diff --git a/CLAUDE.md b/CLAUDE.md
index 51464a0..5dc2e29 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -7,10 +7,9 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 This project uses the Core CLI (`core` binary), not `go` directly.
 
 ```bash
-core go test                          # run all tests
-core go test --run TestConfig_Get_Good  # run a single test
-core go cov                           # test with coverage
-core go cov --open                    # coverage + open HTML report
+go test ./...                                    # run all tests
+go test -run TestConfig_Get_Good ./...           # run a single test
+go test -cover ./...                             # test with coverage
 
 core go qa                            # format, vet, lint, test
 core go qa full                       # adds race detector, vuln scan, security audit
diff --git a/config.go b/config.go
index 35c384c..2dbe4ce 100644
--- a/config.go
+++ b/config.go
@@ -11,7 +11,6 @@
 package config
 
 import (
-	"fmt"
 	"iter"
 	"maps"
 	"os"
@@ -109,7 +108,7 @@ func (c *Config) LoadFile(m coreio.Medium, path string) error {
 
 	content, err := m.Read(path)
 	if err != nil {
-		return coreerr.E("config.LoadFile", fmt.Sprintf("failed to read config file: %s", path), err)
+		return coreerr.E("config.LoadFile", "failed to read config file: "+path, err)
 	}
 
 	ext := filepath.Ext(path)
@@ -123,13 +122,13 @@ func (c *Config) LoadFile(m coreio.Medium, path string) error {
 	// Load into file-backed viper
 	c.f.SetConfigType(configType)
 	if err := c.f.MergeConfig(strings.NewReader(content)); err != nil {
-		return coreerr.E("config.LoadFile", fmt.Sprintf("failed to parse config file (f): %s", path), err)
+		return coreerr.E("config.LoadFile", "failed to parse config file (f): "+path, err)
 	}
 
 	// Load into full viper
 	c.v.SetConfigType(configType)
 	if err := c.v.MergeConfig(strings.NewReader(content)); err != nil {
-		return coreerr.E("config.LoadFile", fmt.Sprintf("failed to parse config file (v): %s", path), err)
+		return coreerr.E("config.LoadFile", "failed to parse config file (v): "+path, err)
 	}
 
 	return nil
@@ -150,11 +149,11 @@ func (c *Config) Get(key string, out any) error {
 	}
 
 	if !c.v.IsSet(key) {
-		return coreerr.E("config.Get", fmt.Sprintf("key not found: %s", key), nil)
+		return coreerr.E("config.Get", "key not found: "+key, nil)
 	}
 
 	if err := c.v.UnmarshalKey(key, out); err != nil {
-		return coreerr.E("config.Get", fmt.Sprintf("failed to unmarshal key: %s", key), err)
+		return coreerr.E("config.Get", "failed to unmarshal key: "+key, err)
 	}
 	return nil
 }