fix(mcp): forward extra advertised input fields as tweaks during tool execution

[Juanmidev1]

Summary

Fixes #14002

When a flow is exposed as an MCP tool, json_schema_from_flow() advertises all visible input-node fields (e.g. backend_token, backend_url) in the tool's inputSchema. However, handle_call_tool() was constructing SimplifiedAPIRequest with only input_value and session_id, silently discarding every other argument supplied by the MCP client.

Root cause: handle_call_tool (line 313) called processed_inputs.get("input_value", "") without removing input_value, and never forwarded the remaining keys.

Fix: Pop input_value and session_id from processed_inputs first, then pass the remaining keys as tweaks to SimplifiedAPIRequest.

process_tweaks already handles a flat {field_name: value} dict and applies it to every node whose template contains a matching field. Nodes that don't have that field skip it silently (apply_tweaks, if tweak_name not in template_data: continue), so the change is safe for all existing flows.

Changes

• src/backend/base/langflow/api/v1/mcp_utils.py — 8 lines changed in handle_call_tool
• src/backend/tests/unit/api/v1/test_mcp_utils.py — 2 new tests added

Test plan

• uv run pytest src/backend/tests/unit/api/v1/test_mcp_utils.py -v — 21/21 pass
• test_handle_call_tool_forwards_extra_fields_as_tweaks — extra fields arrive as tweaks
• test_handle_call_tool_tweaks_none_when_no_extra_fields — backward-compatible: tweaks is None when only input_value is supplied
• Manual: create a flow with a custom input field, expose it via MCP, call the tool with that field — verify it reaches the component

Summary by CodeRabbit

• Bug Fixes

  • Tool calls now correctly forward additional input fields as execution settings.
  • Tool calls containing only the primary input continue to work without additional settings.

• Tests

  • Added coverage for forwarding extra tool inputs and preserving existing behavior when none are provided.

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Walkthrough

MCP tool execution now removes input_value from submitted arguments and forwards remaining fields as tweaks. Unit tests cover both extra arguments and calls with no additional fields.

Changes

MCP tweak forwarding

Layer / File(s)	Summary
Forward tool arguments and validate request construction src/backend/base/langflow/api/v1/mcp_utils.py, src/backend/tests/unit/api/v1/test_mcp_utils.py	execute_tool extracts input_value, forwards remaining arguments as tweaks, and sets tweaks to None when no extra fields exist. Tests cover both cases.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 9

✅ Passed checks (9 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: forwarding extra MCP inputs as tweaks during tool execution.
Linked Issues check	✅ Passed	The change addresses #14002 by forwarding advertised extra arguments to the flow and adding tests for both extra fields and empty tweaks.
Out of Scope Changes check	✅ Passed	The PR stays focused on MCP tool argument forwarding and matching unit tests, with no obvious unrelated changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Test Coverage For New Implementations	✅ Passed	Added backend regression unit tests in test_mcp_utils.py for extra-field forwarding to tweaks and None fallback, matching naming conventions.
Test Quality And Coverage	✅ Passed	New async pytest unit tests assert forwarded input_request data, cover extra tweaks and None fallback, and match backend test patterns.
Test File Naming And Structure	✅ Passed	The new backend test file follows pytest naming/layout, uses descriptive test names, isolates setup via a helper, and covers both positive and negative cases.
Excessive Mock Usage Warning	✅ Passed	Mocks are limited to external deps (simple_run_flow, session, graph), while assertions inspect forwarded_request.tweaks/input_value directly; not excessive.

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• Create PR with unit tests

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🧹 Nitpick comments (1)

src/backend/tests/unit/api/v1/test_mcp_utils.py (1)

535-578: 📐 Maintainability & Code Quality | 🔵 Trivial | 💤 Low value

Remove unnecessary @pytest.mark.asyncio decorators.

The repo is configured with asyncio_mode = 'auto' in pyproject.toml, so async test functions are auto-detected without explicit decorators. The @pytest.mark.asyncio decorators on lines 537 and 565 are redundant.

Based on learnings, in the langflow-ai/langflow repository, pytest-asyncio is configured with asyncio_mode = 'auto' in pyproject.toml, meaning @pytest.mark.asyncio decorators are unnecessary.

♻️ Remove redundant decorators

 `@pytest.mark.asyncio`
 async def test_handle_call_tool_forwards_extra_fields_as_tweaks(monkeypatch):

to:

 async def test_handle_call_tool_forwards_extra_fields_as_tweaks(monkeypatch):

and

 `@pytest.mark.asyncio`
 async def test_handle_call_tool_tweaks_none_when_no_extra_fields(monkeypatch):

to:

 async def test_handle_call_tool_tweaks_none_when_no_extra_fields(monkeypatch):

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@src/backend/tests/unit/api/v1/test_mcp_utils.py` around lines 535 - 578,
Remove the redundant `@pytest.mark.asyncio` decorators from
test_handle_call_tool_forwards_extra_fields_as_tweaks and
test_handle_call_tool_tweaks_none_when_no_extra_fields, leaving both async test
functions and their existing assertions unchanged.

Source: Learnings

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@src/backend/tests/unit/api/v1/test_mcp_utils.py`:
- Around line 535-578: Remove the redundant `@pytest.mark.asyncio` decorators from
test_handle_call_tool_forwards_extra_fields_as_tweaks and
test_handle_call_tool_tweaks_none_when_no_extra_fields, leaving both async test
functions and their existing assertions unchanged.

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📥 Commits

Reviewing files that changed from the base of the PR and between 7f28257 and f3899e9.

📒 Files selected for processing (2)

• src/backend/base/langflow/api/v1/mcp_utils.py
• src/backend/tests/unit/api/v1/test_mcp_utils.py