diff --git a/src/fd_spy.erl b/src/fd_spy.erl
new file mode 100644
index 0000000..24b2e9d
--- /dev/null
+++ b/src/fd_spy.erl
@@ -0,0 +1,177 @@
+% @doc spy: knows constraints {recipient, amount, payload}
+%
+% spies the chain for transactions that satisfy that constraint
+%
+-module(fd_spy).
+-vsn("0.2.0").
+
+% MVP: register search patterns
+% simply print to console when one of them is found
+
+-behavior(gen_server).
+
+-export_type([
+]).
+
+-export([
+    %% caller context
+    reg/3,
+
+    %% api
+    start_link/0,
+
+    %% process context
+    init/1, handle_call/3, handle_cast/2, handle_info/2,
+    code_change/3, terminate/2
+]).
+
+% recipient is a string
+-type pubkey32() :: <<_:256>>.
+
+-record(sp,
+        {recipient  :: pubkey32(),
+         amount     :: pos_integer(),
+         payload    :: binary()}).
+-type search_pattern() :: #sp{}.
+
+-record(s,
+        {searching_for = [] :: search_pattern()}).
+
+-type state() :: #s{}.
+
+-include("$zx_include/zx_logger.hrl").
+
+%%-----------------------------------------------------------------------------
+%% caller context
+%%-----------------------------------------------------------------------------
+
+-spec reg(Recipient, Amount, Payload) -> ok | {error, Reason}
+    when Recipient :: pubkey32(),
+         Amount    :: pos_integer(),
+         Payload   :: binary(),
+         Reason    :: any().
+
+reg(R, A, P) when is_binary(R), byte_size(R) =:= 32,
+                  is_integer(A), A >= 0,
+                  is_binary(P) ->
+    gen_server:call(?MODULE, {reg, R, A, P}).
+
+
+
+%% gen_server callbacks
+start_link() ->
+    gen_server:start_link({local, ?MODULE}, ?MODULE, none, []).
+
+
+%%-----------------------------------------------------------------------------
+%% process context below this line
+%%-----------------------------------------------------------------------------
+
+%% gen_server callbacks
+
+-spec init(none) -> {ok, state()}.
+
+init(none) ->
+    tell("starting fd_spy"),
+    InitState = #s{},
+    {ok, InitState}.
+
+
+
+-spec handle_call(Msg, From, State) -> Result
+    when Msg      :: any(),
+         From     :: {pid(), reference()},
+         State    :: state(),
+         Result   :: {reply, Reply, NewState}
+                   | {noreply, NewState},
+         Reply    :: any(),
+         NewState :: state().
+
+handle_call({reg, Recipient, Amount, Payload}, _From, State) ->
+    {Reply, NewState} = do_reg(Recipient, Amount, Payload, State),
+    {reply, Reply, NewState};
+handle_call(Unexpected, From, State) ->
+    tell("~tp: unexpected call from ~tp: ~tp", [?MODULE, Unexpected, From]),
+    {noreply, State}.
+
+
+
+handle_cast(Unexpected, State) ->
+    tell("~tp: unexpected cast: ~tp", [?MODULE, Unexpected]),
+    {noreply, State}.
+
+
+
+handle_info(Unexpected, State) ->
+    tell("~tp: unexpected info: ~tp", [?MODULE, Unexpected]),
+    {noreply, State}.
+
+
+
+code_change(_, State, _) ->
+    {ok, State}.
+
+
+
+terminate(_, _) ->
+    ok.
+
+
+
+%%-----------------------------------------------------------------------------
+%% internals
+%%-----------------------------------------------------------------------------
+
+-spec do_reg(Recipient, Amount, Payload, State) -> {Reply, NewState}
+    when Recipient :: pubkey32(),
+         Amount    :: pos_integer(),
+         Payload   :: binary(),
+         State     :: state(),
+         Reply     :: ok
+                    | {error, Reason},
+         Reason    :: any(),
+         NewState  :: state().
+
+do_reg(Recipient, Amount, Payload, State) ->
+    case already_registered(Recipient, Payload, State) of
+        true  -> {error, already_registered};
+        false -> really_register(Recipient, Amount, Payload, State)
+    end.
+
+
+
+-spec already_registered(Recipient, Payload, State) -> boolean()
+    when Recipient :: pubkey32(),
+         Payload   :: binary(),
+         State     :: state().
+
+already_registered(Recipient, Payload, _State = #s{searching_for = SPs}) ->
+    ar(Recipient, Payload, SPs).
+
+ar(Recipient, Payload, [#sp{recipient=Recipient, payload=Payload} | _]) ->
+    true;
+ar(Recipient, Payload, [_ | Rest]) ->
+    ar(Recipient, Payload, Rest);
+ar(_, _, []) ->
+    false.
+
+
+
+-spec really_register(Recipient, Amount, Payload, State) -> {Reply, NewState}
+    when Recipient :: pubkey32(),
+         Amount    :: pos_integer(),
+         Payload   :: binary(),
+         State     :: state(),
+         Reply     :: ok
+                    | {error, Reason},
+         Reason    :: any(),
+         NewState  :: state().
+
+really_register(R, A, P, State = #s{searching_for = SPs}) ->
+    NewSearchPattern = #sp{recipient = R,
+                           amount    = A,
+                           payload   = P},
+    tell("~tp: really_register(~tp,~tp,~tp)", [?MODULE, R, A, P]),
+    NewSearchPatterns = [NewSearchPattern | SPs],
+    NewState = State#s{searching_for = NewSearchPatterns},
+    {reply, ok, NewState}.
diff --git a/src/fd_sup.erl b/src/fd_sup.erl
index 3a31653..e8804c8 100644
--- a/src/fd_sup.erl
+++ b/src/fd_sup.erl
@@ -36,6 +36,12 @@ start_link() ->
 
 init([]) ->
     RestartStrategy = {one_for_one, 1, 60},
+    Spy       = {fd_spy,
+                 {fd_spy, start_link, []},
+                 permanent,
+                 5000,
+                 worker,
+                 [fd_spy]},
     GridsD    = {fd_gridsd,
                  {fd_gridsd, start_link, []},
                  permanent,