diff --git a/common/json_command.h b/common/json_command.h
index e4a2dbd67..8586ba72d 100644
--- a/common/json_command.h
+++ b/common/json_command.h
@@ -15,6 +15,9 @@ struct command_result *command_fail(struct command *cmd, enum jsonrpc_errcode co
 				    const char *fmt, ...)
 	PRINTF_FMT(3, 4) WARN_UNUSED_RESULT RETURNS_NONNULL;
 
+/* Caller supplies this too: must provide this to reach into cmd */
+struct json_filter **command_filter_ptr(struct command *cmd);
+
 /* Convenient wrapper for "paramname: msg: invalid token '.*%s'" */
 static inline struct command_result *
 command_fail_badparam(struct command *cmd,
diff --git a/common/json_filter.c b/common/json_filter.c
index ed422f32d..bda4b91eb 100644
--- a/common/json_filter.c
+++ b/common/json_filter.c
@@ -2,6 +2,7 @@
 #include <assert.h>
 #include <ccan/strmap/strmap.h>
 #include <ccan/tal/str/str.h>
+#include <common/json_command.h>
 #include <common/json_filter.h>
 #include <common/utils.h>
 
@@ -190,3 +191,61 @@ const char *json_filter_misused(const tal_t *ctx, const struct json_filter *f)
 		return tal_steal(ctx, ret);
 	}
 }
+
+/* Recursively populate filter.  NULL on success.
+ *
+ * Example for listtransactions to include output type, amount_msat,
+ *   {"transactions": [{"outputs": [{"amount_msat": true, "type": true}]}]}
+ */
+static struct command_result *
+build_filter(struct command *cmd,
+	     const char *name,
+	     const char *buffer,
+	     const jsmntok_t *tok,
+	     struct json_filter *filter)
+{
+	struct command_result *ret;
+	size_t i;
+	const jsmntok_t *t;
+	struct json_filter *subf;
+
+	if (tok->type == JSMN_ARRAY) {
+		if (tok->size != 1)
+			return command_fail_badparam(cmd, name, buffer, tok,
+						     "Arrays can only have one element");
+		subf = json_filter_subarr(filter);
+		return build_filter(cmd, name, buffer, tok + 1, subf);
+	}
+
+	json_for_each_obj(i, t, tok) {
+		bool is_true;
+		const jsmntok_t *val = t + 1;
+
+		if (t->type != JSMN_STRING)
+			return command_fail_badparam(cmd, name, buffer, t,
+						     "expected string key");
+		subf = json_filter_subobj(filter, buffer + t->start, t->end - t->start);
+		if (val->type == JSMN_OBJECT || val->type == JSMN_ARRAY) {
+			ret = build_filter(cmd, name, buffer, val, subf);
+			if (ret)
+				return ret;
+		} else if (!json_to_bool(buffer, val, &is_true) || !is_true)
+			return command_fail_badparam(cmd, name, buffer, val, "value must be true");
+	}
+	return NULL;
+}
+
+struct command_result *parse_filter(struct command *cmd,
+				    const char *name,
+				    const char *buffer,
+				    const jsmntok_t *tok)
+{
+	struct json_filter **filter = command_filter_ptr(cmd);
+
+	if (tok->type != JSMN_OBJECT)
+		return command_fail_badparam(cmd, name, buffer, tok,
+					     "Expected object");
+
+	*filter = json_filter_new(cmd);
+	return build_filter(cmd, name, buffer, tok, *filter);
+}
diff --git a/common/json_filter.h b/common/json_filter.h
index 0bf298687..dee177aeb 100644
--- a/common/json_filter.h
+++ b/common/json_filter.h
@@ -5,8 +5,10 @@
 #define LIGHTNING_COMMON_JSON_FILTER_H
 #include "config.h"
 #include <ccan/tal/tal.h>
+#include <common/json_parse_simple.h>
 #include <stdbool.h>
 
+struct command;
 struct json_filter;
 
 /* Print this? */
@@ -31,4 +33,9 @@ struct json_filter *json_filter_subobj(struct json_filter *filter,
 				       size_t fieldnamelen);
 struct json_filter *json_filter_subarr(struct json_filter *filter);
 
+/* Turn this "filter" field into cmd->filter and return NULL, or fail command */
+struct command_result *parse_filter(struct command *cmd,
+				    const char *name,
+				    const char *buffer,
+				    const jsmntok_t *tok);
 #endif /* LIGHTNING_COMMON_JSON_FILTER_H */
diff --git a/lightningd/jsonrpc.c b/lightningd/jsonrpc.c
index 5c799e4ea..56cbea077 100644
--- a/lightningd/jsonrpc.c
+++ b/lightningd/jsonrpc.c
@@ -22,6 +22,7 @@
 #include <ccan/tal/str/str.h>
 #include <common/configdir.h>
 #include <common/json_command.h>
+#include <common/json_filter.h>
 #include <common/json_param.h>
 #include <common/memleak.h>
 #include <common/timeout.h>
@@ -493,6 +494,11 @@ struct command_result *command_fail(struct command *cmd, enum jsonrpc_errcode co
 	return command_failed(cmd, r);
 }
 
+struct json_filter **command_filter_ptr(struct command *cmd)
+{
+	return &cmd->filter;
+}
+
 struct command_result *command_still_pending(struct command *cmd)
 {
 	notleak_with_children(cmd);
@@ -909,6 +915,7 @@ parse_request(struct json_connection *jcon, const jsmntok_t tok[])
 	c->id_is_string = (id->type == JSMN_STRING);
 	c->id = json_strdup(c, jcon->buffer, id);
 	c->mode = CMD_NORMAL;
+	c->filter = NULL;
 	list_add_tail(&jcon->commands, &c->list);
 	tal_add_destructor(c, destroy_command);
 
diff --git a/lightningd/jsonrpc.h b/lightningd/jsonrpc.h
index 38bff3cae..2310b079b 100644
--- a/lightningd/jsonrpc.h
+++ b/lightningd/jsonrpc.h
@@ -41,6 +41,8 @@ struct command {
 	enum command_mode mode;
 	/* Have we started a json stream already?  For debugging. */
 	struct json_stream *json_stream;
+	/* Optional output field filter. */
+	struct json_filter *filter;
 };
 
 /**
diff --git a/plugins/libplugin.c b/plugins/libplugin.c
index ef68530eb..8f2f0b4b0 100644
--- a/plugins/libplugin.c
+++ b/plugins/libplugin.c
@@ -132,6 +132,12 @@ struct command_result *command_done(void)
 	return &complete;
 }
 
+/* Don't ask for _filter, we will crash! */
+struct json_filter **command_filter_ptr(struct command *cmd)
+{
+	return NULL;
+}
+
 static void ld_send(struct plugin *plugin, struct json_stream *stream)
 {
 	struct jstream *jstr = tal(plugin, struct jstream);