diff --git a/.changeset/old-seals-visit.md b/.changeset/old-seals-visit.md
new file mode 100644
index 000000000..8eaccd6b9
--- /dev/null
+++ b/.changeset/old-seals-visit.md
@@ -0,0 +1,5 @@
+---
+'@hey-api/client-axios': patch
+---
+
+fix: handle special Axios keywords when merging headers
diff --git a/packages/client-axios/src/__tests__/utils.test.ts b/packages/client-axios/src/__tests__/utils.test.ts
index 337dc7119..088338555 100644
--- a/packages/client-axios/src/__tests__/utils.test.ts
+++ b/packages/client-axios/src/__tests__/utils.test.ts
@@ -1,6 +1,11 @@
 import { describe, expect, it, vi } from 'vitest';
 
-import { getAuthToken, setAuthParams } from '../utils';
+import {
+  axiosHeadersKeywords,
+  getAuthToken,
+  mergeHeaders,
+  setAuthParams,
+} from '../utils';
 
 describe('getAuthToken', () => {
   it('returns access token', async () => {
@@ -70,6 +75,30 @@ describe('getAuthToken', () => {
   });
 });
 
+describe('mergeHeaders', () => {
+  it.each(axiosHeadersKeywords)(
+    'handles "%s" Axios special keyword',
+    (keyword) => {
+      const headers = mergeHeaders(
+        {
+          foo: 'foo',
+        },
+        {
+          [keyword]: {
+            foo: 'foo',
+          },
+        },
+      );
+      expect(headers).toEqual({
+        foo: 'foo',
+        [keyword]: {
+          foo: 'foo',
+        },
+      });
+    },
+  );
+});
+
 describe('setAuthParams', () => {
   it('sets access token in headers', async () => {
     const accessToken = vi.fn().mockReturnValue('foo');
diff --git a/packages/client-axios/src/utils.ts b/packages/client-axios/src/utils.ts
index 196928cf7..d5dc4e6f7 100644
--- a/packages/client-axios/src/utils.ts
+++ b/packages/client-axios/src/utils.ts
@@ -423,6 +423,19 @@ export const mergeConfigs = (a: Config, b: Config): Config => {
   return config;
 };
 
+/**
+ * Special Axios headers keywords allowing to set headers by request method.
+ */
+export const axiosHeadersKeywords = [
+  'common',
+  'delete',
+  'get',
+  'head',
+  'patch',
+  'post',
+  'put',
+] as const;
+
 export const mergeHeaders = (
   ...headers: Array<Required<Config>['headers'] | undefined>
 ): Record<any, unknown> => {
@@ -435,7 +448,17 @@ export const mergeHeaders = (
     const iterator = Object.entries(header);
 
     for (const [key, value] of iterator) {
-      if (value === null) {
+      if (
+        axiosHeadersKeywords.includes(
+          key as (typeof axiosHeadersKeywords)[number],
+        ) &&
+        typeof value === 'object'
+      ) {
+        mergedHeaders[key] = {
+          ...(mergedHeaders[key] as Record<any, unknown>),
+          ...value,
+        };
+      } else if (value === null) {
         delete mergedHeaders[key];
       } else if (Array.isArray(value)) {
         for (const v of value) {
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-axios-bundle/client/utils.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-axios-bundle/client/utils.ts.snap
index 196928cf7..d5dc4e6f7 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-axios-bundle/client/utils.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-axios-bundle/client/utils.ts.snap
@@ -423,6 +423,19 @@ export const mergeConfigs = (a: Config, b: Config): Config => {
   return config;
 };
 
+/**
+ * Special Axios headers keywords allowing to set headers by request method.
+ */
+export const axiosHeadersKeywords = [
+  'common',
+  'delete',
+  'get',
+  'head',
+  'patch',
+  'post',
+  'put',
+] as const;
+
 export const mergeHeaders = (
   ...headers: Array<Required<Config>['headers'] | undefined>
 ): Record<any, unknown> => {
@@ -435,7 +448,17 @@ export const mergeHeaders = (
     const iterator = Object.entries(header);
 
     for (const [key, value] of iterator) {
-      if (value === null) {
+      if (
+        axiosHeadersKeywords.includes(
+          key as (typeof axiosHeadersKeywords)[number],
+        ) &&
+        typeof value === 'object'
+      ) {
+        mergedHeaders[key] = {
+          ...(mergedHeaders[key] as Record<any, unknown>),
+          ...value,
+        };
+      } else if (value === null) {
         delete mergedHeaders[key];
       } else if (Array.isArray(value)) {
         for (const v of value) {
diff --git a/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-axios-bundle_transform/client/utils.ts.snap b/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-axios-bundle_transform/client/utils.ts.snap
index 196928cf7..d5dc4e6f7 100644
--- a/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-axios-bundle_transform/client/utils.ts.snap
+++ b/packages/openapi-ts/test/__snapshots__/test/generated/v3-hey-api-client-axios-bundle_transform/client/utils.ts.snap
@@ -423,6 +423,19 @@ export const mergeConfigs = (a: Config, b: Config): Config => {
   return config;
 };
 
+/**
+ * Special Axios headers keywords allowing to set headers by request method.
+ */
+export const axiosHeadersKeywords = [
+  'common',
+  'delete',
+  'get',
+  'head',
+  'patch',
+  'post',
+  'put',
+] as const;
+
 export const mergeHeaders = (
   ...headers: Array<Required<Config>['headers'] | undefined>
 ): Record<any, unknown> => {
@@ -435,7 +448,17 @@ export const mergeHeaders = (
     const iterator = Object.entries(header);
 
     for (const [key, value] of iterator) {
-      if (value === null) {
+      if (
+        axiosHeadersKeywords.includes(
+          key as (typeof axiosHeadersKeywords)[number],
+        ) &&
+        typeof value === 'object'
+      ) {
+        mergedHeaders[key] = {
+          ...(mergedHeaders[key] as Record<any, unknown>),
+          ...value,
+        };
+      } else if (value === null) {
         delete mergedHeaders[key];
       } else if (Array.isArray(value)) {
         for (const v of value) {