diff --git a/Documentation/git-mktag.txt b/Documentation/git-mktag.txt
index 20af1915e9..fa070e21a0 100644
--- a/Documentation/git-mktag.txt
+++ b/Documentation/git-mktag.txt
@@ -25,7 +25,14 @@ write a tag found in `my-tag`:
     git hash-object -t tag -w --stdin <my-tag
 
 The difference is that mktag will die before writing the tag if the
-tag doesn't pass a sanity check.
+tag doesn't pass a linkgit:git-fsck[1] check.
+
+The "fsck" check done mktag is stricter than what linkgit:git-fsck[1]
+would run by default in that all `fsck.<msg-id>` messages are promoted
+from warnings to errors (so e.g. a missing "tagger" line is an error).
+
+Extra headers in the object are also an error under mktag, but ignored
+by linkgit:git-fsck[1]
 
 Tag Format
 ----------
diff --git a/builtin/mktag.c b/builtin/mktag.c
index d89a3c201d..4dd35bc79e 100644
--- a/builtin/mktag.c
+++ b/builtin/mktag.c
@@ -2,160 +2,60 @@
 #include "tag.h"
 #include "replace-object.h"
 #include "object-store.h"
+#include "fsck.h"
 
-/*
- * A signature file has a very simple fixed format: four lines
- * of "object <sha1>" + "type <typename>" + "tag <tagname>" +
- * "tagger <committer>", followed by a blank line, a free-form tag
- * message and a signature block that git itself doesn't care about,
- * but that can be verified with gpg or similar.
- *
- * The first four lines are guaranteed to be at least 83 bytes:
- * "object <sha1>\n" is 48 bytes, "type tag\n" at 9 bytes is the
- * shortest possible type-line, "tag .\n" at 6 bytes is the shortest
- * single-character-tag line, and "tagger . <> 0 +0000\n" at 20 bytes is
- * the shortest possible tagger-line.
- */
-
-/*
- * We refuse to tag something we can't verify. Just because.
- */
-static int verify_object(const struct object_id *oid, const char *expected_type)
+static int mktag_fsck_error_func(struct fsck_options *o,
+				 const struct object_id *oid,
+				 enum object_type object_type,
+				 int msg_type, const char *message)
 {
-	int ret = -1;
-	enum object_type type;
-	unsigned long size;
-	void *buffer = read_object_file(oid, &type, &size);
-	const struct object_id *repl = lookup_replace_object(the_repository, oid);
-
-	if (buffer) {
-		if (type == type_from_string(expected_type)) {
-			ret = check_object_signature(the_repository, repl,
-						     buffer, size,
-						     expected_type);
-		}
-		free(buffer);
+	switch (msg_type) {
+	case FSCK_WARN:
+	case FSCK_ERROR:
+		/*
+		 * We treat both warnings and errors as errors, things
+		 * like missing "tagger" lines are "only" warnings
+		 * under fsck, we've always considered them an error.
+		 */
+		fprintf_ln(stderr, "error: tag input does not pass fsck: %s", message);
+		return 1;
+	default:
+		BUG("%d (FSCK_IGNORE?) should never trigger this callback",
+		    msg_type);
 	}
-	return ret;
 }
 
-static int verify_tag(char *buffer, unsigned long size)
+static int verify_object_in_tag(struct object_id *tagged_oid, int *tagged_type)
 {
-	int typelen;
-	char type[20];
-	struct object_id oid;
-	const char *object, *type_line, *tag_line, *tagger_line, *lb, *rb, *p;
-	size_t len;
+	int ret;
+	enum object_type type;
+	unsigned long size;
+	void *buffer;
+	const struct object_id *repl;
 
-	if (size < 84)
-		return error("wanna fool me ? you obviously got the size wrong !");
+	buffer = read_object_file(tagged_oid, &type, &size);
+	if (!buffer)
+		die("could not read tagged object '%s'",
+		    oid_to_hex(tagged_oid));
+	if (type != *tagged_type)
+		die("object '%s' tagged as '%s', but is a '%s' type",
+		    oid_to_hex(tagged_oid),
+		    type_name(*tagged_type), type_name(type));
 
-	buffer[size] = 0;
+	repl = lookup_replace_object(the_repository, tagged_oid);
+	ret = check_object_signature(the_repository, repl,
+				     buffer, size, type_name(*tagged_type));
+	free(buffer);
 
-	/* Verify object line */
-	object = buffer;
-	if (memcmp(object, "object ", 7))
-		return error("char%d: does not start with \"object \"", 0);
-
-	if (parse_oid_hex(object + 7, &oid, &p))
-		return error("char%d: could not get SHA1 hash", 7);
-
-	/* Verify type line */
-	type_line = p + 1;
-	if (memcmp(type_line - 1, "\ntype ", 6))
-		return error("char%d: could not find \"\\ntype \"", 47);
-
-	/* Verify tag-line */
-	tag_line = strchr(type_line, '\n');
-	if (!tag_line)
-		return error("char%"PRIuMAX": could not find next \"\\n\"",
-				(uintmax_t) (type_line - buffer));
-	tag_line++;
-	if (memcmp(tag_line, "tag ", 4) || tag_line[4] == '\n')
-		return error("char%"PRIuMAX": no \"tag \" found",
-				(uintmax_t) (tag_line - buffer));
-
-	/* Get the actual type */
-	typelen = tag_line - type_line - strlen("type \n");
-	if (typelen >= sizeof(type))
-		return error("char%"PRIuMAX": type too long",
-				(uintmax_t) (type_line+5 - buffer));
-
-	memcpy(type, type_line+5, typelen);
-	type[typelen] = 0;
-
-	/* Verify that the object matches */
-	if (verify_object(&oid, type))
-		return error("char%d: could not verify object %s", 7, oid_to_hex(&oid));
-
-	/* Verify the tag-name: we don't allow control characters or spaces in it */
-	tag_line += 4;
-	for (;;) {
-		unsigned char c = *tag_line++;
-		if (c == '\n')
-			break;
-		if (c > ' ')
-			continue;
-		return error("char%"PRIuMAX": could not verify tag name",
-				(uintmax_t) (tag_line - buffer));
-	}
-
-	/* Verify the tagger line */
-	tagger_line = tag_line;
-
-	if (memcmp(tagger_line, "tagger ", 7))
-		return error("char%"PRIuMAX": could not find \"tagger \"",
-			(uintmax_t) (tagger_line - buffer));
-
-	/*
-	 * Check for correct form for name and email
-	 * i.e. " <" followed by "> " on _this_ line
-	 * No angle brackets within the name or email address fields.
-	 * No spaces within the email address field.
-	 */
-	tagger_line += 7;
-	if (!(lb = strstr(tagger_line, " <")) || !(rb = strstr(lb+2, "> ")) ||
-		strpbrk(tagger_line, "<>\n") != lb+1 ||
-		strpbrk(lb+2, "><\n ") != rb)
-		return error("char%"PRIuMAX": malformed tagger field",
-			(uintmax_t) (tagger_line - buffer));
-
-	/* Check for author name, at least one character, space is acceptable */
-	if (lb == tagger_line)
-		return error("char%"PRIuMAX": missing tagger name",
-			(uintmax_t) (tagger_line - buffer));
-
-	/* timestamp, 1 or more digits followed by space */
-	tagger_line = rb + 2;
-	if (!(len = strspn(tagger_line, "0123456789")))
-		return error("char%"PRIuMAX": missing tag timestamp",
-			(uintmax_t) (tagger_line - buffer));
-	tagger_line += len;
-	if (*tagger_line != ' ')
-		return error("char%"PRIuMAX": malformed tag timestamp",
-			(uintmax_t) (tagger_line - buffer));
-	tagger_line++;
-
-	/* timezone, 5 digits [+-]hhmm, max. 1400 */
-	if (!((tagger_line[0] == '+' || tagger_line[0] == '-') &&
-	      strspn(tagger_line+1, "0123456789") == 4 &&
-	      tagger_line[5] == '\n' && atoi(tagger_line+1) <= 1400))
-		return error("char%"PRIuMAX": malformed tag timezone",
-			(uintmax_t) (tagger_line - buffer));
-	tagger_line += 6;
-
-	/* Verify the blank line separating the header from the body */
-	if (*tagger_line != '\n')
-		return error("char%"PRIuMAX": trailing garbage in tag header",
-			(uintmax_t) (tagger_line - buffer));
-
-	/* The actual stuff afterwards we don't care about.. */
-	return 0;
+	return ret;
 }
 
 int cmd_mktag(int argc, const char **argv, const char *prefix)
 {
 	struct strbuf buf = STRBUF_INIT;
+	struct fsck_options fsck_options = FSCK_OPTIONS_STRICT;
+	struct object_id tagged_oid;
+	int tagged_type;
 	struct object_id result;
 
 	if (argc != 1)
@@ -164,10 +64,14 @@ int cmd_mktag(int argc, const char **argv, const char *prefix)
 	if (strbuf_read(&buf, 0, 0) < 0)
 		die_errno("could not read from stdin");
 
-	/* Verify it for some basic sanity: it needs to start with
-	   "object <sha1>\ntype\ntagger " */
-	if (verify_tag(buf.buf, buf.len) < 0)
-		die("invalid tag signature file");
+	fsck_options.error_func = mktag_fsck_error_func;
+	fsck_set_msg_type(&fsck_options, "extraheaderentry", "warn");
+	if (fsck_tag_standalone(NULL, buf.buf, buf.len, &fsck_options,
+				&tagged_oid, &tagged_type))
+		die("tag on stdin did not pass our strict fsck check");
+
+	if (verify_object_in_tag(&tagged_oid, &tagged_type))
+		die("tag on stdin did not refer to a valid object");
 
 	if (write_object_file(buf.buf, buf.len, tag_type, &result) < 0)
 		die("unable to write tag file");
diff --git a/fsck.c b/fsck.c
index f82e2fe9e3..bed5e20e03 100644
--- a/fsck.c
+++ b/fsck.c
@@ -80,7 +80,9 @@ static struct oidset gitmodules_done = OIDSET_INIT;
 	/* infos (reported as warnings, but ignored by default) */ \
 	FUNC(GITMODULES_PARSE, INFO) \
 	FUNC(BAD_TAG_NAME, INFO) \
-	FUNC(MISSING_TAGGER_ENTRY, INFO)
+	FUNC(MISSING_TAGGER_ENTRY, INFO) \
+	/* ignored (elevated when requested) */ \
+	FUNC(EXTRA_HEADER_ENTRY, IGNORE)
 
 #define MSG_ID(id, msg_type) FSCK_MSG_##id,
 enum fsck_msg_id {
@@ -911,6 +913,16 @@ static int fsck_tag(const struct object_id *oid, const char *buffer,
 		    unsigned long size, struct fsck_options *options)
 {
 	struct object_id tagged_oid;
+	int tagged_type;
+	return fsck_tag_standalone(oid, buffer, size, options, &tagged_oid,
+				   &tagged_type);
+}
+
+int fsck_tag_standalone(const struct object_id *oid, const char *buffer,
+			unsigned long size, struct fsck_options *options,
+			struct object_id *tagged_oid,
+			int *tagged_type)
+{
 	int ret = 0;
 	char *eol;
 	struct strbuf sb = STRBUF_INIT;
@@ -924,7 +936,7 @@ static int fsck_tag(const struct object_id *oid, const char *buffer,
 		ret = report(options, oid, OBJ_TAG, FSCK_MSG_MISSING_OBJECT, "invalid format - expected 'object' line");
 		goto done;
 	}
-	if (parse_oid_hex(buffer, &tagged_oid, &p) || *p != '\n') {
+	if (parse_oid_hex(buffer, tagged_oid, &p) || *p != '\n') {
 		ret = report(options, oid, OBJ_TAG, FSCK_MSG_BAD_OBJECT_SHA1, "invalid 'object' line format - bad sha1");
 		if (ret)
 			goto done;
@@ -940,7 +952,8 @@ static int fsck_tag(const struct object_id *oid, const char *buffer,
 		ret = report(options, oid, OBJ_TAG, FSCK_MSG_MISSING_TYPE, "invalid format - unexpected end after 'type' line");
 		goto done;
 	}
-	if (type_from_string_gently(buffer, eol - buffer, 1) < 0)
+	*tagged_type = type_from_string_gently(buffer, eol - buffer, 1);
+	if (*tagged_type < 0)
 		ret = report(options, oid, OBJ_TAG, FSCK_MSG_BAD_TYPE, "invalid 'type' value");
 	if (ret)
 		goto done;
@@ -975,6 +988,19 @@ static int fsck_tag(const struct object_id *oid, const char *buffer,
 	else
 		ret = fsck_ident(&buffer, oid, OBJ_TAG, options);
 
+	if (!starts_with(buffer, "\n")) {
+		/*
+		 * The verify_headers() check will allow
+		 * e.g. "[...]tagger <tagger>\nsome
+		 * garbage\n\nmessage" to pass, thinking "some
+		 * garbage" could be a custom header. E.g. "mktag"
+		 * doesn't want any unknown headers.
+		 */
+		ret = report(options, oid, OBJ_TAG, FSCK_MSG_EXTRA_HEADER_ENTRY, "invalid format - extra header(s) after 'tagger'");
+		if (ret)
+			goto done;
+	}
+
 done:
 	strbuf_release(&sb);
 	return ret;
diff --git a/fsck.h b/fsck.h
index 69cf715e79..29ee4c45e8 100644
--- a/fsck.h
+++ b/fsck.h
@@ -62,6 +62,15 @@ int fsck_walk(struct object *obj, void *data, struct fsck_options *options);
 int fsck_object(struct object *obj, void *data, unsigned long size,
 	struct fsck_options *options);
 
+/*
+ * fsck a tag, and pass info about it back to the caller. This is
+ * exposed fsck_object() internals for git-mktag(1).
+ */
+int fsck_tag_standalone(const struct object_id *oid, const char *buffer,
+			unsigned long size, struct fsck_options *options,
+			struct object_id *tagged_oid,
+			int *tag_type);
+
 /*
  * Some fsck checks are context-dependent, and may end up queued; run this
  * after completing all fsck_object() calls in order to resolve any remaining
diff --git a/t/t1006-cat-file.sh b/t/t1006-cat-file.sh
index 2f501d2dc9..5d2dc99b74 100755
--- a/t/t1006-cat-file.sh
+++ b/t/t1006-cat-file.sh
@@ -166,7 +166,7 @@ tag_content="$tag_header_without_timestamp 0000000000 +0000
 
 $tag_description"
 
-tag_sha1=$(echo_without_newline "$tag_content" | git mktag)
+tag_sha1=$(echo_without_newline "$tag_content" | git hash-object -t tag --stdin -w)
 tag_size=$(strlen "$tag_content")
 
 run_tests 'tag' $tag_sha1 $tag_size "$tag_content" "$tag_content" 1
diff --git a/t/t3800-mktag.sh b/t/t3800-mktag.sh
index 8bf0e88115..5e96f69e69 100755
--- a/t/t3800-mktag.sh
+++ b/t/t3800-mktag.sh
@@ -53,7 +53,7 @@ too short for a tag
 EOF
 
 check_verify_failure 'Tag object length check' \
-	'^error: .*size wrong.*$'
+	'^error:.* missingObject:'
 
 ############################################################
 #  2. object line label check
@@ -66,7 +66,7 @@ tagger . <> 0 +0000
 
 EOF
 
-check_verify_failure '"object" line label check' '^error: char0: .*"object "$'
+check_verify_failure '"object" line label check' '^error:.* missingObject:'
 
 ############################################################
 #  3. object line hash check
@@ -79,7 +79,7 @@ tagger . <> 0 +0000
 
 EOF
 
-check_verify_failure '"object" line SHA1 check' '^error: char7: .*SHA1 hash$'
+check_verify_failure '"object" line check' '^error:.* badObjectSha1:'
 
 ############################################################
 #  4. type line label check
@@ -92,7 +92,7 @@ tagger . <> 0 +0000
 
 EOF
 
-check_verify_failure '"type" line label check' '^error: char.*: .*"\\ntype "$'
+check_verify_failure '"type" line label check' '^error:.* missingTypeEntry:'
 
 ############################################################
 #  5. type line eol check
@@ -100,7 +100,7 @@ check_verify_failure '"type" line label check' '^error: char.*: .*"\\ntype "$'
 echo "object $head" >tag.sig
 printf "type tagsssssssssssssssssssssssssssssss" >>tag.sig
 
-check_verify_failure '"type" line eol check' '^error: char.*: .*"\\n"$'
+check_verify_failure '"type" line eol check' '^error:.* unterminatedHeader:'
 
 ############################################################
 #  6. tag line label check #1
@@ -114,7 +114,7 @@ tagger . <> 0 +0000
 EOF
 
 check_verify_failure '"tag" line label check #1' \
-	'^error: char.*: no "tag " found$'
+	'^error:.* missingTagEntry:'
 
 ############################################################
 #  7. tag line label check #2
@@ -126,7 +126,7 @@ tag
 EOF
 
 check_verify_failure '"tag" line label check #2' \
-	'^error: char.*: no "tag " found$'
+	'^error:.* badType:'
 
 ############################################################
 #  8. type line type-name length check
@@ -138,7 +138,7 @@ tag mytag
 EOF
 
 check_verify_failure '"type" line type-name length check' \
-	'^error: char.*: type too long$'
+	'^error:.* badType:'
 
 ############################################################
 #  9. verify object (hash/type) check
@@ -152,7 +152,7 @@ tagger . <> 0 +0000
 EOF
 
 check_verify_failure 'verify object (hash/type) check -- correct type, nonexisting object' \
-	'^error: char7: could not verify object.*$'
+	'^fatal: could not read tagged object'
 
 cat >tag.sig <<EOF
 object $head
@@ -163,7 +163,7 @@ tagger . <> 0 +0000
 EOF
 
 check_verify_failure 'verify object (hash/type) check -- made-up type, valid object' \
-	'^fatal: invalid object type'
+	'^error:.* badType:'
 
 cat >tag.sig <<EOF
 object $(test_oid deadbeef)
@@ -174,7 +174,7 @@ tagger . <> 0 +0000
 EOF
 
 check_verify_failure 'verify object (hash/type) check -- made-up type, nonexisting object' \
-	'^error: char7: could not verify object.*$'
+	'^error:.* badType:'
 
 cat >tag.sig <<EOF
 object $head
@@ -185,7 +185,7 @@ tagger . <> 0 +0000
 EOF
 
 check_verify_failure 'verify object (hash/type) check -- mismatched type, valid object' \
-	'^error: char7: could not verify object'
+	'^fatal: object.*tagged as.*tree.*but is.*commit'
 
 ############################################################
 #  9.5. verify object (hash/type) check -- replacement
@@ -214,7 +214,7 @@ tagger . <> 0 +0000
 EOF
 
 check_verify_failure 'verify object (hash/type) check -- mismatched type, valid object' \
-	'^error: char7: could not verify object'
+	'^fatal: object.*tagged as.*tree.*but is.*blob'
 
 ############################################################
 # 10. verify tag-name check
@@ -228,7 +228,7 @@ tagger . <> 0 +0000
 EOF
 
 check_verify_failure 'verify tag-name check' \
-	'^error: char.*: could not verify tag name$'
+	'^error:.* badTagName:'
 
 ############################################################
 # 11. tagger line label check #1
@@ -242,7 +242,7 @@ This is filler
 EOF
 
 check_verify_failure '"tagger" line label check #1' \
-	'^error: char.*: could not find "tagger "$'
+	'^error:.* missingTaggerEntry:'
 
 ############################################################
 # 12. tagger line label check #2
@@ -257,10 +257,10 @@ This is filler
 EOF
 
 check_verify_failure '"tagger" line label check #2' \
-	'^error: char.*: could not find "tagger "$'
+	'^error:.* missingTaggerEntry:'
 
 ############################################################
-# 13. disallow missing tag author name
+# 13. allow missing tag author name like fsck
 
 cat >tag.sig <<EOF
 object $head
@@ -271,8 +271,7 @@ tagger  <> 0 +0000
 This is filler
 EOF
 
-check_verify_failure 'disallow missing tag author name' \
-	'^error: char.*: missing tagger name$'
+test_expect_mktag_success 'allow missing tag author name'
 
 ############################################################
 # 14. disallow missing tag author name
@@ -287,7 +286,7 @@ tagger T A Gger <
 EOF
 
 check_verify_failure 'disallow malformed tagger' \
-	'^error: char.*: malformed tagger field$'
+	'^error:.* badEmail:'
 
 ############################################################
 # 15. allow empty tag email
@@ -303,7 +302,7 @@ EOF
 test_expect_mktag_success 'allow empty tag email'
 
 ############################################################
-# 16. disallow spaces in tag email
+# 16. allow spaces in tag email like fsck
 
 cat >tag.sig <<EOF
 object $head
@@ -313,8 +312,7 @@ tagger T A Gger <tag ger@example.com> 0 +0000
 
 EOF
 
-check_verify_failure 'disallow spaces in tag email' \
-	'^error: char.*: malformed tagger field$'
+test_expect_mktag_success 'allow spaces in tag email like fsck'
 
 ############################################################
 # 17. disallow missing tag timestamp
@@ -328,7 +326,7 @@ tagger T A Gger <tagger@example.com>__
 EOF
 
 check_verify_failure 'disallow missing tag timestamp' \
-	'^error: char.*: missing tag timestamp$'
+	'^error:.* badDate:'
 
 ############################################################
 # 18. detect invalid tag timestamp1
@@ -342,7 +340,7 @@ tagger T A Gger <tagger@example.com> Tue Mar 25 15:47:44 2008
 EOF
 
 check_verify_failure 'detect invalid tag timestamp1' \
-	'^error: char.*: missing tag timestamp$'
+	'^error:.* badDate:'
 
 ############################################################
 # 19. detect invalid tag timestamp2
@@ -356,7 +354,7 @@ tagger T A Gger <tagger@example.com> 2008-03-31T12:20:15-0500
 EOF
 
 check_verify_failure 'detect invalid tag timestamp2' \
-	'^error: char.*: malformed tag timestamp$'
+	'^error:.* badDate:'
 
 ############################################################
 # 20. detect invalid tag timezone1
@@ -370,7 +368,7 @@ tagger T A Gger <tagger@example.com> 1206478233 GMT
 EOF
 
 check_verify_failure 'detect invalid tag timezone1' \
-	'^error: char.*: malformed tag timezone$'
+	'^error:.* badTimezone:'
 
 ############################################################
 # 21. detect invalid tag timezone2
@@ -384,10 +382,10 @@ tagger T A Gger <tagger@example.com> 1206478233 +  30
 EOF
 
 check_verify_failure 'detect invalid tag timezone2' \
-	'^error: char.*: malformed tag timezone$'
+	'^error:.* badTimezone:'
 
 ############################################################
-# 22. detect invalid tag timezone3
+# 22. allow invalid tag timezone3 (the maximum is -1200/+1400)
 
 cat >tag.sig <<EOF
 object $head
@@ -397,8 +395,7 @@ tagger T A Gger <tagger@example.com> 1206478233 -1430
 
 EOF
 
-check_verify_failure 'detect invalid tag timezone3' \
-	'^error: char.*: malformed tag timezone$'
+test_expect_mktag_success 'allow invalid tag timezone'
 
 ############################################################
 # 23. detect invalid header entry
@@ -413,7 +410,7 @@ this line should not be here
 EOF
 
 check_verify_failure 'detect invalid header entry' \
-	'^error: char.*: trailing garbage in tag header$'
+	'^error:.* extraHeaderEntry:'
 
 cat >tag.sig <<EOF
 object $head
@@ -445,7 +442,7 @@ tagger T A Gger <tagger@example.com> 1206478233 -0500
 EOF
 
 check_verify_failure 'require a blank line before an empty body (2)' \
-	'^error: char.*: trailing garbage in tag header$'
+	'^error:.* extraHeaderEntry:'
 
 ############################################################
 # 24. create valid tag