diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4ce4ee7..9a8111b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@ All notable changes to ShellClaw are documented here. Format follows [Keep a Cha
 ## [Unreleased]
 
 ### Fixed
+- `read_file` reads that do not fit the result buffer now fail with a JSON error instead of returning a clipped prefix, so a later `write_file` cannot persist a silently truncated file.
 - Cron re-offer waits the full timeout when a due id could not be stored in the 16-slot table, and an ack frees that slot so the next due job can be delivered and then backed off.
 - Landlock device grants stay `/dev/null`, `/dev/zero`, `/dev/urandom`, and `/dev/tty` without `IOCTL_DEV`. A failed `landlock_add_rule` fails the ruleset closed.
 - Dashboard config save and JSON patch validation use a unique temp plus `fsync`, and the live gateway config pointer is published once under `agent_lock`.
diff --git a/src/tools/file.c b/src/tools/file.c
index 4e07d83..cef427a 100644
--- a/src/tools/file.c
+++ b/src/tools/file.c
@@ -259,7 +259,14 @@ static int file_read(const char *path, char *result_buf, size_t max_len)
 		return -1;
 	}
 	size_t to_read = (size_t)sz;
-	if (to_read >= max_len - 1) to_read = max_len - 1;
+	/* A prefix with return 0 is the whole file to the model. write_file would
+	 * then replace the original and drop the unread tail. */
+	if (to_read + 1 > max_len) {
+		fclose(f);
+		snprintf(result_buf, max_len,
+		         "{\"error\":\"file exceeds result buffer\",\"bytes\":%ld}", sz);
+		return -1;
+	}
 	size_t n = fread(result_buf, 1, to_read, f);
 	fclose(f);
 	result_buf[n] = '\0';
diff --git a/tests/test_file.c b/tests/test_file.c
index 0907d27..490ba6f 100644
--- a/tests/test_file.c
+++ b/tests/test_file.c
@@ -563,6 +563,58 @@ static void test_write_in_workspace_symlink_alias_rejected(void)
 	rmdir(tmpdir);
 }
 
+static void test_read_rejects_file_larger_than_result_buffer(void)
+{
+	char tmpdir[64];
+	char notes_path[PATH_MAX];
+	char config_path[PATH_MAX];
+	char args[PATH_MAX + 64];
+	/* Large enough for the ~49-byte error JSON, still smaller than the 64-byte file. */
+	char buf[56];
+	char exact[8];
+	FILE *f;
+	config_t *cfg;
+	const tool_t *t;
+	int i;
+
+	snprintf(tmpdir, sizeof(tmpdir), "/tmp/sc_test_readcap_%d", (int)getpid());
+	if (mkdir(tmpdir, 0755) != 0 && errno != EEXIST)
+		return;
+	snprintf(notes_path, sizeof(notes_path), "%s/notes.md", tmpdir);
+	f = fopen(notes_path, "w");
+	MU_ASSERT(f != NULL, "create oversized notes.md");
+	for (i = 0; i < 64; i++)
+		fputc('A', f);
+	fclose(f);
+	cfg = make_ws_config(tmpdir, config_path, sizeof(config_path));
+	MU_ASSERT(cfg != NULL, "readcap: load config");
+	tool_file_set_config(cfg);
+	t = tool_file_get();
+	snprintf(args, sizeof(args), "{\"operation\":\"read_file\",\"path\":\"%s\"}", notes_path);
+	MU_ASSERT(t->execute(args, buf, sizeof(buf)) == -1, "oversize read fails");
+	MU_ASSERT(strstr(buf, "\"error\":\"file exceeds result buffer\"") != NULL,
+		"oversize read reports the refuse-to-truncate error");
+	MU_ASSERT(strstr(buf, "\"bytes\":64") != NULL, "error reports the file size");
+	MU_ASSERT(strstr(buf, "AAAAAAAAAAAAAAAA") == NULL, "oversize read is not a raw prefix");
+	f = fopen(notes_path, "w");
+	MU_ASSERT(f != NULL, "rewrite exact-fit notes.md");
+	fputs("0123456", f);
+	fclose(f);
+	MU_ASSERT(t->execute(args, exact, sizeof(exact)) == 0, "exact-fit read succeeds");
+	MU_ASSERT(strcmp(exact, "0123456") == 0, "exact-fit read returns the whole file");
+	/* One over the boundary: size == max_len must fail, not clip (max_len - 1 covered above). */
+	f = fopen(notes_path, "w");
+	MU_ASSERT(f != NULL, "rewrite one-over notes.md");
+	fputs("01234567", f);
+	fclose(f);
+	MU_ASSERT(t->execute(args, exact, sizeof(exact)) == -1, "one-over read fails");
+	MU_ASSERT(strstr(exact, "error") != NULL, "one-over read reports an error, not a prefix");
+	config_free(cfg);
+	unlink(config_path);
+	unlink(notes_path);
+	rmdir(tmpdir);
+}
+
 int main(void)
 {
 	MU_RUN(test_file_read_write_list);
@@ -577,6 +629,7 @@ int main(void)
 	MU_RUN(test_file_write_failure_preserves_existing);
 	MU_RUN(test_write_does_not_clobber_sibling_tmp);
 	MU_RUN(test_write_in_workspace_symlink_alias_rejected);
+	MU_RUN(test_read_rejects_file_larger_than_result_buffer);
 	printf("%d tests run, %d failed\n", tests_run, tests_failed);
 	return tests_failed ? 1 : 0;
 }