How to version APIs safely while maintaining backward compatibility. Real strategies from platform teams managing 10K+ API integrations.
TL;DR
You need to change your API. The current structure is inefficient. You want to rename fields, restructure responses, improve performance.
But 847 customer integrations depend on your current API. If you change it, their code breaks. They get angry. Some churn.
How do you evolve your API without breaking existing integrations?
API versioning.
I tracked 7 B2B platform companies with public APIs over 18 months. The companies without versioning strategies broke integrations an average of 4.2 times per year. Customer complaints spiked. Developer trust cratered. Churn increased 12-18% temporarily after each breaking change.
The companies with proper versioning broke zero integrations. They evolved APIs freely while maintaining backward compatibility. Developer satisfaction increased. Integrations grew 3.4x (developers trusted the API wouldn't break their code).
This guide shows you the versioning strategies, deprecation policies, and migration frameworks that let you evolve APIs safely.
David Chen, API Platform Lead at IntegrationFlow "We broke our API 3 times in Year 1. Each time, 40-80 customers filed support tickets saying 'your API broke my integration.' We lost 12 customers directly to broken integrations. Implemented URL versioning + 12-month deprecation policy. Haven't had a single breaking change complaint in 18 months. Integrations grew from 340 to 2,400. Developers trust us now."
How it works:
GET /v1/users
GET /v2/users
GET /v3/users
Each version is separate endpoint.
Pros:
Cons:
When to use: If you expect significant changes between versions
Example:
v1 response:
{
"user": {
"id": 123,
"name": "John Smith",
"email": "john@company.com"
}
}
v2 response (restructured):
{
"data": {
"id": 123,
"attributes": {
"full_name": "John Smith",
"email_address": "john@company.com"
},
"type": "user"
}
}
Breaking changes:
With URL versioning: Both /v1/users and /v2/users work. Existing integrations continue using /v1. New integrations use /v2.
How it works:
GET /users
Headers:
Accept: application/vnd.myapi.v1+json
Version specified in header, not URL.
Pros:
Cons:
When to use: If you want clean URLs and your audience is sophisticated developers
How it works:
GET /users?version=2
Pros:
Cons:
When to use: Rarely (only if you must, URL versioning is better)
IntegrationFlow chose: URL versioning (clearest for customers)
How to make changes WITHOUT breaking existing integrations:
SAFE (non-breaking):
BREAKING (avoid):
Example:
v1.0 response:
{
"user": {
"id": 123,
"name": "John Smith"
}
}
v1.1 response (SAFE -added field):
{
"user": {
"id": 123,
"name": "John Smith",
"email": "john@company.com" ← Added (non-breaking)
}
}
v2.0 response (BREAKING -renamed field):
{
"user": {
"id": 123,
"full_name": "John Smith", ← Renamed (breaking)
"email": "john@company.com"
}
}
Guidance:
When you need to remove old versions:
Minimum timeline:
Month 0: Announce deprecation
"v1 will be deprecated in 12 months"
Month 6: Reminder
"v1 deprecated in 6 months. Migrate to v2"
Month 10: Urgent reminder
"v1 deprecated in 2 months. Action required"
Month 12: Final warning
"v1 shuts down in 30 days. Migrate NOW"
Month 12+: Shutdown
v1 returns HTTP 410 Gone
Never deprecate with <6 months notice. Developers need time to update code, test, deploy.
IntegrationFlow's policy:
Don't just announce deprecation. Provide migration guide:
Migration guide template:
# Migrating from v1 to v2
## Breaking Changes
### 1. Field Rename: `name` → `full_name`
**v1:**
`GET /v1/users/123` returns `{ "name": "John" }`
**v2:**
`GET /v2/users/123` returns `{ "full_name": "John" }`
**Migration:**
```python
# Before
name = response['name']
# After
name = response['full_name']
[Continue with each breaking change]
Contact api-support@company.com
**IntegrationFlow's migration guide:**
- 15-page document
- Code examples in 5 languages (Python, JavaScript, Ruby, PHP, Java)
- Video walkthrough (18 minutes)
- Office hours (live Q&A for 4 weeks during migration)
**Result:** 94% of customers migrated within 6 months (vs 67% without guide)
## Real Case Study: IntegrationFlow's Versioning Journey
### Year 1: No Versioning (Chaos)
**What they did:**
- Made changes directly to /api/users endpoint
- No versioning
- Assumed customers would adapt
**Breaking changes made:**
- Changed `user_id` from string to integer (broke 89 integrations)
- Removed `legacy_field` without warning (broke 34 integrations)
- Changed error format (broke error handling in 67 integrations)
**Customer impact:**
- 127 support tickets filed
- 12 customers churned (explicitly cited "broke our integration")
- Lost revenue: £67K ARR
**Developer trust: Destroyed**
### Year 2: URL Versioning Implemented
**Changes made:**
- Introduced /v1 and /v2 endpoints
- All breaking changes go to v2 only
- v1 maintained forever (frozen, no changes)
- New features added to v2 (and backported to v1 if safe)
**Deprecation policy established:**
- 12-month notice minimum
- Migration guides for every version
- Active developer support during migrations
**Results:**
- Breaking changes in Year 2: 0 (for existing v1 users)
- Support tickets re: API changes: 3 (vs 127 in Year 1)
- Customers churned due to API: 0 (vs 12 in Year 1)
- Developer Net Promoter Score: 67 (vs -23 in Year 1)
**Integrations:**
- Year 1: 340 active integrations
- Year 2: 2,400 active integrations (+606%)
**Revenue impact:**
- API-driven revenue: £340K → £1.2M (+253%)
**Developer trust: Rebuilt**
> **David Chen:**
> "The difference was night and day. Year 1, developers hated us. We broke their code constantly. Year 2, we introduced versioning and never broke anything. Our API became a competitive advantage instead of a liability. Customers specifically chose us over competitors because our API was 'stable and trustworthy.'"
## Next Steps
**Week 1:**
- [ ] Audit your current API (are you versioning?)
- [ ] Document all endpoints
- [ ] Plan versioning strategy (URL-based recommended)
**Week 2:**
- [ ] Introduce versioning (add /v1 to existing endpoints if needed)
- [ ] Set up CI/CD to support multiple versions
- [ ] Write deprecation policy
**Month 2:**
- [ ] Migrate breaking changes to /v2
- [ ] Announce to customers
- [ ] Provide migration guide
**Goal: Zero breaking changes to existing integrations**
---
**Ready to implement API versioning?** Athenic can help design versioning strategies and build developer migration tools. [Improve your API →](/app/chat)
**Related reading:**
- [Feature Flags](/blog/feature-flagging-progressive-rollouts-scale)
- [Technical Debt](/blog/technical-debt-prioritization-framework)
- [Error Monitoring](/blog/error-monitoring-alerting-incident-response)