diff --git a/server/api/simkl/index.test.ts b/server/api/simkl/index.test.ts
new file mode 100644
index 0000000000..118de3726b
--- /dev/null
+++ b/server/api/simkl/index.test.ts
@@ -0,0 +1,84 @@
+import SimklAPI, {
+  SimklRequestError,
+  simklErrorDetail,
+} from '@server/api/simkl';
+import axios, { AxiosError, type AxiosAdapter } from 'axios';
+import assert from 'node:assert/strict';
+import { afterEach, describe, it } from 'node:test';
+
+const previousAxiosAdapter = axios.defaults.adapter;
+
+afterEach(() => {
+  axios.defaults.adapter = previousAxiosAdapter;
+});
+
+/** The client builds its axios instance on construction, so stub first. */
+const clientAnswering = (status: number, data: unknown) => {
+  const urls: string[] = [];
+  axios.defaults.adapter = (async (config) => {
+    urls.push(String(config.url));
+    const response = {
+      status,
+      statusText: '',
+      headers: {},
+      config,
+      data,
+    };
+    if (status < 400) return response;
+    throw new AxiosError(
+      `Request failed with status code ${status}`,
+      AxiosError.ERR_BAD_REQUEST,
+      config,
+      undefined,
+      response
+    );
+  }) as AxiosAdapter;
+  return { client: new SimklAPI({ clientId: 'test-client' }), urls };
+};
+
+describe('SimklAPI errors', () => {
+  it('keeps Simkl’s own reason for a rejected request', async () => {
+    const { client } = clientAnswering(412, {
+      error: 'client_id_failed',
+      code: 412,
+      message: 'Your client_id is wrong. Try another one',
+    });
+    await assert.rejects(client.requestPinCode(), (error: unknown) => {
+      assert.ok(error instanceof SimklRequestError);
+      assert.equal(error.status, 412);
+      assert.equal(
+        error.message,
+        'Simkl rejected the request (HTTP 412): client_id_failed — Your client_id is wrong. Try another one'
+      );
+      return true;
+    });
+  });
+
+  it('reduces a proxy’s HTML error page to its text', () => {
+    assert.equal(
+      simklErrorDetail(
+        '<html><body><h1>400 Bad Request</h1>\n<p>Invalid header</p></body></html>'
+      ),
+      '400 Bad Request Invalid header'
+    );
+    assert.equal(simklErrorDetail(''), undefined);
+    assert.equal(simklErrorDetail({}), undefined);
+  });
+
+  it('caps a long JSON reason like a text one', () => {
+    assert.equal(
+      simklErrorDetail({ error: 'bad', message: 'x'.repeat(500) })?.length,
+      200
+    );
+  });
+
+  it('validates the Client ID against the PIN endpoint', async () => {
+    const { client, urls } = clientAnswering(200, {
+      result: 'OK',
+      user_code: 'ABCDE',
+    });
+    await client.validateClientId();
+    assert.equal(urls.length, 1);
+    assert.match(urls[0], /^\/oauth\/pin\?client_id=test-client&/);
+  });
+});
diff --git a/server/api/simkl/index.ts b/server/api/simkl/index.ts
index 81058e5488..16b56d12ae 100644
--- a/server/api/simkl/index.ts
+++ b/server/api/simkl/index.ts
@@ -49,6 +49,42 @@ export class SimklTemporarilyUnavailableError extends Error {
   }
 }
 
+/** Simkl (or a proxy on the way) refused the request; keeps its explanation. */
+export class SimklRequestError extends Error {
+  constructor(
+    public readonly status: number,
+    detail?: string,
+    cause?: unknown
+  ) {
+    super(
+      `Simkl rejected the request (HTTP ${status})${detail ? `: ${detail}` : ''}`
+    );
+    this.name = 'SimklRequestError';
+    this.cause = cause;
+  }
+}
+
+const SIMKL_ERROR_DETAIL_LIMIT = 200;
+
+/** Simkl answers `{ error, message }`; proxies tend to answer HTML or text. */
+export const simklErrorDetail = (data: unknown): string | undefined => {
+  let text = '';
+  if (data && typeof data === 'object') {
+    const { message, error } = data as { message?: unknown; error?: unknown };
+    text = [error, message]
+      .filter(
+        (part): part is string => typeof part === 'string' && part.length > 0
+      )
+      .join(' — ');
+  } else if (typeof data === 'string') {
+    text = data
+      .replace(/<[^>]*>/g, ' ')
+      .replace(/\s+/g, ' ')
+      .trim();
+  }
+  return text ? text.slice(0, SIMKL_ERROR_DETAIL_LIMIT) : undefined;
+};
+
 type SimklOptions = {
   clientId?: string;
   accessToken?: string;
@@ -138,14 +174,22 @@ export default class SimklAPI extends ExternalAPI {
         }
         if (!status || status >= 500)
           throw new SimklTemporarilyUnavailableError(error);
-        throw error;
+        throw new SimklRequestError(
+          status,
+          simklErrorDetail(error.response?.data),
+          error
+        );
       }
     }
   }
 
-  /** Requires a real Client ID; `/movies/trending` succeeds without one. */
+  /**
+   * The PIN endpoint is the only one found to check the Client ID (412
+   * `client_id_failed`); catalogs and title lookups accept any value. The
+   * code it issues just expires unused.
+   */
   public async validateClientId(): Promise<void> {
-    await this.getCdnCatalog('/discover/trending/tv/week_100.json');
+    await this.requestPinCode();
   }
 
   public async requestPinCode(): Promise<SimklPinCodeResponse> {
diff --git a/server/lib/integrationHealth.ts b/server/lib/integrationHealth.ts
index 34aa3a4489..17983fec64 100644
--- a/server/lib/integrationHealth.ts
+++ b/server/lib/integrationHealth.ts
@@ -237,7 +237,11 @@ const checkSimkl = async (): Promise<IntegrationHealth> => {
   try {
     await new SimklAPI().validateClientId();
     return healthy('Simkl is reachable and the Client ID was accepted.');
-  } catch {
+  } catch (e) {
+    logger.warn('Simkl health check failed', {
+      label: 'Integration Health',
+      errorMessage: e instanceof Error ? e.message : 'unknown error',
+    });
     return degraded('Simkl could not accept the saved Client ID.');
   }
 };
diff --git a/server/routes/settings/index.ts b/server/routes/settings/index.ts
index 1a548186c8..6237c88e34 100644
--- a/server/routes/settings/index.ts
+++ b/server/routes/settings/index.ts
@@ -3,7 +3,7 @@ import type { JellyfinLibrary } from '@server/api/jellyfin';
 import JellyfinAPI from '@server/api/jellyfin';
 import PlexAPI from '@server/api/plexapi';
 import PlexTvAPI from '@server/api/plextv';
-import SimklAPI from '@server/api/simkl';
+import SimklAPI, { SimklRequestError } from '@server/api/simkl';
 import TautulliAPI from '@server/api/tautulli';
 import TraktAPI from '@server/api/trakt';
 import { ApiErrorCode } from '@server/constants/error';
@@ -749,6 +749,8 @@ settingsRoutes.post('/simkl', async (req, res, next) => {
       label: 'Settings',
       errorMessage: e instanceof Error ? e.message : 'unknown error',
     });
+    if (e instanceof SimklRequestError)
+      return next({ status: 400, message: e.message });
     return next({ status: 500, message: 'Unable to save Simkl settings.' });
   }
 });
diff --git a/server/routes/user/usersettings.ts b/server/routes/user/usersettings.ts
index 463179e92f..760ef1e147 100644
--- a/server/routes/user/usersettings.ts
+++ b/server/routes/user/usersettings.ts
@@ -1148,6 +1148,10 @@ userSettingsRoutes.post<{ id: string }>(
         deviceCode: userCode,
       });
     } catch (e) {
+      logger.error('Failed to start Simkl PIN authorization', {
+        label: 'API',
+        errorMessage: e instanceof Error ? e.message : 'unknown error',
+      });
       return res.status(e instanceof SimklNotConfiguredError ? 400 : 500).json({
         message:
           e instanceof Error
@@ -1223,6 +1227,10 @@ userSettingsRoutes.post<{ id: string }>(
     } catch (e) {
       if (e instanceof SimklAccountAlreadyLinkedError)
         return res.status(409).json({ message: e.message });
+      logger.error('Failed to complete Simkl PIN authorization', {
+        label: 'API',
+        errorMessage: e instanceof Error ? e.message : 'unknown error',
+      });
       return res.status(e instanceof SimklNotConfiguredError ? 400 : 500).json({
         message:
           e instanceof Error