feat(llm-gateway): add system prompt injection to OpenAI chat proxy

- Add injectSystemPromptOpenAI() for OpenAI messages format (role: system)
- Integrate injection into createOpenAIChatProxy before upstream call
- Update audit logs to track injection status
- Enables brand identity override for both Anthropic and OpenAI endpoints

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/services/llm-gateway/src/injection/system-prompt-injector.ts

@@ -113,6 +113,52 @@ export async function injectSystemPrompt(
   return { system, applied: false, ruleIds: [] };
 }
 
+// ─── Inject into OpenAI messages format ───
+
+export async function injectSystemPromptOpenAI(
+  messages: Array<{ role: string; content: any }>,
+  model: string,
+  apiKeyId: string,
+): Promise<{ messages: Array<{ role: string; content: any }>; applied: boolean; ruleIds: string[] }> {
+  const allRules = await loadRules();
+
+  const applicableRules = allRules.filter(
+    (r) => matchesModel(r.matchModels, model) && matchesKey(r.matchKeyIds, apiKeyId),
+  );
+
+  if (applicableRules.length === 0) {
+    return { messages, applied: false, ruleIds: [] };
+  }
+
+  const prependRules = applicableRules.filter((r) => r.position === 'prepend');
+  const appendRules = applicableRules.filter((r) => r.position === 'append');
+
+  const prependText = prependRules.map((r) => r.content).join('\n\n');
+  const appendText = appendRules.map((r) => r.content).join('\n\n');
+  const ruleIds = applicableRules.map((r) => r.id);
+
+  if (!prependText && !appendText) {
+    return { messages, applied: false, ruleIds: [] };
+  }
+
+  const injectedContent = [prependText, appendText].filter(Boolean).join('\n\n');
+  const result = [...messages];
+
+  // Find existing system message
+  const systemIdx = result.findIndex((m) => m.role === 'system');
+  if (systemIdx >= 0) {
+    // Merge into existing system message
+    const existing = typeof result[systemIdx].content === 'string' ? result[systemIdx].content : '';
+    const parts = [prependText, existing, appendText].filter(Boolean);
+    result[systemIdx] = { ...result[systemIdx], content: parts.join('\n\n') };
+  } else {
+    // Insert new system message at the beginning
+    result.unshift({ role: 'system', content: injectedContent });
+  }
+
+  return { messages: result, applied: true, ruleIds };
+}
+
 export function clearInjectionRulesCache(): void {
   cachedRules = [];
   cacheTimestamp = 0;

packages/services/llm-gateway/src/proxy/openai-proxy.ts

@@ -4,6 +4,7 @@ import { ApiKeyRecord } from '../types';
 import { isModelAllowed } from '../middleware/auth';
 import { recordFromOpenAIResponse } from '../logging/usage-tracker';
 import { recordAudit } from '../logging/audit-logger';
+import { injectSystemPromptOpenAI } from '../injection/system-prompt-injector';
 import { pipeSSEStream } from './stream-pipe';
 import { sanitizeOpenAIResponse, sanitizeOpenAIEmbeddingResponse, buildOpenAIStreamTransform } from './response-sanitizer';
 
@@ -63,7 +64,7 @@ export function createOpenAIEmbeddingsProxy(config: GatewayConfig) {
         requestIp: clientIp,
         contentFiltered: false,
         filterRuleId: null,
-        injectionApplied: false,
+        injectionApplied: injection.applied,
         responseStatus: 502,
         durationMs: Date.now() - startTime,
       });
@@ -105,7 +106,7 @@ export function createOpenAIEmbeddingsProxy(config: GatewayConfig) {
           requestIp: clientIp,
           contentFiltered: false,
           filterRuleId: null,
-          injectionApplied: false,
+          injectionApplied: injection.applied,
           responseStatus: upstreamResponse.status,
           durationMs,
         });
@@ -175,6 +176,10 @@ export function createOpenAIChatProxy(config: GatewayConfig) {
     // Replace model for upstream
     body.model = effectiveModel;
 
+    // Inject system prompt (identity, regulatory content)
+    const injection = await injectSystemPromptOpenAI(body.messages || [], effectiveModel, apiKeyRecord.id);
+    body.messages = injection.messages;
+
     let upstreamResponse: Response;
     try {
       // openaiUpstreamUrl may already include /v1 (e.g., "https://host:8443/v1")
@@ -196,7 +201,7 @@ export function createOpenAIChatProxy(config: GatewayConfig) {
         requestIp: clientIp,
         contentFiltered: false,
         filterRuleId: null,
-        injectionApplied: false,
+        injectionApplied: injection.applied,
         responseStatus: 502,
         durationMs: Date.now() - startTime,
       });
@@ -236,7 +241,7 @@ export function createOpenAIChatProxy(config: GatewayConfig) {
         requestIp: clientIp,
         contentFiltered: false,
         filterRuleId: null,
-        injectionApplied: false,
+        injectionApplied: injection.applied,
         responseStatus: upstreamResponse.status,
         durationMs: Date.now() - startTime,
       });
@@ -259,7 +264,7 @@ export function createOpenAIChatProxy(config: GatewayConfig) {
             requestIp: clientIp,
             contentFiltered: false,
             filterRuleId: null,
-            injectionApplied: false,
+            injectionApplied: injection.applied,
             responseStatus: upstreamResponse.status,
             durationMs,
           });
@@ -282,7 +287,7 @@ export function createOpenAIChatProxy(config: GatewayConfig) {
         requestIp: clientIp,
         contentFiltered: false,
         filterRuleId: null,
-        injectionApplied: false,
+        injectionApplied: injection.applied,
         responseStatus: upstreamResponse.status,
         durationMs,
       });