Function diagnoseMatchFailure

test/harness/replayingCapiProxy.ts:527–586

* Produces a human-readable explanation of why no stored conversation matched * a given request. For each stored conversation it reports the first reason * matching failed, mirroring the logic in findAssistantIndexAfterPrefix.

(
  requestMessages: NormalizedMessage[],
  rawMessages: unknown[],
  storedData: NormalizedData | undefined,
)

 * matching failed, mirroring the logic in {@link findAssistantIndexAfterPrefix}.
 */
function diagnoseMatchFailure(
  requestMessages: NormalizedMessage[],
  rawMessages: unknown[],
  storedData: NormalizedData | undefined,
): string {
  const lines: string[] = [];
  lines.push(
    `Request has ${requestMessages.length} normalized messages (${rawMessages.length} raw).`,
  );

  if (!storedData || storedData.conversations.length === 0) {
    lines.push("No stored conversations to match against.");
    return lines.join("\n");
  }

  for (let c = 0; c < storedData.conversations.length; c++) {
    const saved = storedData.conversations[c].messages;

    // Same check as findAssistantIndexAfterPrefix: request must be a strict prefix
    if (requestMessages.length >= saved.length) {
      lines.push(
        `Conversation ${c} (${saved.length} messages): ` +
          `skipped — request has ${requestMessages.length} messages, need fewer than ${saved.length}.`,
      );
      continue;
    }

    // Find the first message that doesn't match
    let mismatchIndex = -1;
    for (let i = 0; i < requestMessages.length; i++) {
      if (JSON.stringify(requestMessages[i]) !== JSON.stringify(saved[i])) {
        mismatchIndex = i;
        break;
      }
    }

    if (mismatchIndex >= 0) {
      const raw =
        mismatchIndex < rawMessages.length
          ? JSON.stringify(rawMessages[mismatchIndex]).slice(0, 300)
          : "(no raw message)";
      lines.push(
        `Conversation ${c} (${saved.length} messages): mismatch at message ${mismatchIndex}:`,
        `  request:    ${JSON.stringify(requestMessages[mismatchIndex]).slice(0, 200)}`,
        `  saved:      ${JSON.stringify(saved[mismatchIndex]).slice(0, 200)}`,
        `  raw (pre-normalization): ${raw}`,
      );
    } else {
      // Prefix matched, but the next saved message isn't an assistant turn
      const nextRole =
        saved[requestMessages.length]?.role ?? "(end of conversation)";
      lines.push(
        `Conversation ${c} (${saved.length} messages): ` +
          `prefix matched, but next saved message is "${nextRole}" (need "assistant").`,
      );
    }
  }