Incremental repository sync fails when a commit changes files in a disabled language (e.g. YAML) #111

Summary

repository sync (incremental) fails with Sync failed: Staged rebuild failed (RuntimeError) whenever the commit range touches a tracked file whose language has no enabled plugin. YAML is disabled-by-default (see docs/SUPPORT_MATRIX.md), so any range that changes a workflow or Compose .yml file breaks incremental sync. The full-index path handles the same files without problems.

Found on the dev0 pilot (#107) while syncing from 85da6e1 to 1a61533. That range changes 12 tracked .yml files. Main at 1a61533 has the same code, so it isn't fixed there.

Root cause

• _incremental_index_update calls dispatcher.index_file for each changed file. For an unsupported language, EnhancedDispatcher._match_plugin raises RuntimeError("No plugin for …"), and the result is recorded as a failure (result.failed += 1, in mcp_server/storage/git_index_manager.py).
• UpdateResult.clean requires failed == skipped == 0. That makes _rebuild_repository_index_locked raise RuntimeError("Staged full index did not complete cleanly").
• The full-index walk (dispatcher_enhanced.py) classifies the same ERROR + "No plugin for " outcome as an ignored file, through _is_non_indexable_result. So the two paths disagree about the same file.

Secondary: the error message is lost

The except in _rebuild_repository_index_locked keeps only type(exc).__name__ ("Staged rebuild failed (RuntimeError)"). It drops both the message and the per-file UpdateResult.errors, and the CLI logs neither, so this took an isolated reproduction to diagnose.

Reproduction

Index a repository at commit A. Commit a change to a .yml file (plus any Python change) to get commit B. Then run repository sync --repo-id <id> without --force-full: it fails as above. With --force-full the same sync succeeds.

Suggested fix

• In the incremental mutation-result path, treat _is_non_indexable_result outcomes as ignored, the same way the full walk does. An in-process patch doing exactly that made the incremental sync succeed: 21 files, and the same file and symbol set as a full rebuild.
• Include the exception message (and a bounded summary of UpdateResult.errors) in the stored sync error and the CLI output.
• Add a regression test: an incremental sync whose diff includes a file in a disabled language.

Refs #107

🤖 Generated with Claude Code