`allowed_tools=[]` (empty list) is treated as falsy, conflating "no auto-allowed tools" with "unset"

[drillan]

Describe

When passing allowed_tools=[] to explicitly set no tools as auto-allowed (i.e., all tools should require permission prompting), the empty list is treated as falsy in Python, causing the --allowedTools flag to be omitted entirely from the CLI command. This makes allowed_tools=[] indistinguishable from allowed_tools=None (unset).

Note: --allowedTools controls which tools execute without prompting for permission, not which tools are available. To restrict tool availability, use --tools instead (see CLI reference).

Location

In subprocess_cli.py, the condition uses a truthiness check:

if self._options.allowed_tools:
    cmd.extend(["--allowedTools", ",".join(self._options.allowed_tools)])

Since [] is falsy in Python, this condition evaluates to False, and --allowedTools is never passed to the CLI subprocess.

Expected behavior

allowed_tools=[] should mean "no tools are auto-allowed — all tools require permission prompting."

The distinction between None (not specified / use defaults) and [] (explicitly no auto-allowed tools) should be preserved.

Suggested fix

if self._options.allowed_tools is not None:
    cmd.extend(["--allowedTools", ",".join(self._options.allowed_tools)])

Impact

This bug affects any use case where the caller wants to ensure all tools require explicit permission prompting. Instead of enforcing permission prompts for every tool, the --allowedTools flag is silently omitted, leaving the default permission behavior unchanged.

Reproduction

from claude_agent_sdk import AgentOptions

options = AgentOptions(allowed_tools=[])
# When building the CLI command, --allowedTools is silently omitted
# allowed_tools=[] is indistinguishable from allowed_tools=None (unset)

[EdanStarfire]

I think this is a weird naming challenge, so it depends on what you're intending here.

If you're intending allowed_tools to prevent access to tools through omission, that's not quite how I believe it works. This is "what tools will be auto-allowed to run" settings. There's a separate --tools parameter you might be wanting instead. --tools is configured right above the change you made in your PR in subprocess_cli.py, and this specifically is intended to limit the total set of tools available to the agent, so if you want to say "Read" is not allowed to even be present, regardless of allow/deny permissions, for example, this is how you would do it. That'll prevent skills from being able to use that tool even if they have Read in the allowed_tools in their frontmatter.

This is documented in the CLI reference with clarification:

• --allowedTools: Tools that execute without prompting for permission. See permission rule syntax for pattern matching. To restrict which tools are available, use --tools instead
• --disallowedTools Tools that are removed from the model’s context and cannot be used
• --tools Restrict which built-in tools Claude can use (works in both interactive and print modes). Use "" to disable all, "default" for all, or tool names like "Bash,Edit,Read"

I believe the last one gives you the behavior you're looking for.

[drillan]

@EdanStarfire Thanks for the clarification! You're right — I was conflating --allowedTools (permission prompting control) with --tools (tool availability).

I've updated the issue title/body and the PR #524 description to reflect the correct semantics. The underlying bug (None vs [] conflation due to Python's truthiness check) is still valid, so the PR is maintained with corrected wording.