Bug: Setting skills=[] does not opt agent out of default skills

[Qasimk555]

Bug Description

Setting skills=[] on an agent decorator does not prevent the agent from receiving default skills. The agent still gets execute (bash) and read_skill tools attached.

Root Cause

There are two fallback mechanisms in the SDK that apply default skills:

1. fastagent.py (line 947-949)

if config_obj.skills is None:
    if not resolved:
        resolved = list(default_skills)

✅ Setting skills=[] bypasses this (since [] is not None)

2. mcp_agent.py (line 116-121)

manifests: list[SkillManifest] = list(getattr(self.config, "skill_manifests", []) or [])
if not manifests and context and context.skill_registry:
    try:
        manifests = list(context.skill_registry.load_manifests())

❌ Setting skills=[] does NOT bypass this because an empty list is falsy (not [] is True)

Impact

• When skills=[] is set, skill_manifests becomes []
• not [] evaluates to True
• The fallback loads skills from context.skill_registry
• This enables shell runtime (execute tool) and skill reader (read_skill tool)

For agents using @fast.agent() without a custom class, there is no way to opt out of skills.

Expected Behavior

Setting skills=[] should explicitly disable skills for an agent, preventing:

• Shell runtime activation
• execute tool attachment
• read_skill tool attachment

Proposed Fix

In mcp_agent.py, check if skills was explicitly configured before applying the fallback:

manifests: list[SkillManifest] = list(getattr(self.config, "skill_manifests", []) or [])
# Only fall back to global registry if skills was NOT explicitly configured
skills_explicitly_configured = self.config.skills is not None
if not manifests and not skills_explicitly_configured and context and context.skill_registry:
    try:
        manifests = list(context.skill_registry.load_manifests())
    except Exception:
        manifests = []

Reproduction

from fast_agent.core.fastagent import FastAgent

fast = FastAgent("test")

@fast.agent(
    name="minimal_agent",
    instruction="You are a minimal agent.",
    skills=[],  # Explicitly opt out of skills
)
async def minimal():
    pass

# When running, minimal_agent still has execute and read_skill tools attached

Environment

• fast-agent-mcp version: 0.4.16
• Python version: 3.13+

[Qasimk555]

Additional Observation

When setting skills=[] on an agent:

What happens:

1. Skills do NOT appear in system prompt - The {{agentSkills}} placeholder (if present) is empty or not populated
2. BUT execute and read_skill tools ARE still attached - These tools exist only to support skill execution

Inconsistent state:

Setting	Skills in prompt	Tools attached
skills=[]	❌ None	⚠️ execute, read_skill

Verification from SDK source:

The execute tool is NOT attached to all agents by default. It is only attached when:

1. --shell CLI flag is provided, OR
2. skills_configured = bool(self._skill_manifests) is True

The bug causes skills_configured to become True due to the fallback loading skills from the global registry, even when skills=[] is explicitly set.

Code path (mcp_agent.py):

# Line 134-145
shell_flag_requested = bool(context and getattr(context, "shell_runtime", False))
skills_configured = bool(self._skill_manifests)  # TRUE due to fallback bug
...
elif skills_configured:
    self._shell_runtime_activation_reason = "because agent skills are configured"

This causes ShellRuntime.enabled = True and creates the execute tool, even though:

• The user explicitly set skills=[]
• No skills appear in the system prompt
• The tools serve no purpose without skills

Expected behavior:

When skills=[]:

• skills_configured should be False
• No execute tool
• No read_skill tool
• Consistent with the empty skills in the prompt

[iqdoctor]

Thanks! I opened PR #588 - Respect skills=None as explicit disable implementing a SKILLS_DEFAULT sentinel. Now skills=None means disable for that agent, while the default sentinel keeps current behavior (load from context/global registry). Let me know if you want any tweaks.

[iqdoctor]

Discussion: https://discord.gg/xg5cJ7ndN6

[iqdoctor]

PR #588 Merged.

[iqdoctor]

@Qasimk555 pls check solution

[Qasimk555]

@iqdoctor Working!