# Tool limits

> The runtime, time, memory, size, network, concurrency and log limits that apply to tool packages and remote MCP tools.

These limits apply to every workspace. A call or build that exceeds one fails with the error code shown, which `caplo tool logs`, test output or the build log reports.

## Runtime

| Limit             | Value                                                                                                                                                                               |
| ----------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Runtime           | Node.js 24 (`runtime: node24`). Local `caplo tool test` and `caplo tool dev` also need Node 24.                                                                                     |
| Module format     | Your code is bundled into one ES module. Native `.node` addons are refused.                                                                                                         |
| Forbidden imports | `child_process`, `cluster`, `worker_threads`, `inspector`.                                                                                                                          |
| Environment       | No environment variables or secrets from the environment. Read secrets from the [call context](/tools/define-tool-and-context/#the-call-context).                                   |
| Memory            | Your workspace’s tools run in workers with a 256 MB JavaScript heap, shared by the packages loaded in that worker. Module-level state can persist between calls in the same worker. |

## Calls

| Limit               | Value                                                                                     | Error                       |
| ------------------- | ----------------------------------------------------------------------------------------- | --------------------------- |
| Timeout per tool    | 1 to 30 seconds, set by `timeoutSeconds` (remote MCP tools: set in review, 20 by default) | `tool_timeout`              |
| Result size         | 256 KiB, after any [files](/tools/present-results/#return-files) are taken out            | `output_too_large`          |
| Structured result   | Must match the tool’s `outputSchema`, if it has one                                       | `output_schema_violation`   |
| Files per result    | 3, each up to the tool’s `maxBytes`, at most 10 MiB; JPEG, PNG, GIF, WebP or PDF          | `output_file_invalid`       |
| Form questions      | 3 per call; up to 12 declared fields                                                      |                             |
| Concurrent calls    | Up to 16 of a workspace’s tool calls at a time                                            | `tool_concurrency_exceeded` |
| Write status checks | Up to 8 per write                                                                         |                             |

## Packages and builds

| Limit                            | Value                                                                                  | Error               |
| -------------------------------- | -------------------------------------------------------------------------------------- | ------------------- |
| Package name                     | 2 to 40 characters: lowercase letters, digits and dashes, starting with a letter       | `config_invalid`    |
| Tool name                        | Up to 48 characters: lowercase letters, digits and underscores, starting with a letter | `tool_name_invalid` |
| Model name (`<package>__<tool>`) | Up to 64 characters                                                                    | `tool_name_invalid` |
| Tools per MCP package            | 64                                                                                     | `config_invalid`    |
| Function package                 | Exactly one tool                                                                       | `schema_invalid`    |
| Description                      | 500 characters                                                                         | `config_invalid`    |
| Files in a draft                 | 1,000, with paths up to 512 characters                                                 | `too_many_files`    |
| File size                        | 1 MiB per file; 10 MiB for a lockfile                                                  | `file_too_large`    |
| Draft size                       | 5 MiB in total, not counting lockfiles                                                 | `draft_too_large`   |
| Bundle size                      | 10 MiB                                                                                 | `bundle_too_large`  |
| Dependency install               | 90 seconds                                                                             | `install_timeout`   |
| Build                            | 120 seconds                                                                            | `build_timeout`     |

## Network and secrets

| Limit               | Value                                                                                                                       |
| ------------------- | --------------------------------------------------------------------------------------------------------------------------- |
| Egress hosts        | Up to 50 per package. No list means no network access.                                                                      |
| Reachable addresses | Public addresses only. Private, loopback, link-local and cloud metadata addresses are refused even when the host is listed. |
| Secrets             | Up to 50 names per package; names up to 64 characters.                                                                      |
| Secret changes      | Live calls use a new, rotated or bound value within a minute.                                                               |

## Presentation

| Limit                           | Value                                   |
| ------------------------------- | --------------------------------------- |
| Confirmation and receipt fields | 12 each                                 |
| Labels and titles               | 80 characters; action labels 20         |
| `list`, `table`, `cards`        | 20 items                                |
| `choices`                       | 10 items                                |
| `comparison`                    | 4 items                                 |
| `timeline`                      | 12 steps                                |
| Progress label                  | 3 to 40 characters; spoken line 3 to 90 |

## Logs

| Limit                | Value                                                                              |
| -------------------- | ---------------------------------------------------------------------------------- |
| Retention            | Live invocations from the last hour, up to about the 1,000 most recent per package |
| Lines per invocation | 200; a line over 8 KiB is cut short                                                |
| `caplo tool logs -n` | At most 200 invocations at a time                                                  |