[BUG] AgentSkills plugin destroys cache points in structured system prompts

[mattdai01]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.34.1

Python Version

3.10.16

Operating System

macOS

Installation Method

pip

Steps to Reproduce

1. Create an agent with a structured system prompt containing cache points:

from strands import Agent
from strands.vended_plugins.skills import Skill, AgentSkills

skill = Skill(name="my-skill", description="A skill", instructions="Do the thing")
plugin = AgentSkills(skills=[skill])

agent = Agent(
    system_prompt=[
        {"text": "Base instructions."},
        {"cachePoint": {"type": "default"}},
        {"text": "More instructions."},
    ],
    plugins=[plugin],
)

# Before any invocation, check:
print(agent._system_prompt_content)
# [{"text": "Base instructions."}, {"cachePoint": {"type": "default"}}, {"text": "More instructions."}]

# After the agent runs (triggering _on_before_invocation):
agent("hello")

print(agent._system_prompt_content)
# Expected: cache points preserved
# Actual: all structure collapsed into a single text block

Expected Behavior

_system_prompt_content should preserve all structured blocks including cachePoint entries after the AgentSkills hook injects skills XML. Cache points should continue to work for prompt caching optimization.

Actual Behavior

AgentSkills._on_before_invocation reads the system prompt as a string via agent.system_prompt, concatenates the skills XML, and writes it back as a plain string. The setter then converts this to [{"text": full_string}], destroying all cachePoint blocks and any other structured content.

After the hook runs, _system_prompt_content becomes a single flat text block with no cache points.

Additional Context

No response

Possible Solution

No response

Related Issues

No response