Pw Systematic Debugging

ProcessWire Systematic Debugging

Random fixes waste time and mask underlying issues. ProcessWire notoriously swallows exceptions inside hooks and template renders, making "guessing the bug" a guaranteed failure.

Phase 1: Context & Log Gathering (Mandatory)

Before running any fix, you must retrieve the actual error stack.

1. Read System Logs (MCP Server) ProcessWire stores fatal errors and exceptions in textual log files (site/assets/logs/).

   • Run MCP tool: pw_system_get_logs to list available logs.
   • Run MCP tool: pw_system_logs_tail_last on errors or exceptions.
   • Never skip past stack traces. Look precisely at the failing file and line number.

2. Verify Debug Mode If logs are empty but the system is failing (e.g. HTTP 500 or Blank White Screen), ensure debug mode is on. Check site/config.php for $config->debug = true;.

3. Hook Tracing If an event isn't firing (e.g., Pages::saved), ensure your Hook isn't intentionally bypassing execution or failing silently. Insert explicit logging right inside the hook temporarily:

   $event->wire()->log->save('debug', "Hook fired for page ID: " . $event->arguments(0)->id);
   

Phase 2: Hypothesis Formation

Once evidence is gathered from the logs, form a specific hypothesis.

• Example: "The log says Method name() does not exist or is not callable in this context. This means $pages->get('template=foo') returned a NullPage, and we didn't verify $page->id before interacting with it."

Phase 3: Minimal Fix & Verification

• Apply the SMALLEST possible fix to test the hypothesis (e.g., Wrap logic in if($page && $page->id)).
• Verify the fix using a ProcessWire validation command (pw_execute, CLI script, or reloading the endpoint).
• Do NOT bundle code refactoring ("Since I am fixing this bug, I will rewrite the whole hook class") with the bug fix.

Anti-Patterns & Loophole Closing

• ❌ Guess-and-Check: Proposing "Let's change this string/variable and see if it works" before reading errors.txt via MCP.
• ❌ Symptom Patching: Using @ (error suppressor) or burying problems in generic try-catch blocks without understanding why the ProcessWire API failed.
• ❌ Ignoring NullPage Logic: pw-expert rule violation. $pages->get() returns an empty NullPage object, not null or false. if($page === null) will fail. You must check if($page->id).
• ❌ Multiple Fixes at Once: Changing the Template, the Hook, and the Module Controller simultaneously. You are masking which layer actually caused the crash.

Triggering Execution

Once the root cause is successfully verified via minimal steps, either apply the fix immediately or invoke writing-plans if the refactor is substantial.