From 7f62cffe44e59eda418cf0612252dcd8794be665 Mon Sep 17 00:00:00 2001
From: Tokuhiro Matsuno <tokuhirom@gmail.com>
Date: Mon, 13 Apr 2026 19:00:07 +0900
Subject: [PATCH 1/2] fix(compiler): preserve dangling comments in empty
 operation parameter list

`tsp format` crashed on `op find(/* conditions */): unknown;` because
`printModelExpression` discarded dangling comments when the property
list was empty, tripping prettier's `ensureAllCommentsPrinted` check.
Print dangling comments in that branch so the block comment survives
formatting.
---
 ...r-dangling-comment-empty-params-2026-04-13.md |  7 +++++++
 packages/compiler/src/formatter/print/printer.ts | 16 ++++++++++------
 .../compiler/test/formatter/formatter.test.ts    | 15 +++++++++++++++
 3 files changed, 32 insertions(+), 6 deletions(-)
 create mode 100644 .chronus/changes/fix-formatter-dangling-comment-empty-params-2026-04-13.md

diff --git a/.chronus/changes/fix-formatter-dangling-comment-empty-params-2026-04-13.md b/.chronus/changes/fix-formatter-dangling-comment-empty-params-2026-04-13.md
new file mode 100644
index 00000000000..ea8ee168957
--- /dev/null
+++ b/.chronus/changes/fix-formatter-dangling-comment-empty-params-2026-04-13.md
@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/compiler"
+---
+
+Fix formatter crash when an operation's parameter list contains only a block comment (e.g. `op find(/* conditions */): unknown;`). Dangling comments in empty parameter lists are now preserved instead of being dropped.
diff --git a/packages/compiler/src/formatter/print/printer.ts b/packages/compiler/src/formatter/print/printer.ts
index 5ceb7b70abc..b018bcb98e6 100644
--- a/packages/compiler/src/formatter/print/printer.ts
+++ b/packages/compiler/src/formatter/print/printer.ts
@@ -969,12 +969,16 @@ export function printModelExpression(
   if (inBlock) {
     return group(printModelPropertiesBlock(path, options, print));
   } else {
-    const properties =
-      node.properties.length === 0
-        ? ""
-        : indent(
-            joinMembersInBlock(path, "properties", options, print, ifBreak(",", ", "), softline),
-          );
+    const nodeHasComments = hasComments(node, CommentCheckFlags.Dangling);
+    if (node.properties.length === 0) {
+      if (nodeHasComments) {
+        return group([indent(printDanglingComments(path, options, { sameIndent: true })), softline]);
+      }
+      return group(["", softline]);
+    }
+    const properties = indent(
+      joinMembersInBlock(path, "properties", options, print, ifBreak(",", ", "), softline),
+    );
     return group([properties, softline]);
   }
 }
diff --git a/packages/compiler/test/formatter/formatter.test.ts b/packages/compiler/test/formatter/formatter.test.ts
index 1360a82e543..6be43cc8ded 100644
--- a/packages/compiler/test/formatter/formatter.test.ts
+++ b/packages/compiler/test/formatter/formatter.test.ts
@@ -737,6 +737,21 @@ op foo(
         });
       });
 
+      it("keeps block comment in empty parameter list", async () => {
+        await assertFormat({
+          code: `
+namespace MyApp;
+
+op find( /* conditions */) : unknown;
+`,
+          expected: `
+namespace MyApp;
+
+op find(/* conditions */): unknown;
+`,
+        });
+      });
+
       it("wrap in new lines parameters with block comments", async () => {
         await assertFormat({
           code: `

From 0b4b4d9b2878c163c5c8ed5b30407c093b88f835 Mon Sep 17 00:00:00 2001
From: Tokuhiro Matsuno <tokuhirom@gmail.com>
Date: Mon, 13 Apr 2026 22:18:39 +0900
Subject: [PATCH 2/2] pnpm format

---
 packages/compiler/src/formatter/print/printer.ts | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/packages/compiler/src/formatter/print/printer.ts b/packages/compiler/src/formatter/print/printer.ts
index b018bcb98e6..0a104e021d7 100644
--- a/packages/compiler/src/formatter/print/printer.ts
+++ b/packages/compiler/src/formatter/print/printer.ts
@@ -972,7 +972,10 @@ export function printModelExpression(
     const nodeHasComments = hasComments(node, CommentCheckFlags.Dangling);
     if (node.properties.length === 0) {
       if (nodeHasComments) {
-        return group([indent(printDanglingComments(path, options, { sameIndent: true })), softline]);
+        return group([
+          indent(printDanglingComments(path, options, { sameIndent: true })),
+          softline,
+        ]);
       }
       return group(["", softline]);
     }