diff --git a/backend/apps/agents/agent_manager.py b/backend/apps/agents/agent_manager.py
index 5270b927..7a46d44d 100644
--- a/backend/apps/agents/agent_manager.py
+++ b/backend/apps/agents/agent_manager.py
@@ -138,6 +138,7 @@ class AgentManager(SessionLifecycle, SessionPersistence, Messaging, SessionContr
         # Read BEFORE build_agent_options consumes these flags: a fresh-session/fork request must force the persistent client to respawn (same branch id would otherwise fingerprint-match a client still holding the old transcript).
         p_force_respawn = bool(session.needs_fresh_session or session.needs_fork or fork_session)
         try:
+            logger.info(f"[SPAWN-PHASE] run-loop start session={session_id[:8]}")
             (options, options_kwargs, prompt_content, p_stderr_buffer,
              global_settings) = await self.build_agent_options(
                 session, session_id, prompt, prompt_content, builtin_perms,
@@ -148,7 +149,9 @@ class AgentManager(SessionLifecycle, SessionPersistence, Messaging, SessionContr
 
             thinking = ThinkingState()
             # Gate the CLI turn (spawn + stream) behind the admission slot so a burst can't run every turn at once; the slot is held ONLY for run_turn_with_retry, so the context-valve retry below re-acquires cleanly instead of nesting.
+            logger.info(f"[SPAWN-PHASE] admission-wait session={session_id[:8]}")
             async with self.turn_admission_slot(session, session_id):
+                logger.info(f"[SPAWN-PHASE] admitted session={session_id[:8]}")
                 await self.run_turn_with_retry(
                     session, session_id, prompt_content, options, options_kwargs,
                     turn, thinking, p_stderr_buffer, resolved_model, api_type, global_settings,
diff --git a/backend/apps/agents/agents.py b/backend/apps/agents/agents.py
index 730882c2..d25a8da3 100644
--- a/backend/apps/agents/agents.py
+++ b/backend/apps/agents/agents.py
@@ -132,6 +132,9 @@ async def get_session(session_id: str):
 @agents.router.post("/launch")
 async def launch_agent(config: AgentConfig):
     session = await agent_manager.launch_agent(config)
+    # A launch that carries a prompt runs it as the first turn through the same path /message uses.
+    if config.prompt:
+        asyncio.create_task(agent_manager.send_message(session.id, config.prompt))
     return {"session_id": session.id, "session": session.model_dump(mode="json")}
 
 @agents.router.post("/sessions/{session_id}/message")
diff --git a/backend/apps/agents/core/models.py b/backend/apps/agents/core/models.py
index 3bec810e..a6dcfc0d 100644
--- a/backend/apps/agents/core/models.py
+++ b/backend/apps/agents/core/models.py
@@ -5,6 +5,8 @@ from uuid import uuid4
 
 class AgentConfig(BaseModel):
     name: str = ""
+    # First-turn prompt. Launch used to silently DROP this (pydantic ignores unknown fields), leaving the session claiming "running" forever with zero messages and no error, the ENG-131 ghost hang.
+    prompt: Optional[str] = None
     model: str = "sonnet"
     mode: str = "agent"
     provider: str = "anthropic"
diff --git a/backend/apps/agents/manager/run/RunOptions.py b/backend/apps/agents/manager/run/RunOptions.py
index 08ce61d5..500d5f55 100644
--- a/backend/apps/agents/manager/run/RunOptions.py
+++ b/backend/apps/agents/manager/run/RunOptions.py
@@ -50,6 +50,8 @@ class RunOptions(AgentManagerProtocol):
         from claude_agent_sdk import ClaudeAgentOptions
         from claude_agent_sdk.types import HookMatcher
 
+        logger.info(f"[SPAWN-PHASE] options-build start session={session_id[:8]}")
+
         # Per-SESSION hook context, updated in place each turn: with a persistent client the hooks the
         # CLI holds were bound at connect, so they must read this stable object, not a per-turn rebuild.
         hook_ctx = self.hook_ctxs.get(session_id)
@@ -115,7 +117,9 @@ class RunOptions(AgentManagerProtocol):
         set_framework_overhead(session, composed_prompt)
 
         # Pass session.active_mcps as the activation filter. Empty list ⇒ no MCP tools shipped to the SDK; the model must MCPSearch and MCPActivate first. The product invariant lives here at the dispatch layer (see build_mcp_servers docstring).
+        logger.info(f"[SPAWN-PHASE] mcp-build start session={session_id[:8]}")
         mcp_servers = await self.build_mcp_servers(session.allowed_tools, session.active_mcps)
+        logger.info(f"[SPAWN-PHASE] mcp-build done session={session_id[:8]}")
 
         browser_delegation_tools, invoke_agent_tools = register_builtin_mcp_servers(
             mcp_servers, session, builtin_perms, selected_browser_ids, selected_app_output_ids
@@ -192,9 +196,11 @@ class RunOptions(AgentManagerProtocol):
             "include_partial_messages": True,
         }
         # cc/cx/gc/ag/gemini/openrouter prefixes force 9Router; route="api" bypasses to the provider's host directly; otherwise Pro proxy or key.
+        logger.info(f"[SPAWN-PHASE] provider-env start session={session_id[:8]}")
         await configure_provider_env(
             options_kwargs, session, resolved_model, api_type, global_settings
         )
+        logger.info(f"[SPAWN-PHASE] provider-env done session={session_id[:8]}")
         if mcp_servers:
             options_kwargs["mcp_servers"] = mcp_servers
             mcp_json_len = len(json.dumps({"mcpServers": mcp_servers}))
@@ -264,7 +270,9 @@ class RunOptions(AgentManagerProtocol):
             # Distill the dropped span into a cached aux summary so a rebuild keeps the gist of old turns instead of hard-dropping them. Fail-open: "" -> the plain recap above, exactly today's behavior.
             from backend.apps.agents.manager.session.distill_history import distilled_history_summary
             from backend.apps.agents.manager.session.history_compaction import wrap_platform_note
+            logger.info(f"[SPAWN-PHASE] distill start session={session_id[:8]}")
             distilled = await distilled_history_summary(session, global_settings)
+            logger.info(f"[SPAWN-PHASE] distill done session={session_id[:8]}")
             if distilled:
                 fenced = wrap_platform_note(f"Summary of earlier conversation (older turns compacted):\n{distilled}")
                 history = f"{fenced}\n\n{history}" if history else fenced
@@ -275,7 +283,9 @@ class RunOptions(AgentManagerProtocol):
                     prompt_content.insert(0, {"type": "text", "text": history})
 
         # Compaction trigger (Phase 2). Driven by live ctx_used ratio rather than turn count, fires when input_tokens/context_window crosses session.compact_threshold_pct (default 0.65). Cheap, programmatic summarization (no aux LLM call) so this adds zero latency on the user's turn.
+        logger.info(f"[SPAWN-PHASE] context-guard start session={session_id[:8]}")
         await pre_send_context_guard(self, session, session_id)
+        logger.info(f"[SPAWN-PHASE] context-guard done session={session_id[:8]}")
 
         logger.info(f"[MCP-DEBUG] Creating ClaudeAgentOptions short={session.model} resolved={resolved_model} api_type={api_type}")
         options = ClaudeAgentOptions(**options_kwargs)
diff --git a/backend/apps/agents/manager/run/TurnRunner.py b/backend/apps/agents/manager/run/TurnRunner.py
index 90df2b49..64b0c0e5 100644
--- a/backend/apps/agents/manager/run/TurnRunner.py
+++ b/backend/apps/agents/manager/run/TurnRunner.py
@@ -131,10 +131,13 @@ class TurnRunner(AgentManagerProtocol):
 
             async def p_connect():
                 p_client = ClaudeSDKClient(options=options)
+                logger.info(f"[SPAWN-PHASE] cli-connect start session={session_id[:8]}")
                 await p_client.connect()
+                logger.info(f"[SPAWN-PHASE] cli-connect done session={session_id[:8]}")
                 return p_client
 
             fp = boot_fingerprint(options_kwargs, session)
+            logger.info(f"[SPAWN-PHASE] client-acquire start session={session_id[:8]}")
             handle = await acquire_client(
                 self.client_pool, session_id, fp, p_connect, force_respawn=force_respawn,
             )
diff --git a/backend/tests/test_launch_with_prompt.py b/backend/tests/test_launch_with_prompt.py
new file mode 100644
index 00000000..bb92b1c0
--- /dev/null
+++ b/backend/tests/test_launch_with_prompt.py
@@ -0,0 +1,53 @@
+"""A launch that carries a prompt must RUN it.
+
+The trap this seals (ENG-131): AgentConfig had no `prompt` field, so pydantic silently dropped it
+from `POST /api/agents/launch`. The session was created, broadcast as "running", and then nothing
+was ever scheduled: a permanent silent spinner with 0 messages, indistinguishable from a real hang.
+Five sessions were forensically chased through pool, router, and scheduler before the launch body
+turned out to be the whole story.
+"""
+
+import asyncio
+
+from pytest import MonkeyPatch
+
+from backend.apps.agents import agents as agents_module
+from backend.apps.agents.core.models import AgentConfig, AgentSession
+
+
+def p_launch(monkeypatch: MonkeyPatch, config: AgentConfig) -> list:
+    sent: list = []
+    session = AgentSession(name="probe")
+
+    async def fake_launch(cfg: AgentConfig) -> AgentSession:
+        return session
+
+    async def fake_send(session_id: str, prompt: str, **kwargs) -> None:
+        sent.append((session_id, prompt))
+
+    monkeypatch.setattr(agents_module.agent_manager, "launch_agent", fake_launch)
+    monkeypatch.setattr(agents_module.agent_manager, "send_message", fake_send)
+
+    async def run() -> None:
+        await agents_module.launch_agent(config)
+        # The first turn is fire-and-forget; drain it before asserting.
+        await asyncio.sleep(0)
+        await asyncio.sleep(0)
+
+    asyncio.run(run())
+    return sent
+
+
+def test_launch_with_prompt_schedules_the_first_turn(monkeypatch: MonkeyPatch) -> None:
+    sent = p_launch(monkeypatch, AgentConfig(name="probe", prompt="say ready"))
+    assert len(sent) == 1
+    assert sent[0][1] == "say ready"
+
+
+def test_launch_without_prompt_schedules_nothing(monkeypatch: MonkeyPatch) -> None:
+    assert p_launch(monkeypatch, AgentConfig(name="probe")) == []
+
+
+def test_prompt_survives_the_launch_body_parse() -> None:
+    # The original failure: this field VANISHED in validation, so the route could never see it.
+    assert AgentConfig(**{"prompt": "hello", "name": "x"}).prompt == "hello"