A MUST-have plugin

Automatically replaces text patterns in your prompts before they're sent to the LLM.

What It Does

Performs case-insensitive string replacements on user-typed prompts. The primary use case is auto-capitalizing RFC2119 keywords (MUST, SHOULD, MAY, etc.) in technical specifications.

Example: Typing "the system must validate input" becomes "the system MUST validate input".

Features

• Case-insensitive matching: must, Must, and MUST all match
• Word boundary aware: Won't replace may inside maybe
• Multi-word phrases: must not is matched as a unit (before must alone)
• Hot reload: Config changes take effect immediately (no restart needed)
• JSONC support: Comments and trailing commas allowed in config file

Scope

• Only replaces text in user-typed prompts
• Does NOT modify file content attached via @ mentions
• Does NOT modify slash command output

Configuration

Config file: ~/.config/opencode/MUST-have-plugin.jsonc

If the config file doesn't exist, it's automatically created with RFC2119 defaults.

Default Configuration

{
  // Uncomment to enable debug logging (logs appear in OpenCode's log file)
  // "debug": true,

  "replacements": {
    "must": "MUST",
    "must not": "MUST NOT",
    "required": "REQUIRED",
    "shall": "SHALL",
    "shall not": "SHALL NOT",
    "should": "SHOULD",
    "should not": "SHOULD NOT",
    "recommended": "RECOMMENDED",
    "not recommended": "NOT RECOMMENDED",
    "may": "MAY",
    "optional": "OPTIONAL"
  }
}

Custom Replacements

Add your own replacement pairs to the replacements object:

{
  // Uncomment to enable debug logging (view with: tail -f /tmp/opencode-replacer-debug.log)
  // "debug": true,

  "replacements": {
    "rfc": "The key words \"**MUST**\", \"**MUST NOT**\", \"**REQUIRED**\", \"**SHALL**\", \"**SHALL NOT**\", \"**SHOULD**\", \"**SHOULD NOT**\", \"**RECOMMENDED**\", \"**MAY**\", and \"**OPTIONAL**\" in this message are to be interpreted as described in RFC2119.\n\n",
    "dfp": "Diagnose and fix this problem:",
    "mnm": "Make no mistakes!",
"always": "**ALWAYS**",
    "may": "**MAY**",
    "must always": "**MUST ALWAYS**",
    "must never" : "**MUST NEVER**",
    "must not" : "**MUST NOT**",
    "mustn't" : "**MUST NOT**",
    "must": "**MUST**",
    "never": "**NEVER**",
    "not recommended": "**NOT RECOMMENDED**",
    "not": "**NOT**",
    "optional": "**OPTIONAL**",
    "ought": "**SHOULD**",
    "oughtn't": "**SHOULD NOT**",
    "recommended": "**RECOMMENDED**",
    "required": "**REQUIRED**",
    "shall not": "**SHALL NOT**",
    "shan'st": "**SHALL NOT**",
    "shall": "**SHALL**",
    "should not": "**SHOULD NOT**",
    "shouldn't": "**SHOULD NOT**",
    "should": "**SHOULD**",
}

Configuration Options

Option	Type	Default	Description
debug	boolean	false	Enable debug logging to OpenCode's log file
replacements	object	RFC2119 keywords	Key-value pairs for text replacement

Debug Logging

Logs are written to OpenCode's unified log file using the SDK logging system.

Log location: ~/.local/share/opencode/log/dev.log

Enable debug mode to see what replacements are being made:

1. Edit ~/.config/opencode/MUST-have-plugin.jsonc
2. Uncomment or add "debug": true
3. View logs in real-time (filtering by this plugin):

tail -f ~/.local/share/opencode/log/dev.log | grep "MUST-have-plugin"

Or view all recent plugin logs:

grep "MUST-have-plugin" ~/.local/share/opencode/log/dev.log | tail -20

Log Format

Logs use OpenCode's standard format with structured metadata:

INFO  2026-01-20T15:30:42 +2ms service=MUST-have-plugin Plugin loaded
INFO  2026-01-20T15:31:05 +5ms service=MUST-have-plugin Applied 3 replacement(s) replacements={"must":{"value":"MUST","count":2},"should":{"value":"SHOULD","count":1}}

RFC2119 Keywords

The default configuration includes all keywords from RFC 2119, which defines requirement levels for use in technical specifications:

Keyword	Meaning
MUST / REQUIRED / SHALL	Absolute requirement
MUST NOT / SHALL NOT	Absolute prohibition
SHOULD / RECOMMENDED	Recommended, but valid reasons may exist to ignore
SHOULD NOT / NOT RECOMMENDED	Not recommended, but may be acceptable in some cases
MAY / OPTIONAL	Truly optional

Troubleshooting

Replacements not working

1. Check that the config file exists: cat ~/.config/opencode/MUST-have-plugin.jsonc
2. Verify JSONC syntax is valid (comments and trailing commas are allowed)
3. Enable debug mode and check the log file

Unexpected replacements

• Replacements use word boundaries, so must won't match inside customer
• Multi-word phrases are matched first, so must not won't become MUST not
• Check for typos in your replacement keys

Config changes not taking effect

The plugin re-reads the config on every message, so changes should be immediate. If not:

1. Verify you saved the config file
2. Check for JSONC syntax errors
3. Restart OpenCode as a last resort