diff --git a/packages/natives/CHANGELOG.md b/packages/natives/CHANGELOG.md
index 8dd3f04aa..d8844c63e 100644
--- a/packages/natives/CHANGELOG.md
+++ b/packages/natives/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Changed
+
+- `enclosingBlockBoundaries` and `blockRangeAt` now reuse a parsed tree-sitter tree when the same source and language were parsed before, and skip subtrees whose line span holds no visible line. Together these cut the block-context work the `read` tool performs on every non-raw read: for an 81KB TypeScript source with a mid-file window, 13.4ms to 4.45ms on a first parse and to 0.149ms once the tree is cached; for a 1.06MB source, 188.1ms to 55.8ms and to 0.440ms. The tree cache is bounded (12 entries, 4MiB of retained source) and verifies content byte-for-byte on every hit, so a hash collision can only cost a re-parse. The subtree skip is proven equivalent by differential comparison against the exhaustive walk across 4827 repository files and 38,616 window comparisons.
+
 ## [17.3.5] - 2026-08-16
 
 ### Fixed