Keep already-empty ModelRequests in LimitWarner._strip_old_warnings (#311)

LimitWarner rebuilds history each request, dropping any ModelRequest left with no
parts after warning markers are stripped. An already-empty ModelRequest -- which
pydantic-ai's empty-response retry appends when a model returns an empty response
against a non-optional output type -- has no parts to begin with, so it was dropped
too. That left history ending on a ModelResponse, and the next request failed the
_agent_graph precondition with `UserError: Processed history must end with a
ModelRequest`, killing the run.

Keep a request that was already empty; only drop one that became empty because every
part was a warning marker. Regression test covers the empty-response-retry tail.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

pydantic_ai_harness/compaction/_limit_warner.py

@@ -121,6 +121,13 @@ class LimitWarner(AbstractCapability[AgentDepsT]):
             if not isinstance(msg, ModelRequest):
                 cleaned.append(msg)
                 continue
+            # An already-empty request has no warnings to strip; keep it. pydantic-ai's
+            # empty-response retry appends a `ModelRequest` with no parts, and dropping it
+            # would leave history ending on a `ModelResponse`, which fails the next
+            # request's "must end with a ModelRequest" precondition.
+            if not msg.parts:
+                cleaned.append(msg)
+                continue
             parts = [p for p in msg.parts if not self._is_marker_part(p)]
             if not parts:
                 continue

tests/compaction/test_compaction.py

@@ -807,6 +807,20 @@ class TestLimitWarnerEdgeCases:
         assert isinstance(first, ModelRequest)
         assert len(first.parts) == 1
 
+    @pytest.mark.anyio
+    async def test_keeps_already_empty_request(self):
+        """An already-empty ModelRequest must survive stripping so history still ends with a
+        ModelRequest. pydantic-ai's empty-response retry appends an empty ModelRequest;
+        dropping it left history ending on a ModelResponse and crashed the next request with
+        `Processed history must end with a ModelRequest`."""
+        lw = LimitWarner(max_iterations=100)
+        messages: list[ModelMessage] = [_user('real'), ModelResponse(parts=[]), ModelRequest(parts=[])]
+        rc = _make_request_context(messages)
+        ctx = _make_ctx(requests=5)
+        result = await lw.before_model_request(ctx, rc)
+        assert len(result.messages) == 3
+        assert isinstance(result.messages[-1], ModelRequest)
+
     @pytest.mark.anyio
     async def test_context_warning_below_threshold(self):
         """Context window should not warn when below threshold."""