fix(chat): resolve model switch lag by tracking model/provider key (#180)

* fix(chat): resolve model switch lag by tracking model/provider key for cache invalidation

* style: format set_chat_model function for improved readability

* fix(chat): improve model switching logic to prevent unnecessary cache rebuilds

* fix(model): ensure globals are restored on agent load failure to prevent model switch issues

EvoScientist/EvoScientist.py

@@ -46,6 +46,12 @@ SKILLS_DIR = str(Path(__file__).parent / "skills")
 
 _config = None
 _chat_model = None
+# Track the (model, provider) binding of _chat_model so cache invalidates
+# when config.model/provider change (e.g. via /model). Without this,
+# _ensure_chat_model() returns the stale cached instance even after
+# _ensure_config(new_cfg) has overwritten the active config — causing
+# /model switch to lag one step (see issue #179).
+_chat_model_key: tuple[str | None, str | None] | None = None
 
 # Cache MCP tools by the effective config signature to avoid reconnecting
 # to MCP servers on every `/new` when config is unchanged.
@@ -75,29 +81,58 @@ def _ensure_config(config=None):
     return _config
 
 
-def _ensure_chat_model():
-    """Return cached chat model, creating it on first call."""
-    global _chat_model
-    if _chat_model is None:
-        from .llm import get_chat_model
+def _replace_chat_model(instance, key: tuple[str | None, str | None]) -> None:
+    """Install a new chat model and propagate the related invariants.
 
-        cfg = _ensure_config()
-        _chat_model = get_chat_model(model=cfg.model, provider=cfg.provider)
+    Single write point for ``_chat_model`` / ``_chat_model_key`` /
+    ``_EvoScientist_agent``: both ``_ensure_chat_model`` (cache-miss
+    rebuild) and ``set_chat_model`` (explicit switch via ``/model``)
+    funnel through here so the three globals can never drift.
+    """
+    global _chat_model, _chat_model_key, _EvoScientist_agent
+    _chat_model = instance
+    _chat_model_key = key
+    # The lazy default agent captured a reference to the previous
+    # ``_chat_model`` at build time, so it must be rebuilt on next access.
+    _EvoScientist_agent = None
+
+
+def _ensure_chat_model():
+    """Return cached chat model, rebuilding if cfg.model/provider changed.
+
+    The cache key is the current config's ``(model, provider)``. If it
+    differs from the key that built ``_chat_model``, rebuild — this makes
+    ``create_cli_agent(config=temp_cfg)`` bind the freshly requested model
+    into the new agent without requiring callers to interleave
+    ``set_chat_model()`` calls in any particular order.
+    """
+    from .llm import get_chat_model
+
+    cfg = _ensure_config()
+    key = (cfg.model, cfg.provider)
+    if _chat_model is None or _chat_model_key != key:
+        _replace_chat_model(
+            get_chat_model(model=cfg.model, provider=cfg.provider),
+            key,
+        )
     return _chat_model
 
 
 def set_chat_model(model: str, provider: str | None = None):
     """Replace the cached chat model with a new one.
 
-    Called by ``/model`` to switch the LLM mid-session.
-    Returns the new chat model instance.
+    Called by ``/model`` to switch the LLM mid-session.  No-op when the
+    cache already holds the requested ``(model, provider)`` — avoids
+    spawning a second ``get_chat_model`` instance (and its HTTP client)
+    under the ``/model`` flow where ``_ensure_chat_model`` has already
+    rebuilt ``_chat_model`` during the preceding ``_load_agent`` call.
+    Returns the current chat model instance.
     """
-    global _chat_model, _EvoScientist_agent
     from .llm import get_chat_model
 
-    _chat_model = get_chat_model(model=model, provider=provider)
-    # Invalidate the cached default agent so it gets rebuilt with the new model.
-    _EvoScientist_agent = None
+    key = (model, provider)
+    if _chat_model is None or _chat_model_key != key:
+        _replace_chat_model(get_chat_model(model=model, provider=provider), key)
     return _chat_model
 
 
@@ -168,8 +203,8 @@ def _inject_subagent_middleware(subs: list[dict]) -> None:
     for sa in subs:
         sa.setdefault("middleware", []).extend(
             [
-                # Uses main agent's model for trigger — subagents currently
-                # share the same model, so context window matches.
+                # No ``model=`` — subagents share the main agent's model,
+                # so defer to the factory's ``_ensure_chat_model()`` fallback.
                 create_context_editing_middleware(),
                 ToolErrorHandlerMiddleware(),
                 ContextOverflowMapperMiddleware(),
@@ -326,7 +361,7 @@ def _get_default_middleware():
         create_context_editing_middleware(model),
         ContextOverflowMapperMiddleware(),
         ToolErrorHandlerMiddleware(),
-        *create_tool_selector_middleware(),
+        *create_tool_selector_middleware(model=model),
         create_memory_middleware(memory_dir, extraction_model=model),
     ]
 
@@ -460,7 +495,7 @@ def create_cli_agent(
         create_context_editing_middleware(model),
         ContextOverflowMapperMiddleware(),
         ToolErrorHandlerMiddleware(),
-        *create_tool_selector_middleware(),
+        *create_tool_selector_middleware(model=model),
         create_memory_middleware(_mem_dir, extraction_model=model),
     ]
     if cfg.enable_ask_user and not cfg.auto_mode:

EvoScientist/commands/implementation/model.py

@@ -111,6 +111,7 @@ class ModelCommand(Command):
     ) -> None:
         import copy
 
+        from ... import EvoScientist as _mod
         from ...cli.agent import _load_agent
         from ...EvoScientist import _ensure_config, set_chat_model
 
@@ -122,6 +123,22 @@ class ModelCommand(Command):
         temp_cfg.model = model_name
         temp_cfg.provider = provider
 
+        # create_cli_agent(config=temp_cfg) calls _ensure_config and
+        # _ensure_chat_model before finishing, so a failure further
+        # down (middleware build, MCP reconnect, deepagents wiring)
+        # would leave the session pointing at the new model. Snapshot
+        # the four globals those helpers write so we can restore on
+        # error. Best-effort: references already captured by concurrent
+        # readers (e.g. a channel thread mid-turn) are not retroactively
+        # patched, but /model is user-initiated from an idle prompt in
+        # practice.
+        snap = (
+            _mod._config,
+            _mod._chat_model,
+            _mod._chat_model_key,
+            _mod._EvoScientist_agent,
+        )
+
         try:
             new_agent = _load_agent(
                 workspace_dir=ctx.workspace_dir,
@@ -129,6 +146,12 @@ class ModelCommand(Command):
                 config=temp_cfg,
             )
         except Exception as e:
+            (
+                _mod._config,
+                _mod._chat_model,
+                _mod._chat_model_key,
+                _mod._EvoScientist_agent,
+            ) = snap
             ctx.ui.append_system(f"Failed to switch model: {e}", style="red")
             return
 

tests/test_model_command.py

@@ -262,6 +262,240 @@ class TestModelCommandFailure:
         assert call_args[1]["style"] == "red"
 
 
+@pytest.fixture
+def evo_module_state():
+    """Snapshot and restore ``EvoScientist.EvoScientist`` module globals.
+
+    The chat-model cache tests mutate ``_chat_model`` / ``_chat_model_key``
+    / ``_config`` / ``_EvoScientist_agent`` directly.  This fixture
+    guarantees all four are restored — even if a test body grows an early
+    return — so later tests in the suite see a clean module state.
+    """
+    import EvoScientist.EvoScientist as mod
+
+    snapshot = (
+        mod._chat_model,
+        mod._chat_model_key,
+        mod._config,
+        mod._EvoScientist_agent,
+    )
+    try:
+        yield mod
+    finally:
+        (
+            mod._chat_model,
+            mod._chat_model_key,
+            mod._config,
+            mod._EvoScientist_agent,
+        ) = snapshot
+
+
+class TestEnsureChatModelCacheInvalidation:
+    """Regression tests for issue #179: /model switch lagged by one step.
+
+    Root cause: ``_ensure_chat_model()`` returned a cached ``_chat_model``
+    without checking whether ``cfg.model`` / ``cfg.provider`` had changed
+    since the cache was populated. ``ModelCommand._apply_model`` builds
+    a new agent *before* ``set_chat_model`` runs, so the new agent was
+    bound to the *previous* cached model.
+
+    Fix: track a ``(model, provider)`` key alongside ``_chat_model`` and
+    rebuild on mismatch.
+    """
+
+    def test_cache_rebuilds_when_config_model_changes(self, evo_module_state):
+        """After cfg.model changes, _ensure_chat_model must return a new instance."""
+        mod = evo_module_state
+
+        cfg = SimpleNamespace(model="claude-sonnet-4-6", provider="anthropic")
+        m1 = MagicMock(name="model-1")
+        m2 = MagicMock(name="model-2")
+
+        mod._chat_model = None
+        mod._chat_model_key = None
+        mod._config = cfg
+        mod._EvoScientist_agent = None
+
+        with patch("EvoScientist.llm.get_chat_model", side_effect=[m1, m2]) as gm:
+            first = mod._ensure_chat_model()
+            assert first is m1
+            # Same config → cache hit, no rebuild.
+            again = mod._ensure_chat_model()
+            assert again is m1
+            assert gm.call_count == 1
+
+            # Simulate /model switch writing the new choice into cfg.
+            cfg.model = "minimax-m2.7"
+            cfg.provider = "openrouter"
+
+            second = mod._ensure_chat_model()
+            # Must be the NEW model instance, not the cached one.
+            assert second is m2
+            assert second is not first
+            assert gm.call_count == 2
+            # Second call used the new model name + provider.
+            _, kwargs = gm.call_args
+            assert kwargs == {"model": "minimax-m2.7", "provider": "openrouter"}
+
+    def test_cache_rebuilds_when_only_provider_changes(self, evo_module_state):
+        """Same model name, different provider (openrouter vs anthropic) must rebuild."""
+        mod = evo_module_state
+
+        cfg = SimpleNamespace(model="claude-sonnet-4-6", provider="anthropic")
+        m1 = MagicMock(name="anthropic-model")
+        m2 = MagicMock(name="openrouter-model")
+
+        mod._chat_model = None
+        mod._chat_model_key = None
+        mod._config = cfg
+        mod._EvoScientist_agent = None
+
+        with patch("EvoScientist.llm.get_chat_model", side_effect=[m1, m2]) as gm:
+            assert mod._ensure_chat_model() is m1
+            cfg.provider = "openrouter"
+            assert mod._ensure_chat_model() is m2
+            assert gm.call_count == 2
+
+    def test_set_chat_model_updates_key(self, evo_module_state):
+        """set_chat_model must keep _chat_model_key in sync to avoid
+        an extra rebuild on the very next _ensure_chat_model() call."""
+        mod = evo_module_state
+
+        cfg = SimpleNamespace(model="claude-sonnet-4-6", provider="anthropic")
+        m_set = MagicMock(name="explicit-set-model")
+
+        mod._chat_model = None
+        mod._chat_model_key = None
+        mod._config = cfg
+        mod._EvoScientist_agent = None
+
+        with patch("EvoScientist.llm.get_chat_model", return_value=m_set) as gm:
+            mod.set_chat_model("minimax-m2.7", provider="openrouter")
+            assert mod._chat_model is m_set
+            assert mod._chat_model_key == ("minimax-m2.7", "openrouter")
+
+            # Align cfg to what set_chat_model was called with.
+            cfg.model = "minimax-m2.7"
+            cfg.provider = "openrouter"
+            # Now _ensure_chat_model should NOT rebuild (key matches cfg).
+            assert mod._ensure_chat_model() is m_set
+            assert gm.call_count == 1
+
+    def test_set_chat_model_is_no_op_when_key_already_matches(self, evo_module_state):
+        """set_chat_model must NOT rebuild when the cache already holds the
+        requested (model, provider).
+
+        Under the /model flow, ``_load_agent`` already rebuilt ``_chat_model``
+        via ``_ensure_chat_model`` before ``set_chat_model`` is reached, so
+        the subsequent set should be idempotent — reusing the same Python
+        instance (and thus the same underlying HTTP client) that ``ctx.agent``
+        is already bound to.
+        """
+        mod = evo_module_state
+
+        existing = MagicMock(name="existing-model")
+        mod._chat_model = existing
+        mod._chat_model_key = ("minimax-m2.7", "openrouter")
+        mod._config = SimpleNamespace(model="minimax-m2.7", provider="openrouter")
+        mod._EvoScientist_agent = None
+
+        with patch("EvoScientist.llm.get_chat_model") as gm:
+            returned = mod.set_chat_model("minimax-m2.7", provider="openrouter")
+
+        # Fast path: returned the EXISTING instance, no rebuild.
+        assert returned is existing
+        assert mod._chat_model is existing
+        gm.assert_not_called()
+
+
+class TestApplyModelIntegration:
+    """End-to-end regression for #179: `_apply_model` must produce an agent
+    bound to the NEW model, not a stale cached one.
+
+    Exercises the real chain:
+        ``_apply_model → _load_agent → _ensure_config → _ensure_chat_model``
+
+    Only ``_load_agent`` is replaced by a minimal fake that mirrors the
+    exact two globals ``create_cli_agent`` mutates (``_ensure_config``
+    + ``_ensure_chat_model``) — so the bug path is fully exercised
+    without having to spin up deepagents, MCP tools, middleware, and
+    subagent YAML. ``get_chat_model`` returns a distinct sentinel per
+    ``(model, provider)`` pair so we can assert on identity.
+    """
+
+    def test_new_agent_is_bound_to_newly_selected_model(self, evo_module_state):
+        from EvoScientist.commands.implementation.model import ModelCommand
+        from EvoScientist.config.settings import EvoScientistConfig
+
+        mod = evo_module_state
+
+        sentinels: dict[tuple[str, str | None], MagicMock] = {}
+
+        def _fake_get_chat_model(model, provider=None):
+            key = (model, provider)
+            if key not in sentinels:
+                sentinels[key] = MagicMock(name=f"chat_model[{model}|{provider}]")
+                sentinels[key]._bound_model = model
+                sentinels[key]._bound_provider = provider
+            return sentinels[key]
+
+        def _fake_load_agent(
+            workspace_dir=None, checkpointer=None, config=None, *, on_mcp_progress=None
+        ):
+            # Replicates the two create_cli_agent side effects that
+            # reveal the bug: ``_ensure_config`` writes the new cfg,
+            # then ``_ensure_chat_model`` must rebuild to match it.
+            mod._ensure_config(config)
+            agent = MagicMock(name="fake-agent")
+            agent._bound_model = mod._ensure_chat_model()
+            return agent
+
+        cfg = EvoScientistConfig(model="claude-sonnet-4-6", provider="anthropic")
+        ctx = MagicMock()
+        ctx.ui = MagicMock()
+        ctx.ui.supports_interactive = True
+        ctx.workspace_dir = "/tmp/test_integration"
+        ctx.checkpointer = None
+
+        # Prime: _chat_model already holds the OLD (default) model —
+        # this is the state that caused the off-by-one in production.
+        mod._config = cfg
+        mod._chat_model = _fake_get_chat_model("claude-sonnet-4-6", "anthropic")
+        mod._chat_model_key = ("claude-sonnet-4-6", "anthropic")
+        mod._EvoScientist_agent = None
+        old_model = mod._chat_model
+
+        with (
+            patch(
+                "EvoScientist.llm.get_chat_model",
+                side_effect=_fake_get_chat_model,
+            ),
+            patch(
+                "EvoScientist.cli.agent._load_agent",
+                side_effect=_fake_load_agent,
+            ),
+        ):
+            cmd = ModelCommand()
+            _run(cmd._apply_model(ctx, "minimax-m2.7", "openrouter"))
+
+        # The agent produced by _apply_model must be bound to the
+        # NEWLY requested model, not the previously cached one.
+        assert ctx.agent._bound_model is not old_model
+        assert ctx.agent._bound_model._bound_model == "minimax-m2.7"
+        assert ctx.agent._bound_model._bound_provider == "openrouter"
+
+        # Global state reflects the switch end-to-end.
+        assert mod._chat_model_key == ("minimax-m2.7", "openrouter")
+        assert mod._chat_model is sentinels[("minimax-m2.7", "openrouter")]
+        assert cfg.model == "minimax-m2.7"
+        assert cfg.provider == "openrouter"
+
+        # User-visible success message.
+        msg = ctx.ui.append_system.call_args[0][0]
+        assert "minimax-m2.7" in msg
+        assert "openrouter" in msg
+
+
 class TestModelCommandLoadAgentFailure:
     """Verify the transactional ordering: when ``_load_agent`` raises,
     nothing downstream (``set_chat_model``, ``cfg`` mutation,
@@ -319,3 +553,83 @@ class TestModelCommandLoadAgentFailure:
         call_args = ui.append_system.call_args
         assert "Failed to switch model" in call_args[0][0]
         assert call_args[1]["style"] == "red"
+
+
+class TestApplyModelLoadAgentFailureTransactional:
+    """Regression: if ``create_cli_agent`` raises AFTER partially mutating
+    module globals via ``_ensure_config`` + ``_ensure_chat_model``,
+    ``_apply_model`` must roll those back so the session stays on the
+    original model.
+
+    Complements :class:`TestModelCommandLoadAgentFailure`, which tests
+    the early-failure path where ``_load_agent`` never reaches
+    ``create_cli_agent`` and no globals get mutated.
+    """
+
+    def test_globals_restored_after_create_cli_agent_partial_mutation(
+        self, evo_module_state
+    ):
+        from EvoScientist.commands.implementation.model import ModelCommand
+        from EvoScientist.config.settings import EvoScientistConfig
+
+        mod = evo_module_state
+        sentinels: dict[tuple[str, str | None], MagicMock] = {}
+
+        def _fake_get_chat_model(model, provider=None):
+            key = (model, provider)
+            sentinels.setdefault(key, MagicMock(name=f"chat_model[{model}|{provider}]"))
+            return sentinels[key]
+
+        def _fake_load_agent(
+            workspace_dir=None,
+            checkpointer=None,
+            config=None,
+            *,
+            on_mcp_progress=None,
+        ):
+            # Mimic ``create_cli_agent``: mutate globals via
+            # ``_ensure_config`` + ``_ensure_chat_model``, then raise
+            # (as if middleware construction or deepagents wiring failed).
+            mod._ensure_config(config)
+            mod._ensure_chat_model()
+            raise RuntimeError("middleware build failed")
+
+        cfg = EvoScientistConfig(model="claude-sonnet-4-6", provider="anthropic")
+        old_model = _fake_get_chat_model("claude-sonnet-4-6", "anthropic")
+        old_agent = MagicMock(name="old-default-agent")
+
+        mod._config = cfg
+        mod._chat_model = old_model
+        mod._chat_model_key = ("claude-sonnet-4-6", "anthropic")
+        mod._EvoScientist_agent = old_agent
+
+        ctx = MagicMock()
+        ctx.ui = MagicMock()
+        ctx.ui.supports_interactive = True
+        ctx.workspace_dir = "/tmp/test_rollback"
+        ctx.checkpointer = None
+
+        with (
+            patch(
+                "EvoScientist.llm.get_chat_model",
+                side_effect=_fake_get_chat_model,
+            ),
+            patch(
+                "EvoScientist.cli.agent._load_agent",
+                side_effect=_fake_load_agent,
+            ),
+        ):
+            cmd = ModelCommand()
+            _run(cmd._apply_model(ctx, "minimax-m2.7", "openrouter"))
+
+        # All four globals restored to their pre-call state.
+        assert mod._config is cfg
+        assert mod._chat_model is old_model
+        assert mod._chat_model_key == ("claude-sonnet-4-6", "anthropic")
+        assert mod._EvoScientist_agent is old_agent
+        # The ``cfg`` object itself was not mutated.
+        assert cfg.model == "claude-sonnet-4-6"
+        assert cfg.provider == "anthropic"
+        # User sees an error message.
+        msg = ctx.ui.append_system.call_args[0][0]
+        assert "Failed to switch model" in msg

tests/test_tool_selector_middleware.py

@@ -152,7 +152,7 @@ def test_tracker_captures_tools():
 
 @patch(
     "EvoScientist.middleware.create_tool_selector_middleware",
-    side_effect=lambda: _patched_create(),
+    side_effect=lambda *a, **kw: _patched_create(),
 )
 @patch("EvoScientist.EvoScientist._ensure_chat_model")
 @patch("EvoScientist.EvoScientist._ensure_config")
@@ -186,7 +186,7 @@ def test_subagent_no_tool_selector(mock_model):
 
 @patch(
     "EvoScientist.middleware.create_tool_selector_middleware",
-    side_effect=lambda: _patched_create(),
+    side_effect=lambda *a, **kw: _patched_create(),
 )
 @patch("EvoScientist.EvoScientist._ensure_chat_model")
 @patch("EvoScientist.EvoScientist._ensure_config")