diff --git a/AGENTS.md b/AGENTS.md
index a4d9b2e..35f5934 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -47,6 +47,7 @@ NOTE: Nostr and NIP specs are available in `~/nostr/` and `~/nips/`.
 - Predicate function names should not start with `is_` and should end in a question mark. Names like `is_thing` should be reserved for guards
 - Elixir's builtin OTP primitives like `DynamicSupervisor` and `Registry`, require names in the child spec, such as `{DynamicSupervisor, name: MyApp.MyDynamicSup}`, then you can use `DynamicSupervisor.start_child(MyApp.MyDynamicSup, child_spec)`
 - Use `Task.async_stream(collection, callback, options)` for concurrent enumeration with back-pressure. The majority of times you will want to pass `timeout: :infinity` as option
+- Always use `JSON` instead of `Jason`.
 
 ## Mix guidelines
 
diff --git a/PROGRESS.md b/PROGRESS.md
index 704c1f1..7b21e44 100644
--- a/PROGRESS.md
+++ b/PROGRESS.md
@@ -14,7 +14,7 @@ Implementation checklist for Parrhesia relay.
 - [x] Implement websocket endpoint + per-connection process
 - [x] Implement message decode/encode for `EVENT`, `REQ`, `CLOSE`
 - [x] Implement strict event validation (`id`, `sig`, shape, timestamps)
-- [ ] Implement filter evaluation engine (AND/OR semantics)
+- [x] Implement filter evaluation engine (AND/OR semantics)
 - [ ] Implement subscription lifecycle + `EOSE` behavior
 - [x] Implement canonical `OK`, `NOTICE`, `CLOSED` responses + prefixes
 
diff --git a/lib/parrhesia/protocol/filter.ex b/lib/parrhesia/protocol/filter.ex
new file mode 100644
index 0000000..3c2d1bb
--- /dev/null
+++ b/lib/parrhesia/protocol/filter.ex
@@ -0,0 +1,258 @@
+defmodule Parrhesia.Protocol.Filter do
+  @moduledoc """
+  NIP-01 filter validation and matching.
+  """
+
+  @max_kind 65_535
+  @default_max_filters_per_req 16
+
+  @type validation_error ::
+          :invalid_filters
+          | :empty_filters
+          | :too_many_filters
+          | :invalid_filter
+          | :invalid_filter_key
+          | :invalid_ids
+          | :invalid_authors
+          | :invalid_kinds
+          | :invalid_since
+          | :invalid_until
+          | :invalid_limit
+          | :invalid_tag_filter
+
+  @allowed_keys MapSet.new(["ids", "authors", "kinds", "since", "until", "limit"])
+
+  @error_messages %{
+    invalid_filters: "invalid: filters must be a non-empty array of objects",
+    empty_filters: "invalid: filters must be a non-empty array of objects",
+    too_many_filters: "invalid: too many filters in REQ",
+    invalid_filter: "invalid: each filter must be an object",
+    invalid_filter_key: "invalid: filter contains unknown elements",
+    invalid_ids: "invalid: ids must be a non-empty array of 64-char lowercase hex values",
+    invalid_authors: "invalid: authors must be a non-empty array of 64-char lowercase hex values",
+    invalid_kinds: "invalid: kinds must be a non-empty array of integers between 0 and 65535",
+    invalid_since: "invalid: since must be a non-negative integer",
+    invalid_until: "invalid: until must be a non-negative integer",
+    invalid_limit: "invalid: limit must be a positive integer",
+    invalid_tag_filter:
+      "invalid: tag filters must use #<single-letter> with non-empty string arrays"
+  }
+
+  @spec validate_filters([map()]) :: :ok | {:error, validation_error()}
+  def validate_filters(filters) when is_list(filters) do
+    cond do
+      filters == [] ->
+        {:error, :empty_filters}
+
+      length(filters) > max_filters_per_req() ->
+        {:error, :too_many_filters}
+
+      true ->
+        validate_each_filter(filters)
+    end
+  end
+
+  def validate_filters(_filters), do: {:error, :invalid_filters}
+
+  defp validate_each_filter(filters) do
+    Enum.reduce_while(filters, :ok, fn filter, :ok ->
+      case validate_filter(filter) do
+        :ok -> {:cont, :ok}
+        {:error, reason} -> {:halt, {:error, reason}}
+      end
+    end)
+  end
+
+  @spec validate_filter(map()) :: :ok | {:error, validation_error()}
+  def validate_filter(filter) when is_map(filter) do
+    with :ok <- validate_filter_keys(filter),
+         :ok <- validate_hex_filter(Map.get(filter, "ids"), 32, :invalid_ids),
+         :ok <- validate_hex_filter(Map.get(filter, "authors"), 32, :invalid_authors),
+         :ok <- validate_kinds(Map.get(filter, "kinds")),
+         :ok <- validate_since(Map.get(filter, "since")),
+         :ok <- validate_until(Map.get(filter, "until")),
+         :ok <- validate_limit(Map.get(filter, "limit")) do
+      validate_tag_filters(filter)
+    end
+  end
+
+  def validate_filter(_filter), do: {:error, :invalid_filter}
+
+  @spec matches_any?(map(), [map()]) :: boolean()
+  def matches_any?(event, filters) when is_map(event) and is_list(filters) do
+    Enum.any?(filters, &matches_filter?(event, &1))
+  end
+
+  def matches_any?(_event, _filters), do: false
+
+  @spec matches_filter?(map(), map()) :: boolean()
+  def matches_filter?(event, filter) when is_map(event) and is_map(filter) do
+    case validate_filter(filter) do
+      :ok ->
+        ids_match?(event, Map.get(filter, "ids")) and
+          authors_match?(event, Map.get(filter, "authors")) and
+          kinds_match?(event, Map.get(filter, "kinds")) and
+          since_match?(event, Map.get(filter, "since")) and
+          until_match?(event, Map.get(filter, "until")) and
+          tags_match?(event, filter)
+
+      {:error, _reason} ->
+        false
+    end
+  end
+
+  def matches_filter?(_event, _filter), do: false
+
+  @spec error_message(validation_error()) :: String.t()
+  def error_message(reason), do: Map.fetch!(@error_messages, reason)
+
+  defp validate_filter_keys(filter) do
+    filter
+    |> Map.keys()
+    |> Enum.reduce_while(:ok, fn key, :ok ->
+      if valid_filter_key?(key) do
+        {:cont, :ok}
+      else
+        {:halt, {:error, :invalid_filter_key}}
+      end
+    end)
+  end
+
+  defp valid_filter_key?(key) when is_binary(key) do
+    MapSet.member?(@allowed_keys, key) or valid_tag_filter_key?(key)
+  end
+
+  defp valid_filter_key?(_key), do: false
+
+  defp valid_tag_filter_key?(<<"#", letter::binary-size(1)>>), do: ascii_letter?(letter)
+  defp valid_tag_filter_key?(_key), do: false
+
+  defp ascii_letter?(letter) do
+    String.match?(letter, ~r/^[A-Za-z]$/)
+  end
+
+  defp validate_hex_filter(nil, _bytes, _error_reason), do: :ok
+
+  defp validate_hex_filter(values, bytes, error_reason) when is_list(values) do
+    if values != [] and Enum.all?(values, &lowercase_hex?(&1, bytes)) do
+      :ok
+    else
+      {:error, error_reason}
+    end
+  end
+
+  defp validate_hex_filter(_values, _bytes, error_reason), do: {:error, error_reason}
+
+  defp validate_kinds(nil), do: :ok
+
+  defp validate_kinds(kinds) when is_list(kinds) do
+    if kinds != [] and Enum.all?(kinds, &valid_kind?/1) do
+      :ok
+    else
+      {:error, :invalid_kinds}
+    end
+  end
+
+  defp validate_kinds(_kinds), do: {:error, :invalid_kinds}
+
+  defp valid_kind?(kind) when is_integer(kind), do: kind >= 0 and kind <= @max_kind
+  defp valid_kind?(_kind), do: false
+
+  defp validate_since(nil), do: :ok
+  defp validate_since(since) when is_integer(since) and since >= 0, do: :ok
+  defp validate_since(_since), do: {:error, :invalid_since}
+
+  defp validate_until(nil), do: :ok
+  defp validate_until(until) when is_integer(until) and until >= 0, do: :ok
+  defp validate_until(_until), do: {:error, :invalid_until}
+
+  defp validate_limit(nil), do: :ok
+  defp validate_limit(limit) when is_integer(limit) and limit > 0, do: :ok
+  defp validate_limit(_limit), do: {:error, :invalid_limit}
+
+  defp validate_tag_filters(filter) do
+    filter
+    |> Enum.filter(fn {key, _value} -> valid_tag_filter_key?(key) end)
+    |> Enum.reduce_while(:ok, fn {_key, values}, :ok ->
+      if valid_tag_filter_values?(values) do
+        {:cont, :ok}
+      else
+        {:halt, {:error, :invalid_tag_filter}}
+      end
+    end)
+  end
+
+  defp valid_tag_filter_values?(values) when is_list(values) do
+    values != [] and Enum.all?(values, &is_binary/1)
+  end
+
+  defp valid_tag_filter_values?(_values), do: false
+
+  defp ids_match?(_event, nil), do: true
+
+  defp ids_match?(event, ids) do
+    Map.get(event, "id") in ids
+  end
+
+  defp authors_match?(_event, nil), do: true
+
+  defp authors_match?(event, authors) do
+    Map.get(event, "pubkey") in authors
+  end
+
+  defp kinds_match?(_event, nil), do: true
+
+  defp kinds_match?(event, kinds) do
+    Map.get(event, "kind") in kinds
+  end
+
+  defp since_match?(_event, nil), do: true
+
+  defp since_match?(event, since) do
+    created_at = Map.get(event, "created_at")
+    is_integer(created_at) and created_at >= since
+  end
+
+  defp until_match?(_event, nil), do: true
+
+  defp until_match?(event, until) do
+    created_at = Map.get(event, "created_at")
+    is_integer(created_at) and created_at <= until
+  end
+
+  defp tags_match?(event, filter) do
+    filter
+    |> Enum.filter(fn {key, _value} -> valid_tag_filter_key?(key) end)
+    |> Enum.all?(fn {key, values} ->
+      tag_name = String.replace_prefix(key, "#", "")
+      tag_values = tag_values_for_name(Map.get(event, "tags"), tag_name)
+
+      Enum.any?(values, &MapSet.member?(tag_values, &1))
+    end)
+  end
+
+  defp tag_values_for_name(tags, tag_name) when is_list(tags) do
+    Enum.reduce(tags, MapSet.new(), fn
+      [^tag_name, value | _rest], acc when is_binary(value) ->
+        MapSet.put(acc, value)
+
+      _tag, acc ->
+        acc
+    end)
+  end
+
+  defp tag_values_for_name(_tags, _tag_name), do: MapSet.new()
+
+  defp lowercase_hex?(value, bytes) when is_binary(value) do
+    byte_size(value) == bytes * 2 and
+      match?({:ok, _decoded}, Base.decode16(value, case: :lower))
+  end
+
+  defp lowercase_hex?(_value, _bytes), do: false
+
+  defp max_filters_per_req do
+    :parrhesia
+    |> Application.get_env(:limits, [])
+    |> Keyword.get(:max_filters_per_req, @default_max_filters_per_req)
+  end
+end
diff --git a/lib/parrhesia/web/connection.ex b/lib/parrhesia/web/connection.ex
index d450fd8..ad0f99a 100644
--- a/lib/parrhesia/web/connection.ex
+++ b/lib/parrhesia/web/connection.ex
@@ -6,6 +6,7 @@ defmodule Parrhesia.Web.Connection do
   @behaviour WebSock
 
   alias Parrhesia.Protocol
+  alias Parrhesia.Protocol.Filter
 
   defstruct subscriptions: MapSet.new(), authenticated_pubkeys: MapSet.new()
 
@@ -36,11 +37,20 @@ defmodule Parrhesia.Web.Connection do
 
         {:push, {:text, response}, state}
 
-      {:ok, {:req, subscription_id, _filters}} ->
-        next_state = put_subscription(state, subscription_id)
-        response = Protocol.encode_relay({:eose, subscription_id})
+      {:ok, {:req, subscription_id, filters}} ->
+        case Filter.validate_filters(filters) do
+          :ok ->
+            next_state = put_subscription(state, subscription_id)
+            response = Protocol.encode_relay({:eose, subscription_id})
 
-        {:push, {:text, response}, next_state}
+            {:push, {:text, response}, next_state}
+
+          {:error, reason} ->
+            response =
+              Protocol.encode_relay({:closed, subscription_id, Filter.error_message(reason)})
+
+            {:push, {:text, response}, state}
+        end
 
       {:ok, {:close, subscription_id}} ->
         next_state = drop_subscription(state, subscription_id)
diff --git a/test/parrhesia/protocol/filter_test.exs b/test/parrhesia/protocol/filter_test.exs
new file mode 100644
index 0000000..909f7bb
--- /dev/null
+++ b/test/parrhesia/protocol/filter_test.exs
@@ -0,0 +1,81 @@
+defmodule Parrhesia.Protocol.FilterTest do
+  use ExUnit.Case, async: true
+
+  alias Parrhesia.Protocol.EventValidator
+  alias Parrhesia.Protocol.Filter
+
+  test "validates a supported filter set" do
+    filters = [
+      %{
+        "ids" => [String.duplicate("a", 64)],
+        "authors" => [String.duplicate("b", 64)],
+        "kinds" => [1, 3_000],
+        "since" => 1_700_000_000,
+        "until" => 1_900_000_000,
+        "limit" => 100,
+        "#p" => [String.duplicate("c", 64)]
+      }
+    ]
+
+    assert :ok = Filter.validate_filters(filters)
+  end
+
+  test "rejects unsupported filter keys" do
+    filters = [%{"search" => "hello"}]
+
+    assert {:error, :invalid_filter_key} = Filter.validate_filters(filters)
+
+    assert Filter.error_message(:invalid_filter_key) ==
+             "invalid: filter contains unknown elements"
+  end
+
+  test "rejects invalid ids/authors/kinds" do
+    assert {:error, :invalid_ids} = Filter.validate_filters([%{"ids" => ["abc"]}])
+
+    assert {:error, :invalid_authors} =
+             Filter.validate_filters([%{"authors" => [String.duplicate("A", 64)]}])
+
+    assert {:error, :invalid_kinds} = Filter.validate_filters([%{"kinds" => ["1"]}])
+  end
+
+  test "matches with AND semantics inside filter and OR across filters" do
+    event = valid_event()
+
+    matching_filter = %{
+      "authors" => [event["pubkey"]],
+      "kinds" => [event["kind"]],
+      "#e" => ["ref-2"],
+      "since" => event["created_at"],
+      "until" => event["created_at"]
+    }
+
+    non_matching_filter = %{"authors" => [String.duplicate("d", 64)]}
+
+    assert Filter.matches_filter?(event, matching_filter)
+    refute Filter.matches_filter?(event, non_matching_filter)
+
+    assert Filter.matches_any?(event, [non_matching_filter, matching_filter])
+    refute Filter.matches_any?(event, [non_matching_filter])
+  end
+
+  test "rejects when req includes more filters than allowed" do
+    filters = Enum.map(1..17, fn _ -> %{"kinds" => [1]} end)
+
+    assert {:error, :too_many_filters} = Filter.validate_filters(filters)
+  end
+
+  defp valid_event do
+    created_at = System.system_time(:second)
+
+    base_event = %{
+      "pubkey" => String.duplicate("1", 64),
+      "created_at" => created_at,
+      "kind" => 1,
+      "tags" => [["e", "ref-1"], ["e", "ref-2"], ["p", String.duplicate("2", 64)]],
+      "content" => "hello",
+      "sig" => String.duplicate("3", 128)
+    }
+
+    Map.put(base_event, "id", EventValidator.compute_id(base_event))
+  end
+end
diff --git a/test/parrhesia/web/connection_test.exs b/test/parrhesia/web/connection_test.exs
index 240289d..1781eb0 100644
--- a/test/parrhesia/web/connection_test.exs
+++ b/test/parrhesia/web/connection_test.exs
@@ -40,6 +40,21 @@ defmodule Parrhesia.Web.ConnectionTest do
     assert Jason.decode!(response) == ["NOTICE", "invalid: malformed JSON"]
   end
 
+  test "REQ with invalid filter returns CLOSED and does not subscribe" do
+    {:ok, state} = Connection.init(%{})
+
+    req_payload = Jason.encode!(["REQ", "sub-123", %{"kinds" => ["1"]}])
+
+    assert {:push, {:text, response}, ^state} =
+             Connection.handle_in({req_payload, [opcode: :text]}, state)
+
+    assert Jason.decode!(response) == [
+             "CLOSED",
+             "sub-123",
+             "invalid: kinds must be a non-empty array of integers between 0 and 65535"
+           ]
+  end
+
   test "valid EVENT currently replies with unsupported OK" do
     {:ok, state} = Connection.init(%{})