Negative Constraints in Prompts: Banned APIs That Safer Codegen Respects

# banned_apis.yaml — generated from eslint/deny + security ADRs
version: 17
banned:
 - id: no-exec-string
 match: { module: "child_process", exports: ["exec", "execSync"] }
 reason: "Use execFile with arg arrays; see ADR-0142"
 - id: no-aws-sdk-v2
 match: { module: "aws-sdk" }
 reason: "Use @aws-sdk/* v3 clients"
 - id: no-hardcoded-secrets
 match: { regex: "(AKIA[0-9A-Z]{16}|secretAccessKey\\s*[:=])" }
 reason: "Secrets belong in SSM/Secrets Manager"
 - id: no-verifalse-tls
 match: { regex: "rejectUnauthorized\\s*:\\s*false" }
 reason: "Never disable TLS verification"
deprecated_modules:
 - "request"
 - "moment"

## NEGATIVES (must not generate)
- Do NOT import aws-sdk (v2). Use @aws-sdk/* v3.
- Do NOT call child_process.exec/execSync with string commands.
- Do NOT disable TLS (rejectUnauthorized:false).
- Do NOT embed AKIA… keys or secretAccessKey literals.
If a task seems to require a banned API, explain and propose the approved alternative.

import banned from "./banned_apis.json";

function systemPrompt(task: string): string {
 return [
 BASE_CODER_PROMPT,
 renderNegatives(banned), // versioned fragment
 `Task:\n${task}`,
 ].join("\n\n");
}

function assertNoBanned(patch: string): void {
 for (const rule of banned.banned) {
 if (rule.match.module && importUses(patch, rule.match)) {
 throw new Error(`banned:${rule.id}:${rule.reason}`);
 }
 if (rule.match.regex && new RegExp(rule.match.regex).test(patch)) {
 throw new Error(`banned:${rule.id}:${rule.reason}`);
 }
 }
}

async function safeApply(task: string) {
 const patch = await generatePatch(systemPrompt(task));
 try {
 assertNoBanned(patch);
 } catch (e) {
 // One repair loop with the violation named
 const repaired = await generatePatch(
 systemPrompt(task) + `\nREPAIR: ${String(e)}. Use approved alternatives only.`,
 );
 assertNoBanned(repaired); // fail closed on second violation
 return applyPatch(repaired);
 }
 return applyPatch(patch);
}

# canary: agent must refuse or rewrite banned patterns
cases = load_yaml("evals/banned_api_tasks.yaml")
for c in cases:
 patch = run_agent(c["task"])
 assert not violates(patch, banned), c["id"]
 assert c["approved_alternative_marker"] in patch