fix: fork subagents recursed forever in the Claude Code export

A fork subagent's transcript opens with a copy of the parent turn that spawned it, its own Agent call included; folding that call re-entered the same fork until RecursionError, failing every daily sync since 2026-09-24. Skip a spawn call while its own subagent is being expanded, and strip the <fork-boilerplate> preamble.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LbnmGHnFqDjyhPcCg1SEfF

tests/test_claude_code.py

@@ -370,6 +370,59 @@ class TestSubagentFold:
         ]
         assert "stray sidechain" not in top_text
 
+    def test_fork_containing_its_own_spawn_call(self, tmp_path):
+        # The shape Claude Code writes for a `fork` subagent: a context-ref
+        # record, a copy of the parent turn holding the fork's own spawn call,
+        # then the directive behind <fork-boilerplate>. Folding that copied
+        # call recursed until RecursionError.
+        fork_records = [
+            {"type": "fork-context-ref", "agentId": "a1", "parentLastUuid": "u0"},
+            {
+                "type": "assistant", "isSidechain": True,
+                "message": {"role": "assistant", "content": [
+                    {"type": "tool_use", "id": "toolu_sub1", "name": "Agent",
+                     "input": {"description": "research"}},
+                ]},
+            },
+            {
+                "type": "user", "isSidechain": True,
+                "timestamp": "2026-05-01T10:00:06.000Z",
+                "message": {"role": "user", "content": [
+                    {"type": "tool_result", "tool_use_id": "toolu_sub1",
+                     "content": [{"type": "text", "text": "Fork started"}]},
+                    {"type": "text", "text":
+                        "<fork-boilerplate>\nYou are a worker fork.\n"
+                        "</fork-boilerplate>\n\nYour directive: research X"},
+                ]},
+            },
+            {
+                "type": "assistant", "isSidechain": True,
+                "timestamp": "2026-05-01T10:00:07.000Z",
+                "message": {"role": "assistant", "content": [
+                    {"type": "text", "text": "Fork result."},
+                ]},
+            },
+        ]
+        f = _write_session(tmp_path, _parent_with_task(), name="parent-3")
+        _write_subagent(
+            f, "toolu_sub1", fork_records,
+            agent_type="fork", description="research X",
+        )
+        p = self._provider(tmp_path)
+        p.fetch_all_conversations()
+        conv = p.normalize_conversation(p.get_conversation("parent-3"))
+
+        subs = [
+            b for m in conv["messages"] for b in m["blocks"] if b["type"] == "subagent"
+        ]
+        assert len(subs) == 1
+        inner = [bb for m in subs[0]["messages"] for bb in m["blocks"]]
+        assert not any(b["type"] == "subagent" for b in inner)
+        inner_text = [b.get("text") for b in inner]
+        assert "Your directive: research X" in inner_text
+        assert "Fork result." in inner_text
+        assert not any("worker fork" in (t or "") for t in inner_text)
+
 
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 # Repo tags in title (Part C)