diff --git a/src/core/skill.c b/src/core/skill.c
index 6cb0490..eb5c8c7 100644
--- a/src/core/skill.c
+++ b/src/core/skill.c
@@ -333,6 +333,64 @@ int skill_get_content(const config_t *cfg, const char *name, char *out_buf, size
 	return 0;
 }
 
+static void discard_skill_tmp(int fd, char *tmp_path)
+{
+	if (fd >= 0)
+		(void)close(fd);
+	if (tmp_path) {
+		unlink(tmp_path);
+		free(tmp_path);
+	}
+}
+
+/*
+ * Replace skill markdown via temp+rename so fopen("w") cannot wipe an existing
+ * skill before the new content is fully on disk (ENOSPC / EFBIG / crash).
+ */
+static int write_skill_atomic(const char *path, const char *content)
+{
+	size_t content_len;
+	size_t off;
+	size_t path_len;
+	char *tmp_path;
+	int fd;
+
+	if (!path || !content) return -1;
+	content_len = strlen(content);
+	path_len = strlen(path);
+	tmp_path = malloc(path_len + 8);
+	if (!tmp_path) return -1;
+	snprintf(tmp_path, path_len + 8, "%s.tmp", path);
+	fd = open(tmp_path, O_WRONLY | O_CREAT | O_TRUNC, 0600);
+	if (fd < 0) {
+		free(tmp_path);
+		return -1;
+	}
+	off = 0;
+	while (off < content_len) {
+		ssize_t n = write(fd, content + off, content_len - off);
+		if (n <= 0) {
+			discard_skill_tmp(fd, tmp_path);
+			return -1;
+		}
+		off += (size_t)n;
+	}
+	if (fsync(fd) != 0) {
+		discard_skill_tmp(fd, tmp_path);
+		return -1;
+	}
+	if (close(fd) != 0) {
+		discard_skill_tmp(-1, tmp_path);
+		return -1;
+	}
+	if (rename(tmp_path, path) != 0) {
+		discard_skill_tmp(-1, tmp_path);
+		return -1;
+	}
+	free(tmp_path);
+	return 0;
+}
+
 int skill_create(const config_t *cfg, const char *name, const char *content)
 {
 	if (!cfg || !name || !content) return -1;
@@ -343,12 +401,7 @@ int skill_create(const config_t *cfg, const char *name, const char *content)
 		fclose(f);
 		return -1;
 	}
-	f = fopen(path, "w");
-	if (!f) return -1;
-	size_t len = strlen(content);
-	size_t written = fwrite(content, 1, len, f);
-	fclose(f);
-	return (written == len) ? 0 : -1;
+	return write_skill_atomic(path, content);
 }
 
 int skill_update(const config_t *cfg, const char *name, const char *content)
@@ -356,12 +409,7 @@ int skill_update(const config_t *cfg, const char *name, const char *content)
 	if (!cfg || !name || !content) return -1;
 	char path[MAX_PATH_LEN];
 	if (build_skill_path(cfg, name, path, sizeof(path)) != 0) return -1;
-	FILE *f = fopen(path, "w");
-	if (!f) return -1;
-	size_t len = strlen(content);
-	size_t written = fwrite(content, 1, len, f);
-	fclose(f);
-	return (written == len) ? 0 : -1;
+	return write_skill_atomic(path, content);
 }
 
 int skill_delete(const config_t *cfg, const char *name)
diff --git a/tests/test_skill.c b/tests/test_skill.c
index 7fde121..dd49bf3 100644
--- a/tests/test_skill.c
+++ b/tests/test_skill.c
@@ -5,10 +5,13 @@
 
 #include "core/config.h"
 #include "core/skill.h"
+#include <errno.h>
+#include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#include <errno.h>
+#include <sys/resource.h>
+#include <unistd.h>
 
 #define ASSERT(c) do { if (!(c)) { fprintf(stderr, "FAIL: %s:%d %s\n", __FILE__, __LINE__, #c); return 1; } } while (0)
 #define RUN(t) do { int r = (t); if (r) return r; } while (0)
@@ -194,6 +197,58 @@ static int test_skill_crud(void)
 	return 0;
 }
 
+/*
+ * skill_update used fopen("w") which truncates before write. A failed write
+ * (ENOSPC/EFBIG) wiped the live skill. Atomic temp+rename must preserve it.
+ */
+static int test_skill_update_write_failure_preserves_existing(void)
+{
+	char cmd[256];
+	char content[256];
+	char oversized[512];
+	struct rlimit old_lim;
+	struct rlimit new_lim;
+	int update_ret;
+	size_t i;
+	config_t *cfg = NULL;
+	char errbuf[256];
+
+	snprintf(cmd, sizeof(cmd), "rm -rf \"%s\" && mkdir -p \"%s\"", TMP_DIR, TMP_DIR);
+	ASSERT(system(cmd) == 0);
+	ASSERT(write_minimal_config(TMP_DIR) == 0);
+	ASSERT(config_load(TMP_CONFIG, &cfg, errbuf, sizeof(errbuf)) == 0);
+	ASSERT(skill_create(cfg, "keepme", "# Keep\nOriginal skill body that must survive") == 0);
+	ASSERT(skill_get_content(cfg, "keepme", content, sizeof(content)) == 0);
+	ASSERT(strstr(content, "Original skill body that must survive") != NULL);
+
+	for (i = 0; i < sizeof(oversized) - 1; i++)
+		oversized[i] = 'A';
+	oversized[sizeof(oversized) - 1] = '\0';
+
+	ASSERT(getrlimit(RLIMIT_FSIZE, &old_lim) == 0);
+	new_lim = old_lim;
+	new_lim.rlim_cur = 8;
+	(void)signal(SIGXFSZ, SIG_IGN);
+	ASSERT(setrlimit(RLIMIT_FSIZE, &new_lim) == 0);
+
+	update_ret = skill_update(cfg, "keepme", oversized);
+	ASSERT(setrlimit(RLIMIT_FSIZE, &old_lim) == 0);
+
+	ASSERT(update_ret != 0);
+	ASSERT(skill_get_content(cfg, "keepme", content, sizeof(content)) == 0);
+	ASSERT(strstr(content, "Original skill body that must survive") != NULL);
+
+	ASSERT(skill_update(cfg, "keepme", "# Keep\nRecovered after limit") == 0);
+	ASSERT(skill_get_content(cfg, "keepme", content, sizeof(content)) == 0);
+	ASSERT(strstr(content, "Recovered after limit") != NULL);
+
+	config_free(cfg);
+	remove(TMP_CONFIG);
+	snprintf(cmd, sizeof(cmd), "rm -rf \"%s\"", TMP_DIR);
+	(void)system(cmd);
+	return 0;
+}
+
 static int test_hot_reload_watch(void)
 {
 	char cmd[256];
@@ -231,6 +286,7 @@ int main(void)
 	RUN(test_missing_dir_no_crash());
 	RUN(test_system_prompt_base_order());
 	RUN(test_skill_crud());
+	RUN(test_skill_update_write_failure_preserves_existing());
 	RUN(test_hot_reload_watch());
 	printf("test_skill: all tests passed\n");
 	return 0;