BC Prompt Diff
Semantic diff for system prompts and agent instructions. Compare two versions of any SKILL.md, AGENTS.md, system prompt, or LLM instruction file and get a behavioral changelog — not just line diffs. Identifies added/removed/mutated instructions, detects contradictions introduced between versions, flags guardrail removals and permission expansions, measures instruction density change, and produces a commit-ready changelog entry. Zero external API — pure local file analysis. Triggers on "diff these prompts", "what changed in my system prompt", "compare prompt versions", "prompt changelog", "what did I change in AGENTS.md", "/prompt-diff".
As a process C 50/100 · Has gaps — weak spots: result and completion, inputs and preconditions, consistency
How to improve
- Say in the description WHEN to use the skill ("use when…", example requests): that is the agent's main cue.
- Your own cases (evals/evals.json, 4–6 real requests with expected answers): the full check would then run those instead of a model-drafted suite.
- A spec.yaml with trigger phrases and assertions — a behaviour contract for CI; `skilltest init` writes a template.
Guard findings · 0
✓ No critical or high findings
Files scanned: 2. Evidence is masked. Grey chips explain why severity was lowered.
Against the Agent Skills spec
- warning
name-formatname should be kebab-case (lowercase letters, digits, hyphens) - warning
description-no-whendescription does not say WHEN to use the skill (no "use when") - note
frontmatter-keyunknown frontmatter key "homepage"
Process rating: all ten parameters 50/100
- 0Result and completion. Does not say what the result is
- 0Inputs and preconditions. Does not say what the process needs to start
- 0Progress reporting. Says nothing while it works
- 30Running it twice. 10 mutating operations with no state check
- 40Consistency. Frontmatter name (Prompt Diff) differs from the folder (phy-prompt-diff)
- 55Failures and branches. 1 branches
- 60Tools and files. Uses tools (python) that frontmatter does not declare
- 70When it triggers. States when to use, but not when not to
- 100Steps. 32 steps
- 100Execution cost. Instruction body is 3151 tokens
- medium Safety rules and hard prohibitions inside a skill: they belong in the system prompt, here they protect nothing
- low 12 top-level sections: this looks like several domains in one skill
Everything here is measured from the skill text rather than judged by a model, so the numbers are checkable. A parameter weighs more when it is a more common reason for the process to stall.
Quality signals
- +4Description does not say when NOT to use the skill (false activations)
- +3Output format is not stated: the model decides each time
- +2Single-language instructions
- +5Description quotes 6 example trigger phrases
- +3Description length 644: enough signal without eating the budget
- +4Structure: 23 headings
- +3Step-by-step instructions: 32 items
- +4Has examples (11 code blocks)
- +1License stated
Quality base 70; lint remarks subtract, signals add up to 100. Result: 72.