Context limits

Context limits bound external instructions and metadata before fx adds them to a model request. Values are byte counts, not token counts.

Available limits

Name	Default	Bounds
`skill_description_bytes`	1 KiB	One discovered skill description
`skill_catalog_bytes`	16 KiB	Combined skill catalog
`skill_chunk_bytes`	20 KiB	One chunk loaded from a skill
`skill_file_bytes`	1 MiB	One skill file
`mcp_description_bytes`	1 KiB	One MCP tool description
`mcp_search_result_bytes`	16 KiB	MCP tool search results
`mcp_server_instructions_bytes`	2 KiB	Instructions from one MCP server
`mcp_selected_schema_bytes`	64 KiB	Schema for a selected MCP tool
`project_instruction_file_bytes`	64 KiB	One project instruction file
`project_instructions_total_bytes`	128 KiB	All applicable project instructions
`image_adapter_output_bytes`	20 KiB	Text produced by an image adapter

Settings

context_limits is supported in global and per-workspace entries in ~/.fx/settings.json. It is not accepted in project .fx.json.

{
  "context_limits": {
    "skill_catalog_bytes": 32768,
    "mcp_selected_schema_bytes": "off"
  },
  "workspaces": {
    "/absolute/path/to/project": {
      "context_limits": {
        "project_instructions_total_bytes": 262144
      }
    }
  }
}

A value is a non-negative integer or the string "off". "off" disables the normal limit, but fx still applies a 64 MiB emergency ceiling.

Command-line overrides

Use a leading, repeatable --context-limit flag:

$fx --context-limit skill_catalog_bytes=32768

$fx --context-limit mcp_description_bytes=off acp

Command-line values override workspace and global settings for the current process.

Larger limits increase request size

Raising or disabling a limit can increase latency and model input usage. Prefer the smallest value that preserves the instructions or schema you need.