diff --git a/docs/usage.ja.md b/docs/usage.ja.md
index ca335fd..89157c6 100644
--- a/docs/usage.ja.md
+++ b/docs/usage.ja.md
@@ -353,7 +353,7 @@ install が失敗しても、残りの install は続けます。失敗したコ
 | | `Pipfile.lock` | `pipenv install` |
 | | `pyproject.toml` / `requirements.txt` | `pip install -e .` |
 | Rust | `Cargo.toml`、または Tauri プロジェクトの `src-tauri/Cargo.toml`（このときは `src-tauri/` で実行） | `cargo update` |
-| Go | `go.mod` | `go mod download` |
+| Go | `go.mod` | `go mod tidy` |
 | Ruby | `Gemfile` | `bundle install` |
 | PHP | `composer.json` | `composer update` |
 | Java | `gradlew` | `./gradlew dependencies` |
@@ -365,6 +365,8 @@ install が失敗しても、残りの install は続けます。失敗したコ
 
 PHP プロジェクトを `--install` で処理するとき、depup は `composer install` ではなく `composer update` を実行します。`composer install` は既存のロックファイルを再利用するため、depup が直前に `composer.json` へ書いた制約を反映できません。`composer update` で制約を解決し直し、`composer.lock` を更新します。
 
+Go では `go mod tidy` で、テストやプラットフォーム固有の import も含めて、ソースコードと `go.mod`・`go.sum` を揃えます。`go.mod` のハッシュに加えてモジュール本体のハッシュも記録し、後続のビルドに必要なチェックサムを揃えます。引数なしの `go mod download` と異なり、不足する間接依存を追加し、使われなくなった依存やハッシュを削除します。`// indirect` コメントが変わるほか、依存先が新しい Go を要求すると `go`・`toolchain` ディレクティブが変わる場合もあります。`vendor/` をコミットするプロジェクトでは、その後に `go mod vendor` で更新してください。tidy が失敗した場合は install 失敗として報告します。
+
 ### 推移的依存と age フィルター
 
 age フィルターは、depup がマニフェストに書き込むバージョンを決めるものです。`--install` で解決される推移的依存にも効くかどうかは、パッケージマネージャーによって違います。複数のマニフェストをまとめて解決する install には、更新がなかったメンバーも含めて最も厳しい設定を渡します（[適用される age の優先順位](#適用される-age-の優先順位)）。
diff --git a/docs/usage.md b/docs/usage.md
index b3a43d9..951110f 100644
--- a/docs/usage.md
+++ b/docs/usage.md
@@ -352,7 +352,7 @@ The package manager is detected from files in the directory where the install ru
 | | `Pipfile.lock` | `pipenv install` |
 | | `pyproject.toml` / `requirements.txt` | `pip install -e .` |
 | Rust | `Cargo.toml`, or `src-tauri/Cargo.toml` in a Tauri project (`cargo update` then runs in `src-tauri/`) | `cargo update` |
-| Go | `go.mod` | `go mod download` |
+| Go | `go.mod` | `go mod tidy` |
 | Ruby | `Gemfile` | `bundle install` |
 | PHP | `composer.json` | `composer update` |
 | Java | `gradlew` | `./gradlew dependencies` |
@@ -364,6 +364,8 @@ If none of the listed files is present, no install runs for that language, and n
 
 When `--install` processes a PHP project, depup runs `composer update` rather than `composer install`. `composer install` reuses the existing lock file and cannot reflect constraints that depup has just changed in `composer.json`; `composer update` resolves those constraints and refreshes `composer.lock`.
 
+For Go, `go mod tidy` synchronizes `go.mod` and `go.sum` with the source code, including tests and platform-specific imports. It records module content checksums as well as `go.mod` checksums so that subsequent builds have the required sums. Unlike argumentless `go mod download`, it also adds missing indirect requirements and removes unused requirements and sums. This may change `// indirect` comments and, when dependencies require a newer Go release, the `go` or `toolchain` directives. Projects that commit `vendor/` must run `go mod vendor` afterwards to refresh it. A tidy failure is reported as an install failure.
+
 ### Transitive Dependencies and the Age Filter
 
 The age filter decides which versions depup writes into manifests. Whether it also reaches transitive dependencies during `--install` depends on the package manager. An install that resolves multiple manifests uses the strictest policy of those manifests, including members with no updates ([Resolution Priority](#resolution-priority)):
diff --git a/src/package_manager.rs b/src/package_manager.rs
index ffd026e..c2111e7 100644
--- a/src/package_manager.rs
+++ b/src/package_manager.rs
@@ -277,8 +277,9 @@ impl SystemPackageManager {
             "pipenv" => vec!["pipenv", "install"],
             // Rust の処理
             "cargo" => vec!["cargo", "update"],
-            // Go 言語
-            "go" => vec!["go", "mod", "download"],
+            // Go 1.17 以降の引数なし download は本体のハッシュを go.sum に保存しない。
+            // tidy でハッシュと、新版が必要とする間接依存の require を揃える。
+            "go" => vec!["go", "mod", "tidy"],
             // Ruby の処理
             "bundle" => vec!["bundle", "install"],
             // PHP の処理。
@@ -617,7 +618,123 @@ mod tests {
     fn test_get_install_command_go() {
         let pm = SystemPackageManager::new();
         let cmd = pm.get_install_command("go");
-        assert_eq!(cmd, vec!["go", "mod", "download"]);
+        assert_eq!(cmd, vec!["go", "mod", "tidy"]);
+    }
+
+    #[test]
+    fn go_install_completes_checksums_and_indirect_requirements() {
+        use std::fs;
+
+        if which::which("go").is_err() {
+            eprintln!("Skipping Go install regression: go is not installed");
+            return;
+        }
+
+        let temp = tempfile::tempdir().unwrap();
+        let project = temp.path().join("project");
+        let proxy = temp.path().join("proxy");
+        fs::create_dir_all(&project).unwrap();
+        for (name, version, archive, manifest) in [
+            (
+                "direct",
+                "v1.0.0",
+                include_bytes!("../tests/fixtures/go-install/direct-v1.0.0.zip").as_slice(),
+                "module example.com/direct\n\ngo 1.17\n",
+            ),
+            (
+                "direct",
+                "v1.1.0",
+                include_bytes!("../tests/fixtures/go-install/direct-v1.1.0.zip").as_slice(),
+                "module example.com/direct\n\ngo 1.17\n\nrequire example.com/indirect v1.0.0\n",
+            ),
+            (
+                "indirect",
+                "v1.0.0",
+                include_bytes!("../tests/fixtures/go-install/indirect-v1.0.0.zip").as_slice(),
+                "module example.com/indirect\n\ngo 1.17\n",
+            ),
+        ] {
+            let versions = proxy.join(format!("example.com/{name}/@v"));
+            fs::create_dir_all(&versions).unwrap();
+            fs::write(versions.join(format!("{version}.zip")), archive).unwrap();
+            fs::write(versions.join(format!("{version}.mod")), manifest).unwrap();
+            fs::write(
+                versions.join(format!("{version}.info")),
+                format!(r#"{{"Version":"{version}","Time":"2020-01-01T00:00:00Z"}}"#),
+            )
+            .unwrap();
+        }
+
+        let env = vec![
+            (
+                "GOPROXY".into(),
+                reqwest::Url::from_directory_path(&proxy)
+                    .unwrap()
+                    .to_string(),
+            ),
+            ("GOSUMDB".into(), "off".into()),
+            ("GOPRIVATE".into(), String::new()),
+            ("GONOPROXY".into(), "none".into()),
+            ("GOENV".into(), "off".into()),
+            ("GOFLAGS".into(), String::new()),
+            ("GO111MODULE".into(), "on".into()),
+            ("GOWORK".into(), "off".into()),
+            ("GOTOOLCHAIN".into(), "local".into()),
+            (
+                "GOMODCACHE".into(),
+                temp.path().join("modcache").to_str().unwrap().into(),
+            ),
+            (
+                "GOCACHE".into(),
+                temp.path().join("buildcache").to_str().unwrap().into(),
+            ),
+        ];
+        let pm = SystemPackageManager::new();
+        let run = |args: &[&str]| pm.run_command(args, &project, &env).unwrap();
+        let assert_success = |output: Output| {
+            assert!(
+                output.status.success(),
+                "Go command failed: {}",
+                String::from_utf8_lossy(&output.stderr)
+            );
+        };
+        fs::write(
+            project.join("go.mod"),
+            "module example.com/app\n\ngo 1.17\n\nrequire example.com/direct v1.0.0\n",
+        )
+        .unwrap();
+        fs::write(
+            project.join("main.go"),
+            "package main\nimport \"example.com/direct\"\nfunc main() { println(direct.Message()) }\n",
+        )
+        .unwrap();
+        assert_success(run(&["go", "mod", "tidy"]));
+        let manifest_path = project.join("go.mod");
+        let manifest = fs::read_to_string(&manifest_path).unwrap();
+        fs::write(&manifest_path, manifest.replace("v1.0.0", "v1.1.0")).unwrap();
+
+        // 旧コマンドは成功しても本体のハッシュを保存せず、readonly build は失敗する。
+        assert_success(run(&["go", "mod", "download"]));
+        let sums = fs::read_to_string(project.join("go.sum")).unwrap();
+        assert!(sums.contains("example.com/direct v1.1.0/go.mod h1:"));
+        assert!(!sums.contains("example.com/direct v1.1.0 h1:"));
+        assert!(
+            !run(&["go", "build", "-mod=readonly", "./..."])
+                .status
+                .success()
+        );
+
+        assert_success(run(&pm.get_install_command("go")));
+        let sums = fs::read_to_string(project.join("go.sum")).unwrap();
+        assert!(sums.contains("example.com/direct v1.1.0 h1:"));
+        assert!(sums.contains("example.com/indirect v1.0.0 h1:"));
+        assert!(
+            fs::read_to_string(&manifest_path)
+                .unwrap()
+                .contains("example.com/indirect v1.0.0 // indirect")
+        );
+        assert_success(run(&["go", "build", "-mod=readonly", "./..."]));
+        assert_success(run(&["go", "test", "-mod=readonly", "./..."]));
     }
 
     #[test]
diff --git a/tests/fixtures/go-install/README.md b/tests/fixtures/go-install/README.md
new file mode 100644
index 0000000..f423ee5
--- /dev/null
+++ b/tests/fixtures/go-install/README.md
@@ -0,0 +1,17 @@
+# Offline Go install fixtures
+
+These archives contain fictional modules for the Go install regression test in
+`src/package_manager.rs`. Version `v1.1.0` of `example.com/direct` imports a new
+transitive dependency, `example.com/indirect v1.0.0`; version `v1.0.0` does not.
+
+Regenerate the deterministic, uncompressed ZIP archives with:
+
+```sh
+python3 tests/fixtures/go-install/generate.py
+```
+
+The Rust test creates the proxy metadata and a sample application in a temporary
+directory. It uses this file proxy with the checksum server disabled and isolated
+module/build caches, so it needs Go 1.17 or later but no network access.
+The test is skipped when Go is unavailable. Python is only needed to regenerate
+the committed archives.
diff --git a/tests/fixtures/go-install/direct-v1.0.0.zip b/tests/fixtures/go-install/direct-v1.0.0.zip
new file mode 100644
index 0000000..f27925c
Binary files /dev/null and b/tests/fixtures/go-install/direct-v1.0.0.zip differ
diff --git a/tests/fixtures/go-install/direct-v1.1.0.zip b/tests/fixtures/go-install/direct-v1.1.0.zip
new file mode 100644
index 0000000..105f8b9
Binary files /dev/null and b/tests/fixtures/go-install/direct-v1.1.0.zip differ
diff --git a/tests/fixtures/go-install/generate.py b/tests/fixtures/go-install/generate.py
new file mode 100644
index 0000000..0dba7e2
--- /dev/null
+++ b/tests/fixtures/go-install/generate.py
@@ -0,0 +1,39 @@
+"""Regenerate the offline Go proxy archives with Python's standard library."""
+
+from pathlib import Path
+from zipfile import ZIP_STORED, ZipFile, ZipInfo
+
+MODULES = [
+    (
+        "direct",
+        "v1.0.0",
+        "module example.com/direct\n\ngo 1.17\n",
+        'package direct\nfunc Message() string { return "old" }\n',
+    ),
+    (
+        "direct",
+        "v1.1.0",
+        "module example.com/direct\n\ngo 1.17\n\nrequire example.com/indirect v1.0.0\n",
+        (
+            'package direct\nimport "example.com/indirect"\n'
+            "func Message() string { return indirect.Message() }\n"
+        ),
+    ),
+    (
+        "indirect",
+        "v1.0.0",
+        "module example.com/indirect\n\ngo 1.17\n",
+        'package indirect\nfunc Message() string { return "new" }\n',
+    ),
+]
+
+
+if __name__ == "__main__":
+    for name, version, manifest, source in MODULES:
+        archive = Path(__file__).parent / f"{name}-{version}.zip"
+        with ZipFile(archive, "w", compression=ZIP_STORED) as output:
+            for filename, contents in [("go.mod", manifest), ("message.go", source)]:
+                entry = ZipInfo(f"example.com/{name}@{version}/{filename}")
+                entry.create_system = 3
+                entry.external_attr = 0o100644 << 16
+                output.writestr(entry, contents.encode("utf-8"))
diff --git a/tests/fixtures/go-install/indirect-v1.0.0.zip b/tests/fixtures/go-install/indirect-v1.0.0.zip
new file mode 100644
index 0000000..77955f1
Binary files /dev/null and b/tests/fixtures/go-install/indirect-v1.0.0.zip differ