diff --git a/lib/Makefile.local b/lib/Makefile.local
index 5dc057c0..8aa03891 100644
--- a/lib/Makefile.local
+++ b/lib/Makefile.local
@@ -58,8 +58,7 @@ libnotmuch_cxx_srcs =		\
 	$(dir)/query-fp.cc      \
 	$(dir)/config.cc	\
 	$(dir)/regexp-fields.cc	\
-	$(dir)/thread.cc \
-	$(dir)/thread-fp.cc
+	$(dir)/thread.cc
 
 libnotmuch_modules := $(libnotmuch_c_srcs:.c=.o) $(libnotmuch_cxx_srcs:.cc=.o)
 
diff --git a/lib/database.cc b/lib/database.cc
index 9cf8062c..02444e09 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -21,7 +21,6 @@
 #include "database-private.h"
 #include "parse-time-vrp.h"
 #include "query-fp.h"
-#include "thread-fp.h"
 #include "regexp-fields.h"
 #include "string-util.h"
 
@@ -259,8 +258,7 @@ prefix_t prefix_table[] = {
     { "directory",		"XDIRECTORY",	NOTMUCH_FIELD_NO_FLAGS },
     { "file-direntry",		"XFDIRENTRY",	NOTMUCH_FIELD_NO_FLAGS },
     { "directory-direntry",	"XDDIRENTRY",	NOTMUCH_FIELD_NO_FLAGS },
-    { "thread",			"G",		NOTMUCH_FIELD_EXTERNAL |
-						NOTMUCH_FIELD_PROCESSOR },
+    { "thread",			"G",		NOTMUCH_FIELD_EXTERNAL },
     { "tag",			"K",		NOTMUCH_FIELD_EXTERNAL |
 						NOTMUCH_FIELD_PROCESSOR },
     { "is",			"K",		NOTMUCH_FIELD_EXTERNAL |
@@ -319,8 +317,6 @@ _setup_query_field (const prefix_t *prefix, notmuch_database_t *notmuch)
 	    fp = (new DateFieldProcessor())->release ();
 	else if (STRNCMP_LITERAL(prefix->name, "query") == 0)
 	    fp = (new QueryFieldProcessor (*notmuch->query_parser, notmuch))->release ();
-	else if (STRNCMP_LITERAL(prefix->name, "thread") == 0)
-	    fp = (new ThreadFieldProcessor (*notmuch->query_parser, notmuch))->release ();
 	else
 	    fp = (new RegexpFieldProcessor (prefix->name, prefix->flags,
 					    *notmuch->query_parser, notmuch))->release ();
diff --git a/lib/thread-fp.cc b/lib/thread-fp.cc
deleted file mode 100644
index 1ab2ae35..00000000
--- a/lib/thread-fp.cc
+++ /dev/null
@@ -1,68 +0,0 @@
-/* thread-fp.cc - "thread:" field processor glue
- *
- * This file is part of notmuch.
- *
- * Copyright © 2017 David Bremner
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program.  If not, see https://www.gnu.org/licenses/ .
- *
- * Author: David Bremner <david@tethera.net>
- */
-
-#include "database-private.h"
-#include "thread-fp.h"
-#include <iostream>
-
-#if HAVE_XAPIAN_FIELD_PROCESSOR
-
-Xapian::Query
-ThreadFieldProcessor::operator() (const std::string & str)
-{
-    notmuch_status_t status;
-
-    if (str.at (0) == '{') {
-	if (str.length() > 1 && str.at (str.size () - 1) == '}'){
-	    std::string subquery_str = str.substr(1,str.size () - 2);
-	    notmuch_query_t *subquery = notmuch_query_create (notmuch, subquery_str.c_str());
-	    notmuch_messages_t *messages;
-	    std::set<std::string> terms;
-
-	    if (!subquery)
-		throw Xapian::QueryParserError ("failed to create subquery for '" + subquery_str + "'");
-
-
-	    status = notmuch_query_search_messages (subquery, &messages);
-	    if (status)
-		throw Xapian::QueryParserError ("failed to search messages for '" + subquery_str + "'");
-
-
-	    for (; notmuch_messages_valid (messages);  notmuch_messages_move_to_next (messages)) {
-		std::string term = "G";
-		notmuch_message_t *message;
-		message = notmuch_messages_get (messages);
-		term += notmuch_message_get_thread_id (message);
-		terms.insert (term);
-	    }
-	    return Xapian::Query (Xapian::Query::OP_OR, terms.begin(), terms.end());
-	} else {
-	    throw Xapian::QueryParserError ("missing } in '" + str + "'");
-	}
-    } else {
-	/* literal thread id */
-	std::string term = "G"+str;
-	return Xapian::Query (term);
-    }
-
-}
-#endif
diff --git a/lib/thread-fp.h b/lib/thread-fp.h
deleted file mode 100644
index 13725978..00000000
--- a/lib/thread-fp.h
+++ /dev/null
@@ -1,42 +0,0 @@
-/* thread-fp.h - thread field processor glue
- *
- * This file is part of notmuch.
- *
- * Copyright © 2017 David Bremner
- *
- * This program is free software: you can redistribute it and/or modify
- * it under the terms of the GNU General Public License as published by
- * the Free Software Foundation, either version 3 of the License, or
- * (at your option) any later version.
- *
- * This program is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
- * GNU General Public License for more details.
- *
- * You should have received a copy of the GNU General Public License
- * along with this program.  If not, see https://www.gnu.org/licenses/ .
- *
- * Author: David Bremner <david@tethera.net>
- */
-
-#ifndef NOTMUCH_THREAD_FP_H
-#define NOTMUCH_THREAD_FP_H
-
-#include <xapian.h>
-#include "notmuch.h"
-
-#if HAVE_XAPIAN_FIELD_PROCESSOR
-class ThreadFieldProcessor : public Xapian::FieldProcessor {
- protected:
-    Xapian::QueryParser &parser;
-    notmuch_database_t *notmuch;
-
- public:
-    ThreadFieldProcessor (Xapian::QueryParser &parser_, notmuch_database_t *notmuch_)
-	: parser(parser_), notmuch(notmuch_) { };
-
-    Xapian::Query operator()(const std::string & str);
-};
-#endif
-#endif /* NOTMUCH_THREAD_FP_H */