diff --git a/lib/tool_kit/github/client.ex b/lib/tool_kit/github/client.ex
new file mode 100644
index 0000000..169940e
--- /dev/null
+++ b/lib/tool_kit/github/client.ex
@@ -0,0 +1,397 @@
+defmodule ToolKit.GitHub.Client do
+  @moduledoc """
+  GitHub REST API への薄い HTTP ラッパ(Req ベース)。
+
+  各ツールが共通で使う endpoint ヘルパとエラー分類を提供する。
+  orchestration とレスポンスの解釈(パース)はツール側の責務とし、
+  本モジュールは HTTP 境界に徹する。
+
+  ## 認証
+
+  トークンは `:token_provider` オプションで注入する。省略時は
+  GitHub CLI(`gh auth token`)から取得する(`gh_cli_token/0`)。
+
+  ## オプション
+
+  すべての関数が共通で受け取る:
+
+    * `:token_provider` - `(-> {:ok, token} | {:error, reason})` 形式の関数。
+      既定は `gh_cli_token/0`
+    * `:base_url` - API のベース URL(既定 `"https://api.github.com"`)
+    * `:receive_timeout` - 応答タイムアウト(ミリ秒、既定 30_000)
+    * `:user_agent` - User-Agent ヘッダ(既定 `"elixir-tool-kit"`)
+    * `:req_options` - Req にそのまま渡す追加オプション
+      (テストでの `plug: {Req.Test, Name}` 差し替えなど)
+
+  ## 戻り値とエラー分類
+
+    * `{:ok, body}` - 2xx。ボディは Req がデコードした値
+    * `{:error, :not_found}` - 404
+    * `{:error, :unauthorized}` - 401 / 403(トークンの権限不足)。
+      404 と区別して返すため、呼び出し側は「存在しない」と
+      「権限がない」を混同せずに扱える
+    * `{:error, {:http_error, status, message}}` - その他の 4xx / 5xx
+    * `{:error, {:request_failed, reason}}` - 通信自体の失敗
+    * `{:error, {:token_error, reason}}` - トークン取得の失敗
+  """
+
+  @default_base_url "https://api.github.com"
+  @default_receive_timeout 30_000
+  @default_user_agent "elixir-tool-kit"
+  # REST API の日付バージョン(X-GitHub-Api-Version)
+  @api_version "2022-11-28"
+
+  @type token_provider :: (-> {:ok, String.t()} | {:error, term()})
+
+  @type error ::
+          :not_found
+          | :unauthorized
+          | {:http_error, non_neg_integer(), String.t()}
+          | {:request_failed, term()}
+          | {:token_error, term()}
+
+  @type result :: {:ok, term()} | {:error, error()}
+
+  @type method :: :get | :post | :put | :patch | :delete
+
+  @typedoc "Req のレスポンス相当(`Req.Response.t()` を含む)"
+  @type http_response :: %{
+          :status => non_neg_integer(),
+          :body => term(),
+          optional(atom()) => term()
+        }
+
+  # ---------------------------------------------------------------
+  # 汎用リクエスト
+  # ---------------------------------------------------------------
+
+  @doc """
+  GET リクエストを送る。
+  """
+  @spec get(String.t(), keyword()) :: result()
+  def get(path, opts \\ []), do: request(:get, path, opts)
+
+  @doc """
+  POST リクエストを送る(`body` は JSON として送信)。
+  """
+  @spec post(String.t(), term(), keyword()) :: result()
+  def post(path, body, opts \\ []), do: request(:post, path, Keyword.put(opts, :json, body))
+
+  @doc """
+  PUT リクエストを送る(`body` は JSON として送信)。
+  """
+  @spec put(String.t(), term(), keyword()) :: result()
+  def put(path, body, opts \\ []), do: request(:put, path, Keyword.put(opts, :json, body))
+
+  @doc """
+  PATCH リクエストを送る(`body` は JSON として送信)。
+  """
+  @spec patch(String.t(), term(), keyword()) :: result()
+  def patch(path, body, opts \\ []), do: request(:patch, path, Keyword.put(opts, :json, body))
+
+  @doc """
+  任意のメソッドでリクエストを送る。
+
+  `path` は `"/repos/owner/repo"` のようなベース URL からの相対パス。
+  クエリパラメータは `:params`(keyword)で渡す。
+  """
+  @spec request(method(), String.t(), keyword()) :: result()
+  def request(method, path, opts \\ []) do
+    case fetch_token(opts) do
+      {:ok, token} ->
+        method
+        |> run_request(path, token, opts)
+        |> classify_response()
+
+      {:error, reason} ->
+        {:error, {:token_error, reason}}
+    end
+  end
+
+  # ---------------------------------------------------------------
+  # contents API
+  # ---------------------------------------------------------------
+
+  @doc """
+  ファイル内容を取得する(contents API)。
+
+  レスポンスには base64 の `"content"` と楽観ロック用の `"sha"` が
+  含まれる。テキストが必要なら `decode_content/1` か `get_file_text/3`
+  を使う。`:ref` オプションでブランチ・タグ・SHA を指定できる。
+  """
+  @spec get_file_contents(String.t(), String.t(), keyword()) :: result()
+  def get_file_contents(repo, file_path, opts \\ []) do
+    {ref, opts} = Keyword.pop(opts, :ref)
+    get("/repos/#{repo}/contents/#{file_path}", put_params(opts, ref: ref))
+  end
+
+  @doc """
+  ファイルを作成・更新する(contents API)。
+
+  `content` は生テキストを受け取り、内部で base64 エンコードする。
+  更新時は `:sha` オプションに取得済みの blob SHA を渡すこと
+  (楽観ロック。競合すると 409 が返る)。新規作成時は `:sha` を
+  省略する。`:branch` オプションでコミット先ブランチを指定できる。
+  """
+  @spec put_file_contents(String.t(), String.t(), String.t(), String.t(), keyword()) :: result()
+  def put_file_contents(repo, file_path, content, commit_message, opts \\ []) do
+    {sha, opts} = Keyword.pop(opts, :sha)
+    {branch, opts} = Keyword.pop(opts, :branch)
+
+    body =
+      %{message: commit_message, content: Base.encode64(content)}
+      |> put_present(:sha, sha)
+      |> put_present(:branch, branch)
+
+    put("/repos/#{repo}/contents/#{file_path}", body, opts)
+  end
+
+  @doc """
+  ファイルを取得してテキストにデコードするところまで行う。
+
+  blob SHA も必要な場合(更新の前段)は `get_file_contents/3` を使い、
+  `decode_content/1` と組み合わせること。
+  """
+  @spec get_file_text(String.t(), String.t(), keyword()) :: result()
+  def get_file_text(repo, file_path, opts \\ []) do
+    with {:ok, body} <- get_file_contents(repo, file_path, opts) do
+      decode_content(body)
+    end
+  end
+
+  @doc """
+  contents API のレスポンスからテキストを取り出す(純関数)。
+
+  `"content"` は 60 桁ごとに改行が入った base64 で返るため、
+  改行を除去してからデコードする。
+
+  ## Examples
+
+      iex> ToolKit.GitHub.Client.decode_content(%{"content" => "aGVsbG8=", "encoding" => "base64"})
+      {:ok, "hello"}
+
+  """
+  @spec decode_content(term()) :: {:ok, String.t()} | {:error, :invalid_content}
+  def decode_content(%{"content" => content, "encoding" => "base64"}) when is_binary(content) do
+    decoded =
+      content
+      |> String.replace(["\n", "\r"], "")
+      |> Base.decode64()
+
+    case decoded do
+      {:ok, text} -> {:ok, text}
+      :error -> {:error, :invalid_content}
+    end
+  end
+
+  def decode_content(_body), do: {:error, :invalid_content}
+
+  # ---------------------------------------------------------------
+  # repo / commits / pulls / issues
+  # ---------------------------------------------------------------
+
+  @doc """
+  リポジトリ情報を取得する。
+  """
+  @spec get_repository(String.t(), keyword()) :: result()
+  def get_repository(repo, opts \\ []), do: get("/repos/#{repo}", opts)
+
+  @doc """
+  ブランチ一覧を取得する。`:per_page` を指定できる。
+  """
+  @spec list_branches(String.t(), keyword()) :: result()
+  def list_branches(repo, opts \\ []) do
+    {params, opts} = Keyword.split(opts, [:per_page])
+    get("/repos/#{repo}/branches", put_params(opts, params))
+  end
+
+  @doc """
+  コミット一覧を取得する。`:since`(ISO8601)・`:author`・`:per_page`
+  を指定できる。
+  """
+  @spec list_commits(String.t(), keyword()) :: result()
+  def list_commits(repo, opts \\ []) do
+    {params, opts} = Keyword.split(opts, [:since, :author, :per_page])
+    get("/repos/#{repo}/commits", put_params(opts, params))
+  end
+
+  @doc """
+  プルリクエスト一覧を取得する。`:state`(open / closed / all)・
+  `:per_page` を指定できる。
+  """
+  @spec list_pull_requests(String.t(), keyword()) :: result()
+  def list_pull_requests(repo, opts \\ []) do
+    {params, opts} = Keyword.split(opts, [:state, :per_page])
+    get("/repos/#{repo}/pulls", put_params(opts, params))
+  end
+
+  @doc """
+  プルリクエストのレビュー一覧を取得する。`:per_page` を指定できる。
+  """
+  @spec list_pull_request_reviews(String.t(), pos_integer(), keyword()) :: result()
+  def list_pull_request_reviews(repo, pr_number, opts \\ []) do
+    {params, opts} = Keyword.split(opts, [:per_page])
+    get("/repos/#{repo}/pulls/#{pr_number}/reviews", put_params(opts, params))
+  end
+
+  @doc """
+  プルリクエストの保留中レビューリクエスト(依頼済みレビュアー)を取得する。
+  """
+  @spec get_requested_reviewers(String.t(), pos_integer(), keyword()) :: result()
+  def get_requested_reviewers(repo, pr_number, opts \\ []) do
+    get("/repos/#{repo}/pulls/#{pr_number}/requested_reviewers", opts)
+  end
+
+  @doc """
+  Issue / プルリクエストにコメントを投稿する。
+  """
+  @spec create_issue_comment(String.t(), pos_integer(), String.t(), keyword()) :: result()
+  def create_issue_comment(repo, issue_number, comment_body, opts \\ []) do
+    post("/repos/#{repo}/issues/#{issue_number}/comments", %{body: comment_body}, opts)
+  end
+
+  @doc """
+  プルリクエストをクローズする(archive 前の整理などに使う)。
+  """
+  @spec close_pull_request(String.t(), pos_integer(), keyword()) :: result()
+  def close_pull_request(repo, pr_number, opts \\ []) do
+    patch("/repos/#{repo}/pulls/#{pr_number}", %{state: "closed"}, opts)
+  end
+
+  @doc """
+  リポジトリを archive する。
+  """
+  @spec archive_repository(String.t(), keyword()) :: result()
+  def archive_repository(repo, opts \\ []) do
+    patch("/repos/#{repo}", %{archived: true}, opts)
+  end
+
+  # ---------------------------------------------------------------
+  # エラー分類
+  # ---------------------------------------------------------------
+
+  @doc """
+  Req の結果をエラー分類済みの `t:result/0` に写す(純関数)。
+  """
+  @spec classify_response({:ok, http_response()} | {:error, term()}) :: result()
+  def classify_response({:ok, %{status: status, body: body}}) when status in 200..299,
+    do: {:ok, body}
+
+  def classify_response({:ok, %{status: 404}}), do: {:error, :not_found}
+
+  def classify_response({:ok, %{status: status}}) when status in [401, 403],
+    do: {:error, :unauthorized}
+
+  def classify_response({:ok, %{status: status, body: body}}),
+    do: {:error, {:http_error, status, extract_error_message(body, status)}}
+
+  def classify_response({:error, reason}), do: {:error, {:request_failed, reason}}
+
+  @doc """
+  エラーが 404(Not Found)かを判定する。
+
+  分類済みの reason(`:not_found`)と `{:error, reason}` タプルの
+  どちらも受け取れる。
+  """
+  @spec not_found_error?(term()) :: boolean()
+  def not_found_error?(:not_found), do: true
+  def not_found_error?({:error, :not_found}), do: true
+  def not_found_error?(_other), do: false
+
+  @doc """
+  エラーが 401 / 403(認証・権限不足)かを判定する。
+
+  分類済みの reason(`:unauthorized`)と `{:error, reason}` タプルの
+  どちらも受け取れる。
+  """
+  @spec unauthorized_error?(term()) :: boolean()
+  def unauthorized_error?(:unauthorized), do: true
+  def unauthorized_error?({:error, :unauthorized}), do: true
+  def unauthorized_error?(_other), do: false
+
+  # ---------------------------------------------------------------
+  # トークン取得・URL 構築
+  # ---------------------------------------------------------------
+
+  @doc """
+  既定のトークンプロバイダ。GitHub CLI(`gh auth token`)から取得する。
+
+  外部コマンド実行のためテストカバレッジの対象外。
+  """
+  @spec gh_cli_token() :: {:ok, String.t()} | {:error, String.t()}
+  def gh_cli_token do
+    case System.cmd("gh", ["auth", "token"], stderr_to_stdout: true) do
+      {token, 0} ->
+        {:ok, String.trim(token)}
+
+      {output, _exit_code} ->
+        {:error, "GitHub CLI authentication failed (run 'gh auth login'): #{String.trim(output)}"}
+    end
+  rescue
+    ErlangError -> {:error, "GitHub CLI (gh) not found in PATH"}
+  end
+
+  @doc """
+  ベース URL とパスをスラッシュ 1 個で結合する(純関数)。
+  """
+  @spec build_url(String.t(), String.t()) :: String.t()
+  def build_url(base_url, path) do
+    String.trim_trailing(base_url, "/") <> "/" <> String.trim_leading(path, "/")
+  end
+
+  # ---------------------------------------------------------------
+  # プライベート関数
+  # ---------------------------------------------------------------
+
+  defp fetch_token(opts) do
+    provider = Keyword.get(opts, :token_provider, &gh_cli_token/0)
+    provider.()
+  end
+
+  defp run_request(method, path, token, opts) do
+    base_url = Keyword.get(opts, :base_url, @default_base_url)
+
+    req_opts =
+      [
+        method: method,
+        url: build_url(base_url, path),
+        headers: build_headers(token, opts),
+        receive_timeout: Keyword.get(opts, :receive_timeout, @default_receive_timeout),
+        retry: false
+      ]
+      |> put_present(:params, opts[:params])
+      |> put_present(:json, opts[:json])
+      |> Keyword.merge(Keyword.get(opts, :req_options, []))
+
+    Req.request(req_opts)
+  end
+
+  defp build_headers(token, opts) do
+    [
+      {"accept", "application/vnd.github+json"},
+      {"x-github-api-version", @api_version},
+      {"authorization", "Bearer " <> token},
+      {"user-agent", Keyword.get(opts, :user_agent, @default_user_agent)}
+    ]
+  end
+
+  defp extract_error_message(%{"message" => message}, _status), do: message
+  defp extract_error_message(body, status) when is_binary(body), do: "#{status} - #{body}"
+  defp extract_error_message(_body, status), do: "HTTP #{status}"
+
+  # クエリパラメータ(nil の値は落とす)を opts の :params にマージする。
+  # 呼び出し元が :params を渡していた場合は保持し、同名キーはヘルパ側を優先する
+  defp put_params(opts, params) do
+    case Enum.reject(params, fn {_key, value} -> is_nil(value) end) do
+      [] -> opts
+      present -> Keyword.update(opts, :params, present, &Keyword.merge(&1, present))
+    end
+  end
+
+  defp put_present(map, _key, nil) when is_map(map), do: map
+  defp put_present(map, key, value) when is_map(map), do: Map.put(map, key, value)
+  defp put_present(keyword, _key, nil) when is_list(keyword), do: keyword
+
+  defp put_present(keyword, key, value) when is_list(keyword),
+    do: Keyword.put(keyword, key, value)
+end
diff --git a/mix.exs b/mix.exs
index dbcb9cf..ad8d855 100644
--- a/mix.exs
+++ b/mix.exs
@@ -35,6 +35,8 @@ defmodule ToolKit.MixProject do
       {:jason, "~> 1.4"},
       {:yaml_elixir, "~> 2.9"},
       {:req, "~> 0.5"},
+      # Req.Test(Plug ベースの HTTP スタブ)用
+      {:plug, "~> 1.16", only: [:test]},
       {:dialyxir, "~> 1.4", only: [:dev], runtime: false},
       {:credo, "~> 1.7", only: [:dev, :test], runtime: false}
     ]
diff --git a/mix.lock b/mix.lock
index 0f7376f..8931a92 100644
--- a/mix.lock
+++ b/mix.lock
@@ -11,6 +11,8 @@
   "mint": {:hex, :mint, "1.9.3", "3337184d69179695c7a9f1714d92c11e629d36c8c037a21cf490131d3d150554", [:mix], [{:castore, "~> 0.1.0 or ~> 1.0", [hex: :castore, repo: "hexpm", optional: true]}, {:hpax, "~> 0.1.1 or ~> 0.2.0 or ~> 1.0", [hex: :hpax, repo: "hexpm", optional: false]}], "hexpm", "5f7c9342480c069dbbc4eeac3490303c9e01870ff01a7f1d29b6107054fc1e74"},
   "nimble_options": {:hex, :nimble_options, "1.1.1", "e3a492d54d85fc3fd7c5baf411d9d2852922f66e69476317787a7b2bb000a61b", [:mix], [], "hexpm", "821b2470ca9442c4b6984882fe9bb0389371b8ddec4d45a9504f00a66f650b44"},
   "nimble_pool": {:hex, :nimble_pool, "1.1.0", "bf9c29fbdcba3564a8b800d1eeb5a3c58f36e1e11d7b7fb2e084a643f645f06b", [:mix], [], "hexpm", "af2e4e6b34197db81f7aad230c1118eac993acc0dae6bc83bac0126d4ae0813a"},
+  "plug": {:hex, :plug, "1.20.3", "56c480c633ec2ce10140e236e15233bf576e1d323887d7c96711bd02ab5160db", [:mix], [{:mime, "~> 1.0 or ~> 2.0", [hex: :mime, repo: "hexpm", optional: false]}, {:plug_crypto, "~> 1.1.1 or ~> 1.2 or ~> 2.0", [hex: :plug_crypto, repo: "hexpm", optional: false]}, {:telemetry, "~> 0.4.3 or ~> 1.0", [hex: :telemetry, repo: "hexpm", optional: false]}], "hexpm", "be266aee1b8536ef6409d58cf39a3121319f0ec47cfa1b24024485aa0e76ad76"},
+  "plug_crypto": {:hex, :plug_crypto, "2.1.1", "19bda8184399cb24afa10be734f84a16ea0a2bc65054e23a62bb10f06bc89491", [:mix], [], "hexpm", "6470bce6ffe41c8bd497612ffde1a7e4af67f36a15eea5f921af71cf3e11247c"},
   "req": {:hex, :req, "0.6.3", "7fe5e68792ff0546e45d5919104fa1764a13694cfe3e48c8a0f32ad051ae77e4", [:mix], [{:brotli, "~> 0.3.1", [hex: :brotli, repo: "hexpm", optional: true]}, {:ezstd, "~> 1.0", [hex: :ezstd, repo: "hexpm", optional: true]}, {:finch, "~> 0.21", [hex: :finch, repo: "hexpm", optional: false]}, {:jason, "~> 1.0", [hex: :jason, repo: "hexpm", optional: false]}, {:mime, "~> 2.0.6 or ~> 2.1", [hex: :mime, repo: "hexpm", optional: false]}, {:nimble_csv, "~> 1.0", [hex: :nimble_csv, repo: "hexpm", optional: true]}, {:plug, "~> 1.0", [hex: :plug, repo: "hexpm", optional: true]}], "hexpm", "e85b5c6c990e6c3f52bbba68e6f099118f2b8252825f96c7c3636b97a3de307d"},
   "telemetry": {:hex, :telemetry, "1.4.2", "a0cb522801dffb1c49fe6e30561badffc7b6d0e180db1300df759faa22062855", [:rebar3], [], "hexpm", "928f6495066506077862c0d1646609eed891a4326bee3126ba54b60af61febb1"},
   "yamerl": {:hex, :yamerl, "0.10.0", "4ff81fee2f1f6a46f1700c0d880b24d193ddb74bd14ef42cb0bcf46e81ef2f8e", [:rebar3], [], "hexpm", "346adb2963f1051dc837a2364e4acf6eb7d80097c0f53cbdc3046ec8ec4b4e6e"},
diff --git a/test/tool_kit/github/client_gh_cli_test.exs b/test/tool_kit/github/client_gh_cli_test.exs
new file mode 100644
index 0000000..9e61c0f
--- /dev/null
+++ b/test/tool_kit/github/client_gh_cli_test.exs
@@ -0,0 +1,28 @@
+defmodule ToolKit.GitHub.ClientGhCliTest do
+  # PATH 環境変数を書き換えるテストを含むため、他のテストと並列実行しない
+  use ExUnit.Case, async: false
+
+  alias ToolKit.GitHub.Client
+
+  describe "gh_cli_token/0(既定プロバイダ)" do
+    # 環境依存(gh の有無・認証状態)のため、戻り値の形だけを検証する
+    test "認証状態にかかわらず {:ok, token} か {:error, message} を返す" do
+      case Client.gh_cli_token() do
+        {:ok, token} -> assert is_binary(token) and token != ""
+        {:error, message} -> assert is_binary(message)
+      end
+    end
+
+    test "gh が見つからない場合はエラーメッセージを返す" do
+      original_path = System.get_env("PATH")
+
+      try do
+        System.put_env("PATH", "")
+        assert {:error, message} = Client.gh_cli_token()
+        assert message =~ "gh"
+      after
+        System.put_env("PATH", original_path)
+      end
+    end
+  end
+end
diff --git a/test/tool_kit/github/client_test.exs b/test/tool_kit/github/client_test.exs
new file mode 100644
index 0000000..9b17650
--- /dev/null
+++ b/test/tool_kit/github/client_test.exs
@@ -0,0 +1,403 @@
+defmodule ToolKit.GitHub.ClientTest do
+  use ExUnit.Case, async: true
+
+  alias ToolKit.GitHub.Client
+
+  doctest ToolKit.GitHub.Client
+
+  @stub ToolKit.GitHub.ClientStub
+
+  # 実 HTTP は呼ばず、Req.Test の Plug スタブへ差し替える共通オプション
+  defp opts(extra \\ []) do
+    Keyword.merge(
+      [
+        token_provider: fn -> {:ok, "test-token"} end,
+        req_options: [plug: {Req.Test, @stub}]
+      ],
+      extra
+    )
+  end
+
+  defp stub_capture(test_pid, response_body) do
+    Req.Test.stub(@stub, fn conn ->
+      {:ok, raw_body, conn} = Plug.Conn.read_body(conn)
+      conn = Plug.Conn.fetch_query_params(conn)
+
+      send(
+        test_pid,
+        {:request,
+         %{method: conn.method, path: conn.request_path, query: conn.query_params, body: raw_body}}
+      )
+
+      Req.Test.json(conn, response_body)
+    end)
+  end
+
+  describe "request/3 の共通挙動" do
+    test "Bearer トークンと GitHub API 用ヘッダを送る" do
+      Req.Test.stub(@stub, fn conn ->
+        assert Plug.Conn.get_req_header(conn, "authorization") == ["Bearer test-token"]
+        assert Plug.Conn.get_req_header(conn, "accept") == ["application/vnd.github+json"]
+        assert Plug.Conn.get_req_header(conn, "x-github-api-version") == ["2022-11-28"]
+        assert [user_agent] = Plug.Conn.get_req_header(conn, "user-agent")
+        assert user_agent =~ "elixir-tool-kit"
+        Req.Test.json(conn, %{"ok" => true})
+      end)
+
+      assert {:ok, %{"ok" => true}} = Client.get("/repos/smkwlab/demo", opts())
+    end
+
+    test ":user_agent オプションで User-Agent を差し替えられる" do
+      Req.Test.stub(@stub, fn conn ->
+        assert Plug.Conn.get_req_header(conn, "user-agent") == ["my-tool/1.0"]
+        Req.Test.json(conn, %{})
+      end)
+
+      assert {:ok, _} = Client.get("/user", opts(user_agent: "my-tool/1.0"))
+    end
+
+    test "token_provider の失敗は {:token_error, reason} に包んで返す" do
+      failing = fn -> {:error, :no_token} end
+
+      assert {:error, {:token_error, :no_token}} =
+               Client.get("/repos/smkwlab/demo", opts(token_provider: failing))
+    end
+
+    test "post/2 は JSON ボディを送る" do
+      stub_capture(self(), %{"id" => 1})
+
+      assert {:ok, %{"id" => 1}} =
+               Client.post("/repos/smkwlab/demo/labels", %{name: "bug"}, opts())
+
+      assert_received {:request, request}
+      assert request.method == "POST"
+      assert Jason.decode!(request.body) == %{"name" => "bug"}
+    end
+
+    test "put/2 と patch/2 は対応する HTTP メソッドを使う" do
+      stub_capture(self(), %{})
+      assert {:ok, _} = Client.put("/x", %{a: 1}, opts())
+      assert_received {:request, %{method: "PUT"}}
+
+      stub_capture(self(), %{})
+      assert {:ok, _} = Client.patch("/x", %{a: 1}, opts())
+      assert_received {:request, %{method: "PATCH"}}
+    end
+
+    test "通信自体の失敗は {:request_failed, reason} を返す" do
+      Req.Test.stub(@stub, fn conn ->
+        Req.Test.transport_error(conn, :timeout)
+      end)
+
+      assert {:error, {:request_failed, %Req.TransportError{reason: :timeout}}} =
+               Client.get("/repos/smkwlab/demo", opts())
+    end
+  end
+
+  describe "エラー分類(HTTP 経由)" do
+    test "404 は {:error, :not_found}" do
+      Req.Test.stub(@stub, fn conn ->
+        conn |> Plug.Conn.put_status(404) |> Req.Test.json(%{"message" => "Not Found"})
+      end)
+
+      assert {:error, :not_found} = Client.get("/repos/smkwlab/none", opts())
+    end
+
+    test "401 と 403 は {:error, :unauthorized}" do
+      for status <- [401, 403] do
+        Req.Test.stub(@stub, fn conn ->
+          conn |> Plug.Conn.put_status(status) |> Req.Test.json(%{"message" => "Bad credentials"})
+        end)
+
+        assert {:error, :unauthorized} = Client.get("/repos/smkwlab/private", opts())
+      end
+    end
+
+    test "その他のエラーは {:http_error, status, message}" do
+      Req.Test.stub(@stub, fn conn ->
+        conn |> Plug.Conn.put_status(422) |> Req.Test.json(%{"message" => "Validation Failed"})
+      end)
+
+      assert {:error, {:http_error, 422, "Validation Failed"}} =
+               Client.get("/repos/smkwlab/demo", opts())
+    end
+  end
+
+  describe "classify_response/1(純関数)" do
+    test "2xx はボディをそのまま返す" do
+      assert Client.classify_response({:ok, %{status: 200, body: %{"a" => 1}}}) ==
+               {:ok, %{"a" => 1}}
+
+      assert Client.classify_response({:ok, %{status: 201, body: nil}}) == {:ok, nil}
+    end
+
+    test "404 / 401 / 403 を分類する" do
+      assert Client.classify_response({:ok, %{status: 404, body: %{}}}) == {:error, :not_found}
+      assert Client.classify_response({:ok, %{status: 401, body: %{}}}) == {:error, :unauthorized}
+      assert Client.classify_response({:ok, %{status: 403, body: %{}}}) == {:error, :unauthorized}
+    end
+
+    test "エラーメッセージはボディ形式に応じて抽出する" do
+      assert Client.classify_response({:ok, %{status: 500, body: %{"message" => "boom"}}}) ==
+               {:error, {:http_error, 500, "boom"}}
+
+      assert Client.classify_response({:ok, %{status: 500, body: "internal error"}}) ==
+               {:error, {:http_error, 500, "500 - internal error"}}
+
+      assert Client.classify_response({:ok, %{status: 502, body: %{"other" => true}}}) ==
+               {:error, {:http_error, 502, "HTTP 502"}}
+    end
+
+    test "Req のエラーは {:request_failed, reason} に包む" do
+      assert Client.classify_response({:error, :nxdomain}) ==
+               {:error, {:request_failed, :nxdomain}}
+    end
+  end
+
+  describe "build_url/2(純関数)" do
+    test "base_url と path をスラッシュ 1 個で結合する" do
+      assert Client.build_url("https://api.github.com", "/repos/a/b") ==
+               "https://api.github.com/repos/a/b"
+
+      assert Client.build_url("https://api.github.com/", "repos/a/b") ==
+               "https://api.github.com/repos/a/b"
+
+      assert Client.build_url("https://ghe.example.com/api/v3/", "/repos/a/b") ==
+               "https://ghe.example.com/api/v3/repos/a/b"
+    end
+  end
+
+  describe "contents ヘルパ" do
+    test "get_file_contents/3 は contents API を GET する" do
+      stub_capture(self(), %{"content" => "e30=", "encoding" => "base64", "sha" => "abc"})
+
+      assert {:ok, %{"sha" => "abc"}} =
+               Client.get_file_contents("smkwlab/repo", "data/registry.json", opts())
+
+      assert_received {:request, request}
+      assert request.method == "GET"
+      assert request.path == "/repos/smkwlab/repo/contents/data/registry.json"
+      assert request.query == %{}
+    end
+
+    test "get_file_contents/3 は :ref をクエリに載せる" do
+      stub_capture(self(), %{})
+      assert {:ok, _} = Client.get_file_contents("smkwlab/repo", "README.md", opts(ref: "main"))
+
+      assert_received {:request, %{query: %{"ref" => "main"}}}
+    end
+
+    test "put_file_contents/5 は base64 化した内容と SHA を PUT する" do
+      stub_capture(self(), %{"commit" => %{"sha" => "new"}})
+
+      assert {:ok, _} =
+               Client.put_file_contents(
+                 "smkwlab/repo",
+                 "data/registry.json",
+                 ~s({"students": []}),
+                 "chore: update registry",
+                 opts(sha: "oldsha", branch: "main")
+               )
+
+      assert_received {:request, request}
+      assert request.method == "PUT"
+      assert request.path == "/repos/smkwlab/repo/contents/data/registry.json"
+
+      body = Jason.decode!(request.body)
+      assert body["message"] == "chore: update registry"
+      assert body["sha"] == "oldsha"
+      assert body["branch"] == "main"
+      assert Base.decode64!(body["content"]) == ~s({"students": []})
+    end
+
+    test "put_file_contents/5 は SHA なし(新規作成)なら sha キーを送らない" do
+      stub_capture(self(), %{})
+
+      assert {:ok, _} =
+               Client.put_file_contents("smkwlab/repo", "new.txt", "hello", "add file", opts())
+
+      assert_received {:request, request}
+      body = Jason.decode!(request.body)
+      refute Map.has_key?(body, "sha")
+      refute Map.has_key?(body, "branch")
+    end
+
+    test "get_file_text/3 は取得とデコードをまとめて行う" do
+      encoded = Base.encode64("hello world")
+      stub_capture(self(), %{"content" => encoded, "encoding" => "base64"})
+
+      assert {:ok, "hello world"} = Client.get_file_text("smkwlab/repo", "hello.txt", opts())
+    end
+
+    test "get_file_text/3 は取得エラーをそのまま返す" do
+      Req.Test.stub(@stub, fn conn ->
+        conn |> Plug.Conn.put_status(404) |> Req.Test.json(%{"message" => "Not Found"})
+      end)
+
+      assert {:error, :not_found} = Client.get_file_text("smkwlab/repo", "none.txt", opts())
+    end
+  end
+
+  describe "decode_content/1(純関数)" do
+    test "60 桁ごとの改行入り base64 をデコードする" do
+      text = String.duplicate("あいうえお", 30)
+
+      wrapped =
+        text
+        |> Base.encode64()
+        |> String.codepoints()
+        |> Enum.chunk_every(60)
+        |> Enum.map_join("\n", &Enum.join/1)
+
+      assert Client.decode_content(%{"content" => wrapped, "encoding" => "base64"}) == {:ok, text}
+    end
+
+    test "不正な base64 は {:error, :invalid_content}" do
+      assert Client.decode_content(%{"content" => "%%%", "encoding" => "base64"}) ==
+               {:error, :invalid_content}
+    end
+
+    test "想定外の形式は {:error, :invalid_content}" do
+      assert Client.decode_content(%{"encoding" => "base64"}) == {:error, :invalid_content}
+
+      assert Client.decode_content(%{"content" => "e30=", "encoding" => "utf-8"}) ==
+               {:error, :invalid_content}
+
+      assert Client.decode_content("not a map") == {:error, :invalid_content}
+    end
+  end
+
+  describe "repo / commits / pulls ヘルパ" do
+    test "get_repository/2 はリポジトリ情報を GET する" do
+      stub_capture(self(), %{"full_name" => "smkwlab/repo"})
+
+      assert {:ok, %{"full_name" => "smkwlab/repo"}} =
+               Client.get_repository("smkwlab/repo", opts())
+
+      assert_received {:request, %{method: "GET", path: "/repos/smkwlab/repo"}}
+    end
+
+    test "list_branches/2 はブランチ一覧を GET する" do
+      stub_capture(self(), [%{"name" => "main"}])
+
+      assert {:ok, [%{"name" => "main"}]} =
+               Client.list_branches("smkwlab/repo", opts(per_page: 100))
+
+      assert_received {:request, request}
+      assert request.path == "/repos/smkwlab/repo/branches"
+      assert request.query == %{"per_page" => "100"}
+    end
+
+    test "list_commits/2 は since / author / per_page をクエリに載せる" do
+      stub_capture(self(), [])
+
+      assert {:ok, []} =
+               Client.list_commits(
+                 "smkwlab/repo",
+                 opts(since: "2026-07-01T00:00:00Z", author: "student", per_page: 10)
+               )
+
+      assert_received {:request, request}
+      assert request.path == "/repos/smkwlab/repo/commits"
+
+      assert request.query == %{
+               "since" => "2026-07-01T00:00:00Z",
+               "author" => "student",
+               "per_page" => "10"
+             }
+    end
+
+    test "list_commits/2 は nil のパラメータを送らない" do
+      stub_capture(self(), [])
+      assert {:ok, []} = Client.list_commits("smkwlab/repo", opts(author: nil, per_page: 1))
+
+      assert_received {:request, %{query: %{"per_page" => "1"} = query}}
+      refute Map.has_key?(query, "author")
+    end
+
+    test "ヘルパは呼び出し元の :params を保持したままマージする" do
+      stub_capture(self(), [])
+
+      assert {:ok, []} = Client.list_commits("smkwlab/repo", opts(per_page: 5, params: [page: 2]))
+
+      assert_received {:request, %{query: %{"per_page" => "5", "page" => "2"}}}
+    end
+
+    test "list_pull_requests/2 は state / per_page をクエリに載せる" do
+      stub_capture(self(), [])
+
+      assert {:ok, []} =
+               Client.list_pull_requests("smkwlab/repo", opts(state: "all", per_page: 100))
+
+      assert_received {:request, request}
+      assert request.path == "/repos/smkwlab/repo/pulls"
+      assert request.query == %{"state" => "all", "per_page" => "100"}
+    end
+
+    test "list_pull_request_reviews/3 はレビュー一覧を GET する" do
+      stub_capture(self(), [])
+      assert {:ok, []} = Client.list_pull_request_reviews("smkwlab/repo", 12, opts(per_page: 100))
+
+      assert_received {:request, request}
+      assert request.path == "/repos/smkwlab/repo/pulls/12/reviews"
+      assert request.query == %{"per_page" => "100"}
+    end
+
+    test "get_requested_reviewers/3 はレビューリクエストを GET する" do
+      stub_capture(self(), %{"users" => []})
+      assert {:ok, %{"users" => []}} = Client.get_requested_reviewers("smkwlab/repo", 12, opts())
+
+      assert_received {:request, %{path: "/repos/smkwlab/repo/pulls/12/requested_reviewers"}}
+    end
+
+    test "create_issue_comment/4 はコメントを POST する" do
+      stub_capture(self(), %{"id" => 1})
+
+      assert {:ok, _} = Client.create_issue_comment("smkwlab/repo", 34, "対応しました", opts())
+
+      assert_received {:request, request}
+      assert request.method == "POST"
+      assert request.path == "/repos/smkwlab/repo/issues/34/comments"
+      assert Jason.decode!(request.body) == %{"body" => "対応しました"}
+    end
+
+    test "close_pull_request/3 は state: closed を PATCH する" do
+      stub_capture(self(), %{"state" => "closed"})
+
+      assert {:ok, _} = Client.close_pull_request("smkwlab/repo", 5, opts())
+
+      assert_received {:request, request}
+      assert request.method == "PATCH"
+      assert request.path == "/repos/smkwlab/repo/pulls/5"
+      assert Jason.decode!(request.body) == %{"state" => "closed"}
+    end
+
+    test "archive_repository/2 は archived: true を PATCH する" do
+      stub_capture(self(), %{"archived" => true})
+
+      assert {:ok, _} = Client.archive_repository("smkwlab/repo", opts())
+
+      assert_received {:request, request}
+      assert request.method == "PATCH"
+      assert request.path == "/repos/smkwlab/repo"
+      assert Jason.decode!(request.body) == %{"archived" => true}
+    end
+  end
+
+  describe "エラー述語" do
+    test "not_found_error?/1 は :not_found と {:error, :not_found} を真とする" do
+      assert Client.not_found_error?(:not_found)
+      assert Client.not_found_error?({:error, :not_found})
+      refute Client.not_found_error?(:unauthorized)
+      refute Client.not_found_error?({:error, {:http_error, 500, "boom"}})
+      refute Client.not_found_error?("GitHub API error (404)")
+    end
+
+    test "unauthorized_error?/1 は :unauthorized と {:error, :unauthorized} を真とする" do
+      assert Client.unauthorized_error?(:unauthorized)
+      assert Client.unauthorized_error?({:error, :unauthorized})
+      refute Client.unauthorized_error?(:not_found)
+      refute Client.unauthorized_error?(nil)
+    end
+  end
+end