fix(anthropic): merge consecutive assistant replay turns (#87346)

Jefsky · web-flow · commit 462c076a24e2 · 2026-06-13T22:34:38.000-07:00
Merge adjacent Anthropic assistant turns before dangling tool-use validation so signed tool calls remain immediately paired with their tool results. Preserve contributor credit. Fixes #87329.
diff --git a/src/agents/embedded-agent-helpers.validate-turns.test.ts b/src/agents/embedded-agent-helpers.validate-turns.test.ts
@@ -323,7 +323,7 @@ describe("validateAnthropicTurns", () => {
     ]);
   });
 
-  it("should not merge consecutive assistant messages", () => {
+  it("merges consecutive assistant messages", () => {
     const msgs = asMessages([
       { role: "user", content: [{ type: "text", text: "Question" }] },
       {
@@ -338,7 +338,60 @@ describe("validateAnthropicTurns", () => {
 
     const result = validateAnthropicTurns(msgs);
 
-    expect(result).toEqual(msgs);
+    expect(result).toEqual([
+      { role: "user", content: [{ type: "text", text: "Question" }] },
+      {
+        role: "assistant",
+        content: [
+          { type: "text", text: "Answer 1" },
+          { type: "text", text: "Answer 2" },
+        ],
+      },
+    ]);
+  });
+
+  it("merges an injected assistant turn before validating signed tool-result pairing", () => {
+    const msgs = asMessages([
+      { role: "user", content: [{ type: "text", text: "Use the gateway" }] },
+      {
+        role: "assistant",
+        content: makeSignedThinkingGatewayToolCall("tool-1"),
+        stopReason: "toolUse",
+      },
+      {
+        role: "assistant",
+        content: [{ type: "text", text: "Subagent completion delivered." }],
+        stopReason: "stop",
+      },
+      {
+        role: "toolResult",
+        toolUseId: "tool-1",
+        toolName: "gateway",
+        content: [{ type: "text", text: "done" }],
+        isError: false,
+      },
+    ]);
+
+    const result = validateAnthropicTurns(msgs);
+
+    expect(result).toEqual([
+      { role: "user", content: [{ type: "text", text: "Use the gateway" }] },
+      {
+        role: "assistant",
+        content: [
+          ...makeSignedThinkingGatewayToolCall("tool-1"),
+          { type: "text", text: "Subagent completion delivered." },
+        ],
+        stopReason: "stop",
+      },
+      {
+        role: "toolResult",
+        toolUseId: "tool-1",
+        toolName: "gateway",
+        content: [{ type: "text", text: "done" }],
+        isError: false,
+      },
+    ]);
   });
 
   it("should handle mixed scenario with steering messages", () => {
diff --git a/src/agents/embedded-agent-helpers/turns.ts b/src/agents/embedded-agent-helpers/turns.ts
@@ -386,8 +386,15 @@ function normalizeUserContentForMerge(content: unknown): UserContentBlock[] {
  * Also strips dangling tool_use blocks that lack corresponding tool_result blocks.
  */
 export function validateAnthropicTurns(messages: AgentMessage[]): AgentMessage[] {
-  // First, strip dangling tool-call blocks from assistant messages.
-  const stripped = stripDanglingAnthropicToolUses(messages);
+  // Merge first so an injected assistant turn cannot hide the tool result that
+  // resolves the preceding signed tool call. Stripping first would destroy the
+  // active Anthropic tool-use turn before the adjacent turns can be repaired.
+  const mergedAssistant = validateTurnsWithConsecutiveMerge({
+    messages,
+    role: "assistant",
+    merge: mergeConsecutiveAssistantTurns,
+  });
+  const stripped = stripDanglingAnthropicToolUses(mergedAssistant);
 
   return validateTurnsWithConsecutiveMerge({
     messages: stripped,
