Claude Code skills can disappear when the system prompt gets too long
Claude Code assembles a list of installed skills and their descriptions into the system prompt so the model knows what tools are available. When that injected prompt grows too large — either from many skills or from lengthy description fields — it can get truncated. If a skill is omitted from the prompt, the model is instructed not to call skills that aren't listed, which effectively leaves those skills unused.
What’s happening under the hood
- Claude Code 2.0.70 enforces a limit on the combined length of skill and command descriptions: 15,000 characters (roughly 4,000 tokens).
- There is no in-app warning when the limit is exceeded, so skills can silently stop being available.
Immediate mitigation
A practical workaround is to increase the character budget via an environment variable. Running the client with:
SLASH_COMMAND_TOOL_CHAR_BUDGET=30000 claudewill roughly double the headroom for skill and command descriptions and restore availability for larger skill sets.
Longer-term fix in progress
Efforts are underway to reduce the system-prompt footprint by compressing and trimming skill metadata. The Superpowers 4.0 update aims to combine infrequently used skills and shorten multiple descriptions to avoid hitting the limit as often.
For developers managing many custom skills or long descriptions, the environment-variable workaround provides a quick stopgap until the upcoming optimizations are released. Read the full writeup and additional context on the original post: https://blog.fsck.com/2025/12/17/claude-code-skills-not-triggering/
