diff --git a/src/assumeRole.ts b/src/assumeRole.ts
index 4506594..1f05923 100644
--- a/src/assumeRole.ts
+++ b/src/assumeRole.ts
@@ -81,6 +81,62 @@ export interface assumeRoleParams {
   customTags?: string;
 }
 
+const TAG_KEY_REGEX = /^[\p{L}\p{Z}\p{N}_.:/=+\-@]+$/u;
+const TAG_VALUE_REGEX = /^[\p{L}\p{Z}\p{N}_.:/=+\-@]*$/u;
+const MAX_TAG_KEY_LENGTH = 128;
+const MAX_TAG_VALUE_LENGTH = 256;
+const MAX_SESSION_TAGS = 50;
+
+export function parseAndValidateCustomTags(customTags: string, existingTags: Tag[]): Tag[] {
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(customTags);
+  } catch {
+    throw new Error('custom-tags: input is not valid JSON');
+  }
+
+  if (typeof parsed !== 'object' || parsed === null || Array.isArray(parsed)) {
+    throw new Error('custom-tags: input must be a JSON object (not an array or primitive)');
+  }
+
+  const reservedKeys = new Set(existingTags.map((tag) => tag.Key));
+  const newTags: Tag[] = [];
+
+  for (const [key, value] of Object.entries(parsed)) {
+    if (typeof value === 'object' && value !== null) {
+      throw new Error(`custom-tags: value for key '${key}' must be a string, number, or boolean (not an object or array)`);
+    }
+
+    const stringValue = String(value);
+
+    if (key.length === 0 || key.length > MAX_TAG_KEY_LENGTH) {
+      throw new Error(`custom-tags: key '${key}' must be between 1 and ${MAX_TAG_KEY_LENGTH} characters`);
+    }
+    if (stringValue.length > MAX_TAG_VALUE_LENGTH) {
+      throw new Error(`custom-tags: value for key '${key}' exceeds maximum length of ${MAX_TAG_VALUE_LENGTH} characters`);
+    }
+    if (!TAG_KEY_REGEX.test(key)) {
+      throw new Error(`custom-tags: key '${key}' contains invalid characters. Allowed: unicode letters, digits, spaces, and _.:/=+-@`);
+    }
+    if (stringValue.length > 0 && !TAG_VALUE_REGEX.test(stringValue)) {
+      throw new Error(`custom-tags: value for key '${key}' contains invalid characters. Allowed: unicode letters, digits, spaces, and _.:/=+-@`);
+    }
+    if (reservedKeys.has(key)) {
+      throw new Error(`custom-tags: key '${key}' conflicts with a default session tag set by this action and cannot be overridden`);
+    }
+
+    newTags.push({ Key: key, Value: stringValue });
+  }
+
+  if (existingTags.length + newTags.length > MAX_SESSION_TAGS) {
+    throw new Error(
+      `custom-tags: total session tags (${existingTags.length + newTags.length}) would exceed the AWS limit of ${MAX_SESSION_TAGS}`,
+    );
+  }
+
+  return newTags;
+}
+
 export async function assumeRole(params: assumeRoleParams) {
   const {
     credentialsClient,
@@ -122,19 +178,8 @@ export async function assumeRole(params: assumeRoleParams) {
   }
 
   if (customTags) {
-    try {
-      const parsed = JSON.parse(customTags);
-
-      // Then do the mapping
-      const newTags = Object.entries(parsed).map(([Key, Value]) => ({
-        Key,
-        Value: String(Value),
-      }));
-
-      tagArray.push(...newTags);
-    } catch {
-      throw new Error('Invalid custom-tags, json is not valid');
-    }
+    const parsed = parseAndValidateCustomTags(customTags, tagArray);
+    tagArray.push(...parsed);
   }
 
   const tags = roleSkipSessionTagging ? undefined : tagArray;
diff --git a/src/index.ts b/src/index.ts
index 47657f8..b7e5c9d 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -191,6 +191,13 @@ export async function run() {
       await credentialsClient.validateCredentials(AccessKeyId, roleChaining, expectedAccountIds);
       sourceAccountId = await exportAccountId(credentialsClient, maskAccountId);
     }
+    if (customTags && (useGitHubOIDCProvider() || webIdentityTokenFile)) {
+      core.warning(
+        "'custom-tags' is set but will be ignored because session tags cannot be applied when using OIDC or web identity token authentication. " +
+          'Tags are controlled by the identity provider token claims in these authentication flows.',
+      );
+    }
+
     // Get role credentials if configured to do so
     if (roleToAssume) {
       let roleCredentials: AssumeRoleCommandOutput;
diff --git a/test/helpers.test.ts b/test/helpers.test.ts
index ca9929a..09e768a 100644
--- a/test/helpers.test.ts
+++ b/test/helpers.test.ts
@@ -14,7 +14,7 @@ describe('Configure AWS Credentials helpers', {}, () => {
   });
   it('can sleep', {}, async () => {
     const sleep = helpers.defaultSleep(10);
-    expect(Promise.race([sleep, new Promise((_, reject) => setTimeout(reject, 20))])).resolves.toBe(undefined);
+    await expect(Promise.race([sleep, new Promise((_, reject) => setTimeout(reject, 20))])).resolves.toBe(undefined);
   });
   it('removes special characters from workflow names', {}, () => {
     expect(helpers.sanitizeGitHubVariables('sdf234@#$%$^&*()_+{}|:"<>?')).toEqual('sdf234@__________+___:____');
diff --git a/test/index.test.ts b/test/index.test.ts
index 4db19dc..84e25a1 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -248,13 +248,13 @@ describe('Configure AWS Credentials', {}, () => {
         .mockResolvedValueOnce({ accessKeyId: 'STSAWSACCESSKEYID' });
     });
     it('rejects invalid JSON in custom tags', {}, async () => {
-      vi.spyOn(core, 'getInput').mockImplementation(mocks.getInput(mocks.CUSTOM_TAGS_INVALID_JSON_INPUTS));
+      vi.mocked(core.getInput).mockImplementation(mocks.getInput(mocks.CUSTOM_TAGS_INVALID_JSON_INPUTS));
       await run();
-      expect(core.setFailed).toHaveBeenCalledWith('Invalid custom-tags, json is not valid');
-      //expect(mockedSTSClient.commandCalls(AssumeRoleCommand)).toHaveLength(0);
+      expect(core.setFailed).toHaveBeenCalledWith('custom-tags: input is not valid JSON');
+      expect(mockedSTSClient.commandCalls(AssumeRoleCommand)).toHaveLength(0);
     });
-    it('handles object custom tags', {}, async () => {
-      vi.spyOn(core, 'getInput').mockImplementation(mocks.getInput(mocks.CUSTOM_TAGS_OBJECT_INPUTS));
+    it('handles valid custom tags', {}, async () => {
+      vi.mocked(core.getInput).mockImplementation(mocks.getInput(mocks.CUSTOM_TAGS_OBJECT_INPUTS));
       await run();
       expect(core.info).toHaveBeenCalledWith('Assuming role with user credentials');
       expect(core.info).toHaveBeenCalledWith('Authenticated as assumedRoleId AROAFAKEASSUMEDROLEID');
@@ -268,9 +268,53 @@ describe('Configure AWS Credentials', {}, () => {
           { Key: 'Commit', Value: 'MY-COMMIT-ID' },
           { Key: 'Environment', Value: 'Production' },
           { Key: 'Team', Value: 'DevOps' },
-        ])
+        ]),
       });
     });
+    it('rejects array input for custom tags', {}, async () => {
+      vi.mocked(core.getInput).mockImplementation(mocks.getInput(mocks.CUSTOM_TAGS_ARRAY_INPUTS));
+      await run();
+      expect(core.setFailed).toHaveBeenCalledWith(
+        'custom-tags: input must be a JSON object (not an array or primitive)',
+      );
+      expect(mockedSTSClient.commandCalls(AssumeRoleCommand)).toHaveLength(0);
+    });
+    it('rejects custom tags that conflict with default session tags', {}, async () => {
+      vi.mocked(core.getInput).mockImplementation(mocks.getInput(mocks.CUSTOM_TAGS_RESERVED_KEY_INPUTS));
+      await run();
+      expect(core.setFailed).toHaveBeenCalledWith(
+        "custom-tags: key 'Repository' conflicts with a default session tag set by this action and cannot be overridden",
+      );
+      expect(mockedSTSClient.commandCalls(AssumeRoleCommand)).toHaveLength(0);
+    });
+    it('rejects custom tags with invalid key characters', {}, async () => {
+      vi.mocked(core.getInput).mockImplementation(mocks.getInput(mocks.CUSTOM_TAGS_INVALID_KEY_CHARS_INPUTS));
+      await run();
+      expect(core.setFailed).toHaveBeenCalledWith(
+        expect.stringContaining("custom-tags: key 'invalid{key}' contains invalid characters"),
+      );
+      expect(mockedSTSClient.commandCalls(AssumeRoleCommand)).toHaveLength(0);
+    });
+    it('warns when custom tags are used with OIDC', {}, async () => {
+      vi.mocked(core.getInput).mockImplementation(
+        mocks.getInput({
+          ...mocks.GH_OIDC_INPUTS,
+          'custom-tags': JSON.stringify({ MyTag: 'value' }),
+        }),
+      );
+      vi.mocked(core.getIDToken).mockResolvedValue('testoidctoken');
+      mockedSTSClient.on(AssumeRoleWithWebIdentityCommand).resolvesOnce(mocks.outputs.STS_CREDENTIALS);
+      mockedSTSClient.on(GetCallerIdentityCommand).resolves({ ...mocks.outputs.GET_CALLER_IDENTITY });
+      // biome-ignore lint/suspicious/noExplicitAny: any required to mock private method
+      vi.spyOn(CredentialsClient.prototype as any, 'loadCredentials').mockResolvedValue({
+        accessKeyId: 'STSAWSACCESSKEYID',
+      });
+      process.env.ACTIONS_ID_TOKEN_REQUEST_TOKEN = 'fake-token';
+      await run();
+      expect(core.warning).toHaveBeenCalledWith(
+        expect.stringContaining("'custom-tags' is set but will be ignored"),
+      );
+    });
   });
 
   describe('Odd inputs', {}, () => {
@@ -305,7 +349,7 @@ describe('Configure AWS Credentials', {}, () => {
       expect(core.setFailed).toHaveBeenCalled();
     });
 
-    it('handles improper retry-max-attempts input', async () => {
+    it('handles improper retry-max-attempts input', {}, async () => {
       // This should mean we retry one time
       vi.mocked(core.getInput).mockImplementation(
         mocks.getInput({
@@ -748,7 +792,7 @@ describe('Configure AWS Credentials', {}, () => {
 
       // Get the timeout callback function
       const timeoutCallback = setTimeoutSpy.mock.calls[0][0] as () => void;
-      
+
       // Execute the timeout callback
       timeoutCallback();
 
@@ -1051,4 +1095,3 @@ describe('Configure AWS Credentials', {}, () => {
     });
   });
 });
-
diff --git a/test/mockinputs.test.ts b/test/mockinputs.test.ts
index f83ec60..a8bdd2a 100644
--- a/test/mockinputs.test.ts
+++ b/test/mockinputs.test.ts
@@ -6,13 +6,6 @@ const inputs = {
     'aws-region': 'fake-region-1',
     'special-characters-workaround': 'true',
   },
-  CUSTOM_TAGS_JSON_INPUTS: {
-    'aws-access-key-id': 'MYAWSACCESSKEYID',
-    'aws-secret-access-key': 'MYAWSSECRETACCESSKEY',
-    'role-to-assume': 'arn:aws:iam::111111111111:role/MY-ROLE',
-    'aws-region': 'fake-region-1',
-    'custom-tags': '{"Environment": "Production", "Team": "DevOps"}',
-  },
   CUSTOM_TAGS_INVALID_JSON_INPUTS: {
     'aws-access-key-id': 'MYAWSACCESSKEYID',
     'aws-secret-access-key': 'MYAWSSECRETACCESSKEY',
@@ -21,6 +14,30 @@ const inputs = {
     'retry-max-attempts': '1',
     'custom-tags': 'not a json',
   },
+  CUSTOM_TAGS_ARRAY_INPUTS: {
+    'aws-access-key-id': 'MYAWSACCESSKEYID',
+    'aws-secret-access-key': 'MYAWSSECRETACCESSKEY',
+    'role-to-assume': 'arn:aws:iam::111111111111:role/MY-ROLE',
+    'aws-region': 'fake-region-1',
+    'retry-max-attempts': '1',
+    'custom-tags': '[1, 2, 3]',
+  },
+  CUSTOM_TAGS_RESERVED_KEY_INPUTS: {
+    'aws-access-key-id': 'MYAWSACCESSKEYID',
+    'aws-secret-access-key': 'MYAWSSECRETACCESSKEY',
+    'role-to-assume': 'arn:aws:iam::111111111111:role/MY-ROLE',
+    'aws-region': 'fake-region-1',
+    'retry-max-attempts': '1',
+    'custom-tags': JSON.stringify({ Repository: 'evil-repo' }),
+  },
+  CUSTOM_TAGS_INVALID_KEY_CHARS_INPUTS: {
+    'aws-access-key-id': 'MYAWSACCESSKEYID',
+    'aws-secret-access-key': 'MYAWSSECRETACCESSKEY',
+    'role-to-assume': 'arn:aws:iam::111111111111:role/MY-ROLE',
+    'aws-region': 'fake-region-1',
+    'retry-max-attempts': '1',
+    'custom-tags': JSON.stringify({ 'invalid{key}': 'value' }),
+  },
   CUSTOM_TAGS_OBJECT_INPUTS: {
     'aws-access-key-id': 'MYAWSACCESSKEYID',
     'aws-secret-access-key': 'MYAWSSECRETACCESSKEY',