diff --git a/backend/apps/agents/agent_manager.py b/backend/apps/agents/agent_manager.py
index c7426fe4..a68a4777 100644
--- a/backend/apps/agents/agent_manager.py
+++ b/backend/apps/agents/agent_manager.py
@@ -28,6 +28,7 @@ from backend.apps.agents.manager.RunSupport import RunSupport
 from backend.apps.agents.manager.run.handle_run_error import handle_run_error
 from backend.apps.agents.manager.run.TurnRunner import TurnRunner
 from backend.apps.agents.manager.run.RunOptions import RunOptions
+from backend.apps.agents.manager.ttft_probe import ttft_probe
 
 logger = logging.getLogger(__name__)
 
@@ -51,13 +52,15 @@ class AgentManager(SessionLifecycle, SessionPersistence, Messaging, SessionContr
         session = self.sessions.get(session_id)
         if not session:
             return
-        
+        ttft_probe(session_id, "loop_start", fork=fork_session, model=session.model, msgs=len(session.messages))
+
         from backend.apps.agents.providers.registry import get_api_type as p_get_api_type
         p_api = p_get_api_type(session.model)
         prompt_content = self.build_prompt_content(
             prompt, images, context_paths, forced_tools, attached_skills,
             api_type=p_api, model=session.model,
         )
+        ttft_probe(session_id, "prompt_built")
 
         try:
             # SDK presence check: fall to mock mode here, before the options build, so a missing SDK is a clean mock run, not an error card. The real use is in run_options / turn_runner (lazy-imported there).
@@ -88,6 +91,7 @@ class AgentManager(SessionLifecycle, SessionPersistence, Messaging, SessionContr
                 session, session_id, prompt, prompt_content, builtin_perms,
                 selected_browser_ids, selected_app_output_ids, selected_setting_ids,
                 fork_session, p_router_model_id, p_api_type_for_session)
+            ttft_probe(session_id, "options_built")
             resolved_model = p_router_model_id
             api_type = p_api_type_for_session
 
diff --git a/backend/apps/agents/manager/run/TurnRunner.py b/backend/apps/agents/manager/run/TurnRunner.py
index fe8029db..8a979c02 100644
--- a/backend/apps/agents/manager/run/TurnRunner.py
+++ b/backend/apps/agents/manager/run/TurnRunner.py
@@ -16,6 +16,7 @@ from backend.apps.agents.manager.streaming.state import ThinkingState, TurnState
 from backend.apps.agents.manager.streaming.handle_stream_event import handle_stream_event
 from backend.apps.agents.manager.streaming.handle_assistant_message import handle_assistant_message
 from backend.apps.agents.manager.streaming.handle_result_message import handle_result_message
+from backend.apps.agents.manager.ttft_probe import ttft_probe
 from backend.apps.agents.manager.streaming import thinking as thinking_mod
 from backend.apps.settings.models import AppSettings
 
@@ -35,6 +36,7 @@ class TurnRunner(AgentManagerProtocol):
                                     global_settings: AppSettings) -> None:
         from claude_agent_sdk import query, ClaudeAgentOptions, AssistantMessage, ResultMessage
         from claude_agent_sdk.types import StreamEvent, SystemMessage
+        ttft_probe(session_id, "query_enter")
 
         async def prompt_stream():
             yield {
@@ -88,6 +90,7 @@ class TurnRunner(AgentManagerProtocol):
                             logger.exception("pre-emit thinking pill failed; continuing")
 
                 if turn.first_event:
+                    ttft_probe(session_id, "first_event", type=type(message).__name__)
                     logger.info(f"[MCP-DEBUG] First event received: {type(message).__name__}")
                     turn.first_event = False
 
diff --git a/backend/apps/agents/manager/ttft_probe.py b/backend/apps/agents/manager/ttft_probe.py
new file mode 100644
index 00000000..3472d453
--- /dev/null
+++ b/backend/apps/agents/manager/ttft_probe.py
@@ -0,0 +1,25 @@
+"""Temporary time-to-first-token phase probe for the send->first-token A/B sweep. A no-op unless
+OSW_TTFT_PROBE=1, so it never spams a normal run. Strip once the persistent-client work lands."""
+
+import logging
+import os
+import time
+
+from typeguard import typechecked
+
+logger = logging.getLogger(__name__)
+
+P_TTFT_ENABLED = os.environ.get("OSW_TTFT_PROBE") == "1"
+
+
+@typechecked
+def ttft_probe(session_id: str, phase: str, **extra: object) -> None:
+    """One monotonic phase stamp for the TTFT breakdown; the A/B parser reads `phase=<name> mono=<t>`.
+    A no-op unless OSW_TTFT_PROBE=1, and it swallows any error so instrumentation can NEVER break a turn."""
+    if not P_TTFT_ENABLED:
+        return
+    try:
+        tail = " ".join(f"{k}={v}" for k, v in extra.items())
+        logger.warning(f"[TTFT] sid={session_id} phase={phase} mono={time.monotonic():.4f} {tail}".rstrip())
+    except Exception:
+        pass