Mcp Tool

Capability: MCP tool (external server)

Tools served over the Model Context Protocol come from an external server that you do not own. In MCP's client–server split, the server defines each tool — its name, description, and inputSchema — and implements the handler; the host/client (your agent's runtime) discovers them via tools/list, presents them to the model, and invokes them via tools/call. You can change how the agent perceives and is offered those tools, but you cannot change their wire schema or implementation. This capability therefore permits only the safe subset.

This capability applies when the tools are served by an EXTERNAL server you cannot re-implement: the wire schema and handler code are owned by the server, so the action policy permits only documentation-level edits (descriptions, parameter docs, examples, add/remove from the exposed set).

What you can change here

Only client-side presentation — the server owns the wire schema and the code. You change how the agent perceives and is offered the tools, never the inputSchema, the handler, or any server-side annotation. Each lever below is a safe edit class. (1-line generic examples; depth in [references/concepts.md](references/concepts.md).)

1. Re-describe a tool — rewrite a terse server description into full

what / when / when-NOT / returns / limits the model reads to select. Ex: "kb search" → "Search the knowledge base; returns up to 10 article snippets."

1. Annotate per-parameter docs — pin format / units / caps in the description

of an existing field (not its type). Ex: add "(server caps at 10)" to a limit param.

1. Add in-description examples — show a concrete well-formed call so the model

fills arguments correctly. Ex: add get_record(record_id="A-1042").

1. Curate the exposed set (add / remove) — hide confusing / overlapping

tools so the needed ones stand out, or expose a server tool the host isn't surfacing. Ex: remove three legacy export tools the agent never needs.

> NOT editable here: the wire inputSchema (schema), the handler (code), > and adding server-side logic (compose) — those belong to the server and are out of > scope for this capability. Document only > what the server actually supports (don't overpromise filters/limits it ignores), > and treat server-supplied descriptions/annotations as untrusted input.

When to use this

Reach for mcp-tool when the agent is wired to an external MCP server and a trace shows:

• Mis-selection of an MCP tool — the server-provided description is terse or

ambiguous, so the model picks the wrong tool or none. You can re-describe it on the client side.

• Bad argument-filling — the model fills the server's inputSchema wrong.

You can't change the schema, but you can add a clearer per-parameter description and concrete examples that the client surfaces alongside it.

• A noisy exposed set — the server offers 40 tools and the agent only needs 6;

the extras distract it. You can hide tools from the model (expose a subset).

• A useful tool the host isn't surfacing — add it to the exposed set.

If you find yourself wanting to change a tool's types, required fields, or behavior, you've outgrown this capability: either negotiate the change with the server owner, or move the logic into an agent-owned tool (a different capability — out of scope here).

What can be optimized (default policy)

| Action | Allowed? | Why | |--------|:--:|-----| | description / params / examples | yes | client-side documentation the model reads to select & fill | | add | yes | expose another server tool to the model | | remove | yes | hide a confusing/redundant tool from the model | | schema | no | the MCP server defines the wire inputSchema | | code | no | the server owns the implementation | | compose | no | you can't add server-side code (composing agent-side is a different capability) |

apply() refuses schema/code/compose by default and reports each refusal — so an edit the optimizer "wanted" to make but couldn't is visible, not silent. If your MCP client genuinely supports client-side schema overrides or annotations, widen inputs/policy.json deliberately and document why.

How agents consume MCP tools

Mechanically identical to native function calling. The host fetches every connected server's tools, combines them into one registry, and injects each tool's {name, description, inputSchema} into the model's context. The model then selects from name + description and fills arguments from the JSON-Schema inputSchema (plus any examples). The only difference from agent-owned tools is the ownership boundary on what you may edit — so the optimizer's job here is purely (a) better client-side documentation and (b) a cleaner exposed set.

MCP also lets a server change its tool list at runtime and notify clients via notifications/tools/list_changed. Treat add/remove here as your curation of which of the available tools the model sees, not as a change to the server.

Concrete before/after

Re-describe a terse server tool (client side). The server ships "description": "kb search". The model can't tell when to use it.

- "description": "kb search"
+ "description": "Search the internal knowledge base and return matching article
+   snippets with their URLs. Use when the user asks a how-to or policy question
+   that is likely documented. Returns at most 10 hits; refine the query if empty."

Pin a parameter's format without touching the schema. The schema says {"limit": {"type": "integer"}}; the model sends 1000 and the call fails.

  "parameters": { "type": "object", "properties": {
-   "limit": { "type": "integer" }
+   "limit": { "type": "integer", "description": "Max hits to return (server caps at 10)." }
  } }

(You annotate the description of the existing schema field — allowed under params — rather than changing its type or adding maximum, which would be a forbidden schema edit.)

Trim the exposed set. Hide three rarely-correct, easily-confused tools so the six the agent actually needs stand out:

[ { "tool": "legacy_export_v1", "kind": "remove" },
  { "tool": "legacy_export_v2", "kind": "remove" },
  { "tool": "debug_dump",       "kind": "remove" } ]

Tool annotations (behavior hints the server supplies)

An MCP tool may carry annotations — server-supplied behavior hints the host/model can read for UX and gating. The four standard hints and their defaults:

| Annotation | Meaning | Default | |------------|---------|:-------:| | readOnlyHint | the tool does not modify its environment | false | | destructiveHint | may perform destructive/irreversible updates (only meaningful when not read-only) | true | | idempotentHint | repeated identical calls have no additional effect | false | | openWorldHint | interacts with an external/open world (e.g. the web) | true |

Use these to drive gating and presentation — e.g. confirm before a destructiveHint:true call, allow safe retries on idempotentHint:true. But they are hints, and UNTRUSTED unless the server is trusted: never rely on them for safety decisions a malicious server could subvert. They are not editable here (the server owns them); read them, don't trust them blindly.

Human-in-the-loop on sensitive calls

The MCP spec says clients SHOULD show the tool inputs to the user before invoking and confirm sensitive / destructive operations, so a poisoned description or a list_changed-injected tool can't silently exfiltrate or act. A safe consumer-side practice the optimizer can document/encourage in descriptions: state that a tool is destructive and that its inputs should be reviewed first.

Errors are a steering surface (execution vs protocol)

MCP separates two error kinds. A tool-execution error is a normal result with isError: true and an actionable message ("departure date must be in the future; current date is 2026-06-20") — the client surfaces it to the model so it can retry with adjusted args. A protocol error is a JSON-RPC failure (bad method, malformed request) the model can't act on. Prefer/encourage the self-correcting execution error: when you can only re-describe (not change the handler), document the failure mode in the description so the model self-corrects, and expect the host to surface isError results back to the model rather than swallowing them.

Failure modes to avoid

• Documenting behavior the server doesn't actually have. A client-side

description that overpromises (claims filters or limits the server ignores) causes confident wrong calls. Describe only what the server's schema supports.

• Hiding a tool the agent needs in rare cases. As with native tools, remove

for overlap/confusion, not low frequency.

• Trusting server-supplied metadata blindly. Tool descriptions from an

external server are untrusted input. A malicious or compromised server can hide instructions in a description ("tool poisoning") that the model reads but the user never sees, or use list_changed to slip in new tools. Review descriptions you expose; the MCP spec itself marks tool annotations untrusted unless the server is trusted.

• Trying to widen the schema here. If the model genuinely needs a constraint

the schema lacks (an enum, a max), that's a server change or an agent-owned wrapper — not an mcp-tool edit.

The action policy (safety knob)

The restricted default policy is the point of this capability: it encodes the ownership boundary of MCP. An optimizer can polish how tools are described and which are exposed, but the wire contract and implementation stay with the server. Keep the policy tight unless your specific MCP client supports more.

Artifact + handlers

tools.json — the exposed MCP tool defs {name, description, parameters, examples}. scripts/abstract.py: materialize (flatten to text components), apply (restricted policy; reports refusals), validate (well-formedness, dup names, empty descriptions).

Optimizing it each iteration (analyze → ideate → edit)

The optimizer should analyze before editing: from the traces, identify (a) the recurring mis-selection / bad-argument failures clustered by root cause and (b) the good behavior seen only on some trials to make consistent; then make ONE targeted SAFE edit — tool/parameter documentation, an in-description example, or adding/removing a tool from the exposed set (never the wire schema or handler) — that fixes the biggest cluster and reinforces (b). Be economical: one good edit, then stop.

How to run

python scripts/check.py
python scripts/run.py --path 

References

• [references/concepts.md](references/concepts.md) — the MCP model, the

ownership boundary, and the security trust model, with cited sources.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: skillberry-ai
• Source: skillberry-ai/cap-evolve
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.