fix(turns): deduplicate toolResult blocks in mergeConsecutiveUserTurns

When consecutive user messages are merged for Anthropic's alternating
turn requirement, duplicate toolResult blocks with the same toolUseId
could appear. This causes Anthropic API to reject with:
'each tool_use must have a single result. Found multiple tool_result
blocks with id: <id>'

Root cause: retry logic ('Continue where you left off') injects a
second user message containing the same tool_result. The naive array
concat in mergeConsecutiveUserTurns creates duplicates.

Fix: filter by seen toolUseId set, keeping the first occurrence.

Author: MuhlmannAI

src/agents/pi-embedded-helpers.validate-turns.test.ts

@@ -511,4 +511,77 @@ describe("validateAnthropicTurns strips dangling tool_use blocks", () => {
     const result = validateAnthropicTurns(msgs);
     expect(result).toHaveLength(3);
   });
+
+  it("deduplicates toolResult blocks with the same toolUseId when merging consecutive user messages", () => {
+    // Simulates the "Continue where you left off" retry scenario:
+    // two consecutive user messages both contain a toolResult for the same tool call
+    const msgs = asMessages([
+      { role: "user", content: [{ type: "text", text: "Do something" }] },
+      {
+        role: "assistant",
+        content: [{ type: "toolUse", id: "tool-abc", name: "read", input: {} }],
+      },
+      // First user message with tool result
+      {
+        role: "user",
+        content: [{ type: "toolResult", toolUseId: "tool-abc", text: "file contents" }],
+      },
+      // Second user message (retry) with the SAME tool result
+      {
+        role: "user",
+        content: [
+          { type: "toolResult", toolUseId: "tool-abc", text: "file contents" },
+          { type: "text", text: "Continue where you left off." },
+        ],
+      },
+    ]);
+
+    const result = validateAnthropicTurns(msgs);
+
+    // The two consecutive user messages should be merged into one
+    expect(result).toHaveLength(3); // user, assistant, merged-user
+
+    const mergedUser = result[2];
+    expect(mergedUser.role).toBe("user");
+    const content = mergedUser.content as Array<{ type: string; toolUseId?: string }>;
+
+    // Should have exactly ONE toolResult for "tool-abc", plus the text block
+    const toolResults = content.filter((b) => b.type === "toolResult");
+    expect(toolResults).toHaveLength(1);
+    expect(toolResults[0].toolUseId).toBe("tool-abc");
+
+    // The text block should still be there
+    const textBlocks = content.filter((b) => b.type === "text");
+    expect(textBlocks.length).toBeGreaterThanOrEqual(1);
+  });
+
+  it("keeps distinct toolResult blocks when merging consecutive user messages", () => {
+    // Two consecutive user messages with DIFFERENT tool results should both be kept
+    const msgs = asMessages([
+      { role: "user", content: [{ type: "text", text: "Do something" }] },
+      {
+        role: "assistant",
+        content: [
+          { type: "toolUse", id: "tool-1", name: "read", input: {} },
+          { type: "toolUse", id: "tool-2", name: "exec", input: {} },
+        ],
+      },
+      {
+        role: "user",
+        content: [{ type: "toolResult", toolUseId: "tool-1", text: "result 1" }],
+      },
+      {
+        role: "user",
+        content: [{ type: "toolResult", toolUseId: "tool-2", text: "result 2" }],
+      },
+    ]);
+
+    const result = validateAnthropicTurns(msgs);
+    const mergedUser = result[2];
+    const content = mergedUser.content as Array<{ type: string; toolUseId?: string }>;
+    const toolResults = content.filter((b) => b.type === "toolResult");
+    expect(toolResults).toHaveLength(2);
+    expect(toolResults[0].toolUseId).toBe("tool-1");
+    expect(toolResults[1].toolUseId).toBe("tool-2");
+  });
 });

src/agents/pi-embedded-helpers/turns.ts

@@ -171,11 +171,26 @@ export function mergeConsecutiveUserTurns(
   previous: Extract<AgentMessage, { role: "user" }>,
   current: Extract<AgentMessage, { role: "user" }>,
 ): Extract<AgentMessage, { role: "user" }> {
-  const mergedContent = [
+  const rawContent = [
     ...(Array.isArray(previous.content) ? previous.content : []),
     ...(Array.isArray(current.content) ? current.content : []),
   ];
 
+  // Deduplicate toolResult blocks by toolUseId to prevent
+  // "each tool_use must have a single result" Anthropic API errors.
+  // This can happen when retry logic ("Continue where you left off")
+  // injects a second user message containing the same tool_result.
+  const seenToolUseIds = new Set<string>();
+  const mergedContent = rawContent.filter((block) => {
+    if (block && block.type === "toolResult" && block.toolUseId) {
+      if (seenToolUseIds.has(block.toolUseId)) {
+        return false;
+      }
+      seenToolUseIds.add(block.toolUseId);
+    }
+    return true;
+  });
+
   return {
     ...current,
     content: mergedContent,