From ac4d4dffdbd12d99d1b11b34ad4566cf453d54ea Mon Sep 17 00:00:00 2001
From: Sravan Vadla <44364763+sravan-v@users.noreply.github.com>
Date: Mon, 12 Oct 2026 21:25:29 -0700
Subject: [PATCH] fix(core): match URI template fragment delimiters correctly

---
 .changeset/uri-template-fragment-delimiter.md     |  5 +++++
 packages/core-internal/src/shared/uriTemplate.ts  |  7 +++++--
 .../core-internal/test/shared/uriTemplate.test.ts | 15 +++++++++++++++
 3 files changed, 25 insertions(+), 2 deletions(-)
 create mode 100644 .changeset/uri-template-fragment-delimiter.md

diff --git a/.changeset/uri-template-fragment-delimiter.md b/.changeset/uri-template-fragment-delimiter.md
new file mode 100644
index 0000000000..4462d01bce
--- /dev/null
+++ b/.changeset/uri-template-fragment-delimiter.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/core': patch
+---
+
+Match URI template fragment expressions with their required `#` delimiter outside the captured variable, including empty fragment values.
diff --git a/packages/core-internal/src/shared/uriTemplate.ts b/packages/core-internal/src/shared/uriTemplate.ts
index 5ffe213acd..820145d049 100644
--- a/packages/core-internal/src/shared/uriTemplate.ts
+++ b/packages/core-internal/src/shared/uriTemplate.ts
@@ -230,11 +230,14 @@ export class UriTemplate {
                 pattern = part.exploded ? '([^/,]+(?:,[^/,]+)*)' : '([^/,]+)';
                 break;
             }
-            case '+':
-            case '#': {
+            case '+': {
                 pattern = '(.+)';
                 break;
             }
+            case '#': {
+                pattern = '#(.*)';
+                break;
+            }
             case '.': {
                 pattern = String.raw`\.([^/,]+)`;
                 break;
diff --git a/packages/core-internal/test/shared/uriTemplate.test.ts b/packages/core-internal/test/shared/uriTemplate.test.ts
index bfc3237872..81412bac49 100644
--- a/packages/core-internal/test/shared/uriTemplate.test.ts
+++ b/packages/core-internal/test/shared/uriTemplate.test.ts
@@ -51,6 +51,21 @@ describe('UriTemplate', () => {
             expect(template.expand({ var: '/test' })).toBe('X#/test');
             expect(template.variableNames).toEqual(['var']);
         });
+
+        it.each(['intro', '/chapter?view=full', ''])('should match fragment value %j without its delimiter', section => {
+            const template = new UriTemplate('file:///doc{#section}');
+            expect(template.match(template.expand({ section }))).toEqual({ section });
+        });
+
+        it('should reject a fragment value without the fragment delimiter', () => {
+            const template = new UriTemplate('file:///doc{#section}');
+            expect(template.match('file:///docintro')).toBeNull();
+        });
+
+        it('should preserve reserved expansion matching without a fragment delimiter', () => {
+            const template = new UriTemplate('file:///doc{+section}');
+            expect(template.match('file:///docintro')).toEqual({ section: 'intro' });
+        });
     });
 
     describe('label expansion', () => {