diff --git a/CHANGELOG.md b/CHANGELOG.md
index 2236b708..b8c5a7ad 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -2,6 +2,15 @@
 
 ## [Unreleased]
 
+## v0.50.215 — 2026-04-26
+
+### Added
+- **Real `/steer` command** — wires `/steer <text>` through the agent's thread-safe `agent.steer()` method rather than falling back to interrupt. Steer text is stashed in `_pending_steer` and injected into the next tool-result boundary without interrupting the current run, giving the agent a mid-turn course correction. New `/api/chat/steer` POST endpoint with five graceful fallback reasons (`no_cached_agent`, `agent_lacks_steer`, `session_not_found`, `not_running`, `stream_dead`) — any fallback transparently falls back to the existing interrupt+queue mechanism. (`api/routes.py`, `api/streaming.py`, `static/commands.js`, `static/messages.js`, `static/i18n.js`) Closes #720 follow-up [#1066 @nesquena]
+- **Steer leftover delivery** — if the agent finishes its turn before hitting a tool boundary, the stashed steer text is drained and emitted as a `pending_steer_leftover` SSE event; the frontend queues it as a next-turn message, mirroring the CLI's existing leftover path. (`api/streaming.py`, `static/messages.js`) [#1066]
+
+### Fixed
+- **Pending files preserved on steer→interrupt fallback** — the busy-mode steer path in `send()` now defers `S.pendingFiles=[]` until after `_trySteer()` returns, so staged file attachments are not lost when the steer endpoint falls back to interrupt+queue. (`static/messages.js`)
+
 ## v0.50.214 — 2026-04-26
 
 ### Added
diff --git a/ROADMAP.md b/ROADMAP.md
index 38ff5387..523f73a1 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -3,7 +3,7 @@
 > Goal: Full 1:1 parity with the Hermes CLI experience via a clean dark web UI.
 > Everything you can do from the CLI terminal, you can do from this UI.
 >
-> Last updated: v0.50.214 (April 26, 2026) — 2299 tests collected
+> Last updated: v0.50.215 (April 26, 2026) — 2319 tests collected
 > Tests: 2107 collected (`pytest tests/ --collect-only -q`)
 > Source: <repo>/
 
diff --git a/TESTING.md b/TESTING.md
index ea140b7f..cbb814ab 100644
--- a/TESTING.md
+++ b/TESTING.md
@@ -8,7 +8,7 @@
 > Prerequisites: SSH tunnel is active on port 8787. Open http://localhost:8787 in browser.
 > Server health check: curl http://127.0.0.1:8787/health should return {"status":"ok"}.
 >
-> Automated coverage: 2276 tests collected via `pytest tests/ --collect-only -q`. Includes onboarding coverage for bootstrap/static wizard presence, real provider config persistence (`config.yaml` + `.env`), the `/api/onboarding/*` backend, the onboarding skip/existing-config guard, and CSS regression coverage for smooth thinking/tool card disclosure animation.
+> Automated coverage: 2319 tests collected via `pytest tests/ --collect-only -q`. Includes onboarding coverage for bootstrap/static wizard presence, real provider config persistence (`config.yaml` + `.env`), the `/api/onboarding/*` backend, the onboarding skip/existing-config guard, and CSS regression coverage for smooth thinking/tool card disclosure animation.
 > Run: `pytest tests/ -v --timeout=60`
 >
 > Local regression focus: verify that a previously closed workspace panel stays visually closed from first paint through boot completion on desktop refresh; there should be no brief open-then-close flash.
diff --git a/api/routes.py b/api/routes.py
index ecdcbd39..f7389af8 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -1345,6 +1345,10 @@ def handle_post(handler, parsed) -> bool:
     if parsed.path == "/api/chat":
         return _handle_chat_sync(handler, body)
 
+    if parsed.path == "/api/chat/steer":
+        from api.streaming import _handle_chat_steer
+        return _handle_chat_steer(handler, body)
+
     # ── Cron API (POST) ──
     if parsed.path == "/api/crons/create":
         return _handle_cron_create(handler, body)
diff --git a/api/streaming.py b/api/streaming.py
index a4b0e360..c6218119 100644
--- a/api/streaming.py
+++ b/api/streaming.py
@@ -1936,6 +1936,23 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
                 usage['threshold_tokens'] = getattr(_cc, 'threshold_tokens', 0) or 0
                 usage['last_prompt_tokens'] = getattr(_cc, 'last_prompt_tokens', 0) or 0
             # (reasoning trace already attached + saved above, before s.save())
+            # Leftover-steer delivery: if a /steer was queued (via
+            # api/chat/steer) but the agent finished its turn before
+            # reaching a tool-result boundary that would consume it,
+            # the text is still stashed in agent._pending_steer. Drain
+            # it now and emit a pending_steer_leftover SSE event so the
+            # frontend can queue it for the next turn — same fallback
+            # path as the CLI in cli.py:8788-8794.
+            try:
+                _drain_pending_steer = getattr(agent, '_drain_pending_steer', None)
+                _leftover = _drain_pending_steer() if _drain_pending_steer else None
+                if _leftover:
+                    put('pending_steer_leftover', {
+                        'session_id': session_id,
+                        'text': str(_leftover),
+                    })
+            except Exception:
+                logger.debug("Failed to drain pending steer for session %s", session_id)
             raw_session = s.compact() | {'messages': s.messages, 'tool_calls': tool_calls}
             put('done', {'session': redact_session_data(raw_session), 'usage': usage})
             # Emit metering stats for the header TPS label
@@ -2098,6 +2115,82 @@ def _run_agent_streaming(session_id, msg_text, model, workspace, stream_id, atta
 # ============================================================
 
 
+def _handle_chat_steer(handler, body: dict) -> bool:
+    """Inject a /steer payload into the active agent for a session.
+
+    Mirrors the CLI's `/steer <text>` command (cli.py:6140-6155):
+      - Look up the cached AIAgent for the session (PR #1051's
+        SESSION_AGENT_CACHE).
+      - Verify a stream is currently active for this session.
+      - Call agent.steer(text) — thread-safe, stashes text in
+        _pending_steer for application at the next tool-result boundary.
+
+    The agent's loop calls _apply_pending_steer_to_tool_results() at the
+    end of every tool batch and appends the steer text to the last tool
+    result's content with a marker, so the model sees the steer as part
+    of the tool output on its next iteration. The user's stream is NOT
+    interrupted.
+
+    If no agent is cached, the agent is too old to support steer, or no
+    stream is active, return {"accepted": False, "fallback": "<reason>"}
+    so the frontend can fall back to interrupt or queue mode. The
+    fallback path is the existing behaviour from PR #1062.
+
+    Returns 200 with {"accepted": bool, "fallback": str|None,
+    "stream_id": str|None}.
+    """
+    from api.helpers import j, bad
+    from api.config import SESSION_AGENT_CACHE, SESSION_AGENT_CACHE_LOCK
+
+    sid = str((body or {}).get("session_id", "") or "").strip()
+    text = str((body or {}).get("text", "") or "").strip()
+    if not sid:
+        return bad(handler, "session_id required")
+    if not text:
+        return bad(handler, "text required")
+
+    with SESSION_AGENT_CACHE_LOCK:
+        cached = SESSION_AGENT_CACHE.get(sid)
+    if not cached:
+        # No active agent for this session — caller falls back to interrupt
+        return j(handler, {"accepted": False, "fallback": "no_cached_agent",
+                           "stream_id": None})
+    agent = cached[0]
+    if not hasattr(agent, "steer"):
+        # Older hermes-agent that pre-dates the steer() method
+        return j(handler, {"accepted": False, "fallback": "agent_lacks_steer",
+                           "stream_id": None})
+
+    # Verify the agent is currently running. Use the session's
+    # active_stream_id rather than calling load_session_locked() which
+    # would block on the streaming thread's lock.
+    try:
+        s = get_session(sid)
+    except KeyError:
+        return j(handler, {"accepted": False, "fallback": "session_not_found",
+                           "stream_id": None})
+    active_stream_id = getattr(s, "active_stream_id", None) or None
+    if not active_stream_id:
+        return j(handler, {"accepted": False, "fallback": "not_running",
+                           "stream_id": None})
+    with STREAMS_LOCK:
+        stream_alive = active_stream_id in STREAMS
+    if not stream_alive:
+        # Active stream id is stale — stream has ended; caller falls back
+        return j(handler, {"accepted": False, "fallback": "stream_dead",
+                           "stream_id": None})
+
+    try:
+        accepted = bool(agent.steer(text))
+    except Exception as exc:
+        logger.debug("agent.steer() raised for session=%s: %s", sid, exc)
+        return j(handler, {"accepted": False, "fallback": "steer_error",
+                           "stream_id": active_stream_id})
+
+    return j(handler, {"accepted": accepted, "fallback": None,
+                       "stream_id": active_stream_id})
+
+
 def cancel_stream(stream_id: str) -> bool:
     """Signal an in-flight stream to cancel. Returns True if the stream existed.
 
diff --git a/static/commands.js b/static/commands.js
index bbb9f973..e7d7cfdf 100644
--- a/static/commands.js
+++ b/static/commands.js
@@ -573,23 +573,68 @@ async function cmdInterrupt(args){
 }
 
 /**
- * /steer <message> — Inject a steering hint mid-task.
- * Currently falls back to interrupt behaviour because the WebUI cannot
- * inject messages into an in-flight agent thread.  Shows a toast to
- * inform the user that true steering is not yet available.
+ * /steer <message> — Inject a steering hint mid-task without interrupting.
+ *
+ * Calls POST /api/chat/steer which looks up the cached AIAgent for this
+ * session and calls agent.steer(text). The agent's run loop appends the
+ * steer text to the next tool-result message so the model sees it on its
+ * next iteration — same pathway as the CLI's /steer command.
+ *
+ * Falls back to interrupt mode when the agent isn't running, isn't cached,
+ * or doesn't support steer (older hermes-agent versions).
  */
 async function cmdSteer(args){
   const msg=(args||'').trim();
   if(!msg){showToast(t('cmd_steer_no_msg'));return;}
   if(!S.busy||!S.activeStreamId){showToast(t('no_active_task'));return;}
   if(!S.session){showToast(t('no_active_session'));return;}
-  // True steer (inject without cancelling) requires agent-side support
-  // that is not yet available in the WebUI.  Fall back to interrupt.
+  await _trySteer(msg, /*explicitSteer=*/true);
+}
+
+/**
+ * Shared implementation for /steer and the busy_input_mode='steer' path.
+ *
+ * Tries the real steer endpoint first. On any non-accept response (no cached
+ * agent, agent lacks steer, stream dead, etc.) falls back to interrupt mode:
+ * queue the message + cancel the stream so the existing drain re-sends.
+ *
+ * @param {string} msg - The steer text.
+ * @param {boolean} explicitSteer - True if the user explicitly invoked /steer
+ *   (vs the busy-mode auto-fallback). Affects toast wording only.
+ */
+async function _trySteer(msg, explicitSteer){
+  let result=null;
+  try{
+    result=await api('/api/chat/steer',{
+      method:'POST',
+      body:JSON.stringify({session_id:S.session.session_id,text:msg}),
+    });
+  }catch(e){
+    // Network or server error — fall back to interrupt
+    result={accepted:false, fallback:'network_error'};
+  }
+  if(result&&result.accepted){
+    showToast(t('cmd_steer_delivered'),2500);
+    return;
+  }
+  // Fall back to interrupt: queue the message + cancel the stream so the
+  // drain in setBusy(false) re-sends it as a fresh turn.
   queueSessionMessage(S.session.session_id,{text:msg,files:[...S.pendingFiles],model:S.session&&S.session.model||($('modelSelect')&&$('modelSelect').value)||'',profile:S.activeProfile||'default'});
   updateQueueBadge(S.session.session_id);
   S.pendingFiles=[];renderTray();
   if(typeof cancelStream==='function'){await cancelStream();}
-  showToast(t('cmd_steer_fallback'),2500);
+  // Toast wording differs based on why we're falling back so the user
+  // understands what just happened.
+  const reason=(result&&result.fallback)||'unknown';
+  if(explicitSteer){
+    showToast(t('cmd_steer_fallback'),2500);
+  } else if(reason==='no_cached_agent'||reason==='not_running'||reason==='stream_dead'){
+    // Busy mode hit the steer path before the agent was ready —
+    // interrupt is the natural fallback, no need to call out steer.
+    showToast(t('busy_interrupt_confirm'),2000);
+  } else {
+    showToast(t('busy_steer_fallback'),2500);
+  }
 }
 
 async function cmdTitle(args){
diff --git a/static/i18n.js b/static/i18n.js
index 467d00c3..f2c8c3b8 100644
--- a/static/i18n.js
+++ b/static/i18n.js
@@ -108,7 +108,9 @@ const LOCALES = {
     cmd_interrupt_no_msg: 'Usage: /interrupt <message>',
     cmd_interrupt_confirm: 'Interrupted — sending new message',
     cmd_steer_no_msg: 'Usage: /steer <message>',
-    cmd_steer_fallback: 'Steer is not yet available — interrupted and queued instead',
+    cmd_steer_fallback: 'Steer unavailable — interrupted and queued instead',
+    cmd_steer_delivered: 'Steer delivered — agent will see it on its next tool result',
+    steer_leftover_queued: 'Steer queued for next turn',
     busy_steer_fallback: 'Steer not available — interrupted instead',
     busy_interrupt_confirm: 'Interrupted — sending new message',
     settings_label_busy_input_mode: 'Busy input mode',
@@ -734,7 +736,9 @@ const LOCALES = {
     cmd_interrupt_no_msg: 'Использование: /interrupt <сообщение>',
     cmd_interrupt_confirm: 'Прервано — отправка нового сообщения',
     cmd_steer_no_msg: 'Использование: /steer <сообщение>',
-    cmd_steer_fallback: 'Steer пока недоступен — прервано и поставлено в очередь',
+    cmd_steer_fallback: 'Steer недоступен — прервано и поставлено в очередь',
+    cmd_steer_delivered: 'Steer доставлен — агент увидит его в следующем ответе инструмента',
+    steer_leftover_queued: 'Steer поставлен в очередь на следующий ход',
     busy_steer_fallback: 'Steer недоступен — прервано',
     busy_interrupt_confirm: 'Прервано — отправка нового сообщения',
     settings_label_busy_input_mode: 'Режим ввода при занятости',
@@ -1343,7 +1347,9 @@ const LOCALES = {
     cmd_interrupt_no_msg: 'Uso: /interrupt <mensaje>',
     cmd_interrupt_confirm: 'Interrumpido \u2014 enviando nuevo mensaje',
     cmd_steer_no_msg: 'Uso: /steer <mensaje>',
-    cmd_steer_fallback: 'Steer no disponible a\u00fan \u2014 interrumpido y encolado',
+    cmd_steer_fallback: 'Steer no disponible \u2014 interrumpido y encolado',
+    cmd_steer_delivered: 'Steer entregado \u2014 el agente lo ver\u00e1 en su pr\u00f3ximo resultado de herramienta',
+    steer_leftover_queued: 'Steer en cola para el pr\u00f3ximo turno',
     busy_steer_fallback: 'Steer no disponible \u2014 interrumpido',
     busy_interrupt_confirm: 'Interrumpido \u2014 enviando nuevo mensaje',
     settings_label_busy_input_mode: 'Modo de entrada ocupada',
@@ -1921,7 +1927,9 @@ const LOCALES = {
     cmd_interrupt_no_msg: 'Verwendung: /interrupt <Nachricht>',
     cmd_interrupt_confirm: 'Unterbrochen \u2014 neue Nachricht wird gesendet',
     cmd_steer_no_msg: 'Verwendung: /steer <Nachricht>',
-    cmd_steer_fallback: 'Steer noch nicht verf\u00fcgbar \u2014 unterbrochen und eingereiht',
+    cmd_steer_fallback: 'Steer nicht verf\u00fcgbar \u2014 unterbrochen und eingereiht',
+    cmd_steer_delivered: 'Steer geliefert \u2014 der Agent sieht es bei seinem n\u00e4chsten Tool-Ergebnis',
+    steer_leftover_queued: 'Steer f\u00fcr n\u00e4chsten Durchgang eingereiht',
     busy_steer_fallback: 'Steer nicht verf\u00fcgbar \u2014 unterbrochen',
     busy_interrupt_confirm: 'Unterbrochen \u2014 neue Nachricht wird gesendet',
     settings_label_busy_input_mode: 'Eingabemodus bei Besch\u00e4ftigung',
@@ -2289,7 +2297,9 @@ const LOCALES = {
     cmd_interrupt_no_msg: '\u7528\u6cd5\uff1a/interrupt <\u6d88\u606f>',
     cmd_interrupt_confirm: '\u5df2\u4e2d\u65ad \u2014 \u6b63\u5728\u53d1\u9001\u65b0\u6d88\u606f',
     cmd_steer_no_msg: '\u7528\u6cd5\uff1a/steer <\u6d88\u606f>',
-    cmd_steer_fallback: 'Steer \u5c1a\u4e0d\u53ef\u7528 \u2014 \u5df2\u4e2d\u65ad\u5e76\u52a0\u5165\u961f\u5217',
+    cmd_steer_fallback: 'Steer \u4e0d\u53ef\u7528 \u2014 \u5df2\u4e2d\u65ad\u5e76\u52a0\u5165\u961f\u5217',
+    cmd_steer_delivered: 'Steer \u5df2\u4ea4\u4ed8 \u2014 \u4ee3\u7406\u5c06\u5728\u4e0b\u4e00\u4e2a\u5de5\u5177\u7ed3\u679c\u4e2d\u770b\u5230',
+    steer_leftover_queued: 'Steer \u5df2\u52a0\u5165\u4e0b\u8f6e\u961f\u5217',
     busy_steer_fallback: 'Steer \u4e0d\u53ef\u7528 \u2014 \u5df2\u4e2d\u65ad',
     busy_interrupt_confirm: '\u5df2\u4e2d\u65ad \u2014 \u6b63\u5728\u53d1\u9001\u65b0\u6d88\u606f',
     settings_label_busy_input_mode: '\u5fd9\u788c\u8f93\u5165\u6a21\u5f0f',
@@ -3212,7 +3222,9 @@ const LOCALES = {
     cmd_interrupt_no_msg: '\u7528\u6cd5\uff1a/interrupt <\u8a0a\u606f>',
     cmd_interrupt_confirm: '\u5df2\u4e2d\u65ad \u2014 \u6b63\u5728\u767c\u9001\u65b0\u8a0a\u606f',
     cmd_steer_no_msg: '\u7528\u6cd5\uff1a/steer <\u8a0a\u606f>',
-    cmd_steer_fallback: 'Steer \u5c1a\u4e0d\u53ef\u7528 \u2014 \u5df2\u4e2d\u65ad\u4e26\u52a0\u5165\u4f47\u5217',
+    cmd_steer_fallback: 'Steer \u4e0d\u53ef\u7528 \u2014 \u5df2\u4e2d\u65ad\u4e26\u52a0\u5165\u4f47\u5217',
+    cmd_steer_delivered: 'Steer \u5df2\u9001\u9054 \u2014 \u4ee3\u7406\u5c07\u5728\u4e0b\u4e00\u500b\u5de5\u5177\u7d50\u679c\u4e2d\u770b\u5230',
+    steer_leftover_queued: 'Steer \u5df2\u52a0\u5165\u4e0b\u4e00\u8f2a\u4f47\u5217',
     busy_steer_fallback: 'Steer \u4e0d\u53ef\u7528 \u2014 \u5df2\u4e2d\u65ad',
     busy_interrupt_confirm: '\u5df2\u4e2d\u65ad \u2014 \u6b63\u5728\u767c\u9001\u65b0\u8a0a\u606f',
     settings_label_busy_input_mode: '\u5fd9\u788c\u8f38\u5165\u6a21\u5f0f',
diff --git a/static/index.html b/static/index.html
index 1f05fe7e..1d63ca13 100644
--- a/static/index.html
+++ b/static/index.html
@@ -669,7 +669,7 @@
                 <option value="interrupt" data-i18n="settings_busy_input_mode_interrupt">Interrupt current turn</option>
                 <option value="steer" data-i18n="settings_busy_input_mode_steer">Steer (interrupt + send)</option>
               </select>
-              <div style="font-size:11px;color:var(--muted);margin-top:4px" data-i18n="settings_desc_busy_input_mode">Controls what happens when you send a message while the agent is running. Queue waits for the current task; Interrupt cancels and starts fresh; Steer sends a correction (currently falls back to interrupt).</div>
+              <div style="font-size:11px;color:var(--muted);margin-top:4px" data-i18n="settings_desc_busy_input_mode">Controls what happens when you send a message while the agent is running. Queue waits for the current task; Interrupt cancels and starts fresh; Steer injects a mid-turn correction without interrupting (falls back to interrupt when the agent is not yet cached or the stream has ended).</div>
             </div>
             <div class="settings-field">
               <label style="display:flex;align-items:center;gap:8px;cursor:pointer">
diff --git a/static/messages.js b/static/messages.js
index fe82b975..64535066 100644
--- a/static/messages.js
+++ b/static/messages.js
@@ -15,24 +15,34 @@ async function send(){
     if(text){
       if(!S.session){await newSession();await renderSessionList();}
       const busyMode=window._busyInputMode||'queue';
-      if(busyMode==='interrupt'||busyMode==='steer'){
-        // Cancel the current stream, then queue so drain sends it after cleanup
+      if(busyMode==='steer'&&S.activeStreamId&&typeof _trySteer==='function'){
+        // Real steer: clear the input first so the user gets immediate
+        // feedback, then ship the steer payload via /api/chat/steer.
+        // _trySteer falls back to queue+cancel internally if the agent
+        // isn't running / cached / steer-capable.
+        $('msg').value='';autoResize();
+        // Do NOT clear pendingFiles yet — _trySteer may fall back to
+        // interrupt+queue and needs the files for queueSessionMessage.
+        // _trySteer clears pendingFiles itself in the fallback path, and
+        // the server returns accepted:true (no files sent) on success.
+        await _trySteer(text, /*explicitSteer=*/false);
+        // After _trySteer: clear any remaining files (success path).
+        S.pendingFiles=[];renderTray();
+      } else if(busyMode==='interrupt'){
+        // Queue the message, then cancel so drain re-sends it.
         queueSessionMessage(S.session.session_id,{text,files:[...S.pendingFiles],model:S.session&&S.session.model||($('modelSelect')&&$('modelSelect').value)||'',profile:S.activeProfile||'default'});
         updateQueueBadge(S.session.session_id);
         $('msg').value='';autoResize();
         S.pendingFiles=[];renderTray();
         if(S.activeStreamId&&typeof cancelStream==='function'){
-          if(busyMode==='steer'){
-            showToast(t('busy_steer_fallback'),2500);
-          } else {
-            showToast(t('busy_interrupt_confirm'),2000);
-          }
+          showToast(t('busy_interrupt_confirm'),2000);
           await cancelStream();
         } else {
           showToast(`Queued: "${text.slice(0,40)}${text.length>40?'…':''}"`,2000);
         }
       } else {
-        // Default: queue mode (current behavior)
+        // Default: queue mode (current behavior). Also the fallback for
+        // 'steer' mode when no stream is active or _trySteer is unavailable.
         queueSessionMessage(S.session.session_id,{text,files:[...S.pendingFiles],model:S.session&&S.session.model||($('modelSelect')&&$('modelSelect').value)||'',profile:S.activeProfile||'default'});
         $('msg').value='';autoResize();
         S.pendingFiles=[];renderTray();
@@ -764,6 +774,28 @@ function attachLiveStream(activeSid, streamId, uploaded=[], options={}){
       source.close();
     });
 
+    source.addEventListener('pending_steer_leftover',e=>{
+      // The agent finished its turn with steer text still stashed (no
+      // tool-result boundary fired). Match the CLI's leftover-delivery
+      // behaviour: queue the leftover text as a next-turn user message
+      // so the existing drain in setBusy(false) ships it.
+      try{
+        const d=JSON.parse(e.data||'{}');
+        const sid=d.session_id||activeSid;
+        const txt=String(d.text||'').trim();
+        if(!txt||sid!==activeSid) return;
+        if(typeof queueSessionMessage==='function'){
+          queueSessionMessage(sid,{
+            text:txt,files:[],
+            model:S.session&&S.session.model||'',
+            profile:S.activeProfile||'default',
+          });
+          if(typeof updateQueueBadge==='function') updateQueueBadge(sid);
+          showToast(t('steer_leftover_queued'),3000);
+        }
+      }catch(_){}
+    });
+
     source.addEventListener('compressed',e=>{
       // Context was auto-compressed during this turn -- show a system message
       if(!S.session||S.session.session_id!==activeSid) return;
diff --git a/tests/test_1062_busy_input_modes.py b/tests/test_1062_busy_input_modes.py
index 0c04e6f3..fc1fa735 100644
--- a/tests/test_1062_busy_input_modes.py
+++ b/tests/test_1062_busy_input_modes.py
@@ -90,35 +90,60 @@ class TestSlashCommandHandlers:
         assert "queueSessionMessage" in body, "/interrupt must queue the new message before cancelling"
         assert "cancelStream" in body, "/interrupt must call cancelStream() so the drain re-sends"
 
-    def test_cmd_steer_falls_back_to_interrupt(self):
-        """Steer is a placeholder — currently uses interrupt semantics with a different toast."""
+    def test_cmd_steer_delegates_to_try_steer(self):
+        """/steer delegates to _trySteer which calls /api/chat/steer with
+        a queue+cancel fallback. The fallback path is exercised by tests
+        in test_real_steer.py — this test just pins the delegation."""
         idx = COMMANDS_JS.find("async function cmdSteer(")
         assert idx >= 0
         body = COMMANDS_JS[idx:idx + 800]
-        assert "queueSessionMessage" in body
-        assert "cancelStream" in body
-        # Toast should differ from interrupt to signal the placeholder
-        assert "cmd_steer_fallback" in body or "steer_fallback" in body
+        # cmdSteer now delegates to _trySteer; the fallback (queueSessionMessage
+        # + cancelStream) lives inside _trySteer.
+        assert "_trySteer" in body, "cmdSteer must call _trySteer to use the real /api/chat/steer endpoint"
+        # The shared helper must contain the fallback path
+        helper_idx = COMMANDS_JS.find("async function _trySteer(")
+        assert helper_idx >= 0, "_trySteer helper must exist"
+        helper_body = COMMANDS_JS[helper_idx:helper_idx + 1500]
+        assert "queueSessionMessage" in helper_body
+        assert "cancelStream" in helper_body
+        # Toast should differ from interrupt to signal it's the steer path
+        assert "cmd_steer_fallback" in helper_body or "steer_fallback" in helper_body
 
 
 # ── send() busy branch ───────────────────────────────────────────────────
 
     def test_slash_commands_clear_pending_files(self):
-        """All three slash command handlers must clear S.pendingFiles and call
-        renderTray() after enqueuing, so staged files are not duplicated when
-        the next send() fires.
+        """All three busy command handlers must clear S.pendingFiles (directly
+        or via _trySteer) after enqueuing, so staged files are not duplicated.
+
+        cmdQueue and cmdInterrupt call queueSessionMessage themselves and clear
+        S.pendingFiles directly.  cmdSteer delegates to _trySteer.  The fallback/interrupt path clears
+        S.pendingFiles inside _trySteer; the success path returns early and
+        send() handles the post-await clear.  Either way files are not
+        duplicated — we verify by checking _trySteer body for the clearing.
         """
-        for fn_name in ("cmdQueue", "cmdInterrupt", "cmdSteer"):
+        # cmdQueue and cmdInterrupt clear pendingFiles directly
+        for fn_name in ("cmdQueue", "cmdInterrupt"):
             idx = COMMANDS_JS.find(f"function {fn_name}(")
             assert idx >= 0, f"{fn_name} not found"
             body = COMMANDS_JS[idx:idx + 800]
             assert "S.pendingFiles=[]" in body, (
-                f"{fn_name} must clear S.pendingFiles after queueSessionMessage — "
-                "otherwise staged files are re-attached on the next send()"
+                f"{fn_name} must clear S.pendingFiles after queueSessionMessage"
             )
             assert "renderTray()" in body, (
                 f"{fn_name} must call renderTray() after clearing pendingFiles"
             )
+        # cmdSteer delegates to _trySteer; that helper clears pendingFiles
+        idx_try = COMMANDS_JS.find("function _trySteer(")
+        assert idx_try >= 0, "_trySteer not found"
+        try_body = COMMANDS_JS[idx_try:idx_try + 1200]
+        assert "S.pendingFiles=[]" in try_body, (
+            "_trySteer must clear S.pendingFiles in its fallback path — "
+            "without this, files are lost on steer→interrupt fallback"
+        )
+        assert "renderTray()" in try_body, (
+            "_trySteer must call renderTray() after clearing pendingFiles"
+        )
 
 
 class TestSendBusyBranchDispatch:
diff --git a/tests/test_real_steer.py b/tests/test_real_steer.py
new file mode 100644
index 00000000..5584ace9
--- /dev/null
+++ b/tests/test_real_steer.py
@@ -0,0 +1,347 @@
+"""Tests for real /steer functionality (follow-up to PR #1062).
+
+Covers the new POST /api/chat/steer endpoint which mirrors the CLI's /steer
+command (cli.py:6140-6155): the endpoint looks up the cached AIAgent for the
+session, calls agent.steer(text), and the agent's run loop appends the steer
+text to the next tool-result message — no interruption.
+
+Falls back to {"accepted": false, "fallback": "<reason>"} when the agent
+isn't running, isn't cached, or doesn't support steer (older agent versions).
+The frontend uses the fallback signal to drop back to interrupt mode.
+
+Plus a leftover-delivery flow: if the agent finishes its turn before the
+steer is consumed (no tool-call boundary), _drain_pending_steer is called
+after run_conversation returns and a `pending_steer_leftover` SSE event is
+emitted so the frontend can queue the leftover text as a next-turn message.
+"""
+import sys
+import os
+import unittest
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
+
+
+@pytest.fixture(autouse=True)
+def _restore_auth_sessions():
+    """Snapshot and restore api.auth._sessions — see test_1058 for the rationale."""
+    import api.auth as _auth
+    snapshot = dict(_auth._sessions)
+    yield
+    _auth._sessions.clear()
+    _auth._sessions.update(snapshot)
+
+
+@pytest.fixture
+def _clear_caches():
+    """Snapshot SESSION_AGENT_CACHE and STREAMS so tests don't bleed."""
+    from api.config import SESSION_AGENT_CACHE, SESSION_AGENT_CACHE_LOCK, STREAMS, STREAMS_LOCK
+    with SESSION_AGENT_CACHE_LOCK:
+        cache_snap = dict(SESSION_AGENT_CACHE)
+        SESSION_AGENT_CACHE.clear()
+    with STREAMS_LOCK:
+        streams_snap = dict(STREAMS)
+        STREAMS.clear()
+    yield
+    with SESSION_AGENT_CACHE_LOCK:
+        SESSION_AGENT_CACHE.clear()
+        SESSION_AGENT_CACHE.update(cache_snap)
+    with STREAMS_LOCK:
+        STREAMS.clear()
+        STREAMS.update(streams_snap)
+
+
+def _make_handler():
+    """Minimal handler stub matching the methods api.helpers.j() touches."""
+    h = MagicMock()
+    h.wfile = MagicMock()
+    h.headers = MagicMock()
+    h.headers.get = MagicMock(return_value="")
+    return h
+
+
+def _captured_response(handler):
+    """Pull the JSON body that j() wrote to handler.wfile."""
+    import json as _json
+    # j() calls handler.wfile.write(body)
+    write_calls = handler.wfile.write.call_args_list
+    assert write_calls, "no body was written to handler.wfile"
+    body = write_calls[-1][0][0]
+    return _json.loads(body.decode("utf-8"))
+
+
+def _captured_status(handler):
+    """Pull the HTTP status passed to handler.send_response()."""
+    calls = handler.send_response.call_args_list
+    assert calls, "no status was sent"
+    return calls[-1][0][0]
+
+
+# ── Backend: the /api/chat/steer endpoint ─────────────────────────────────
+
+class TestHandleChatSteerHappyPath:
+    """Endpoint accepts text and calls agent.steer() when all gates pass."""
+
+    def test_accepts_when_agent_cached_and_running(self, _clear_caches):
+        from api.streaming import _handle_chat_steer
+        from api.config import SESSION_AGENT_CACHE, SESSION_AGENT_CACHE_LOCK, STREAMS, STREAMS_LOCK
+        sid, stream_id = "sid_happy", "stream_happy"
+        agent = MagicMock()
+        agent.steer = MagicMock(return_value=True)
+        with SESSION_AGENT_CACHE_LOCK:
+            SESSION_AGENT_CACHE[sid] = (agent, "sig")
+        with STREAMS_LOCK:
+            import queue as _q
+            STREAMS[stream_id] = _q.Queue()
+
+        sess = MagicMock()
+        sess.active_stream_id = stream_id
+        with patch("api.streaming.get_session", return_value=sess):
+            handler = _make_handler()
+            _handle_chat_steer(handler, {"session_id": sid, "text": "Use Python instead"})
+
+        agent.steer.assert_called_once_with("Use Python instead")
+        body = _captured_response(handler)
+        assert body == {"accepted": True, "fallback": None, "stream_id": stream_id}
+
+
+class TestHandleChatSteerFallbacks:
+    """Each gate that fails returns a structured fallback the frontend can branch on."""
+
+    def test_no_cached_agent(self, _clear_caches):
+        from api.streaming import _handle_chat_steer
+        handler = _make_handler()
+        _handle_chat_steer(handler, {"session_id": "sid_x", "text": "hint"})
+        body = _captured_response(handler)
+        assert body["accepted"] is False
+        assert body["fallback"] == "no_cached_agent"
+
+    def test_agent_lacks_steer_method(self, _clear_caches):
+        from api.streaming import _handle_chat_steer
+        from api.config import SESSION_AGENT_CACHE, SESSION_AGENT_CACHE_LOCK
+        sid = "sid_old"
+        # Older agent without steer() — use spec to suppress MagicMock auto-create
+        agent = MagicMock(spec=["interrupt", "run_conversation"])
+        with SESSION_AGENT_CACHE_LOCK:
+            SESSION_AGENT_CACHE[sid] = (agent, "sig")
+        handler = _make_handler()
+        _handle_chat_steer(handler, {"session_id": sid, "text": "hint"})
+        body = _captured_response(handler)
+        assert body["accepted"] is False
+        assert body["fallback"] == "agent_lacks_steer"
+
+    def test_session_not_found(self, _clear_caches):
+        from api.streaming import _handle_chat_steer
+        from api.config import SESSION_AGENT_CACHE, SESSION_AGENT_CACHE_LOCK
+        sid = "sid_missing"
+        agent = MagicMock()
+        agent.steer = MagicMock(return_value=True)
+        with SESSION_AGENT_CACHE_LOCK:
+            SESSION_AGENT_CACHE[sid] = (agent, "sig")
+        with patch("api.streaming.get_session", side_effect=KeyError(sid)):
+            handler = _make_handler()
+            _handle_chat_steer(handler, {"session_id": sid, "text": "hint"})
+        body = _captured_response(handler)
+        assert body["accepted"] is False
+        assert body["fallback"] == "session_not_found"
+        agent.steer.assert_not_called()  # never reached the steer call
+
+    def test_session_not_running(self, _clear_caches):
+        from api.streaming import _handle_chat_steer
+        from api.config import SESSION_AGENT_CACHE, SESSION_AGENT_CACHE_LOCK
+        sid = "sid_idle"
+        agent = MagicMock()
+        agent.steer = MagicMock(return_value=True)
+        with SESSION_AGENT_CACHE_LOCK:
+            SESSION_AGENT_CACHE[sid] = (agent, "sig")
+        sess = MagicMock()
+        sess.active_stream_id = None  # idle session
+        with patch("api.streaming.get_session", return_value=sess):
+            handler = _make_handler()
+            _handle_chat_steer(handler, {"session_id": sid, "text": "hint"})
+        body = _captured_response(handler)
+        assert body["accepted"] is False
+        assert body["fallback"] == "not_running"
+        agent.steer.assert_not_called()
+
+    def test_stream_dead(self, _clear_caches):
+        """Session has active_stream_id but the stream is gone from STREAMS (e.g. crashed)."""
+        from api.streaming import _handle_chat_steer
+        from api.config import SESSION_AGENT_CACHE, SESSION_AGENT_CACHE_LOCK
+        sid = "sid_zombie"
+        agent = MagicMock()
+        agent.steer = MagicMock(return_value=True)
+        with SESSION_AGENT_CACHE_LOCK:
+            SESSION_AGENT_CACHE[sid] = (agent, "sig")
+        sess = MagicMock()
+        sess.active_stream_id = "stream_zombie"
+        with patch("api.streaming.get_session", return_value=sess):
+            handler = _make_handler()
+            _handle_chat_steer(handler, {"session_id": sid, "text": "hint"})
+        body = _captured_response(handler)
+        assert body["accepted"] is False
+        assert body["fallback"] == "stream_dead"
+        agent.steer.assert_not_called()
+
+    def test_steer_raises(self, _clear_caches):
+        """If agent.steer() raises, return steer_error rather than 500."""
+        from api.streaming import _handle_chat_steer
+        from api.config import SESSION_AGENT_CACHE, SESSION_AGENT_CACHE_LOCK, STREAMS, STREAMS_LOCK
+        sid, stream_id = "sid_throws", "stream_throws"
+        agent = MagicMock()
+        agent.steer = MagicMock(side_effect=RuntimeError("boom"))
+        with SESSION_AGENT_CACHE_LOCK:
+            SESSION_AGENT_CACHE[sid] = (agent, "sig")
+        with STREAMS_LOCK:
+            import queue as _q
+            STREAMS[stream_id] = _q.Queue()
+        sess = MagicMock()
+        sess.active_stream_id = stream_id
+        with patch("api.streaming.get_session", return_value=sess):
+            handler = _make_handler()
+            _handle_chat_steer(handler, {"session_id": sid, "text": "hint"})
+        body = _captured_response(handler)
+        assert body["accepted"] is False
+        assert body["fallback"] == "steer_error"
+
+
+class TestHandleChatSteerInputValidation:
+    """Bad input → 400 Bad Request, not silent acceptance."""
+
+    def test_missing_session_id(self, _clear_caches):
+        from api.streaming import _handle_chat_steer
+        handler = _make_handler()
+        _handle_chat_steer(handler, {"text": "hint"})
+        assert _captured_status(handler) == 400
+
+    def test_missing_text(self, _clear_caches):
+        from api.streaming import _handle_chat_steer
+        handler = _make_handler()
+        _handle_chat_steer(handler, {"session_id": "sid"})
+        assert _captured_status(handler) == 400
+
+    def test_empty_text_after_strip(self, _clear_caches):
+        from api.streaming import _handle_chat_steer
+        handler = _make_handler()
+        _handle_chat_steer(handler, {"session_id": "sid", "text": "   \n\t  "})
+        assert _captured_status(handler) == 400
+
+
+# ── Routing ───────────────────────────────────────────────────────────────
+
+class TestRouting:
+    """The POST handler must dispatch /api/chat/steer to _handle_chat_steer."""
+
+    def test_route_registered(self):
+        src = (Path(__file__).parent.parent / "api" / "routes.py").read_text(encoding="utf-8")
+        assert '/api/chat/steer' in src
+        assert '_handle_chat_steer' in src
+
+
+# ── Frontend: cmdSteer + busy-mode steer use the new endpoint ────────────
+
+class TestFrontendWiring:
+    """The slash command and busy-mode steer paths must call /api/chat/steer."""
+
+    @classmethod
+    def setup_class(cls):
+        cls.cmds = (Path(__file__).parent.parent / "static" / "commands.js").read_text(encoding="utf-8")
+        cls.msgs = (Path(__file__).parent.parent / "static" / "messages.js").read_text(encoding="utf-8")
+        cls.i18n = (Path(__file__).parent.parent / "static" / "i18n.js").read_text(encoding="utf-8")
+
+    def test_cmd_steer_calls_endpoint(self):
+        idx = self.cmds.find("async function cmdSteer(")
+        assert idx >= 0
+        body = self.cmds[idx:idx + 600]
+        # Should call _trySteer (which calls the endpoint), not directly cancelStream
+        assert "_trySteer" in body, "cmdSteer must delegate to _trySteer"
+
+    def test_try_steer_calls_endpoint(self):
+        idx = self.cmds.find("async function _trySteer(")
+        assert idx >= 0
+        body = self.cmds[idx:idx + 1500]
+        assert "/api/chat/steer" in body, "_trySteer must POST to /api/chat/steer"
+        assert "method:'POST'" in body or 'method:"POST"' in body
+
+    def test_try_steer_handles_fallback(self):
+        idx = self.cmds.find("async function _trySteer(")
+        body = self.cmds[idx:idx + 1500]
+        # Must check result.accepted and fall back via queueSessionMessage + cancelStream
+        assert "result&&result.accepted" in body or "result.accepted" in body
+        assert "queueSessionMessage" in body
+        assert "cancelStream" in body, "fallback path must cancel the stream"
+
+    def test_send_busy_steer_uses_try_steer(self):
+        # send() in messages.js: when busyMode === 'steer', should call _trySteer
+        idx = self.msgs.find("busyMode==='steer'")
+        assert idx >= 0
+        block = self.msgs[idx:idx + 800]
+        assert "_trySteer" in block, "send()'s steer branch must delegate to _trySteer"
+
+    def test_pending_steer_leftover_listener(self):
+        """Frontend must listen for pending_steer_leftover SSE events and queue them."""
+        idx = self.msgs.find("addEventListener('pending_steer_leftover'")
+        assert idx >= 0, "messages.js must add a listener for pending_steer_leftover"
+        block = self.msgs[idx:idx + 600]
+        assert "queueSessionMessage" in block, (
+            "pending_steer_leftover handler must queue the leftover text for the next turn"
+        )
+
+
+# ── i18n keys ─────────────────────────────────────────────────────────────
+
+class TestI18nKeys:
+    """The two new keys (cmd_steer_delivered, steer_leftover_queued) must be in all 6 locales."""
+
+    @classmethod
+    def setup_class(cls):
+        cls.i18n = (Path(__file__).parent.parent / "static" / "i18n.js").read_text(encoding="utf-8")
+
+    def test_cmd_steer_delivered_in_all_locales(self):
+        assert self.i18n.count("cmd_steer_delivered:") >= 6, (
+            f"cmd_steer_delivered appears {self.i18n.count('cmd_steer_delivered:')} times; "
+            f"expected ≥6 (one per locale)"
+        )
+
+    def test_steer_leftover_queued_in_all_locales(self):
+        assert self.i18n.count("steer_leftover_queued:") >= 6, (
+            f"steer_leftover_queued appears {self.i18n.count('steer_leftover_queued:')} times; "
+            f"expected ≥6 (one per locale)"
+        )
+
+
+# ── Leftover SSE delivery: streaming.py emits pending_steer_leftover ─────
+
+class TestLeftoverDelivery:
+    """After run_conversation returns, _drain_pending_steer is called and a
+    pending_steer_leftover SSE event is emitted if there's still text stashed."""
+
+    def test_leftover_drain_call_in_streaming(self):
+        """Verify the streaming.py source contains the drain call before put('done', ...)."""
+        src = (Path(__file__).parent.parent / "api" / "streaming.py").read_text(encoding="utf-8")
+        assert "_drain_pending_steer" in src, (
+            "_run_agent_streaming must call agent._drain_pending_steer() to deliver leftovers"
+        )
+        assert "pending_steer_leftover" in src, (
+            "_run_agent_streaming must emit a pending_steer_leftover SSE event"
+        )
+
+    def test_leftover_drain_runs_before_done_event(self):
+        """The drain must happen BEFORE put('done', ...) so frontend gets both events
+        on the same turn."""
+        src = (Path(__file__).parent.parent / "api" / "streaming.py").read_text(encoding="utf-8")
+        # Find the drain invocation and the next put('done', ...) AFTER it
+        drain_idx = src.find("_drain_pending_steer()")
+        assert drain_idx >= 0
+        done_idx = src.find("put('done'", drain_idx)
+        assert done_idx >= 0
+        # No put('done', ...) should appear BEFORE the drain in the same code block
+        # (we already check the drain is in the file; ordering matters within the
+        # non-ephemeral success path)
+        assert drain_idx < done_idx, (
+            "_drain_pending_steer must run before put('done', ...) so the SSE listener "
+            "sees the leftover before stream_end fires"
+        )