check

A command that checks Mermaid diagrams, which are text-based diagrams embedded in Markdown files, for common syntax errors.

In plain words
What is it for?
Use it to check changed Markdown files, inspect one named file, scan all Markdown files, or automatically fix common Mermaid syntax problems.
Why use it?
It catches broken diagram notation that could prevent a Markdown renderer from displaying the diagram correctly.

Command

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add commands/shdennlin/agent-plugins/check
Clone the repo
git clone --depth 1 https://github.com/shdennlin/agent-plugins
Per session 9 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 746 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00009 $0.00746
Opus 5 $0.00005 $0.00373
Sonnet 5 $0.00002 $0.00149
Haiku 4.5 $0.00001 $0.00075

Measured yesterday against content hash 8918d9b09139, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

check scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured yesterday.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

plugins/mermaid-validator/commands/check.md · 111 lines

How it starts

The opening of the file, as written. The whole thing — 111 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Mermaid Check Command

Validate and fix Mermaid diagram syntax in Markdown files.

Arguments

  • [file] - Optional: Specific file to check (e.g., README.md)
  • --fix - Auto-fix common syntax errors
  • --all - Check all .md files in project (not just git changes)

Default behavior: Only check git changed/staged/untracked .md files.

Instructions

Step 1: Determine Files to Check

Based on arguments ($ARGUMENTS):

Argument Action
No args Get git changed .md files
filename.md Check only that file
--all Check all .md files

Git commands to find changed files:

git diff --name-only --cached --diff-filter=ACMR | grep '\.md$'  # staged
git diff --name-only --diff-filter=ACMR | grep '\.md$'           # modified
git ls-files --others --exclude-standard | grep '\.md$'          # untracked

Step 2: Find Mermaid Blocks

For each file, search for mermaid code blocks:

  • Pattern: ```mermaid ... ```
  • Record file path and line number of each block

Step 3: Validate Syntax

Check for common errors:

  • Unclosed brackets: A[Start --> B → should be A[Start] --> B
  • Missing graph type: No graph/flowchart/sequenceDiagram declaration
  • Invalid arrows: -> instead of -->
  • Unquoted special chars: A[Hello!]A["Hello!"]

If mmdc is available, use for deep validation:

echo "diagram content" > /tmp/test.mmd
mmdc -i /tmp/test.mmd -o /tmp/test.svg 2>&1

Step 4: Report Results

Show results in format:

📊 Mermaid Validation Results

❌ Error in file.md:15
   graph TD
       A[Start --> B[End]

   Problem: Unclosed bracket in node A
   Fix: A[Start] --> B[End]

Step 5: Fix (if --fix)

If --fix flag present or user requests:

  1. Propose the fix
  2. Ask for confirmation (unless obvious fix)
  3. Use Edit tool to apply
  4. Re-validate after fixing

Complex Operations

For --fix or --all flags, delegate to the mermaid-validator agent:

Read the full file on GitHub · 111 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. yesterday First seen · 111 lines · 9 tokens per session scan A 8918d9b09139

Subscribe to this mod's changes

check is a command published in the GitHub repository shdennlin/agent-plugins (2 stars, last pushed 7d ago), licensed MIT. It adds 9 tokens to every session and 746 once invoked, about $0.0000 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.