diff --git a/test/lib/LibCodeGen.commentPrefix.t.sol b/test/lib/LibCodeGen.commentPrefix.t.sol
new file mode 100644
index 0000000..73580df
--- /dev/null
+++ b/test/lib/LibCodeGen.commentPrefix.t.sol
@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: LicenseRef-DCL-1.0
+// SPDX-FileCopyrightText: Copyright (c) 2020 Rain Open Source Software Ltd
+pragma solidity =0.8.25;
+
+import {Test} from "forge-std-1.16.1/src/Test.sol";
+import {LibCodeGen} from "src/lib/LibCodeGen.sol";
+import {LibCodeGenSlow} from "./LibCodeGenSlow.sol";
+
+/// @title LibCodeGenCommentPrefixTest
+/// @notice `commentPrefix` is the one rule every constant declaration shares:
+/// a blank line always, and a comment line only when there is a comment. The
+/// library states it as a choice between two whole prefix literals, so these
+/// assert it against a reference that assembles the prefix a line at a time,
+/// and pin that reference's own two cases to literals so agreement between the
+/// two is not the only thing holding either.
+contract LibCodeGenCommentPrefixTest is Test {
+    /// No comment is one blank line and nothing else. Two consecutive newlines
+    /// here would be a blank line `forge fmt` collapses, which makes a
+    /// regenerated file fail the consumer's `forge fmt --check`.
+    function testCommentPrefixSlowEmptyComment() external pure {
+        assertEq(LibCodeGenSlow.commentPrefixSlow(""), "\n");
+    }
+
+    /// A comment is a blank line, then the comment on a line of its own. The
+    /// comment is emitted as given, so the caller owns whether it carries
+    /// `///` and where it breaks.
+    function testCommentPrefixSlowWithComment() external pure {
+        assertEq(LibCodeGenSlow.commentPrefixSlow("/// @dev Some comment."), "\n/// @dev Some comment.\n");
+    }
+
+    /// Whatever the comment, the library emits the lines the reference
+    /// assembles. Fuzzed over the comment because emptiness is the only thing
+    /// the rule branches on and every other comment has to come back untouched.
+    function testCommentPrefixMatchesSlow(string memory comment) external pure {
+        assertEq(LibCodeGen.commentPrefix(comment), LibCodeGenSlow.commentPrefixSlow(comment));
+    }
+}
diff --git a/test/lib/LibCodeGenSlow.sol b/test/lib/LibCodeGenSlow.sol
index bd11101..d097e5d 100644
--- a/test/lib/LibCodeGenSlow.sol
+++ b/test/lib/LibCodeGenSlow.sol
@@ -34,11 +34,10 @@ string constant SLOW_TAIL_ALPHABET = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnop
 /// than imported, so a change to a literal in `LibCodeGen` shows up as a
 /// disagreement instead of moving both sides at once.
 ///
-/// The empty comment case is the one place the two sides state the same rule
-/// rather than deriving it independently: a declaration is preceded by one
-/// blank line, and by a comment line only when there is a comment. The exact
-/// text of both cases is pinned separately by literal assertions, so this
-/// reference is not the only thing holding it.
+/// The lines that precede a declaration are assembled here from the rule they
+/// follow: one blank line always, and a comment line only when there is a
+/// comment. The exact text of both cases is pinned separately by literal
+/// assertions, so this reference is not the only thing holding it.
 library LibCodeGenSlow {
     /// `vm.toString` on a `bytes` always prefixes `0x`, which a `hex"..."`
     /// literal must not carry. Dropped by copying the tail one byte at a time so
@@ -63,13 +62,31 @@ library LibCodeGenSlow {
         return oneLine;
     }
 
+    /// The lines that precede a declaration, listed out and then joined one
+    /// newline terminated line at a time: a blank line always, and a comment
+    /// line only when there is a comment. `LibCodeGen.commentPrefix` instead
+    /// chooses between two whole prefix literals, so the two agree only when
+    /// both the number of lines and the text of each one is right.
+    function commentPrefixSlow(string memory comment) internal pure returns (string memory) {
+        string[] memory lines = new string[](bytes(comment).length == 0 ? 1 : 2);
+        lines[0] = "";
+        if (lines.length == 2) {
+            lines[1] = comment;
+        }
+        string memory prefix = "";
+        for (uint256 i = 0; i < lines.length; i++) {
+            prefix = string.concat(prefix, lines[i], "\n");
+        }
+        return prefix;
+    }
+
     function bytesConstantStringSlow(Vm vm, string memory comment, string memory name, bytes memory data)
         internal
         pure
         returns (string memory)
     {
         return string.concat(
-            bytes(comment).length == 0 ? "\n" : string.concat("\n", comment, "\n"),
+            commentPrefixSlow(comment),
             joinSlow(string.concat("bytes constant ", name, " ="), string.concat("hex\"", hexOfSlow(vm, data), "\";")),
             "\n"
         );
@@ -81,7 +98,7 @@ library LibCodeGenSlow {
         returns (string memory)
     {
         return string.concat(
-            bytes(comment).length == 0 ? "\n" : string.concat("\n", comment, "\n"),
+            commentPrefixSlow(comment),
             joinSlow(string.concat("uint8 constant ", name, " ="), string.concat(vm.toString(uint256(data)), ";")),
             "\n"
         );
@@ -93,7 +110,7 @@ library LibCodeGenSlow {
         returns (string memory)
     {
         return string.concat(
-            bytes(comment).length == 0 ? "\n" : string.concat("\n", comment, "\n"),
+            commentPrefixSlow(comment),
             joinSlow(
                 string.concat("bytes32 constant ", name, " ="), string.concat("bytes32(", vm.toString(data), ");")
             ),
@@ -107,7 +124,7 @@ library LibCodeGenSlow {
         returns (string memory)
     {
         return string.concat(
-            bytes(comment).length == 0 ? "\n" : string.concat("\n", comment, "\n"),
+            commentPrefixSlow(comment),
             joinSlow(
                 string.concat("address constant ", name, " ="), string.concat("address(", vm.toString(data), ");")
             ),