diff --git a/.changeset/oauth-start-failure-detail.md b/.changeset/oauth-start-failure-detail.md
new file mode 100644
index 00000000..47098fe6
--- /dev/null
+++ b/.changeset/oauth-start-failure-detail.md
@@ -0,0 +1,5 @@
+---
+'seamless-auth-api': patch
+---
+
+A failed OAuth start now says why. The `oauth_login_failed` event with reason `start_failed` carries a `detail` of `redirect_not_allowed` or `provider_unusable`, and the error is logged. A refused redirect is logged by scheme and host only. The answer to the client is unchanged.
diff --git a/src/controllers/oauth.ts b/src/controllers/oauth.ts
index 35a3e219..4c0568de 100644
--- a/src/controllers/oauth.ts
+++ b/src/controllers/oauth.ts
@@ -25,6 +25,7 @@ import {
   OAuthProfileError,
   oauthProfileFromIdToken,
   OAuthProviderRetiredError,
+  OAuthRedirectNotAllowedError,
   resolveOAuthProvider,
   resolveOAuthRedirectUri,
   resolveOAuthUser,
@@ -98,11 +99,21 @@ export async function startOAuthLogin(req: RouteRequest, res: Response) {
         ...(codeChallenge ? { codeChallenge } : {}),
       }),
     });
-  } catch {
+  } catch (error) {
+    // The client only ever sees a generic answer, so this is the one place an operator
+    // can learn why. A redirect refusal is a client or allowlist problem and anything
+    // else is the provider's configuration or discovery, and those are fixed differently.
+    const detail =
+      error instanceof OAuthRedirectNotAllowedError ? 'redirect_not_allowed' : 'provider_unusable';
+
+    // The provider is on the event below. The error names a refused redirect by its parsed
+    // scheme and host only, so nothing the caller sent reaches the log as written.
+    logger.warn(`OAuth start failed: ${String(error)}`);
+
     await AuthEventService.log({
       type: 'oauth_login_failed',
       req,
-      metadata: { providerId: provider.id, reason: 'start_failed' },
+      metadata: { providerId: provider.id, reason: 'start_failed', detail },
     });
 
     return res.status(400).json({
diff --git a/src/services/oauthService.ts b/src/services/oauthService.ts
index 3a08f635..a47e27f0 100644
--- a/src/services/oauthService.ts
+++ b/src/services/oauthService.ts
@@ -87,6 +87,24 @@ export class OAuthProfileError extends Error {
   }
 }
 
+/**
+ * The client asked for a redirect the provider config does not allow. The message names
+ * the scheme and host only: the rest of the value is whatever the caller sent.
+ */
+export class OAuthRedirectNotAllowedError extends Error {
+  constructor(requestedRedirectUri: string) {
+    let target = 'an unparsable value';
+    try {
+      const url = new URL(requestedRedirectUri);
+      target = `${url.protocol}//${url.host}`;
+    } catch {
+      // Named as unparsable above.
+    }
+    super(`OAuth redirect URI is not allowed: ${target}`);
+    this.name = 'OAuthRedirectNotAllowedError';
+  }
+}
+
 export class OAuthProviderRetiredError extends Error {
   readonly code = 'oauth_provider_retired';
   readonly userId: string;
@@ -259,7 +277,7 @@ export async function resolveOAuthRedirectUri(
 
   if (requestedRedirectUri) {
     if (!allowedRedirect(requestedRedirectUri, providerAllowlist, config.origins)) {
-      throw new Error('OAuth redirect URI is not allowed');
+      throw new OAuthRedirectNotAllowedError(requestedRedirectUri);
     }
 
     return requestedRedirectUri;
diff --git a/tests/integration/oauth/oauth.spec.ts b/tests/integration/oauth/oauth.spec.ts
index 62273c90..1c7f6002 100644
--- a/tests/integration/oauth/oauth.spec.ts
+++ b/tests/integration/oauth/oauth.spec.ts
@@ -570,6 +570,12 @@ describe('OAuth routes', () => {
     vi.unstubAllGlobals();
     expect(res.status).toBe(400);
     expect(res.body.error).toBe('OAuth start failed');
+    expect(AuthEventService.log).toHaveBeenCalledWith(
+      expect.objectContaining({
+        type: 'oauth_login_failed',
+        metadata: { providerId: 'idp', reason: 'start_failed', detail: 'provider_unusable' },
+      }),
+    );
   });
 
   it('refuses an ID token minted for a different sign-in', async () => {
@@ -621,7 +627,22 @@ describe('OAuth routes', () => {
     expect(AuthEventService.log).toHaveBeenCalledWith(
       expect.objectContaining({
         type: 'oauth_login_failed',
-        metadata: { providerId: 'google', reason: 'start_failed' },
+        metadata: { providerId: 'google', reason: 'start_failed', detail: 'redirect_not_allowed' },
+      }),
+    );
+  });
+
+  it('refuses an app scheme the provider does not allow, and says that is why', async () => {
+    const res = await request(app).post('/oauth/google/start').send({
+      redirectUri: 'myapp://oauth/callback?code=secret',
+    });
+
+    expect(res.status).toBe(400);
+    expect(res.body.error).toBe('OAuth start failed');
+    expect(AuthEventService.log).toHaveBeenCalledWith(
+      expect.objectContaining({
+        type: 'oauth_login_failed',
+        metadata: { providerId: 'google', reason: 'start_failed', detail: 'redirect_not_allowed' },
       }),
     );
   });