Replace an agent's system prompt with the full new prompt text to immediately change its workflow and instructions. Use this to correct or refine agent behavior.
Update an agent's system prompt / instructions. Pass the full new prompt text. This updates the agent's workflow with the new prompt.
| Name | Required | Description | Default |
|---|---|---|---|
| prompt | Yes | The new system prompt for the agent | |
| agent_id | Yes | The agent ID to update |
Changes observed during successful MCP inspections.
v0.1.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the burden of disclosing behavior. It states that the operation updates the agent's prompt and workflow, and the 'full new prompt text' wording indicates wholesale replacement. It does not mention permissions, side effects, reversibility, or return behavior, but for a simple setter the replacement semantics are the most important trait.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
The description is compact and front-loads the action and target. The final sentence is slightly redundant with the first, but the overall length is appropriate and there is little wasted text.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
For a simple two-parameter mutation tool, the description is mostly adequate: it names the target, the value to pass, and the effect. However, it lacks any mention of alternatives, return values, or edge cases, and with no annotations there is no safety or side-effect context beyond the basic replacement behavior.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
Schema description coverage is 100%, so both parameters are already documented. The description adds a small nuance by stressing 'full new prompt text,' which clarifies that the prompt parameter is a complete replacement, but it does not add substantial meaning beyond the schema.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description clearly states a specific verb and resource: 'Update an agent's system prompt / instructions.' This distinguishes the tool from general agent config tools, though it does not explicitly call out sibling tools such as update_agent_config. The phrase 'Pass the full new prompt text' also signals replace semantics.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
The description gives a practical usage hint: 'Pass the full new prompt text,' implying the caller must provide the complete replacement rather than a partial update. However, it provides no explicit guidance on when to use this tool versus alternatives like update_agent_config or when not to use it.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.