Mythos Harness — Development skill for Claude Code

MYTHOS Harness

**One file. Four rules. Opus 4.7 + MYTHOS Harness won 7 of 8 tasks against base Opus 4.7** in an 8-task pilot. No SDK, no runtime — paste one Markdown file into your coding agent's system prompt.

MYTHOS Harness is a prompt doctrine you paste into a coding agent's system prompt. Four short rules push the agent to **name the target before acting**, **tag claims as known / inferred / guessed**, **resist unrequested abstraction**, and **report what was actually run before declaring done**.

It ships no model, no runtime, and no SDK — just a single Markdown file. The clean proof artifact: **Opus 4.7 + MYTHOS Harness won 7 of 8 tasks (22.88 avg) vs base Opus 4.7 at 1 of 8 (21.00 avg)** in an 8-task run (see `benchmarks/`). The average-score gap is modest; the task-win flip is the louder signal. Small-sample and setup-dependent — re-run on your own suite before relying on it.

Core 8-task run. Same task family, same single-judge setup, same environment. Bronze = Opus 4.7 + MYTHOS Harness. Slate = base Opus 4.7.

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Why you might want this

Most coding-agent failures in practice are not reasoning failures. They are:

• Acting on the wrong target (wrong file, wrong caller, wrong symptom).
• Upgrading a guessed claim into a confident one by repeating it.
• Building an abstraction the code did not need.
• Declaring done on unverified output.

MYTHOS Harness is four short sections that push back on exactly those four patterns. No config, no knobs. Copy the file, paste it in, done.

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Quick start (30 seconds)

# Cla