---
title: "Self-validation before printing"
description: "Why monedula-sim-link refuses to print a link it cannot prove safe: the lossless round-trip check, the 2048-character limit and the compressed form."
url: https://monedula.dev/flock/docs/sim-link/concepts/self-validation/
---
The playground's decoder drops malformed entries silently, so a subtly wrong link opens a session that looks fine and shows something other than your cluster. `connect`, `demo` and the MCP tools refuse to print a link that fails the checks below.

## The two checks

1. **The action log must round-trip.** The tool encodes the action log, decodes the result and encodes it again, and the two encodings must be identical. This check is never overridable: a link that would lose an action never ships.
2. **The link must be at most 2048 characters.** Browsers and chat clients truncate longer links, and a truncated `actions` parameter loses actions without any error. `--force` downgrades this check to a warning, for the case where you accept the risk.

When the second check fails, the message gives the length and the way out: lower `--max-records-per-partition`, narrow the topic selection, or pass `--force`. [Handle large clusters](/flock/docs/sim-link/how-to/handle-large-clusters/) shows the first two.

## Plain and compressed links

The tool keeps the action log in its plain, human-readable form while it is under the 1500-character compression threshold. Past that, the log is emitted in the compressed form, which starts with `~1`. A compressed link passes the checks like a plain one.

The playground writes compressed links in a different `~1` layout from the tool's, and decodes both. The tool decodes both as well, so `validate_actions` reads a link that the playground made.

## The MCP tools

The MCP tools validate every action field one by one, check `config_change` paths against the whitelist of the [playground URL format](/flock/docs/kafka-simulator/reference/url-format/), and enforce the action cap and the free-play size limits. They return no link unless its `actions` value decodes back to the identical list, and they return that decoded list next to the link, in `decodedActions`, so the caller can compare it with its input. A validation error names the offending field, separator or config path, so a calling agent can fix its input.

## Related

- [The MCP tools reference](/flock/docs/sim-link/reference/mcp-tools/) describes `validate_actions`, which reports what the playground would drop from a link you already have.
- The [`connect` command reference](/flock/docs/sim-link/reference/cli/connect/) lists `--force`.