langchain-ai/deepagents · error · TypeError

Server '{server_name}' '{field_name}' must be a list of stri

Error message

Server '{server_name}' '{field_name}' must be a list of strings

What it means

When a server config defines `allowedTools` or `disabledTools`, the value must be a list containing only strings. `_validate_tool_filter_fields` raises this TypeError when the field is a non-list (e.g. a string or dict) or when any element is not a string. It's a config schema check that runs before any MCP server is contacted.

Source

Thrown at libs/code/deepagents_code/mcp_tools.py:1000

    has_disabled = "disabledTools" in server_config
    if has_allowed and has_disabled:
        error_msg = (
            f"Server '{server_name}' cannot set both 'allowedTools' and"
            " 'disabledTools' — pick one."
        )
        raise ValueError(error_msg)

    for field_name in ("allowedTools", "disabledTools"):
        if field_name not in server_config:
            continue
        value = server_config[field_name]
        if not isinstance(value, list) or not all(
            isinstance(item, str) for item in value
        ):
            error_msg = (
                f"Server '{server_name}' '{field_name}' must be a list of strings"
            )
            raise TypeError(error_msg)
        if not value:
            error_msg = (
                f"Server '{server_name}' '{field_name}' must be non-empty;"
                " omit the field to disable filtering."
            )
            raise ValueError(error_msg)


def _looks_like_comment(doc: str, lineno: int) -> bool:
    """Return `True` if the offending line *begins* with `//` or `/*`.

    Only the failing line is checked, and only its leading characters (after
    stripping indentation). A `url` value such as `"url": "https://..."`
    begins with a quote, not `//`, so a URL scheme inside a quoted string
    never triggers a false comment hint.

    Args:
        doc: Full source text that failed to parse.

View on GitHub (pinned to a1af029e6e)

Solutions

  1. Wrap the value in a JSON array of strings: "allowedTools": ["read_file"].
  2. Remove or stringify non-string elements (numbers, booleans, null) from the list.
  3. If the field is not needed, delete the key entirely rather than passing an empty or malformed value.

Example fix

// before
{"mcpServers": {"fs": {"command": "mcp-server-fs", "allowedTools": "read_file"}}}
// after
{"mcpServers": {"fs": {"command": "mcp-server-fs", "allowedTools": ["read_file"]}}}
Defensive patterns

Strategy: validation

Validate before calling

def check_filter_value(server, field):
    value = server.get(field)
    if value is not None and not (isinstance(value, list) and all(isinstance(i, str) for i in value)):
        raise TypeError(f"{field} must be a list of strings")

Type guard

def is_str_list(value: object) -> bool:
    return isinstance(value, list) and all(isinstance(i, str) for i in value)

Try / catch

try:
    load_mcp_config(path)
except TypeError as e:
    if "must be a list of strings" in str(e):
        repair_filter_field(path, server_name=e, field=...)
    else:
        raise

Prevention

When it happens

Trigger: Server config with a non-list value like "allowedTools": "read_file" (string instead of list), or a list with non-string entries like "disabledTools": ["write_file", 42] or ["tool", null].

Common situations: Hand-edited JSON where quotes around a single tool name were dropped; YAML-derived configs where a single-item list becomes a scalar; programmatic config generation that inserts tool IDs as integers; copying an object of tool->settings instead of an array of names.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

Related errors

AI-assisted analysis of langchain-ai/deepagents@a1af029e6e (2026-08-29). Data as JSON: /api/errors/b8845e8705af79cd. Report an issue: GitHub.