diff --git a/common/hsm_version.h b/common/hsm_version.h
index 7d3f8fb80..44927bd30 100644
--- a/common/hsm_version.h
+++ b/common/hsm_version.h
@@ -23,6 +23,7 @@
  * v5 with hsmd_revoke_commitment_tx: 5742538f87ef5d5bf55b66dc19e52c8683cfeb1b887d3e64ba530ba9a4d8e638
  * v5 with sign_any_cannouncement: 5fdb9068c43a21887dc03f7dce410d2e3eeff6277f0d49b4fc56595a798fd4a4
  * v5 drop init v2: 5024454532fe5a78bb7558000cb344190888b9915360d3d56ddca22eaba9b872
+ * v5 with dev_preinit: b93e18534a468a4aa9f7015db42e9c363c32aeee5f9146b36dc953ebbdc3d33c
 */
 #define HSM_MIN_VERSION 5
 #define HSM_MAX_VERSION 5
diff --git a/hsmd/hsmd.c b/hsmd/hsmd.c
index e41033c19..6599f777d 100644
--- a/hsmd/hsmd.c
+++ b/hsmd/hsmd.c
@@ -425,6 +425,24 @@ static void load_hsm(const struct secret *encryption_key)
 	close(fd);
 }
 
+/*~ We have a pre-init call in developer mode, to set dev flags */
+static struct io_plan *preinit_hsm(struct io_conn *conn,
+				   struct client *c,
+				   const u8 *msg_in)
+{
+	struct tlv_hsmd_dev_preinit_tlvs *tlv;
+	assert(developer);
+
+	if (!fromwire_hsmd_dev_preinit(tmpctx, msg_in, &tlv))
+		return bad_req(conn, c, msg_in);
+
+	if (tlv->fail_preapprove)
+		dev_fail_preapprove = *tlv->fail_preapprove;
+
+	/* We don't send a reply, just read next */
+	return client_read_next(conn, c);
+}
+
 /*~ This is the response to lightningd's HSM_INIT request, which is the first
  * thing it sends. */
 static struct io_plan *init_hsm(struct io_conn *conn,
@@ -634,6 +652,9 @@ static struct io_plan *handle_client(struct io_conn *conn, struct client *c)
 
 	/* Now actually go and do what the client asked for */
 	switch (t) {
+	case WIRE_HSMD_DEV_PREINIT:
+		return preinit_hsm(conn, c, c->msg_in);
+
 	case WIRE_HSMD_INIT:
 		return init_hsm(conn, c, c->msg_in);
 
diff --git a/hsmd/hsmd_wire.csv b/hsmd/hsmd_wire.csv
index cd7583562..f92a16af1 100644
--- a/hsmd/hsmd_wire.csv
+++ b/hsmd/hsmd_wire.csv
@@ -5,6 +5,14 @@ msgdata,hsmstatus_client_bad_request,description,wirestring,
 msgdata,hsmstatus_client_bad_request,len,u16,
 msgdata,hsmstatus_client_bad_request,msg,u8,len
 
+# We use this to set dev flags (before init!).  It's a TLV, so we can
+# extend easily.  Normal TLV rules apply: ignore odd fields you don't
+# understand.
+msgtype,hsmd_dev_preinit,99
+msgdata,hsmd_dev_preinit,tlvs,hsmd_dev_preinit_tlvs,
+tlvtype,hsmd_dev_preinit_tlvs,fail_preapprove,1
+tlvdata,hsmd_dev_preinit_tlvs,fail_preapprove,fail,bool,
+
 #include <bitcoin/chainparams.h>
 # Start the HSM.
 msgtype,hsmd_init,11
diff --git a/hsmd/libhsmd.c b/hsmd/libhsmd.c
index c90edc7e1..522e67c52 100644
--- a/hsmd/libhsmd.c
+++ b/hsmd/libhsmd.c
@@ -33,6 +33,9 @@ struct {
 /* Have we initialized the secretstuff? */
 bool initialized = false;
 
+/* Do we fail all preapprove requests? */
+bool dev_fail_preapprove = false;
+
 struct hsmd_client *hsmd_client_new_main(const tal_t *ctx, u64 capabilities,
 					 void *extra)
 {
@@ -117,6 +120,7 @@ bool hsmd_check_client_capabilities(struct hsmd_client *client,
 		return (client->capabilities & HSM_PERM_LOCK_OUTPOINT) != 0;
 
 	case WIRE_HSMD_INIT:
+	case WIRE_HSMD_DEV_PREINIT:
 	case WIRE_HSMD_NEW_CHANNEL:
  	case WIRE_HSMD_FORGET_CHANNEL:
 	case WIRE_HSMD_CLIENT_HSMFD:
@@ -774,8 +778,8 @@ static u8 *handle_preapprove_invoice(struct hsmd_client *c, const u8 *msg_in)
 	if (!fromwire_hsmd_preapprove_invoice(tmpctx, msg_in, &invstring))
 		return hsmd_status_malformed_request(c, msg_in);
 
-	/* This stub always approves */
-	approved = true;
+	/* This stub always approves unless overridden */
+	approved = !dev_fail_preapprove;
 
 	return towire_hsmd_preapprove_invoice_reply(NULL, approved);
 }
@@ -792,8 +796,8 @@ static u8 *handle_preapprove_keysend(struct hsmd_client *c, const u8 *msg_in)
 	if (!fromwire_hsmd_preapprove_keysend(msg_in, &destination, &payment_hash, &amount_msat))
 		return hsmd_status_malformed_request(c, msg_in);
 
-	/* This stub always approves */
-	approved = true;
+	/* This stub always approves unless overridden */
+	approved = !dev_fail_preapprove;
 
 	return towire_hsmd_preapprove_keysend_reply(NULL, approved);
 }
@@ -2012,6 +2016,7 @@ u8 *hsmd_handle_client_message(const tal_t *ctx, struct hsmd_client *client,
 
 	/* Now actually go and do what the client asked for */
 	switch (t) {
+	case WIRE_HSMD_DEV_PREINIT:
 	case WIRE_HSMD_INIT:
 	case WIRE_HSMD_CLIENT_HSMFD:
 		/* Not implemented yet. Should not have been passed here yet. */
diff --git a/hsmd/libhsmd.h b/hsmd/libhsmd.h
index 756c6c2f5..a722926cf 100644
--- a/hsmd/libhsmd.h
+++ b/hsmd/libhsmd.h
@@ -93,5 +93,6 @@ bool hsmd_check_client_capabilities(struct hsmd_client *client,
 extern struct privkey *dev_force_privkey;
 /* If they specify --dev-force-bip32-seed it ends up in here. */
 extern struct secret *dev_force_bip32_seed;
-
+/* If they specify --dev-hsmd-fail-preapprove it ends up in here. */
+extern bool dev_fail_preapprove;
 #endif /* LIGHTNING_HSMD_LIBHSMD_H */