diff --git a/src/llm/backends/openai.py b/src/llm/backends/openai.py
index d8a2fc60..2ab35846 100644
--- a/src/llm/backends/openai.py
+++ b/src/llm/backends/openai.py
@@ -30,12 +30,14 @@ logger = logging.getLogger(__name__)
 # does not hold a reference to the keyed BaseModel so that when a dynamically created
 # type is no longer referenced it becomes eligible for garbage collection. This avoids a
 # memory leak.
-_json_object_instruction_cache: weakref.WeakKeyDictionary[type[BaseModel], str] = (
-    weakref.WeakKeyDictionary()
-)
+_json_object_instruction_cache: weakref.WeakKeyDictionary[
+    type[BaseModel], dict[bool, str]
+] = weakref.WeakKeyDictionary()
 
 
-def _json_object_instruction(response_format: type[BaseModel]) -> str:
+def _json_object_instruction(
+    response_format: type[BaseModel], *, tools_present: bool = False
+) -> str:
     """Schema-injection instruction for json_object mode.
 
     The JSON schema is static per response_format class, so cache the serialized
@@ -43,17 +45,25 @@ def _json_object_instruction(response_format: type[BaseModel]) -> str:
     it every call.
     """
     cached = _json_object_instruction_cache.get(response_format)
-    if cached is not None:
-        return cached
+    if cached is not None and tools_present in cached:
+        return cached[tools_present]
     # Some OpenAI-compatible providers enforce this JSON-object precondition with
     # a case-sensitive substring check, so include lowercase "json" explicitly.
+    requirement = (
+        "If not responding with a tool call, respond with"
+        if tools_present
+        else "You must respond with"
+    )
     instruction = (
-        "You must respond with a single JSON object (json) that conforms "
-        "exactly to the following JSON schema. Do not include any text, "
-        "markdown, or code fences outside the JSON object.\n\nJSON schema:\n"
+        f"{requirement} a single JSON object (json) that conforms exactly to "
+        "the following JSON schema. Do not include any text, markdown, or code "
+        "fences outside the JSON object.\n\nJSON schema:\n"
         f"{json.dumps(response_format.model_json_schema())}"
     )
-    _json_object_instruction_cache[response_format] = instruction
+    if cached is None:
+        cached = {}
+        _json_object_instruction_cache[response_format] = cached
+    cached[tools_present] = instruction
     return instruction
 
 
@@ -536,7 +546,9 @@ class OpenAIBackend:
             isinstance(response_format, type)
             and self._structured_output_mode(extra_params) == "json_object"
         ):
-            schema_instruction = _json_object_instruction(response_format)
+            schema_instruction = _json_object_instruction(
+                response_format, tools_present=bool(tools)
+            )
             instructions = "\n\n".join(
                 part for part in (instructions, schema_instruction) if part
             )
@@ -699,8 +711,9 @@ class OpenAIBackend:
                             }
                         )
                     else:
+                        block_name = block_type or "<missing>"
                         raise ValidationException(
-                            f"Unsupported Responses content block type: {block_type or '<missing>'}"
+                            f"Unsupported Responses content block type: {block_name}"
                         )
             elif content is not None:
                 raise ValidationException(
diff --git a/tests/llm/test_backends/test_openai.py b/tests/llm/test_backends/test_openai.py
index 60fb0bcb..630a3fe9 100644
--- a/tests/llm/test_backends/test_openai.py
+++ b/tests/llm/test_backends/test_openai.py
@@ -1701,6 +1701,27 @@ def test_responses_json_object_mode_injects_schema_instructions() -> None:
     assert "answer" in params["instructions"]
 
 
+def test_responses_json_object_mode_keeps_tool_calls_available() -> None:
+    backend = OpenAIBackend(Mock())
+    params = backend._build_responses_params(  # pyright: ignore[reportPrivateUsage]
+        model="gpt-5.6-luna",
+        messages=[{"role": "user", "content": "Search, then answer."}],
+        max_tokens=100,
+        tools=[AGENT_TOOL],
+        tool_choice="auto",
+        response_format=_StructuredResponse,
+        thinking_effort=None,
+        extra_params={
+            "api_mode": "responses",
+            "structured_output_mode": "json_object",
+        },
+    )
+
+    assert params["text"]["format"] == {"type": "json_object"}
+    assert params["tools"][0]["name"] == "search"
+    assert "If not responding with a tool call" in params["instructions"]
+
+
 def test_openai_backend_responses_mode_can_omit_max_output_tokens() -> None:
     backend = OpenAIBackend(Mock())
     params = backend._build_responses_params(  # pyright: ignore[reportPrivateUsage]