Your MCP server changed last night. Did anything break for the agents using it?
MCP Regression Lab snapshots your server's tool contract every week, diffs it against the last one and tells you about breaking changes before your users' agents find them.
"name": "search_leads","inputSchema": {"properties": {"limit": { "type": "number" },"limit": { "type": "string" },"region": { "type": "string" }},"required": ["region"]"required": ["region", "limit"]}
What it catches
- Removed or renamed tools
- An agent keeps calling a tool that no longer exists.
- Argument type changes
- number became string, and every stored prompt breaks.
- New required arguments
- Calls that worked yesterday now fail validation.
- Required arguments made optional
- Quietly changes what agents think they must send.
- Description-only changes
- The schema is identical but the model now picks a different tool.
- Undocumented arguments
- The model has to guess what a field means.
- Suspicious descriptions
- Text that tries to instruct the model — a prompt-injection smell.
- Endpoint down or auth rejected
- Reported as a failed check, never as a clean one.
Contract diffs are deterministic: the same two snapshots always give the same answer. Description checks are heuristics and are labelled as such.
How it works
- 1
Add your endpoint
Paste the https URL of your remote MCP server. Add a bearer token if it needs one — it is encrypted at rest.
- 2
Get a baseline
We call tools/list and store the full contract. You see hygiene findings straight away.
- 3
Hear about breaks
Every week we snapshot again, diff it against your approved baseline, rerun your golden tests and post regressions to your Slack or Discord.
Prefer CI? The same engine ships as a GitHub Action that fails a pull request on breaking changes and can run tool-selection tests against your own local model.
Pricing
| Free | Monitor | |
|---|---|---|
| Price | €0 | €149 per server / month |
| Servers | 1 | As many as you subscribe for |
| On-demand checks | Yes | Yes |
| Weekly scheduled checks | — | Yes |
| Slack / Discord alerts | — | Yes |
| Snapshot history and reports | Yes | Yes |