diff --git a/mercurial/revlogutils/deltas.py b/mercurial/revlogutils/deltas.py
--- a/mercurial/revlogutils/deltas.py
+++ b/mercurial/revlogutils/deltas.py
@@ -1147,7 +1147,13 @@ class deltacomputer:
                 if deltainfo is not None:
                     prev = deltainfo.base
 
-                if p1 in candidaterevs or p2 in candidaterevs:
+                if (
+                    cachedelta is not None
+                    and len(candidaterevs) == 1
+                    and cachedelta[0] in candidaterevs
+                ):
+                    round_type = b"cached-delta"
+                elif p1 in candidaterevs or p2 in candidaterevs:
                     round_type = b"parents"
                 elif prev is not None and all(c < prev for c in candidaterevs):
                     round_type = b"refine-down"
@@ -1246,10 +1252,19 @@ class deltacomputer:
 
         if self._write_debug is not None:
             end = util.timer()
+            assert deltainfo is not None  # please pytype
+            used_cached = (
+                cachedelta is not None
+                and dbg_try_rounds == 1
+                and dbg_try_count == 1
+                and deltainfo.base == cachedelta[0]
+            )
             dbg = {
                 'duration': end - start,
                 'revision': target_rev,
+                'delta-base': deltainfo.base,
                 'search_round_count': dbg_try_rounds,
+                'using-cached-base': used_cached,
                 'delta_try_count': dbg_try_count,
                 'type': dbg_type,
                 'p1-chain-len': p1_chain_len,
@@ -1283,7 +1298,9 @@ class deltacomputer:
                 b"DBG-DELTAS:"
                 b" %-12s"
                 b" rev=%d:"
-                b" search-rounds=%d"
+                b" delta-base=%d"
+                b" is-cached=%d"
+                b" - search-rounds=%d"
                 b" try-count=%d"
                 b" - delta-type=%-6s"
                 b" snap-depth=%d"
@@ -1295,6 +1312,8 @@ class deltacomputer:
             msg %= (
                 dbg["target-revlog"],
                 dbg["revision"],
+                dbg["delta-base"],
+                dbg["using-cached-base"],
                 dbg["search_round_count"],
                 dbg["delta_try_count"],
                 dbg["type"],
diff --git a/tests/test-bundle.t b/tests/test-bundle.t
--- a/tests/test-bundle.t
+++ b/tests/test-bundle.t
@@ -1048,17 +1048,17 @@ Test the debug output when applying delt
   > --config storage.revlog.reuse-external-delta=no \
   > --config storage.revlog.reuse-external-delta-parent=no
   adding changesets
-  DBG-DELTAS: CHANGELOG:   rev=0: search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
-  DBG-DELTAS: CHANGELOG:   rev=1: search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=0 p2-chain-length=-1 - duration=* (glob)
-  DBG-DELTAS: CHANGELOG:   rev=2: search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=0 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: CHANGELOG:   rev=0: delta-base=0 is-cached=0 - search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: CHANGELOG:   rev=1: delta-base=1 is-cached=0 - search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=0 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: CHANGELOG:   rev=2: delta-base=2 is-cached=0 - search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=0 p2-chain-length=-1 - duration=* (glob)
   adding manifests
-  DBG-DELTAS: MANIFESTLOG: rev=0: search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
-  DBG-DELTAS: MANIFESTLOG: rev=1: search-rounds=1 try-count=1 - delta-type=delta  snap-depth=0 - p1-chain-length=0 p2-chain-length=-1 - duration=* (glob)
-  DBG-DELTAS: MANIFESTLOG: rev=2: search-rounds=1 try-count=1 - delta-type=delta  snap-depth=0 - p1-chain-length=1 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: MANIFESTLOG: rev=0: delta-base=0 is-cached=0 - search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: MANIFESTLOG: rev=1: delta-base=0 is-cached=1 - search-rounds=1 try-count=1 - delta-type=delta  snap-depth=0 - p1-chain-length=0 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: MANIFESTLOG: rev=2: delta-base=1 is-cached=1 - search-rounds=1 try-count=1 - delta-type=delta  snap-depth=0 - p1-chain-length=1 p2-chain-length=-1 - duration=* (glob)
   adding file changes
-  DBG-DELTAS: FILELOG:a:   rev=0: search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
-  DBG-DELTAS: FILELOG:b:   rev=0: search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
-  DBG-DELTAS: FILELOG:c:   rev=0: search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:a:   rev=0: delta-base=0 is-cached=0 - search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:b:   rev=0: delta-base=0 is-cached=0 - search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:c:   rev=0: delta-base=0 is-cached=0 - search-rounds=0 try-count=0 - delta-type=full   snap-depth=0 - p1-chain-length=-1 p2-chain-length=-1 - duration=* (glob)
   added 3 changesets with 3 changes to 3 files
   new changesets 4fe08cd4693e:4652c276ac4f (3 drafts)
   (run 'hg update' to get a working copy)
diff --git a/tests/test-sparse-revlog.t b/tests/test-sparse-revlog.t
--- a/tests/test-sparse-revlog.t
+++ b/tests/test-sparse-revlog.t
@@ -189,7 +189,7 @@ Test `debug-delta-find`
   DBG-DELTAS-SEARCH:     uncompressed-delta-size=82661
   DBG-DELTAS-SEARCH:     delta-search-time=* (glob)
   DBG-DELTAS-SEARCH:     DELTA: length=49132 (BAD)
-  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: delta-base=4930 is-cached=0 - search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
 
   $ cat << EOF >>.hg/hgrc
   > [storage]
@@ -198,7 +198,7 @@ Test `debug-delta-find`
   > EOF
 
   $ hg debug-delta-find SPARSE-REVLOG-TEST-FILE 4971 --quiet
-  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: delta-base=4930 is-cached=0 - search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
   $ hg debug-delta-find SPARSE-REVLOG-TEST-FILE 4971 --source full
   DBG-DELTAS-SEARCH: SEARCH rev=4971
   DBG-DELTAS-SEARCH: ROUND #1 - 2 candidates - search-down
@@ -231,10 +231,10 @@ Test `debug-delta-find`
   DBG-DELTAS-SEARCH:     uncompressed-delta-size=82661
   DBG-DELTAS-SEARCH:     delta-search-time=* (glob)
   DBG-DELTAS-SEARCH:     DELTA: length=49132 (BAD)
-  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: delta-base=4930 is-cached=0 - search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
   $ hg debug-delta-find SPARSE-REVLOG-TEST-FILE 4971 --source storage
   DBG-DELTAS-SEARCH: SEARCH rev=4971
-  DBG-DELTAS-SEARCH: ROUND #1 - 1 candidates - search-down
+  DBG-DELTAS-SEARCH: ROUND #1 - 1 candidates - cached-delta
   DBG-DELTAS-SEARCH:   CANDIDATE: rev=4930
   DBG-DELTAS-SEARCH:     type=snapshot-3
   DBG-DELTAS-SEARCH:     size=39228
@@ -242,7 +242,7 @@ Test `debug-delta-find`
   DBG-DELTAS-SEARCH:     uncompressed-delta-size=33050
   DBG-DELTAS-SEARCH:     delta-search-time=* (glob)
   DBG-DELTAS-SEARCH:     DELTA: length=19179 (GOOD)
-  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: search-rounds=1 try-count=1 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: delta-base=4930 is-cached=1 - search-rounds=1 try-count=1 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
   $ hg debug-delta-find SPARSE-REVLOG-TEST-FILE 4971 --source p1
   DBG-DELTAS-SEARCH: SEARCH rev=4971
   DBG-DELTAS-SEARCH: ROUND #1 - 2 candidates - search-down
@@ -275,7 +275,7 @@ Test `debug-delta-find`
   DBG-DELTAS-SEARCH:     uncompressed-delta-size=82661
   DBG-DELTAS-SEARCH:     delta-search-time=* (glob)
   DBG-DELTAS-SEARCH:     DELTA: length=49132 (BAD)
-  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: delta-base=4930 is-cached=0 - search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
   $ hg debug-delta-find SPARSE-REVLOG-TEST-FILE 4971 --source p2
   DBG-DELTAS-SEARCH: SEARCH rev=4971
   DBG-DELTAS-SEARCH: ROUND #1 - 2 candidates - search-down
@@ -308,7 +308,7 @@ Test `debug-delta-find`
   DBG-DELTAS-SEARCH:     uncompressed-delta-size=82661
   DBG-DELTAS-SEARCH:     delta-search-time=* (glob)
   DBG-DELTAS-SEARCH:     DELTA: length=49132 (BAD)
-  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: delta-base=4930 is-cached=0 - search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
   $ hg debug-delta-find SPARSE-REVLOG-TEST-FILE 4971 --source prev
   DBG-DELTAS-SEARCH: SEARCH rev=4971
   DBG-DELTAS-SEARCH: ROUND #1 - 2 candidates - search-down
@@ -341,6 +341,6 @@ Test `debug-delta-find`
   DBG-DELTAS-SEARCH:     uncompressed-delta-size=82661
   DBG-DELTAS-SEARCH:     delta-search-time=* (glob)
   DBG-DELTAS-SEARCH:     DELTA: length=49132 (BAD)
-  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
+  DBG-DELTAS: FILELOG:SPARSE-REVLOG-TEST-FILE: rev=4971: delta-base=4930 is-cached=0 - search-rounds=3 try-count=3 - delta-type=snapshot snap-depth=4 - p1-chain-length=15 p2-chain-length=-1 - duration=* (glob)
 
   $ cd ..