diff --git a/packages/core/src/agent/runner.ts b/packages/core/src/agent/runner.ts
index ddfd3ac11..b27d8fc36 100644
--- a/packages/core/src/agent/runner.ts
+++ b/packages/core/src/agent/runner.ts
@@ -39,8 +39,8 @@ import {
 } from "./system-prompt/index.js";
 import type { AuthProfileFailureReason } from "./auth-profiles/index.js";
 import {
-  sanitizeToolCallInputs,
-  sanitizeToolUseResultPairing,
+  repairToolCallInputs,
+  repairToolUseResultPairing,
 } from "./session/session-transcript-repair.js";
 
 // ============================================================
@@ -75,6 +75,10 @@ export function isRotatableError(reason: AuthProfileFailureReason): boolean {
   return reason === "auth" || reason === "rate_limit" || reason === "billing" || reason === "timeout";
 }
 
+function shouldInjectInvalidToolCallIdForDebug(): boolean {
+  return process.env.MULTICA_DEBUG_INJECT_INVALID_TOOL_CALL_ID === "1";
+}
+
 export class Agent {
   private readonly agent: PiAgentCore;
   private output;
@@ -186,8 +190,43 @@ export class Agent {
         return this.currentApiKey;
       },
       transformContext: async (messages) => {
-        const sanitizedInputs = sanitizeToolCallInputs(messages);
-        return sanitizeToolUseResultPairing(sanitizedInputs);
+        let workingMessages = messages;
+
+        // Debug-only fault injection:
+        // Simulate a poisoned transcript with an empty toolCallId so we can
+        // verify sanitize logic prevents provider-side tool_call_id failures.
+        if (shouldInjectInvalidToolCallIdForDebug()) {
+          workingMessages = [
+            ...workingMessages,
+            {
+              role: "toolResult",
+              toolCallId: "",
+              toolName: "debug-invalid-tool-call-id",
+              content: [{ type: "text", text: "[debug] injected invalid toolCallId" }],
+              isError: true,
+              timestamp: Date.now(),
+            } as AgentMessage,
+          ];
+        }
+
+        const inputRepair = repairToolCallInputs(workingMessages);
+        const pairingRepair = repairToolUseResultPairing(inputRepair.messages);
+
+        if (
+          shouldInjectInvalidToolCallIdForDebug()
+          || inputRepair.droppedToolCalls > 0
+          || pairingRepair.droppedOrphanCount > 0
+          || pairingRepair.droppedDuplicateCount > 0
+        ) {
+          console.error(
+            `[Agent] context sanitize: droppedToolCalls=${inputRepair.droppedToolCalls}, ` +
+            `droppedOrphanToolResults=${pairingRepair.droppedOrphanCount}, ` +
+            `droppedDuplicateToolResults=${pairingRepair.droppedDuplicateCount}, ` +
+            `insertedMissingToolResults=${pairingRepair.added.length}`,
+          );
+        }
+
+        return pairingRepair.messages;
       },
     });