core/agent+tools: apply a batch's same-file writes in call order

tests/test_agent/test_parallel_tools.py

@@ -22,7 +22,7 @@ from plyngent.lmproto.openai_compatible.model import (
 )
 
 if TYPE_CHECKING:
-    from collections.abc import AsyncIterator
+    from collections.abc import AsyncIterator, Mapping
 
 
 def _response(message: AssistantChatMessage) -> ChatCompletionResponse:
@@ -156,3 +156,102 @@ async def test_parallel_tool_calls_run_concurrently() -> None:
     tool_msgs = [e for e in agent.messages if isinstance(e, ToolChatMessage)]
     assert {m.content for m in tool_msgs} == {"a", "b"}
     assert any(getattr(e, "content", None) == "done" for e in events)
+
+
+async def test_same_file_writes_apply_in_call_order() -> None:
+    """A batch's writes to one file run one at a time, in call order."""
+    order: list[str] = []
+
+    @tool(register=False, writes=("path",))
+    async def write_a(path: str) -> str:
+        order.append(f"a start {path}")
+        await asyncio.sleep(0)  # an unserialized sibling would interleave here
+        order.append(f"a end {path}")
+        return "a"
+
+    @tool(register=False, writes=("path",))
+    async def write_b(path: str) -> str:
+        order.append(f"b start {path}")
+        await asyncio.sleep(0)
+        order.append(f"b end {path}")
+        return "b"
+
+    registry = ToolRegistry([write_a, write_b])
+    results = await asyncio.gather(
+        registry.execute("write_a", '{"path": "a.txt"}'),
+        registry.execute("write_b", '{"path": "./a.txt"}'),
+    )
+    assert results == ["a", "b"]
+    assert order == ["a start a.txt", "a end a.txt", "b start ./a.txt", "b end ./a.txt"]
+
+
+async def test_different_files_still_write_concurrently() -> None:
+    """Serialization is per file: two files of one batch still overlap."""
+    overlaps = 0
+    gate = asyncio.Event()
+
+    async def _rendezvous() -> None:
+        nonlocal overlaps
+        overlaps += 1
+        if overlaps == 2:
+            _ = gate.set()
+        _ = await asyncio.wait_for(gate.wait(), timeout=2.0)
+
+    @tool(register=False, writes=("path",))
+    async def write_a(path: str) -> str:
+        del path
+        await _rendezvous()
+        return "a"
+
+    @tool(register=False, writes=("path",))
+    async def write_b(path: str) -> str:
+        del path
+        await _rendezvous()
+        return "b"
+
+    registry = ToolRegistry([write_a, write_b])
+    results = await asyncio.gather(
+        registry.execute("write_a", '{"path": "a.txt"}'),
+        registry.execute("write_b", '{"path": "b.txt"}'),
+    )
+    assert results == ["a", "b"]
+
+
+async def test_a_write_holds_its_file_across_the_confirm() -> None:
+    """The file is locked before the prompt, so a sibling write cannot overtake it."""
+    order: list[str] = []
+    prompt_shown = asyncio.Event()
+    answered = asyncio.Event()
+
+    @tool(register=False, writes=("path",))
+    async def write_a(path: str) -> str:
+        del path
+        order.append("a")
+        return "a"
+
+    @tool(register=False, writes=("path",))
+    async def write_b(path: str) -> str:
+        del path
+        order.append("b")
+        return "b"
+
+    async def on_confirm(name: str, args: Mapping[str, object], reason: str) -> bool:
+        del args, reason
+        order.append(f"confirm {name}")
+        prompt_shown.set()
+        await answered.wait()
+        return True
+
+    def danger(name: str, args: Mapping[str, object]) -> str | None:
+        del args
+        return "overwrite" if name == "write_a" else None
+
+    registry = ToolRegistry([write_a, write_b], danger=danger, on_confirm=on_confirm)
+    first = asyncio.create_task(registry.execute("write_a", '{"path": "a.txt"}'))
+    await prompt_shown.wait()
+    second = asyncio.create_task(registry.execute("write_b", '{"path": "a.txt"}'))
+    await asyncio.sleep(0)  # the second write waits for the file, not for the human
+    assert order == ["confirm write_a"]
+    answered.set()
+    assert await asyncio.gather(first, second) == ["a", "b"]
+    assert order == ["confirm write_a", "a", "b"]