diff --git a/cmd/ww/main.c b/cmd/ww/main.c
index 9f74588d..0cf32f61 100644
--- a/cmd/ww/main.c
+++ b/cmd/ww/main.c
@@ -1309,6 +1309,7 @@ struct sepproduct {
 	int directory_product;
 	int no_tests;
 	int build_action;	/* loaded product retained in the action list */
+	int public_out;
 	int context;
 	int root;
 	int variant_root;	/* retained single-unit root outside directory products */
@@ -4983,6 +4984,7 @@ struct septxnentry {
 	char *backup;
 	int had_old;
 	int installed;
+	int public_output;
 };
 
 struct septxn {
@@ -4991,7 +4993,8 @@ struct septxn {
 };
 
 static int
-sep_txn_add(struct septxn *tx, const char *stage, const char *dst)
+sep_txn_add_mode(struct septxn *tx, const char *stage, const char *dst,
+    int public_output)
 {
 	if (strcmp(stage, dst) == 0) {
 		fprintf(stderr, "ww: transaction path collision: %s\n", dst);
@@ -5013,6 +5016,7 @@ sep_txn_add(struct septxn *tx, const char *stage, const char *dst)
 	e->stage = strdup(stage);
 	e->dst = strdup(dst);
 	e->backup = sep_sprintf("%s.wwtxn.%ld.old", dst, (long)getpid());
+	e->public_output = public_output;
 	if (e->stage == NULL || e->dst == NULL || e->backup == NULL) {
 		sep_fail_nomem();
 		free(e->backup); free(e->dst); free(e->stage);
@@ -5029,6 +5033,18 @@ sep_txn_add(struct septxn *tx, const char *stage, const char *dst)
 	return 0;
 }
 
+static int
+sep_txn_add(struct septxn *tx, const char *stage, const char *dst)
+{
+	return sep_txn_add_mode(tx, stage, dst, 0);
+}
+
+static int
+sep_txn_add_public(struct septxn *tx, const char *stage, const char *dst)
+{
+	return sep_txn_add_mode(tx, stage, dst, 1);
+}
+
 static void
 sep_txn_discard(struct septxn *tx)
 {
@@ -5049,8 +5065,87 @@ sep_txn_free(struct septxn *tx)
 	memset(tx, 0, sizeof *tx);
 }
 
+struct sep_output_magic {
+	const unsigned char *bytes;
+	size_t n;
+};
+
+#define SEP_OUTPUT_MAGIC(s) { (const unsigned char *)(s), sizeof(s) - 1 }
+
+/* Go 1.26.5 work.objectMagic. WW interfaces are an additional public output
+ * kind and always begin with the compiler-owned module directive. */
+static const struct sep_output_magic sep_output_magic[] = {
+	SEP_OUTPUT_MAGIC("!<arch>\n"),
+	SEP_OUTPUT_MAGIC("<bigaf>\n"),
+	SEP_OUTPUT_MAGIC("\x7f" "ELF"),
+	SEP_OUTPUT_MAGIC("\xfe\xed\xfa\xce"),
+	SEP_OUTPUT_MAGIC("\xfe\xed\xfa\xcf"),
+	SEP_OUTPUT_MAGIC("\xce\xfa\xed\xfe"),
+	SEP_OUTPUT_MAGIC("\xcf\xfa\xed\xfe"),
+	SEP_OUTPUT_MAGIC("\x4d\x5a\x90\x00\x03\x00"),
+	SEP_OUTPUT_MAGIC("\x4d\x5a\x78\x00\x01\x00"),
+	SEP_OUTPUT_MAGIC("\x00\x00\x01\xeb"),
+	SEP_OUTPUT_MAGIC("\x00\x00\x8a\x97"),
+	SEP_OUTPUT_MAGIC("\x00\x00\x06\x47"),
+	SEP_OUTPUT_MAGIC("\x00\x61\x73\x6d"),
+	SEP_OUTPUT_MAGIC("\x01\xdf"),
+	SEP_OUTPUT_MAGIC("\x01\xf7"),
+	SEP_OUTPUT_MAGIC("//ww:module "),
+};
+
+#undef SEP_OUTPUT_MAGIC
+
+static int
+sep_is_object_output(const char *path)
+{
+	unsigned char buf[64] = {0};
+	int fd = open(path, O_RDONLY);
+	if (fd < 0) return 0;
+	size_t got = 0;
+	int bad = 0;
+	while (got < sizeof buf) {
+		ssize_t n = read(fd, buf + got, sizeof buf - got);
+		if (n > 0) got += (size_t)n;
+		else if (n == 0) break;
+		else if (errno != EINTR) { bad = 1; break; }
+	}
+	(void)close(fd);
+	if (bad) return 0;
+	for (size_t i = 0; i < nelem(sep_output_magic); i++)
+		if (got >= sep_output_magic[i].n
+		    && memcmp(buf, sep_output_magic[i].bytes,
+			sep_output_magic[i].n) == 0)
+			return 1;
+	return 0;
+}
+
+/* Pinned Go's checkDstOverwrite follows symlinks with stat and protects only
+ * directories and nonempty regular non-object files. Other file kinds remain
+ * eligible for the install operation itself to replace. */
+static int
+sep_check_dst_overwrite(const char *dst)
+{
+	struct stat st;
+	if (stat(dst, &st) != 0) return 0;
+	if (S_ISDIR(st.st_mode)) {
+		fputs("ww: build output ", stderr);
+		sep_put_quoted(dst);
+		fputs(" already exists and is a directory\n", stderr);
+		return -1;
+	}
+	if (S_ISREG(st.st_mode) && st.st_size != 0
+	    && !sep_is_object_output(dst)) {
+		fputs("ww: build output ", stderr);
+		sep_put_quoted(dst);
+		fputs(" already exists and is not an object file\n", stderr);
+		return -1;
+	}
+	return 0;
+}
+
 /* One request-wide rollback group: producers and linkers finish first; only
- * then are old destinations parked and all staged files installed. */
+ * then are public destinations checked, old destinations parked, and all
+ * staged files installed. */
 static int
 sep_txn_commit(struct septxn *tx)
 {
@@ -5060,6 +5155,10 @@ sep_txn_commit(struct septxn *tx)
 			    tx->v[i].stage);
 			goto rollback;
 		}
+	for (int i = 0; i < tx->n; i++)
+		if (tx->v[i].public_output
+		    && sep_check_dst_overwrite(tx->v[i].dst) != 0)
+			goto rollback;
 	for (int i = 0; i < tx->n; i++)
 		if (path_exists_nofollow(tx->v[i].backup) != 0) {
 			fprintf(stderr, "ww: transaction backup already exists: %s\n",
@@ -5476,6 +5575,52 @@ sep_mkdirs(const char *path, mode_t mode, struct sep_created_dirs *created)
 	return 0;
 }
 
+/* A retained running test is built and executed from request-private storage.
+ * Its Go-like install action runs only after that test succeeds, but reuses
+ * the same guarded publisher as build and compile-only test products. */
+static int
+sep_install_test_output(const char *stage, const char *dst)
+{
+	char *install_stage = sep_sprintf("%s.install", stage);
+	if (install_stage == NULL) return 1;
+	if (strlen(install_stage) + 1 > (size_t)PATH_MAX
+	    || path_exists_nofollow(install_stage) != 0
+	    || copy_executable_stage(stage, install_stage) != 0) {
+		fputs("ww: cannot stage retained test output\n", stderr);
+		free(install_stage);
+		return 1;
+	}
+	char *parent = sep_parent_path(dst);
+	if (parent == NULL) {
+		(void)unlink(install_stage);
+		free(install_stage);
+		return 1;
+	}
+	struct sep_created_dirs created = {0};
+	if (sep_mkdirs(parent, 0777, &created) != 0) {
+		fprintf(stderr, "ww: cannot create test output directory %s\n",
+		    parent);
+		free(parent);
+		(void)unlink(install_stage);
+		free(install_stage);
+		return 1;
+	}
+	free(parent);
+	struct septxn tx = {0};
+	if (sep_txn_add_public(&tx, install_stage, dst) != 0
+	    || sep_txn_commit(&tx) != 0) {
+		sep_txn_discard(&tx);
+		sep_txn_free(&tx);
+		sep_rollback_dirs(&created);
+		(void)unlink(install_stage);
+		free(install_stage);
+		return 1;
+	}
+	sep_txn_free(&tx);
+	free(install_stage);
+	return 0;
+}
+
 /* build_sep_plan — discover dependencies for every requested product in one
  * package universe, compile the dependency-first union once, then link each
  * root from its own complete reachable archive closure. The dependency-first
@@ -6713,11 +6858,14 @@ prepare_transaction:
 	}
 	for (int i = 0; i < nproducts; i++) {
 		if (products[i].stage_out != NULL
-		    && sep_txn_add(&tx, products[i].stage_out,
-			products[i].out) < 0)
+		    && (products[i].public_out
+			? sep_txn_add_public(&tx, products[i].stage_out,
+			    products[i].out)
+			: sep_txn_add(&tx, products[i].stage_out,
+			    products[i].out)) < 0)
 			goto request_fail;
 		if (products[i].stage_publish != NULL
-		    && sep_txn_add(&tx, products[i].stage_publish,
+		    && sep_txn_add_public(&tx, products[i].stage_publish,
 			products[i].publish) < 0)
 			goto request_fail;
 		if (products[i].stage_iface != NULL) {
@@ -6725,7 +6873,7 @@ prepare_transaction:
 			int on = snprintf(outiface, sizeof outiface, "%s.wwi",
 			    products[i].out);
 			if (on < 0 || (size_t)on >= sizeof outiface
-			    || sep_txn_add(&tx, products[i].stage_iface,
+			    || sep_txn_add_public(&tx, products[i].stage_iface,
 				outiface) < 0)
 				goto request_fail;
 		}
@@ -6771,7 +6919,8 @@ build_one_sep(const char *src, int entry_is_dir, const char *root_identity,
     const struct seplinkflags *linkflags, int publish_package,
     int require_command, int is_test,
     int root_variant, const char *test_package, int emit_asm,
-    int keepscratch, const char *workdir, const char *create_output_dir,
+    int public_output, int keepscratch, const char *workdir,
+    const char *create_output_dir,
     const char *default_output_dir, int output_path_error)
 {
 	char scratch[PATH_MAX] = {0};
@@ -6786,6 +6935,7 @@ build_one_sep(const char *src, int entry_is_dir, const char *root_identity,
 		.artifact = NULL,
 		.variant = root_variant,
 		.build_action = 1,
+		.public_out = public_output,
 		.root = -1,
 		.variant_root = -1,
 		.support = -1,
@@ -7225,7 +7375,7 @@ do_build(int argc, char **argv)
 		 * library compilation still need request-private product paths. */
 		int rc = build_one_sep(resolved, is_dir, root_identity, tmp, tmp, incs,
 		    &linkflags, 0, 0, 0, SEP_VARIANT_PRODUCTION, NULL,
-		    emit_asm, 0, workdir, NULL, NULL, 0);
+		    emit_asm, 0, 0, workdir, NULL, NULL, 0);
 		int cleanfail = 0;
 		if (unlink(tmp) != 0 && errno != ENOENT) {
 			fputs("ww: cannot remove temporary output\n", stderr);
@@ -7241,7 +7391,7 @@ do_build(int argc, char **argv)
 	}
 	int rc = build_one_sep(resolved, is_dir, root_identity, out, objstem, incs,
 	    &linkflags, outflag[0] != '\0', 0, 0, SEP_VARIANT_PRODUCTION, NULL,
-	    emit_asm, 1, workdir, create_output_dir, default_output_dir,
+	    emit_asm, 1, 1, workdir, create_output_dir, default_output_dir,
 	    output_path_error);
 	free(incs);
 	return rc;
@@ -7287,7 +7437,7 @@ do_run(int argc, char **argv)
 	const char *root_identity = !literal && is_dir ? src : NULL;
 	int buildrc = build_one_sep(resolved, is_dir, root_identity, tmp, tmp, incs,
 	    &linkflags,
-	    0, 1, 0, SEP_VARIANT_PRODUCTION, NULL, 0, 0, NULL, NULL, NULL, 0);
+	    0, 1, 0, SEP_VARIANT_PRODUCTION, NULL, 0, 0, 0, NULL, NULL, NULL, 0);
 	free(incs);
 	if (buildrc != 0) {
 		if (unlink(tmp) != 0 && errno != ENOENT)
@@ -7334,6 +7484,9 @@ do_run(int argc, char **argv)
 static int
 do_test(int argc, char **argv)
 {
+	if (argc == 3
+	    && strcmp(argv[0], "--ww-install-test-output") == 0)
+		return sep_install_test_output(argv[1], argv[2]);
 	const char *src = NULL;
 	struct sepproduct *products = NULL;
 	int nproducts = 0, productcap = 0;
@@ -7475,7 +7628,10 @@ do_test(int argc, char **argv)
 				const char *publish = argv[++i];
 				const char *status = argv[++i];
 				size_t pn = strlen(name);
-				int build_product = strcmp(kind, "build") == 0;
+				int build_product = strcmp(kind, "build") == 0
+				    || strcmp(kind, "build-public") == 0;
+				int public_build_product =
+				    strcmp(kind, "build-public") == 0;
 				int test_product = strcmp(kind, "test") == 0;
 				int has_production = strcmp(production, "-") != 0;
 				int has_internal = strcmp(internal, "-") != 0;
@@ -7527,6 +7683,7 @@ do_test(int argc, char **argv)
 				products[nproducts].no_tests = test_product
 				    && !has_internal && !has_external;
 				products[nproducts].build_action = 1;
+				products[nproducts].public_out = public_build_product;
 				products[nproducts].root = -1;
 				products[nproducts].variant_root = -1;
 				products[nproducts].production_root = -1;
@@ -7788,9 +7945,12 @@ do_test(int argc, char **argv)
 		}
 		char tmpdir[PATH_MAX] = {0}, tmp[PATH_MAX];
 		const char *outp;
-		int owntmp = (!outstem[0] || discard_output) && !workdir[0];
-		if (outstem[0] && !discard_output) outp = outstem;
-		else if (workdir[0]) {
+		int retain_output = outstem[0] && !discard_output;
+		int deferred_install = retain_output && !compileonly && !emit_asm;
+		int owntmp = deferred_install
+		    || ((!outstem[0] || discard_output) && !workdir[0]);
+		if (retain_output && !deferred_install) outp = outstem;
+		else if (workdir[0] && !deferred_install) {
 			/* The workdir owns the persistent test binary the same
 			 * way it owns the package artifacts. */
 			int tn = snprintf(tmp, sizeof tmp, "%s/main", workdir);
@@ -7816,7 +7976,8 @@ do_test(int argc, char **argv)
 		    outstem[0] && !discard_output ? outstem : tmp,
 		    incs, NULL, 0, 0, 1,
 		    SEP_VARIANT_PRODUCTION, NULL, emit_asm,
-		    outstem[0] && !discard_output ? 1 : 0, workdir, NULL, NULL, 0);
+		    retain_output && !deferred_install, retain_output ? 1 : 0,
+		    workdir, NULL, NULL, 0);
 		if (br != 0) {
 			if (owntmp && unlink(outp) != 0 && errno != ENOENT)
 				fputs("ww: cannot remove temporary output\n", stderr);
@@ -7837,6 +7998,9 @@ do_test(int argc, char **argv)
 			return cleanfail ? 1 : 0;
 		}
 		int rc = run_test_bin(outp, pattern);
+		if (rc == 0 && deferred_install
+		    && sep_install_test_output(outp, outstem) != 0)
+			rc = 1;
 		if (owntmp && unlink(outp) != 0 && errno != ENOENT) {
 			fputs("ww: cannot remove temporary output\n", stderr);
 			if (rc == 0) rc = 1;
@@ -7860,9 +8024,12 @@ do_test(int argc, char **argv)
 		}
 		char tmpdir[PATH_MAX] = {0}, tmp[PATH_MAX];
 		const char *outp;
-		int owntmp = (!outstem[0] || discard_output) && !workdir[0];
-		if (outstem[0] && !discard_output) outp = outstem;
-		else if (workdir[0]) {
+		int retain_output = outstem[0] && !discard_output;
+		int deferred_install = retain_output && !compileonly && !emit_asm;
+		int owntmp = deferred_install
+		    || ((!outstem[0] || discard_output) && !workdir[0]);
+		if (retain_output && !deferred_install) outp = outstem;
+		else if (workdir[0] && !deferred_install) {
 			int tn = snprintf(tmp, sizeof tmp, "%s/main", workdir);
 			if (tn < 0 || (size_t)tn >= sizeof tmp) {
 				fputs("ww: workdir path is too long\n", stderr);
@@ -7884,7 +8051,8 @@ do_test(int argc, char **argv)
 		int br = build_one_sep(target, 0, NULL, outp,
 		    outstem[0] && !discard_output ? outstem : tmp,
 		    incs, NULL, 0, 0, 1, SEP_VARIANT_PRODUCTION, NULL, emit_asm,
-		    outstem[0] && !discard_output ? 1 : 0, workdir, NULL, NULL, 0);
+		    retain_output && !deferred_install, retain_output ? 1 : 0,
+		    workdir, NULL, NULL, 0);
 		if (br != 0) {
 			if (owntmp && unlink(outp) != 0 && errno != ENOENT)
 				fputs("ww: cannot remove temporary output\n", stderr);
@@ -7905,6 +8073,9 @@ do_test(int argc, char **argv)
 			return cleanfail ? 1 : 0;
 		}
 		int rc = run_test_bin(outp, pattern);
+		if (rc == 0 && deferred_install
+		    && sep_install_test_output(outp, outstem) != 0)
+			rc = 1;
 		if (owntmp && unlink(outp) != 0 && errno != ENOENT) {
 			fputs("ww: cannot remove temporary output\n", stderr);
 			if (rc == 0) rc = 1;
diff --git a/docs/build-system.md b/docs/build-system.md
index 780fb696..7968d074 100644
--- a/docs/build-system.md
+++ b/docs/build-system.md
@@ -6214,11 +6214,10 @@ The authority is official Go 1.26.5 at commit
 The separation between saved and executed paths is a conclusion derived from
 the pinned action dependencies: the run consumes the temporary link action,
 not the install action. Duplicate names are likewise a materialization
-collision, not package identity. Go's sources do not assert that a set of
-retained binaries is installed as one rollback transaction; WW keeps its
-existing stronger request-wide transaction while matching the observable
-accepted, rejected, and preserved outputs. No installed host Go behavior was
-used as authority.
+collision, not package identity. Section 11.31 completes the later install
+dependency: compile-only products retain the request transaction, while a
+running retained product installs independently only after its successful run.
+No installed host Go behavior was used as authority.
 
 Before this slice, direct native measurements of both Cstage and WWstage showed
 that single-package `-c -o FILE` retained and did not run, but `-o FILE`
@@ -6268,23 +6267,24 @@ performs ordinary production validation, reports `[no tests]`, and creates no
 binary or otherwise-unused output directory. Successful compile-only products
 are silent, matching Go's no-op print action.
 
-After linking, the driver copies the private runnable bytes to a distinct
-`.new` inode opened with executable mode `0777` subject to umask. Temporary
-runnable, retained copy, statuses, changed persistent actions, tool records,
-and stamp then enter the existing one-request transaction. All producers and
-linkers complete before installation. Any load, compile, assemble, archive,
-link, stage, or install failure preserves old retained binaries and persistent
-bytes, discards all stages, removes cold scratch, and rolls back only output
-prefixes created by that request. Occupied or dangling `.new` paths reject
-before tools and are never overwritten.
+For `-c`, the driver copies the private runnable bytes to a distinct `.new`
+inode opened with executable mode `0777` subject to umask. Temporary runnable,
+retained copy, statuses, changed persistent actions, tool records, and stamp
+then enter the existing one-request transaction. All producers and linkers
+complete before installation. Any load, compile, assemble, archive, link,
+stage, or install failure preserves old retained binaries and persistent bytes,
+discards all stages, removes cold scratch, and rolls back only output prefixes
+created by that request. Occupied or dangling `.new` paths reject before tools
+and are never overwritten.
 
-Execution begins only after publication commits. Assertion failure, signal,
-timeout, or child-setup failure therefore leaves an explicitly retained binary
-while retaining the established stdout/stderr result routing and sibling
-isolation. Parallel products stage independent runnable/copy pairs; the shared
-transaction prevents partial sibling publication and canonical result emission
-order remains unchanged. Direct invocation of a retained binary continues to
-inherit caller cwd, environment, and separate standard descriptors.
+For running `-o`, the build transaction commits only the private runnable,
+status, and semantic actions. The coordinator executes that runnable and, on a
+successful result, invokes the selected driver stage's public install action.
+Assertion failure, signal, timeout, interruption, or child-setup failure skips
+that action and preserves any prior binary. Successful parallel products
+install independently after their runs; canonical result emission order stays
+unchanged. Direct invocation of a retained binary continues to inherit caller
+cwd, environment, and separate standard descriptors.
 
 `-c` and `-o` can accompany `-w`: the workdir owns only semantic actions while
 the invocation/output path owns only the retained copy. Unchanged actions are
@@ -6302,9 +6302,9 @@ declared-name versus import-leaf naming; executable mode and direct execution;
 temporary argv versus retained path; no-test omission; duplicate and
 non-directory rejection; occupied stages; serial and parallel sibling
 publication; injected late-link rollback over old files and newly created
-parents; runtime-failure retention; persistent cold/warm/invalidation behavior;
-diagnostic equality; retained binary byte identity; and absence of `.new`
-residue. Existing package tests continue to own all action/test variants,
+parents; runtime-failure preservation; persistent cold/warm/invalidation
+behavior; diagnostic equality; retained binary byte identity; and absence of
+`.new` residue. Existing package tests continue to own all action/test variants,
 graph identity, output ordering, cwd/environment/stdin, timeout, and broader
 transaction behavior.
 
@@ -6760,6 +6760,150 @@ output umasks, occupied stages, generalized multi-product transactions, test
 runtime failure and timeout, null discard, and broader package/import graph
 matrices.
 
+### 11.31 Implemented Go-like public-output overwrite safety
+
+Every caller-visible build and retained-test install now protects an existing
+destination at the same late boundary as Go 1.26.5. After applicable producers
+finish, ordinary `stat` rejects a directory and rejects a nonempty regular file
+whose leading bytes do not identify a toolchain output. Absent paths, empty
+regular reservations, recognized outputs, and non-directory non-regular paths
+remain replaceable. Exact `/dev/null` and assembly-only `-S` have no install
+action and never enter this rule.
+
+#### Pinned Go evidence and classification
+
+The sole authority is official Go 1.26.5 at commit
+`c19862e5f8415b4f24b189d065ed739517c548ba`:
+
+- `Shell.moveOrCopyFile` and `Shell.CopyFile` call `checkDstOverwrite` before
+  replacing the destination
+  ([`cmd/go/internal/work/shell.go`, lines 119–235](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/work/shell.go#L119-L235)).
+  `checkDstOverwrite` uses `os.Stat`, rejects a directory, and—unless forced—
+  rejects a nonempty regular file for which `isObject` is false
+  ([lines 248–261](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/work/shell.go#L248-L261)).
+- `BuildInstallFunc` creates the destination parent and reaches
+  `moveOrCopyFile(..., false)` only after its build producer
+  ([`cmd/go/internal/work/exec.go`, lines 1904–2000](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/work/exec.go#L1904-L2000)).
+  `objectMagic` and `isObject` read the first 64 bytes and recognize archive,
+  ELF, Mach-O, PE, Plan 9, WASM, and XCOFF prefixes without consulting a file
+  extension
+  ([lines 2118–2150](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/work/exec.go#L2118-L2150)).
+- `runBuild` loads and checks every selected package before constructing the
+  output/install action
+  ([`cmd/go/internal/work/build.go`, lines 459–558](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/work/build.go#L459-L558)).
+- `builderTest` makes `-c` depend directly on the install action. For a running
+  retained test, the run consumes the private build action and the install
+  action additionally depends on that run
+  ([`cmd/go/internal/test/test.go`, lines 1257–1364](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/test/test.go#L1257-L1364)).
+  `Builder.Do` invokes an actor only when dependency failure has not propagated
+  (unless the action explicitly ignores failure), so a failed test run skips
+  `BuildInstallFunc`
+  ([`cmd/go/internal/work/exec.go`, lines 72–207](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/work/exec.go#L72-L207)).
+- Official `build_output_overwrite.txt` requires refusal to replace a
+  nonempty source file and preservation of its contents
+  ([lines 1–20](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/testdata/script/build_output_overwrite.txt#L1-L20)).
+  Official `test_compile_tempfile.txt` requires an existing empty reservation
+  to be accepted and replaced
+  ([lines 1–11](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/testdata/script/test_compile_tempfile.txt#L1-L11)).
+  Official `build_output.txt` separately pins executable command and archive
+  products
+  ([lines 47–57 and 64–76](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/testdata/script/build_output.txt#L47-L76)).
+
+The destination predicate, complete magic table, load-before-install ordering,
+producer-before-check ordering, and run-before-install dependency are directly
+implemented by the pinned source. Non-overwrite and empty-file acceptance are
+directly asserted by official testdata. Applying ELF and archive recognition
+to WW's byte-identical output forms is derived from that implementation. Go
+has no WW interface sidecar; recognizing only the compiler-owned
+`//ww:module ` prefix is the derived local application that permits ordinary
+repeat publication without letting arbitrary sidecar text be overwritten. No
+installed host Go behavior was used as authority.
+
+#### Fresh four-axis audit and pre-fix measurements
+
+The bounded audit selected this one gap on the build axis and the shared
+retained-test install axis. The package control selected the same canonical
+command root twice and measured one deduplicated command action plus one
+dependency action, with byte-identical Cstage/WWstage units, interfaces, and
+archives. The import control placed a used alias in one source file and an
+unused alias for the same dependency in a sibling; both stages emitted the
+same file-local unused-import diagnostic and committed no work. Those package
+and import candidates were aligned and were not changed.
+
+Fresh public Cstage and WWstage probes directly measured the same pre-fix
+behavior. Explicit command, raw command, output-directory child, library
+archive/interface, compile-only test, and running retained-test destinations
+containing arbitrary nonempty text were replaced successfully. A nonempty
+directory at a build or test child destination was renamed to a PID-bearing
+transaction backup, replaced by the executable, and left stranded because
+backup cleanup could not unlink the directory. Empty reservations were already
+accepted. Missing-import rejection already preceded destination handling. All
+measured successful executables, archives, interfaces, diagnostics, runtime
+results, and semantic artifacts were stage-identical. Those are directly
+measured WW facts, not source inferences.
+
+#### Ownership, action order, rollback, and identity
+
+The Cstage `sep_txn_commit` and WWstage `septxncommit` publishers own the byte
+predicate. Each transaction entry now explicitly distinguishes a public
+install from internal status, tool-identity, stamp, and persistent-action
+state. Only command/archive output, retained-copy, and published `.wwi` entries
+are checked. Producers still finish before transaction commit; a rejected
+destination discards all staged outputs and preserves every prior public and
+persistent byte. A library archive and interface remain one rollback group, so
+arbitrary text in either destination changes neither.
+
+`internal/wwpackage.packagecommand` continues to own directory-test naming and
+scheduling. A running retained descriptor withholds its public destination
+from the build child. After a successful private run, the coordinator invokes
+a private action in the selected driver, which stages an executable copy and
+re-enters the same guarded publisher. A failed, signalled, timed-out,
+interrupted, or unstartable run never invokes that action. Successful products
+in a multi-package running request install independently; compile-only products
+retain the established request transaction. The driver-owned raw single-file
+route applies the same private build, run, and guarded-install sequence.
+
+Package-build descriptors use `build-public` only for caller-visible command or
+archive products. Private package-build placeholders, test runnables, `ww run`,
+workdir-owned test binaries, null-discard products, and assembly-only products
+remain internal entries. Destination path, file kind, magic, declared name,
+requested alias, import leaf, physical directory, and publication order do not
+enter package/import identity, graph edges, action keys, symbols, artifacts,
+`.wwi` contents, or persistence keys.
+
+The guard follows symlinks for classification, matching `os.Stat`; the existing
+transaction still replaces the destination directory entry itself. It permits
+FIFO and other non-directory non-regular destinations because the pinned guard
+does. Diagnostics are exactly
+`ww: build output "PATH" already exists and is a directory` and
+`ww: build output "PATH" already exists and is not an object file` in both
+stages. No guard is preflighted during loading: package/import errors still win,
+and compiler, assembler, archive, or linker failure prevents the install action
+from being reached.
+
+Build runtime is inapplicable because `ww build` starts no program. Test
+runtime is applicable and owns the post-run dependency above. Producer failure,
+linker interruption, output-parent rollback, concurrency, occupied stages,
+prior-state preservation, and residue cleanup remain governed by the existing
+request/private-action transactions; the new check adds no process-global
+state. Public artifact bytes and modes are unchanged on accepted installs.
+There is no persisted-byte contract change: build workdir format remains `18`,
+test workdir format remains `19`, and semantic storage format remains `3`.
+
+The WW-native `public_output_overwrite_safety` observer covers both stages:
+direct, default, raw, package-output-directory, library, compile-only test, and
+running-test routes; late linker activity and load precedence; absent/empty,
+ELF, archive, interface, arbitrary regular, directory, symlink, and FIFO
+destinations; cold, warm, and invalidated persistent rollback; run-before-check
+and failed-run no-install behavior; exact null and assembly-only exclusions;
+runtime results; modes; diagnostic identity; public and semantic artifact-byte
+identity; and `.new`, install-stage, and transaction-backup cleanup.
+`test_binary_publication_transaction` pins the changed failed-run behavior and
+the existing linker failure, output-parent rollback, multi-product, persistent,
+and retained-binary contracts. Existing request-transaction, timeout,
+interruption, and concurrent-driver owners continue to cover those unchanged
+dimensions.
+
 ## 12. Candidate architectures and hard-gate decision
 
 Five candidates were developed as coherent systems, not as feature bins.
diff --git a/docs/spec.md b/docs/spec.md
index 008b5a7d..df62ea14 100644
--- a/docs/spec.md
+++ b/docs/spec.md
@@ -335,6 +335,22 @@ ImportPath    = ident { "." ident } .
   output. Output paths and directory metadata never become package, import,
   graph, action,
   symbol, artifact, `.wwi`, or persistence identity.
+
+  Every caller-visible build installation checks its destination after all
+  applicable compile, assemble, archive, and link producers finish. Ordinary
+  `stat` follows symlinks. An existing directory rejects as
+  `ww: build output "PATH" already exists and is a directory`; an existing
+  nonempty regular file rejects as `... is not an object file` unless its
+  leading bytes identify a Go 1.26.5 object/output form. The recognized table
+  is archive, ELF, Mach-O, PE, Plan 9, WASM, and XCOFF magic; WW additionally
+  recognizes its compiler-owned `//ww:module ` interface prefix. An absent
+  path, an empty regular reservation, or a non-directory non-regular path may
+  be replaced. A published non-main archive and its `.wwi` sidecar are checked
+  as one WW request transaction, so arbitrary caller text in either
+  destination preserves both old outputs and the committed persistent
+  generation. This safety check is output disposition only: it does not enter
+  package/import loading, graph or action identity, artifact bytes, or
+  invalidation.
   Assembly-only `-S` retains the directory form's command-action selection and
   no-main rejection, but it reaches no install action: destination length,
   duplicate publication names, implicit destination collision, and output
@@ -714,12 +730,17 @@ paths, and retained binary names remain presentation or loader metadata and do
 not become canonical package or action identity.
 
 The retained file is an executable, byte-identical copy of the private
-runnable. It joins package artifacts and statuses in the request-wide atomic
-publication transaction. Build, link, stage, or install failure preserves old
-destinations and removes temporary stages and invocation-created output
-prefixes. Test execution starts only after that transaction commits, so a
-runtime failure leaves an explicitly retained binary. A no-test product
-publishes no binary and does not create a directory solely for one.
+runnable. A compile-only retained binary joins package artifacts and statuses
+in the request-wide atomic publication transaction. Build, link, stage, or
+install failure preserves old destinations and removes temporary stages and
+invocation-created output prefixes. For a running `-o` request, the private
+binary executes first. Only a successful run enters the guarded install
+action; a failed, signalled, timed-out, interrupted, or unstartable run
+publishes no new copy and preserves any prior destination. The post-run guard
+uses the same directory/nonempty-regular/object-magic rule as `ww build`.
+Successful products in a multi-package running request install independently;
+their visible result order remains package order. A no-test product publishes
+no binary and does not create a directory solely for one.
 `-w` may persist the unchanged semantic actions for either `-c` or running
 retention without changing publication identity or introducing a test-result
 cache.
diff --git a/docs/test-system-v2.md b/docs/test-system-v2.md
index 9061e616..de75926f 100644
--- a/docs/test-system-v2.md
+++ b/docs/test-system-v2.md
@@ -258,11 +258,25 @@ install-only destination validation or output-directory creation.
 
 The retained executable is byte-identical to the temporary runnable and has
 executable mode `0777` filtered by the caller's umask, but it is never the path
-executed by the coordinator. Publication participates in the driver's one
-request-wide transaction: producer, linker, staging, or installation failure
-preserves every prior destination and removes stages and newly created output
-prefixes. A runtime failure occurs after commit and therefore leaves the saved
-binary. The language runtime owns individual `@test` functions.
+executed by the coordinator. Compile-only publication participates in the
+driver's one request-wide transaction: producer, linker, staging, or
+installation failure preserves every prior destination and removes stages and
+newly created output prefixes. A running retained request instead withholds
+the public path from that build transaction. The coordinator runs the private
+binary, then invokes the selected stage driver's guarded install action only
+after a successful process result. Failure, signal, timeout, interruption, or
+child-start failure therefore preserves any prior retained binary and creates
+no new one. Successful products install independently after their runs. The
+language runtime owns individual `@test` functions.
+
+Every build or retained-test public install follows Go 1.26.5's late
+destination safety rule. After applicable producers (and, for running tests,
+after the successful run), ordinary `stat` rejects a directory and rejects a
+nonempty regular non-object file. Empty reservations, recognized prior
+outputs, and non-directory non-regular paths remain replaceable. Recognition
+uses Go's archive/ELF/Mach-O/PE/Plan 9/WASM/XCOFF magic plus WW's narrow
+`//ww:module ` interface prefix. Package/import rejection keeps its earlier
+diagnostic precedence; exact `/dev/null` and `-S` never enter the guard.
 
 Every actually executed directory product gives its single generated binary
 the product's canonical absolute physical source directory as child cwd. A
@@ -468,8 +482,9 @@ with one fresh `mkdir` and refuses an existing path; it never clears a
 collision. A caller keeps only the exact artifacts it observes and removes
 that exact tree on every later success or failure. Directory-package test
 plans instead keep their cold semantic-action scratch and runnable binary
-inside the coordinator's temporary root; only the optional retained executable
-escapes through the transaction above. `ww run` and no-output single-file
+inside the coordinator's temporary root; a compile-only retained executable
+escapes through the build transaction, while a running retained executable
+uses the post-run install action above. `ww run` and no-output single-file
 `ww test` use driver-owned scratch instead; both driver stages place that
 scratch and their temporary executable beneath one freshly acquired directory,
 remove both after every build result, and make cleanup failure fail the
diff --git a/internal/wwpackage/package.ww b/internal/wwpackage/package.ww
index bbdd5d33..66c722be 100644
--- a/internal/wwpackage/package.ww
+++ b/internal/wwpackage/package.ww
@@ -39,9 +39,13 @@ type pkggroup = struct {
 	bin: str,
 	buildok: str,
 	runoutput: str,
+	installoutput: str,
 	state: i32,
 	runstartfailed: bool,
 	runres: exec.result,
+	publicbin: bool,
+	installattempted: bool,
+	installres: exec.result,
 };
 
 type pkgplan = struct {
@@ -64,6 +68,7 @@ type pkgplan = struct {
 	outputcollisionbase: str,
 	outputcollisiondir: str,
 	suppressbuildreports: bool,
+	deferpublish: bool,
 };
 
 def PKG_COUNT_MAX: i32 = 2147483647;
@@ -1373,16 +1378,23 @@ fn pkgsetplanpaths(p: *pkgplan, groups: []pkggroup, root: str, index: i32,
 			pkgfailpath(g.root, "cannot create product temporary path");
 			return false;
 		};
+		g.publicbin = false;
+		g.installattempted = false;
 		if (buildonly) {
 			if (outputdir && strings.compare(g.pkg, "main") == 0
 				&& !p.outputpatherror && !p.emitasm) {
 				if (!pkgjoinpath(outname, g.basename, &g.bin)) { return false; };
-			} else if (outname.len != 0 && !outputdir) { g.bin = outname; }
+				g.publicbin = true;
+			} else if (outname.len != 0 && !outputdir) {
+				g.bin = outname;
+				g.publicbin = true;
+			}
 			else if (!pkgstring(&g.bin, g.root, "/package.build")) { return false; };
 		} else {
 			if (!pkgstring(&g.bin, g.root, "/package.test")) { return false; };
 		};
 		if (!pkgstring(&g.runoutput, g.root, "/test.output")
+			|| !pkgstring(&g.installoutput, g.root, "/install.output")
 			|| !pkgstring(&g.buildok, g.root, "/build.ok")) { return false; };
 		i += 1;
 	};
@@ -1484,7 +1496,8 @@ fn pkgstartbuild(p: *pkgplan, groups: []pkggroup, builder: str, includes: []str,
 	for (i < p.end) {
 		let g: *pkggroup = &groups[i];
 		append(ba, "--ww-package-test");
-		if (p.buildonly) { append(ba, "build"); }
+		if (p.buildonly && g.publicbin) { append(ba, "build-public"); }
+		else if (p.buildonly) { append(ba, "build"); }
 		else { append(ba, "test"); };
 		append(ba, g.pkg);
 		if (g.prodpkg.len != 0) { append(ba, g.prodpkg); }
@@ -1495,7 +1508,7 @@ fn pkgstartbuild(p: *pkgplan, groups: []pkggroup, builder: str, includes: []str,
 		else { append(ba, "-"); };
 		append(ba, g.dir);
 		append(ba, g.bin);
-		if (g.publish.len != 0) { append(ba, g.publish); }
+		if (g.publish.len != 0 && !p.deferpublish) { append(ba, g.publish); }
 		else { append(ba, "-"); };
 		append(ba, g.buildok);
 		i += 1;
@@ -1591,6 +1604,31 @@ fn pkgstartrun(g: *pkggroup, filters: []str, timeoutarg: str,
 	return true;
 };
 
+// Pinned builderTest orders a retained running test as build -> run ->
+// BuildInstallFunc. Re-enter the selected stage driver for the final action so
+// build and test share one overwrite predicate instead of copying object magic
+// into the coordinator.
+fn pkginstalloutput(g: *pkggroup, builder: str) void = {
+	let ia: []str = [builder, "test", "--ww-install-test-output",
+		g.bin, g.publish];
+	let env: []str = os.getenvs();
+	let icmd: exec.command;
+	icmd.path = builder;
+	icmd.argv = ia;
+	icmd.env = env;
+	icmd.dir = "";
+	icmd.stdoutpath = g.installoutput;
+	icmd.stderrpath = g.installoutput;
+	icmd.deadline.sec = 0i64;
+	icmd.deadline.nsec = 0i64;
+	icmd.grace = 0i64: time.duration;
+	let h: exec.process;
+	g.installattempted = true;
+	exec.start(&h, &icmd);
+	for (!exec.poll(&h)) { time.sleep(pkgpoll, time.clock.monotonic); };
+	g.installres = h.result;
+};
+
 fn pkgproductbuilt(g: *pkggroup, buildonly: bool) bool = {
 	if (buildonly || g.notests) { return pkgisreg(g.buildok); };
 	return pkgisreg(g.buildok) && pkgisreg(g.bin);
@@ -1602,6 +1640,35 @@ fn pkgrunok(g: *pkggroup) bool = {
 		&& g.runres.code == 0;
 };
 
+fn pkginstallok(g: *pkggroup) bool = {
+	return g.installres.errno == 0 && g.installres.cleanuperrno == 0
+		&& g.installres.termination == exec.termination.EXIT
+		&& g.installres.code == 0;
+};
+
+fn pkgemitinstall(g: *pkggroup) bool = {
+	if (!g.installattempted) { return true; };
+	let output: str;
+	if (!pkgread(g.installoutput, &output)) {
+		pkgfailpath(g.root, "cannot read install capture");
+		return false;
+	};
+	if (output.len != 0) {
+		pkgput(os.STDERR_FILENO, output);
+		if (output[output.len - 1] != '\n') {
+			pkgput(os.STDERR_FILENO, "\n");
+		};
+	};
+	if (!pkginstallok(g)) {
+		if (output.len == 0) {
+			pkgreportcommand(os.STDERR_FILENO, "install", g,
+				&g.installres);
+		};
+		return false;
+	};
+	return true;
+};
+
 fn pkgemitgroup(g: *pkggroup, compileonly: bool) bool = {
 	if (g.notests) {
 		pkgput(os.STDOUT_FILENO, "?    ");
@@ -1653,8 +1720,9 @@ fn pkgemitplan(p: *pkgplan, groups: []pkggroup,
 			failed += 1;
 		} else if (buildonly) {
 			void;
-		} else if (!pkgemitgroup(&groups[i], compileonly)) {
-			failed += 1;
+		} else {
+			if (!pkgemitgroup(&groups[i], compileonly)) { failed += 1; };
+			if (!compileonly && !pkgemitinstall(&groups[i])) { failed += 1; };
 		};
 		i += 1;
 	};
@@ -2385,11 +2453,12 @@ export fn packagecommand(args: []str) int = {
 	plan.outputcollisiondir = outputcollisiondir;
 	plan.suppressbuildreports = buildonly
 		&& (outputdir || defaultoutputdir.len != 0);
+	plan.deferpublish = !buildonly && !compileonly && testretain && !testnull;
 	append(plans, plan);
 	let createdir: str = "";
 	if (buildonly && outputdir) {
 		createdir = outname;
-	} else if (!buildonly && testretain && !testnull) {
+	} else if (!buildonly && compileonly && testretain && !testnull) {
 		i = 0;
 		for (i < groups.len) {
 			if (groups[i].publish.len != 0) {
@@ -2550,6 +2619,22 @@ export fn packagecommand(args: []str) int = {
 		if (active > 0) { time.sleep(pkgpoll, time.clock.monotonic); };
 	};
 
+	// A failed, signalled, timed-out, or unstartable run propagates to its Go
+	// install action, so it preserves any prior retained binary. Successful
+	// products install independently after their private execution.
+	if (!compileonly) {
+		i = 0;
+		for (i < groups.len) {
+			let g: *pkggroup = &groups[i];
+			if (g.publish.len != 0 && !g.notests
+			    && pkgproductbuilt(g, buildonly) && !g.runstartfailed
+			    && pkgrunok(g)) {
+				pkginstalloutput(g, builder);
+			};
+			i += 1;
+		};
+	};
+
 	i = 0;
 	for (i < plans.len) {
 		failed += pkgemitplan(&plans[i], groups, compileonly, buildonly);
diff --git a/selfhost/cmd/ww/main.ww b/selfhost/cmd/ww/main.ww
index 0a6a4115..743f2956 100644
--- a/selfhost/cmd/ww/main.ww
+++ b/selfhost/cmd/ww/main.ww
@@ -1515,6 +1515,7 @@ type sepproduct = struct {
 	directoryproduct: bool,
 	notests: bool,
 	buildaction: bool,	// loaded product retained in the action list
+	publicout: bool,
 	context: i32,
 	root: i32,
 	variantroot: i32,
@@ -6134,6 +6135,7 @@ type septxnentry = struct {
 	backup: *u8,
 	hadold: bool,
 	installed: bool,
+	publicoutput: bool,
 };
 
 fn sepalloctxnentries(cap: i32) ([]septxnentry | nomem) = {
@@ -6207,8 +6209,8 @@ fn septxnbackup(dst: *u8) *u8 = {
 	return buf.ptr;
 };
 
-fn septxnadd(entries: *[]septxnentry, n: *i32,
-	stage: *u8, dst: *u8) bool = {
+fn septxnaddmode(entries: *[]septxnentry, n: *i32,
+	stage: *u8, dst: *u8, publicoutput: bool) bool = {
 	if (cstreq(stage, dst)) {
 		cerrpath("ww: transaction path collision: ", dst, "\n");
 		return false;
@@ -6253,10 +6255,21 @@ fn septxnadd(entries: *[]septxnentry, n: *i32,
 	(*entries)[*n].backup = backup;
 	(*entries)[*n].hadold = false;
 	(*entries)[*n].installed = false;
+	(*entries)[*n].publicoutput = publicoutput;
 	*n += 1;
 	return true;
 };
 
+fn septxnadd(entries: *[]septxnentry, n: *i32,
+	stage: *u8, dst: *u8) bool = {
+	return septxnaddmode(entries, n, stage, dst, false);
+};
+
+fn septxnaddpublic(entries: *[]septxnentry, n: *i32,
+	stage: *u8, dst: *u8) bool = {
+	return septxnaddmode(entries, n, stage, dst, true);
+};
+
 fn septxnaddpkgsuffix(entries: *[]septxnentry, n: *i32,
 	g: *sepgraph, pi: i32, scratch: *u8,
 	stagesuffix: str, dstsuffix: str) bool = {
@@ -6271,6 +6284,91 @@ fn septxndiscard(entries: []septxnentry, n: i32) void = {
 	for (i < n) { os.remove(pathstr(entries[i].stage)); i += 1; };
 };
 
+fn sepoutputprefix(buf: *u8, n: i64, magic: str) bool = {
+	if (n < 0i64 || n < magic.len: i64) { return false; };
+	let i: i32 = 0;
+	for (i < magic.len) {
+		if (buf[i] != magic.ptr[i]) { return false; };
+		i += 1;
+	};
+	return true;
+};
+
+fn sepoutput2(buf: *u8, n: i64, a: u8, b: u8) bool = {
+	return n >= 2i64 && buf[0u64] == a && buf[1u64] == b;
+};
+
+fn sepoutput4(buf: *u8, n: i64, a: u8, b: u8, c: u8, d: u8) bool = {
+	return n >= 4i64 && buf[0u64] == a && buf[1u64] == b
+		&& buf[2u64] == c && buf[3u64] == d;
+};
+
+fn sepoutput6(buf: *u8, n: i64, a: u8, b: u8, c: u8, d: u8,
+	e: u8, f: u8) bool = {
+	return n >= 6i64 && buf[0u64] == a && buf[1u64] == b
+		&& buf[2u64] == c && buf[3u64] == d
+		&& buf[4u64] == e && buf[5u64] == f;
+};
+
+// Go 1.26.5 work.objectMagic, plus WW's compiler-owned interface prefix.
+fn sepisobjectoutput(path: *u8) bool = {
+	let fd: i32 = os.open(pathstr(path), os.flag.RDONLY, 0i32);
+	if (fd < 0) { return false; };
+	let buf: [64]u8;
+	let got: u64 = 0u64;
+	let bad: bool = false;
+	for (got < 64u64) {
+		let n: i64 = os.read(fd, &buf[0] + got, 64u64 - got);
+		if (n > 0) { got += n: u64; }
+		else if (n == 0) { break; }
+		else if (n != -4i64) { bad = true; break; };
+	};
+	os.close(fd);
+	if (bad) { return false; };
+	let n: i64 = got: i64;
+	if (sepoutputprefix(&buf[0], n, "!<arch>\n")
+	    || sepoutputprefix(&buf[0], n, "<bigaf>\n")
+	    || sepoutput4(&buf[0], n, 127u8, 69u8, 76u8, 70u8)
+	    || sepoutput4(&buf[0], n, 254u8, 237u8, 250u8, 206u8)
+	    || sepoutput4(&buf[0], n, 254u8, 237u8, 250u8, 207u8)
+	    || sepoutput4(&buf[0], n, 206u8, 250u8, 237u8, 254u8)
+	    || sepoutput4(&buf[0], n, 207u8, 250u8, 237u8, 254u8)
+	    || sepoutput6(&buf[0], n, 77u8, 90u8, 144u8, 0u8, 3u8, 0u8)
+	    || sepoutput6(&buf[0], n, 77u8, 90u8, 120u8, 0u8, 1u8, 0u8)
+	    || sepoutput4(&buf[0], n, 0u8, 0u8, 1u8, 235u8)
+	    || sepoutput4(&buf[0], n, 0u8, 0u8, 138u8, 151u8)
+	    || sepoutput4(&buf[0], n, 0u8, 0u8, 6u8, 71u8)
+	    || sepoutput4(&buf[0], n, 0u8, 97u8, 115u8, 109u8)
+	    || sepoutput2(&buf[0], n, 1u8, 223u8)
+	    || sepoutput2(&buf[0], n, 1u8, 247u8)
+	    || sepoutputprefix(&buf[0], n, "//ww:module ")) {
+		return true;
+	};
+	return false;
+};
+
+fn sepcheckdstoverwrite(dst: *u8) bool = {
+	let fi: os.filestat;
+	match (os.stat(&fi, pathstr(dst))) {
+	case void => {
+		let typ: u32 = (fi.mode: u32) & 61440u32;
+		if (typ == os.mode.DIR: u32) {
+			cerr("ww: build output "); sepputquoted(dst);
+			cerr(" already exists and is a directory\n");
+			return false;
+		};
+		if (typ == os.mode.REG: u32 && fi.sz != 0u64
+		    && !sepisobjectoutput(dst)) {
+			cerr("ww: build output "); sepputquoted(dst);
+			cerr(" already exists and is not an object file\n");
+			return false;
+		};
+	};
+	case let e: os.oserror => void;
+	};
+	return true;
+};
+
 fn septxncommit(entries: []septxnentry, n: i32) bool = {
 	let i: i32 = 0;
 	let valid: bool = true;
@@ -6283,6 +6381,17 @@ fn septxncommit(entries: []septxnentry, n: i32) bool = {
 		};
 		i += 1;
 	};
+	if (valid) {
+		i = 0;
+		for (i < n) {
+			if (entries[i].publicoutput
+			    && !sepcheckdstoverwrite(entries[i].dst)) {
+				valid = false;
+				break;
+			};
+			i += 1;
+		};
+	};
 	if (valid) {
 		i = 0;
 		for (i < n) {
@@ -6614,6 +6723,50 @@ fn septxnrelease(entries: *[]septxnentry, n: i32) void = {
 	entries.cap = 0;
 };
 
+// Running retained tests enter this Go-like install action only after their
+// request-private executable has returned successfully.
+fn sepinstalltestoutput(stage: *u8, dst: *u8) i32 = {
+	let installstage: *u8 = sepappendlit(stage, ".install");
+	if (installstage == nil) { return 1; };
+	if (cstrlen(installstage) + 1u64 > os.PATH_MAX: u64
+	    || pathexistsnofollow(installstage) != 0
+	    || copyexecutablestage(stage, installstage) != 0) {
+		cerr("ww: cannot stage retained test output\n");
+		os.free(installstage: *void, cstrlen(installstage) + 1u64);
+		return 1;
+	};
+	let parent: *u8 = seplexicalparent(dst);
+	if (parent == nil) {
+		os.remove(pathstr(installstage));
+		os.free(installstage: *void, cstrlen(installstage) + 1u64);
+		return 1;
+	};
+	let created: sepcreateddirs;
+	created.n = 0;
+	if (sepmkdirsrecord(parent, 511, &created) != 0) {
+		cerrpath("ww: cannot create test output directory ", parent, "\n");
+		os.free(parent: *void, cstrlen(parent) + 1u64);
+		os.remove(pathstr(installstage));
+		os.free(installstage: *void, cstrlen(installstage) + 1u64);
+		return 1;
+	};
+	os.free(parent: *void, cstrlen(parent) + 1u64);
+	let entries: []septxnentry;
+	let n: i32 = 0;
+	if (!septxnaddpublic(&entries, &n, installstage, dst)
+	    || !septxncommit(entries, n)) {
+		septxndiscard(entries, n);
+		septxnrelease(&entries, n);
+		seprollbackdirs(&created);
+		os.remove(pathstr(installstage));
+		os.free(installstage: *void, cstrlen(installstage) + 1u64);
+		return 1;
+	};
+	septxnrelease(&entries, n);
+	os.free(installstage: *void, cstrlen(installstage) + 1u64);
+	return 0;
+};
+
 fn sepfinishfail(entries: *[]septxnentry, n: i32) i32 = {
 	septxndiscard(*entries, n);
 	septxnrelease(entries, n);
@@ -6924,19 +7077,24 @@ fn sepfinishrequest(selfdir: *u8, l6: *u8, c6: *u8, a6: *u8,
 	producti = 0;
 	for (producti < nproducts) {
 		if (products[producti].stageout != nil
-		    && !septxnadd(&entries, &ntxn, products[producti].stageout,
-			products[producti].out)) {
-			return sepfinishfail(&entries, ntxn);
+		    && ((products[producti].publicout
+			&& !septxnaddpublic(&entries, &ntxn,
+				products[producti].stageout, products[producti].out))
+		    || (!products[producti].publicout
+			&& !septxnadd(&entries, &ntxn,
+				products[producti].stageout, products[producti].out)))) {
+		return sepfinishfail(&entries, ntxn);
 		};
 		if (products[producti].stagepublish != nil
-		    && !septxnadd(&entries, &ntxn, products[producti].stagepublish,
+		    && !septxnaddpublic(&entries, &ntxn,
+			products[producti].stagepublish,
 			products[producti].publish)) {
 			return sepfinishfail(&entries, ntxn);
 		};
 		if (products[producti].stageiface != nil) {
 			let outiface: *u8 = sepappendlit(products[producti].out, ".wwi");
 			if (outiface == nil
-			    || !septxnadd(&entries, &ntxn,
+			    || !septxnaddpublic(&entries, &ntxn,
 				products[producti].stageiface, outiface)) {
 				return sepfinishfail(&entries, ntxn);
 			};
@@ -8213,7 +8371,8 @@ fn buildonesep(selfdir: *u8, src: *u8, entryisdir: i32,
     objstem: *u8, incs: *u8, lf: *lflags, publishpackage: i32,
     requirecommand: i32, istest: i32,
     rootvariant: i32, testpackage: *u8, emitasm: i32,
-    keepscratch: i32, workdir: *u8, createoutputdir: *u8,
+    publicoutput: bool, keepscratch: i32, workdir: *u8,
+    createoutputdir: *u8,
     defaultoutputdir: *u8, outputpatherror: bool) i32 = {
 	let scratch: *u8 = nil;
 	let g: *sepgraph = nil;
@@ -8235,6 +8394,7 @@ fn buildonesep(selfdir: *u8, src: *u8, entryisdir: i32,
 	product.directoryproduct = false;
 	product.notests = false;
 	product.buildaction = true;
+	product.publicout = publicoutput;
 	product.root = -1;
 	product.variantroot = -1;
 	product.support = -1;
@@ -8691,7 +8851,7 @@ fn dobuild(selfdir: *u8, argv: **u8, argc: i32, start: i32) i32 = {
 		let rc: i32 = buildonesep(selfdir, resolved, isdir, rootidentity,
 			outp, outp, incs.ptr, &lf,
 			0i32, 0i32, 0i32, SEP_VARIANT_PRODUCTION, nil,
-			emitasm, 0i32, workdir, nil, nil, false);
+			emitasm, false, 0i32, workdir, nil, nil, false);
 		let cleanbad: bool = false;
 		let cleanrc: i32 = os.remove(pathstr(outp));
 		if (cleanrc != 0 && cleanrc != -2i32) {
@@ -8710,7 +8870,7 @@ fn dobuild(selfdir: *u8, argv: **u8, argc: i32, start: i32) i32 = {
 	return buildonesep(selfdir, resolved, isdir, rootidentity,
 	    out, objstem, incs.ptr, &lf,
 	    publishpackage, 0i32, 0i32, SEP_VARIANT_PRODUCTION, nil,
-	    emitasm, 1i32, workdir, createoutputdir, defaultoutputdir,
+	    emitasm, true, 1i32, workdir, createoutputdir, defaultoutputdir,
 	    outputpatherror);
 };
 
@@ -8893,7 +9053,7 @@ fn dorun(selfdir: *u8, argv: **u8, argc: i32, start: i32) i32 = {
 	if (buildonesep(selfdir, resolved, isdir, rootidentity,
 	    outp, outp, incs.ptr, &lf,
 	    0i32, 1i32, 0i32, SEP_VARIANT_PRODUCTION, nil,
-	    0i32, 0i32, nil, nil, nil, false) != 0) {
+	    0i32, false, 0i32, nil, nil, nil, false) != 0) {
 		let cleanrc: i32 = os.remove(pathstr(outp));
 		if (cleanrc != 0 && cleanrc != -2i32) {
 			cerr("ww: cannot remove temporary output\n");
@@ -8958,10 +9118,12 @@ fn runsingletest(selfdir: *u8, src: *u8, incs: *u8, compileonly: i32,
 	let owntmp: bool = false;
 	let retainout: bool = outstem != nil
 		&& !cstreqlit(outstem, "/dev/null");
-	if (retainout) {
+	let deferredinstall: bool = retainout && compileonly == 0
+		&& emitasm == 0;
+	if (retainout && !deferredinstall) {
 		outp = outstem;
 		objstem = outstem;
-	} else { if (workdir != nil) {
+	} else { if (workdir != nil && !deferredinstall) {
 		// The workdir owns the persistent test binary the same way it
 		// owns the package artifacts.
 		outp = joinpathlit(workdir, "main");
@@ -8975,6 +9137,7 @@ fn runsingletest(selfdir: *u8, src: *u8, incs: *u8, compileonly: i32,
 		};
 		outp = joinpathlit(tmp.ptr, "main");
 		objstem = outp;
+		if (retainout) { objstem = outstem; };
 	}; };
 	// E3-C1: separate compilation is the sole build path (task #87).
 	let lf: lflags;
@@ -8986,7 +9149,8 @@ fn runsingletest(selfdir: *u8, src: *u8, incs: *u8, compileonly: i32,
 	if (retainout) { keep = 1; };
 	let bres: i32 = buildonesep(selfdir, src, 0, nil, outp, objstem, incs, &lf,
 	    0i32, 0i32, 1i32, SEP_VARIANT_PRODUCTION, nil,
-	    emitasm, keep, workdir, nil, nil, false);
+	    emitasm, retainout && !deferredinstall, keep, workdir,
+	    nil, nil, false);
 	if (bres != 0) {
 		if (owntmp) {
 			let cleanrc: i32 = os.remove(pathstr(outp));
@@ -9037,6 +9201,8 @@ fn runsingletest(selfdir: *u8, src: *u8, incs: *u8, compileonly: i32,
 			rc = -1;
 		};
 	}; };
+	if (rc == 0 && deferredinstall
+	    && sepinstalltestoutput(outp, outstem) != 0) { rc = 1; };
 	if (owntmp) {
 		let cleanrc: i32 = os.remove(pathstr(outp));
 		if (cleanrc != 0 && cleanrc != -2i32) {
@@ -9052,6 +9218,10 @@ fn runsingletest(selfdir: *u8, src: *u8, incs: *u8, compileonly: i32,
 };
 
 fn dotest(selfdir: *u8, argv: **u8, argc: i32, start: i32) i32 = {
+	if (argc - start == 3
+	    && cstreqlit(argv[start], "--ww-install-test-output")) {
+		return sepinstalltestoutput(argv[start + 1], argv[start + 2]);
+	};
 	// -I parsing mirrors dobuild/dorun so a single-file test can resolve
 	// transitive imports (e.g. 905_nkname asttest → tok); coupled to the
 	// -T flip (task #5/#10).
@@ -9203,7 +9373,10 @@ fn dotest(selfdir: *u8, argv: **u8, argc: i32, start: i32) i32 = {
 				let publish: *u8 = argv[i + 8];
 				let status: *u8 = argv[i + 9];
 				let pn: u64 = cstrlen(name);
-				let buildproduct: bool = cstreqlit(kind, "build");
+				let publicbuildproduct: bool =
+					cstreqlit(kind, "build-public");
+				let buildproduct: bool = cstreqlit(kind, "build")
+					|| publicbuildproduct;
 				let testproduct: bool = cstreqlit(kind, "test");
 				let hasproduction: bool = !cstreqlit(production, "-");
 				let hasinternal: bool = !cstreqlit(internal, "-");
@@ -9248,6 +9421,7 @@ fn dotest(selfdir: *u8, argv: **u8, argc: i32, start: i32) i32 = {
 				product.directoryproduct = true;
 				product.notests = testproduct && !hasinternal && !hasexternal;
 				product.buildaction = true;
+				product.publicout = publicbuildproduct;
 				product.root = -1;
 				product.variantroot = -1;
 				product.productionroot = -1;
diff --git a/test/package/package_test.ww b/test/package/package_test.ww
index d4a60ba9..6cabaca8 100644
--- a/test/package/package_test.ww
+++ b/test/package/package_test.ww
@@ -12443,7 +12443,6 @@ fn runtimepath(relative: str) str = {
 	let rollbackdiag: str = "";
 	let missingdiag: str = "";
 	let occupieddiag: str = "";
-	let failurebytes: str = "";
 	let persistbasebytes: str = "";
 	let persistchangedbytes: str = "";
 	let baseenv: []str = os.getenvs();
@@ -12595,29 +12594,23 @@ fn runtimepath(relative: str) str = {
 			" [no tests]\n")));
 		assert(out.stderr.len == 0 && !os.exists(noneroot));
 
-		// Publication commits before execution. A failing test therefore leaves
-		// a runnable retained binary while its product failure stays on stdout.
+		// Pinned builderTest makes the install action depend on the run action.
+		// A failing run therefore preserves a prior destination and never enters
+		// the retained-output overwrite guard.
 		let failurebin: str = strings.concat(root, "/failure-", tags[si],
 			".test");
+		writefile(failurebin, "failure-sentinel\n");
 		let failureav: []str = [driver(stages[si]), "test", "-I", suite,
 			"-o", failurebin, failed];
 		runcommandenvdir(root, strings.concat("publish-runtime-", tags[si]),
 			failureav, env, root,
 			(60i64 * (time.second: i64)): time.duration, &out);
 		expectexit(&out, 1);
-		assert(os.exists(failurebin) && !os.exists(strings.concat(failurebin,
-			".new")));
+		assert(same(readfile(failurebin), "failure-sentinel\n"));
+		assert(!os.exists(strings.concat(failurebin, ".new")));
 		assert(has(out.stdout,
 			"failed_decl.runtime_failure ... FAIL (exit 1)\n"));
 		assert(out.stderr.len == 0);
-		let fi: os.filestat;
-		match (os.stat(&fi, failurebin)) {
-		case void => void;
-		case let e: os.oserror => abort("retained failure binary missing");
-		};
-		assert(((fi.mode: u32) & 73u32) != 0u32);
-		if (si == 0) { failurebytes = readfile(failurebin); }
-		else { assert(same(failurebytes, readfile(failurebin))); };
 
 		// -w keeps semantic actions persistent while -c and -o remain execution
 		// and presentation policy. Compile-only can cold-build and invalidate;
@@ -12681,6 +12674,377 @@ fn runtimepath(relative: str) str = {
 	clean(root);
 };
 
+// Go 1.26.5's BuildInstallFunc protects caller data at the install boundary:
+// after producers, and after a successful run for retained running tests.
+// Exercise that one predicate through direct, raw, package, and test routes.
+@test fn public_output_overwrite_safety() void = {
+	let root: str = fresh();
+	let source: str = strings.concat(root, "/source");
+	let dep: str = strings.concat(source, "/lib/dep");
+	let app: str = strings.concat(source, "/cmd/app");
+	let bad: str = strings.concat(source, "/cmd/bad");
+	let checked: str = strings.concat(source, "/pkg/check");
+	let failed: str = strings.concat(source, "/pkg/failed");
+	let invocation: str = strings.concat(root, "/invoke");
+	mkdirall(dep); mkdirall(app); mkdirall(bad); mkdirall(checked);
+	mkdirall(failed); mkdirall(invocation);
+	writefile(strings.concat(dep, "/dep.ww"), strings.concat(
+		"package renamed;\n",
+		"export fn value() int = { return 9; };\n"));
+	let appsource: str = strings.concat(app, "/main.ww");
+	let appbase: str = strings.concat(
+		"package main;\n",
+		"import stable lib.dep;\n",
+		"export fn main() int = { return stable.value(); };\n");
+	let appchanged: str = strings.concat(
+		"package main;\n",
+		"import stable lib.dep;\n",
+		"export fn main() int = { return stable.value() + 1; };\n");
+	writefile(appsource, appbase);
+	writefile(strings.concat(bad, "/main.ww"), strings.concat(
+		"package main;\n",
+		"import missing nowhere.present;\n",
+		"export fn main() int = { return missing.value(); };\n"));
+	writefile(strings.concat(checked, "/check.ww"),
+		"package checked;\nfn value() int = { return 4; };\n");
+	writefile(strings.concat(checked, "/check_test.ww"), strings.concat(
+		"package checked;\n",
+		"@test fn guarded_run() void = { assert(value() == 4); };\n"));
+	writefile(strings.concat(failed, "/failed_test.ww"), strings.concat(
+		"package failed;\n",
+		"@test fn failed_run() void = { assert(false); };\n"));
+	let raw: str = strings.concat(root, "/raw.ww");
+	writefile(raw,
+		"package main;\nexport fn main() int = { return 3; };\n");
+
+	let linkerwrapper: str = strings.concat(root, "/overwrite-w6l.sh");
+	writeexecutable(linkerwrapper, strings.concat(
+		"#!/bin/sh\n",
+		"printf 'link\\n' >> \"$WW_OVERWRITE_TRACE\"\n",
+		"exec \"$WW_OVERWRITE_REAL_L\" \"$@\"\n"));
+	let drivers: []str = ["ww", "ww_ww"];
+	let linkers: []str = ["w6l", "w6l_ww"];
+	let tags: []str = ["c", "ww"];
+	let commandout: str = strings.concat(root, "/command.bin");
+	let rawout: str = strings.concat(root, "/raw.bin");
+	let defaultout: str = strings.concat(invocation, "/app");
+	let libout: str = strings.concat(root, "/library.a");
+	let testbin: str = strings.concat(root, "/check.test");
+	let symlinktarget: str = strings.concat(root, "/symlink-target");
+	let symlinkout: str = strings.concat(root, "/symlink.bin");
+	let fifoout: str = strings.concat(root, "/fifo.bin");
+	writefile(symlinktarget, "symlink-sentinel\n");
+	assert(os.symlink(symlinktarget, symlinkout) == 0);
+	let builddir: str = strings.concat(root, "/build-output");
+	let buildchild: str = strings.concat(builddir, "/app");
+	let testdir: str = strings.concat(root, "/test-output");
+	let testchild: str = strings.concat(testdir, "/check.test");
+	mkdirall(buildchild); mkdirall(testchild);
+	writefile(strings.concat(buildchild, "/marker"), "build-directory\n");
+	writefile(strings.concat(testchild, "/marker"), "test-directory\n");
+
+	let commanddiag: str = "";
+	let rawdiag: str = "";
+	let defaultdiag: str = "";
+	let symlinkdiag: str = "";
+	let builddirdiag: str = "";
+	let librarydiag: str = "";
+	let libraryarchivediag: str = "";
+	let testcompilediag: str = "";
+	let testrundiag: str = "";
+	let testdirdiag: str = "";
+	let loaddiag: str = "";
+	let commandbytes: str = "";
+	let librarybytes: str = "";
+	let interfacebytes: str = "";
+	let testbytes: str = "";
+	let unitbytes: str = "";
+	let baseenv: []str = os.getenvs();
+	let out: commandout;
+	let si: i32 = 0;
+	for (si < drivers.len) {
+		let trace: str = strings.concat(root, "/overwrite-", tags[si],
+			".trace");
+		writefile(trace, "");
+		let env: []str = alloc([], (baseenv.len + 3): u64)!;
+		let ei: i32 = 0;
+		for (ei < baseenv.len) {
+			if (!strings.hasprefix(baseenv[ei], "WW_W6L=")
+				&& !strings.hasprefix(baseenv[ei], "WW_OVERWRITE_TRACE=")
+				&& !strings.hasprefix(baseenv[ei], "WW_OVERWRITE_REAL_L=")) {
+				append(env, baseenv[ei]);
+			};
+			ei += 1;
+		};
+		append(env, strings.concat("WW_W6L=", linkerwrapper));
+		append(env, strings.concat("WW_OVERWRITE_TRACE=", trace));
+		append(env, strings.concat("WW_OVERWRITE_REAL_L=",
+			driver(linkers[si])));
+		let work: str = strings.concat(root, "/command-work-", tags[si]);
+		let badwork: str = strings.concat(root, "/bad-work-", tags[si]);
+		let rawwork: str = strings.concat(root, "/raw-work-", tags[si]);
+		let defaultwork: str = strings.concat(root, "/default-work-", tags[si]);
+		let symlinkwork: str = strings.concat(root, "/symlink-work-", tags[si]);
+		let fifowork: str = strings.concat(root, "/fifo-work-", tags[si]);
+		let dirwork: str = strings.concat(root, "/dir-work-", tags[si]);
+		let libwork: str = strings.concat(root, "/lib-work-", tags[si]);
+		let testwork: str = strings.concat(root, "/test-work-", tags[si]);
+		let testdirwork: str = strings.concat(root, "/test-dir-work-", tags[si]);
+		mkdirall(work); mkdirall(badwork); mkdirall(rawwork);
+		mkdirall(defaultwork); mkdirall(symlinkwork); mkdirall(fifowork);
+		mkdirall(dirwork);
+		mkdirall(libwork); mkdirall(testwork); mkdirall(testdirwork);
+
+		// A nonempty caller file rejects only after the linker ran, and a cold
+		// persistent generation remains unpublished.
+		if (os.exists(commandout)) { assert(os.remove(commandout) == 0); };
+		writefile(commandout, "caller-command\n");
+		rewritefile(trace, "");
+		let commandav: []str = [driver(drivers[si]), "build", "-w", work,
+			"-I", source, "-o", commandout, "cmd.app"];
+		runcommandenv(root, strings.concat("overwrite-command-", tags[si]),
+			commandav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(has(out.stderr, strings.concat("ww: build output \"", commandout,
+			"\" already exists and is not an object file\n")));
+		assert(same(readfile(commandout), "caller-command\n"));
+		assert(permissionmode(commandout) == 384u32);
+		assert(readfile(trace).len != 0 && directoryisempty(work));
+		assert(!os.exists(strings.concat(commandout, ".new"))
+			&& !directoryhasfragment(root, "command.bin.wwtxn."));
+		if (si == 0) { commanddiag = strings.dup(out.stderr); }
+		else { assert(same(commanddiag, out.stderr)); };
+
+		// Package/import failure keeps load precedence and starts no linker.
+		rewritefile(trace, "");
+		let badav: []str = [driver(drivers[si]), "build", "-w", badwork,
+			"-I", source, "-o", commandout, "cmd.bad"];
+		runcommandenv(root, strings.concat("overwrite-load-", tags[si]), badav,
+			env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(has(out.stderr, "cannot find package nowhere.present"));
+		assert(!has(out.stderr, "already exists and is not an object file"));
+		assert(readfile(trace).len == 0 && directoryisempty(badwork));
+		assert(same(readfile(commandout), "caller-command\n"));
+		if (si == 0) { loaddiag = strings.dup(out.stderr); }
+		else { assert(same(loaddiag, out.stderr)); };
+
+		// Empty reservations and recognized ELF outputs are replaceable. A
+		// rejected invalidation preserves the prior persistent generation.
+		rewritefile(commandout, ""); rewritefile(trace, "");
+		runcommandenv(root, strings.concat("overwrite-empty-", tags[si]),
+			commandav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		let built: str = readfile(commandout);
+		assert(strings.hasprefix(built, "\x7fELF"));
+		let unit: str = readfile(strings.concat(work, "/cmd.app.unit.ww"));
+		if (si == 0) {
+			commandbytes = strings.dup(built);
+			unitbytes = strings.dup(unit);
+		} else {
+			assert(same(commandbytes, built) && same(unitbytes, unit));
+		};
+		rewritefile(appsource, appchanged);
+		rewritefile(commandout, "caller-after-change\n");
+		rewritefile(trace, "");
+		runcommandenv(root, strings.concat("overwrite-invalidated-", tags[si]),
+			commandav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(readfile(trace).len != 0);
+		assert(same(readfile(commandout), "caller-after-change\n"));
+		assert(same(readfile(strings.concat(work, "/cmd.app.unit.ww")), unit));
+		rewritefile(appsource, appbase);
+		rewritefile(commandout, built);
+		runcommandenv(root, strings.concat("overwrite-elf-", tags[si]),
+			commandav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		let runav: []str = [commandout];
+		runcommand(root, strings.concat("overwrite-command-run-", tags[si]),
+			runav, (10i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 9);
+
+		// Default, raw, symlink-following, and output-directory child routes
+		// use the same late guard and never strand rollback backups.
+		if (os.exists(defaultout)) { rewritefile(defaultout, "default-sentinel\n"); }
+		else { writefile(defaultout, "default-sentinel\n"); };
+		let defaultav: []str = [driver(drivers[si]), "build", "-w",
+			defaultwork, "-I", source, "cmd.app"];
+		runcommandenvdir(root, strings.concat("overwrite-default-", tags[si]),
+			defaultav, env, invocation,
+			(60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(same(readfile(defaultout), "default-sentinel\n"));
+		if (si == 0) { defaultdiag = strings.dup(out.stderr); }
+		else { assert(same(defaultdiag, out.stderr)); };
+		if (os.exists(rawout)) { rewritefile(rawout, "raw-sentinel\n"); }
+		else { writefile(rawout, "raw-sentinel\n"); };
+		let rawav: []str = [driver(drivers[si]), "build", "-w", rawwork,
+			"-o", rawout, raw];
+		runcommandenv(root, strings.concat("overwrite-raw-", tags[si]), rawav,
+			env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(same(readfile(rawout), "raw-sentinel\n"));
+		if (si == 0) { rawdiag = strings.dup(out.stderr); }
+		else { assert(same(rawdiag, out.stderr)); };
+		let symlinkav: []str = [driver(drivers[si]), "build", "-w",
+			symlinkwork, "-I", source, "-o", symlinkout, "cmd.app"];
+		runcommandenv(root, strings.concat("overwrite-symlink-", tags[si]),
+			symlinkav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(same(readfile(symlinkout), "symlink-sentinel\n"));
+		if (si == 0) { symlinkdiag = strings.dup(out.stderr); }
+		else { assert(same(symlinkdiag, out.stderr)); };
+		if (os.exists(fifoout)) { assert(os.remove(fifoout) == 0); };
+		let fifoav: []str = ["/usr/bin/mkfifo", fifoout];
+		runcommand(root, strings.concat("overwrite-mkfifo-", tags[si]), fifoav,
+			(10i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		let fifobuildav: []str = [driver(drivers[si]), "build", "-w",
+			fifowork, "-I", source, "-o", fifoout, "cmd.app"];
+		runcommandenv(root, strings.concat("overwrite-fifo-", tags[si]),
+			fifobuildav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		assert(strings.hasprefix(readfile(fifoout), "\x7fELF"));
+		let dirav: []str = [driver(drivers[si]), "build", "-w", dirwork,
+			"-I", source, "-o", strings.concat(builddir, "/"), "cmd.app"];
+		runcommandenv(root, strings.concat("overwrite-build-dir-", tags[si]),
+			dirav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(has(out.stderr, "already exists and is a directory"));
+		assert(same(readfile(strings.concat(buildchild, "/marker")),
+			"build-directory\n"));
+		assert(!directoryhasfragment(builddir, ".wwtxn."));
+		if (si == 0) { builddirdiag = strings.dup(out.stderr); }
+		else { assert(same(builddirdiag, out.stderr)); };
+
+		// Archive and WW-interface outputs each participate in the guard. An
+		// ordinary repeat accepts both recognized prior outputs; arbitrary
+		// interface or archive text rejects without changing either member.
+		if (os.exists(libout)) { assert(os.remove(libout) == 0); };
+		if (os.exists(strings.concat(libout, ".wwi"))) {
+			assert(os.remove(strings.concat(libout, ".wwi")) == 0);
+		};
+		let libav: []str = [driver(drivers[si]), "build", "-w", libwork,
+			"-I", source, "-o", libout, "lib.dep"];
+		runcommandenv(root, strings.concat("overwrite-library-new-", tags[si]),
+			libav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		let archive: str = readfile(libout);
+		let iface: str = readfile(strings.concat(libout, ".wwi"));
+		assert(strings.hasprefix(archive, "!<arch>\n"));
+		assert(strings.hasprefix(iface, "//ww:module "));
+		runcommandenv(root, strings.concat("overwrite-library-repeat-", tags[si]),
+			libav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		assert(same(archive, readfile(libout))
+			&& same(iface, readfile(strings.concat(libout, ".wwi"))));
+		if (si == 0) {
+			librarybytes = strings.dup(archive);
+			interfacebytes = strings.dup(iface);
+		} else {
+			assert(same(librarybytes, archive)
+				&& same(interfacebytes, iface));
+		};
+		rewritefile(strings.concat(libout, ".wwi"), "caller-interface\n");
+		runcommandenv(root, strings.concat("overwrite-library-reject-", tags[si]),
+			libav, env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(same(readfile(libout), archive));
+		assert(same(readfile(strings.concat(libout, ".wwi")),
+			"caller-interface\n"));
+		if (si == 0) { librarydiag = strings.dup(out.stderr); }
+		else { assert(same(librarydiag, out.stderr)); };
+		rewritefile(libout, "caller-archive\n");
+		rewritefile(strings.concat(libout, ".wwi"), iface);
+		runcommandenv(root, strings.concat("overwrite-library-archive-",
+			tags[si]), libav, env,
+			(60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(same(readfile(libout), "caller-archive\n"));
+		assert(same(readfile(strings.concat(libout, ".wwi")), iface));
+		if (si == 0) { libraryarchivediag = strings.dup(out.stderr); }
+		else { assert(same(libraryarchivediag, out.stderr)); };
+
+		// Compile-only installs check after linking. Running retention executes
+		// the private test first; a successful run then checks, while a failed
+		// run never installs or diagnoses the caller destination.
+		if (os.exists(testbin)) { rewritefile(testbin, "caller-test\n"); }
+		else { writefile(testbin, "caller-test\n"); };
+		let testcompileav: []str = [driver(drivers[si]), "test", "-c", "-w",
+			testwork, "-I", source, "-o", testbin, "pkg.check"];
+		runcommandenv(root, strings.concat("overwrite-test-c-", tags[si]),
+			testcompileav, env,
+			(90i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(same(readfile(testbin), "caller-test\n"));
+		if (si == 0) { testcompilediag = strings.dup(out.stderr); }
+		else { assert(same(testcompilediag, out.stderr)); };
+		rewritefile(testbin, "");
+		runcommandenv(root, strings.concat("overwrite-test-empty-", tags[si]),
+			testcompileav, env,
+			(90i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		let compiledtest: str = readfile(testbin);
+		assert(strings.hasprefix(compiledtest, "\x7fELF"));
+		if (si == 0) { testbytes = strings.dup(compiledtest); }
+		else { assert(same(testbytes, compiledtest)); };
+		rewritefile(testbin, "caller-running-test\n");
+		let testrunav: []str = [driver(drivers[si]), "test", "-w", testwork,
+			"-I", source, "-o", testbin, "pkg.check"];
+		runcommandenv(root, strings.concat("overwrite-test-run-", tags[si]),
+			testrunav, env, (90i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(has(out.stdout, "checked.guarded_run ... ok\n"));
+		assert(same(readfile(testbin), "caller-running-test\n"));
+		if (si == 0) { testrundiag = strings.dup(out.stderr); }
+		else { assert(same(testrundiag, out.stderr)); };
+		rewritefile(testbin, compiledtest);
+		runcommandenv(root, strings.concat("overwrite-test-elf-", tags[si]),
+			testrunav, env, (90i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		assert(has(out.stdout, "checked.guarded_run ... ok\n"));
+		let beforefailure: str = readfile(testbin);
+		let failav: []str = [driver(drivers[si]), "test", "-w", testwork,
+			"-I", source, "-o", testbin, "pkg.failed"];
+		runcommandenv(root, strings.concat("overwrite-test-fail-", tags[si]),
+			failav, env, (90i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(has(out.stdout, "failed.failed_run ... FAIL (exit 1)\n"));
+		assert(out.stderr.len == 0 && same(readfile(testbin), beforefailure));
+		assert(!os.exists(strings.concat(testbin, ".new"))
+			&& !os.exists(strings.concat(testbin, ".install")));
+		let testdirav: []str = [driver(drivers[si]), "test", "-c", "-w",
+			testdirwork, "-I", source, "-o", strings.concat(testdir, "/"),
+			"pkg.check"];
+		runcommandenv(root, strings.concat("overwrite-test-dir-", tags[si]),
+			testdirav, env, (90i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 1);
+		assert(has(out.stderr, "already exists and is a directory"));
+		assert(same(readfile(strings.concat(testchild, "/marker")),
+			"test-directory\n"));
+		assert(!directoryhasfragment(testdir, ".wwtxn."));
+		if (si == 0) { testdirdiag = strings.dup(out.stderr); }
+		else { assert(same(testdirdiag, out.stderr)); };
+
+		// Assembly and exact-null routes have no public install action.
+		let asmout: str = strings.concat(root, "/assembly-", tags[si]);
+		writefile(asmout, "assembly-sentinel\n");
+		let asmav: []str = [driver(drivers[si]), "build", "-S", "-w",
+			work, "-I", source, "-o", asmout, "cmd.app"];
+		runcommandenv(root, strings.concat("overwrite-asm-", tags[si]), asmav,
+			env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		assert(same(readfile(asmout), "assembly-sentinel\n"));
+		let nullav: []str = [driver(drivers[si]), "build", "-I", source,
+			"-o", "/dev/null", "cmd.app"];
+		runcommandenv(root, strings.concat("overwrite-null-", tags[si]), nullav,
+			env, (60i64 * (time.second: i64)): time.duration, &out);
+		expectexit(&out, 0);
+		si += 1;
+	};
+	clean(root);
+};
+
 // A single directory root used to bypass the package coordinator and treat
 // every non-null -o spelling as one file.  Exercise the Go output-directory
 // branch at that dispatch boundary while the established graph transaction