diff --git a/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter.go b/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter.go
index 76fd53d0a0..a106f81a9e 100644
--- a/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter.go
+++ b/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter.go
@@ -39,6 +39,16 @@ const (
 	// configOptionIDModel is the well-known ConfigOption ID/Category value used
 	// by ACP agents to surface the active model as a selectable option.
 	configOptionIDModel = "model"
+
+	// Codex surfaces reasoning effort as a separate config option while its
+	// availableModels list uses model/effort IDs such as gpt-5.5/medium.
+	configOptionIDReasoningEffort    = "reasoning_effort"
+	configOptionCategoryThoughtLevel = "thought_level"
+
+	reasoningEffortLow    = "low"
+	reasoningEffortMedium = "medium"
+	reasoningEffortHigh   = "high"
+	reasoningEffortXHigh  = "xhigh"
 )
 
 // wakeupPromptTimeout bounds how long a synthetic wakeup prompt can run.
diff --git a/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter_session.go b/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter_session.go
index 1556a94036..f8432dcf05 100644
--- a/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter_session.go
+++ b/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter_session.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"errors"
 	"fmt"
+	"strings"
 
 	"github.com/coder/acp-go-sdk"
 	"github.com/kandev/kandev/internal/agentctl/server/adapter/transport/shared"
@@ -378,7 +379,7 @@ func (a *Adapter) emitSessionModels(sessionID string, models *acp.SessionModelSt
 	// CurrentModelId nor a configOption surface a value, emit empty and let
 	// the frontend fall through to its profile/snapshot resolution.
 	if currentModelID == "" {
-		currentModelID = resolveCurrentModelFromConfig(configOptions)
+		currentModelID = resolveCurrentModelFromConfig(configOptions, models.AvailableModels)
 	}
 
 	// Cache config options so emitSetModelEvent can include them in the
@@ -422,9 +423,19 @@ func (a *Adapter) emitSetModelEvent(sessionID, modelID string, cachedModels []ac
 		// deep copy to avoid aliasing the caller's backing array.
 		outConfig = make([]streams.ConfigOption, len(cachedConfig))
 		copy(outConfig, cachedConfig)
+		baseModelID, reasoningEffort, splitReasoningModel := splitReasoningModelID(modelID, outConfig)
 		for i := range outConfig {
 			if outConfig[i].ID == configOptionIDModel || outConfig[i].Category == configOptionIDModel {
-				outConfig[i].CurrentValue = modelID
+				if splitReasoningModel {
+					outConfig[i].CurrentValue = baseModelID
+				} else {
+					outConfig[i].CurrentValue = modelID
+				}
+			}
+			if splitReasoningModel &&
+				(outConfig[i].ID == configOptionIDReasoningEffort ||
+					outConfig[i].Category == configOptionCategoryThoughtLevel) {
+				outConfig[i].CurrentValue = reasoningEffort
 			}
 		}
 	}
@@ -443,13 +454,82 @@ func (a *Adapter) emitSetModelEvent(sessionID, modelID string, cachedModels []ac
 }
 
 // resolveCurrentModelFromConfig extracts current model ID from configOptions.
-func resolveCurrentModelFromConfig(options []streams.ConfigOption) string {
+func resolveCurrentModelFromConfig(options []streams.ConfigOption, available []acp.ModelInfo) string {
+	modelID := ""
+	reasoningEffort := ""
 	for _, opt := range options {
 		if opt.ID == configOptionIDModel || opt.Category == configOptionIDModel {
-			return opt.CurrentValue
+			modelID = opt.CurrentValue
+		}
+		if opt.ID == configOptionIDReasoningEffort || opt.Category == configOptionCategoryThoughtLevel {
+			reasoningEffort = opt.CurrentValue
 		}
 	}
-	return ""
+	if modelID == "" {
+		return ""
+	}
+	if reasoningEffort != "" {
+		combined := modelID + "/" + reasoningEffort
+		if modelIDExists(combined, available) {
+			return combined
+		}
+	}
+	if modelIDExists(modelID, available) {
+		return modelID
+	}
+	// Keep the agent-reported config value as a best-effort fallback for
+	// providers that expose the current model only through configOptions.
+	return modelID
+}
+
+func splitReasoningModelID(modelID string, options []streams.ConfigOption) (string, string, bool) {
+	allowedReasoningEfforts := map[string]bool{}
+	hasReasoningOption := false
+	for _, opt := range options {
+		if opt.ID == configOptionIDReasoningEffort || opt.Category == configOptionCategoryThoughtLevel {
+			hasReasoningOption = true
+			for _, optionValue := range opt.Options {
+				if optionValue.Value != "" {
+					allowedReasoningEfforts[optionValue.Value] = true
+				}
+			}
+		}
+	}
+	if hasReasoningOption && len(allowedReasoningEfforts) == 0 {
+		for _, effort := range []string{
+			reasoningEffortLow,
+			reasoningEffortMedium,
+			reasoningEffortHigh,
+			reasoningEffortXHigh,
+		} {
+			allowedReasoningEfforts[effort] = true
+		}
+	}
+	if len(allowedReasoningEfforts) == 0 {
+		return "", "", false
+	}
+	slashIndex := strings.LastIndex(modelID, "/")
+	if slashIndex < 1 || slashIndex == len(modelID)-1 {
+		return "", "", false
+	}
+	baseModelID := modelID[:slashIndex]
+	reasoningEffort := modelID[slashIndex+1:]
+	if !allowedReasoningEfforts[reasoningEffort] {
+		return "", "", false
+	}
+	return baseModelID, reasoningEffort, true
+}
+
+func modelIDExists(modelID string, available []acp.ModelInfo) bool {
+	if len(available) == 0 {
+		return false
+	}
+	for _, model := range available {
+		if string(model.ModelId) == modelID {
+			return true
+		}
+	}
+	return false
 }
 
 // SetMode changes the agent's session mode via ACP session/set_mode.
diff --git a/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter_updates.go b/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter_updates.go
index fc56649064..456f918dd2 100644
--- a/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter_updates.go
+++ b/apps/backend/internal/agentctl/server/adapter/transport/acp/adapter_updates.go
@@ -230,7 +230,7 @@ func (a *Adapter) convertNotification(n acp.SessionNotification) *AgentEvent {
 			cachedModels := a.availableModels
 			a.availableConfigOptions = configOptions
 			a.mu.Unlock()
-			currentModelID := resolveCurrentModelFromConfig(configOptions)
+			currentModelID := resolveCurrentModelFromConfig(configOptions, cachedModels)
 			return &AgentEvent{
 				Type:           streams.EventTypeSessionModels,
 				SessionID:      sessionID,
diff --git a/apps/backend/internal/agentctl/server/adapter/transport/acp/session_models_test.go b/apps/backend/internal/agentctl/server/adapter/transport/acp/session_models_test.go
index ade5735c36..1a1081ab4e 100644
--- a/apps/backend/internal/agentctl/server/adapter/transport/acp/session_models_test.go
+++ b/apps/backend/internal/agentctl/server/adapter/transport/acp/session_models_test.go
@@ -73,6 +73,123 @@ func TestEmitSessionModels_EmptyCurrentIDFromConfigOption(t *testing.T) {
 	}
 }
 
+// TestEmitSessionModels_EmptyCurrentIDComposesReasoningEffort pins Codex's
+// split config-option shape: configOptions reports model="gpt-5.5" and
+// reasoning_effort="medium", while availableModels carries the actual
+// selectable ID "gpt-5.5/medium".
+func TestEmitSessionModels_EmptyCurrentIDComposesReasoningEffort(t *testing.T) {
+	a := newTestAdapter()
+	models := &acp.SessionModelState{
+		CurrentModelId: "",
+		AvailableModels: []acp.ModelInfo{
+			{ModelId: "gpt-5.5/low", Name: "GPT-5.5 (low)"},
+			{ModelId: "gpt-5.5/medium", Name: "GPT-5.5 (medium)"},
+		},
+	}
+	meta := map[string]any{
+		"configOptions": []any{
+			map[string]any{
+				"type":         "select",
+				"id":           "model",
+				"name":         "Model",
+				"category":     "model",
+				"currentValue": "gpt-5.5",
+			},
+			map[string]any{
+				"type":         "select",
+				"id":           "reasoning_effort",
+				"name":         "Reasoning Effort",
+				"category":     "thought_level",
+				"currentValue": "medium",
+			},
+		},
+	}
+
+	a.emitSessionModels("sess-1", models, meta, nil)
+
+	ev := findSessionModelsEvent(t, drainEvents(a))
+	if ev.CurrentModelID != "gpt-5.5/medium" {
+		t.Errorf("CurrentModelID = %q, want %q", ev.CurrentModelID, "gpt-5.5/medium")
+	}
+}
+
+func TestEmitSessionModels_EmptyCurrentIDComposesReasoningEffortFromTypedOptions(t *testing.T) {
+	a := newTestAdapter()
+	modelCategory := acp.SessionConfigOptionCategoryModel
+	thoughtCategory := acp.SessionConfigOptionCategoryThoughtLevel
+	modelOptions := acp.SessionConfigSelectOptionsUngrouped{
+		{Name: "GPT-5.5", Value: "gpt-5.5"},
+	}
+	effortOptions := acp.SessionConfigSelectOptionsUngrouped{
+		{Name: "Low", Value: reasoningEffortLow},
+		{Name: "Medium", Value: reasoningEffortMedium},
+	}
+	models := &acp.SessionModelState{
+		CurrentModelId: "",
+		AvailableModels: []acp.ModelInfo{
+			{ModelId: "gpt-5.5/low", Name: "GPT-5.5 (low)"},
+			{ModelId: "gpt-5.5/medium", Name: "GPT-5.5 (medium)"},
+		},
+	}
+	configOptions := []acp.SessionConfigOption{
+		{Select: &acp.SessionConfigOptionSelect{
+			Type:         "select",
+			Id:           "model",
+			Name:         "Model",
+			Category:     &modelCategory,
+			CurrentValue: "gpt-5.5",
+			Options:      acp.SessionConfigSelectOptions{Ungrouped: &modelOptions},
+		}},
+		{Select: &acp.SessionConfigOptionSelect{
+			Type:         "select",
+			Id:           "reasoning_effort",
+			Name:         "Reasoning Effort",
+			Category:     &thoughtCategory,
+			CurrentValue: reasoningEffortMedium,
+			Options:      acp.SessionConfigSelectOptions{Ungrouped: &effortOptions},
+		}},
+	}
+
+	a.emitSessionModels("sess-1", models, nil, configOptions)
+
+	ev := findSessionModelsEvent(t, drainEvents(a))
+	if ev.CurrentModelID != "gpt-5.5/medium" {
+		t.Errorf("CurrentModelID = %q, want %q", ev.CurrentModelID, "gpt-5.5/medium")
+	}
+}
+
+func TestResolveCurrentModelFromConfig_ComposesReasoningEffort(t *testing.T) {
+	options := []streams.ConfigOption{
+		{Type: "select", ID: "model", Category: "model", CurrentValue: "gpt-5.5"},
+		{Type: "select", ID: "reasoning_effort", Category: "thought_level", CurrentValue: reasoningEffortMedium},
+	}
+	available := []acp.ModelInfo{
+		{ModelId: "gpt-5.5/low", Name: "GPT-5.5 (low)"},
+		{ModelId: "gpt-5.5/medium", Name: "GPT-5.5 (medium)"},
+	}
+
+	got := resolveCurrentModelFromConfig(options, available)
+	if got != "gpt-5.5/medium" {
+		t.Errorf("resolveCurrentModelFromConfig() = %q, want %q", got, "gpt-5.5/medium")
+	}
+}
+
+func TestResolveCurrentModelFromConfig_PrefersReasoningModelWhenBaseAlsoAvailable(t *testing.T) {
+	options := []streams.ConfigOption{
+		{Type: "select", ID: "model", Category: "model", CurrentValue: "gpt-5.5"},
+		{Type: "select", ID: "reasoning_effort", Category: "thought_level", CurrentValue: reasoningEffortMedium},
+	}
+	available := []acp.ModelInfo{
+		{ModelId: "gpt-5.5", Name: "GPT-5.5"},
+		{ModelId: "gpt-5.5/medium", Name: "GPT-5.5 (medium)"},
+	}
+
+	got := resolveCurrentModelFromConfig(options, available)
+	if got != "gpt-5.5/medium" {
+		t.Errorf("resolveCurrentModelFromConfig() = %q, want %q", got, "gpt-5.5/medium")
+	}
+}
+
 // TestEmitSessionModels_NonEmptyCurrentIDPreserved checks the happy path:
 // when the agent populates CurrentModelId, we propagate it verbatim.
 func TestEmitSessionModels_NonEmptyCurrentIDPreserved(t *testing.T) {
@@ -149,6 +266,150 @@ func TestEmitSetModelEvent_EmitsSessionModelsWithCachedState(t *testing.T) {
 	}
 }
 
+func TestEmitSetModelEvent_RewritesSplitReasoningOptions(t *testing.T) {
+	a := newTestAdapter()
+
+	reasoningOptions := []streams.ConfigOptionValue{
+		{Name: "Medium", Value: reasoningEffortMedium},
+		{Name: "High", Value: reasoningEffortHigh},
+	}
+	cachedModels := []acp.ModelInfo{
+		{ModelId: "gpt-5.5/medium", Name: "GPT-5.5 (medium)"},
+		{ModelId: "gpt-5.5/high", Name: "GPT-5.5 (high)"},
+	}
+	cachedConfig := []streams.ConfigOption{
+		{Type: "select", ID: "model", Category: "model", Name: "Model", CurrentValue: "gpt-5.5"},
+		{
+			Type:         "select",
+			ID:           "reasoning_effort",
+			Category:     "thought_level",
+			Name:         "Reasoning Effort",
+			CurrentValue: reasoningEffortMedium,
+			Options:      reasoningOptions,
+		},
+	}
+
+	a.emitSetModelEvent("sess-1", "gpt-5.5/high", cachedModels, cachedConfig)
+
+	ev := findSessionModelsEvent(t, drainEvents(a))
+	if ev.CurrentModelID != "gpt-5.5/high" {
+		t.Errorf("CurrentModelID = %q, want %q", ev.CurrentModelID, "gpt-5.5/high")
+	}
+	if len(ev.ConfigOptions) != 2 {
+		t.Fatalf("ConfigOptions len = %d, want 2", len(ev.ConfigOptions))
+	}
+	for _, opt := range ev.ConfigOptions {
+		switch opt.ID {
+		case "model":
+			if opt.CurrentValue != "gpt-5.5" {
+				t.Errorf("model option CurrentValue = %q, want %q", opt.CurrentValue, "gpt-5.5")
+			}
+		case "reasoning_effort":
+			if opt.CurrentValue != reasoningEffortHigh {
+				t.Errorf("reasoning option CurrentValue = %q, want %q", opt.CurrentValue, reasoningEffortHigh)
+			}
+		default:
+			t.Errorf("unexpected option ID %q in ConfigOptions", opt.ID)
+		}
+	}
+}
+
+func TestEmitSetModelEvent_RewritesSplitReasoningOptionsWithSlashInBaseModel(t *testing.T) {
+	a := newTestAdapter()
+
+	reasoningOptions := []streams.ConfigOptionValue{
+		{Name: "Medium", Value: reasoningEffortMedium},
+		{Name: "High", Value: reasoningEffortHigh},
+	}
+	cachedModels := []acp.ModelInfo{
+		{ModelId: "vendor/gpt-5.5/medium", Name: "Vendor GPT-5.5 (medium)"},
+		{ModelId: "vendor/gpt-5.5/high", Name: "Vendor GPT-5.5 (high)"},
+	}
+	cachedConfig := []streams.ConfigOption{
+		{Type: "select", ID: "model", Category: "model", Name: "Model", CurrentValue: "vendor/gpt-5.5"},
+		{
+			Type:         "select",
+			ID:           "reasoning_effort",
+			Category:     "thought_level",
+			Name:         "Reasoning Effort",
+			CurrentValue: reasoningEffortMedium,
+			Options:      reasoningOptions,
+		},
+	}
+
+	a.emitSetModelEvent("sess-1", "vendor/gpt-5.5/high", cachedModels, cachedConfig)
+
+	ev := findSessionModelsEvent(t, drainEvents(a))
+	if ev.CurrentModelID != "vendor/gpt-5.5/high" {
+		t.Errorf("CurrentModelID = %q, want %q", ev.CurrentModelID, "vendor/gpt-5.5/high")
+	}
+	if len(ev.ConfigOptions) != 2 {
+		t.Fatalf("ConfigOptions len = %d, want 2", len(ev.ConfigOptions))
+	}
+	for _, opt := range ev.ConfigOptions {
+		switch opt.ID {
+		case "model":
+			if opt.CurrentValue != "vendor/gpt-5.5" {
+				t.Errorf("model option CurrentValue = %q, want %q", opt.CurrentValue, "vendor/gpt-5.5")
+			}
+		case "reasoning_effort":
+			if opt.CurrentValue != reasoningEffortHigh {
+				t.Errorf("reasoning option CurrentValue = %q, want %q", opt.CurrentValue, reasoningEffortHigh)
+			}
+		default:
+			t.Errorf("unexpected option ID %q in ConfigOptions", opt.ID)
+		}
+	}
+}
+
+func TestEmitSetModelEvent_DoesNotSplitSlashModelWithoutReasoningSuffix(t *testing.T) {
+	a := newTestAdapter()
+
+	reasoningOptions := []streams.ConfigOptionValue{
+		{Name: "Low", Value: reasoningEffortLow},
+		{Name: "Medium", Value: reasoningEffortMedium},
+		{Name: "High", Value: reasoningEffortHigh},
+	}
+	cachedModels := []acp.ModelInfo{
+		{ModelId: "vendor/gpt-5.5", Name: "Vendor GPT-5.5"},
+	}
+	cachedConfig := []streams.ConfigOption{
+		{Type: "select", ID: "model", Category: "model", Name: "Model", CurrentValue: "old-model"},
+		{
+			Type:         "select",
+			ID:           "reasoning_effort",
+			Category:     "thought_level",
+			Name:         "Reasoning Effort",
+			CurrentValue: reasoningEffortMedium,
+			Options:      reasoningOptions,
+		},
+	}
+
+	a.emitSetModelEvent("sess-1", "vendor/gpt-5.5", cachedModels, cachedConfig)
+
+	ev := findSessionModelsEvent(t, drainEvents(a))
+	if ev.CurrentModelID != "vendor/gpt-5.5" {
+		t.Errorf("CurrentModelID = %q, want %q", ev.CurrentModelID, "vendor/gpt-5.5")
+	}
+	if len(ev.ConfigOptions) != 2 {
+		t.Fatalf("ConfigOptions len = %d, want 2", len(ev.ConfigOptions))
+	}
+	for _, opt := range ev.ConfigOptions {
+		switch opt.ID {
+		case "model":
+			if opt.CurrentValue != "vendor/gpt-5.5" {
+				t.Errorf("model option CurrentValue = %q, want %q", opt.CurrentValue, "vendor/gpt-5.5")
+			}
+		case "reasoning_effort":
+			if opt.CurrentValue != reasoningEffortMedium {
+				t.Errorf("reasoning option CurrentValue = %q, want %q", opt.CurrentValue, reasoningEffortMedium)
+			}
+		default:
+			t.Errorf("unexpected option ID %q in ConfigOptions", opt.ID)
+		}
+	}
+}
+
 // TestConfigOptionUpdate_RefreshesCachedConfig pins that an inbound
 // ConfigOptionUpdate notification refreshes the adapter's availableConfigOptions
 // cache, so a subsequent SetModel convergence event emits the latest options
@@ -194,3 +455,55 @@ func TestConfigOptionUpdate_RefreshesCachedConfig(t *testing.T) {
 		t.Errorf("availableConfigOptions = %+v, want one option with CurrentValue=fresh", got)
 	}
 }
+
+func TestConfigOptionUpdate_ComposesReasoningEffortCurrentModel(t *testing.T) {
+	a := newTestAdapter()
+	thoughtCategory := acp.SessionConfigOptionCategoryThoughtLevel
+	modelOptions := acp.SessionConfigSelectOptionsUngrouped{
+		{Name: "GPT-5.5", Value: "gpt-5.5"},
+	}
+	effortOptions := acp.SessionConfigSelectOptionsUngrouped{
+		{Name: "Medium", Value: reasoningEffortMedium},
+		{Name: "High", Value: reasoningEffortHigh},
+	}
+
+	a.mu.Lock()
+	a.availableModels = []acp.ModelInfo{
+		{ModelId: "gpt-5.5/medium", Name: "GPT-5.5 (medium)"},
+		{ModelId: "gpt-5.5/high", Name: "GPT-5.5 (high)"},
+	}
+	a.mu.Unlock()
+
+	notif := acp.SessionNotification{
+		SessionId: "sess-1",
+		Update: acp.SessionUpdate{
+			ConfigOptionUpdate: &acp.SessionConfigOptionUpdate{
+				ConfigOptions: []acp.SessionConfigOption{
+					{Select: &acp.SessionConfigOptionSelect{
+						Type:         "select",
+						Id:           "model",
+						Name:         "Model",
+						CurrentValue: "gpt-5.5",
+						Options:      acp.SessionConfigSelectOptions{Ungrouped: &modelOptions},
+					}},
+					{Select: &acp.SessionConfigOptionSelect{
+						Type:         "select",
+						Id:           "reasoning_effort",
+						Name:         "Reasoning Effort",
+						Category:     &thoughtCategory,
+						CurrentValue: reasoningEffortHigh,
+						Options:      acp.SessionConfigSelectOptions{Ungrouped: &effortOptions},
+					}},
+				},
+			},
+		},
+	}
+
+	ev := a.convertNotification(notif)
+	if ev == nil {
+		t.Fatalf("expected a session_models event from ConfigOptionUpdate")
+	}
+	if ev.CurrentModelID != "gpt-5.5/high" {
+		t.Errorf("event CurrentModelID = %q, want %q", ev.CurrentModelID, "gpt-5.5/high")
+	}
+}