diff --git a/.github/workflows/checks.yml b/.github/workflows/checks.yml
index 4cf1439..fb2b575 100644
--- a/.github/workflows/checks.yml
+++ b/.github/workflows/checks.yml
@@ -84,6 +84,9 @@ jobs:
         with:
           elixir-version: "1.15"
           otp-version: "26"
+      - name: mix deps.get
+        working-directory: elixir
+        run: mix deps.get
       - name: Run tests
         working-directory: elixir
         run: mix test
diff --git a/.gitignore b/.gitignore
index cfd2fc5..d67578e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,4 @@ node_modules/
 __pycache__/
 *.py[cod]
 _build/
+deps/
diff --git a/elixir/AGENTS.md b/elixir/AGENTS.md
index e5d1ba6..8df26fe 100644
--- a/elixir/AGENTS.md
+++ b/elixir/AGENTS.md
@@ -11,13 +11,26 @@ Runtime target: Elixir 1.15 or newer.
 Key files:
 
 - `lib/client_signals.ex` — runtime implementation and exported API.
+- `lib/client_signals/plug.ex` — `ClientSignals.Plug`, the consumer side:
+  reads `Fly-Client-*` request headers and attaches them as OTel span
+  attributes. Only defined when both optional deps are loaded (see below).
 - `test/client_signals_test.exs` — ExUnit suite, including shared fixture
   checks.
+- `test/client_signals/plug_test.exs` — `ClientSignals.Plug` tests.
 - `mix.exs` — package metadata.
 
 ## Constraints
 
-- No runtime dependencies.
+- The core signal-detection API (`lib/client_signals.ex`) has no runtime
+  dependencies.
+- `:plug` and `:opentelemetry_api` are declared as `optional: true` deps
+  solely so `ClientSignals.Plug` can exist. This is a deliberate, narrow
+  exception to "no runtime dependencies" — justified because
+  `ClientSignals.Plug` is a server-side consumer of the headers this
+  package produces, only used by services that already depend on Plug and
+  OpenTelemetry (e.g. a Phoenix app), and the whole module is skipped at
+  compile time (`Code.ensure_loaded?/1` guard) for anyone who doesn't have
+  both. Do not add further optional or hard dependencies without asking.
 - Do not shell out for parent-process lookup.
 - `detect_once/0` must cache the first detected value.
 - Keep `known_markers/0` aligned with `../spec/markers.json`.
diff --git a/elixir/README.md b/elixir/README.md
index 4272892..972eef8 100644
--- a/elixir/README.md
+++ b/elixir/README.md
@@ -45,6 +45,21 @@ ClientSignals.apply_headers(headers, signals, "Acme")
   `ClientSignals.classify_parent_name/1` are exposed for tests and
   advanced consumers that need the shared contract helpers.
 
+## Server-side: recording signals on request spans
+
+If your application already depends on `:plug` and `:opentelemetry_api`
+(e.g. a Phoenix app), `ClientSignals.Plug` reads the `Fly-Client-*`
+headers off incoming requests and attaches them as `fly.client.*`
+attributes on the current OTel span:
+
+```elixir
+# in your endpoint or router
+plug ClientSignals.Plug
+```
+
+This module is only defined when both dependencies are present, so it
+has no effect on consumers that only use the header-generation API above.
+
 ## Development
 
 ```sh
diff --git a/elixir/lib/client_signals/plug.ex b/elixir/lib/client_signals/plug.ex
new file mode 100644
index 0000000..e23fb55
--- /dev/null
+++ b/elixir/lib/client_signals/plug.ex
@@ -0,0 +1,90 @@
+if Code.ensure_loaded?(Plug.Conn) and Code.ensure_loaded?(OpenTelemetry.Tracer) do
+  defmodule ClientSignals.Plug do
+    @moduledoc """
+    Reads the coarse, self-reported `Fly-Client-*` headers (as produced by
+    `ClientSignals.headers_for/2`) off an incoming request and attaches
+    them to the current request's OTel span as `fly.client.*` attributes.
+
+    This gives services a rough human-vs-agent traffic estimate for
+    capacity planning and product decisions. These values are self-reported
+    by the caller and must never be used for gating, enforcement, or any
+    per-request trust decision.
+
+    This module is only defined when both `:plug` and `:opentelemetry_api`
+    are loaded; both are optional dependencies of `:client_signals`.
+    """
+
+    import Plug.Conn
+    require OpenTelemetry.Tracer, as: Tracer
+
+    @behaviour Plug
+
+    @interactive_header "fly-client-interactive"
+    @parent_header "fly-client-parent"
+    @agent_header "fly-client-agent"
+    @agent_source_header "fly-client-agent-source"
+    @ci_header "fly-client-ci"
+
+    # Headers are caller-controlled; bound how much of them we keep so a
+    # malicious or misbehaving client can't bloat span payloads.
+    @max_value_length 256
+
+    @impl true
+    def init(opts), do: opts
+
+    @impl true
+    def call(conn, _opts) do
+      attrs = client_signal_attributes(conn)
+
+      if attrs != [] do
+        Tracer.set_attributes(attrs)
+      end
+
+      conn
+    end
+
+    @doc false
+    def client_signal_attributes(conn) do
+      []
+      |> put_bool_attr(conn, @interactive_header, "fly.client.interactive")
+      |> put_string_attr(conn, @parent_header, "fly.client.parent")
+      |> put_string_attr(conn, @agent_header, "fly.client.agent")
+      |> put_string_attr(conn, @agent_source_header, "fly.client.agent_source")
+      |> put_bool_attr(conn, @ci_header, "fly.client.ci")
+    end
+
+    defp put_string_attr(attrs, conn, header, key) do
+      case get_req_header(conn, header) do
+        [value | _] ->
+          case String.trim(value) do
+            "" -> attrs
+            trimmed -> [{key, String.slice(trimmed, 0, @max_value_length)} | attrs]
+          end
+
+        _ ->
+          attrs
+      end
+    end
+
+    defp put_bool_attr(attrs, conn, header, key) do
+      case get_req_header(conn, header) do
+        [value | _] ->
+          case parse_bool(value) do
+            {:ok, bool} -> [{key, bool} | attrs]
+            :error -> attrs
+          end
+
+        _ ->
+          attrs
+      end
+    end
+
+    defp parse_bool(value) do
+      case value |> String.trim() |> String.downcase() do
+        v when v in ~w(1 t true) -> {:ok, true}
+        v when v in ~w(0 f false) -> {:ok, false}
+        _ -> :error
+      end
+    end
+  end
+end
diff --git a/elixir/mix.exs b/elixir/mix.exs
index fa3d290..b25a9f2 100644
--- a/elixir/mix.exs
+++ b/elixir/mix.exs
@@ -7,7 +7,7 @@ defmodule ClientSignals.MixProject do
       version: "0.0.0",
       elixir: "~> 1.15",
       start_permanent: Mix.env() == :prod,
-      deps: [],
+      deps: deps(),
       description: "Privacy-safe client signals for CLI HTTP traffic.",
       package: package()
     ]
@@ -19,6 +19,17 @@ defmodule ClientSignals.MixProject do
     ]
   end
 
+  # :plug and :opentelemetry_api are optional: the core signal-detection API
+  # has no runtime dependencies, but including apps that already depend on
+  # both (e.g. a Phoenix app) can use ClientSignals.Plug, which is only
+  # defined when both are loaded. See elixir/AGENTS.md.
+  defp deps do
+    [
+      {:plug, "~> 1.14", optional: true},
+      {:opentelemetry_api, "~> 1.4", optional: true}
+    ]
+  end
+
   defp package do
     [
       licenses: ["MIT"],
diff --git a/elixir/mix.lock b/elixir/mix.lock
new file mode 100644
index 0000000..902c625
--- /dev/null
+++ b/elixir/mix.lock
@@ -0,0 +1,7 @@
+%{
+  "mime": {:hex, :mime, "2.0.7", "b8d739037be7cd402aee1ba0306edfdef982687ee7e9859bee6198c1e7e2f128", [:mix], [], "hexpm", "6171188e399ee16023ffc5b76ce445eb6d9672e2e241d2df6050f3c771e80ccd"},
+  "opentelemetry_api": {:hex, :opentelemetry_api, "1.5.0", "1a676f3e3340cab81c763e939a42e11a70c22863f645aa06aafefc689b5550cf", [:mix, :rebar3], [], "hexpm", "f53ec8a1337ae4a487d43ac89da4bd3a3c99ddf576655d071deed8b56a2d5dda"},
+  "plug": {:hex, :plug, "1.20.2", "adbee2441232412e37fbb357fd5e4cd533fdd253b29f2e1992262b0f1fb01462", [: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", "b16baf55877d60891002ffc1ce0b3ff7d6f30a38a23e02e4d4293c4ac266f136"},
+  "plug_crypto": {:hex, :plug_crypto, "2.1.1", "19bda8184399cb24afa10be734f84a16ea0a2bc65054e23a62bb10f06bc89491", [:mix], [], "hexpm", "6470bce6ffe41c8bd497612ffde1a7e4af67f36a15eea5f921af71cf3e11247c"},
+  "telemetry": {:hex, :telemetry, "1.4.2", "a0cb522801dffb1c49fe6e30561badffc7b6d0e180db1300df759faa22062855", [:rebar3], [], "hexpm", "928f6495066506077862c0d1646609eed891a4326bee3126ba54b60af61febb1"},
+}
diff --git a/elixir/test/client_signals/plug_test.exs b/elixir/test/client_signals/plug_test.exs
new file mode 100644
index 0000000..27b4a4c
--- /dev/null
+++ b/elixir/test/client_signals/plug_test.exs
@@ -0,0 +1,99 @@
+defmodule ClientSignals.PlugTest do
+  use ExUnit.Case, async: true
+
+  alias ClientSignals.Plug, as: ClientSignalsPlug
+
+  defp build_conn(headers) do
+    conn = Plug.Test.conn(:get, "/")
+
+    Enum.reduce(headers, conn, fn {k, v}, conn ->
+      Plug.Conn.put_req_header(conn, k, v)
+    end)
+  end
+
+  describe "client_signal_attributes/1" do
+    test "collects all headers when present" do
+      conn =
+        build_conn([
+          {"fly-client-interactive", "true"},
+          {"fly-client-parent", "shell"},
+          {"fly-client-agent", "claude-code"},
+          {"fly-client-agent-source", "env:CLAUDECODE"},
+          {"fly-client-ci", "true"}
+        ])
+
+      attrs = ClientSignalsPlug.client_signal_attributes(conn) |> Map.new()
+
+      assert attrs == %{
+               "fly.client.interactive" => true,
+               "fly.client.parent" => "shell",
+               "fly.client.agent" => "claude-code",
+               "fly.client.agent_source" => "env:CLAUDECODE",
+               "fly.client.ci" => true
+             }
+    end
+
+    test "returns nothing when no headers are present" do
+      assert ClientSignalsPlug.client_signal_attributes(build_conn([])) == []
+    end
+
+    test "only includes attributes for headers actually present" do
+      conn =
+        build_conn([{"fly-client-interactive", "false"}, {"fly-client-parent", "python"}])
+
+      attrs = ClientSignalsPlug.client_signal_attributes(conn) |> Map.new()
+
+      assert attrs == %{
+               "fly.client.interactive" => false,
+               "fly.client.parent" => "python"
+             }
+    end
+
+    test "ignores unparseable boolean values" do
+      conn = build_conn([{"fly-client-interactive", "maybe"}, {"fly-client-ci", "nope"}])
+
+      assert ClientSignalsPlug.client_signal_attributes(conn) == []
+    end
+
+    test "ignores an empty or whitespace-only string value" do
+      conn = build_conn([{"fly-client-parent", "   "}])
+
+      assert ClientSignalsPlug.client_signal_attributes(conn) == []
+    end
+
+    test "trims whitespace from string and boolean values" do
+      conn =
+        build_conn([{"fly-client-parent", "  shell  "}, {"fly-client-interactive", " true\n"}])
+
+      attrs = ClientSignalsPlug.client_signal_attributes(conn) |> Map.new()
+
+      assert attrs == %{
+               "fly.client.parent" => "shell",
+               "fly.client.interactive" => true
+             }
+    end
+
+    test "truncates string values longer than the max attribute length" do
+      long_value = String.duplicate("a", 500)
+      conn = build_conn([{"fly-client-parent", long_value}])
+
+      attrs = ClientSignalsPlug.client_signal_attributes(conn) |> Map.new()
+
+      assert String.length(attrs["fly.client.parent"]) == 256
+    end
+  end
+
+  describe "call/2" do
+    test "returns the conn unchanged" do
+      conn = build_conn([{"fly-client-interactive", "true"}])
+
+      assert ClientSignalsPlug.call(conn, ClientSignalsPlug.init([])) == conn
+    end
+
+    test "does not raise when there are no client signal headers" do
+      conn = build_conn([])
+
+      assert ClientSignalsPlug.call(conn, ClientSignalsPlug.init([])) == conn
+    end
+  end
+end