diff --git a/Documentation/technical/sparse-index.txt b/Documentation/technical/sparse-index.txt
index 8d3d8080460..3b24c1a219f 100644
--- a/Documentation/technical/sparse-index.txt
+++ b/Documentation/technical/sparse-index.txt
@@ -85,8 +85,41 @@ index, as well.
 
 Next, consumers of the index will be guarded against operating on a
 sparse-index by inserting calls to `ensure_full_index()` or
-`expand_index_to_path()`. After these guards are in place, we can begin
-leaving sparse-directory entries in the in-memory index structure.
+`expand_index_to_path()`. If a specific path is requested, then those will
+be protected from within the `index_file_exists()` and `index_name_pos()`
+API calls: they will call `ensure_full_index()` if necessary. The
+intention here is to preserve existing behavior when interacting with a
+sparse-checkout. We don't want a change to happen by accident, without
+tests. Many of these locations may not need any change before removing the
+guards, but we should not do so without tests to ensure the expected
+behavior happens.
+
+It may be desirable to _change_ the behavior of some commands in the
+presence of a sparse index or more generally in any sparse-checkout
+scenario. In such cases, these should be carefully communicated and
+tested. No such behavior changes are intended during this phase.
+
+During a scan of the codebase, not every iteration of the cache entries
+needs an `ensure_full_index()` check. The basic reasons include:
+
+1. The loop is scanning for entries with non-zero stage. These entries
+   are not collapsed into a sparse-directory entry.
+
+2. The loop is scanning for submodules. These entries are not collapsed
+   into a sparse-directory entry.
+
+3. The loop is part of the index API, especially around reading or
+   writing the format.
+
+4. The loop is checking for correct order of cache entries and that is
+   correct if and only if the sparse-directory entries are in the correct
+   location.
+
+5. The loop ignores entries with the `SKIP_WORKTREE` bit set, or is
+   otherwise already aware of sparse directory entries.
+
+6. The sparse-index is disabled at this point when using the split-index
+   feature, so no effort is made to protect the split-index API.
 
 Even after inserting these guards, we will keep expanding sparse-indexes
 for most Git commands using the `command_requires_full_index` repository
diff --git a/attr.c b/attr.c
index ac8ec7ce51e..9e897e43f53 100644
--- a/attr.c
+++ b/attr.c
@@ -733,7 +733,7 @@ static struct attr_stack *read_attr_from_file(const char *path, unsigned flags)
 	return res;
 }
 
-static struct attr_stack *read_attr_from_index(const struct index_state *istate,
+static struct attr_stack *read_attr_from_index(struct index_state *istate,
 					       const char *path,
 					       unsigned flags)
 {
@@ -763,7 +763,7 @@ static struct attr_stack *read_attr_from_index(const struct index_state *istate,
 	return res;
 }
 
-static struct attr_stack *read_attr(const struct index_state *istate,
+static struct attr_stack *read_attr(struct index_state *istate,
 				    const char *path, unsigned flags)
 {
 	struct attr_stack *res = NULL;
@@ -855,7 +855,7 @@ static void push_stack(struct attr_stack **attr_stack_p,
 	}
 }
 
-static void bootstrap_attr_stack(const struct index_state *istate,
+static void bootstrap_attr_stack(struct index_state *istate,
 				 struct attr_stack **stack)
 {
 	struct attr_stack *e;
@@ -894,7 +894,7 @@ static void bootstrap_attr_stack(const struct index_state *istate,
 	push_stack(stack, e, NULL, 0);
 }
 
-static void prepare_attr_stack(const struct index_state *istate,
+static void prepare_attr_stack(struct index_state *istate,
 			       const char *path, int dirlen,
 			       struct attr_stack **stack)
 {
@@ -1094,7 +1094,7 @@ static void determine_macros(struct all_attrs_item *all_attrs,
  * If check->check_nr is non-zero, only attributes in check[] are collected.
  * Otherwise all attributes are collected.
  */
-static void collect_some_attrs(const struct index_state *istate,
+static void collect_some_attrs(struct index_state *istate,
 			       const char *path,
 			       struct attr_check *check)
 {
@@ -1123,7 +1123,7 @@ static void collect_some_attrs(const struct index_state *istate,
 	fill(path, pathlen, basename_offset, check->stack, check->all_attrs, rem);
 }
 
-void git_check_attr(const struct index_state *istate,
+void git_check_attr(struct index_state *istate,
 		    const char *path,
 		    struct attr_check *check)
 {
@@ -1140,7 +1140,7 @@ void git_check_attr(const struct index_state *istate,
 	}
 }
 
-void git_all_attrs(const struct index_state *istate,
+void git_all_attrs(struct index_state *istate,
 		   const char *path, struct attr_check *check)
 {
 	int i;
diff --git a/attr.h b/attr.h
index 404548f028a..3732505edae 100644
--- a/attr.h
+++ b/attr.h
@@ -190,14 +190,14 @@ void attr_check_free(struct attr_check *check);
  */
 const char *git_attr_name(const struct git_attr *);
 
-void git_check_attr(const struct index_state *istate,
+void git_check_attr(struct index_state *istate,
 		    const char *path, struct attr_check *check);
 
 /*
  * Retrieve all attributes that apply to the specified path.
  * check holds the attributes and their values.
  */
-void git_all_attrs(const struct index_state *istate,
+void git_all_attrs(struct index_state *istate,
 		   const char *path, struct attr_check *check);
 
 enum git_attr_direction {
diff --git a/builtin/add.c b/builtin/add.c
index ea762a41e3a..afccf2fd554 100644
--- a/builtin/add.c
+++ b/builtin/add.c
@@ -141,6 +141,8 @@ static int renormalize_tracked_files(const struct pathspec *pathspec, int flags)
 {
 	int i, retval = 0;
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&the_index);
 	for (i = 0; i < active_nr; i++) {
 		struct cache_entry *ce = active_cache[i];
 
diff --git a/builtin/checkout-index.c b/builtin/checkout-index.c
index c0bf4ac1b2d..c9a3c71914b 100644
--- a/builtin/checkout-index.c
+++ b/builtin/checkout-index.c
@@ -120,6 +120,8 @@ static void checkout_all(const char *prefix, int prefix_length)
 	int i, errs = 0;
 	struct cache_entry *last_ce = NULL;
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&the_index);
 	for (i = 0; i < active_nr ; i++) {
 		struct cache_entry *ce = active_cache[i];
 		if (ce_stage(ce) != checkout_stage
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 4c696ef4805..5bd9128d1a3 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -369,6 +369,9 @@ static int checkout_worktree(const struct checkout_opts *opts,
 			       NULL);
 
 	enable_delayed_checkout(&state);
+
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&the_index);
 	for (pos = 0; pos < active_nr; pos++) {
 		struct cache_entry *ce = active_cache[pos];
 		if (ce->ce_flags & CE_MATCHED) {
@@ -513,6 +516,8 @@ static int checkout_paths(const struct checkout_opts *opts,
 	 * Make sure all pathspecs participated in locating the paths
 	 * to be checked out.
 	 */
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&the_index);
 	for (pos = 0; pos < active_nr; pos++)
 		if (opts->overlay_mode)
 			mark_ce_for_checkout_overlay(active_cache[pos],
diff --git a/builtin/commit.c b/builtin/commit.c
index 55d50a8891b..190d215d43b 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -261,6 +261,8 @@ static int list_paths(struct string_list *list, const char *with_tree,
 		free(max_prefix);
 	}
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&the_index);
 	for (i = 0; i < active_nr; i++) {
 		const struct cache_entry *ce = active_cache[i];
 		struct string_list_item *item;
@@ -976,6 +978,8 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 		if (get_oid(parent, &oid)) {
 			int i, ita_nr = 0;
 
+			/* TODO: audit for interaction with sparse-index. */
+			ensure_full_index(&the_index);
 			for (i = 0; i < active_nr; i++)
 				if (ce_intent_to_add(active_cache[i]))
 					ita_nr++;
diff --git a/builtin/difftool.c b/builtin/difftool.c
index ef25729d496..0202a430520 100644
--- a/builtin/difftool.c
+++ b/builtin/difftool.c
@@ -585,6 +585,9 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 		setenv("GIT_DIFFTOOL_DIRDIFF", "true", 1);
 	rc = run_command_v_opt(helper_argv, flags);
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&wtindex);
+
 	/*
 	 * If the diff includes working copy files and those
 	 * files were modified during the diff, then the changes
diff --git a/builtin/fsck.c b/builtin/fsck.c
index e6a80e54042..87a99b0108e 100644
--- a/builtin/fsck.c
+++ b/builtin/fsck.c
@@ -881,6 +881,8 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 		verify_index_checksum = 1;
 		verify_ce_order = 1;
 		read_cache();
+		/* TODO: audit for interaction with sparse-index. */
+		ensure_full_index(&the_index);
 		for (i = 0; i < active_nr; i++) {
 			unsigned int mode;
 			struct blob *blob;
diff --git a/builtin/grep.c b/builtin/grep.c
index 5de725f9044..b71b4a2de65 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -504,6 +504,8 @@ static int grep_cache(struct grep_opt *opt,
 	if (repo_read_index(repo) < 0)
 		die(_("index file corrupt"));
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(repo->index);
 	for (nr = 0; nr < repo->index->cache_nr; nr++) {
 		const struct cache_entry *ce = repo->index->cache[nr];
 
diff --git a/builtin/ls-files.c b/builtin/ls-files.c
index 60a2913a01e..a0b4e54d114 100644
--- a/builtin/ls-files.c
+++ b/builtin/ls-files.c
@@ -57,7 +57,7 @@ static const char *tag_modified = "";
 static const char *tag_skip_worktree = "";
 static const char *tag_resolve_undo = "";
 
-static void write_eolinfo(const struct index_state *istate,
+static void write_eolinfo(struct index_state *istate,
 			  const struct cache_entry *ce, const char *path)
 {
 	if (show_eol) {
@@ -122,7 +122,7 @@ static void print_debug(const struct cache_entry *ce)
 	}
 }
 
-static void show_dir_entry(const struct index_state *istate,
+static void show_dir_entry(struct index_state *istate,
 			   const char *tag, struct dir_entry *ent)
 {
 	int len = max_prefix_len;
@@ -139,7 +139,7 @@ static void show_dir_entry(const struct index_state *istate,
 	write_name(ent->name);
 }
 
-static void show_other_files(const struct index_state *istate,
+static void show_other_files(struct index_state *istate,
 			     const struct dir_struct *dir)
 {
 	int i;
@@ -152,7 +152,7 @@ static void show_other_files(const struct index_state *istate,
 	}
 }
 
-static void show_killed_files(const struct index_state *istate,
+static void show_killed_files(struct index_state *istate,
 			      const struct dir_struct *dir)
 {
 	int i;
@@ -254,7 +254,7 @@ static void show_ce(struct repository *repo, struct dir_struct *dir,
 	}
 }
 
-static void show_ru_info(const struct index_state *istate)
+static void show_ru_info(struct index_state *istate)
 {
 	struct string_list_item *item;
 
@@ -317,6 +317,8 @@ static void show_files(struct repository *repo, struct dir_struct *dir)
 
 	if (!(show_cached || show_stage || show_deleted || show_modified))
 		return;
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(repo->index);
 	for (i = 0; i < repo->index->cache_nr; i++) {
 		const struct cache_entry *ce = repo->index->cache[i];
 		struct stat st;
@@ -494,6 +496,8 @@ void overlay_tree_on_index(struct index_state *istate,
 		die("bad tree-ish %s", tree_name);
 
 	/* Hoist the unmerged entries up to stage #3 to make room */
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(istate);
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct cache_entry *ce = istate->cache[i];
 		if (!ce_stage(ce))
diff --git a/builtin/merge-index.c b/builtin/merge-index.c
index 38ea6ad6ca2..c0383fe9df9 100644
--- a/builtin/merge-index.c
+++ b/builtin/merge-index.c
@@ -58,6 +58,8 @@ static void merge_one_path(const char *path)
 static void merge_all(void)
 {
 	int i;
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&the_index);
 	for (i = 0; i < active_nr; i++) {
 		const struct cache_entry *ce = active_cache[i];
 		if (!ce_stage(ce))
@@ -80,6 +82,9 @@ int cmd_merge_index(int argc, const char **argv, const char *prefix)
 
 	read_cache();
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&the_index);
+
 	i = 1;
 	if (!strcmp(argv[i], "-o")) {
 		one_shot = 1;
diff --git a/builtin/rm.c b/builtin/rm.c
index 4858631e0f0..5559a0b453a 100644
--- a/builtin/rm.c
+++ b/builtin/rm.c
@@ -293,6 +293,8 @@ int cmd_rm(int argc, const char **argv, const char *prefix)
 
 	seen = xcalloc(pathspec.nr, 1);
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&the_index);
 	for (i = 0; i < active_nr; i++) {
 		const struct cache_entry *ce = active_cache[i];
 		if (!ce_path_match(&the_index, ce, &pathspec, seen))
diff --git a/builtin/stash.c b/builtin/stash.c
index c56fed33546..d68ed784d2a 100644
--- a/builtin/stash.c
+++ b/builtin/stash.c
@@ -1412,6 +1412,8 @@ static int do_push_stash(const struct pathspec *ps, const char *stash_msg, int q
 		int i;
 		char *ps_matched = xcalloc(ps->nr, 1);
 
+		/* TODO: audit for interaction with sparse-index. */
+		ensure_full_index(&the_index);
 		for (i = 0; i < active_nr; i++)
 			ce_path_match(&the_index, active_cache[i], ps,
 				      ps_matched);
diff --git a/builtin/update-index.c b/builtin/update-index.c
index 79087bccea4..f1f16f2de52 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -745,6 +745,8 @@ static int do_reupdate(int ac, const char **av,
 		 */
 		has_head = 0;
  redo:
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(&the_index);
 	for (pos = 0; pos < active_nr; pos++) {
 		const struct cache_entry *ce = active_cache[pos];
 		struct cache_entry *old = NULL;
diff --git a/cache.h b/cache.h
index d81582e6ecf..b785ffb3831 100644
--- a/cache.h
+++ b/cache.h
@@ -350,6 +350,7 @@ void add_name_hash(struct index_state *istate, struct cache_entry *ce);
 void remove_name_hash(struct index_state *istate, struct cache_entry *ce);
 void free_name_hash(struct index_state *istate);
 
+void ensure_full_index(struct index_state *istate);
 
 /* Cache entry creation and cleanup */
 
@@ -800,7 +801,7 @@ struct cache_entry *index_file_exists(struct index_state *istate, const char *na
  * index_name_pos(&index, "f", 1) -> -3
  * index_name_pos(&index, "g", 1) -> -5
  */
-int index_name_pos(const struct index_state *, const char *name, int namelen);
+int index_name_pos(struct index_state *, const char *name, int namelen);
 
 /*
  * Some functions return the negative complement of an insert position when a
@@ -850,8 +851,8 @@ int add_file_to_index(struct index_state *, const char *path, int flags);
 int chmod_index_entry(struct index_state *, struct cache_entry *ce, char flip);
 int ce_same_name(const struct cache_entry *a, const struct cache_entry *b);
 void set_object_name_for_intent_to_add_entry(struct cache_entry *ce);
-int index_name_is_other(const struct index_state *, const char *, int);
-void *read_blob_data_from_index(const struct index_state *, const char *, unsigned long *);
+int index_name_is_other(struct index_state *, const char *, int);
+void *read_blob_data_from_index(struct index_state *, const char *, unsigned long *);
 
 /* do stat comparison even if CE_VALID is true */
 #define CE_MATCH_IGNORE_VALID		01
diff --git a/convert.c b/convert.c
index 45ac75f80c8..fd9c84b0257 100644
--- a/convert.c
+++ b/convert.c
@@ -127,7 +127,7 @@ static const char *gather_convert_stats_ascii(const char *data, unsigned long si
 	}
 }
 
-const char *get_cached_convert_stats_ascii(const struct index_state *istate,
+const char *get_cached_convert_stats_ascii(struct index_state *istate,
 					   const char *path)
 {
 	const char *ret;
@@ -211,7 +211,7 @@ static void check_global_conv_flags_eol(const char *path,
 	}
 }
 
-static int has_crlf_in_index(const struct index_state *istate, const char *path)
+static int has_crlf_in_index(struct index_state *istate, const char *path)
 {
 	unsigned long sz;
 	void *data;
@@ -485,7 +485,7 @@ static int encode_to_worktree(const char *path, const char *src, size_t src_len,
 	return 1;
 }
 
-static int crlf_to_git(const struct index_state *istate,
+static int crlf_to_git(struct index_state *istate,
 		       const char *path, const char *src, size_t len,
 		       struct strbuf *buf,
 		       enum convert_crlf_action crlf_action, int conv_flags)
@@ -1293,7 +1293,7 @@ static int git_path_check_ident(struct attr_check_item *check)
 
 static struct attr_check *check;
 
-void convert_attrs(const struct index_state *istate,
+void convert_attrs(struct index_state *istate,
 		   struct conv_attrs *ca, const char *path)
 {
 	struct attr_check_item *ccheck = NULL;
@@ -1355,7 +1355,7 @@ void reset_parsed_attributes(void)
 	user_convert_tail = NULL;
 }
 
-int would_convert_to_git_filter_fd(const struct index_state *istate, const char *path)
+int would_convert_to_git_filter_fd(struct index_state *istate, const char *path)
 {
 	struct conv_attrs ca;
 
@@ -1374,7 +1374,7 @@ int would_convert_to_git_filter_fd(const struct index_state *istate, const char
 	return apply_filter(path, NULL, 0, -1, NULL, ca.drv, CAP_CLEAN, NULL, NULL);
 }
 
-const char *get_convert_attr_ascii(const struct index_state *istate, const char *path)
+const char *get_convert_attr_ascii(struct index_state *istate, const char *path)
 {
 	struct conv_attrs ca;
 
@@ -1400,7 +1400,7 @@ const char *get_convert_attr_ascii(const struct index_state *istate, const char
 	return "";
 }
 
-int convert_to_git(const struct index_state *istate,
+int convert_to_git(struct index_state *istate,
 		   const char *path, const char *src, size_t len,
 		   struct strbuf *dst, int conv_flags)
 {
@@ -1434,7 +1434,7 @@ int convert_to_git(const struct index_state *istate,
 	return ret | ident_to_git(src, len, dst, ca.ident);
 }
 
-void convert_to_git_filter_fd(const struct index_state *istate,
+void convert_to_git_filter_fd(struct index_state *istate,
 			      const char *path, int fd, struct strbuf *dst,
 			      int conv_flags)
 {
@@ -1511,7 +1511,7 @@ int convert_to_working_tree_ca(const struct conv_attrs *ca,
 						   meta, NULL);
 }
 
-int renormalize_buffer(const struct index_state *istate, const char *path,
+int renormalize_buffer(struct index_state *istate, const char *path,
 		       const char *src, size_t len, struct strbuf *dst)
 {
 	struct conv_attrs ca;
@@ -1972,7 +1972,7 @@ struct stream_filter *get_stream_filter_ca(const struct conv_attrs *ca,
 	return filter;
 }
 
-struct stream_filter *get_stream_filter(const struct index_state *istate,
+struct stream_filter *get_stream_filter(struct index_state *istate,
 					const char *path,
 					const struct object_id *oid)
 {
diff --git a/convert.h b/convert.h
index 43e567a59b0..5ee1c322058 100644
--- a/convert.h
+++ b/convert.h
@@ -84,19 +84,19 @@ struct conv_attrs {
 	const char *working_tree_encoding; /* Supported encoding or default encoding if NULL */
 };
 
-void convert_attrs(const struct index_state *istate,
+void convert_attrs(struct index_state *istate,
 		   struct conv_attrs *ca, const char *path);
 
 extern enum eol core_eol;
 extern char *check_roundtrip_encoding;
-const char *get_cached_convert_stats_ascii(const struct index_state *istate,
+const char *get_cached_convert_stats_ascii(struct index_state *istate,
 					   const char *path);
 const char *get_wt_convert_stats_ascii(const char *path);
-const char *get_convert_attr_ascii(const struct index_state *istate,
+const char *get_convert_attr_ascii(struct index_state *istate,
 				   const char *path);
 
 /* returns 1 if *dst was used */
-int convert_to_git(const struct index_state *istate,
+int convert_to_git(struct index_state *istate,
 		   const char *path, const char *src, size_t len,
 		   struct strbuf *dst, int conv_flags);
 int convert_to_working_tree_ca(const struct conv_attrs *ca,
@@ -108,7 +108,7 @@ int async_convert_to_working_tree_ca(const struct conv_attrs *ca,
 				     size_t len, struct strbuf *dst,
 				     const struct checkout_metadata *meta,
 				     void *dco);
-static inline int convert_to_working_tree(const struct index_state *istate,
+static inline int convert_to_working_tree(struct index_state *istate,
 					  const char *path, const char *src,
 					  size_t len, struct strbuf *dst,
 					  const struct checkout_metadata *meta)
@@ -117,7 +117,7 @@ static inline int convert_to_working_tree(const struct index_state *istate,
 	convert_attrs(istate, &ca, path);
 	return convert_to_working_tree_ca(&ca, path, src, len, dst, meta);
 }
-static inline int async_convert_to_working_tree(const struct index_state *istate,
+static inline int async_convert_to_working_tree(struct index_state *istate,
 						const char *path, const char *src,
 						size_t len, struct strbuf *dst,
 						const struct checkout_metadata *meta,
@@ -129,20 +129,20 @@ static inline int async_convert_to_working_tree(const struct index_state *istate
 }
 int async_query_available_blobs(const char *cmd,
 				struct string_list *available_paths);
-int renormalize_buffer(const struct index_state *istate,
+int renormalize_buffer(struct index_state *istate,
 		       const char *path, const char *src, size_t len,
 		       struct strbuf *dst);
-static inline int would_convert_to_git(const struct index_state *istate,
+static inline int would_convert_to_git(struct index_state *istate,
 				       const char *path)
 {
 	return convert_to_git(istate, path, NULL, 0, NULL, 0);
 }
 /* Precondition: would_convert_to_git_filter_fd(path) == true */
-void convert_to_git_filter_fd(const struct index_state *istate,
+void convert_to_git_filter_fd(struct index_state *istate,
 			      const char *path, int fd,
 			      struct strbuf *dst,
 			      int conv_flags);
-int would_convert_to_git_filter_fd(const struct index_state *istate,
+int would_convert_to_git_filter_fd(struct index_state *istate,
 				   const char *path);
 
 /*
@@ -176,7 +176,7 @@ void reset_parsed_attributes(void);
 
 struct stream_filter; /* opaque */
 
-struct stream_filter *get_stream_filter(const struct index_state *istate,
+struct stream_filter *get_stream_filter(struct index_state *istate,
 					const char *path,
 					const struct object_id *);
 struct stream_filter *get_stream_filter_ca(const struct conv_attrs *ca,
diff --git a/dir.c b/dir.c
index 3474e67e8f3..d81b5df8030 100644
--- a/dir.c
+++ b/dir.c
@@ -306,7 +306,7 @@ static int do_read_blob(const struct object_id *oid, struct oid_stat *oid_stat,
  * [1] Only if DO_MATCH_DIRECTORY is passed; otherwise, this is NOT a match.
  * [2] Only if DO_MATCH_LEADING_PATHSPEC is passed; otherwise, not a match.
  */
-static int match_pathspec_item(const struct index_state *istate,
+static int match_pathspec_item(struct index_state *istate,
 			       const struct pathspec_item *item, int prefix,
 			       const char *name, int namelen, unsigned flags)
 {
@@ -429,7 +429,7 @@ static int match_pathspec_item(const struct index_state *istate,
  * pathspec did not match any names, which could indicate that the
  * user mistyped the nth pathspec.
  */
-static int do_match_pathspec(const struct index_state *istate,
+static int do_match_pathspec(struct index_state *istate,
 			     const struct pathspec *ps,
 			     const char *name, int namelen,
 			     int prefix, char *seen,
@@ -500,7 +500,7 @@ static int do_match_pathspec(const struct index_state *istate,
 	return retval;
 }
 
-static int match_pathspec_with_flags(const struct index_state *istate,
+static int match_pathspec_with_flags(struct index_state *istate,
 				     const struct pathspec *ps,
 				     const char *name, int namelen,
 				     int prefix, char *seen, unsigned flags)
@@ -516,7 +516,7 @@ static int match_pathspec_with_flags(const struct index_state *istate,
 	return negative ? 0 : positive;
 }
 
-int match_pathspec(const struct index_state *istate,
+int match_pathspec(struct index_state *istate,
 		   const struct pathspec *ps,
 		   const char *name, int namelen,
 		   int prefix, char *seen, int is_dir)
@@ -529,7 +529,7 @@ int match_pathspec(const struct index_state *istate,
 /**
  * Check if a submodule is a superset of the pathspec
  */
-int submodule_path_match(const struct index_state *istate,
+int submodule_path_match(struct index_state *istate,
 			 const struct pathspec *ps,
 			 const char *submodule_name,
 			 char *seen)
@@ -892,7 +892,7 @@ void add_pattern(const char *string, const char *base,
 	add_pattern_to_hashsets(pl, pattern);
 }
 
-static int read_skip_worktree_file_from_index(const struct index_state *istate,
+static int read_skip_worktree_file_from_index(struct index_state *istate,
 					      const char *path,
 					      size_t *size_out, char **data_out,
 					      struct oid_stat *oid_stat)
@@ -3542,6 +3542,8 @@ static void connect_wt_gitdir_in_nested(const char *sub_worktree,
 	if (repo_read_index(&subrepo) < 0)
 		die(_("index file corrupt in repo %s"), subrepo.gitdir);
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(subrepo.index);
 	for (i = 0; i < subrepo.index->cache_nr; i++) {
 		const struct cache_entry *ce = subrepo.index->cache[i];
 
diff --git a/dir.h b/dir.h
index 04d886cfce7..35963f9e2ce 100644
--- a/dir.h
+++ b/dir.h
@@ -354,7 +354,7 @@ int count_slashes(const char *s);
 int simple_length(const char *match);
 int no_wildcard(const char *string);
 char *common_prefix(const struct pathspec *pathspec);
-int match_pathspec(const struct index_state *istate,
+int match_pathspec(struct index_state *istate,
 		   const struct pathspec *pathspec,
 		   const char *name, int namelen,
 		   int prefix, char *seen, int is_dir);
@@ -493,12 +493,12 @@ int git_fnmatch(const struct pathspec_item *item,
 		const char *pattern, const char *string,
 		int prefix);
 
-int submodule_path_match(const struct index_state *istate,
+int submodule_path_match(struct index_state *istate,
 			 const struct pathspec *ps,
 			 const char *submodule_name,
 			 char *seen);
 
-static inline int ce_path_match(const struct index_state *istate,
+static inline int ce_path_match(struct index_state *istate,
 				const struct cache_entry *ce,
 				const struct pathspec *pathspec,
 				char *seen)
@@ -507,7 +507,7 @@ static inline int ce_path_match(const struct index_state *istate,
 			      S_ISDIR(ce->ce_mode) || S_ISGITLINK(ce->ce_mode));
 }
 
-static inline int dir_path_match(const struct index_state *istate,
+static inline int dir_path_match(struct index_state *istate,
 				 const struct dir_entry *ent,
 				 const struct pathspec *pathspec,
 				 int prefix, char *seen)
diff --git a/entry.c b/entry.c
index 2dc94ba5cc2..9fdc843135f 100644
--- a/entry.c
+++ b/entry.c
@@ -423,6 +423,8 @@ static void mark_colliding_entries(const struct checkout *state,
 
 	ce->ce_flags |= CE_MATCHED;
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(state->istate);
 	for (i = 0; i < state->istate->cache_nr; i++) {
 		struct cache_entry *dup = state->istate->cache[i];
 
diff --git a/merge-ort.c b/merge-ort.c
index b1795d838ea..6c2792b10e1 100644
--- a/merge-ort.c
+++ b/merge-ort.c
@@ -2564,7 +2564,7 @@ static int blob_unchanged(struct merge_options *opt,
 	struct strbuf basebuf = STRBUF_INIT;
 	struct strbuf sidebuf = STRBUF_INIT;
 	int ret = 0; /* assume changed for safety */
-	const struct index_state *idx = &opt->priv->attr_index;
+	struct index_state *idx = &opt->priv->attr_index;
 
 	if (!idx->initialized)
 		initialize_attr_index(opt);
diff --git a/merge-recursive.c b/merge-recursive.c
index 7618303f7b4..27b222ae490 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -522,6 +522,8 @@ static struct string_list *get_unmerged(struct index_state *istate)
 
 	unmerged->strdup_strings = 1;
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(istate);
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct string_list_item *item;
 		struct stage_data *e;
@@ -3014,7 +3016,7 @@ static int blob_unchanged(struct merge_options *opt,
 	struct strbuf obuf = STRBUF_INIT;
 	struct strbuf abuf = STRBUF_INIT;
 	int ret = 0; /* assume changed for safety */
-	const struct index_state *idx = opt->repo->index;
+	struct index_state *idx = opt->repo->index;
 
 	if (a->mode != o->mode)
 		return 0;
diff --git a/name-hash.c b/name-hash.c
index ce28f3f0708..7487d331240 100644
--- a/name-hash.c
+++ b/name-hash.c
@@ -8,6 +8,7 @@
 #include "cache.h"
 #include "thread-utils.h"
 #include "trace2.h"
+#include "sparse-index.h"
 
 struct dir_entry {
 	struct hashmap_entry ent;
@@ -109,8 +110,11 @@ static void hash_index_entry(struct index_state *istate, struct cache_entry *ce)
 	if (ce->ce_flags & CE_HASHED)
 		return;
 	ce->ce_flags |= CE_HASHED;
-	hashmap_entry_init(&ce->ent, memihash(ce->name, ce_namelen(ce)));
-	hashmap_add(&istate->name_hash, &ce->ent);
+
+	if (!S_ISSPARSEDIR(ce->ce_mode)) {
+		hashmap_entry_init(&ce->ent, memihash(ce->name, ce_namelen(ce)));
+		hashmap_add(&istate->name_hash, &ce->ent);
+	}
 
 	if (ignore_case)
 		add_dir_entry(istate, ce);
@@ -680,6 +684,7 @@ int index_dir_exists(struct index_state *istate, const char *name, int namelen)
 	struct dir_entry *dir;
 
 	lazy_init_name_hash(istate);
+	expand_to_path(istate, name, namelen, 0);
 	dir = find_dir_entry(istate, name, namelen);
 	return dir && dir->nr;
 }
@@ -690,6 +695,7 @@ void adjust_dirname_case(struct index_state *istate, char *name)
 	const char *ptr = startPtr;
 
 	lazy_init_name_hash(istate);
+	expand_to_path(istate, name, strlen(name), 0);
 	while (*ptr) {
 		while (*ptr && *ptr != '/')
 			ptr++;
@@ -713,6 +719,7 @@ struct cache_entry *index_file_exists(struct index_state *istate, const char *na
 	unsigned int hash = memihash(name, namelen);
 
 	lazy_init_name_hash(istate);
+	expand_to_path(istate, name, namelen, icase);
 
 	ce = hashmap_get_entry_from_hash(&istate->name_hash, hash, NULL,
 					 struct cache_entry, ent);
diff --git a/pathspec.c b/pathspec.c
index 18b3be362ae..14c7e9fbe33 100644
--- a/pathspec.c
+++ b/pathspec.c
@@ -20,7 +20,7 @@
  * to use find_pathspecs_matching_against_index() instead.
  */
 void add_pathspec_matches_against_index(const struct pathspec *pathspec,
-					const struct index_state *istate,
+					struct index_state *istate,
 					char *seen)
 {
 	int num_unmatched = 0, i;
@@ -36,6 +36,8 @@ void add_pathspec_matches_against_index(const struct pathspec *pathspec,
 			num_unmatched++;
 	if (!num_unmatched)
 		return;
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(istate);
 	for (i = 0; i < istate->cache_nr; i++) {
 		const struct cache_entry *ce = istate->cache[i];
 		ce_path_match(istate, ce, pathspec, seen);
@@ -51,7 +53,7 @@ void add_pathspec_matches_against_index(const struct pathspec *pathspec,
  * given pathspecs achieves against all items in the index.
  */
 char *find_pathspecs_matching_against_index(const struct pathspec *pathspec,
-					    const struct index_state *istate)
+					    struct index_state *istate)
 {
 	char *seen = xcalloc(pathspec->nr, 1);
 	add_pathspec_matches_against_index(pathspec, istate, seen);
@@ -702,7 +704,7 @@ void clear_pathspec(struct pathspec *pathspec)
 	pathspec->nr = 0;
 }
 
-int match_pathspec_attrs(const struct index_state *istate,
+int match_pathspec_attrs(struct index_state *istate,
 			 const char *name, int namelen,
 			 const struct pathspec_item *item)
 {
diff --git a/pathspec.h b/pathspec.h
index 454ce364fac..2ccc8080b6c 100644
--- a/pathspec.h
+++ b/pathspec.h
@@ -150,11 +150,11 @@ static inline int ps_strcmp(const struct pathspec_item *item,
 }
 
 void add_pathspec_matches_against_index(const struct pathspec *pathspec,
-					const struct index_state *istate,
+					struct index_state *istate,
 					char *seen);
 char *find_pathspecs_matching_against_index(const struct pathspec *pathspec,
-					    const struct index_state *istate);
-int match_pathspec_attrs(const struct index_state *istate,
+					    struct index_state *istate);
+int match_pathspec_attrs(struct index_state *istate,
 			 const char *name, int namelen,
 			 const struct pathspec_item *item);
 
diff --git a/read-cache.c b/read-cache.c
index d68e7479af4..72a1d339c97 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -549,7 +549,7 @@ int cache_name_stage_compare(const char *name1, int len1, int stage1, const char
 	return 0;
 }
 
-static int index_name_stage_pos(const struct index_state *istate, const char *name, int namelen, int stage)
+static int index_name_stage_pos(struct index_state *istate, const char *name, int namelen, int stage)
 {
 	int first, last;
 
@@ -567,10 +567,31 @@ static int index_name_stage_pos(const struct index_state *istate, const char *na
 		}
 		first = next+1;
 	}
+
+	if (istate->sparse_index &&
+	    first > 0) {
+		/* Note: first <= istate->cache_nr */
+		struct cache_entry *ce = istate->cache[first - 1];
+
+		/*
+		 * If we are in a sparse-index _and_ the entry before the
+		 * insertion position is a sparse-directory entry that is
+		 * an ancestor of 'name', then we need to expand the index
+		 * and search again. This will only trigger once, because
+		 * thereafter the index is fully expanded.
+		 */
+		if (S_ISSPARSEDIR(ce->ce_mode) &&
+		    ce_namelen(ce) < namelen &&
+		    !strncmp(name, ce->name, ce_namelen(ce))) {
+			ensure_full_index(istate);
+			return index_name_stage_pos(istate, name, namelen, stage);
+		}
+	}
+
 	return -first-1;
 }
 
-int index_name_pos(const struct index_state *istate, const char *name, int namelen)
+int index_name_pos(struct index_state *istate, const char *name, int namelen)
 {
 	return index_name_stage_pos(istate, name, namelen, 0);
 }
@@ -1556,6 +1577,8 @@ int refresh_index(struct index_state *istate, unsigned int flags,
 	 */
 	preload_index(istate, pathspec, 0);
 	trace2_region_enter("index", "refresh", NULL);
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(istate);
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct cache_entry *ce, *new_entry;
 		int cache_errno = 0;
@@ -2478,6 +2501,8 @@ int repo_index_has_changes(struct repository *repo,
 		diff_flush(&opt);
 		return opt.flags.has_changes != 0;
 	} else {
+		/* TODO: audit for interaction with sparse-index. */
+		ensure_full_index(istate);
 		for (i = 0; sb && i < istate->cache_nr; i++) {
 			if (i)
 				strbuf_addch(sb, ' ');
@@ -3390,8 +3415,8 @@ int repo_read_index_unmerged(struct repository *repo)
  * We helpfully remove a trailing "/" from directories so that
  * the output of read_directory can be used as-is.
  */
-int index_name_is_other(const struct index_state *istate, const char *name,
-		int namelen)
+int index_name_is_other(struct index_state *istate, const char *name,
+			int namelen)
 {
 	int pos;
 	if (namelen && name[namelen - 1] == '/')
@@ -3409,7 +3434,7 @@ int index_name_is_other(const struct index_state *istate, const char *name,
 	return 1;
 }
 
-void *read_blob_data_from_index(const struct index_state *istate,
+void *read_blob_data_from_index(struct index_state *istate,
 				const char *path, unsigned long *size)
 {
 	int pos, len;
diff --git a/resolve-undo.c b/resolve-undo.c
index bbd2e57fe41..e81096e2d45 100644
--- a/resolve-undo.c
+++ b/resolve-undo.c
@@ -172,6 +172,8 @@ void unmerge_marked_index(struct index_state *istate)
 	if (!istate->resolve_undo)
 		return;
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(istate);
 	for (i = 0; i < istate->cache_nr; i++) {
 		const struct cache_entry *ce = istate->cache[i];
 		if (ce->ce_flags & CE_MATCHED)
@@ -186,6 +188,8 @@ void unmerge_index(struct index_state *istate, const struct pathspec *pathspec)
 	if (!istate->resolve_undo)
 		return;
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(istate);
 	for (i = 0; i < istate->cache_nr; i++) {
 		const struct cache_entry *ce = istate->cache[i];
 		if (!ce_path_match(istate, ce, pathspec, NULL))
diff --git a/revision.c b/revision.c
index 7e73dafd96e..4853c85d0b7 100644
--- a/revision.c
+++ b/revision.c
@@ -1680,6 +1680,8 @@ static void do_add_index_objects_to_pending(struct rev_info *revs,
 {
 	int i;
 
+	/* TODO: audit for interaction with sparse-index. */
+	ensure_full_index(istate);
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct cache_entry *ce = istate->cache[i];
 		struct blob *blob;
diff --git a/sparse-index.c b/sparse-index.c
index 95ea17174da..6f21397e2ee 100644
--- a/sparse-index.c
+++ b/sparse-index.c
@@ -283,3 +283,76 @@ void ensure_full_index(struct index_state *istate)
 
 	trace2_region_leave("index", "ensure_full_index", istate->repo);
 }
+
+/*
+ * This static global helps avoid infinite recursion between
+ * expand_to_path() and index_file_exists().
+ */
+static int in_expand_to_path = 0;
+
+void expand_to_path(struct index_state *istate,
+		    const char *path, size_t pathlen, int icase)
+{
+	struct strbuf path_mutable = STRBUF_INIT;
+	size_t substr_len;
+
+	/* prevent extra recursion */
+	if (in_expand_to_path)
+		return;
+
+	if (!istate || !istate->sparse_index)
+		return;
+
+	if (!istate->repo)
+		istate->repo = the_repository;
+
+	in_expand_to_path = 1;
+
+	/*
+	 * We only need to actually expand a region if the
+	 * following are both true:
+	 *
+	 * 1. 'path' is not already in the index.
+	 * 2. Some parent directory of 'path' is a sparse directory.
+	 */
+
+	if (index_file_exists(istate, path, pathlen, icase))
+		goto cleanup;
+
+	strbuf_add(&path_mutable, path, pathlen);
+	strbuf_addch(&path_mutable, '/');
+
+	/* Check the name hash for all parent directories */
+	substr_len = 0;
+	while (substr_len < pathlen) {
+		char temp;
+		char *replace = strchr(path_mutable.buf + substr_len, '/');
+
+		if (!replace)
+			break;
+
+		/* replace the character _after_ the slash */
+		replace++;
+		temp = *replace;
+		*replace = '\0';
+		if (index_file_exists(istate, path_mutable.buf,
+				      path_mutable.len, icase)) {
+			/*
+			 * We found a parent directory in the name-hash
+			 * hashtable, because only sparse directory entries
+			 * have a trailing '/' character.  Since "path" wasn't
+			 * in the index, perhaps it exists within this
+			 * sparse-directory.  Expand accordingly.
+			 */
+			ensure_full_index(istate);
+			break;
+		}
+
+		*replace = temp;
+		substr_len = replace - path_mutable.buf;
+	}
+
+cleanup:
+	strbuf_release(&path_mutable);
+	in_expand_to_path = 0;
+}
diff --git a/sparse-index.h b/sparse-index.h
index 39dcc859735..1115a0d7dd9 100644
--- a/sparse-index.h
+++ b/sparse-index.h
@@ -2,9 +2,21 @@
 #define SPARSE_INDEX_H__
 
 struct index_state;
-void ensure_full_index(struct index_state *istate);
 int convert_to_sparse(struct index_state *istate);
 
+/*
+ * Some places in the codebase expect to search for a specific path.
+ * This path might be outside of the sparse-checkout definition, in
+ * which case a sparse-index may not contain a path for that index.
+ *
+ * Given an index and a path, check to see if a leading directory for
+ * 'path' exists in the index as a sparse directory. In that case,
+ * expand that sparse directory to a full range of cache entries and
+ * populate the index accordingly.
+ */
+void expand_to_path(struct index_state *istate,
+		    const char *path, size_t pathlen, int icase);
+
 struct repository;
 int set_sparse_index_config(struct repository *repo, int enable);
 
diff --git a/submodule.c b/submodule.c
index 9767ba9893c..83809a4f7bd 100644
--- a/submodule.c
+++ b/submodule.c
@@ -33,7 +33,7 @@ static struct oid_array ref_tips_after_fetch;
  * will be disabled because we can't guess what might be configured in
  * .gitmodules unless the user resolves the conflict.
  */
-int is_gitmodules_unmerged(const struct index_state *istate)
+int is_gitmodules_unmerged(struct index_state *istate)
 {
 	int pos = index_name_pos(istate, GITMODULES_FILE, strlen(GITMODULES_FILE));
 	if (pos < 0) { /* .gitmodules not found or isn't merged */
@@ -301,7 +301,7 @@ int is_submodule_populated_gently(const char *path, int *return_error_code)
 /*
  * Dies if the provided 'prefix' corresponds to an unpopulated submodule
  */
-void die_in_unpopulated_submodule(const struct index_state *istate,
+void die_in_unpopulated_submodule(struct index_state *istate,
 				  const char *prefix)
 {
 	int i, prefixlen;
@@ -331,7 +331,7 @@ void die_in_unpopulated_submodule(const struct index_state *istate,
 /*
  * Dies if any paths in the provided pathspec descends into a submodule
  */
-void die_path_inside_submodule(const struct index_state *istate,
+void die_path_inside_submodule(struct index_state *istate,
 			       const struct pathspec *ps)
 {
 	int i, j;
diff --git a/submodule.h b/submodule.h
index 4ac6e31cf1f..84640c49c11 100644
--- a/submodule.h
+++ b/submodule.h
@@ -39,7 +39,7 @@ struct submodule_update_strategy {
 };
 #define SUBMODULE_UPDATE_STRATEGY_INIT {SM_UPDATE_UNSPECIFIED, NULL}
 
-int is_gitmodules_unmerged(const struct index_state *istate);
+int is_gitmodules_unmerged(struct index_state *istate);
 int is_writing_gitmodules_ok(void);
 int is_staging_gitmodules_ok(struct index_state *istate);
 int update_path_in_gitmodules(const char *oldpath, const char *newpath);
@@ -60,9 +60,9 @@ int is_submodule_active(struct repository *repo, const char *path);
  * Otherwise the return error code is the same as of resolve_gitdir_gently.
  */
 int is_submodule_populated_gently(const char *path, int *return_error_code);
-void die_in_unpopulated_submodule(const struct index_state *istate,
+void die_in_unpopulated_submodule(struct index_state *istate,
 				  const char *prefix);
-void die_path_inside_submodule(const struct index_state *istate,
+void die_path_inside_submodule(struct index_state *istate,
 			       const struct pathspec *ps);
 enum submodule_update_type parse_submodule_update_type(const char *value);
 int parse_submodule_update_strategy(const char *value,