MCP integration silently skipped on all JSON-config agents: 'json5' grammar no longer exists in tree-sitter-language-pack 1.6.2

[Mudit-Lal]

On semble 0.5.2, semble install reports the MCP server integration as skipped for every agent whose config is JSON:

Claude Code
  – IntegrationType.MCP (skipped) — JSON5 grammar unavailable — add manually (see README) → C:\Users\...\.claude.json
  ✓ IntegrationType.INSTRUCTIONS (updated) → C:\Users\...\.claude\CLAUDE.md
  ✓ IntegrationType.SUBAGENT (created) → C:\Users\...\.claude\agents\semble-search.md

So the two integrations that describe the MCP tools land, but the MCP server itself is never registered — the generated CLAUDE.md tells the agent to call mcp__semble__search, and that tool does not exist. The install looks like it succeeded (Done!, exit 0).

Root cause

semble/installer/config.py::_json5_parser does:

try:
    download(["json5"])
    _json5_parser_cache = get_parser(cast(SupportedLanguage, "json5"))
except Exception:
    _json5_parser_cache = None

with the comment "json5 ships in tree-sitter-language-pack but isn't in its typed language list". On the pinned tree-sitter-language-pack==1.6.2 that is no longer true:

>>> from tree_sitter_language_pack import get_parser
>>> get_parser("json")
<OK>
>>> get_parser("json5")
LanguageNotFoundError: Language 'json5' not found
>>> get_parser("jsonc")
LanguageNotFoundError: Download error: Language 'jsonc' not available for download. Available groups: ["all"]

json5 is not bundled and is not downloadable — the name isn't known to the pack at all (note it doesn't even produce the "not available for download" message that jsonc does).

Impact

This is not Windows-specific and not network-specific. On any OS, any install resolving tree-sitter-language-pack==1.6.2, the JSON5 parser is None, _json5_object returns "skipped", and merge_mcp never writes. That covers every agent using the JSON path — Claude Code, Cursor, VS Code, Windsurf, opencode. Codex is unaffected because it takes the TOML text path.

Worth confirming on CI: if the test suite has a pinned or cached grammar it may not surface there.

Workaround

Register the server manually. For Claude Code:

claude mcp add semble -s user -- uvx --from "semble[mcp]" semble

(I pointed it at the already-installed executable instead — claude mcp add semble -s user -- /path/to/semble — which avoids a uvx resolve on every launch, since bare semble with no subcommand starts the MCP server.)

Suggested fixes

1. Don't require a JSON5 grammar for this. These config files are JSON with occasional comments/trailing commas. A json5/pyjson5 pure-Python dependency, or json.loads with a comment-stripping fallback, would remove a tree-sitter grammar download from the install path entirely.
2. If the grammar approach stays, fall back to plain json when json5 is unavailable — .claude.json is valid strict JSON in practice, so the merge would succeed for most users.
3. Don't swallow the cause: except Exception hides LanguageNotFoundError, and the user-facing string "JSON5 grammar unavailable" reads like a transient download problem rather than a permanent dependency mismatch. Logging the exception would have made this a one-minute diagnosis.
4. Exit non-zero, or at minimum print a prominent warning with the exact claude mcp add command, when MCP is skipped but instructions/subagent were written — the current combination produces a config that references tools that don't exist.

Possibly the same underlying area as #224 (grammar download failures reported misleadingly and silently degrading behaviour), but the blast radius here is the MCP install path rather than chunking.