fix(agent): scope SOUL.md load to the agent's own profile home (#50233)

load_soul_md resolved the home ambiently, so a build thread that lost the
HERMES_HOME ContextVar read the launch profile's SOUL.md into another
profile's prompt — same class as the skills-index leak fixed in #86313.
load_soul_md and build_context_files_prompt now accept home_override, and
build_system_prompt_parts passes the agent's own home (from session_db)
at both SOUL call sites. Ambient behavior unchanged when no override.

agent/prompt_builder.py

@@ -2154,12 +2154,21 @@ def _truncate_content(
     return head + marker + tail
 
 
-def load_soul_md(context_length: Optional[int] = None) -> Optional[str]:
+def load_soul_md(
+    context_length: Optional[int] = None,
+    home_override: "Path | None" = None,
+) -> Optional[str]:
     """Load SOUL.md from HERMES_HOME and return its content, or None.
 
     Used as the agent identity (slot #1 in the system prompt).  When this
     returns content, ``build_context_files_prompt`` should be called with
     ``skip_soul=True`` so SOUL.md isn't injected twice.
+
+    ``home_override`` scopes the read to an explicit profile home (the agent
+    knows its own home from its session_db path). Without it, resolution is
+    ambient — which on a thread that lost the HERMES_HOME ContextVar falls
+    back to the launch home and reads the wrong profile's SOUL.md (#50233,
+    same class as the skills-index leak fixed in #86313).
     """
     try:
         from hermes_cli.config import ensure_hermes_home
@@ -2167,7 +2176,8 @@ def load_soul_md(context_length: Optional[int] = None) -> Optional[str]:
     except Exception as e:
         logger.debug("Could not ensure HERMES_HOME before loading SOUL.md: %s", e)
 
-    soul_path = get_hermes_home() / "SOUL.md"
+    _home = Path(home_override) if home_override is not None else get_hermes_home()
+    soul_path = _home / "SOUL.md"
     if not soul_path.exists():
         return None
     try:
@@ -2350,6 +2360,7 @@ def build_context_files_prompt(
     skip_soul: bool = False,
     context_length: Optional[int] = None,
     allow_install_tree_fallback: bool = False,
+    home_override: "Path | None" = None,
 ) -> str:
     """Discover and load context files for the system prompt.
 
@@ -2413,7 +2424,7 @@ def build_context_files_prompt(
 
     # SOUL.md from HERMES_HOME only — skip when already loaded as identity
     if not skip_soul:
-        soul_content = load_soul_md(context_length)
+        soul_content = load_soul_md(context_length, home_override=home_override)
         if soul_content:
             sections.append(soul_content)
 

agent/system_prompt.py

@@ -352,7 +352,10 @@ def build_system_prompt_parts(agent: Any, system_message: Optional[str] = None)
     # cwd project instructions disabled.
     _soul_loaded = False
     if agent.load_soul_identity or not agent.skip_context_files:
-        _soul_content = _r.load_soul_md(_ctx_len)
+        # Scope the SOUL.md read to the agent's OWN home (see _agent_home) —
+        # ambient resolution on a thread that lost the HERMES_HOME ContextVar
+        # reads the launch profile's SOUL.md instead (#50233).
+        _soul_content = _r.load_soul_md(_ctx_len, home_override=_agent_home(agent))
         if _soul_content:
             stable_parts.append(_soul_content)
             _soul_loaded = True
@@ -692,7 +695,8 @@ def build_system_prompt_parts(agent: Any, system_message: Optional[str] = None)
         context_files_prompt = _r.build_context_files_prompt(
             cwd=resolve_context_cwd(), skip_soul=_soul_loaded,
             context_length=_ctx_len,
-            allow_install_tree_fallback=agent.platform in ("cli", "tui"))
+            allow_install_tree_fallback=agent.platform in ("cli", "tui"),
+            home_override=_agent_home(agent))
         if context_files_prompt:
             context_parts.append(context_files_prompt)
 

tests/agent/test_soul_md_profile_isolation.py

@@ -0,0 +1,62 @@
+"""Regression (#50233, SOUL.md half): a profile agent's SOUL.md must load from
+ITS OWN home, never the ambient/launch home — even on a thread that did not
+bind the HERMES_HOME ContextVar. Same bug class as the skills-index leak
+fixed in #86313; load_soul_md now accepts home_override.
+"""
+
+import threading
+
+
+def test_soul_md_scoped_to_home_override_not_ambient(tmp_path, monkeypatch):
+    from agent import prompt_builder
+
+    default_home = tmp_path / "default"
+    default_home.mkdir()
+    (default_home / "SOUL.md").write_text(
+        "DEFAULT SOUL — must never leak into a profile prompt", encoding="utf-8"
+    )
+
+    bot_home = tmp_path / "profiles" / "mybot"
+    bot_home.mkdir(parents=True)
+    (bot_home / "SOUL.md").write_text("BOT SOUL", encoding="utf-8")
+
+    # Ambient home points at default (mimics a build thread that lost the
+    # profile's ContextVar override and fell back to launch).
+    monkeypatch.setenv("HERMES_HOME", str(default_home))
+
+    result = {}
+
+    def build():
+        result["soul"] = prompt_builder.load_soul_md(home_override=bot_home)
+
+    t = threading.Thread(target=build)
+    t.start()
+    t.join()
+
+    assert result["soul"] == "BOT SOUL"
+
+
+def test_soul_md_override_missing_file_returns_none(tmp_path, monkeypatch):
+    from agent import prompt_builder
+
+    default_home = tmp_path / "default"
+    default_home.mkdir()
+    (default_home / "SOUL.md").write_text("DEFAULT SOUL", encoding="utf-8")
+    monkeypatch.setenv("HERMES_HOME", str(default_home))
+
+    empty_bot = tmp_path / "profiles" / "emptybot"
+    empty_bot.mkdir(parents=True)
+
+    # No SOUL.md in the bot home -> None, NOT default's soul.
+    assert prompt_builder.load_soul_md(home_override=empty_bot) is None
+
+
+def test_soul_md_ambient_unchanged_without_override(tmp_path, monkeypatch):
+    from agent import prompt_builder
+
+    home = tmp_path / "home"
+    home.mkdir()
+    (home / "SOUL.md").write_text("AMBIENT SOUL", encoding="utf-8")
+    monkeypatch.setenv("HERMES_HOME", str(home))
+
+    assert prompt_builder.load_soul_md() == "AMBIENT SOUL"

tests/agent/test_system_prompt.py

@@ -35,7 +35,7 @@ def _captured_context_cwd(agent):
 
     def fake_context_files(
         cwd=None, skip_soul=False, context_length=None,
-        allow_install_tree_fallback=False,
+        allow_install_tree_fallback=False, home_override=None,
     ):
         captured["cwd"] = cwd
         return ""