diff --git a/apps/api/src/auth/localAuthMiddleware.ts b/apps/api/src/auth/localAuthMiddleware.ts
index f4eef0b..b952137 100644
--- a/apps/api/src/auth/localAuthMiddleware.ts
+++ b/apps/api/src/auth/localAuthMiddleware.ts
@@ -33,6 +33,7 @@ import { eq } from 'drizzle-orm';
 import { db } from '../db/client.js';
 import { users } from '../db/schema.js';
 import { verifyLocalSessionCookie } from './localSession.js';
+import type { DEV_USER } from './devBypass.js';
 
 /**
  * Returns a Hono MiddlewareHandler that:
@@ -82,14 +83,15 @@ export function localAuthMiddleware(): MiddlewareHandler {
 
     // Populate c.get('user') with the same shape as DEV_USER (devBypass.ts ContextVariableMap).
     // oidcIss/oidcSub: local users have nullable oidcIss/oidcSub — use fallback strings so the
-    // shape matches typeof DEV_USER (all required fields, no undefined in the value object).
+    // shape is compatible with typeof DEV_USER at runtime. Cast required because ContextVariableMap
+    // is narrowed to the const DEV_USER literal type.
     c.set('user', {
       id: row.id,
       oidcIss: row.oidcIss ?? 'local',
       oidcSub: row.oidcSub ?? String(row.id),
       displayName: row.displayName ?? null,
       color: row.color ?? '#4A90D9',
-    });
+    } as typeof DEV_USER);
 
     await next();
   };
diff --git a/apps/api/src/auth/middleware.ts b/apps/api/src/auth/middleware.ts
index 1737635..d730a94 100644
--- a/apps/api/src/auth/middleware.ts
+++ b/apps/api/src/auth/middleware.ts
@@ -1,12 +1,12 @@
 /**
  * OIDC authentication middleware wiring.
  *
- * Configures @hono/oidc-auth for Authelia as the identity provider.
+ * Configures @hono/oidc-auth for the generic OIDC identity provider (D-06).
  *
  * Required env vars:
  *   OIDC_AUTH_SECRET          — 32+ char random string for JWT cookie signing (T-02-03)
- *   OIDC_ISSUER               — Authelia base URL (middleware fetches /.well-known/openid-configuration)
- *   OIDC_CLIENT_ID            — registered client ID in Authelia
+ *   OIDC_ISSUER               — OIDC issuer URL (middleware fetches /.well-known/openid-configuration)
+ *   OIDC_CLIENT_ID            — registered client ID at the OIDC provider
  *   OIDC_CLIENT_SECRET        — PLAIN text secret (NOT the pbkdf2 hash — see Pitfall 7)
  *   OIDC_REDIRECT_URI         — https://familysync.<domain>/callback
  *   OIDC_AUTH_EXTERNAL_URL    — https://familysync.<domain> — MANDATORY behind Pangolin (Pitfall 1)
@@ -30,7 +30,7 @@
  *   Every OIDC_AUTH_REFRESH_INTERVAL (default 15 min) the middleware calls
  *   the token endpoint with the stored refresh token — no iframe required (D-12).
  *   Session lifespan is governed by OIDC_AUTH_EXPIRES (default 1 day) and
- *   Authelia's refresh_token_lifespan.
+ *   the OIDC provider's refresh_token_lifespan.
  *
  * Scopes: openid, profile, email only — no 'groups' scope (D-11).
  *
diff --git a/apps/api/src/index.ts b/apps/api/src/index.ts
index 6bd5dd3..205afe7 100644
--- a/apps/api/src/index.ts
+++ b/apps/api/src/index.ts
@@ -25,6 +25,9 @@ import { startBrokerPoller } from './broker/poller.js';
 import { startOutboxWorker, initOutboxTrigger } from './broker/outboxWorker.js';
 import { startReminderScheduler } from './broker/reminderScheduler.js';
 import { assertNotDevBypassInProduction, assertLocalSessionSecretSet } from './lib/bootGuards.js';
+import { getAuth } from './auth/middleware.js';
+import { Jwt } from 'hono/utils/jwt';
+import { linkOidcToUser, OidcLinkConflictError } from './auth/linkOidc.js';
 import webpush from 'web-push';
 
 export const app = new Hono();
@@ -39,8 +42,63 @@ if (devBypassActive) {
 }
 
 // OIDC callback — must be registered BEFORE oidcAuthMiddleware so the
-// authorization-code exchange is not itself intercepted by the auth check (T-02-02)
-app.get('/callback', (c) => processOAuthCallback(c));
+// authorization-code exchange is not itself intercepted by the auth check (T-02-02).
+//
+// Phase 19 — link mode (AUTH-LOCAL-10, T-19-15):
+// When POST /api/me/link-oidc initiates a link flow, a signed JWT state is included
+// in the authorization URL as the `state` parameter. On callback, we read that raw URL
+// state param, try to decode it as our signed JWT, and if it carries `linkUserId`, we
+// call linkOidcToUser after processOAuthCallback establishes the OIDC session.
+//
+// Security: the signed state JWT prevents CSRF (T-19-09); linkOidcToUser preflight
+// prevents account takeover via conflict (T-19-15 / T-19-08).
+//
+// Normal (non-link) callbacks are unaffected — processOAuthCallback is called in all paths.
+app.get('/callback', async (c) => {
+  // Attempt to extract linkUserId from the signed state param BEFORE processOAuthCallback
+  // consumes it. The state param may be our signed JWT (link mode) or a random string (normal).
+  let linkUserId: number | null = null;
+  const rawState = c.req.query('state');
+  if (rawState) {
+    const secret = process.env.LOCAL_SESSION_SECRET;
+    if (secret) {
+      try {
+        const payload = await Jwt.verify(rawState, secret, 'HS256');
+        if (typeof payload.linkUserId === 'number') {
+          linkUserId = payload.linkUserId;
+        }
+      } catch {
+        // Not our signed link state — normal OIDC callback, proceed normally.
+      }
+    }
+  }
+
+  // Process the OIDC authorization-code exchange (sets the OIDC session cookie + redirects).
+  const callbackResponse = await processOAuthCallback(c);
+
+  // Link mode: after session is established, bind the OIDC identity to the local user.
+  if (linkUserId !== null) {
+    try {
+      const auth = await getAuth(c);
+      if (auth) {
+        const iss = (auth.iss as string | undefined) ?? '';
+        const sub = auth.sub ?? '';
+        await linkOidcToUser(linkUserId, iss, sub);
+        // On success: user is now OIDC-only; normal redirect via callbackResponse proceeds.
+      }
+    } catch (err) {
+      if (err instanceof OidcLinkConflictError) {
+        // 409: iss+sub already linked to a different user — redirect to conflict error page.
+        // UI-SPEC Surface 13 error copy: "This OIDC identity is already linked to another account."
+        return c.redirect('/?error=oidc-link-conflict');
+      }
+      // Unexpected error during link binding — log and continue with normal redirect.
+      console.error('[callback] linkOidcToUser unexpected error:', err instanceof Error ? err.message : String(err));
+    }
+  }
+
+  return callbackResponse;
+});
 
 // GET /health — unauthenticated, mounted BEFORE the OIDC guard (T-01-03, T-02-05)
 app.route('/health', healthRouter);
@@ -68,7 +126,7 @@ app.use('/api/*', devAuthBypass());
 app.use('/api/*', localAuthMiddleware());
 
 // Protect all /api/* routes with OIDC session middleware (AUTH-01, T-02-05).
-// Skipped entirely when devBypassActive so that local dev works without Authelia.
+// Skipped entirely when devBypassActive so that local dev works without the OIDC provider.
 // In production devBypassActive is always false — OIDC is unconditionally mounted.
 // Unauthenticated requests receive a 302 redirect to the OIDC authorize endpoint.
 // OIDC_AUTH_EXTERNAL_URL is MANDATORY behind Pangolin to construct the correct
@@ -84,12 +142,17 @@ if (!devBypassActive) {
   // Phase 19 / D-03: OIDC guard wrapped to skip when c.get('user') is already set.
   // A valid local-session (or dev-bypass) user must NOT be 302-redirected to the OIDC
   // provider — the skip-when-set wrapper is the coexistence seam (D-03 / RESEARCH Pitfall 1).
+  //
+  // IMPORTANT: oidcAuthMiddleware() factory is called ONCE at app construction time (not per
+  // request) to match the prior behavior and keep test assertions about "called once" valid.
+  // The returned handler is stored and invoked per-request inside the wrapper.
+  const oidcHandler = oidcAuthMiddleware();
   app.use('/api/*', async (c, next) => {
     if (c.get('user')) {
       await next();
       return;
     }
-    await oidcAuthMiddleware()(c, next);
+    await oidcHandler(c, next);
   });
   // Re-issues the session-scoped oidc-auth cookie as persistent so PWA sessions survive close (AUTH-02).
   app.use('/api/*', persistSessionCookie());
@@ -99,7 +162,7 @@ if (!devBypassActive) {
 
 // GET /api/login — login entry point for the PWA.
 // Flow (production): unauthenticated top-level nav hits the OIDC guard above,
-// which 302-redirects to Authelia. After login, Authelia POSTs to /callback,
+// which 302-redirects to the OIDC provider. After login, the provider POSTs to /callback,
 // the middleware sets a `continue` cookie pointing back to /api/login, and the
 // browser follows it here — now authenticated. The handler then redirects to /
 // so the SPA boots with a valid session. Under DEV_AUTH_BYPASS the guard is not