diff --git a/packages/sync-server/src/load-config.js b/packages/sync-server/src/load-config.js
index 4de7763172..43794ad52f 100644
--- a/packages/sync-server/src/load-config.js
+++ b/packages/sync-server/src/load-config.js
@@ -35,7 +35,28 @@ convict.addFormat({
   validate(val) {
     if (val === 'never' || val === 'openid-provider') return;
     if (typeof val === 'number' && Number.isFinite(val) && val >= 0) return;
-    throw new Error(`Invalid token_expiration value: ${val}`);
+
+    // Handle string values that can be converted to numbers (from env vars)
+    if (typeof val === 'string') {
+      const numVal = Number(val);
+      if (Number.isFinite(numVal) && numVal >= 0) return;
+    }
+
+    throw new Error(
+      `Invalid token_expiration value: ${val}: value was "${val}"`,
+    );
+  },
+  coerce(val) {
+    if (val === 'never' || val === 'openid-provider') return val;
+    if (typeof val === 'number') return val;
+
+    // Convert string values to numbers for environment variables
+    if (typeof val === 'string') {
+      const numVal = Number(val);
+      if (Number.isFinite(numVal) && numVal >= 0) return numVal;
+    }
+
+    return val; // Let validate() handle invalid values
   },
 });
 
diff --git a/packages/sync-server/src/load-config.test.js b/packages/sync-server/src/load-config.test.js
new file mode 100644
index 0000000000..0c85d84ac1
--- /dev/null
+++ b/packages/sync-server/src/load-config.test.js
@@ -0,0 +1,112 @@
+import convict from 'convict';
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+
+// Import the custom format
+import './load-config.js';
+
+describe('tokenExpiration format', () => {
+  let originalEnv;
+
+  beforeEach(() => {
+    originalEnv = { ...process.env };
+  });
+
+  afterEach(() => {
+    process.env = originalEnv;
+  });
+
+  it('should accept string numbers from environment variables', () => {
+    // Test string number
+    process.env.TEST_TOKEN_EXPIRATION = '86400';
+    const testSchema = convict({
+      token_expiration: {
+        format: 'tokenExpiration',
+        default: 'never',
+        env: 'TEST_TOKEN_EXPIRATION',
+      },
+    });
+    expect(() => testSchema.validate()).not.toThrow();
+    expect(testSchema.get('token_expiration')).toBe(86400);
+    expect(typeof testSchema.get('token_expiration')).toBe('number');
+  });
+
+  it('should accept different string numbers', () => {
+    const testSchema = convict({
+      token_expiration: {
+        format: 'tokenExpiration',
+        default: 'never',
+        env: 'TEST_TOKEN_EXPIRATION',
+      },
+    });
+
+    // Test different string numbers
+    const testCases = ['3600', '7200', '0'];
+
+    for (const testValue of testCases) {
+      process.env.TEST_TOKEN_EXPIRATION = testValue;
+      testSchema.load({});
+      expect(() => testSchema.validate()).not.toThrow();
+      expect(testSchema.get('token_expiration')).toBe(Number(testValue));
+      expect(typeof testSchema.get('token_expiration')).toBe('number');
+    }
+  });
+
+  it('should accept special string values', () => {
+    const testSchema = convict({
+      token_expiration: {
+        format: 'tokenExpiration',
+        default: 'never',
+        env: 'TEST_TOKEN_EXPIRATION',
+      },
+    });
+
+    // Test 'never' value
+    process.env.TEST_TOKEN_EXPIRATION = 'never';
+    testSchema.load({});
+    expect(() => testSchema.validate()).not.toThrow();
+    expect(testSchema.get('token_expiration')).toBe('never');
+
+    // Test 'openid-provider' value
+    process.env.TEST_TOKEN_EXPIRATION = 'openid-provider';
+    testSchema.load({});
+    expect(() => testSchema.validate()).not.toThrow();
+    expect(testSchema.get('token_expiration')).toBe('openid-provider');
+  });
+
+  it('should accept numeric values directly', () => {
+    const testSchema = convict({
+      token_expiration: {
+        format: 'tokenExpiration',
+        default: 'never',
+      },
+    });
+
+    testSchema.set('token_expiration', 86400);
+    expect(() => testSchema.validate()).not.toThrow();
+    expect(testSchema.get('token_expiration')).toBe(86400);
+  });
+
+  it('should reject invalid string values', () => {
+    const testSchema = convict({
+      token_expiration: {
+        format: 'tokenExpiration',
+        default: 'never',
+        env: 'TEST_TOKEN_EXPIRATION',
+      },
+    });
+
+    // Test invalid string
+    process.env.TEST_TOKEN_EXPIRATION = 'invalid';
+    testSchema.load({});
+    expect(() => testSchema.validate()).toThrow(
+      /Invalid token_expiration value/,
+    );
+
+    // Test negative number as string
+    process.env.TEST_TOKEN_EXPIRATION = '-100';
+    testSchema.load({});
+    expect(() => testSchema.validate()).toThrow(
+      /Invalid token_expiration value/,
+    );
+  });
+});
diff --git a/upcoming-release-notes/5782.md b/upcoming-release-notes/5782.md
new file mode 100644
index 0000000000..cd183ccada
--- /dev/null
+++ b/upcoming-release-notes/5782.md
@@ -0,0 +1,7 @@
+---
+category: Bugfix
+authors: [MatissJanis]
+---
+
+Fix token expiration parsing to accept numeric strings and validate special string formats.
+