diff --git a/CHANGELOG.md b/CHANGELOG.md
index c24f65d..244afe7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -18,6 +18,7 @@
 
 ## Bug Fixes
 
+- fix: `Cache.Sandbox` no longer raises `"Not Implemented"` from the Redis-only surface. `smembers/3` and `sadd/4` are real in-memory sets with Redis semantics (`SADD` returns the new-member count, `SMEMBERS` of a missing key is `[]`). `command/3` and `pipeline/3` handle `PING`, `GET`, `EXISTS` and `DEL` against the sandbox map and return `{:error, %ErrorMessage{code: :not_implemented}}` for anything else — the error shape the Redis adapter itself produces — with `command!/3`/`pipeline!/3` raising only on that branch. Under `sandbox?: Mix.env() === :test` the `use Cache` wrappers delegate to the sandbox, so dialyzer (running in test) had been correctly reporting every Redis cache module's injected `command/1`, `pipeline/1`, `sadd/2`, `smembers/2` (and their default-arg arities) as having no local return, forcing a `:no_return` ignore entry per Redis cache module in every consuming app. Those ignores can now be deleted.
 - fix: `:compression_level` is reachable. It was unusable on every path — no adapter declares it, so `NimbleOptions` rejected it on compile-time adapter opts, and it resolved to `nil` before it could reach the encoder otherwise. It is now an option on the `use Cache` line (`compression_level: 6`), it is taken off the adapter opts before they are validated so the `opts: [compression_level: 6]` spelling works too, and it is never handed to the adapter. Setting it forces encoding on adapters that hold terms natively — asking for compression is asking for bytes. A cache using a strategy adapter raises at compile time rather than ignoring the option.
 - fix: `Cache.ConCache.get_or_store/3` followed by `get/1` no longer raises. `get_or_store/3` writes through ConCache directly, bypassing the encode in `put/3`, so the matching `get/1` tried to `binary_to_term/1` a raw term.
 - fix: a binary value wrapped in braces but not valid JSON (eg `"{oops}"`) no longer raises `Jason.DecodeError` on read. `Cache.TermEncoder.decode/1` used `Jason.decode!/1`, and now falls back to returning the binary unchanged.
diff --git a/lib/cache/sandbox.ex b/lib/cache/sandbox.ex
index c72404e..f022efe 100644
--- a/lib/cache/sandbox.ex
+++ b/lib/cache/sandbox.ex
@@ -397,22 +397,66 @@ defmodule Cache.Sandbox do
 
   # SECTION: Redis compatibility
 
-  def pipeline(_cache_name, _commands, _opts) do
-    raise "Not Implemented"
+  # Raw command passthrough. The sandbox is a term map, not a Redis server, so
+  # only the commands whose semantics map directly onto that map are supported;
+  # anything else is a `:not_implemented` error rather than a raise, so the
+  # `use Cache` wrappers keep a real return type and callers see the same
+  # `{:error, %ErrorMessage{}}` shape the Redis adapter produces on failure.
+  def pipeline(cache_name, commands, opts) do
+    with {:ok, replies} <- run_commands(cache_name, commands, opts) do
+      {:ok, Enum.reverse(replies)}
+    end
+  end
+
+  def pipeline!(cache_name, commands, opts) do
+    case pipeline(cache_name, commands, opts) do
+      {:ok, replies} -> replies
+      {:error, %ErrorMessage{message: message}} -> raise message
+    end
+  end
+
+  def command(_cache_name, ["PING"], _opts), do: {:ok, "PONG"}
+
+  def command(cache_name, ["GET", key], _opts) do
+    scoped_agent_get(cache_name, fn sub -> {:ok, Map.get(sub, key)} end)
   end
 
-  def pipeline!(_cache_name, _commands, _opts) do
-    raise "Not Implemented"
+  def command(cache_name, ["EXISTS", key], _opts) do
+    scoped_agent_get(cache_name, fn sub -> {:ok, boolean_to_integer(Map.has_key?(sub, key))} end)
   end
 
-  def command(_cache_name, _command, _opts) do
-    raise "Not Implemented"
+  def command(cache_name, ["DEL", key], _opts) do
+    scoped_agent_get_and_update(cache_name, fn sub ->
+      {{:ok, boolean_to_integer(Map.has_key?(sub, key))}, Map.delete(sub, key)}
+    end)
   end
 
-  def command!(_cache_name, _command, _opts) do
-    raise "Not Implemented"
+  def command(_cache_name, [command | _args], _opts) do
+    {:error,
+     ErrorMessage.not_implemented("#{command} is not implemented by the cache sandbox", %{
+       command: command
+     })}
   end
 
+  def command!(cache_name, command, opts) do
+    case command(cache_name, command, opts) do
+      {:ok, reply} -> reply
+      {:error, %ErrorMessage{message: message}} -> raise message
+    end
+  end
+
+  defp run_commands(cache_name, commands, opts) do
+    Enum.reduce_while(commands, {:ok, []}, fn command, {:ok, replies} ->
+      case command(cache_name, command, opts) do
+        {:ok, reply} -> {:cont, {:ok, [reply | replies]}}
+        {:error, _} = error -> {:halt, error}
+      end
+    end)
+  end
+
+  defp boolean_to_integer(true), do: 1
+  defp boolean_to_integer(false), do: 0
+
   def scan(cache_name, scan_opts, _opts) do
     match = scan_opts[:match] || "*"
     count = scan_opts[:count]
@@ -1171,12 +1215,22 @@ defmodule Cache.Sandbox do
     {Enum.reverse(results), continuation}
   end
 
-  def smembers(_cache_name, _key, _opts) do
-    raise "Not Implemented"
+  # SECTION: set API
+
+  # Sets are stored as MapSets under the key. Like Redis, SMEMBERS of a
+  # missing key is the empty set and SADD returns how many members were new.
+  def smembers(cache_name, key, _opts) do
+    scoped_agent_get(cache_name, fn sub ->
+      {:ok, sub |> Map.get(key, MapSet.new()) |> MapSet.to_list()}
+    end)
   end
 
-  def sadd(_cache_name, _key, _value, _opts) do
-    raise "Not Implemented"
+  def sadd(cache_name, key, value, _opts) do
+    scoped_agent_get_and_update(cache_name, fn sub ->
+      set = Map.get(sub, key, MapSet.new())
+      added = boolean_to_integer(not MapSet.member?(set, value))
+      {{:ok, added}, Map.put(sub, key, MapSet.put(set, value))}
+    end)
   end
 
   # SECTION: internal helpers
diff --git a/test/cache/redis_sandbox_test.exs b/test/cache/redis_sandbox_test.exs
new file mode 100644
index 0000000..6b3ade5
--- /dev/null
+++ b/test/cache/redis_sandbox_test.exs
@@ -0,0 +1,104 @@
+defmodule Cache.RedisSandboxTest do
+  @moduledoc """
+  Sandbox coverage for the Redis-only surface `use Cache, adapter: Cache.Redis`
+  injects (set ops, raw command/pipeline). These run against `Cache.Sandbox`,
+  so no Redis is needed.
+  """
+  use ExUnit.Case, async: true
+
+  defmodule RedisSandboxCache do
+    use Cache,
+      adapter: Cache.Redis,
+      name: :test_cache_redis_sandbox,
+      opts: [uri: "redis://localhost:6379"],
+      sandbox?: true
+  end
+
+  setup do
+    Cache.SandboxRegistry.start(RedisSandboxCache)
+
+    :ok
+  end
+
+  describe "&sadd/3 and &smembers/2" do
+    test "smembers of a missing key is an empty list" do
+      assert {:ok, []} === RedisSandboxCache.smembers("missing", [])
+    end
+
+    test "sadd returns the number of newly added members and smembers returns the set" do
+      assert {:ok, 1} === RedisSandboxCache.sadd("symbols", "AAPL")
+      assert {:ok, 1} === RedisSandboxCache.sadd("symbols", "TSLA")
+      assert {:ok, 0} === RedisSandboxCache.sadd("symbols", "AAPL")
+
+      assert {:ok, members} = RedisSandboxCache.smembers("symbols", [])
+      assert ["AAPL", "TSLA"] === Enum.sort(members)
+    end
+
+    test "members round-trip as terms, not binaries" do
+      assert {:ok, 1} === RedisSandboxCache.sadd("terms", %{symbol: "AAPL", strike: 150.0})
+      assert {:ok, [%{symbol: "AAPL", strike: 150.0}]} === RedisSandboxCache.smembers("terms", [])
+    end
+
+    test "sets are isolated per sandbox" do
+      assert {:ok, []} === RedisSandboxCache.smembers("symbols", [])
+    end
+  end
+
+  describe "&command/2" do
+    test "PING" do
+      assert {:ok, "PONG"} === RedisSandboxCache.command(["PING"])
+    end
+
+    test "GET / EXISTS / DEL against keys written through the cache API" do
+      assert :ok === RedisSandboxCache.put("key", "value")
+
+      # Raw GET returns the stored binary undecoded, exactly like Redis would —
+      # the Redis-backed sandbox term-encodes on put to round-trip faithfully.
+      assert {:ok, encoded} = RedisSandboxCache.command(["GET", "key"])
+      assert "value" === Cache.TermEncoder.decode(encoded)
+      assert {:ok, 1} === RedisSandboxCache.command(["EXISTS", "key"])
+      assert {:ok, 1} === RedisSandboxCache.command(["DEL", "key"])
+      assert {:ok, nil} === RedisSandboxCache.command(["GET", "key"])
+      assert {:ok, 0} === RedisSandboxCache.command(["EXISTS", "key"])
+      assert {:ok, 0} === RedisSandboxCache.command(["DEL", "key"])
+    end
+
+    test "unsupported commands return a not-implemented error instead of raising" do
+      assert {:error, %ErrorMessage{code: :not_implemented, details: %{command: "ZADD"}}} =
+               RedisSandboxCache.command(["ZADD", "key", "1", "member"])
+    end
+  end
+
+  describe "&pipeline/2" do
+    test "runs each command in order and returns the replies as a list" do
+      assert :ok === RedisSandboxCache.put("key", "value")
+
+      assert {:ok, ["PONG", encoded, 1]} =
+               RedisSandboxCache.pipeline([["PING"], ["GET", "key"], ["DEL", "key"]])
+
+      assert "value" === Cache.TermEncoder.decode(encoded)
+    end
+
+    test "an unsupported command fails the whole pipeline" do
+      assert {:error, %ErrorMessage{code: :not_implemented}} =
+               RedisSandboxCache.pipeline([["PING"], ["ZADD", "key", "1", "member"]])
+    end
+  end
+
+  describe "&command!/2 and &pipeline!/2" do
+    test "return the raw reply on success" do
+      assert "PONG" === RedisSandboxCache.command!(["PING"])
+      assert ["PONG", "PONG"] === RedisSandboxCache.pipeline!([["PING"], ["PING"]])
+    end
+
+    test "raise on an unsupported command" do
+      assert_raise RuntimeError, ~r/not implemented/, fn ->
+        RedisSandboxCache.command!(["ZADD", "key", "1", "member"])
+      end
+
+      assert_raise RuntimeError, ~r/not implemented/, fn ->
+        RedisSandboxCache.pipeline!([["ZADD", "key", "1", "member"]])
+      end
+    end
+  end
+end