diff --git a/api/streaming.py b/api/streaming.py
index f8f63e1e..b3bf664e 100644
--- a/api/streaming.py
+++ b/api/streaming.py
@@ -586,9 +586,25 @@ def _message_text(value) -> str:
     return _strip_thinking_markup(str(value or '').strip())
 
 
-def _strip_workspace_prefix(text: str) -> str:
-    """Remove WebUI's model-facing workspace tag from display identity text."""
-    return re.sub(r'^\s*\[Workspace:[^\]]+\]\s*', '', str(text or '')).strip()
+_WORKSPACE_PREFIX_RE = re.compile(r'^\s*\[Workspace::v1:\s*(?:\\.|[^\]\\])+\]\s*')
+_LEGACY_WORKSPACE_PREFIX_RE = re.compile(r'^\s*\[Workspace:[^\]]+\]\s*')
+
+
+def _escape_workspace_prefix_path(path: str) -> str:
+    return str(path or '').replace('\\', '\\\\').replace(']', '\\]')
+
+
+def _workspace_context_prefix(path: str) -> str:
+    return f"[Workspace::v1: {_escape_workspace_prefix_path(path)}]\n"
+
+
+def _strip_workspace_prefix(text: str, *, include_legacy: bool = False) -> str:
+    """Remove WebUI-injected workspace tags without eating user-typed text."""
+    value = str(text or '')
+    stripped = _WORKSPACE_PREFIX_RE.sub('', value, count=1)
+    if include_legacy and stripped == value:
+        stripped = _LEGACY_WORKSPACE_PREFIX_RE.sub('', value, count=1)
+    return stripped.strip()
 
 
 def _first_exchange_snippets(messages):
@@ -1051,7 +1067,7 @@ def _fallback_title_from_exchange(user_text: str, assistant_text: str) -> Option
     assistant_text = _strip_thinking_markup(assistant_text or '').strip()
     if not user_text:
         return None
-    user_text = re.sub(r'^\[Workspace:[^\]]+\]\s*', '', user_text)
+    user_text = _strip_workspace_prefix(user_text)
     user_text = re.sub(r'\s+', ' ', user_text).strip()
     assistant_text = re.sub(r'\s+', ' ', assistant_text).strip()
     combined = f"{user_text} {assistant_text}".strip().lower()
@@ -1443,7 +1459,7 @@ def _message_identity(msg):
         # visible optimistic bubble contains only the human text. Treat them as
         # the same turn for merge/dedup purposes; otherwise compaction results
         # render two adjacent user bubbles ("Ok" and "[Workspace...]\nOk").
-        text = _strip_workspace_prefix(text)
+        text = _strip_workspace_prefix(text, include_legacy=True)
     if not text and not msg.get('tool_call_id') and not msg.get('tool_calls'):
         return None
     return (
@@ -1482,7 +1498,12 @@ def _find_current_user_turn(messages, msg_text):
         if not isinstance(msg, dict) or msg.get('role') != 'user':
             continue
         fallback = idx
-        text = " ".join(_strip_workspace_prefix(_message_text(msg.get('content', ''))).split())
+        text = " ".join(
+            _strip_workspace_prefix(
+                _message_text(msg.get('content', '')),
+                include_legacy=True,
+            ).split()
+        )
         if needle and (needle in text or text in needle):
             return idx
     return fallback
@@ -2534,15 +2555,15 @@ def _run_agent_streaming(
 
             # Prepend workspace context so the agent always knows which directory
             # to use for file operations, regardless of session age or AGENTS.md defaults.
-            workspace_ctx = f"[Workspace: {s.workspace}]\n"
+            workspace_ctx = _workspace_context_prefix(str(s.workspace))
             workspace_system_msg = (
                 f"Active workspace at session start: {s.workspace}\n"
-                "Every user message is prefixed with [Workspace: /absolute/path] indicating the "
+                "Every user message is prefixed with [Workspace::v1: /absolute/path] indicating the "
                 "workspace the user has selected in the web UI at the time they sent that message. "
                 "This tag is the single authoritative source of the active workspace and updates "
                 "with every message. It overrides any prior workspace mentioned in this system "
                 "prompt, memory, or conversation history. Always use the value from the most recent "
-                "[Workspace: ...] tag as your default working directory for ALL file operations: "
+                "[Workspace::v1: ...] tag as your default working directory for ALL file operations: "
                 "write_file, read_file, search_files, terminal workdir, and patch. "
                 "Never fall back to a hardcoded path when this tag is present."
             )
diff --git a/static/ui.js b/static/ui.js
index 1ceed738..4599dc50 100644
--- a/static/ui.js
+++ b/static/ui.js
@@ -70,7 +70,7 @@ function _isBacktickFenceClose(line,minLen){
  */
 
 function _stripWorkspaceDisplayPrefix(text){
-  return String(text||'').replace(/^\s*\[Workspace:[^\]]+\]\s*/,'').trim();
+  return String(text||'').replace(/^\s*\[Workspace::v1:\s*(?:\\.|[^\]\\])+\]\s*/,'').trim();
 }
 function _renderUserFencedBlocks(text){
   const stash=[];
diff --git a/tests/test_issue1913_workspace_prefix_sentinel.py b/tests/test_issue1913_workspace_prefix_sentinel.py
new file mode 100644
index 00000000..9f5e03f2
--- /dev/null
+++ b/tests/test_issue1913_workspace_prefix_sentinel.py
@@ -0,0 +1,34 @@
+from api.streaming import (
+    _fallback_title_from_exchange,
+    _strip_workspace_prefix,
+    _workspace_context_prefix,
+)
+
+
+def test_workspace_prefix_strips_only_versioned_sentinel():
+    assert _strip_workspace_prefix("[Workspace::v1: /tmp/project]\nHello") == "Hello"
+    assert _strip_workspace_prefix("[Workspace: /tmp/project]\nHello") == "[Workspace: /tmp/project]\nHello"
+
+
+def test_workspace_prefix_escapes_paths_with_closing_brackets():
+    prefix = _workspace_context_prefix("/tmp/proj-[wip]/src")
+
+    assert prefix == "[Workspace::v1: /tmp/proj-[wip\\]/src]\n"
+    assert _strip_workspace_prefix(f"{prefix}Continue") == "Continue"
+
+
+def test_legacy_workspace_prefix_only_strips_for_compatibility_callers():
+    legacy = "[Workspace: /tmp/project]\nContinue"
+
+    assert _strip_workspace_prefix(legacy) == legacy
+    assert _strip_workspace_prefix(legacy, include_legacy=True) == "Continue"
+
+
+def test_user_typed_legacy_workspace_prefix_survives_fallback_title():
+    title = _fallback_title_from_exchange(
+        "[Workspace: /tmp/project]\nExplain this literal prefix",
+        "Sure",
+    )
+
+    assert title is not None
+    assert title.startswith("Workspace tmp/project")
diff --git a/tests/test_workspace_display_prefix.py b/tests/test_workspace_display_prefix.py
index 98528011..7965362b 100644
--- a/tests/test_workspace_display_prefix.py
+++ b/tests/test_workspace_display_prefix.py
@@ -16,7 +16,8 @@ def test_workspace_display_prefix_helper_strips_leading_metadata_only():
     assert end != -1, "user fenced block renderer not found after prefix stripper"
     helper = src[start:end]
 
-    assert r"^\s*\[Workspace:[^\]]+\]\s*" in helper
+    assert r"^\s*\[Workspace::v1:\s*(?:\\.|[^\]\\])+\]\s*" in helper
+    assert "[Workspace:[^\\]]+" not in helper
     assert ".trim()" in helper