diff --git a/src/extension/ipc/network/interpolate-vars.spec.ts b/src/extension/ipc/network/interpolate-vars.spec.ts
new file mode 100644
index 0000000..0653c1c
--- /dev/null
+++ b/src/extension/ipc/network/interpolate-vars.spec.ts
@@ -0,0 +1,85 @@
+import { describe, it, expect } from 'vitest';
+import { interpolateVars } from './interpolate-vars';
+
+const formUrlEncodedHeaders = { 'Content-Type': 'application/x-www-form-urlencoded' };
+
+describe('interpolateVars - form-urlencoded body', () => {
+  it('interpolates values in array form data', () => {
+    const request = {
+      url: 'https://example.com/token',
+      headers: { ...formUrlEncodedHeaders },
+      data: [
+        { name: 'grant_type', value: 'client_credentials', enabled: true },
+        { name: 'client_secret', value: '{{ClientSecret}}', enabled: true }
+      ]
+    };
+
+    const result = interpolateVars(request, {
+      envVars: { ClientSecret: 's3cr3t-value' }
+    });
+
+    expect(result.data[1].value).toBe('s3cr3t-value');
+  });
+
+  it('interpolates variables in a pre-serialized string body (send-http-request flow)', () => {
+    // The send-http-request handler stringifies the form body via qs.stringify
+    // before scripts run, so interpolateVars receives a string with encoded braces
+    const request = {
+      url: 'https://example.com/token',
+      headers: { ...formUrlEncodedHeaders },
+      data: 'grant_type=client_credentials&client_secret=%7B%7BClientSecret%7D%7D&client_id=my-client'
+    };
+
+    const result = interpolateVars(request, {
+      envVars: { ClientSecret: 's3cr3t-value' }
+    });
+
+    const params = new URLSearchParams(result.data as string);
+    expect(params.get('client_secret')).toBe('s3cr3t-value');
+    expect(params.get('grant_type')).toBe('client_credentials');
+    expect(params.get('client_id')).toBe('my-client');
+  });
+
+  it('interpolates unencoded braces in a string body', () => {
+    const request = {
+      url: 'https://example.com/token',
+      headers: { ...formUrlEncodedHeaders },
+      data: 'client_secret={{ClientSecret}}'
+    };
+
+    const result = interpolateVars(request, {
+      runtimeVariables: { ClientSecret: 'runtime-value' }
+    });
+
+    const params = new URLSearchParams(result.data as string);
+    expect(params.get('client_secret')).toBe('runtime-value');
+  });
+
+  it('resolves process.env variables in a string body', () => {
+    const request = {
+      url: 'https://example.com/token',
+      headers: { ...formUrlEncodedHeaders },
+      data: 'client_secret=%7B%7Bprocess.env.CLIENT_SECRET%7D%7D'
+    };
+
+    const result = interpolateVars(request, {
+      processEnvVars: { CLIENT_SECRET: 'env-file-value' }
+    });
+
+    const params = new URLSearchParams(result.data as string);
+    expect(params.get('client_secret')).toBe('env-file-value');
+  });
+
+  it('leaves a string body without variables untouched', () => {
+    const data = 'grant_type=client_credentials&client_secret=plain-value';
+    const request = {
+      url: 'https://example.com/token',
+      headers: { ...formUrlEncodedHeaders },
+      data
+    };
+
+    const result = interpolateVars(request, { envVars: {} });
+
+    expect(result.data).toBe(data);
+  });
+});
diff --git a/src/extension/ipc/network/interpolate-vars.ts b/src/extension/ipc/network/interpolate-vars.ts
index 7d561c9..7e859e1 100644
--- a/src/extension/ipc/network/interpolate-vars.ts
+++ b/src/extension/ipc/network/interpolate-vars.ts
@@ -228,6 +228,20 @@ const interpolateVars = (request: any, options: InterpolationOptions): any => {
           ...d,
           value: _interpolate(d?.value)
         }));
+      } else if (typeof request.data === 'string' && request.data.length && (request.data.includes('{{') || request.data.includes('%7B%7B'))) {
+        // The body may have been serialized to a form-urlencoded string before
+        // script execution (see send-http-request handler). Parse it, interpolate
+        // each name/value pair, then re-serialize so {{variables}} are resolved.
+        try {
+          const parsedParams = new URLSearchParams(request.data);
+          const interpolatedParams = new URLSearchParams();
+          for (const [name, value] of parsedParams) {
+            interpolatedParams.append(_interpolate(name) as string, _interpolate(value) as string);
+          }
+          request.data = interpolatedParams.toString();
+        } catch {
+          // Leave the body as-is if it cannot be parsed
+        }
       }
     } else if (contentType === 'multipart/form-data') {
       if (Array.isArray(request?.data) && !(request.data instanceof FormData)) {