diff --git a/CHANGELOG.md b/CHANGELOG.md
index f34c81b..dcfacb5 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Block messages read as a checklist step and show the `steplock step done` command instead of `sh .../ack.sh`. Claude Code auto mode flagged running `ack.sh` as the agent approving its own gate, so agents in auto mode could not finish any checklist
 - With `allow_preview_request = true`, the first block message lists every step instead of pointing at `preview.sh`
 
+### Fixed
+- `state.json` writes are now fsynced before the atomic rename, so a crash or power loss can no longer leave an empty or truncated session state. A failed write no longer leaves a `state.json.tmp.<pid>` file behind
+
 ### Deprecated
 - `ack.sh` and `preview.sh` are still written to each session dir so old sessions keep working, but no message mentions them. They will be removed in a future release
 
diff --git a/core/Cargo.toml b/core/Cargo.toml
index fcb6712..039eaa1 100644
--- a/core/Cargo.toml
+++ b/core/Cargo.toml
@@ -91,6 +91,7 @@ needless_pass_by_value = "deny"
 trivially_copy_pass_by_ref = "deny"
 verbose_file_reads = "deny"
 unused_self = "deny"
+mem_forget = "deny"
 
 [dependencies]
 cel-interpreter = "0.10"
diff --git a/core/src/state.rs b/core/src/state.rs
index 57915a1..b523c0b 100644
--- a/core/src/state.rs
+++ b/core/src/state.rs
@@ -1,6 +1,7 @@
 //! Session state persistence and core domain types.
 use std::collections::HashMap;
-use std::fs;
+use std::fs::{self, File};
+use std::io::{self, Write};
 use std::path::Path;
 use std::process;
 
@@ -48,18 +49,32 @@ pub fn load_state(path: &Path) -> Result<SessionState> {
     Ok(serde_json::from_str(&content)?)
 }
 
-/// Atomically write `state` to `path` via a PID-suffixed temp file and POSIX `mv`.
+/// Atomically write `state` to `path` via a PID-suffixed temp file and rename.
+///
+/// The temp file is fsynced before the rename, so a crash or power loss leaves
+/// either the old or the new `state.json` — never an empty or truncated one.
+/// The temp file is removed if the write or rename fails.
 ///
 /// # Errors
 ///
-/// Returns an error if serialization fails or the write or rename fails.
+/// Returns an error if serialization fails or the write, sync, or rename fails.
 pub fn save_state(path: &Path, state: &SessionState) -> Result<()> {
     let dir = path.parent().unwrap_or_else(|| Path::new("."));
     let tmp = dir.join(format!("state.json.tmp.{}", process::id()));
     let content = serde_json::to_string_pretty(state)?;
-    fs::write(&tmp, content)?;
-    fs::rename(&tmp, path)?;
-    Ok(())
+    let written = write_synced(&tmp, content.as_bytes()).and_then(|()| fs::rename(&tmp, path));
+    if written.is_err() {
+        // ponytail: best-effort cleanup; the original error is what matters.
+        let _ignored: io::Result<()> = fs::remove_file(&tmp);
+    }
+    Ok(written?)
+}
+
+/// Write `bytes` to a new file at `path` and flush it to disk.
+fn write_synced(path: &Path, bytes: &[u8]) -> io::Result<()> {
+    let mut file = File::create(path)?;
+    file.write_all(bytes)?;
+    file.sync_all()
 }
 
 /// Initialize fresh state for the first (initial) state of a flow.
diff --git a/core/src/state_tests.rs b/core/src/state_tests.rs
index f371bbc..fd167c3 100644
--- a/core/src/state_tests.rs
+++ b/core/src/state_tests.rs
@@ -59,3 +59,40 @@ fn load_state_error_on_missing_file() {
     let path = tmp.path().join("nonexistent.json");
     load_state(&path).unwrap_err();
 }
+
+#[test]
+fn save_state_overwrites_and_leaves_no_temp_file() {
+    let tmp = TempDir::new().unwrap();
+    let path = tmp.path().join("state.json");
+    save_state(&path, &init_state("gate", "first")).unwrap();
+    save_state(&path, &init_state("gate", "second")).unwrap();
+    assert_eq!(
+        load_state(&path).unwrap().current_state,
+        "second",
+        "second save must win"
+    );
+    let names: Vec<_> = fs::read_dir(tmp.path())
+        .unwrap()
+        .map(|e| e.unwrap().file_name())
+        .collect();
+    assert_eq!(names, vec!["state.json"], "temp file must be renamed away");
+}
+
+#[test]
+fn save_state_failure_removes_temp_file() {
+    let tmp = TempDir::new().unwrap();
+    // Renaming a file over a non-empty directory fails on every platform.
+    let path = tmp.path().join("state.json");
+    fs::create_dir(&path).unwrap();
+    fs::write(path.join("keep"), "x").unwrap();
+    save_state(&path, &init_state("gate", "first")).unwrap_err();
+    let names: Vec<_> = fs::read_dir(tmp.path())
+        .unwrap()
+        .map(|e| e.unwrap().file_name())
+        .collect();
+    assert_eq!(
+        names,
+        vec!["state.json"],
+        "temp file must be cleaned up on failure"
+    );
+}