diff --git a/cmd/main.go b/cmd/main.go
index ece9ea7..4b0fd74 100644
--- a/cmd/main.go
+++ b/cmd/main.go
@@ -7,10 +7,10 @@ import (
 	"os/signal"
 	"syscall"
 
-	"github.com/nalgeon/codapi/config"
-	"github.com/nalgeon/codapi/logx"
-	"github.com/nalgeon/codapi/sandbox"
-	"github.com/nalgeon/codapi/server"
+	"github.com/nalgeon/codapi/internal/config"
+	"github.com/nalgeon/codapi/internal/logx"
+	"github.com/nalgeon/codapi/internal/sandbox"
+	"github.com/nalgeon/codapi/internal/server"
 )
 
 var Version string = "main"
diff --git a/config/config.go b/internal/config/config.go
similarity index 100%
rename from config/config.go
rename to internal/config/config.go
diff --git a/config/config_test.go b/internal/config/config_test.go
similarity index 100%
rename from config/config_test.go
rename to internal/config/config_test.go
diff --git a/config/load.go b/internal/config/load.go
similarity index 100%
rename from config/load.go
rename to internal/config/load.go
diff --git a/config/load_test.go b/internal/config/load_test.go
similarity index 100%
rename from config/load_test.go
rename to internal/config/load_test.go
diff --git a/config/testdata/boxes.json b/internal/config/testdata/boxes.json
similarity index 100%
rename from config/testdata/boxes.json
rename to internal/config/testdata/boxes.json
diff --git a/config/testdata/commands.json b/internal/config/testdata/commands.json
similarity index 100%
rename from config/testdata/commands.json
rename to internal/config/testdata/commands.json
diff --git a/config/testdata/config.json b/internal/config/testdata/config.json
similarity index 100%
rename from config/testdata/config.json
rename to internal/config/testdata/config.json
diff --git a/engine/docker.go b/internal/engine/docker.go
similarity index 97%
rename from engine/docker.go
rename to internal/engine/docker.go
index d577931..ae20622 100644
--- a/engine/docker.go
+++ b/internal/engine/docker.go
@@ -13,10 +13,10 @@ import (
 	"strings"
 	"time"
 
-	"github.com/nalgeon/codapi/config"
-	"github.com/nalgeon/codapi/execy"
-	"github.com/nalgeon/codapi/fileio"
-	"github.com/nalgeon/codapi/logx"
+	"github.com/nalgeon/codapi/internal/config"
+	"github.com/nalgeon/codapi/internal/execy"
+	"github.com/nalgeon/codapi/internal/fileio"
+	"github.com/nalgeon/codapi/internal/logx"
 )
 
 var killTimeout = 5 * time.Second
diff --git a/engine/docker_test.go b/internal/engine/docker_test.go
similarity index 96%
rename from engine/docker_test.go
rename to internal/engine/docker_test.go
index 89dff9b..0322ee9 100644
--- a/engine/docker_test.go
+++ b/internal/engine/docker_test.go
@@ -4,9 +4,9 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/nalgeon/codapi/config"
-	"github.com/nalgeon/codapi/execy"
-	"github.com/nalgeon/codapi/logx"
+	"github.com/nalgeon/codapi/internal/config"
+	"github.com/nalgeon/codapi/internal/execy"
+	"github.com/nalgeon/codapi/internal/logx"
 )
 
 var dockerCfg = &config.Config{
diff --git a/engine/engine.go b/internal/engine/engine.go
similarity index 98%
rename from engine/engine.go
rename to internal/engine/engine.go
index 0252350..d3d6584 100644
--- a/engine/engine.go
+++ b/internal/engine/engine.go
@@ -5,7 +5,7 @@ import (
 	"errors"
 	"fmt"
 
-	"github.com/nalgeon/codapi/stringx"
+	"github.com/nalgeon/codapi/internal/stringx"
 )
 
 // A Request initiates code execution.
diff --git a/engine/engine_test.go b/internal/engine/engine_test.go
similarity index 100%
rename from engine/engine_test.go
rename to internal/engine/engine_test.go
diff --git a/engine/exec.go b/internal/engine/exec.go
similarity index 93%
rename from engine/exec.go
rename to internal/engine/exec.go
index 7fc0dd2..bb2e8d3 100644
--- a/engine/exec.go
+++ b/internal/engine/exec.go
@@ -7,8 +7,8 @@ import (
 	"strings"
 	"time"
 
-	"github.com/nalgeon/codapi/execy"
-	"github.com/nalgeon/codapi/logx"
+	"github.com/nalgeon/codapi/internal/execy"
+	"github.com/nalgeon/codapi/internal/logx"
 )
 
 // A Program is an executable program.
diff --git a/engine/exec_test.go b/internal/engine/exec_test.go
similarity index 97%
rename from engine/exec_test.go
rename to internal/engine/exec_test.go
index 3771785..be8400a 100644
--- a/engine/exec_test.go
+++ b/internal/engine/exec_test.go
@@ -5,7 +5,7 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/nalgeon/codapi/execy"
+	"github.com/nalgeon/codapi/internal/execy"
 )
 
 func TestProgram_Run(t *testing.T) {
diff --git a/engine/http.go b/internal/engine/http.go
similarity index 96%
rename from engine/http.go
rename to internal/engine/http.go
index 5a0e307..91c3aba 100644
--- a/engine/http.go
+++ b/internal/engine/http.go
@@ -9,9 +9,9 @@ import (
 	"net/http"
 	"strings"
 
-	"github.com/nalgeon/codapi/config"
-	"github.com/nalgeon/codapi/httpx"
-	"github.com/nalgeon/codapi/logx"
+	"github.com/nalgeon/codapi/internal/config"
+	"github.com/nalgeon/codapi/internal/httpx"
+	"github.com/nalgeon/codapi/internal/logx"
 )
 
 // An HTTP engine sends HTTP requests.
diff --git a/engine/http_test.go b/internal/engine/http_test.go
similarity index 97%
rename from engine/http_test.go
rename to internal/engine/http_test.go
index 2961f78..a166aa7 100644
--- a/engine/http_test.go
+++ b/internal/engine/http_test.go
@@ -5,9 +5,9 @@ import (
 	"net/http"
 	"testing"
 
-	"github.com/nalgeon/codapi/config"
-	"github.com/nalgeon/codapi/httpx"
-	"github.com/nalgeon/codapi/logx"
+	"github.com/nalgeon/codapi/internal/config"
+	"github.com/nalgeon/codapi/internal/httpx"
+	"github.com/nalgeon/codapi/internal/logx"
 )
 
 var httpCfg = &config.Config{
diff --git a/engine/io.go b/internal/engine/io.go
similarity index 100%
rename from engine/io.go
rename to internal/engine/io.go
diff --git a/engine/io_test.go b/internal/engine/io_test.go
similarity index 100%
rename from engine/io_test.go
rename to internal/engine/io_test.go
diff --git a/engine/testdata/example.txt b/internal/engine/testdata/example.txt
similarity index 100%
rename from engine/testdata/example.txt
rename to internal/engine/testdata/example.txt
diff --git a/execy/execy.go b/internal/execy/execy.go
similarity index 100%
rename from execy/execy.go
rename to internal/execy/execy.go
diff --git a/execy/execy_test.go b/internal/execy/execy_test.go
similarity index 100%
rename from execy/execy_test.go
rename to internal/execy/execy_test.go
diff --git a/execy/mock.go b/internal/execy/mock.go
similarity index 95%
rename from execy/mock.go
rename to internal/execy/mock.go
index a816ad9..20e16a7 100644
--- a/execy/mock.go
+++ b/internal/execy/mock.go
@@ -4,7 +4,7 @@ import (
 	"os/exec"
 	"strings"
 
-	"github.com/nalgeon/codapi/logx"
+	"github.com/nalgeon/codapi/internal/logx"
 )
 
 // Mock installs mock outputs for given commands.
diff --git a/execy/mock_test.go b/internal/execy/mock_test.go
similarity index 100%
rename from execy/mock_test.go
rename to internal/execy/mock_test.go
diff --git a/fileio/fileio.go b/internal/fileio/fileio.go
similarity index 100%
rename from fileio/fileio.go
rename to internal/fileio/fileio.go
diff --git a/fileio/fileio_test.go b/internal/fileio/fileio_test.go
similarity index 100%
rename from fileio/fileio_test.go
rename to internal/fileio/fileio_test.go
diff --git a/httpx/httpx.go b/internal/httpx/httpx.go
similarity index 100%
rename from httpx/httpx.go
rename to internal/httpx/httpx.go
diff --git a/httpx/httpx_test.go b/internal/httpx/httpx_test.go
similarity index 100%
rename from httpx/httpx_test.go
rename to internal/httpx/httpx_test.go
diff --git a/httpx/mock.go b/internal/httpx/mock.go
similarity index 100%
rename from httpx/mock.go
rename to internal/httpx/mock.go
diff --git a/httpx/mock_test.go b/internal/httpx/mock_test.go
similarity index 100%
rename from httpx/mock_test.go
rename to internal/httpx/mock_test.go
diff --git a/httpx/testdata/example.json b/internal/httpx/testdata/example.json
similarity index 100%
rename from httpx/testdata/example.json
rename to internal/httpx/testdata/example.json
diff --git a/httpx/testdata/example.txt b/internal/httpx/testdata/example.txt
similarity index 100%
rename from httpx/testdata/example.txt
rename to internal/httpx/testdata/example.txt
diff --git a/logx/logx.go b/internal/logx/logx.go
similarity index 100%
rename from logx/logx.go
rename to internal/logx/logx.go
diff --git a/logx/logx_test.go b/internal/logx/logx_test.go
similarity index 100%
rename from logx/logx_test.go
rename to internal/logx/logx_test.go
diff --git a/logx/memory.go b/internal/logx/memory.go
similarity index 100%
rename from logx/memory.go
rename to internal/logx/memory.go
diff --git a/logx/memory_test.go b/internal/logx/memory_test.go
similarity index 100%
rename from logx/memory_test.go
rename to internal/logx/memory_test.go
diff --git a/sandbox/config.go b/internal/sandbox/config.go
similarity index 93%
rename from sandbox/config.go
rename to internal/sandbox/config.go
index 925bb06..177ca6f 100644
--- a/sandbox/config.go
+++ b/internal/sandbox/config.go
@@ -4,8 +4,8 @@ package sandbox
 import (
 	"fmt"
 
-	"github.com/nalgeon/codapi/config"
-	"github.com/nalgeon/codapi/engine"
+	"github.com/nalgeon/codapi/internal/config"
+	"github.com/nalgeon/codapi/internal/engine"
 )
 
 // A semaphore represents available concurrent workers
diff --git a/sandbox/config_test.go b/internal/sandbox/config_test.go
similarity index 93%
rename from sandbox/config_test.go
rename to internal/sandbox/config_test.go
index 66255dc..1def702 100644
--- a/sandbox/config_test.go
+++ b/internal/sandbox/config_test.go
@@ -3,8 +3,8 @@ package sandbox
 import (
 	"testing"
 
-	"github.com/nalgeon/codapi/config"
-	"github.com/nalgeon/codapi/engine"
+	"github.com/nalgeon/codapi/internal/config"
+	"github.com/nalgeon/codapi/internal/engine"
 )
 
 var cfg = &config.Config{
diff --git a/sandbox/sandbox.go b/internal/sandbox/sandbox.go
similarity index 96%
rename from sandbox/sandbox.go
rename to internal/sandbox/sandbox.go
index 7101a33..5654d33 100644
--- a/sandbox/sandbox.go
+++ b/internal/sandbox/sandbox.go
@@ -7,7 +7,7 @@ import (
 	"strings"
 	"time"
 
-	"github.com/nalgeon/codapi/engine"
+	"github.com/nalgeon/codapi/internal/engine"
 )
 
 var ErrUnknownSandbox = errors.New("unknown sandbox")
diff --git a/sandbox/sandbox_test.go b/internal/sandbox/sandbox_test.go
similarity index 96%
rename from sandbox/sandbox_test.go
rename to internal/sandbox/sandbox_test.go
index 6bc0b6b..6a1b9e0 100644
--- a/sandbox/sandbox_test.go
+++ b/internal/sandbox/sandbox_test.go
@@ -4,8 +4,8 @@ import (
 	"errors"
 	"testing"
 
-	"github.com/nalgeon/codapi/engine"
-	"github.com/nalgeon/codapi/execy"
+	"github.com/nalgeon/codapi/internal/engine"
+	"github.com/nalgeon/codapi/internal/execy"
 )
 
 func TestValidate(t *testing.T) {
diff --git a/sandbox/semaphore.go b/internal/sandbox/semaphore.go
similarity index 100%
rename from sandbox/semaphore.go
rename to internal/sandbox/semaphore.go
diff --git a/sandbox/semaphore_test.go b/internal/sandbox/semaphore_test.go
similarity index 100%
rename from sandbox/semaphore_test.go
rename to internal/sandbox/semaphore_test.go
diff --git a/server/io.go b/internal/server/io.go
similarity index 100%
rename from server/io.go
rename to internal/server/io.go
diff --git a/server/io_test.go b/internal/server/io_test.go
similarity index 97%
rename from server/io_test.go
rename to internal/server/io_test.go
index 13e061d..78cf0b1 100644
--- a/server/io_test.go
+++ b/internal/server/io_test.go
@@ -8,7 +8,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/nalgeon/codapi/engine"
+	"github.com/nalgeon/codapi/internal/engine"
 )
 
 func Test_readJson(t *testing.T) {
diff --git a/server/middleware.go b/internal/server/middleware.go
similarity index 100%
rename from server/middleware.go
rename to internal/server/middleware.go
diff --git a/server/middleware_test.go b/internal/server/middleware_test.go
similarity index 100%
rename from server/middleware_test.go
rename to internal/server/middleware_test.go
diff --git a/server/router.go b/internal/server/router.go
similarity index 90%
rename from server/router.go
rename to internal/server/router.go
index 99b6820..ae5e776 100644
--- a/server/router.go
+++ b/internal/server/router.go
@@ -6,10 +6,10 @@ import (
 	"fmt"
 	"net/http"
 
-	"github.com/nalgeon/codapi/engine"
-	"github.com/nalgeon/codapi/logx"
-	"github.com/nalgeon/codapi/sandbox"
-	"github.com/nalgeon/codapi/stringx"
+	"github.com/nalgeon/codapi/internal/engine"
+	"github.com/nalgeon/codapi/internal/logx"
+	"github.com/nalgeon/codapi/internal/sandbox"
+	"github.com/nalgeon/codapi/internal/stringx"
 )
 
 // NewRouter creates HTTP routes and handlers for them.
diff --git a/server/router_test.go b/internal/server/router_test.go
similarity index 95%
rename from server/router_test.go
rename to internal/server/router_test.go
index 78b0087..6c04cc2 100644
--- a/server/router_test.go
+++ b/internal/server/router_test.go
@@ -7,10 +7,10 @@ import (
 	"net/http/httptest"
 	"testing"
 
-	"github.com/nalgeon/codapi/config"
-	"github.com/nalgeon/codapi/engine"
-	"github.com/nalgeon/codapi/execy"
-	"github.com/nalgeon/codapi/sandbox"
+	"github.com/nalgeon/codapi/internal/config"
+	"github.com/nalgeon/codapi/internal/engine"
+	"github.com/nalgeon/codapi/internal/execy"
+	"github.com/nalgeon/codapi/internal/sandbox"
 )
 
 var cfg = &config.Config{
diff --git a/server/server.go b/internal/server/server.go
similarity index 96%
rename from server/server.go
rename to internal/server/server.go
index 9f0b094..b377871 100644
--- a/server/server.go
+++ b/internal/server/server.go
@@ -8,7 +8,7 @@ import (
 	"sync"
 	"time"
 
-	"github.com/nalgeon/codapi/logx"
+	"github.com/nalgeon/codapi/internal/logx"
 )
 
 // The maximum duration of the server graceful shutdown.
diff --git a/server/server_test.go b/internal/server/server_test.go
similarity index 100%
rename from server/server_test.go
rename to internal/server/server_test.go
diff --git a/stringx/stringx.go b/internal/stringx/stringx.go
similarity index 100%
rename from stringx/stringx.go
rename to internal/stringx/stringx.go
diff --git a/stringx/stringx_test.go b/internal/stringx/stringx_test.go
similarity index 100%
rename from stringx/stringx_test.go
rename to internal/stringx/stringx_test.go