Tools and extensions

Enable executable capabilities, inspect their contracts and manage installed extensions.

Tools let an agent do more than generate text: retrieve information, inspect files, call a service, or perform an approved action. Mellow combines built-in capabilities, installed plugins, and connected MCP services in a catalog, then applies the current agent's exposure and permission rules.

The useful question is not “How many tools are installed?” but “Which operations can this agent use for this task, and under whose permissions?”

Choose the source of a capability

A skill is not a substitute for an unavailable tool. A tool description is not authorization to perform every action it describes.

Set up one capability at a time

Open Tools & MCP to inspect installed tools and services. Then open the agent's abilities and select the capabilities it needs. Read the operation description, required arguments, and permission policy before enabling broad automatic use.

Test a small read-only task first. For a write, prepare a reviewable change and inspect the approval and result. Verify the target resource after execution rather than relying only on the model's summary.

Tool permissions

A tool can be allowed automatically, require a decision, or be denied according to the controls applicable to it. Some built-in operations enforce additional approval rules at execution time. A broad automatic setting cannot remove every built-in restriction.

Permissions can be narrower for an external caller than for an attended local chat. Apple app tools and other host-only classes may be unavailable through an external MCP or HTTP client. An agent receiving a remote task must still satisfy the relevant host restrictions.

Review destination-specific controls for messaging. Channels have their own sending switch, readable and writable conversation lists, and proactive destination policies; turning on a generic tool is not enough to authorize a message.

Discovery during a conversation

Mellow can load capabilities relevant to the current task instead of placing every installed schema in every prompt. This reduces unnecessary context, but means installation count and current tool count can differ.

When a tool is missing, check its source, connection state, current agent settings, and discovery result. Give the agent a concrete task so the intended capability can be selected. If multiple MCP services expose the same operation name, use the explicit namespaced name shown by Mellow.

Inspect an operation

The model proposes a tool name and arguments. Mellow validates the call, applies permissions, executes through the owning implementation, and returns a structured result. An error can arise at any of these stages.

Use the tool feed and Insights to find the actual stage. Repeating a failed operation with the same arguments is rarely useful unless the failure was transient.

Install, update, and remove plugins

Choose a compatible package from the supported catalog or installation workflow. Inspect what it adds and whether it needs network access, local data, or additional credentials. Test after updating, because an implementation or schema can change while the display name stays the same.

Remove unused plugins and review their associated data or credentials. An imported bundle can own several resources; use its bundle-level management controls to avoid leaving an unexplained service or schedule behind.

For developers, use the current Mellow tool scaffold and its generated SDK interfaces. The scaffold is the authority for binary entry points and ABI types; do not derive symbol names from the public product name. A plugin must honor the current tool contract and lifecycle expected by the host.

Troubleshooting

Installed but not selectable: inspect plugin load errors and agent exposure.

Available locally but missing remotely: check the external deny rules; this can be intentional.

Permission keeps appearing: the operation may require approval every time, or a destination-specific gate may apply.

Wrong tool runs: inspect service namespacing and duplicate canonical names.

Long task times out: use an operation designed for that workflow rather than increasing every global timeout.

See Tool contract, Plugin authoring, and MCP services for deeper integration details.