Address high-priority review gaps before merging the dev snapshot

The release branch still had unresolved review comments around payload-rule
atomicity, brand blocking, compact fallback controls, and response
compatibility edge-cases. This patch pulls those behaviors back in line with
the intended contracts and adds focused regression coverage for the fragile
bridges.

Constraint: The fixes must stay narrow to the PR branch so review/merge can continue without reopening the larger dev snapshot conflict set
Rejected: Leave outdated-looking threads unresolved until after merge | reviewers still see concrete unfixed regressions in the current release branch
Rejected: Re-run another broad merge from dev/main | too risky while the PR branch is already green and mergeable
Confidence: medium
Scope-risk: moderate
Directive: If these release-branch fixes need to persist for future release cuts, backport them onto dev instead of re-fighting the same review threads next time
Tested: Focused vitest for settings events, brand matcher, compact responses, codex compatibility, anthropic stream bridge, openai chat response bridge; typecheck; repo drift check; git diff --check
Not-tested: Full npm test; live PR thread resolution state after push

src/server/proxy-core/capabilities/responsesCompact.test.ts

@@ -86,14 +86,17 @@ describe('ensureCompactResponsesJsonAcceptHeader', () => {
     });
   });
 
-  it('preserves existing accept values and unrelated platforms', () => {
+  it('forces compact requests back to application/json even when callers asked for SSE', () => {
     expect(ensureCompactResponsesJsonAcceptHeader({
-      Accept: 'application/json',
+      Accept: 'text/event-stream',
     }, {
       sitePlatform: 'codex',
     })).toEqual({
-      Accept: 'application/json',
+      accept: 'application/json',
     });
+  });
+
+  it('preserves unrelated platforms', () => {
     expect(ensureCompactResponsesJsonAcceptHeader({}, {
       sitePlatform: 'openai',
     })).toEqual({});

src/server/proxy-core/capabilities/responsesCompact.ts

@@ -42,10 +42,11 @@ export function ensureCompactResponsesJsonAcceptHeader(
   },
 ): Record<string, string> {
   if (!shouldForceCompactResponsesJsonAccept(options?.sitePlatform)) return headers;
-  const existingAccept = asTrimmedString(headers.accept) || asTrimmedString(headers.Accept);
-  if (existingAccept) return headers;
+  const nextHeaders = { ...headers };
+  delete (nextHeaders as Record<string, unknown>).Accept;
+  delete (nextHeaders as Record<string, unknown>).accept;
   return {
-    ...headers,
+    ...nextHeaders,
     accept: 'application/json',
   };
 }

src/server/proxy-core/surfaces/openAiResponsesSurface.ts

@@ -105,15 +105,16 @@ function isRecord(value: unknown): value is Record<string, unknown> {
 }
 
 function getCodexSessionHeaderValue(headers: Record<string, string>): string {
-  for (const [rawKey, rawValue] of Object.entries(headers)) {
-    const normalizedKey = rawKey.trim().toLowerCase();
-    if (
-      normalizedKey === 'session_id'
-      || normalizedKey === 'session-id'
-      || normalizedKey === 'conversation_id'
-      || normalizedKey === 'conversation-id'
-    ) {
-      return String(rawValue || '').trim();
+  const normalizedEntries = Object.entries(headers).map(([rawKey, rawValue]) => [
+    rawKey.trim().toLowerCase(),
+    String(rawValue || '').trim(),
+  ] as const);
+  for (const preferredKey of ['session_id', 'session-id', 'conversation_id', 'conversation-id']) {
+    const match = normalizedEntries.find(([normalizedKey, normalizedValue]) => (
+      normalizedKey === preferredKey && normalizedValue
+    ));
+    if (match) {
+      return match[1];
     }
   }
   return '';
@@ -626,7 +627,7 @@ export async function handleOpenAiResponsesSurfaceRequest(
               ctx.rawErrText = await readRuntimeResponseText(recoveredResponse).catch(() => 'unknown error');
             }
           }
-          const compactFallbackEnabled = config.responsesCompactFallbackToResponsesEnabled || !isCodexSite;
+          const compactFallbackEnabled = config.responsesCompactFallbackToResponsesEnabled;
           if (
             isCompactRequest
             && compactFallbackEnabled