diff --git a/ROADMAP.md b/ROADMAP.md
index d8a96bb..58a0378 100644
--- a/ROADMAP.md
+++ b/ROADMAP.md
@@ -21,14 +21,16 @@
        Reverse direction is a known limitation: the original UC2 Pro
        cannot read UC2 v3 archives due to compression bitstream
        differences.)
-- [x] Single-file backward compatibility with original UC2 Pro
-      (listing + extraction verified in DOSBox-X)
-- [x] Single-file backward compatibility with original UC2 Pro
-      (listing + extraction verified in automated DOSBox-X test).
-- [x] Multi-file backward compatibility with original UC2 Pro
-      (listing + extraction verified for 4 files in automated
-      DOSBox-X test).  Fix: always assign custom master indices
-      (>= 2), never SuperMaster (0), matching the original's behavior.
+- [x] Backward compatibility with original UC2 Pro (listing +
+      extraction verified for multi-file archives in both directions
+      in automated DOSBox-X test).
+- [ ] Custom Huffman tree compatibility with original UC2 Pro.
+      Currently uses the default tree for all blocks (~40% worse
+      compression for text, ~0% for random data).  Our treegen
+      produces valid trees but the original's nuke1 ASM kernel
+      depends on the exact tree shapes from the original TreeGen.
+      Improving this requires bit-exact treegen or understanding
+      nuke1's tree assumptions.
 - [ ] Give UC2 a voice: status and progress messages with personality,
       continuing the original's tradition ("Do not worry, you have got
       the tree", "decompression always lightspeed", FAST/TIGHT/S-TIGHT
diff --git a/lib/src/compress.c b/lib/src/compress.c
index deb3f9a..73f8c5d 100644
--- a/lib/src/compress.c
+++ b/lib/src/compress.c
@@ -697,9 +697,11 @@ static int flush_block(struct compressor *c, int is_last)
 	u8 lengths[NumSymbols];
 
 	/* Use the default tree for backward compatibility with the original
-	   UC2 Pro's ASM decompressor kernel.  Custom trees from our treegen
-	   cause the original to hang (the tree encoding is valid but the
-	   ASM kernel has undocumented assumptions about tree shapes). */
+	   UC2 Pro's ASM decompressor (nuke1).  Custom trees from our treegen
+	   produce valid bitstreams but nuke1 hangs on them — the original's
+	   tree generation (TREEGEN.CPP) produces different Huffman code
+	   assignments that nuke1 depends on.  The default tree gives ~40%
+	   worse compression but full backward compatibility. */
 	uc2_default_lengths(lengths);
 
 	/* Emit block-present flag */