diff --git a/tests/integration/_support/mcp.py b/tests/integration/_support/mcp.py
index 6f58a2d5e29..73eb73ab214 100644
--- a/tests/integration/_support/mcp.py
+++ b/tests/integration/_support/mcp.py
@@ -77,7 +77,7 @@ def math_service(
         if ctx.method == "initialize":
             requested: Final = InitializeRequestParams.model_validate(ctx.params).protocol_version
             returned: Final = InitializeResult.model_validate(result).protocol_version
-            record({"body": {}, "negotiation": {"requested": requested, "returned": returned}})
+            record({"body": {}, "headers": {}, "negotiation": {"requested": requested, "returned": returned}})
         return result
 
     service.middleware.append(capture_negotiation)
diff --git a/tests/unit/integration_support/test_conformance.py b/tests/unit/integration_support/test_conformance.py
index e1cf0610411..db102b9cfea 100644
--- a/tests/unit/integration_support/test_conformance.py
+++ b/tests/unit/integration_support/test_conformance.py
@@ -18,6 +18,38 @@ from tests.integration._support.conformance import (
 from pydantic import ValidationError
 
 
+@pytest.mark.asyncio
+async def test_negotiation_records_preserve_the_peer_call_contract() -> None:
+    from integration._support.mcp import math_service
+    from integration.mcp.test_mcp_user_env_vars import UPSTREAM_CALLS
+    from mcp.server.context import ServerRequestContext
+    from mcp.types import InitializeResult
+
+    records: Final[list[dict[str, object]]] = []
+    service: Final = math_service(record=records.append)
+    context: Final = ServerRequestContext(
+        session=Mock(),
+        lifespan_context=None,
+        protocol_version="2025-03-26",
+        method="initialize",
+        params={"protocolVersion": "2025-03-26", "capabilities": {}, "clientInfo": {"name": "client", "version": "1"}},
+    )
+
+    async def initialize(request: ServerRequestContext[object, object]) -> InitializeResult:
+        return InitializeResult.model_validate(
+            {
+                "protocolVersion": request.protocol_version,
+                "capabilities": {},
+                "serverInfo": {"name": "peer", "version": "1"},
+            }
+        )
+
+    await service.middleware[-1](context, initialize)
+    calls: Final = UPSTREAM_CALLS.validate_python(tuple(records))
+    assert len(calls) == 1 and calls[0].headers == {} and calls[0].body == {}
+    assert records[0]["negotiation"] == {"requested": "2025-03-26", "returned": "2025-03-26"}
+
+
 @pytest.mark.parametrize("explicit", (False, True))
 def test_owned_proxy_isolates_automatic_coverage_unless_requested(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch, explicit: bool