Allow configurable inheritance of system prompt and memory for subagents

Description

Currently, subagents always inherit:

• The system prompt
• User memory (personal context)
• Project memory (project-level history)

This behavior leads to huge token overhead, even for very simple subagents.
In one test, a “tiny” subagent that only runs echo success consumed \~60k tokens because it inherited the full parent memory and context.

However, when the same subagent was tested in an environment with no user or project memory, it consumed only \~3k tokens — basically just the system prompt overhead.
This clearly shows how much token usage could be reduced if inheritance were configurable.

Developers need a way to explicitly configure what a subagent inherits, just like how tools are defined.

Here’s a polished description you can place below the “Description” section in the issue to explain the images clearly:

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To illustrate the problem, here are three test scenarios comparing agent-tiny and agent-mega:

• agent-tiny → can only use the Bash tool.
• agent-mega → can use all tools and MCP integrations.

Image 1: Clean environment (no extra context)

• Only the system prompt is present.
• Token usage stays very low (\~3k tokens).

<img width="1101" height="804" alt="Image" src="https://github.com/user-attachments/assets/971d9dba-7192-4819-a5b9-4ae28a86fa89" />

Image 2: MCP added

• MCP tools are included in the context.
• Token usage increases due to the MCP tool metadata.

<img width="712" height="495" alt="Image" src="https://github.com/user-attachments/assets/6a43cbed-76c3-4521-a893-02fa8d841635" />

Image 3: User memory + Project memory + MCP added

• Both user-level and project-level memory, plus MCP tools, are included.
• Token usage skyrockets (\~60k tokens or more) even for trivial subagent calls.

<img width="1093" height="1197" alt="Image" src="https://github.com/user-attachments/assets/a909db88-a619-4b5c-96a3-c5edd75239d2" />

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Steps to Reproduce

1. Create a simple subagent with:

``yaml
tools: Bash
``

1. Invoke the subagent and run /context.
2. Compare the token usage:

• With user/project memory: \~60k tokens consumed.
• Without user/project memory: \~3k tokens consumed.

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Proposed Syntax

tools: Read, Write, Bash
includes: System, User, Project, Subagent

Suggestion: Just like tools, allow a field such as includes or memories to clearly specify what context is inherited.

Descriptions:

• System: Inherit the parent agent’s system prompt (cannot be disabled if core behavior requires it).
• User: Inherit the user-level memory (personal context/history).
• Project: Inherit the project-level memory (project history and context).
• Subagent: Use independent, persistent memory specific to this subagent.

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Benefits

• Token efficiency: Prevent wasted tokens when subagents don’t need user or project context.
• Role isolation: Avoid “context pollution” in specialized subagents.
• Lightweight agents: Create extremely small and efficient agents (\~3k tokens usage).
• Flexibility: Still allow full inheritance when necessary for complex workflows.

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Related Issues

• \#4418 – Discusses subagent memory but doesn’t support explicit inheritance control.
• \#1770 – Focuses on monitoring and parent-child communication, not inheritance configuration.

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Summary

Introducing an includes (or memories) field — modeled after tools — would make configuration intuitive, consistent, and developer-friendly.
This change would let developers build lighter, isolated subagents with minimal token usage, while preserving full inheritance when complex workflows require it.