From 687200c4dd5afa6aa1d86c5739aaf1114ca8c062 Mon Sep 17 00:00:00 2001
From: Sebastian Lorenz <fubhy@fubhy.com>
Date: Sat, 1 Aug 2026 18:57:05 +0000
Subject: [PATCH 1/2] Add reproduction for Trie issue

---
 packages/effect/test/Trie.test.ts | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/packages/effect/test/Trie.test.ts b/packages/effect/test/Trie.test.ts
index f3e25d39872..365798e27a3 100644
--- a/packages/effect/test/Trie.test.ts
+++ b/packages/effect/test/Trie.test.ts
@@ -368,6 +368,16 @@ describe("Trie", () => {
     assertSome(Trie.longestPrefixOf(trie, "shellsort"), ["shells", 0])
   })
 
+  it("longestPrefixOf ignores valued sibling nodes that do not match the input", () => {
+    const trie = Trie.make(["a", 1], ["b", 2])
+
+    strictEqual(
+      Option.isNone(Trie.longestPrefixOf(trie, "c")),
+      true,
+      "a non-matching sibling must not be reported as a prefix"
+    )
+  })
+
   it("map transforms values and can use keys", () => {
     const trie = Trie.empty<number>().pipe(
       Trie.insert("shells", 0),

From acc9713f60d8df6ed54dfdb845dfbc31108716c8 Mon Sep 17 00:00:00 2001
From: Tim Smart <hello@timsmart.co>
Date: Mon, 3 Aug 2026 11:24:13 +1200
Subject: [PATCH 2/2] Fix Trie longest prefix sibling matching

---
 .changeset/fix-trie-longest-prefix.md | 5 +++++
 packages/effect/src/internal/trie.ts  | 7 +++----
 2 files changed, 8 insertions(+), 4 deletions(-)
 create mode 100644 .changeset/fix-trie-longest-prefix.md

diff --git a/.changeset/fix-trie-longest-prefix.md b/.changeset/fix-trie-longest-prefix.md
new file mode 100644
index 00000000000..bf422ac6d4e
--- /dev/null
+++ b/.changeset/fix-trie-longest-prefix.md
@@ -0,0 +1,5 @@
+---
+"effect": patch
+---
+
+Fix `Trie.longestPrefixOf` returning a valued sibling that does not match the input key.
diff --git a/packages/effect/src/internal/trie.ts b/packages/effect/src/internal/trie.ts
index 6cccc73172f..e003b575551 100644
--- a/packages/effect/src/internal/trie.ts
+++ b/packages/effect/src/internal/trie.ts
@@ -693,10 +693,6 @@ export const longestPrefixOf = dual<
     let cIndex = 0
     while (cIndex < key.length) {
       const c = key[cIndex]
-      if (n.value !== undefined) {
-        longestPrefixNode = Option.some([key.slice(0, cIndex + 1), n.value.value])
-      }
-
       if (c > n.key) {
         if (n.right === undefined) {
           break
@@ -710,6 +706,9 @@ export const longestPrefixOf = dual<
           n = n.left
         }
       } else {
+        if (n.value !== undefined) {
+          longestPrefixNode = Option.some([key.slice(0, cIndex + 1), n.value.value])
+        }
         if (n.mid === undefined) {
           break
         } else {