diff --git a/pkg/agent/claude.go b/pkg/agent/claude.go
index 14d6488..ea49a4b 100644
--- a/pkg/agent/claude.go
+++ b/pkg/agent/claude.go
@@ -25,6 +25,7 @@ import (
 
 	workspace "github.com/openkaiden/kdn-api/workspace-configuration/go"
 	kdnconfig "github.com/openkaiden/kdn/pkg/config"
+	"github.com/openkaiden/kdn/pkg/containerurl"
 )
 
 const (
@@ -217,9 +218,12 @@ func (c *claudeAgent) SetMCPServers(settings map[string]SettingsFile, mcp *works
 }
 
 // SetModel configures the model ID in Claude settings.
-// It sets the model field in .claude/settings.json.
-// All other fields in the settings file are preserved.
-func (c *claudeAgent) SetModel(settings map[string]SettingsFile, modelID string, _ string) (map[string]SettingsFile, error) {
+// It sets the model field in .claude/settings.json. When the model ID
+// includes a base URL (provider::model::baseURL), it also sets
+// ANTHROPIC_BASE_URL in the env block so Claude Code routes requests
+// to the custom endpoint. Localhost URLs are rewritten using
+// containerHost so they are reachable from inside the runtime.
+func (c *claudeAgent) SetModel(settings map[string]SettingsFile, modelID string, containerHost string) (map[string]SettingsFile, error) {
 	settings = EnsureSettings(settings)
 	existingContent := GetContent(settings, ClaudeSettingsPath, []byte("{}"))
 
@@ -228,9 +232,22 @@ func (c *claudeAgent) SetModel(settings map[string]SettingsFile, modelID string,
 		return nil, fmt.Errorf("failed to parse existing %s: %w", ClaudeSettingsPath, err)
 	}
 
-	_, modelName, _ := kdnconfig.ParseModelID(modelID)
+	_, modelName, baseURL := kdnconfig.ParseModelID(modelID)
 	config["model"] = modelName
 
+	if baseURL != "" {
+		resolvedURL := containerurl.RewriteURLWithHost(baseURL, containerHost)
+
+		env, _ := config["env"].(map[string]interface{})
+		if env == nil {
+			env = make(map[string]interface{})
+		}
+		env["ANTHROPIC_BASE_URL"] = resolvedURL
+		config["env"] = env
+	} else if env, ok := config["env"].(map[string]interface{}); ok {
+		delete(env, "ANTHROPIC_BASE_URL")
+	}
+
 	modifiedContent, err := json.MarshalIndent(config, "", "  ")
 	if err != nil {
 		return nil, fmt.Errorf("failed to marshal modified %s: %w", ClaudeSettingsPath, err)
diff --git a/pkg/agent/claude_test.go b/pkg/agent/claude_test.go
index b01cbf4..2bd87c7 100644
--- a/pkg/agent/claude_test.go
+++ b/pkg/agent/claude_test.go
@@ -537,6 +537,164 @@ func TestClaude_SetModel_ProviderModelURLFormat(t *testing.T) {
 	if model, ok := config["model"].(string); !ok || model != "gemma2:7b" {
 		t.Errorf("model = %v, want %q", config["model"], "gemma2:7b")
 	}
+
+	env, ok := config["env"].(map[string]interface{})
+	if !ok {
+		t.Fatalf("env is not a map: %v", config["env"])
+	}
+	expectedURL := "http://" + containerurl.ContainerHost + ":11434/v1"
+	if env["ANTHROPIC_BASE_URL"] != expectedURL {
+		t.Errorf("ANTHROPIC_BASE_URL = %v, want %q", env["ANTHROPIC_BASE_URL"], expectedURL)
+	}
+}
+
+func TestClaude_SetModel_BaseURLRewritesLocalhost(t *testing.T) {
+	t.Parallel()
+
+	agent := NewClaude()
+	settings := make(map[string]SettingsFile)
+
+	result, err := agent.SetModel(settings, "anthropic::claude-sonnet-4-20250514::http://127.0.0.1:8080", containerurl.ContainerHost)
+	if err != nil {
+		t.Fatalf("SetModel() error = %v", err)
+	}
+
+	var config map[string]interface{}
+	if err := json.Unmarshal(result[ClaudeSettingsPath].Content, &config); err != nil {
+		t.Fatalf("Failed to parse result JSON: %v", err)
+	}
+
+	env := config["env"].(map[string]interface{})
+	expectedURL := "http://" + containerurl.ContainerHost + ":8080"
+	if env["ANTHROPIC_BASE_URL"] != expectedURL {
+		t.Errorf("ANTHROPIC_BASE_URL = %v, want %q", env["ANTHROPIC_BASE_URL"], expectedURL)
+	}
+}
+
+func TestClaude_SetModel_BaseURLPreservesRemoteHost(t *testing.T) {
+	t.Parallel()
+
+	agent := NewClaude()
+	settings := make(map[string]SettingsFile)
+
+	result, err := agent.SetModel(settings, "anthropic::claude-sonnet-4-20250514::https://proxy.example.com/anthropic", containerurl.ContainerHost)
+	if err != nil {
+		t.Fatalf("SetModel() error = %v", err)
+	}
+
+	var config map[string]interface{}
+	if err := json.Unmarshal(result[ClaudeSettingsPath].Content, &config); err != nil {
+		t.Fatalf("Failed to parse result JSON: %v", err)
+	}
+
+	env := config["env"].(map[string]interface{})
+	if env["ANTHROPIC_BASE_URL"] != "https://proxy.example.com/anthropic" {
+		t.Errorf("ANTHROPIC_BASE_URL = %v, want %q", env["ANTHROPIC_BASE_URL"], "https://proxy.example.com/anthropic")
+	}
+}
+
+func TestClaude_SetModel_NoBaseURL_NoEnvBlock(t *testing.T) {
+	t.Parallel()
+
+	agent := NewClaude()
+	settings := make(map[string]SettingsFile)
+
+	result, err := agent.SetModel(settings, "anthropic::claude-sonnet-4-20250514", containerurl.ContainerHost)
+	if err != nil {
+		t.Fatalf("SetModel() error = %v", err)
+	}
+
+	var config map[string]interface{}
+	if err := json.Unmarshal(result[ClaudeSettingsPath].Content, &config); err != nil {
+		t.Fatalf("Failed to parse result JSON: %v", err)
+	}
+
+	if _, ok := config["env"]; ok {
+		t.Errorf("env block should not be present when no baseURL is specified, got: %v", config["env"])
+	}
+}
+
+func TestClaude_SetModel_NoBaseURL_ClearsStaleBaseURL(t *testing.T) {
+	t.Parallel()
+
+	agent := NewClaude()
+
+	existingSettings := map[string]interface{}{
+		"model": "old-model",
+		"env": map[string]interface{}{
+			"ANTHROPIC_BASE_URL": "http://stale-endpoint:8080",
+			"CUSTOM_VAR":         "keep-me",
+		},
+	}
+	existingJSON, err := json.Marshal(existingSettings)
+	if err != nil {
+		t.Fatalf("Failed to marshal existing settings: %v", err)
+	}
+
+	settings := map[string]SettingsFile{
+		ClaudeSettingsPath: {Content: existingJSON},
+	}
+
+	result, err := agent.SetModel(settings, "anthropic::claude-sonnet-4-20250514", containerurl.ContainerHost)
+	if err != nil {
+		t.Fatalf("SetModel() error = %v", err)
+	}
+
+	var config map[string]interface{}
+	if err := json.Unmarshal(result[ClaudeSettingsPath].Content, &config); err != nil {
+		t.Fatalf("Failed to parse result JSON: %v", err)
+	}
+
+	env, ok := config["env"].(map[string]interface{})
+	if !ok {
+		t.Fatalf("env block should still exist (has other vars): %v", config["env"])
+	}
+	if _, exists := env["ANTHROPIC_BASE_URL"]; exists {
+		t.Errorf("ANTHROPIC_BASE_URL should have been removed, got: %v", env["ANTHROPIC_BASE_URL"])
+	}
+	if env["CUSTOM_VAR"] != "keep-me" {
+		t.Errorf("CUSTOM_VAR = %v, want %q (other env vars should be preserved)", env["CUSTOM_VAR"], "keep-me")
+	}
+}
+
+func TestClaude_SetModel_BaseURLPreservesExistingEnv(t *testing.T) {
+	t.Parallel()
+
+	agent := NewClaude()
+
+	existingSettings := map[string]interface{}{
+		"model": "old-model",
+		"env": map[string]interface{}{
+			"CUSTOM_VAR": "custom-value",
+		},
+	}
+	existingJSON, err := json.Marshal(existingSettings)
+	if err != nil {
+		t.Fatalf("Failed to marshal existing settings: %v", err)
+	}
+
+	settings := map[string]SettingsFile{
+		ClaudeSettingsPath: {Content: existingJSON},
+	}
+
+	result, err := agent.SetModel(settings, "anthropic::new-model::http://localhost:8080", containerurl.ContainerHost)
+	if err != nil {
+		t.Fatalf("SetModel() error = %v", err)
+	}
+
+	var config map[string]interface{}
+	if err := json.Unmarshal(result[ClaudeSettingsPath].Content, &config); err != nil {
+		t.Fatalf("Failed to parse result JSON: %v", err)
+	}
+
+	env := config["env"].(map[string]interface{})
+	if env["CUSTOM_VAR"] != "custom-value" {
+		t.Errorf("CUSTOM_VAR = %v, want %q (existing env should be preserved)", env["CUSTOM_VAR"], "custom-value")
+	}
+	expectedURL := "http://" + containerurl.ContainerHost + ":8080"
+	if env["ANTHROPIC_BASE_URL"] != expectedURL {
+		t.Errorf("ANTHROPIC_BASE_URL = %v, want %q", env["ANTHROPIC_BASE_URL"], expectedURL)
+	}
 }
 
 func TestClaude_SkillsDir(t *testing.T) {