diff --git a/apps/backend/internal/agent/hostutility/public.go b/apps/backend/internal/agent/hostutility/public.go
index 8a93cc5103..11ce5cc03b 100644
--- a/apps/backend/internal/agent/hostutility/public.go
+++ b/apps/backend/internal/agent/hostutility/public.go
@@ -6,6 +6,8 @@ import (
 	"fmt"
 	"time"
 
+	"go.uber.org/zap"
+
 	"github.com/kandev/kandev/internal/agent/agents"
 	"github.com/kandev/kandev/internal/agent/settings/cliflags"
 	agentctlutil "github.com/kandev/kandev/internal/agentctl/server/utility"
@@ -199,6 +201,11 @@ func (m *Manager) resolveModelConfigFlight(
 		if isAuthError(resp.Error) {
 			resolution.Status = StatusAuthRequired
 		}
+		m.log.Warn("model config resolution failed",
+			zap.String("agent_type", agentType),
+			zap.String("model", req.Model),
+			zap.String("status", string(resolution.Status)),
+			zap.String("error", resp.Error))
 		return resolution, nil
 	}
 
diff --git a/apps/backend/internal/agentctl/server/utility/acp_executor.go b/apps/backend/internal/agentctl/server/utility/acp_executor.go
index 9f8b80d2d2..7c8980443c 100644
--- a/apps/backend/internal/agentctl/server/utility/acp_executor.go
+++ b/apps/backend/internal/agentctl/server/utility/acp_executor.go
@@ -796,6 +796,20 @@ func applyProbeModel(
 		return nil, err
 	}
 	if !received {
+		if method == sessionmodel.MethodSetModel {
+			// The legacy session/set_model RPC applied the model, but the agent
+			// surfaces no per-model config options and pushes no follow-up
+			// config-update notification (e.g. auggie, which advertises a flat
+			// model list and answers session/set_model with an empty result).
+			// That is a valid empty resolution, not a failure: the caller keeps
+			// the session-advertised options.
+			return nil, nil
+		}
+		// A typed session/set_config_option that returns neither inline options
+		// nor a config-update notification leaves us without an authoritative
+		// snapshot for the newly selected model. Keeping the pre-switch
+		// session/new snapshot would report the previous model's options as the
+		// current configuration, so treat this as a failure.
 		return nil, fmt.Errorf("ACP model selection returned no configuration options")
 	}
 	return updated, nil
@@ -904,7 +918,9 @@ func (e *ACPInferenceExecutor) probeACPSessionWithContext(
 		if err != nil {
 			return nil, err
 		}
-		sessionResp.ConfigOptions = updated
+		if updated != nil {
+			sessionResp.ConfigOptions = updated
+		}
 	}
 	if updated, err := applyProbeConfigOptions(
 		ctx,
diff --git a/apps/backend/internal/agentctl/server/utility/acp_probe_capabilities_test.go b/apps/backend/internal/agentctl/server/utility/acp_probe_capabilities_test.go
index 7e031cc7e1..bd25f22182 100644
--- a/apps/backend/internal/agentctl/server/utility/acp_probe_capabilities_test.go
+++ b/apps/backend/internal/agentctl/server/utility/acp_probe_capabilities_test.go
@@ -174,7 +174,15 @@ func TestProbeACPSessionWithContextRejectsProviderWithoutModelSelection(t *testi
 	}
 }
 
-func TestProbeACPSessionWithContextTimesOutWithoutConfigSnapshot(t *testing.T) {
+// TestProbeACPSessionWithContextErrorsWhenConfigOptionYieldsNoSnapshot pins that
+// when an agent advertises a typed model config option (so the probe applies the
+// model via session/set_config_option) but returns neither inline options nor a
+// follow-up config-update notification, the probe fails. Keeping the pre-switch
+// session/new snapshot would report the previous model's options as the resolved
+// configuration for the newly selected model, so this must stay an error. The
+// empty-resolution relaxation is scoped to the legacy session/set_model path
+// (auggie); see TestApplyProbeModel_LegacyNoConfigOptionsReturnsEmpty.
+func TestProbeACPSessionWithContextErrorsWhenConfigOptionYieldsNoSnapshot(t *testing.T) {
 	c2aR, c2aW := io.Pipe()
 	a2cR, a2cW := io.Pipe()
 	t.Cleanup(func() {
diff --git a/apps/backend/internal/agentctl/server/utility/model_apply_test.go b/apps/backend/internal/agentctl/server/utility/model_apply_test.go
index 557c2b08b5..6e46d661a6 100644
--- a/apps/backend/internal/agentctl/server/utility/model_apply_test.go
+++ b/apps/backend/internal/agentctl/server/utility/model_apply_test.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"reflect"
 	"testing"
+	"time"
 
 	acp "github.com/coder/acp-go-sdk"
 )
@@ -169,6 +170,67 @@ func TestApplySessionModel_NoOpWhenAgentSupportsNeitherSurface(t *testing.T) {
 	}
 }
 
+// TestApplyProbeModel_LegacyNoConfigOptionsReturnsEmpty pins that when the
+// legacy session/set_model RPC succeeds but the agent surfaces no per-model
+// config options (and pushes no follow-up config-update notification), the
+// probe treats it as a valid empty resolution (nil options, nil error) rather
+// than a hard failure. This is the auggie shape: a flat model list with no
+// reasoning-effort style selectors to resolve.
+func TestApplyProbeModel_LegacyNoConfigOptionsReturnsEmpty(t *testing.T) {
+	t.Parallel()
+
+	conn := &fakeModelConn{}
+	state := newACPProbeNotificationState("auggie")
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	options, err := applyProbeModel(ctx, conn, acp.SessionId("sess-1"), "legacy-model", nil, state)
+	if err != nil {
+		t.Fatalf("applyProbeModel() error = %v, want nil", err)
+	}
+	if options != nil {
+		t.Fatalf("options = %#v, want nil (empty resolution)", options)
+	}
+	wantCalls := []string{"legacy:sess-1:legacy-model"}
+	if !reflect.DeepEqual(conn.calls, wantCalls) {
+		t.Fatalf("calls = %#v, want %#v", conn.calls, wantCalls)
+	}
+}
+
+// TestApplyProbeModel_ConfigOptionNoSnapshotReturnsError pins the other half of
+// the narrowing: when the agent advertises a typed model config option (so the
+// model is applied via session/set_config_option) but returns neither inline
+// options nor a config-update notification, applyProbeModel must fail rather than
+// keep the stale pre-switch snapshot. The empty-resolution relaxation is scoped
+// to the legacy session/set_model path only.
+func TestApplyProbeModel_ConfigOptionNoSnapshotReturnsError(t *testing.T) {
+	t.Parallel()
+
+	modelCat := acp.SessionConfigOptionCategoryModel
+	conn := &fakeModelConn{}
+	state := newACPProbeNotificationState("modern-agent")
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	options, err := applyProbeModel(ctx, conn, acp.SessionId("sess-1"), "model-with-effort", []acp.SessionConfigOption{
+		{Select: &acp.SessionConfigOptionSelect{
+			Id:       "model",
+			Category: &modelCat,
+			Type:     "select",
+		}},
+	}, state)
+	if err == nil {
+		t.Fatalf("applyProbeModel() error = nil, want no-configuration-options error")
+	}
+	if options != nil {
+		t.Fatalf("options = %#v, want nil on error", options)
+	}
+	wantCalls := []string{"config:model:model-with-effort"}
+	if !reflect.DeepEqual(conn.calls, wantCalls) {
+		t.Fatalf("calls = %#v, want %#v", conn.calls, wantCalls)
+	}
+}
+
 // fakeModelConnLegacyErr returns a configurable error from the legacy
 // session/set_model RPC. Used to exercise the -32601 no-op path.
 type fakeModelConnLegacyErr struct {