diff --git a/src/runtime/http/express-router-impl.ts b/src/runtime/http/express-router-impl.ts
index ef7e082..3d4b2b6 100644
--- a/src/runtime/http/express-router-impl.ts
+++ b/src/runtime/http/express-router-impl.ts
@@ -22,6 +22,25 @@ import { extractClientIdentifier } from './client-identifier.ts';
 
 const SEP10_AUTH_OPERATION_SUFFIX = ' auth';
 
+/**
+ * Result of looking up an account's configured signers via an injectable
+ * SEP-10 account lookup. Distinguishes not-found from upstream failures.
+ */
+export type Sep10AccountLookupResult =
+  | {
+      status: 'found';
+      account: {
+        signers: Array<{ key: string; weight: number }>;
+        thresholds: { low: number; medium: number; high: number };
+      };
+    }
+  | { status: 'not_found' }
+  | { status: 'unavailable'; reason?: string };
+
+export interface Sep10AccountLookup {
+  lookup(accountId: string): Promise<Sep10AccountLookupResult>;
+}
+
 /**
  * Stable issuer for bearer access tokens minted by the anchor API.
  * Tokens issued by a different service must not be accepted here, even when
diff --git a/src/runtime/http/express-router.ts b/src/runtime/http/express-router.ts
index 18f098a..e3d8133 100644
--- a/src/runtime/http/express-router.ts
+++ b/src/runtime/http/express-router.ts
@@ -1,7 +1,11 @@
 import type { AnchorConfig } from '@/core/config.ts';
 import { errorHandler } from '../../utils/error-handler.ts';
 import { InMemoryRateLimiter, type RateLimitRule } from './rate-limiter.ts';
-import type { DatabaseAdapter, WebhookProcessor } from '@/runtime/interfaces.ts';
+import type {
+  AccountLookup,
+  DatabaseAdapter,
+  WebhookProcessor,
+} from '@/runtime/interfaces.ts';
 import type { RouteDefinition } from '@/types/foundation.ts';
 import type { AnchorPluginHooks } from '@/types/plugin.ts';
 import { Keypair } from '@stellar/stellar-sdk';
@@ -25,6 +29,7 @@ interface RouterDependencies {
   depositRequestHooks?: NonNullable<AnchorPluginHooks['onDepositRequest']>[];
   sep10ChallengeHooks?: NonNullable<AnchorPluginHooks['onSep10Challenge']>[];
   pluginRoutes?: Array<{ pluginId: string; route: RouteDefinition }>;
+  accountLookup?: AccountLookup;
 }
 
 export class AnchorExpressRouter {
@@ -65,6 +70,7 @@ export class AnchorExpressRouter {
       rateLimiter: new InMemoryRateLimiter(),
       rateRules,
       pluginRoutes,
+      accountLookup: dependencies.accountLookup,
     };
   }
 
diff --git a/src/runtime/interfaces.ts b/src/runtime/interfaces.ts
index 61b2611..3b7a845 100644
--- a/src/runtime/interfaces.ts
+++ b/src/runtime/interfaces.ts
@@ -45,6 +45,30 @@ export interface AccountSigner {
   weight: number;
 }
 
+/** Signers and thresholds for a Stellar account, as reported by an account lookup source. */
+export interface AccountSignerConfiguration {
+  account: string;
+  signers: AccountSigner[];
+  thresholds: {
+    low: number;
+    medium: number;
+    high: number;
+  };
+}
+
+export type AccountLookupResult =
+  | { outcome: 'found'; account: AccountSignerConfiguration }
+  | { outcome: 'not_found'; account: string };
+
+/**
+ * Injectable boundary for loading account signers and thresholds.
+ * Implementations must throw on upstream failure (network, provider outage);
+ * an unknown account must be reported as `not_found`, never as an exception.
+ */
+export interface AccountLookup {
+  lookupAccount(account: string): Promise<AccountLookupResult>;
+}
+
 export interface AuthChallengeRecord {
   id: string;
   account: string;
diff --git a/src/services/stellar-signer.ts b/src/services/stellar-signer.ts
index b20054a..a7a9f88 100644
--- a/src/services/stellar-signer.ts
+++ b/src/services/stellar-signer.ts
@@ -5,6 +5,32 @@ export interface StellarTransactionSigner {
   signTransaction(transactionXdr: string, networkPassphrase: string): Promise<string>;
 }
 
+export interface StellarAccountSigner {
+  key: string;
+  weight: number;
+}
+
+export interface StellarAccountThresholds {
+  low: number;
+  medium: number;
+  high: number;
+}
+
+export interface StellarAccountRecord {
+  accountId: string;
+  signers: StellarAccountSigner[];
+  thresholds: StellarAccountThresholds;
+}
+
+export type StellarAccountLookupResult =
+  | { status: 'found'; account: StellarAccountRecord }
+  | { status: 'not_found' }
+  | { status: 'upstream_error'; message: string };
+
+export interface StellarAccountLookup {
+  lookupAccount(accountId: string): Promise<StellarAccountLookupResult>;
+}
+
 export class LocalKeypairSigner implements StellarTransactionSigner {
   readonly #keypair: Keypair;