diff --git a/src/agents/pi-embedded-runner/run/attempt.ts b/src/agents/pi-embedded-runner/run/attempt.ts
index ea465367a33..49938ba2d6e 100644
--- a/src/agents/pi-embedded-runner/run/attempt.ts
+++ b/src/agents/pi-embedded-runner/run/attempt.ts
@@ -72,6 +72,7 @@ import {
 import type { EmbeddedContextFile } from "../../pi-embedded-helpers.js";
 import {
   downgradeOpenAIFunctionCallReasoningPairs,
+  downgradeOpenAIReasoningBlocks,
   isCloudCodeAssistFormatError,
   resolveBootstrapMaxChars,
   resolveBootstrapPromptTruncationWarningMode,
@@ -1367,7 +1368,12 @@ export async function runEmbeddedAttempt(
           if (!Array.isArray(messages)) {
             return inner(model, context, options);
           }
-          const sanitized = downgradeOpenAIFunctionCallReasoningPairs(messages as AgentMessage[]);
+          const pairSanitized = downgradeOpenAIFunctionCallReasoningPairs(
+            messages as AgentMessage[],
+          );
+          // Also strip orphaned reasoning blocks that lack a required following
+          // content item — OpenAI rejects these with a 400 error.
+          const sanitized = downgradeOpenAIReasoningBlocks(pairSanitized);
           if (sanitized === messages) {
             return inner(model, context, options);
           }