Skills files got Claude Haiku to use Symfony's Mate in 15 of 20 test runs

An MCP client advertises the server's tools for you. The handshake calls tools/list, and whatever comes back sits in the model's context before the first prompt. In Hamdi Laadhari's deprecated bundle that whole path was one match statement over four methods: initialize, tools/list, tools/call and notifications/initialized ^[5]. Symfony announced the official PHP MCP SDK in September 2025, built with The PHP Foundation and Anthropic's MCP team ^[9], and Mate ran on it for roughly eight months ^[4]. The agent only knows a console command exists if something tells it ^[16].

So discovery moved into files. Mate installs SKILL.md files into .agents/skills/ and mirrors them into .claude/skills/, and they tell the agent when to reach for which tool ^[17]. The profiler skill is written as a triage procedure: filter for the failing request instead of scrolling, check which collectors the profile has, then read them in symptom order, exception first for a 5xx, then time, db and memory for a slow request ^[18].

Johannes Wachter, who made the change, measured only whether the agent touched Mate at all ^[19]. The two skills runs add to 15 of 20, three times the MCP server's tally ^[1].

Wachter said "transport alone wasn't the deciding variable. Visibility was." He also calls the result a pattern from a small sample, not a proven effect ^[21][22]. The metric is permissive too. It counts the agent opening the tool, not the agent finding the bug ^[19]. Two things have to hold before the ratio transfers to another codebase: the fault has to be visible in the profiler at all, and the client has to read the directory Mate writes skills into ^[17].

Symfony's spotlight on Mate opens on the premise Laadhari started from, that an agent can read all your code and still not know what happened on the last request ^[12]. His bundle exposed four tools with one-line descriptions, among them "Lists available profiler tokens." ^[24][4]. "That's the half I got wrong. I treated exposing the data as the product," he wrote ^[23]. Rereading the code for the deprecation notice, he found PHPStan reporting an always-true while loop on line 40 and an unreachable statement on line 61, on a CI step he had marked continue-on-error: true the day he set up CI ^[25]. The linter had been finding both since then.

Mate is past 100,000 installs a month ^[13]. The bundle finished with 11 stars, 2 forks and roughly 5,900 Packagist installs ^[6], which at Mate's rate is under two days of downloads ^[2].