diff --git a/src/lib/db/sessionAccountAffinity.ts b/src/lib/db/sessionAccountAffinity.ts
new file mode 100644
index 0000000000..80ba0a729d
--- /dev/null
+++ b/src/lib/db/sessionAccountAffinity.ts
@@ -0,0 +1,31 @@
+// Stubbed functions for session account affinity (PR 1887 pending)
+
+export function getSessionAccountAffinity(sessionKey: string, provider: string): any {
+  return null;
+}
+
+export function upsertSessionAccountAffinity(
+  sessionKey: string,
+  provider: string,
+  connectionId: string,
+  now: number = Date.now()
+): void {}
+
+export function touchSessionAccountAffinity(
+  sessionKey: string,
+  provider: string,
+  now: number = Date.now()
+): void {}
+
+export function deleteSessionAccountAffinity(sessionKey: string, provider: string): void {}
+
+export function cleanupStaleSessionAccountAffinities(
+  ttlMs: number = 30 * 60 * 1000,
+  now: number = Date.now()
+): number {
+  return 0;
+}
+
+export function startSessionAccountAffinityCleanup(): void {}
+
+export function stopSessionAccountAffinityCleanupForTests(): void {}
diff --git a/src/sse/services/auth.ts b/src/sse/services/auth.ts
index 84d3fc5c86..863204ed92 100644
--- a/src/sse/services/auth.ts
+++ b/src/sse/services/auth.ts
@@ -1,4 +1,4 @@
-import { randomUUID } from "crypto";
+import { randomUUID, createHash } from "crypto";
 import {
   getProviderConnections,
   validateApiKey,
@@ -78,6 +78,7 @@ interface CredentialSelectionOptions {
   bypassQuotaPolicy?: boolean;
   forcedConnectionId?: string | null;
   excludeConnectionIds?: string[] | null;
+  sessionKey?: string | null;
 }
 
 interface CooldownInspectionState {
@@ -145,6 +146,94 @@ function toBooleanOrDefault(value: unknown, fallback: boolean): boolean {
   return typeof value === "boolean" ? value : fallback;
 }
 
+function readHeaderValue(
+  headers: Headers | { get?: (name: string) => string | null } | null | undefined,
+  name: string
+): string | null {
+  if (!headers || typeof headers.get !== "function") return null;
+  const value = headers.get(name);
+  return typeof value === "string" && value.trim().length > 0 ? value.trim() : null;
+}
+
+function normalizeSessionKey(value: unknown, prefix: string): string | null {
+  if (typeof value !== "string" || value.trim().length === 0) return null;
+  const trimmed = value.trim();
+  if (trimmed.length <= 180 && /^[A-Za-z0-9._:-]+$/.test(trimmed)) {
+    return `${prefix}:${trimmed}`;
+  }
+  return `${prefix}:sha256:${createHash("sha256").update(trimmed).digest("hex")}`;
+}
+
+function extractTextForSessionHash(value: unknown): string | null {
+  if (typeof value === "string") return value;
+  if (Array.isArray(value)) {
+    const parts = value
+      .map((item) => {
+        if (typeof item === "string") return item;
+        const record = asRecord(item);
+        if (typeof record.text === "string") return record.text;
+        if (typeof record.content === "string") return record.content;
+        return null;
+      })
+      .filter(Boolean) as string[];
+    return parts.length > 0 ? parts.join("\n") : JSON.stringify(value);
+  }
+  if (value && typeof value === "object") return JSON.stringify(value);
+  return null;
+}
+
+function getFirstInputText(body: unknown): string | null {
+  const record = asRecord(body);
+  if (record.input !== undefined) {
+    if (typeof record.input === "string") return record.input;
+    if (Array.isArray(record.input)) {
+      for (const item of record.input) {
+        const itemRecord = asRecord(item);
+        const text = extractTextForSessionHash(itemRecord.content ?? item);
+        if (text && text.trim().length > 0) return text;
+      }
+    }
+    const text = extractTextForSessionHash(record.input);
+    if (text && text.trim().length > 0) return text;
+  }
+
+  if (Array.isArray(record.messages)) {
+    const userMessage = record.messages.find((message) => asRecord(message).role === "user");
+    const firstMessage = userMessage ?? record.messages[0];
+    const text = extractTextForSessionHash(asRecord(firstMessage).content ?? firstMessage);
+    if (text && text.trim().length > 0) return text;
+  }
+
+  return null;
+}
+
+export function extractSessionAffinityKey(
+  body: unknown,
+  headers?: Headers | { get?: (name: string) => string | null } | null
+): string | null {
+  const headerKey = normalizeSessionKey(
+    readHeaderValue(headers, "x-codex-session-id") ??
+      readHeaderValue(headers, "x-session-id") ??
+      readHeaderValue(headers, "x-omniroute-session"),
+    "header"
+  );
+  if (headerKey) return headerKey;
+
+  const record = asRecord(body);
+  const metadata = asRecord(record.metadata);
+  const explicitKey =
+    normalizeSessionKey(metadata.session_id, "metadata") ??
+    normalizeSessionKey(metadata.sessionId, "metadata") ??
+    normalizeSessionKey(record.conversation_id, "conversation") ??
+    normalizeSessionKey(record.session_id, "session") ??
+    normalizeSessionKey(record.prompt_cache_key, "prompt-cache");
+  if (explicitKey) return explicitKey;
+
+  const inputText = getFirstInputText(body);
+  if (!inputText || inputText.trim().length === 0) return null;
+  return `input:sha256:${createHash("sha256").update(inputText.slice(0, 4096)).digest("hex")}`;
+}
+
 function getCodexLimitPolicy(providerSpecificData: JsonRecord): {
   use5h: boolean;
   useWeekly: boolean;