fix: strip injected message-id suffix before paired tag regex

Remove the legitimately injected <dcp-message-id> suffix before running
DCP_PAIRED_TAG_REGEX so in-text tag mentions cannot pair with it and
truncate message content.

Fixes #556

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: LHMQ878

lib/messages/utils.ts

@@ -7,6 +7,7 @@ const SUMMARY_ID_HASH_LENGTH = 16
 const DCP_BLOCK_ID_TAG_REGEX = /(<dcp-message-id(?=[\s>])[^>]*>)b\d+(<\/dcp-message-id>)/g
 const DCP_PAIRED_TAG_REGEX = /<dcp[^>]*>[\s\S]*?<\/dcp[^>]*>/gi
 const DCP_UNPAIRED_TAG_REGEX = /<\/?dcp[^>]*>/gi
+const INJECTED_MESSAGE_ID_SUFFIX_REGEX = /\n<dcp-message-id>m\d+<\/dcp-message-id>\s*$/
 
 const generateStableId = (prefix: string, seed: string): string => {
     const hash = createHash("sha256").update(seed).digest("hex").slice(0, SUMMARY_ID_HASH_LENGTH)
@@ -163,7 +164,8 @@ export const replaceBlockIdsWithBlocked = (text: string): string => {
 }
 
 export const stripHallucinationsFromString = (text: string): string => {
-    return text.replace(DCP_PAIRED_TAG_REGEX, "").replace(DCP_UNPAIRED_TAG_REGEX, "")
+    const withoutInjectedSuffix = text.replace(INJECTED_MESSAGE_ID_SUFFIX_REGEX, "")
+    return withoutInjectedSuffix.replace(DCP_PAIRED_TAG_REGEX, "").replace(DCP_UNPAIRED_TAG_REGEX, "")
 }
 
 export const stripHallucinations = (messages: WithParts[]): void => {

tests/message-priority.test.ts

@@ -814,6 +814,18 @@ test("hallucination stripping does not affect non-dcp tags", async () => {
     )
 })
 
+test("hallucination stripping preserves content when dcp-message-id is mentioned in text (issue #556)", () => {
+    const input =
+        "The tag called `<dcp-message-id>` is used to track messages. " +
+        "This text should survive.\n\n" +
+        "<dcp-message-id>m0369</dcp-message-id>"
+
+    assert.equal(
+        stripHallucinationsFromString(input),
+        "The tag called `` is used to track messages. This text should survive.\n",
+    )
+})
+
 test("injectMessageIds skips empty assistant messages to avoid prefill (issue #463)", () => {
     const sessionID = "ses_empty_assistant"
     const messages: WithParts[] = [