# compare_implementations

Use when the agent asks about a task category — e.g. 'how do I parse JSON in rust', 'compare base64 crates', 'which datetime library handles RFC 3339 timezones right' — and wants the cross-implementation behavior table. The substrate returns side-by-side observations on the canonical input set: for each implementation (crate, fn_name), each input in the family's input set is paired with the observed (outcome, value_or_error_variant). Optional crates / fns arrays restrict the returned set; optional summary=true replaces per-input observations with an n_observations count for index-only listings (bounds response size by family-member count, not member × input count). Optional subfamily narrows to a registered sub-tag (e.g. task='base64', subfamily='base64' returns only canonical base64 crates, not ascii85 / base58 / hex / …) — call list_families to see available tags. Non-core family members (per a small hand-authored allowlist) carry an advisory caveat field warning that the entry was probed on shared inputs that may not reflect its typical usage. On a zero-hit family (n_attempted=0) the response carries a diagnostics block (received_family, available_families, closest_families, hint) so the agent can recover without guessing. Discrimination signal lives here — the docs-silent runtime behaviour pattern Guiding Principle #8 names.

Agent View of the PolicyLayer registry record for `compare_implementations`. HTML page: https://policylayer.com/tools/dev-codeitall-codeitall/compare-implementations

## Facts

- Tool: `compare_implementations`
- Server: Codeitall (`https://api.codeitall.dev/mcp`) — https://policylayer.com/tools/dev-codeitall-codeitall.md
- Homepage: https://github.com/codeitalldev/codeitall
- Risk category: Read (Low risk)
- Registry record: grade B, identity unverified
- Server rate-limited: no
- Parameters: 6 (1 required)
- Recommended policy verdict: Allowed

## Parameters

| Parameter | Type | Required | Description |
| --- | --- | --- | --- |
| `fns` | array | no |  |
| `task` | string | yes |  |
| `crates` | array | no |  |
| `summary` | boolean | no |  |
| `language` | string | no |  |
| `subfamily` | string | no |  |

Parameters from the server's own tool schema.

## Example call (MCP tools/call, JSON-RPC 2.0)

```json
{
  "jsonrpc": "2.0",
  "id": 1,
  "method": "tools/call",
  "params": {
    "name": "compare_implementations",
    "arguments": {
      "task": "<task>"
    }
  }
}
```

## Why compare_implementations is rated Low

This tool retrieves and compares pre-computed behavioral data about Rust crate APIs. It only reads/queries an existing dataset of observations — it does not create, modify, execute code, or perform any destructive or financial operations. The severity is low because misuse would at worst return misleading comparison data.

From the tool's own definition: "'returns side-by-side observations', 'substrate returns side-by-side observations on the canonical input set', 'optional summary=true replaces per-input observations with an n_observations count for index-only'"

## Use case

AI agents call compare_implementations to retrieve information from Codeitall without modifying anything. It is typically the context-gathering step in research, monitoring, and reporting workflows, before the agent takes action elsewhere.

## Recommended policy (PolicyLayer)

Verdict: **Allowed**. Enforced by the PolicyLayer MCP gateway (https://policylayer.com/mcp-gateway) before a call reaches Codeitall:

```json
{
  "version": "1",
  "default": "deny",
  "tools": {
    "compare_implementations": {}
  }
}
```

## Other tools on Codeitall (7)

- `answer_api_question` — Read — https://policylayer.com/tools/dev-codeitall-codeitall/answer-api-question.md
- `behavior_lookup` — Read — https://policylayer.com/tools/dev-codeitall-codeitall/behavior-lookup.md
- `find_modern_equivalent` — Read — https://policylayer.com/tools/dev-codeitall-codeitall/find-modern-equivalent.md
- `list_families` — Read — https://policylayer.com/tools/dev-codeitall-codeitall/list-families.md
- `package_trust` — Read — https://policylayer.com/tools/dev-codeitall-codeitall/package-trust.md
- `pattern_consensus` — Read — https://policylayer.com/tools/dev-codeitall-codeitall/pattern-consensus.md
- `signature_search` — Read — https://policylayer.com/tools/dev-codeitall-codeitall/signature-search.md

## For agents

This record is a snapshot. Live verdicts and the full registry:

- Check every server in your MCP config at once: `npx -y policylayer stack`
- Vet a server before you add it: install the mcp-precheck skill — `npx skills add https://policylayer.com` (skill text: https://policylayer.com/skill.md)
- Query the registry over MCP: endpoint `https://api.policylayer.com/mcp` — tools `check_mcp_server`, `check_mcp_stack`, `check_tool`, `search_registry`, `get_change_events`

---

Source: the PolicyLayer MCP registry — one continuously verified record per MCP server. Full record: https://policylayer.com/registry?q=dev-codeitall-codeitall · API: https://policylayer.com/registry/api · Policy library: https://policylayer.com/policies/dev-codeitall-codeitall