From c4a5a99b6f32f7982df83bb5187e3c5ded4cd853 Mon Sep 17 00:00:00 2001
From: Amolith <amolith@secluded.site>
Date: Sat, 3 Jan 2026 20:55:01 -0700
Subject: [PATCH] feat(acp): implement SetSessionModel

- Wire SetSessionModel stub to parse provider:model IDs, validate, and
update the agent's active model via config and coordinator
- Add buildSessionModelState helper to collect available models from all
configured providers
- Include Models in NewSessionResponse and LoadSessionResponse so
  clients
can display model selection UI

Assisted-by: Claude Sonnet 4 via Crush
---
 internal/acp/agent.go | 87 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 84 insertions(+), 3 deletions(-)

diff --git a/internal/acp/agent.go b/internal/acp/agent.go
index b0b3c6e13530401e4b6ad1f4617bf802d22ce5f4..f3687f0efb3cd7e74193a3ace5ce44bddc47d108 100644
--- a/internal/acp/agent.go
+++ b/internal/acp/agent.go
@@ -2,9 +2,12 @@ package acp
 
 import (
 	"context"
+	"fmt"
 	"log/slog"
+	"strings"
 
 	"github.com/charmbracelet/crush/internal/app"
+	"github.com/charmbracelet/crush/internal/config"
 	"github.com/charmbracelet/crush/internal/csync"
 	"github.com/charmbracelet/crush/internal/message"
 	"github.com/coder/acp-go-sdk"
@@ -81,6 +84,7 @@ func (a *Agent) NewSession(ctx context.Context, params acp.NewSessionRequest) (a
 
 	return acp.NewSessionResponse{
 		SessionId: acp.SessionId(sess.ID),
+		Models:    a.buildSessionModelState(),
 	}, nil
 }
 
@@ -112,7 +116,9 @@ func (a *Agent) LoadSession(ctx context.Context, params acp.LoadSessionRequest)
 		}
 	}
 
-	return acp.LoadSessionResponse{}, nil
+	return acp.LoadSessionResponse{
+		Models: a.buildSessionModelState(),
+	}, nil
 }
 
 // SetSessionMode handles mode switching (stub - Crush doesn't have modes yet).
@@ -121,9 +127,44 @@ func (a *Agent) SetSessionMode(ctx context.Context, params acp.SetSessionModeReq
 	return acp.SetSessionModeResponse{}, nil
 }
 
-// SetSessionModel handles model switching (stub - model selection not yet wired).
+// SetSessionModel handles model switching by parsing the model ID and updating
+// the agent's active model.
 func (a *Agent) SetSessionModel(ctx context.Context, params acp.SetSessionModelRequest) (acp.SetSessionModelResponse, error) {
-	slog.Debug("ACP SetSessionModel", "session_id", params.SessionId, "model_id", params.ModelId)
+	slog.Info("ACP SetSessionModel", "session_id", params.SessionId, "model_id", params.ModelId)
+
+	// Parse model ID (format: "provider:model").
+	parts := strings.SplitN(string(params.ModelId), ":", 2)
+	if len(parts) != 2 || parts[0] == "" || parts[1] == "" {
+		return acp.SetSessionModelResponse{}, fmt.Errorf("invalid model ID format %q: expected provider:model", params.ModelId)
+	}
+	providerID, modelID := parts[0], parts[1]
+
+	// Validate that the model exists.
+	cfg := config.Get()
+	if cfg.GetModel(providerID, modelID) == nil {
+		return acp.SetSessionModelResponse{}, fmt.Errorf("model %q not found for provider %q", modelID, providerID)
+	}
+
+	// Check if the agent is busy.
+	if a.app.AgentCoordinator.IsBusy() {
+		return acp.SetSessionModelResponse{}, fmt.Errorf("agent is busy, cannot switch models")
+	}
+
+	// Update the preferred model in config.
+	selectedModel := config.SelectedModel{
+		Provider: providerID,
+		Model:    modelID,
+	}
+	if err := cfg.UpdatePreferredModel(config.SelectedModelTypeLarge, selectedModel); err != nil {
+		return acp.SetSessionModelResponse{}, fmt.Errorf("failed to update preferred model: %w", err)
+	}
+
+	// Apply the model change to the agent.
+	if err := a.app.UpdateAgentModel(ctx); err != nil {
+		return acp.SetSessionModelResponse{}, fmt.Errorf("failed to apply model change: %w", err)
+	}
+
+	slog.Info("ACP SetSessionModel completed", "provider", providerID, "model", modelID)
 	return acp.SetSessionModelResponse{}, nil
 }
 
@@ -240,3 +281,43 @@ func (a *Agent) translateHistoryPart(role message.MessageRole, part message.Cont
 		return nil
 	}
 }
+
+// buildSessionModelState constructs the model state for session responses,
+// listing all available models and the currently selected one.
+func (a *Agent) buildSessionModelState() *acp.SessionModelState {
+	cfg := config.Get()
+	if cfg == nil {
+		return nil
+	}
+
+	var availableModels []acp.ModelInfo
+	for providerID, providerConfig := range cfg.Providers.Seq2() {
+		if providerConfig.Disable {
+			continue
+		}
+		providerName := providerConfig.Name
+		if providerName == "" {
+			providerName = providerID
+		}
+		for _, model := range providerConfig.Models {
+			modelID := acp.ModelId(providerID + ":" + model.ID)
+			modelName := model.Name
+			if modelName == "" {
+				modelName = model.ID
+			}
+			availableModels = append(availableModels, acp.ModelInfo{
+				ModelId: modelID,
+				Name:    providerName + " / " + modelName,
+			})
+		}
+	}
+
+	// Get current model.
+	currentModel := cfg.Models[config.SelectedModelTypeLarge]
+	currentModelID := acp.ModelId(currentModel.Provider + ":" + currentModel.Model)
+
+	return &acp.SessionModelState{
+		AvailableModels: availableModels,
+		CurrentModelId:  currentModelID,
+	}
+}