Claude Code Plugin Reference

Claude Code Plugin Reference (night-market edition)

This is the domain pack for how Claude Code plugin machinery works in this repository. It covers manifests, skills, commands, agents, hooks, and the marketplace registry, with the local conventions layered on top. Every contract below was checked against a shipped plugin (mostly plugins/imbue, plugins/abstract, and plugins/herald) on 2026-07-02 at repo v1.9.15. When another night-market skill says "see the plugin reference for what a hook payload is," this is the file it means.

Vocabulary

Each term is defined once here and used without redefinition below.

Anatomy of a night-market plugin

Verified tree (imbue, trimmed):

plugins/imbue/
├── .claude-plugin/
│   ├── plugin.json       # Claude Code manifest: registers everything
│   └── metadata.json     # inter-plugin dependency manifest
├── openpackage.yml       # cross-framework export manifest
├── skills/<name>/
│   ├── SKILL.md          # hub document with frontmatter
│   └── modules/*.md      # spokes, loaded on demand
├── commands/*.md         # slash commands
├── agents/*.md           # subagents
├── hooks/
│   ├── hooks.json        # hook registration (auto-loaded)
│   ├── *.py, *.sh        # hook scripts
│   └── shared/           # vendored helpers (hook_io, vow_utils, ...)
├── scripts/              # Python utilities skills shell out to
├── tests/                # pytest suite, run per plugin
├── Makefile              # per-plugin test/lint/typecheck targets
├── pyproject.toml        # package config, per-plugin coverage threshold
└── README.md

plugin.json field contract

metadata.json

Second manifest in .claude-plugin/. Fields verified on imbue: name, version (lockstep), main, a skills subset, dependencies, provides (infrastructure/patterns/tools), and a claude block (skill_prefix, auto_load, categories). One trap: dependency ranges here use their own semver namespace. imbue declares "abstract": ">=2.0.0" while every plugin ships 1.9.15. The range is not compared against the marketplace version, so do not "fix" it to 1.9.15 during a bump. The update_versions.py bumper knows which fields to touch.

openpackage.yml

Cross-framework manifest consumed by the export pipeline (make cross-framework, scripts/clawhub_export.py). Lists the skills exported outside Claude Code and dependency sources in gh@athola/claude-night-market form. Its version is also lockstep.

SKILL.md contract

Frontmatter

Fields verified on plugins/imbue/skills/proof-of-work/SKILL.md. Only name and description are consumed by Claude Code itself. The rest are house conventions read by repo tooling.

Role taxonomy (from docs/skill-integration-guide.md):

Progressive loading (hub-and-spoke)

A skill's SKILL.md is the hub: it carries the always-loaded core and a routing table into modules/*.md spokes. The model loads a spoke only when the hub tells it to. This keeps the initial token cost near the estimated_tokens value instead of the whole directory. The pattern is specified in Skill(leyline:progressive-loading).

Required sections and ratchets

• Every new or modified SKILL.md needs a ## Exit Criteria section with concrete, observable, falsifiable checkboxes (.claude/rules/skill-exit-criteria.md). A pre-commit ratchet, scripts/check_skill_exit_criteria_drift.py, fails when the count of missing sections rises above scripts/skill_exit_criteria_baseline.json.
• Skill(plugin:name) references in prose are scanned by plugins/abstract/scripts/skill_graph.py. A second ratchet, scripts/check_skill_graph_drift.py, blocks commits that add dangling references (the bugs category) above scripts/skill_graph_baseline.json. When your change lowers the count, lower the baseline too so the ratchet tightens.
• CONSTITUTION.md rule 3 exempts skill prose from TDD but requires a structural validation test per new skill (constitution names the pattern test_skill_<name>.py; in practice imbue keeps them at tests/unit/skills/test_<name>.py, e.g. test_proof_of_work.py).

How Skill() invocation resolves

Skill(imbue:proof-of-work) resolves imbue to the plugin registered in the marketplace, then proof-of-work to the directory listed in that plugin's plugin.json skills array. If the directory or the manifest entry is missing, the reference is dangling and the skill-graph ratchet will flag it.

Commands

Frontmatter verified on plugins/imbue/commands/justify.md and plugins/sanctum/commands/fixit.md:

---
name: fixit
description: Fix broken functionality from pasted error output...
usage: /fixit <pasted-error-or-description> [--file <path>] ...
extends: "imbue:proof-of-work"
---

• name and description are required. usage documents the argument shape. extends (optional) names the skill the command routes into.
• $ARGUMENTS in the body is replaced with the raw argument string the user typed after the command (see fixit: "The input is $ARGUMENTS (a paste) or the contents of --file <path>").

When to write a command versus a skill:

The Exit Criteria rule does not apply to command files.

Agents

Frontmatter verified on plugins/imbue/agents/review-analyst.md:

---
name: review-analyst
description: Autonomous agent for conducting structured reviews with
  evidence gathering. Use when detailed code reviews requiring evidence
  trails... Do not use when quick code check without formal review...
model: opus
tools:
- Read
- Glob
- Grep
- Bash
skills: imbue:review-core, imbue:proof-of-work, ...
---

• The description drives dispatch: Claude decides whether to hand a task to the agent by matching it, so it must carry "Use when" and "Do not use when" clauses just like a skill description.
• tools is the allowlist the subagent gets. model pins the model tier.
• skills lists skills the agent should load. The body below the frontmatter is the agent's system prompt.

Hooks

The hardest-won knowledge in the repo. Read this whole section before writing or editing any hook.

Events registered in this repo

Survey of every plugins/*/hooks/hooks.json (2026-07-02):

Registration: hooks/hooks.json, never the plugin.json array

Claude Code auto-loads hooks/hooks.json from each installed plugin. Listing ./hooks/hooks.json in the plugin.json hooks array causes a "Duplicate hooks file" error at session start. The pre-commit guard scripts/check_plugin_hooks.py rejects any manifest that does it. The hooks array exists only for additional hook files beyond the auto-loaded default, and every plugin here keeps it [].

Shape (verified, herald):

{
  "hooks": {
    "Stop": [
      {
        "hooks": [
          {
            "type": "command",
            "command": "${CLAUDE_PLUGIN_ROOT}/hooks/double_shot_latte.py",
            "timeout": 10
          }
        ]
      }
    ]
  }
}

PreToolUse/PostToolUse entries add a matcher regex over tool names (imbue uses "Write|Edit|MultiEdit" and "Bash"). timeout is the per-hook budget in seconds. Always reference the script through ${CLAUDE_PLUGIN_ROOT}.

Payload contract: stdin JSON, never env vars

Claude Code delivers the payload as one JSON object on stdin with fields such as tool_name, tool_input, tool_response, session_id, and hook_event_name. It does NOT set CLAUDE_TOOL_NAME, CLAUDE_TOOL_INPUT, or CLAUDE_TOOL_OUTPUT environment variables. Hooks that read those env vars silently no-op on every real invocation. That exact bug left the skill-execution logger dead for months with no error anywhere (fixed in 1.9.14). Canonical read pattern:

import json
import sys


def main() -> None:
    try:
        payload = json.loads(sys.stdin.read())
    except (json.JSONDecodeError, OSError):
        sys.exit(0)  # fail open: a broken hook must not wedge the session
    tool_name = payload.get("tool_name", "")
    tool_input = payload.get("tool_input", {})
    command = tool_input.get("command", "")

The shared reader plugins/abstract/hooks/shared/hook_io.py (read_hook_payload()) implements stdin-first with a legacy env fallback for the synthetic test harness, and warns on stderr when stdin JSON is malformed. Because ADR-0001 forbids cross-plugin imports, leyline (hooks/noqa_guard.py) and sanctum (hooks/deferred_item_watcher.py) carry parallel copies that must change together (noted in the hook_io docstring). Hooks import their own plugin's shared/ sibling by inserting the script directory on sys.path first:

from pathlib import Path
sys.path.insert(0, str(Path(__file__).parent))
from shared.vow_utils import is_git_commit, shadow_mode_active

Output contract and exit codes

Repo convention is exit 0 always, with the verdict carried as JSON on stdout. Comments in plugins/imbue/hooks/tdd_bdd_gate.py record that exit code 2 has blocked the tool call since Claude Code v2.1.90, so a stray nonzero exit is itself a decision. Fail open on parse errors.

Human-facing diagnostics go to stderr, never stdout (stdout must stay parseable JSON).

Repo-local anomaly, do not copy: imbue's shadow-mode vow hooks (vow_no_ai_attribution.py) emit "warn" in shadow mode and "block" when VOW_SHADOW_MODE=0. Neither value is in the harness contract (allow/deny/ask), so the hook most likely fails open silently even with shadow mode off: the SB9 failure class from night-market-failure-archaeology. vow_bounded_reads.py in the same plugin correctly emits "deny". File an issue rather than imitating the "warn"/"block" output. (Anomaly flagged 2026-07-03.)

Timeout budgets

Any subprocess a hook spawns must finish inside the timeout registered in hooks.json, with headroom. herald once shipped an LLM call whose timeout exceeded its registered Stop-hook budget, so the harness killed the hook before any verdict was emitted (full record: night-market-failure-archaeology SB7). Copy the fix's pattern: assert subprocess_timeout < registered_budget in a guard test, not a comment.

Host Python 3.9 constraint

Plugin package code targets Python 3.12, but hook scripts run under the system interpreter, assumed 3.9. python39-compat.yml enforces two gates with uneven coverage: ruff UP007 under --target-version py39 (bare unions) runs on 12 plugins' hooks/**, while the hook test subtree in a real 3.9 venv runs on only 7 plugins (abstract, conserve, egregore, imbue, leyline, memory-palace, sanctum). herald ships a Stop hook yet is covered by neither gate.

The durable defense against datetime.UTC regressions is an AST-scanning test, not a lint rule: see plugins/leyline/tests/test_python39_compat.py, which walks the source tree and fails on any reintroduction. When a linter fights your fix, add an AST invariant test.

Cache-dir execution

Installed plugins run from the Claude Code plugin cache, not the repo. Consequences:

• No relative paths in hook scripts or hooks.json. Use ${CLAUDE_PLUGIN_ROOT} (config) or Path(__file__) (Python).
• No reaching into sibling plugins or repo-root scripts at runtime. Shared helpers are vendored per plugin: each hook-bearing plugin keeps a byte-identical copy of scripts/shared/json_utils.sh under its own hooks/shared/, and make check-json-utils fails on drift.

Testing hooks

Hook behavior is tested by piping a synthetic payload:

echo '{"tool_name": "Bash", "tool_input": {"command": "git commit -m x"}}' \
  | python3 plugins/imbue/hooks/vow_no_ai_attribution.py

Exit code and stdout JSON are the assertions. Hook test subtrees (for example plugins/imbue/tests/unit/hooks/) run under 3.9 in CI, so keep them stdlib-plus-pytest only.

Marketplace mechanics

• Registry: root .claude-plugin/marketplace.json. Top-level name, version, description, owner, and a plugins array of {name, source: "./plugins/<dir>", description, version, keywords}.
• Install: /plugin marketplace add athola/claude-night-market (README quick start), then /plugin install <name>@claude-night-market. The install form is documented in the book's getting-started pages; re-verify there if it fails.
• Version lockstep: one ecosystem version fans out from marketplace.json to every plugin's plugin.json, metadata.json, openpackage.yml, pyproject.toml, and package __init__.py. Bump with:

uv run python plugins/sanctum/scripts/update_versions.py 1.9.16 --dry-run
uv run python plugins/sanctum/scripts/update_versions.py 1.9.16

• Drift tooling: /sanctum:update-plugins [plugin] [--fix] audits plugin.json arrays against disk contents. make docs-sync-check (and the capabilities-sync.yml CI job) verifies book/src/reference/capabilities-*.md matches registrations. Fix drift with /sanctum:sync-capabilities --fix, never by hand-editing the generated capabilities files.

Where the authoritative docs live

• Repo book: book/src/ (mdBook, published by deploy-book.yml). book/src/reference/capabilities-*.md are generated by the sync tooling. Getting-started and plugin pages are hand-written.
• Local deep dives: docs/plugin-development-guide.md, docs/skill-description-guide.md, docs/skill-integration-guide.md. Note: the dev guide's make create-plugin NAME=... target does not exist in the root Makefile as of 2026-07-02 (stale doc). Scaffold with Skill(abstract:create-skill) and copy an existing plugin's layout.
• Upstream: https://code.claude.com/docs (linked from the dev guide and README badge). Consult upstream when the question is about harness behavior this repo only records as code comments: full event payload schemas, new hook events, or exit-code semantics changing across Claude Code versions.

When NOT to use

Exit Criteria

• Given a new hook idea, you register it by editing that plugin's hooks/hooks.json only, and python3 scripts/check_plugin_hooks.py exits 0 afterward.
• A hook you author reads its payload from stdin JSON (no CLAUDE_TOOL_* env vars) and exits 0 when fed malformed input: echo not-json | python3 <hook>.py; echo $? prints 0.
• Every subprocess timeout inside your hook is asserted in a test to be strictly less than the timeout registered in hooks.json.
• Your hook file starts with from __future__ import annotations, uses timezone.utc rather than datetime.UTC, imports no unguarded third-party package, and the python39-compat.yml gates pass.
• A new skill you add appears in its plugin's plugin.json skills array, its description is at most 160 characters, it has an ## Exit Criteria section, and both ratchet scripts (check_skill_graph_drift.py, check_skill_exit_criteria_drift.py) pass.
• You can state from memory why plugin.json hooks arrays are empty in this repo and what error appears if they are not.

Provenance and maintenance

Compiled 2026-07-02 against repo v1.9.15 (branch discussions-fix-1.9.14). PreToolUse verdict contract corrected to allow|deny|ask and the imbue vow-hook warn/block anomaly flagged on 2026-07-03. Volatile facts and how to re-verify them:

# Hook events per plugin
for f in plugins/*/hooks/hooks.json; do
  echo "$f"
  python3 -c "import json,sys; print(list(json.load(open(sys.argv[1]))['hooks']))" "$f"
done

# All plugin.json hooks arrays empty (exit 0 = clean)
python3 scripts/check_plugin_hooks.py
rg '"hooks": \[' plugins/*/.claude-plugin/plugin.json

# 160-char description cap still in force
rg -n 'DESCRIPTION_MAX' plugins/abstract/scripts/validate_budget.py

# Payload-on-stdin contract (read the module docstring)
head -15 plugins/abstract/hooks/shared/hook_io.py

# Exit-2 semantics comment
rg -n 'exit 2 blocks' plugins/imbue/hooks/tdd_bdd_gate.py

# permissionDecision values in repo hooks (deny is correct; the
# warn/block hits in vow_no_ai_attribution.py are the known anomaly)
rg -n '"permissionDecision"' plugins/*/hooks/*.py

# Herald timeout pairing (registered 10s vs internal 8s)
rg -n 'LLM_TIMEOUT_SECONDS|timeout' plugins/herald/hooks/

# py39 gates and covered plugins
head -50 .github/workflows/python39-compat.yml

# Version lockstep fields touched by the bumper
rg -n 'def main' -A 12 plugins/sanctum/scripts/update_versions.py

# Marketplace entry shape
head -60 .claude-plugin/marketplace.json

# Stale make create-plugin claim (doc says it exists, Makefile disagrees)
rg -n 'create-plugin' Makefile docs/plugin-development-guide.md

Unverified/candidate items are labeled inline: the /plugin install <name>@claude-night-market form (book-documented, not exercised here) and the explanation that metadata.json dependency ranges form a separate semver namespace (observed, not specified anywhere).

Source: athola/claude-night-market → .claude/skills/claude-code-plugin-reference/SKILL.md