diff --git a/CHANGELOG.md b/CHANGELOG.md
index ee42145..7d629f2 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+- `ack.sh` left the previous state's `transitions` in `state.json`, so a second ack before the hook fired again failed with `invalid next state ''`. The agent had to retry the gated command after every step. `ack.sh` now sets `transitions` and `next_state` for the new state, and a session started by an older steplock gets the fixed `ack.sh` on its next block
+
 ## [0.2.0] - 2026-09-29
 
 ### Changed
diff --git a/core/scripts/ack.sh b/core/scripts/ack.sh
index 8553090..4ac944d 100644
--- a/core/scripts/ack.sh
+++ b/core/scripts/ack.sh
@@ -24,10 +24,13 @@ if [ -z "$MATCHED" ]; then
   exit 1
 fi
 
+# Set the new state's transitions from the flow, so the next ack works
+# without the hook firing in between.
 jq --arg cur "$CURRENT" --arg next "$NEXT" '
   .visited      += [$cur] |
   .current_state = $next  |
-  .next_state    = null
+  .transitions   = (.flow_transitions[$next] // []) |
+  .next_state    = (if (.transitions | length) == 1 then .transitions[0] else null end)
 ' "$STATE" > "$TMP" && mv "$TMP" "$STATE"
 
 # Append ack event to audit.log — failures silently ignored (audit must never block).
diff --git a/core/src/ack.sh b/core/src/ack.sh
deleted file mode 100644
index 8553090..0000000
--- a/core/src/ack.sh
+++ /dev/null
@@ -1,41 +0,0 @@
-#!/bin/sh
-DIR="$(cd "$(dirname "$0")" && pwd)"
-STATE="$DIR/state.json"
-TMP="$STATE.tmp.$$"
-
-CURRENT=$(jq -r '.current_state'      "$STATE")
-
-if [ "$CURRENT" = "[*]" ]; then
-  CHECKLIST=$(jq -r '.checklist' "$STATE")
-  echo "steplock: nothing to acknowledge for '$CHECKLIST' (session already complete)"
-  exit 0
-fi
-
-NEXT=$(jq -r '.next_state // empty'   "$STATE")
-NEXT="${1:-$NEXT}"
-
-VALID=$(jq -r '.transitions[]' "$STATE")
-MATCHED=$(printf '%s\n' $VALID | grep -Fx "$NEXT")
-
-if [ -z "$MATCHED" ]; then
-  echo "steplock: invalid next state '$NEXT'" >&2
-  echo "Valid transitions from '$CURRENT':" >&2
-  printf '%s\n' $VALID | sed 's/^/  /' >&2
-  exit 1
-fi
-
-jq --arg cur "$CURRENT" --arg next "$NEXT" '
-  .visited      += [$cur] |
-  .current_state = $next  |
-  .next_state    = null
-' "$STATE" > "$TMP" && mv "$TMP" "$STATE"
-
-# Append ack event to audit.log — failures silently ignored (audit must never block).
-STEPLOCK_DIR="$(cd "$DIR/../../.." && pwd)"
-SESSION="$(basename "$(dirname "$DIR")")"
-CHECKLIST="$(jq -r '.checklist' "$STATE")"
-TS="$(date -u +%Y-%m-%dT%H:%M:%SZ)"
-jq -cn --arg e "ack" --arg c "$CHECKLIST" --arg s "$CURRENT" \
-    --arg sess "$SESSION" --arg ts "$TS" \
-    '{event:$e,checklist:$c,state:$s,session:$sess,ts:$ts}' \
-    >> "$STEPLOCK_DIR/audit.log" 2>/dev/null || true
diff --git a/core/src/gate.rs b/core/src/gate.rs
index 93f5a73..3d79de5 100644
--- a/core/src/gate.rs
+++ b/core/src/gate.rs
@@ -1,4 +1,5 @@
 //! Checklist gate: decides whether one checklist blocks a hook event.
+use std::collections::HashMap;
 use std::fmt::Write as _;
 use std::fs;
 use std::path::Path;
@@ -90,6 +91,7 @@ fn block_reset_always(
         current_state: initial_state.to_owned(),
         next_state,
         transitions,
+        flow_transitions: HashMap::new(),
         visited: vec![],
     };
     audit::append(
@@ -152,6 +154,7 @@ fn block_reset_session(
         .cloned()
         .filter(|_| raw_transitions.len() == 1);
     state.transitions = raw_transitions;
+    state.flow_transitions.clone_from(&flow.transitions);
 
     save_state(&state_path, &state)?;
     scripts::ensure_ack_sh(&session_dir)?;
diff --git a/core/src/run_tests.rs b/core/src/run_tests.rs
index 6286d7b..dd6cf5d 100644
--- a/core/src/run_tests.rs
+++ b/core/src/run_tests.rs
@@ -76,6 +76,7 @@ fn approves_and_resets_state_when_complete() {
         current_state: "[*]".to_owned(),
         next_state: None,
         transitions: vec![],
+        flow_transitions: HashMap::new(),
         visited: vec!["clean_code".to_owned()],
     };
     save_state(&session_dir.join("state.json"), &state).unwrap();
@@ -635,6 +636,7 @@ fn unknown_current_state_in_flow_skips_checklist() {
         current_state: "nonexistent_state".to_owned(),
         next_state: None,
         transitions: vec![],
+        flow_transitions: HashMap::new(),
         visited: vec![],
     };
     save_state(&session_dir.join("state.json"), &state).unwrap();
@@ -660,6 +662,7 @@ fn complete_event_written_to_audit_log() {
         current_state: "[*]".to_owned(),
         next_state: None,
         transitions: vec![],
+        flow_transitions: HashMap::new(),
         visited: vec!["clean_code".to_owned()],
     };
     save_state(&session_dir.join("state.json"), &state).unwrap();
diff --git a/core/src/scripts.rs b/core/src/scripts.rs
index 126245d..6b51279 100644
--- a/core/src/scripts.rs
+++ b/core/src/scripts.rs
@@ -7,14 +7,15 @@ use crate::flow::FlowGraph;
 
 static ACK_SH: &str = include_str!("../scripts/ack.sh");
 
-/// Write ack.sh to `dir` only if it does not already exist.
+/// Write ack.sh to `dir` unless it already holds the current script.
+/// A session started by an older steplock gets the current script on its next block.
 ///
 /// # Errors
 ///
 /// Returns `Err` if writing the file or setting its permissions fails.
 pub fn ensure_ack_sh(dir: &Path) -> Result<()> {
     let path = dir.join("ack.sh");
-    if path.exists() {
+    if fs::read_to_string(&path).is_ok_and(|s| s == ACK_SH) {
         return Ok(());
     }
     write_executable(&path, ACK_SH)
diff --git a/core/src/scripts_tests.rs b/core/src/scripts_tests.rs
index 0992c61..458c6b3 100644
--- a/core/src/scripts_tests.rs
+++ b/core/src/scripts_tests.rs
@@ -41,13 +41,12 @@ fn ack_sh_appends_audit_event() {
 }
 
 #[test]
-fn ensure_ack_sh_is_idempotent() {
+fn ensure_ack_sh_replaces_stale_script() {
     let tmp = TempDir::new().unwrap();
     let path = tmp.path().join("ack.sh");
-    fs::write(&path, "custom content").unwrap();
+    fs::write(&path, "old script").unwrap();
     ensure_ack_sh(tmp.path()).unwrap();
-    // Should not overwrite existing file
-    assert_eq!(fs::read_to_string(&path).unwrap(), "custom content");
+    assert_eq!(fs::read_to_string(&path).unwrap(), ACK_SH);
 }
 
 #[test]
diff --git a/core/src/state.rs b/core/src/state.rs
index e749363..57915a1 100644
--- a/core/src/state.rs
+++ b/core/src/state.rs
@@ -22,6 +22,10 @@ pub struct SessionState {
     pub next_state: Option<String>,
     /// All valid next state names from `current_state` (including `"[*]"`).
     pub transitions: Vec<String>,
+    /// Outgoing transitions for every state in the flow, so `ack.sh` can set
+    /// `transitions` and `next_state` for the state it advances to.
+    #[serde(default, skip_serializing_if = "HashMap::is_empty")]
+    pub flow_transitions: HashMap<String, Vec<String>>,
     /// States that have been acknowledged in this session so far.
     pub visited: Vec<String>,
 }
@@ -66,6 +70,7 @@ pub fn init_state(checklist: &str, initial_state: &str) -> SessionState {
         current_state: initial_state.to_owned(),
         next_state: None,
         transitions: Vec::new(),
+        flow_transitions: HashMap::new(),
         visited: Vec::new(),
     }
 }
diff --git a/core/src/state_tests.rs b/core/src/state_tests.rs
index 0a3e0fe..f371bbc 100644
--- a/core/src/state_tests.rs
+++ b/core/src/state_tests.rs
@@ -1,5 +1,6 @@
 //! Unit tests for `state`.
 use super::*;
+use std::collections::HashMap;
 use tempfile::TempDir;
 
 #[test]
@@ -25,6 +26,7 @@ fn is_complete_true_at_end() {
         current_state: "[*]".to_owned(),
         next_state: None,
         transitions: vec![],
+        flow_transitions: HashMap::new(),
         visited: vec!["step_one".to_owned()],
     };
     assert!(s.is_complete());
@@ -39,6 +41,7 @@ fn save_and_load_roundtrip() {
         current_state: "check_one".to_owned(),
         next_state: Some("check_two".to_owned()),
         transitions: vec!["check_two".to_owned()],
+        flow_transitions: HashMap::new(),
         visited: vec!["prev".to_owned()],
     };
     save_state(&path, &s).unwrap();
diff --git a/core/tests/cli_tests.rs b/core/tests/cli_tests.rs
index 55cf6f0..dbc03e1 100644
--- a/core/tests/cli_tests.rs
+++ b/core/tests/cli_tests.rs
@@ -416,3 +416,50 @@ fn hermes_hook_uses_real_home_global_checklist_when_home_is_sandboxed() {
         "global checklist from the real home must run: {json}"
     );
 }
+
+#[test]
+fn ack_sh_advances_twice_without_a_hook_call_between() {
+    let tmp = TempDir::new().unwrap();
+    let dir = tmp.path().join(".steplock/checklists/gate");
+    fs::create_dir_all(&dir).unwrap();
+    fs::write(
+        dir.join("config.toml"),
+        "on_event = \"tool:before\"\non_tool = \"bash\"\nreset = \"session\"\n",
+    )
+    .unwrap();
+    fs::write(
+        dir.join("flow.mmd"),
+        "stateDiagram-v2\n    [*] --> one\n    one --> two\n    two --> left\n    two --> right\n    left --> three\n    right --> three\n    three --> [*]\n    one: Step one\n    two: Step two\n    left: Go left\n    right: Go right\n    three: Step three\n",
+    )
+    .unwrap();
+
+    let stdin = hook_event("bash", "git push", "sess-ack");
+    let (first_code, _, _) = run_steplock(tmp.path(), &stdin);
+    assert_eq!(first_code, 0, "first call blocks at step one");
+
+    let ack = tmp.path().join(".steplock/sessions/sess-ack/gate/ack.sh");
+    let run_ack = |arg: Option<&str>| {
+        let mut cmd = Command::new("sh");
+        cmd.arg(&ack);
+        if let Some(a) = arg {
+            cmd.arg(a);
+        }
+        cmd.output().unwrap()
+    };
+    // one -> two (linear), two -> right (branch), right -> three (linear): no hook call between.
+    for arg in [None, Some("right"), None] {
+        let out = run_ack(arg);
+        assert!(
+            out.status.success(),
+            "ack {arg:?} failed: {}",
+            String::from_utf8_lossy(&out.stderr)
+        );
+    }
+
+    let (code, stdout, _) = run_steplock(tmp.path(), &stdin);
+    assert_eq!(code, 0);
+    assert!(
+        stdout.contains("Step three"),
+        "expected block at step three, got: {stdout}"
+    );
+}
diff --git a/schemas/session-state.schema.json b/schemas/session-state.schema.json
index a098d50..5853604 100644
--- a/schemas/session-state.schema.json
+++ b/schemas/session-state.schema.json
@@ -24,6 +24,11 @@
       "description": "Valid next state names from current_state. ack.sh validates $1 against this list.",
       "items": { "type": "string" }
     },
+    "flow_transitions": {
+      "type": "object",
+      "description": "Outgoing transitions for every state in the flow. ack.sh reads it to set transitions and next_state for the state it advances to.",
+      "additionalProperties": { "type": "array", "items": { "type": "string" } }
+    },
     "visited": {
       "type": "array",
       "description": "State node names already acknowledged this scope. Written by ack.sh.",