thodg/libgit2

diff --git a/CHANGELOG.md b/CHANGELOG.md
index ec5a0d3..9c50f12 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -29,6 +29,11 @@ v0.23 + 1
 * Rebases can now be performed purely in-memory, without touching the
   repository's workdir.
 
+* When adding objects to the index, or when creating new tree or commit
+  objects, the inputs are validated to ensure that the dependent objects
+  exist and are of the correct type.  This object validation can be
+  disabled with the GIT_OPT_ENABLE_STRICT_OBJECT_CREATION option.
+
 ### API additions
 
 * `git_config_lock()` has been added, which allow for
diff --git a/include/git2/common.h b/include/git2/common.h
index c260308..4f43185 100644
--- a/include/git2/common.h
+++ b/include/git2/common.h
@@ -147,6 +147,7 @@ typedef enum {
 	GIT_OPT_SET_TEMPLATE_PATH,
 	GIT_OPT_SET_SSL_CERT_LOCATIONS,
 	GIT_OPT_SET_USER_AGENT,
+	GIT_OPT_ENABLE_STRICT_OBJECT_CREATION,
 } git_libgit2_opt_t;
 
 /**
@@ -251,6 +252,14 @@ typedef enum {
  *		> - `user_agent` is the value that will be delivered as the
  *		>   User-Agent header on HTTP requests.
  *
+ *	* opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, int enabled)
+ *
+ *		> Enable strict input validation when creating new objects
+ *		> to ensure that all inputs to the new objects are valid.  For
+ *		> example, when this is enabled, the parent(s) and tree inputs
+ *		> will be validated when creating a new commit.  This defaults
+ *		> to disabled.
+ *
  * @param option Option key
  * @param ... value to set the option
  * @return 0 on success, <0 on failure
diff --git a/src/commit.c b/src/commit.c
index 5a05098..685c642 100644
--- a/src/commit.c
+++ b/src/commit.c
@@ -17,6 +17,7 @@
 #include "signature.h"
 #include "message.h"
 #include "refs.h"
+#include "object.h"
 
 void git_commit__free(void *_commit)
 {
@@ -36,7 +37,7 @@ void git_commit__free(void *_commit)
 	git__free(commit);
 }
 
-int git_commit_create_from_callback(
+static int git_commit__create_internal(
 	git_oid *id,
 	git_repository *repo,
 	const char *update_ref,
@@ -46,7 +47,8 @@ int git_commit_create_from_callback(
 	const char *message,
 	const git_oid *tree,
 	git_commit_parent_callback parent_cb,
-	void *parent_payload)
+	void *parent_payload,
+	bool validate)
 {
 	git_reference *ref = NULL;
 	int error = 0, matched_parent = 0;
@@ -58,6 +60,9 @@ int git_commit_create_from_callback(
 
 	assert(id && repo && tree && parent_cb);
 
+	if (validate && !git_object__is_valid(repo, tree, GIT_OBJ_TREE))
+		return -1;
+
 	if (update_ref) {
 		error = git_reference_lookup_resolved(&ref, repo, update_ref, 10);
 		if (error < 0 && error != GIT_ENOTFOUND)
@@ -71,6 +76,11 @@ int git_commit_create_from_callback(
 	git_oid__writebuf(&commit, "tree ", tree);
 
 	while ((parent = parent_cb(i, parent_payload)) != NULL) {
+		if (validate && !git_object__is_valid(repo, parent, GIT_OBJ_COMMIT)) {
+			error = -1;
+			goto on_error;
+		}
+
 		git_oid__writebuf(&commit, "parent ", parent);
 		if (i == 0 && current_id && git_oid_equal(current_id, parent))
 			matched_parent = 1;
@@ -114,10 +124,26 @@ int git_commit_create_from_callback(
 
 on_error:
 	git_buf_free(&commit);
-	giterr_set(GITERR_OBJECT, "Failed to create commit.");
 	return -1;
 }
 
+int git_commit_create_from_callback(
+	git_oid *id,
+	git_repository *repo,
+	const char *update_ref,
+	const git_signature *author,
+	const git_signature *committer,
+	const char *message_encoding,
+	const char *message,
+	const git_oid *tree,
+	git_commit_parent_callback parent_cb,
+	void *parent_payload)
+{
+	return git_commit__create_internal(
+		id, repo, update_ref, author, committer, message_encoding, message,
+		tree, parent_cb, parent_payload, true);
+}
+
 typedef struct {
 	size_t total;
 	va_list args;
@@ -153,10 +179,10 @@ int git_commit_create_v(
 	data.total = parent_count;
 	va_start(data.args, parent_count);
 
-	error = git_commit_create_from_callback(
+	error = git_commit__create_internal(
 		id, repo, update_ref, author, committer,
 		message_encoding, message, git_tree_id(tree),
-		commit_parent_from_varargs, &data);
+		commit_parent_from_varargs, &data, false);
 
 	va_end(data.args);
 	return error;
@@ -187,10 +213,10 @@ int git_commit_create_from_ids(
 {
 	commit_parent_oids data = { parent_count, parents };
 
-	return git_commit_create_from_callback(
+	return git_commit__create_internal(
 		id, repo, update_ref, author, committer,
 		message_encoding, message, tree,
-		commit_parent_from_ids, &data);
+		commit_parent_from_ids, &data, true);
 }
 
 typedef struct {
@@ -227,10 +253,10 @@ int git_commit_create(
 
 	assert(tree && git_tree_owner(tree) == repo);
 
-	return git_commit_create_from_callback(
+	return git_commit__create_internal(
 		id, repo, update_ref, author, committer,
 		message_encoding, message, git_tree_id(tree),
-		commit_parent_from_array, &data);
+		commit_parent_from_array, &data, false);
 }
 
 static const git_oid *commit_parent_for_amend(size_t curr, void *payload)
@@ -290,9 +316,9 @@ int git_commit_amend(
 		}
 	}
 
-	error = git_commit_create_from_callback(
+	error = git_commit__create_internal(
 		id, repo, NULL, author, committer, message_encoding, message,
-		&tree_id, commit_parent_for_amend, (void *)commit_to_amend);
+		&tree_id, commit_parent_for_amend, (void *)commit_to_amend, false);
 
 	if (!error && update_ref) {
 		error = git_reference__update_for_commit(
diff --git a/src/index.c b/src/index.c
index 5704432..b97f809 100644
--- a/src/index.c
+++ b/src/index.c
@@ -1245,17 +1245,22 @@ static void index_existing_and_best(
  * it, then it will return an error **and also free the entry**.  When
  * it replaces an existing entry, it will update the entry_ptr with the
  * actual entry in the index (and free the passed in one).
+ *
  * trust_path is whether we use the given path, or whether (on case
  * insensitive systems only) we try to canonicalize the given path to
  * be within an existing directory.
+ *
  * trust_mode is whether we trust the mode in entry_ptr.
+ *
+ * trust_id is whether we trust the id or it should be validated.
  */
 static int index_insert(
 	git_index *index,
 	git_index_entry **entry_ptr,
 	int replace,
 	bool trust_path,
-	bool trust_mode)
+	bool trust_mode,
+	bool trust_id)
 {
 	int error = 0;
 	size_t path_length, position;
@@ -1288,6 +1293,15 @@ static int index_insert(
 	if (!trust_path)
 		error = canonicalize_directory_path(index, entry, best);
 
+	/* ensure that the given id exists (unless it's a submodule) */
+	if (!error && !trust_id && INDEX_OWNER(index) &&
+		(entry->mode & GIT_FILEMODE_COMMIT) != GIT_FILEMODE_COMMIT) {
+
+		if (!git_object__is_valid(INDEX_OWNER(index), &entry->id,
+			git_object__type_from_filemode(entry->mode)))
+			error = -1;
+	}
+
 	/* look for tree / blob name collisions, removing conflicts if requested */
 	if (!error)
 		error = check_file_directory_collision(index, entry, position, replace);
@@ -1395,7 +1409,7 @@ int git_index_add_frombuffer(
 	git_oid_cpy(&entry->id, &id);
 	entry->file_size = len;
 
-	if ((error = index_insert(index, &entry, 1, true, true)) < 0)
+	if ((error = index_insert(index, &entry, 1, true, true, true)) < 0)
 		return error;
 
 	/* Adding implies conflict was resolved, move conflict entries to REUC */
@@ -1454,7 +1468,7 @@ int git_index_add_bypath(git_index *index, const char *path)
 	assert(index && path);
 
 	if ((ret = index_entry_init(&entry, index, path)) == 0)
-		ret = index_insert(index, &entry, 1, false, false);
+		ret = index_insert(index, &entry, 1, false, false, true);
 
 	/* If we were given a directory, let's see if it's a submodule */
 	if (ret < 0 && ret != GIT_EDIRECTORY)
@@ -1480,7 +1494,7 @@ int git_index_add_bypath(git_index *index, const char *path)
 			if ((ret = add_repo_as_submodule(&entry, index, path)) < 0)
 				return ret;
 
-			if ((ret = index_insert(index, &entry, 1, false, false)) < 0)
+			if ((ret = index_insert(index, &entry, 1, false, false, true)) < 0)
 				return ret;
 		} else if (ret < 0) {
 			return ret;
@@ -1569,7 +1583,7 @@ int git_index_add(git_index *index, const git_index_entry *source_entry)
 	}
 
 	if ((ret = index_entry_dup(&entry, index, source_entry)) < 0 ||
-		(ret = index_insert(index, &entry, 1, true, true)) < 0)
+		(ret = index_insert(index, &entry, 1, true, true, false)) < 0)
 		return ret;
 
 	git_tree_cache_invalidate_path(index->tree, entry->path);
@@ -1731,7 +1745,7 @@ int git_index_conflict_add(git_index *index,
 		/* Make sure stage is correct */
 		GIT_IDXENTRY_STAGE_SET(entries[i], i + 1);
 
-		if ((ret = index_insert(index, &entries[i], 1, true, true)) < 0)
+		if ((ret = index_insert(index, &entries[i], 1, true, true, false)) < 0)
 			goto on_error;
 
 		entries[i] = NULL; /* don't free if later entry fails */
diff --git a/src/object.c b/src/object.c
index b0a8199..ebf77fb 100644
--- a/src/object.c
+++ b/src/object.c
@@ -14,6 +14,8 @@
 #include "blob.h"
 #include "tag.h"
 
+bool git_object__strict_input_validation = true;
+
 typedef struct {
 	const char	*str;	/* type name string */
 	size_t		size;	/* size in bytes of the object structure */
@@ -465,3 +467,27 @@ int git_object_short_id(git_buf *out, const git_object *obj)
 	return error;
 }
 
+bool git_object__is_valid(
+	git_repository *repo, const git_oid *id, git_otype expected_type)
+{
+	git_odb *odb;
+	git_otype actual_type;
+	size_t len;
+	int error;
+
+	if (!git_object__strict_input_validation)
+		return true;
+
+	if ((error = git_repository_odb__weakptr(&odb, repo)) < 0 ||
+		(error = git_odb_read_header(&len, &actual_type, odb, id)) < 0)
+		return false;
+
+	if (expected_type != GIT_OBJ_ANY && expected_type != actual_type) {
+		giterr_set(GITERR_INVALID,
+			"the requested type does not match the type in the ODB");
+		return false;
+	}
+
+	return true;
+}
+
diff --git a/src/object.h b/src/object.h
index d187c55..dd227d1 100644
--- a/src/object.h
+++ b/src/object.h
@@ -7,6 +7,10 @@
 #ifndef INCLUDE_object_h__
 #define INCLUDE_object_h__
 
+#include "repository.h"
+
+extern bool git_object__strict_input_validation;
+
 /** Base git object for inheritance */
 struct git_object {
 	git_cached_obj cached;
@@ -28,4 +32,23 @@ int git_oid__parse(git_oid *oid, const char **buffer_out, const char *buffer_end
 
 void git_oid__writebuf(git_buf *buf, const char *header, const git_oid *oid);
 
+bool git_object__is_valid(
+	git_repository *repo, const git_oid *id, git_otype expected_type);
+
+GIT_INLINE(git_otype) git_object__type_from_filemode(git_filemode_t mode)
+{
+	switch (mode) {
+	case GIT_FILEMODE_TREE:
+		return GIT_OBJ_TREE;
+	case GIT_FILEMODE_COMMIT:
+		return GIT_OBJ_COMMIT;
+	case GIT_FILEMODE_BLOB:
+	case GIT_FILEMODE_BLOB_EXECUTABLE:
+	case GIT_FILEMODE_LINK:
+		return GIT_OBJ_BLOB;
+	default:
+		return GIT_OBJ_BAD;
+	}
+}
+
 #endif
diff --git a/src/refs.c b/src/refs.c
index 7b53865..a15e31b 100644
--- a/src/refs.c
+++ b/src/refs.c
@@ -377,15 +377,9 @@ static int reference__create(
 		return error;
 
 	if (oid != NULL) {
-		git_odb *odb;
-
 		assert(symbolic == NULL);
 
-		/* Sanity check the reference being created - target must exist. */
-		if ((error = git_repository_odb__weakptr(&odb, repo)) < 0)
-			return error;
-
-		if (!git_odb_exists(odb, oid)) {
+		if (!git_object__is_valid(repo, oid, GIT_OBJ_ANY)) {
 			giterr_set(GITERR_REFERENCE,
 				"Target OID for the reference doesn't exist on the repository");
 			return -1;
diff --git a/src/settings.c b/src/settings.c
index d7341ab..88602ba 100644
--- a/src/settings.c
+++ b/src/settings.c
@@ -14,6 +14,7 @@
 #include "sysdir.h"
 #include "cache.h"
 #include "global.h"
+#include "object.h"
 
 void git_libgit2_version(int *major, int *minor, int *rev)
 {
@@ -181,6 +182,11 @@ int git_libgit2_opts(int key, ...)
 		}
 
 		break;
+
+	case GIT_OPT_ENABLE_STRICT_OBJECT_CREATION:
+		git_object__strict_input_validation = (va_arg(ap, int) != 0);
+		break;
+
 	default:
 		giterr_set(GITERR_INVALID, "invalid option key");
 		error = -1;
diff --git a/src/tree.c b/src/tree.c
index cfceb3f..2c31515 100644
--- a/src/tree.c
+++ b/src/tree.c
@@ -726,6 +726,18 @@ on_error:
 	return -1;
 }
 
+static git_otype otype_from_mode(git_filemode_t filemode)
+{
+	switch (filemode) {
+	case GIT_FILEMODE_TREE:
+		return GIT_OBJ_TREE;
+	case GIT_FILEMODE_COMMIT:
+		return GIT_OBJ_COMMIT;
+	default:
+		return GIT_OBJ_BLOB;
+	}
+}
+
 int git_treebuilder_insert(
 	const git_tree_entry **entry_out,
 	git_treebuilder *bld,
@@ -745,6 +757,9 @@ int git_treebuilder_insert(
 	if (!valid_entry_name(bld->repo, filename))
 		return tree_error("Failed to insert entry. Invalid name for a tree entry", filename);
 
+	if (!git_object__is_valid(bld->repo, id, otype_from_mode(filemode)))
+		return tree_error("Failed to insert entry; invalid object specified", filename);
+
 	pos = git_strmap_lookup_index(bld->map, filename);
 	if (git_strmap_valid_index(bld->map, pos)) {
 		entry = git_strmap_value_at(bld->map, pos);
diff --git a/tests/commit/write.c b/tests/commit/write.c
index 176965c..96b7cc3 100644
--- a/tests/commit/write.c
+++ b/tests/commit/write.c
@@ -1,10 +1,12 @@
 #include "clar_libgit2.h"
+#include "git2/sys/commit.h"
 
 static const char *committer_name = "Vicent Marti";
 static const char *committer_email = "vicent@github.com";
 static const char *commit_message = "This commit has been created in memory\n\
    This is a commit created in memory and it will be written back to disk\n";
-static const char *tree_oid = "1810dff58d8a660512d4832e740f692884338ccd";
+static const char *tree_id_str = "1810dff58d8a660512d4832e740f692884338ccd";
+static const char *parent_id_str = "8496071c1b46c854b31185ea97743be6a8774479";
 static const char *root_commit_message = "This is a root commit\n\
    This is a root commit and should be the only one in this branch\n";
 static const char *root_reflog_message = "commit (initial): This is a root commit \
@@ -35,6 +37,8 @@ void test_commit_write__cleanup(void)
 	head_old = NULL;
 
 	cl_git_sandbox_cleanup();
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, 1));
 }
 
 
@@ -46,12 +50,11 @@ void test_commit_write__from_memory(void)
    const git_signature *author1, *committer1;
    git_commit *parent;
    git_tree *tree;
-   const char *commit_id_str = "8496071c1b46c854b31185ea97743be6a8774479";
 
-   git_oid_fromstr(&tree_id, tree_oid);
+   git_oid_fromstr(&tree_id, tree_id_str);
    cl_git_pass(git_tree_lookup(&tree, g_repo, &tree_id));
 
-   git_oid_fromstr(&parent_id, commit_id_str);
+   git_oid_fromstr(&parent_id, parent_id_str);
    cl_git_pass(git_commit_lookup(&parent, g_repo, &parent_id));
 
    /* create signatures */
@@ -106,7 +109,7 @@ void test_commit_write__root(void)
 	git_reflog *log;
 	const git_reflog_entry *entry;
 
-	git_oid_fromstr(&tree_id, tree_oid);
+	git_oid_fromstr(&tree_id, tree_id_str);
 	cl_git_pass(git_tree_lookup(&tree, g_repo, &tree_id));
 
 	/* create signatures */
@@ -158,3 +161,101 @@ void test_commit_write__root(void)
 	git_signature_free(committer);
 	git_reflog_free(log);
 }
+
+static int create_commit_from_ids(
+	git_oid *result,
+	const git_oid *tree_id,
+	const git_oid *parent_id)
+{
+	git_signature *author, *committer;
+	const git_oid *parent_ids[1];
+	int ret;
+
+	cl_git_pass(git_signature_new(
+		&committer, committer_name, committer_email, 123456789, 60));
+	cl_git_pass(git_signature_new(
+		&author, committer_name, committer_email, 987654321, 90));
+
+	parent_ids[0] = parent_id;
+
+	ret = git_commit_create_from_ids(
+		result,
+		g_repo,
+		NULL,
+		author,
+		committer,
+		NULL,
+		root_commit_message,
+		tree_id,
+		1,
+		parent_ids);
+
+	git_signature_free(committer);
+	git_signature_free(author);
+
+	return ret;
+}
+
+void test_commit_write__can_write_invalid_objects(void)
+{
+	git_oid expected_id, tree_id, parent_id, commit_id;
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, 0));
+
+	/* this is a valid tree and parent */
+	git_oid_fromstr(&tree_id, tree_id_str);
+	git_oid_fromstr(&parent_id, parent_id_str);
+
+	git_oid_fromstr(&expected_id, "c8571bbec3a72c4bcad31648902e5a453f1adece");
+	cl_git_pass(create_commit_from_ids(&commit_id, &tree_id, &parent_id));
+	cl_assert_equal_oid(&expected_id, &commit_id);
+
+	/* this is a wholly invented tree id */
+	git_oid_fromstr(&tree_id, "1234567890123456789012345678901234567890");
+	git_oid_fromstr(&parent_id, parent_id_str);
+
+	git_oid_fromstr(&expected_id, "996008340b8e68d69bf3c28d7c57fb7ec3c8e202");
+	cl_git_pass(create_commit_from_ids(&commit_id, &tree_id, &parent_id));
+	cl_assert_equal_oid(&expected_id, &commit_id);
+
+	/* this is a wholly invented parent id */
+	git_oid_fromstr(&tree_id, tree_id_str);
+	git_oid_fromstr(&parent_id, "1234567890123456789012345678901234567890");
+
+	git_oid_fromstr(&expected_id, "d78f660cab89d9791ca6714b57978bf2a7e709fd");
+	cl_git_pass(create_commit_from_ids(&commit_id, &tree_id, &parent_id));
+	cl_assert_equal_oid(&expected_id, &commit_id);
+
+	/* these are legitimate objects, but of the wrong type */
+	git_oid_fromstr(&tree_id, parent_id_str);
+	git_oid_fromstr(&parent_id, tree_id_str);
+
+	git_oid_fromstr(&expected_id, "5d80c07414e3f18792949699dfcacadf7748f361");
+	cl_git_pass(create_commit_from_ids(&commit_id, &tree_id, &parent_id));
+	cl_assert_equal_oid(&expected_id, &commit_id);
+}
+
+void test_commit_write__can_validate_objects(void)
+{
+	git_oid tree_id, parent_id, commit_id;
+
+	/* this is a valid tree and parent */
+	git_oid_fromstr(&tree_id, tree_id_str);
+	git_oid_fromstr(&parent_id, parent_id_str);
+	cl_git_pass(create_commit_from_ids(&commit_id, &tree_id, &parent_id));
+
+	/* this is a wholly invented tree id */
+	git_oid_fromstr(&tree_id, "1234567890123456789012345678901234567890");
+	git_oid_fromstr(&parent_id, parent_id_str);
+	cl_git_fail(create_commit_from_ids(&commit_id, &tree_id, &parent_id));
+
+	/* this is a wholly invented parent id */
+	git_oid_fromstr(&tree_id, tree_id_str);
+	git_oid_fromstr(&parent_id, "1234567890123456789012345678901234567890");
+	cl_git_fail(create_commit_from_ids(&commit_id, &tree_id, &parent_id));
+
+	/* these are legitimate objects, but of the wrong type */
+	git_oid_fromstr(&tree_id, parent_id_str);
+	git_oid_fromstr(&parent_id, tree_id_str);
+	cl_git_fail(create_commit_from_ids(&commit_id, &tree_id, &parent_id));
+}
diff --git a/tests/diff/index.c b/tests/diff/index.c
index df45ad2..0293b78 100644
--- a/tests/diff/index.c
+++ b/tests/diff/index.c
@@ -185,9 +185,9 @@ static void do_conflicted_diff(diff_expects *exp, unsigned long flags)
 	ancestor.path = ours.path = theirs.path = "staged_changes";
 	ancestor.mode = ours.mode = theirs.mode = GIT_FILEMODE_BLOB;
 
-	git_oid_fromstr(&ancestor.id, "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
-	git_oid_fromstr(&ours.id, "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
-	git_oid_fromstr(&theirs.id, "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
+	git_oid_fromstr(&ancestor.id, "d427e0b2e138501a3d15cc376077a3631e15bd46");
+	git_oid_fromstr(&ours.id, "ee3fa1b8c00aff7fe02065fdb50864bb0d932ccf");
+	git_oid_fromstr(&theirs.id, "2bd0a343aeef7a2cf0d158478966a6e587ff3863");
 
 	cl_git_pass(git_index_conflict_add(index, &ancestor, &ours, &theirs));
 	cl_git_pass(git_diff_tree_to_index(&diff, g_repo, a, index, &opts));
@@ -255,7 +255,7 @@ void test_diff_index__not_in_head_conflicted(void)
 
 	theirs.path = "file_not_in_head";
 	theirs.mode = GIT_FILEMODE_BLOB;
-	git_oid_fromstr(&theirs.id, "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
+	git_oid_fromstr(&theirs.id, "2bd0a343aeef7a2cf0d158478966a6e587ff3863");
 	cl_git_pass(git_index_conflict_add(index, NULL, NULL, &theirs));
 
 	cl_git_pass(git_diff_tree_to_index(&diff, g_repo, a, index, NULL));
diff --git a/tests/diff/workdir.c b/tests/diff/workdir.c
index 892c7b7..e1bbce8 100644
--- a/tests/diff/workdir.c
+++ b/tests/diff/workdir.c
@@ -85,9 +85,11 @@ void test_diff_workdir__to_index_with_conflicts(void)
 	/* Adding an entry that represents a rename gets two files in conflict */
 	our_entry.path = "subdir/modified_file";
 	our_entry.mode = 0100644;
+	git_oid_fromstr(&our_entry.id, "ee3fa1b8c00aff7fe02065fdb50864bb0d932ccf");
 
 	their_entry.path = "subdir/rename_conflict";
 	their_entry.mode = 0100644;
+	git_oid_fromstr(&their_entry.id, "2bd0a343aeef7a2cf0d158478966a6e587ff3863");
 
 	cl_git_pass(git_repository_index(&index, g_repo));
 	cl_git_pass(git_index_conflict_add(index, NULL, &our_entry, &their_entry));
@@ -1975,9 +1977,9 @@ void test_diff_workdir__to_index_conflicted(void) {
 
 	ancestor.path = ours.path = theirs.path = "_file";
 	ancestor.mode = ours.mode = theirs.mode = 0100644;
-	git_oid_fromstr(&ancestor.id, "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
-	git_oid_fromstr(&ours.id, "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
-	git_oid_fromstr(&theirs.id, "deadbeefdeadbeefdeadbeefdeadbeefdeadbeef");
+	git_oid_fromstr(&ancestor.id, "d427e0b2e138501a3d15cc376077a3631e15bd46");
+	git_oid_fromstr(&ours.id, "ee3fa1b8c00aff7fe02065fdb50864bb0d932ccf");
+	git_oid_fromstr(&theirs.id, "2bd0a343aeef7a2cf0d158478966a6e587ff3863");
 	cl_git_pass(git_index_conflict_add(index, &ancestor, &ours, &theirs));
 
 	cl_git_pass(git_diff_tree_to_index(&diff1, g_repo, a, index, NULL));
diff --git a/tests/index/add.c b/tests/index/add.c
new file mode 100644
index 0000000..f101ea2
--- /dev/null
+++ b/tests/index/add.c
@@ -0,0 +1,84 @@
+#include "clar_libgit2.h"
+
+static git_repository *g_repo = NULL;
+static git_index *g_index = NULL;
+
+static const char *valid_blob_id = "fa49b077972391ad58037050f2a75f74e3671e92";
+static const char *valid_tree_id = "181037049a54a1eb5fab404658a3a250b44335d7";
+static const char *valid_commit_id = "763d71aadf09a7951596c9746c024e7eece7c7af";
+static const char *invalid_id = "1234567890123456789012345678901234567890";
+
+void test_index_add__initialize(void)
+{
+	g_repo = cl_git_sandbox_init("testrepo");
+	cl_git_pass(git_repository_index(&g_index, g_repo));
+}
+
+void test_index_add__cleanup(void)
+{
+	git_index_free(g_index);
+	cl_git_sandbox_cleanup();
+	g_repo = NULL;
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, 1));
+}
+
+static void test_add_entry(
+	bool should_succeed, const char *idstr, git_filemode_t mode)
+{
+	git_index_entry entry = {{0}};
+
+	cl_git_pass(git_oid_fromstr(&entry.id, idstr));
+
+	entry.path = mode == GIT_FILEMODE_TREE ? "test_folder" : "test_file";
+	entry.mode = mode;
+
+	if (should_succeed)
+		cl_git_pass(git_index_add(g_index, &entry));
+	else
+		cl_git_fail(git_index_add(g_index, &entry));
+}
+
+void test_index_add__invalid_entries_succeeds_by_default(void)
+{
+	/*
+	 * Ensure that there is validation on object ids by default
+	 */
+
+	/* ensure that we can add some actually good entries */
+	test_add_entry(true, valid_blob_id, GIT_FILEMODE_BLOB);
+	test_add_entry(true, valid_blob_id, GIT_FILEMODE_BLOB_EXECUTABLE);
+	test_add_entry(true, valid_blob_id, GIT_FILEMODE_LINK);
+
+	/* test that we fail to add some invalid (missing) blobs and trees */
+	test_add_entry(false, invalid_id, GIT_FILEMODE_BLOB);
+	test_add_entry(false, invalid_id, GIT_FILEMODE_BLOB_EXECUTABLE);
+	test_add_entry(false, invalid_id, GIT_FILEMODE_LINK);
+
+	/* test that we validate the types of objects */
+	test_add_entry(false, valid_commit_id, GIT_FILEMODE_BLOB);
+	test_add_entry(false, valid_tree_id, GIT_FILEMODE_BLOB_EXECUTABLE);
+	test_add_entry(false, valid_commit_id, GIT_FILEMODE_LINK);
+
+	/*
+	 * Ensure that there we can disable validation
+	 */
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, 0));
+
+	/* ensure that we can add some actually good entries */
+	test_add_entry(true, valid_blob_id, GIT_FILEMODE_BLOB);
+	test_add_entry(true, valid_blob_id, GIT_FILEMODE_BLOB_EXECUTABLE);
+	test_add_entry(true, valid_blob_id, GIT_FILEMODE_LINK);
+
+	/* test that we can now add some invalid (missing) blobs and trees */
+	test_add_entry(true, invalid_id, GIT_FILEMODE_BLOB);
+	test_add_entry(true, invalid_id, GIT_FILEMODE_BLOB_EXECUTABLE);
+	test_add_entry(true, invalid_id, GIT_FILEMODE_LINK);
+
+	/* test that we do not validate the types of objects */
+	test_add_entry(true, valid_commit_id, GIT_FILEMODE_BLOB);
+	test_add_entry(true, valid_tree_id, GIT_FILEMODE_BLOB_EXECUTABLE);
+	test_add_entry(true, valid_commit_id, GIT_FILEMODE_LINK);
+}
+
diff --git a/tests/index/bypath.c b/tests/index/bypath.c
index 88a7617..34a7412 100644
--- a/tests/index/bypath.c
+++ b/tests/index/bypath.c
@@ -134,6 +134,7 @@ void test_index_bypath__add_honors_existing_case_2(void)
 		clar__skip();
 
 	dummy.mode = GIT_FILEMODE_BLOB;
+	cl_git_pass(git_oid_fromstr(&dummy.id, "f990a25a74d1a8281ce2ab018ea8df66795cd60b"));
 
 	/* note that `git_index_add` does no checking to canonical directories */
 	dummy.path = "Just_a_dir/file0.txt";
@@ -189,6 +190,7 @@ void test_index_bypath__add_honors_existing_case_3(void)
 		clar__skip();
 
 	dummy.mode = GIT_FILEMODE_BLOB;
+	cl_git_pass(git_oid_fromstr(&dummy.id, "f990a25a74d1a8281ce2ab018ea8df66795cd60b"));
 
 	dummy.path = "just_a_dir/filea.txt";
 	cl_git_pass(git_index_add(g_idx, &dummy));
@@ -219,6 +221,7 @@ void test_index_bypath__add_honors_existing_case_4(void)
 		clar__skip();
 
 	dummy.mode = GIT_FILEMODE_BLOB;
+	cl_git_pass(git_oid_fromstr(&dummy.id, "f990a25a74d1a8281ce2ab018ea8df66795cd60b"));
 
 	dummy.path = "just_a_dir/a/b/c/d/e/file1.txt";
 	cl_git_pass(git_index_add(g_idx, &dummy));
diff --git a/tests/index/cache.c b/tests/index/cache.c
index 3982bf1..56885af 100644
--- a/tests/index/cache.c
+++ b/tests/index/cache.c
@@ -111,7 +111,7 @@ void test_index_cache__read_tree_no_children(void)
 	memset(&entry, 0x0, sizeof(git_index_entry));
 	entry.path = "new.txt";
 	entry.mode = GIT_FILEMODE_BLOB;
-	git_oid_fromstr(&entry.id, "45b983be36b73c0788dc9cbcb76cbb80fc7bb057");
+	git_oid_fromstr(&entry.id, "d4bcc68acd4410bf836a39f20afb2c2ece09584e");
 
 	cl_git_pass(git_index_add(index, &entry));
 	cl_assert_equal_i(-1, index->tree->entry_count);
@@ -191,7 +191,7 @@ void test_index_cache__read_tree_children(void)
 	memset(&entry, 0x0, sizeof(git_index_entry));
 	entry.path = "top-level";
 	entry.mode = GIT_FILEMODE_BLOB;
-	git_oid_fromstr(&entry.id, "45b983be36b73c0788dc9cbcb76cbb80fc7bb057");
+	git_oid_fromstr(&entry.id, "ee3fa1b8c00aff7fe02065fdb50864bb0d932ccf");
 	cl_git_pass(git_index_add(index, &entry));
 
 
@@ -217,7 +217,7 @@ void test_index_cache__read_tree_children(void)
 
 	/* override with a slightly different id, also dummy */
 	entry.path = "subdir/some-file";
-	git_oid_fromstr(&entry.id, "45b983be36b73c0788dc9cbcb76cbb80fc7bb058");
+	git_oid_fromstr(&entry.id, "ee3fa1b8c00aff7fe02065fdb50864bb0d932ccf");
 	cl_git_pass(git_index_add(index, &entry));
 
 	cl_assert_equal_i(-1, index->tree->entry_count);
diff --git a/tests/index/conflicts.c b/tests/index/conflicts.c
index 8e94cd4..d400468 100644
--- a/tests/index/conflicts.c
+++ b/tests/index/conflicts.c
@@ -16,11 +16,6 @@ static git_index *repo_index;
 #define CONFLICTS_TWO_OUR_OID "8b3f43d2402825c200f835ca1762413e386fd0b2"
 #define CONFLICTS_TWO_THEIR_OID "220bd62631c8cf7a83ef39c6b94595f00517211e"
 
-#define TEST_STAGED_OID "beefdadafeedabedcafedeedbabedeadbeaddeaf"
-#define TEST_ANCESTOR_OID "f00ff00ff00ff00ff00ff00ff00ff00ff00ff00f"
-#define TEST_OUR_OID "b44bb44bb44bb44bb44bb44bb44bb44bb44bb44b"
-#define TEST_THEIR_OID "0123456789abcdef0123456789abcdef01234567"
-
 // Fixture setup and teardown
 void test_index_conflicts__initialize(void)
 {
@@ -49,17 +44,17 @@ void test_index_conflicts__add(void)
 	ancestor_entry.path = "test-one.txt";
 	ancestor_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&ancestor_entry, 1);
-	git_oid_fromstr(&ancestor_entry.id, TEST_ANCESTOR_OID);
+	git_oid_fromstr(&ancestor_entry.id, CONFLICTS_ONE_ANCESTOR_OID);
 
 	our_entry.path = "test-one.txt";
 	our_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&our_entry, 2);
-	git_oid_fromstr(&our_entry.id, TEST_OUR_OID);
+	git_oid_fromstr(&our_entry.id, CONFLICTS_ONE_OUR_OID);
 
 	their_entry.path = "test-one.txt";
 	their_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&ancestor_entry, 2);
-	git_oid_fromstr(&their_entry.id, TEST_THEIR_OID);
+	git_oid_fromstr(&their_entry.id, CONFLICTS_ONE_THEIR_OID);
 
 	cl_git_pass(git_index_conflict_add(repo_index, &ancestor_entry, &our_entry, &their_entry));
 
@@ -80,17 +75,17 @@ void test_index_conflicts__add_fixes_incorrect_stage(void)
 	ancestor_entry.path = "test-one.txt";
 	ancestor_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&ancestor_entry, 3);
-	git_oid_fromstr(&ancestor_entry.id, TEST_ANCESTOR_OID);
+	git_oid_fromstr(&ancestor_entry.id, CONFLICTS_ONE_ANCESTOR_OID);
 
 	our_entry.path = "test-one.txt";
 	our_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&our_entry, 1);
-	git_oid_fromstr(&our_entry.id, TEST_OUR_OID);
+	git_oid_fromstr(&our_entry.id, CONFLICTS_ONE_OUR_OID);
 
 	their_entry.path = "test-one.txt";
 	their_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&their_entry, 2);
-	git_oid_fromstr(&their_entry.id, TEST_THEIR_OID);
+	git_oid_fromstr(&their_entry.id, CONFLICTS_ONE_THEIR_OID);
 
 	cl_git_pass(git_index_conflict_add(repo_index, &ancestor_entry, &our_entry, &their_entry));
 
@@ -105,36 +100,33 @@ void test_index_conflicts__add_fixes_incorrect_stage(void)
 
 void test_index_conflicts__add_removes_stage_zero(void)
 {
-	git_index_entry staged, ancestor_entry, our_entry, their_entry;
+	git_index_entry ancestor_entry, our_entry, their_entry;
 	const git_index_entry *conflict_entry[3];
 
 	cl_assert(git_index_entrycount(repo_index) == 8);
 
-	memset(&staged, 0x0, sizeof(git_index_entry));
 	memset(&ancestor_entry, 0x0, sizeof(git_index_entry));
 	memset(&our_entry, 0x0, sizeof(git_index_entry));
 	memset(&their_entry, 0x0, sizeof(git_index_entry));
 
-	staged.path = "test-one.txt";
-	staged.mode = 0100644;
-	git_oid_fromstr(&staged.id, TEST_STAGED_OID);
-	cl_git_pass(git_index_add(repo_index, &staged));
+	cl_git_mkfile("./mergedrepo/test-one.txt", "new-file\n");
+	cl_git_pass(git_index_add_bypath(repo_index, "test-one.txt"));
 	cl_assert(git_index_entrycount(repo_index) == 9);
 
 	ancestor_entry.path = "test-one.txt";
 	ancestor_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&ancestor_entry, 3);
-	git_oid_fromstr(&ancestor_entry.id, TEST_ANCESTOR_OID);
+	git_oid_fromstr(&ancestor_entry.id, CONFLICTS_ONE_ANCESTOR_OID);
 
 	our_entry.path = "test-one.txt";
 	our_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&our_entry, 1);
-	git_oid_fromstr(&our_entry.id, TEST_OUR_OID);
+	git_oid_fromstr(&our_entry.id, CONFLICTS_ONE_OUR_OID);
 
 	their_entry.path = "test-one.txt";
 	their_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&their_entry, 2);
-	git_oid_fromstr(&their_entry.id, TEST_THEIR_OID);
+	git_oid_fromstr(&their_entry.id, CONFLICTS_ONE_THEIR_OID);
 
 	cl_git_pass(git_index_conflict_add(repo_index, &ancestor_entry, &our_entry, &their_entry));
 
@@ -330,7 +322,7 @@ void test_index_conflicts__partial(void)
 	ancestor_entry.path = "test-one.txt";
 	ancestor_entry.mode = 0100644;
 	GIT_IDXENTRY_STAGE_SET(&ancestor_entry, 1);
-	git_oid_fromstr(&ancestor_entry.id, TEST_ANCESTOR_OID);
+	git_oid_fromstr(&ancestor_entry.id, CONFLICTS_ONE_ANCESTOR_OID);
 
 	cl_git_pass(git_index_conflict_add(repo_index, &ancestor_entry, NULL, NULL));
 	cl_assert(git_index_entrycount(repo_index) == 9);
diff --git a/tests/index/filemodes.c b/tests/index/filemodes.c
index 6442d77..2efad5b 100644
--- a/tests/index/filemodes.c
+++ b/tests/index/filemodes.c
@@ -236,12 +236,19 @@ void test_index_filemodes__invalid(void)
 {
 	git_index *index;
 	git_index_entry entry;
+	const git_index_entry *dummy;
 
 	cl_git_pass(git_repository_index(&index, g_repo));
 
+	/* add a dummy file so that we have a valid id */
+	cl_git_mkfile("./filemodes/dummy-file.txt", "new-file\n");
+	cl_git_pass(git_index_add_bypath(index, "dummy-file.txt"));
+	cl_assert((dummy = git_index_get_bypath(index, "dummy-file.txt", 0)));
+
 	GIT_IDXENTRY_STAGE_SET(&entry, 0);
 	entry.path = "foo";
 	entry.mode = GIT_OBJ_BLOB;
+	git_oid_cpy(&entry.id, &dummy->id);
 	cl_git_fail(git_index_add(index, &entry));
 
 	entry.mode = GIT_FILEMODE_BLOB;
diff --git a/tests/index/racy.c b/tests/index/racy.c
index ace84d5..68aa460 100644
--- a/tests/index/racy.c
+++ b/tests/index/racy.c
@@ -178,6 +178,7 @@ static void setup_uptodate_files(void)
 {
 	git_buf path = GIT_BUF_INIT;
 	git_index *index;
+	const git_index_entry *a_entry;
 	git_index_entry new_entry = {{0}};
 
 	cl_git_pass(git_repository_index(&index, g_repo));
@@ -188,9 +189,12 @@ static void setup_uptodate_files(void)
 	/* Put 'A' into the index */
 	cl_git_pass(git_index_add_bypath(index, "A"));
 
+	cl_assert((a_entry = git_index_get_bypath(index, "A", 0)));
+
 	/* Put 'B' into the index */
 	new_entry.path = "B";
 	new_entry.mode = GIT_FILEMODE_BLOB;
+	git_oid_cpy(&new_entry.id, &a_entry->id);
 	cl_git_pass(git_index_add(index, &new_entry));
 
 	/* Put 'C' into the index */
diff --git a/tests/object/tree/write.c b/tests/object/tree/write.c
index 5433e5f..341f5db 100644
--- a/tests/object/tree/write.c
+++ b/tests/object/tree/write.c
@@ -18,6 +18,8 @@ void test_object_tree_write__initialize(void)
 void test_object_tree_write__cleanup(void)
 {
    cl_git_sandbox_cleanup();
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, 1));
 }
 
 void test_object_tree_write__from_memory(void)
@@ -131,15 +133,18 @@ void test_object_tree_write__sorted_subtrees(void)
 	  	{ GIT_FILEMODE_TREE, "vendors"}
 	};
 
-	git_oid blank_oid, tree_oid;
+	git_oid bid, tid, tree_oid;
 
-	memset(&blank_oid, 0x0, sizeof(blank_oid));
+	cl_git_pass(git_oid_fromstr(&bid, blob_oid));
+	cl_git_pass(git_oid_fromstr(&tid, first_tree));
 
 	cl_git_pass(git_treebuilder_new(&builder, g_repo, NULL));
 
 	for (i = 0; i < ARRAY_SIZE(entries); ++i) {
+		git_oid *id = entries[i].attr == GIT_FILEMODE_TREE ?  &tid : &bid; 
+
 		cl_git_pass(git_treebuilder_insert(NULL,
-			builder, entries[i].filename, &blank_oid, entries[i].attr));
+			builder, entries[i].filename, id, entries[i].attr));
 	}
 
 	cl_git_pass(git_treebuilder_write(&tree_oid, builder));
@@ -187,10 +192,10 @@ void test_object_tree_write__removing_and_re_adding_in_treebuilder(void)
 {
 	git_treebuilder *builder;
 	int i, aardvark_i, apple_i, apple_after_i, apple_extra_i, last_i;
-	git_oid blank_oid, tree_oid;
+	git_oid entry_oid, tree_oid;
 	git_tree *tree;
 
-	memset(&blank_oid, 0x0, sizeof(blank_oid));
+	cl_git_pass(git_oid_fromstr(&entry_oid, blob_oid));
 
 	cl_git_pass(git_treebuilder_new(&builder, g_repo, NULL));
 
@@ -198,7 +203,7 @@ void test_object_tree_write__removing_and_re_adding_in_treebuilder(void)
 
 	for (i = 0; _entries[i].filename; ++i)
 		cl_git_pass(git_treebuilder_insert(NULL,
-			builder, _entries[i].filename, &blank_oid, _entries[i].attr));
+			builder, _entries[i].filename, &entry_oid, _entries[i].attr));
 
 	cl_assert_equal_i(6, (int)git_treebuilder_entrycount(builder));
 
@@ -209,12 +214,12 @@ void test_object_tree_write__removing_and_re_adding_in_treebuilder(void)
 	cl_assert_equal_i(4, (int)git_treebuilder_entrycount(builder));
 
 	cl_git_pass(git_treebuilder_insert(
-		NULL, builder, "before_last", &blank_oid, GIT_FILEMODE_BLOB));
+		NULL, builder, "before_last", &entry_oid, GIT_FILEMODE_BLOB));
 	cl_assert_equal_i(5, (int)git_treebuilder_entrycount(builder));
 
 	/* reinsert apple_after */
 	cl_git_pass(git_treebuilder_insert(
-		NULL, builder, "apple_after", &blank_oid, GIT_FILEMODE_BLOB));
+		NULL, builder, "apple_after", &entry_oid, GIT_FILEMODE_BLOB));
 	cl_assert_equal_i(6, (int)git_treebuilder_entrycount(builder));
 
 	cl_git_pass(git_treebuilder_remove(builder, "last"));
@@ -222,11 +227,11 @@ void test_object_tree_write__removing_and_re_adding_in_treebuilder(void)
 
 	/* reinsert last */
 	cl_git_pass(git_treebuilder_insert(
-		NULL, builder, "last", &blank_oid, GIT_FILEMODE_BLOB));
+		NULL, builder, "last", &entry_oid, GIT_FILEMODE_BLOB));
 	cl_assert_equal_i(6, (int)git_treebuilder_entrycount(builder));
 
 	cl_git_pass(git_treebuilder_insert(
-		NULL, builder, "apple_extra", &blank_oid, GIT_FILEMODE_BLOB));
+		NULL, builder, "apple_extra", &entry_oid, GIT_FILEMODE_BLOB));
 	cl_assert_equal_i(7, (int)git_treebuilder_entrycount(builder));
 
 	cl_git_pass(git_treebuilder_write(&tree_oid, builder));
@@ -278,16 +283,16 @@ void test_object_tree_write__filtering(void)
 {
 	git_treebuilder *builder;
 	int i;
-	git_oid blank_oid, tree_oid;
+	git_oid entry_oid, tree_oid;
 	git_tree *tree;
 
-	memset(&blank_oid, 0x0, sizeof(blank_oid));
+	git_oid_fromstr(&entry_oid, blob_oid);
 
 	cl_git_pass(git_treebuilder_new(&builder, g_repo, NULL));
 
 	for (i = 0; _entries[i].filename; ++i)
 		cl_git_pass(git_treebuilder_insert(NULL,
-			builder, _entries[i].filename, &blank_oid, _entries[i].attr));
+			builder, _entries[i].filename, &entry_oid, _entries[i].attr));
 
 	cl_assert_equal_i(6, (int)git_treebuilder_entrycount(builder));
 
@@ -406,6 +411,8 @@ void test_object_tree_write__protect_filesystems(void)
 	git_treebuilder *builder;
 	git_oid bid;
 
+	cl_git_pass(git_oid_fromstr(&bid, "fa49b077972391ad58037050f2a75f74e3671e92"));
+
 	/* Ensure that (by default) we can write objects with funny names on
 	 * platforms that are not affected.
 	 */
@@ -440,3 +447,56 @@ void test_object_tree_write__protect_filesystems(void)
 
 	git_treebuilder_free(builder);
 }
+
+static void test_invalid_objects(bool should_allow_invalid)
+{
+	git_treebuilder *builder;
+	git_oid valid_blob_id, invalid_blob_id, valid_tree_id, invalid_tree_id;
+
+#define assert_allowed(expr) \
+	clar__assert(!(expr) == should_allow_invalid, __FILE__, __LINE__, \
+		(should_allow_invalid ? \
+		 "Expected function call to succeed: " #expr : \
+		 "Expected function call to fail: " #expr), \
+		NULL, 1)
+
+	cl_git_pass(git_oid_fromstr(&valid_blob_id, blob_oid));
+	cl_git_pass(git_oid_fromstr(&invalid_blob_id,
+		"1234567890123456789012345678901234567890"));
+	cl_git_pass(git_oid_fromstr(&valid_tree_id, first_tree));
+	cl_git_pass(git_oid_fromstr(&invalid_tree_id,
+		"0000000000111111111122222222223333333333"));
+
+	cl_git_pass(git_treebuilder_new(&builder, g_repo, NULL));
+
+	/* test valid blobs and trees (these should always pass) */
+	cl_git_pass(git_treebuilder_insert(NULL, builder, "file.txt", &valid_blob_id, GIT_FILEMODE_BLOB));
+	cl_git_pass(git_treebuilder_insert(NULL, builder, "folder", &valid_tree_id, GIT_FILEMODE_TREE));
+
+	/* replace valid files and folders with invalid ones */
+	assert_allowed(git_treebuilder_insert(NULL, builder, "file.txt", &invalid_blob_id, GIT_FILEMODE_BLOB));
+	assert_allowed(git_treebuilder_insert(NULL, builder, "folder", &invalid_blob_id, GIT_FILEMODE_BLOB));
+
+	/* insert new invalid files and folders */
+	assert_allowed(git_treebuilder_insert(NULL, builder, "invalid_file.txt", &invalid_blob_id, GIT_FILEMODE_BLOB));
+	assert_allowed(git_treebuilder_insert(NULL, builder, "invalid_folder", &invalid_blob_id, GIT_FILEMODE_BLOB));
+
+	/* insert valid blobs as trees and trees as blobs */
+	assert_allowed(git_treebuilder_insert(NULL, builder, "file_as_folder", &valid_blob_id, GIT_FILEMODE_TREE));
+	assert_allowed(git_treebuilder_insert(NULL, builder, "folder_as_file.txt", &valid_tree_id, GIT_FILEMODE_BLOB));
+
+#undef assert_allowed
+
+	git_treebuilder_free(builder);
+}
+
+void test_object_tree_write__object_validity(void)
+{
+	/* Ensure that we cannot add invalid objects by default */
+	test_invalid_objects(false);
+
+	/* Ensure that we can turn off validation */
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, 0));
+	test_invalid_objects(true);
+}
+
diff --git a/tests/refs/create.c b/tests/refs/create.c
index 48194ae..a0bc780 100644
--- a/tests/refs/create.c
+++ b/tests/refs/create.c
@@ -18,6 +18,8 @@ void test_refs_create__initialize(void)
 void test_refs_create__cleanup(void)
 {
    cl_git_sandbox_cleanup();
+
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, 0));
 }
 
 void test_refs_create__symbolic(void)
@@ -119,9 +121,27 @@ void test_refs_create__oid(void)
 	git_reference_free(looked_up_ref);
 }
 
-void test_refs_create__oid_unknown(void)
+/* Can by default create a reference that targets at an unknown id */
+void test_refs_create__oid_unknown_succeeds_by_default(void)
+{
+	git_reference *new_reference, *looked_up_ref;
+	git_oid id;
+
+	const char *new_head = "refs/heads/new-head";
+
+	git_oid_fromstr(&id, "deadbeef3f795b2b4353bcce3a527ad0a4f7f644");
+
+	/* Create and write the new object id reference */
+	cl_git_pass(git_reference_create(&new_reference, g_repo, new_head, &id, 0, NULL));
+
+	/* Ensure the reference can't be looked-up... */
+	cl_git_pass(git_reference_lookup(&looked_up_ref, g_repo, new_head));
+	git_reference_free(looked_up_ref);
+}
+
+/* Strict object enforcement enforces valid object id */
+void test_refs_create__oid_unknown_fails_strict_mode(void)
 {
-   // Can not create a new OID reference which targets at an unknown id
 	git_reference *new_reference, *looked_up_ref;
 	git_oid id;
 
@@ -129,6 +149,8 @@ void test_refs_create__oid_unknown(void)
 
 	git_oid_fromstr(&id, "deadbeef3f795b2b4353bcce3a527ad0a4f7f644");
 
+	cl_git_pass(git_libgit2_opts(GIT_OPT_ENABLE_STRICT_OBJECT_CREATION, 1));
+
 	/* Create and write the new object id reference */
 	cl_git_fail(git_reference_create(&new_reference, g_repo, new_head, &id, 0, NULL));
 
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