Fix #4650: Re-run tool pairing and turn validation after history limiting

After limitHistoryTurns or compaction operations, the message history can
end up with:
1. Orphaned tool_result blocks (tool_use IDs not in preceding message)
2. Consecutive assistant messages that violate API format requirements

Root cause: sanitizeToolUseResultPairing and turn validation functions
(validateGeminiTurns, validateAnthropicTurns) were only running BEFORE
limitHistoryTurns, but history limiting can create new issues by cutting
off tool_use blocks or creating consecutive assistant messages during
compaction.

Solution: Re-run both sanitizeToolUseResultPairing and turn validation
AFTER limitHistoryTurns in both main code paths:
- src/agents/pi-embedded-runner/run/attempt.ts
- src/agents/pi-embedded-runner/compact.ts

This ensures tool_use/tool_result pairing is maintained and consecutive
assistant/user messages are properly merged after any history modification.

Changes:
- Added transcriptPolicy checks before applying repairs (respects config)
- Re-run validateGeminiTurns after sanitizeToolUseResultPairing
- Re-run validateAnthropicTurns after validateGeminiTurns
- Added import for sanitizeToolUseResultPairing in compact.ts

src/agents/pi-embedded-runner/compact.ts

@@ -35,6 +35,7 @@ import {
   validateAnthropicTurns,
   validateGeminiTurns,
 } from "../pi-embedded-helpers.js";
+import { sanitizeToolUseResultPairing } from "../session-transcript-repair.js";
 import {
   ensurePiCompactionReserveTokens,
   resolveCompactionReserveTokensFloor,
@@ -421,8 +422,19 @@ export async function compactEmbeddedPiSessionDirect(
           validated,
           getDmHistoryLimitFromSessionKey(params.sessionKey, params.config),
         );
-        if (limited.length > 0) {
+        // Fix: Repair tool_use/tool_result pairings AFTER truncation (issue #4650)
-          session.agent.replaceMessages(limited);
+        const repaired = transcriptPolicy.repairToolUseResultPairing
+          ? sanitizeToolUseResultPairing(limited)
+          : limited;
+        // Re-run turn validation after limiting (issue #4650) to merge consecutive assistant/user messages
+        const revalidatedGemini = transcriptPolicy.validateGeminiTurns
+          ? validateGeminiTurns(repaired)
+          : repaired;
+        const revalidatedAnthropic = transcriptPolicy.validateAnthropicTurns
+          ? validateAnthropicTurns(revalidatedGemini)
+          : revalidatedGemini;
+        if (revalidatedAnthropic.length > 0) {
+          session.agent.replaceMessages(revalidatedAnthropic);
         }
         const result = await session.compact(params.customInstructions);
         // Estimate tokens after compaction by summing token estimates for remaining messages

src/agents/pi-embedded-runner/run/attempt.ts

@@ -536,11 +536,20 @@ export async function runEmbeddedAttempt(
           validated,
           getDmHistoryLimitFromSessionKey(params.sessionKey, params.config),
         );
-        // Fix: Repair tool_use/tool_result pairings AFTER truncation (issue #4367)
+        // Fix: Repair tool_use/tool_result pairings AFTER truncation (issue #4367, #4650)
-        const repaired = sanitizeToolUseResultPairing(limited);
+        const repaired = transcriptPolicy.repairToolUseResultPairing
-        cacheTrace?.recordStage("session:limited", { messages: repaired });
+          ? sanitizeToolUseResultPairing(limited)
-        if (repaired.length > 0) {
+          : limited;
-          activeSession.agent.replaceMessages(repaired);
+        // Re-run turn validation after limiting (issue #4650) to merge consecutive assistant/user messages
+        const revalidatedGemini = transcriptPolicy.validateGeminiTurns
+          ? validateGeminiTurns(repaired)
+          : repaired;
+        const revalidatedAnthropic = transcriptPolicy.validateAnthropicTurns
+          ? validateAnthropicTurns(revalidatedGemini)
+          : revalidatedGemini;
+        cacheTrace?.recordStage("session:limited", { messages: revalidatedAnthropic });
+        if (revalidatedAnthropic.length > 0) {
+          activeSession.agent.replaceMessages(revalidatedAnthropic);
         }
       } catch (err) {
         sessionManager.flushPendingToolResults?.();