (os.getenv(envvar)) {
+	case let p: str => {
+		if (p.len != 0) { return owncstr(p); };
+	};
+	case void => void;
+	};
+	return joinpathlit(selfdir, name);
+};
+
 // execpackagetests — replace the driver with the native WW package
 // coordinator for directory/default invocations.  Explicit generated
 // package.ww roots stay on runsingletest, which is the recursion boundary when
@@ -2018,9 +2028,9 @@ fn buildonesepimpl(selfdir: *u8, src: *u8, entryisdir: i32,
 		};
 		statusi += 1;
 	};
-	let c6: *u8 = joinpathlit(selfdir, "w6c_ww");
-	let a6: *u8 = joinpathlit(selfdir, "w6a_ww");
-	let l6: *u8 = joinpathlit(selfdir, "w6l_ww");
+	let c6: *u8 = toolpath(selfdir, "WW_W6C", "w6c_ww");
+	let a6: *u8 = toolpath(selfdir, "WW_W6A", "w6a_ww");
+	let l6: *u8 = toolpath(selfdir, "WW_W6L", "w6l_ww");
 
 	let dotdotlib: []u8 = alloc([], (os.PATH_MAX: u64))!;
 	dotdotlib.len = os.PATH_MAX;