System Prompt

Capability: system prompt

The system prompt is the cheapest, highest-leverage parameter of most agents: a few words can flip success on a whole class of tasks. This capability treats one or more prompt/policy text files (prompt.txt, policy.md, SYSTEM.md) as the optimizable artifact.

What you can change here

The prompt is HIGH-VALUE to edit, not a last resort. When the traces show the agent doesn't know a rule, a format, or a decision criterion, a sharper prompt is the fastest fix. Each lever below is a safe, bounded edit class — pick the one that fixes the biggest failure cluster. (1-line generic examples; depth in [references/concepts.md](references/concepts.md).)

1. Rewrite a rule for clarity / positive framing — say what TO do, specifically.

Ex: "Don't be vague" → "State the record ID in every reply."

1. Add the WHY to a bare rule — append the reason so the model generalizes.

Ex: "Never use ellipses" → "Never use ellipses — output is read by a TTS engine that can't pronounce them."

1. Consolidate redundant rules — merge duplicates into one, keeping every

distinct constraint. Ex: three "confirm before deleting" lines → one "Confirm before any destructive action (delete, overwrite, send)."

1. Add a missing rule grounded in the source/policy — add a rule the source

requires but the prompt omits (must trace to a real source, never invented). The added rule must be ADDITIVE (a constraint/predicate the prompt lacked) or NARROWING (a stricter condition) — NEVER one that LOOSENS or broadens an existing permission/decision, which regresses the whole class. Ex: source says refunds need a manager code → "Require a manager code before any refund." (A narrowing add. Adding "anyone may refund without a code" would be a loosening — forbidden.)

1. Add an example — 1, up to 3–5, ``-tagged exemplars to pin format.

Ex: add one `` showing the exact JSON envelope expected.

1. Reorder — long context/data to the top, query + instructions last; group

mixed content under headers / XML tags. Ex: move a long reference block above the task instruction.

1. Add a role / goal line — one sentence on who the agent is and its objective.

Ex: "You are a careful support agent; resolve the request in one turn."

1. Tighten the output contract — make the required shape explicit and exact.

Ex: "Reply with only a JSON object {status, reason} — no prose." If the eval scores COMMUNICATION of computed values (totals, refunds, savings, counts, balances), the contract must require the agent to STATE each computed figure explicitly in its final message. The agent often performs the DB action correctly but never reports the number, and the eval marks the omission as a miss. This is a KNOWLEDGE / output-contract gap — prose-fixable here — and is DISTINCT from a missing DB action/write (a behavioral execution failure, which prose cannot fix and is out of scope for this capability). Ex: "After computing a total/refund/savings, state the exact figure in your final message (e.g. 'Your refund is $42.00')."

1. Soften over-strong language — downgrade CRITICAL/MUST/ALWAYS to "Use …

when …" when a cluster shows over-eagerness/over-triggering on current models. Ex: "CRITICAL: you MUST call the tool" → "Use the tool when you need live data."

> NEVER drop a needed rule — change, consolidate, or add; don't delete. When an > edit removes text, every distinct constraint that text carried must survive > somewhere (rewritten, merged, or relocated). Deletion is legitimate only when the > information is genuinely redundant or contradicted by the source — and even then, > prefer rewriting the conflicting rule. Consolidation cuts words, never rules.

The good practices, failure modes, and the full never-drop rule are in [references/concepts.md](references/concepts.md) — read it before a non-trivial edit.

What can be optimized

• Role line — a single sentence stating who the agent is. A known cheap win:

one role sentence focuses behavior and tone.

• Task framing — what "done" means.
• Output contract — exact format the downstream/eval expects (a frequent

silent failure: the agent is capable but formats wrong). Diagnose output-shape failures first — right content / wrong shape scores zero, and is the cheapest class to fix.

• Decision rules — when to call which tool, when to ask vs. act, refusal

rules (many agents are scored on adherence to such decision rules). DANGER — a global decision/permission/refusal rule has UNBOUNDED blast radius. Loosening or broadening one to fix a failing cluster (e.g. relaxing "restricted records may not be modified", or "any role may approve the request") flips behavior for the WHOLE class and regresses every currently-passing task where the original, stricter behavior was the gold answer. NEVER loosen a global decision rule to fix a cluster. If a cluster needs different behavior, state the EXACT discriminating CONDITION that NARROWS the rule to the qualifying cases only. A prompt edit to a decision rule is safe ONLY when it ADDS a predicate the agent lacked or NARROWS the rule — never when it broadens a permission or flips a decision the agent currently gets right.

• Few-shot exemplars / reasoning scaffolds — 3–5 diverse, relevant examples

wrapped in `` tags steer format (caveat: long example dumps can hurt reasoning models — keep it to a handful).

How to write the edit (authoring rules)

These are how to phrase a prompt edit so it actually changes behavior:

• State instructions positively — say what TO do, not just what not to do.

"Respond in flowing prose paragraphs" beats "don't use markdown." Positive phrasing gives the model a target; prohibitions only fence off one wrong path.

• Explain the WHY, not bare MUSTs. A rule with its reason generalizes; a bare

MUST/CRITICAL does not. "Never use ellipses" works far better as "your output is read by a TTS engine that can't pronounce ellipses." Teach the optimizer to write the reason, not the command.

• **Model-sensitivity caveat — sometimes the fix is to REMOVE or soften an

instruction.** Newer models over-comply: stale anti-laziness phrasing (CRITICAL/MUST/ALWAYS) now causes over-eagerness and over-engineering. Prefer plain "Use … when …". If a cluster shows over-doing rather than under-doing, the edit is to cut or soften an instruction, not add one.

• Ordering / structure. Put long context first and the query / output contract

last (end-placement can lift quality on long inputs); separate instructions / context / examples with lightweight `` tags so the model doesn't conflate them.

• Generalize, never hardcode. A prompt rule must state the GENERAL policy that

holds across the whole class of inputs, never a specific task's case or answer. Good: "Reverse the charge to the original payment method on file." Bad: "If the record id is ``, apply the exact amount that one task expects." Baking one task's id/value/date/answer into the prompt overfits, fails the held-out gate, and can mislead other tasks. Use a failing task's specifics only to understand the class, then write the general rule.

• Ground new rules in a source; don't fabricate. You MAY add a rule the source

or policy requires but the prompt omits (menu item 4) — that is a high-value edit. What you must not do is invent a normative rule, exception, or workaround that no source supports and that conflicts with the existing instructions. Trace every added rule to a real source (the policy doc, the runner, the benchmark spec). If two existing rules conflict, rewrite toward the more restrictive one rather than dropping either.

Prose fixes KNOWLEDGE gaps — behavioral failures are OUT OF SCOPE here

The system prompt is the right lever when a failure is a knowledge gap — the agent doesn't know the required output format, a decision criterion, or a rule. Telling it teaches it, and behavior changes. The prompt is the WRONG lever when a failure is behavioral — the agent already "knows" what to do (it analyzes, explains, even confirms) but then skips the action (e.g. stalls before issuing a write and stops). More prose does not fix a behavior the model already agreed to and declined: that class of failure is OUT OF SCOPE for the prompt — only enforcing the action deterministically (outside this capability) fixes it. So before reaching for a prompt edit, classify each cluster as KNOWLEDGE (fix here) vs behavioral (not a prompt fix), and spend prose only on the knowledge gaps.

If a rule is a VIOLATION the agent commits despite knowing it (not a knowledge gap), do NOT add prose. Adding another sentence to a rule the agent already read and broke just grows the prompt without changing behavior — a known-but-broken rule needs deterministic enforcement the prompt cannot provide, so it is out of scope here.

A DECISION / PERMISSION cluster is NOT a knowledge gap — never loosen a global rule to fix it. When a cluster shows the agent making the wrong ACT-vs-REFUSE call, the agent usually already "knows" the rule; the real condition is just more specific than the prose. Relaxing the global rule makes the agent act on the WHOLE class and regresses every task where refusing/escalating was the gold answer (unbounded blast radius). The only safe prompt fix is a NARROWING rule that states the EXACT discriminating predicate that separates the qualifying cases — a prompt edit may only ADD knowledge or NARROW, never broaden a permission or flip a decision the agent currently gets right. If the condition cannot be expressed as a narrowing rule, it is out of scope for the prompt.

Each prompt iteration should also CONSOLIDATE. When a rule is now enforced deterministically elsewhere (no longer dependent on the prompt), REMOVE its now-redundant prose so the prompt stays sharp — the deterministic enforcement is authoritative and the duplicate sentence only dilutes attention. This prevents prose pile-up: the prompt should get shorter as constraints become enforced, not longer. (This is consolidation under the never-drop rule — the constraint still lives, enforced elsewhere, so removing its prose drops no rule.)

How agents use it

The prompt is prepended to context every turn. Agents read it literally and are sensitive to ordering, contradictions, and verbosity. Long preambles dilute attention; conflicting instructions resolve unpredictably.

Common problems (see references/pitfalls.md)

• Over-long, redundant instructions → worse, not better.
• Missing/loose output contract → correct content, wrong shape, zero reward.
• Instructions that fight the model's defaults → inconsistent behavior.

Handlers (scripts/abstract.py)

materialize(dir) -> {file: text} · apply(dir, edits) -> report · validate(dir) -> {ok, files, problems}. Edit ops: set, append, ensure_contains. A project adapter's apply can call these directly.

Optimizing it each iteration (analyze → ideate → edit)

The optimizer should analyze before editing: from the traces + the current prompt, identify (a) the recurring failures clustered by root cause (the rule the agent keeps breaking) and (b) the good behavior seen only on some trials that should be made consistent; then make prompt edits for EVERY knowledge-gap cluster, paired with the tool-code fixes for the behavioral clusters in the SAME candidate, and reinforce (b) — sharpen or correct the offending rules rather than appending more preamble.

How to run

python scripts/check.py
python scripts/run.py --path      # prints the candidate + validity

References

• references/concepts.md — why prompts are high-leverage; what to optimize; pitfalls.

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: skillberry-ai
• Source: skillberry-ai/cap-evolve
• License: Apache-2.0

Install and usage instructions live in the source repository linked above.