studio: deterministic backend tool-calling wiring test (#6836)

* studio: deterministic backend tool-calling wiring test

Add a deterministic, download-free test that exercises the shared tool-calling
seam both inference backends use. InferenceBackend (transformers) and
MLXInferenceBackend both render the prompt through
apply_chat_template_for_generation(..., tools=...) and stream cumulative text
into run_safetensors_tool_loop. The existing test_safetensors_tool_loop.py
covers the parser and the loop state machine with fake generators but does not
cover the backend's own tool-injection seam, so a regression that drops the
tool schema before the tokenizer, or fails to feed a tool result back into
generation, would slip through.

The test drives that seam with fakes: a tokenizer that records the tools it is
handed, a canned tool-call generation, and a stub executor. It asserts the full
chain: tools reach the chat template, the loop parses the call, the tool is
dispatched once with the parsed arguments, the result is fed back, generation
re-enters, and the final answer streams after the tool result. It also guards
that the raw tool-call markup never leaks to the client as content.

The test imports no torch, unsloth, or mlx, so it runs in the portable Backend
CI alongside the tool-call parser tests and stays sub-second. Follow-up to the
parser test PRs #5620 and #5704.

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* studio: assert the tool result is fed back before the final turn

Strengthen the wiring test so single_turn records each turn's conversation and
the test asserts the tool result message is present in the conversation handed
to the final generation turn. Event ordering alone did not catch a loop that
stops appending the tool output before re-entering generation, because the fake
generation ignores the conversation; this closes that gap.

* studio: tighten comments in tool-calling wiring test

* studio: shorten comments in tool-calling wiring test

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

studio/backend/tests/test_safetensors_toolcall_wiring.py

@@ -0,0 +1,179 @@
+# SPDX-License-Identifier: AGPL-3.0-only
+# Copyright 2026-present the Unsloth AI Inc. team. All rights reserved. See /studio/LICENSE.AGPL-3.0
+
+"""Deterministic backend-wiring test for the safetensors / MLX tool-calling path.
+
+The parser and the cumulative-text state machine are already covered exhaustively by
+``test_safetensors_tool_loop.py`` with fake generators. What that suite does not touch is the
+*backend's own tool-injection seam*: both ``InferenceBackend`` (transformers) and
+``MLXInferenceBackend`` render the prompt through the shared
+``apply_chat_template_for_generation(..., tools=...)`` helper and stream cumulative text into the
+shared ``run_safetensors_tool_loop`` (see ``core/inference/inference.py`` and
+``core/inference/mlx_inference.py`` -- both call the same helper and the same loop, so a single CPU
+test of that seam covers the macOS MLX path too).
+
+This test drives that exact seam with deterministic fakes -- a fake tokenizer that records the
+``tools`` it is handed, a canned tool-call generation, and a stub executor -- and asserts the full
+agentic chain end to end:
+
+    tools injected into the template -> loop parses the call -> tool dispatched once ->
+    tool result fed back -> generation re-entered -> final answer streamed.
+
+It is the deterministic, download-free stand-in for the real-model MLX / GGUF browser tool-calling
+end-to-end: it imports no torch / unsloth / mlx, so it runs in the portable Backend CI alongside the
+tool-call parser tests. Follow-up to the parser test PRs (#5620 / #5704).
+"""
+
+from core.inference.chat_template_helpers import apply_chat_template_for_generation
+from core.inference.safetensors_agentic import run_safetensors_tool_loop
+
+TOOL_NAME = "get_weather"
+TOOL_ARGS = {"city": "Paris"}
+FAKE_TOOL = {
+    "type": "function",
+    "function": {
+        "name": TOOL_NAME,
+        "description": "Get the current weather for a city.",
+        "parameters": {
+            "type": "object",
+            "properties": {"city": {"type": "string"}},
+            "required": ["city"],
+        },
+    },
+}
+# Full parser matrix lives in test_safetensors_tool_loop.py.
+TOOL_CALL_TEXT = '<tool_call>{"name": "get_weather", "arguments": {"city": "Paris"}}</tool_call>'
+FINAL_ANSWER = "The weather in Paris is sunny and 22C."
+TOOL_RESULT = "Paris: sunny, 22C"
+
+
+class RecordingTokenizer:
+    """Fake tokenizer that records the ``tools`` handed to ``apply_chat_template``.
+
+    Modelled on ``TestChatTemplateHelper._Tok`` in ``test_safetensors_tool_loop.py``: it accepts the
+    real helper's kwargs and returns a canned prompt, so the test can assert the backend seam actually
+    forwarded the tool schema -- a silent drop on a chat-template fallback would leave ``tools_seen``
+    holding ``None``.
+    """
+
+    def __init__(self):
+        self.tools_seen: list = []
+        self.call_count = 0
+
+    def apply_chat_template(
+        self,
+        messages,
+        *,
+        tokenize = False,
+        add_generation_prompt = True,
+        **kwargs,
+    ):
+        self.call_count += 1
+        self.tools_seen.append(kwargs.get("tools"))
+        return "PROMPT"
+
+
+class StubExecutor:
+    """Stand-in for ``core.inference.tools.execute_tool``: records calls, returns a fixed result.
+
+    A fake tool name plus this stub means no real python / terminal / web / RAG side effect can run.
+    """
+
+    def __init__(self, result: str):
+        self.result = result
+        self.calls: list[tuple[str, dict]] = []
+
+    def __call__(
+        self,
+        name,
+        arguments,
+        *,
+        cancel_event = None,
+        timeout = None,
+        session_id = None,
+        rag_scope = None,
+        disable_sandbox = False,
+    ):
+        self.calls.append((name, arguments))
+        return self.result
+
+
+def _collect(generator, max_events = 200):
+    events = []
+    for ev in generator:
+        events.append(ev)
+        if len(events) >= max_events:
+            break
+    return events
+
+
+def _tool_names(tools):
+    return [(t.get("function") or {}).get("name") for t in (tools or [])]
+
+
+def test_backend_seam_injects_tools_and_drives_full_tool_loop():
+    """The shared backend seam forwards tools into the chat template, and the loop parses the call,
+    dispatches it once, feeds the result back, and re-enters generation for the final answer."""
+    tok = RecordingTokenizer()
+    executor = StubExecutor(TOOL_RESULT)
+    turns = iter([TOOL_CALL_TEXT, FINAL_ANSWER])
+    active_tools_seen: list = []
+    conversations_seen: list = []
+
+    def single_turn(conversation, *, active_tools = None):
+        # Mirror the real _single_turn: render via the shared helper, then yield cumulative snapshots.
+        active_tools_seen.append(active_tools)
+        conversations_seen.append([dict(m) for m in conversation])
+        apply_chat_template_for_generation(tok, conversation, tools = active_tools)
+        text = next(turns)
+        mid = len(text) // 2
+        acc = ""
+        for chunk in (text[:mid], text[mid:]):
+            acc += chunk
+            yield acc
+
+    events = _collect(
+        run_safetensors_tool_loop(
+            single_turn = single_turn,
+            messages = [{"role": "user", "content": "What is the weather in Paris?"}],
+            tools = [FAKE_TOOL],
+            execute_tool = executor,
+            max_tool_iterations = 3,
+        )
+    )
+
+    # 1. Helper forwarded the tool schema to the tokenizer (seam does not drop tools).
+    assert tok.tools_seen, "tokenizer.apply_chat_template was never called"
+    assert tok.tools_seen[0], "tool schema was dropped before reaching the tokenizer"
+    assert TOOL_NAME in _tool_names(tok.tools_seen[0])
+
+    # 2. Loop offered the tool to the first generation turn.
+    assert active_tools_seen and active_tools_seen[0] is not None
+    assert TOOL_NAME in _tool_names(active_tools_seen[0])
+
+    # 3 / 4 / 5. Exactly one tool_start, one dispatch with parsed args, one tool_end with the result.
+    tool_starts = [e for e in events if e["type"] == "tool_start"]
+    tool_ends = [e for e in events if e["type"] == "tool_end"]
+    assert len(tool_starts) == 1 and tool_starts[0]["tool_name"] == TOOL_NAME
+    assert executor.calls == [(TOOL_NAME, TOOL_ARGS)], executor.calls
+    assert len(tool_ends) == 1 and tool_ends[0]["result"] == TOOL_RESULT
+
+    # 6. Final answer streams after the tool result: loop appended it and re-entered generation.
+    contents = [e for e in events if e["type"] == "content"]
+    assert contents and FINAL_ANSWER in contents[-1]["text"]
+    last_tool_end_idx = max(i for i, e in enumerate(events) if e["type"] == "tool_end")
+    last_content_idx = max(i for i, e in enumerate(events) if e["type"] == "content")
+    assert last_content_idx > last_tool_end_idx, "final answer must stream after the tool result"
+
+    # 6b. Tool result fed back into the conversation before the final turn (6 alone misses this:
+    #     the fake generation ignores the conversation).
+    assert len(conversations_seen) >= 2, "loop did not re-enter generation after the tool call"
+    final_turn_convo = conversations_seen[1]
+    assert any(
+        TOOL_RESULT in str(m.get("content", "")) for m in final_turn_convo
+    ), "tool result was not fed back into the conversation before the final generation turn"
+
+    # 7. Guard: raw tool-call markup never leaked to the client as content.
+    for e in contents:
+        assert "<tool_call>" not in e["text"]
+        assert TOOL_NAME not in e["text"]