diff --git a/renderers/base.py b/renderers/base.py
index 3c2eb6af..f9e0c927 100644
--- a/renderers/base.py
+++ b/renderers/base.py
@@ -619,12 +619,22 @@ class ParsedToolCall:
     it back as the call's ``arguments`` re-renders the call with the
     model's own spacing, which ``json.dumps(arguments)`` generally
     doesn't. It's ``None`` for other formats.
+
+    ``content_type`` is the call's content type as the model sampled it in the
+    call's header, for formats whose header carries one (gpt-oss: the word after
+    ``<|constrain|>``, else the header's bare content type such as ``code``;
+    ``None`` when the header has none). The model doesn't always pick the type
+    its arguments would suggest (gpt-oss samples `` code`` for JSON objects too),
+    so passing it back as the tool call's ``content_type`` (on the tool-call dict
+    or its ``function`` dict) re-renders the header as sampled; the gpt-oss
+    renderer honours ``json`` and ``code``. It's ``None`` for other formats.
     """
 
     raw: str
     name: str | None = None
     arguments: dict[str, Any] | str | None = None
     arguments_text: str | None = None
+    content_type: str | None = None
     token_span: tuple[int, int] | None = None
     status: ToolCallParseStatus = ToolCallParseStatus.OK
     id: str | None = None  # native tool-call id when the format carries one (Kimi K2)
diff --git a/renderers/gpt_oss.py b/renderers/gpt_oss.py
index 00f41f21..05d8427f 100644
--- a/renderers/gpt_oss.py
+++ b/renderers/gpt_oss.py
@@ -20,7 +20,8 @@
     verified empirically. Tool calls render as gpt-oss samples them,
     channel-first with a content type
     (``<|channel|>commentary to=functions.x <|constrain|>json``, or ``code``
-    for arguments that aren't a JSON object); tool results keep the
+    for arguments that aren't a JSON object, unless the call carries the
+    ``content_type`` it was sampled with); tool results keep the
     recipient-first header (``to=assistant<|channel|>commentary``).
 
 Special tokens
@@ -850,7 +851,14 @@ def _assistant_to_harmony(
             recipient = name if name.startswith("functions.") else f"functions.{name}"
             m = HarmonyMessage.from_role_and_content(Role.ASSISTANT, args)
             m = m.with_channel("commentary").with_recipient(recipient)
-            out.append(m.with_content_type("json" if _is_json_object(args) else "code"))
+            # The header as sampled when the call carries it; else the type its arguments suggest.
+            sampled = tc.get("content_type") or fn.get("content_type")
+            content_type = (
+                sampled
+                if sampled in ("json", "code")
+                else ("json" if _is_json_object(args) else "code")
+            )
+            out.append(m.with_content_type(content_type))
 
         # Empty assistant (no text and no tool_calls) — emit an empty
         # final-channel message so message_indices stays non-empty.
diff --git a/renderers/parsing.py b/renderers/parsing.py
index 9f7d5db9..6439ee6c 100644
--- a/renderers/parsing.py
+++ b/renderers/parsing.py
@@ -1745,8 +1745,19 @@ def parse_gpt_oss(
         recipient_match = re.search(r"to=([^\s<]+)", header_text)
         recipient = recipient_match.group(1) if recipient_match else None
 
-        if recipient and recipient.startswith("functions."):
+        if recipient_match and recipient and recipient.startswith("functions."):
             tool_name = recipient[len("functions.") :]
+            if constrain_id in header_ids:
+                after = _decode(
+                    tokenizer, header_ids[header_ids.index(constrain_id) + 1 :]
+                ).split()
+            else:
+                # The header's words after the recipient, without its channel (recipient-first headers
+                # put ``<|channel|>commentary`` there).
+                after = re.sub(
+                    r"<\|channel\|>\s*\S+", " ", header_text[recipient_match.end() :]
+                ).split()
+            content_type = after[0] if after else None
             block_end = body_end + 1 if body_closed else body_end
             span = (block_start, block_end)
             try:
@@ -1758,6 +1769,7 @@ def parse_gpt_oss(
                         name=tool_name or None,
                         arguments=body_text,
                         arguments_text=body_text,
+                        content_type=content_type,
                         token_span=span,
                         status=ToolCallParseStatus.INVALID_JSON,
                     )
@@ -1775,6 +1787,7 @@ def parse_gpt_oss(
                         name=tool_name or None,
                         arguments=arguments,
                         arguments_text=body_text,
+                        content_type=content_type,
                         token_span=span,
                         status=status,
                     )
diff --git a/tests/test_gpt_oss.py b/tests/test_gpt_oss.py
index 0f05eb9f..90b7c56e 100644
--- a/tests/test_gpt_oss.py
+++ b/tests/test_gpt_oss.py
@@ -106,3 +106,80 @@ def test_rerendered_tool_calls_equal_sampled_tokens():
                 if header in text
             }
     assert content_types == {"json", "code"}
+
+
+def test_a_code_header_on_json_arguments_round_trips():
+    """gpt-oss sometimes samples `` code`` for a call whose arguments are a JSON
+    object. The parsed call carries the sampled ``content_type``, and a re-render
+    given it reproduces the sampled tokens; without it the header follows the
+    arguments."""
+    tok = load_tokenizer("openai/gpt-oss-20b")
+    renderer = create_renderer(tok, GptOssRendererConfig(reasoning_effort="low"))
+    args = '{"command":"ls -R"}'
+    user = {"role": "user", "content": "List the files."}
+    prompt = renderer.render_ids([user], add_generation_prompt=True)
+    sampled = tok.encode(
+        "List files.<|end|>"
+        "<|start|>assistant<|channel|>commentary to=functions.bash code"
+        f"<|message|>{args}<|call|>",
+        add_special_tokens=False,
+    )
+    (call,) = renderer.parse_response(sampled, prompt_ids=prompt).tool_calls
+    assert (call.arguments_text, call.content_type) == (args, "code")
+
+    def assistant_tokens(**extra):
+        message = {
+            "role": "assistant",
+            "content": None,
+            "reasoning_content": "List files.",
+            "tool_calls": [
+                {
+                    "type": "function",
+                    "id": "call_0",
+                    "function": {"name": "bash", "arguments": args},
+                    **extra,
+                }
+            ],
+        }
+        rendered = renderer.render([user, message])
+        return [
+            t for t, i in zip(rendered.token_ids, rendered.message_indices) if i == 1
+        ]
+
+    # The prompt's generation header plus the sample are the assistant turn's tokens.
+    turn = prompt[len(renderer.render_ids([user])) :] + sampled
+    assert assistant_tokens(content_type=call.content_type) == turn
+    assert assistant_tokens() != turn
+    assert "<|constrain|>json" in tok.decode(assistant_tokens())
+
+
+def test_a_tool_call_header_content_type_is_parsed_as_sampled():
+    tok = load_tokenizer("openai/gpt-oss-20b")
+    renderer = create_renderer(tok, GptOssRendererConfig(reasoning_effort="low"))
+    prompt = renderer.render_ids(
+        [{"role": "user", "content": "q"}], add_generation_prompt=True
+    )
+
+    def content_type(header):
+        sampled = tok.encode(
+            f"Go.<|end|><|start|>assistant{header}<|message|>{{}}<|call|>",
+            add_special_tokens=False,
+        )
+        return (
+            renderer.parse_response(sampled, prompt_ids=prompt)
+            .tool_calls[0]
+            .content_type
+        )
+
+    assert (
+        content_type("<|channel|>commentary to=functions.bash <|constrain|>json")
+        == "json"
+    )
+    assert content_type("<|channel|>commentary to=functions.bash code") == "code"
+    assert (
+        content_type(" to=functions.bash<|channel|>commentary <|constrain|>json")
+        == "json"
+    )
+    # A header without a content type has none.
+    assert content_type("<|channel|>commentary to=functions.bash") is None
+    assert content_type(" to=functions.bash<|channel|>commentary") is None