fix(agents): preserve active tool-turn thinking

src/agents/pi-embedded-runner.sanitize-session-history.test.ts

@@ -1522,6 +1522,59 @@ describe("sanitizeSessionHistory", () => {
     ]);
   });
 
+  it.each([
+    {
+      provider: "anthropic",
+      modelApi: "anthropic-messages",
+      label: "anthropic",
+    },
+    {
+      provider: "amazon-bedrock",
+      modelApi: "bedrock-converse-stream",
+      label: "bedrock",
+    },
+  ])(
+    "preserves active tool-turn thinking signatures for $label even when a tool result follows",
+    async ({ provider, modelApi }) => {
+      setNonGoogleModelApi();
+
+      const messages = castAgentMessages([
+        makeUserMessage("look up the answer"),
+        makeAssistantMessage([
+          {
+            type: "thinking",
+            thinking: "call the tool",
+            signature: "",
+          } as unknown as ThinkingContent,
+          { type: "toolCall", id: "call_1", name: "lookup", arguments: {} },
+        ]),
+        castAgentMessage({
+          role: "toolResult",
+          toolCallId: "call_1",
+          toolName: "lookup",
+          content: [{ type: "text", text: "42" }],
+          isError: false,
+        }),
+      ]);
+
+      const result = await sanitizeAnthropicHistory({
+        provider,
+        modelApi,
+        messages,
+        modelId: "claude-sonnet-4-6",
+      });
+
+      expect((result[1] as Extract<AgentMessage, { role: "assistant" }>).content).toEqual([
+        {
+          type: "thinking",
+          thinking: "call the tool",
+          signature: "",
+        },
+        { type: "toolCall", id: "call_1", name: "lookup", arguments: {} },
+      ]);
+    },
+  );
+
   it.each([
     {
       provider: "anthropic",

src/agents/pi-embedded-runner/replay-history.ts

@@ -50,6 +50,7 @@ import { isZeroUsageEmptyStopAssistantTurn } from "./empty-assistant-turn.js";
 import {
   dropReasoningFromHistory,
   dropThinkingBlocks,
+  shouldPreserveLatestAssistantThinking,
   stripInvalidThinkingSignatures,
 } from "./thinking.js";
 
@@ -727,12 +728,9 @@ export async function sanitizeSessionHistory(params: {
       ...resolveImageSanitizationLimits(params.config),
     },
   );
-  const lastMessage = sanitizedImages[sanitizedImages.length - 1];
   const preserveLatestAssistantThinking =
     params.preserveLatestAssistantThinking ??
-    (!!lastMessage &&
-      typeof lastMessage === "object" &&
-      (lastMessage as { role?: unknown }).role === "assistant");
+    shouldPreserveLatestAssistantThinking(sanitizedImages);
   // Some recovery paths supply a narrow policy with preserveSignatures disabled.
   // Native signed-thinking providers still cannot replay missing/blank
   // signatures once the assistant turn is no longer latest in the outbound

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

@@ -261,6 +261,31 @@ function shouldPreserveCurrentToolTurnReasoning(
   return false;
 }
 
+export function shouldPreserveLatestAssistantThinking(messages: AgentMessage[]): boolean {
+  let latestAssistantIndex = -1;
+  for (let index = messages.length - 1; index >= 0; index -= 1) {
+    if (isAssistantMessageWithContent(messages[index])) {
+      latestAssistantIndex = index;
+      break;
+    }
+  }
+  if (latestAssistantIndex < 0) {
+    return false;
+  }
+  if (latestAssistantIndex === messages.length - 1) {
+    return true;
+  }
+
+  let latestUserIndex = -1;
+  for (let index = messages.length - 1; index >= 0; index -= 1) {
+    if ((messages[index] as { role?: unknown })?.role === "user") {
+      latestUserIndex = index;
+      break;
+    }
+  }
+  return shouldPreserveCurrentToolTurnReasoning(messages, latestAssistantIndex, latestUserIndex);
+}
+
 function stripAllThinkingBlocks(messages: AgentMessage[]): AgentMessage[] {
   let touched = false;
   const out: AgentMessage[] = [];