{"version":"2.1.299","anchor":"unloaded-deferred-mcp-tools-get-a-tool-not-loaded-validation","canonical_anchor":"unloaded-deferred-mcp-tools-get-a-tool-not-loaded-validation","heading":"Calling an unloaded browser or computer-use tool now returns a clear error","tier":"notice","area":"MCP","scope":"individual","heads_up":false,"url":"https:\/\/changelogs.core-directive.com\/v\/2.1.299\/e\/unloaded-deferred-mcp-tools-get-a-tool-not-loaded-validation","release_url":"https:\/\/changelogs.core-directive.com\/v\/2.1.299","markdown":"### Calling an unloaded browser or computer-use tool now returns a clear error\n\nIf the model calls a browser, computer use or simulator MCP tool before loading it, Claude Code now returns an error saying to load it first\n\n**Unclear.** It is not clear whether the new error is returned to the model for everyone or only recorded in some cases.\n\n**What**\n\nSome MCP tools are deferred: they are listed but not loaded until the model fetches them with the tool search tool. Calling a deferred tool before it is loaded already failed with an input error. For browser, computer use and simulator tools, the error now says that the tool was not loaded when it was called and tells the model to load it first. Claude Code also records when this check fails.\n\n**Why**\n\nThe model gets a clear signal about what went wrong and what to do next, which changes how its early calls to these tools fail and recover.\n\n- Area: MCP\n- Tier: You'll notice\n- Useful: 1\/5\n- Signal: 1\/5\n- Scope: individual\n- Heads-up: no"}