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php-xdebug-mcp

A production-grade Model Context Protocol server that exposes Xdebug step debugging via DBGp as typed, safe, model-friendly tools and resources. Treat it as an IDE-grade debug adapter for AI coding agents — not as a raw protocol tunnel.

php-xdebug-mcp lets an AI agent attach to a live PHP process, set breakpoints by local workspace path, step through code, inspect stack frames and variables, and (when explicitly opted in) evaluate expressions against the running runtime. It works for plain CLI scripts, PHP-FPM behind nginx, queue workers, and PHP running inside Docker containers.

It is built on php-mcp/server and the Xdebug DBGp protocol.

Highlights

• 31 typed MCP tools covering session lifecycle, breakpoints (line / conditional / exception / call / return / watch), stepping, inspection, source retrieval, executable-line discovery, stdout/stderr capture, and Docker helpers.
• Adapter-level path mapping with rule-based prefix translation, exact-file overrides, URL encoding, Windows drive letters, and synthetic-frame normalisation (eval / internal / unknown).
• Inverse-mapping diagnostics: when a session connects with a fileuri no rule covers, the adapter attaches a structured PATH_RULE_MISSING warning to the session snapshot — including a best-effort suggested path_rules entry derived from your workspace roots.
• Safety modes (observe / control / full_control) implemented via static omission at registration time so tools/list reflects exactly what the current process can do — never deny stubs that show up just to refuse.
• Docker-aware: optional helpers php_debug_docker_exec and php_debug_docker_run build argv via the proc_open array form, validate identifiers conservatively, and default --add-host host.docker.internal:host-gateway so Linux containers reach the host listener.
• Stdio-clean: stdout is reserved for MCP traffic; logs go to stderr (or any configured PSR-3 sink). A regression test asserts this even on bootstrap failure.
• PHP 8.5 native: the codebase uses final readonly class for value objects, first-class callable syntax for handler registrations, clone($obj, [...]) for immutable updates, and array_find / array_any where they cleanly express intent. CI runs the Unit + Contract suites on PHP 8.5 against every push.
• Tested: 67 unit + contract tests, deterministic and Docker-free. Real-Xdebug integration scenarios are scaffolded as runnable examples.

Quickstart

1. Install

Requires PHP 8.5+. Install via Composer:

Or clone and use directly:

2. Configure Xdebug

Drop a 99-xdebug.ini into your PHP environment:

For Docker, replace 127.0.0.1 with host.docker.internal and add extra_hosts: ["host.docker.internal:host-gateway"] to your compose service. See docs/docker-recipes.md for the full story including the PHP-FPM clear_env trap and the Linux loopback gotcha.

3. Wire the MCP client

Point any MCP client at the binary over stdio. Examples for the most common clients live in examples/agent-clients/:

4. Drive a debug session

The agent's typical flow:

The full set of tools is described inline in docs/architecture.md.

Architecture

Three boundaries make Docker deployments work cleanly:

Long-form: docs/architecture.md.

Safety modes

xdebug_stop and xdebug_detach are independently gated by allow_stop and allow_detach. Docker helpers are gated by docker_helpers_enabled (default false).

Disabled tools are not registered with the MCP server, so tools/list accurately reflects the surface that the current process can use.

See docs/safety-modes.md.

Tools at a glance

Session — xdebug_server_status, xdebug_wait_for_session, xdebug_list_sessions, xdebug_get_session, xdebug_claim_session, xdebug_release_session.

Control — xdebug_continue, xdebug_step_into, xdebug_step_over, xdebug_step_out, xdebug_break_execution (async only), xdebug_wait_for_state, xdebug_stop, xdebug_detach.

Breakpoints — xdebug_set_breakpoint, xdebug_list_breakpoints, xdebug_update_breakpoint, xdebug_remove_breakpoint, xdebug_run_to_cursor.

Inspection — xdebug_get_stack, xdebug_get_contexts, xdebug_get_variables, xdebug_get_property, xdebug_get_source, xdebug_get_executable_lines, xdebug_get_typemap, xdebug_eval (full_control), xdebug_set_property (full_control).

I/O — xdebug_configure_output, xdebug_send_stdin (full_control).

Helpers — php_debug_run_cli, php_debug_http_request, php_debug_docker_exec (gated), php_debug_docker_run (gated).

Resources — xdebug://sessions, xdebug://session/{id}, xdebug://session/{id}/{stack,breakpoints,events,stdout,stderr}, xdebug://session/{id}/source/{path} template.

Configuration

config/php-xdebug-mcp.php ships with sensible defaults; environment variables override (XDEBUG_MCP_*). Notable knobs:

Documentation

• Architecture
• Protocol notes (DBGp)
• Xdebug setup (CLI / FPM / Docker / workers)
• Docker recipes — compose, FPM clear_env, Linux loopback gotcha
• Path mapping cookbook
• Session lifecycle / state machine
• Safety modes & dangerous tools
• Troubleshooting
• Security model
• Limitations and roadmap

Examples

Tests

CI runs Unit + Contract on PHP 8.5 against every push and PR to main.

Contributing

Bug reports and PRs welcome. Please:

• match the existing PSR-12 style (typed properties, single quotes, early returns, no else where early return reads cleaner);
• run the full test suite and add a focused test alongside any behavioural change;
• keep commits focused; the scope of composer.json is the source-of-truth dependency surface and we intentionally keep it small.

License

MIT.