claude-sonnet-4-5-20250929 vs 4-20250514: Version Diff Guide
Anthropic maintains explicit date-stamped model identifiers so developers can pin to specific versions. claude-sonnet-4-5-20250929 (released September 29, 2025) and claude-sonnet-4-20250514 (released May 14, 2025) are the two most-searched Sonnet 4 family snapshots. This guide covers the concrete differences, when to use each, and why most teams should migrate — or skip ahead to Sonnet 4.6. All data verified against Anthropic's official migration guides and model documentation as of April 2026.
For new projects: neither — use claude-sonnet-4-6 (latest Sonnet 4 family, shipped Q1 2026, strongest quality)
For existing code on Sonnet 4: migrate to 4-5 — zero breaking changes, broad quality improvements
For reproducing historical outputs: pin to specific date-stamped ID to guarantee behavior
Version pinning matters for reproducibility. For capability, always prefer the latest.
What Each Model Is
claude-sonnet-4-20250514: The original Claude Sonnet 4, released May 14, 2025. First mid-tier Sonnet of the Claude 4 family. Positioned as the quality-per-dollar workhorse alongside Opus 4 (frontier) and Haiku 4 (budget).
claude-sonnet-4-5-20250929: Sonnet 4.5, released September 29, 2025. A point release that improved benchmarks across the board without changing API surface. Knowledge cutoff pushed to 2025-01-31.
Both are part of the Claude 4 generation architecture. Sonnet 4.6 (2026-Q1) succeeded them both.
Side-by-Side Comparison
Attribute
Sonnet 4 (20250514)
Sonnet 4.5 (20250929)
Release date
May 14, 2025
September 29, 2025
Input price / MTok
$3.00
$3.00 (same)
Output price / MTok
5.00
5.00 (same)
Context window
200K tokens
200K tokens
Max output
64K tokens
64K tokens
Knowledge cutoff
earlier 2025
January 31, 2025
Multimodal (vision)
Yes
Yes
API breaking changes
—
None
Relative quality
baseline
Better on 6 major benchmarks
Pricing is identical. The only reason to stay on Sonnet 4 rather than 4.5 is bit-exact reproducibility of prior outputs — no cost or capability reason.
Benchmark Differences
Sonnet 4.5 improves on Sonnet 4 across specific benchmark categories:
Benchmark
Sonnet 4
Sonnet 4.5
Delta
AIME 2025 (math)
—
Better
✓
GPQA (grad-level QA)
—
Better
✓
MMMLU (multilingual MMLU)
—
Better
✓
TAU-bench Airline
—
Better
✓
TAU-bench Retail
—
Better
✓
Terminal-Bench
—
Better
✓
Anthropic didn't publish exact point deltas, but across the six benchmarks, Sonnet 4.5 leads consistently.
What didn't change meaningfully: overall MMLU, basic coding benchmarks, routine Q&A. If your workload is dominated by these, the upgrade is marginal.
What did change meaningfully: math reasoning (AIME), graduate-level QA (GPQA), and multi-step agent benchmarks (TAU). If your use case is reasoning- or agent-heavy, the upgrade is more noticeable.
API Compatibility and Migration
Zero breaking changes. Migration is a model identifier swap:
# Before
response = client.messages.create(
model="claude-sonnet-4-20250514",
max_tokens=1024,
messages=[{"role": "user", "content": "Hello"}],
)
# After (no other changes)
response = client.messages.create(
model="claude-sonnet-4-5-20250929",
max_tokens=1024,
messages=[{"role": "user", "content": "Hello"}],
)
All request fields, response format, tool use syntax, vision capabilities, streaming behavior work identically.
What to test anyway after migration:
Output format subtle differences (list styles, emoji usage, tone)
Refusal pattern edge cases (slightly different safety calibration)
Response length distribution (may differ slightly)
These are cosmetic, not breaking. For most production apps, migration is a 5-minute change.
Supported LLM Providers and Model Routing
Both versions are accessible via:
Anthropic API (api.anthropic.com) — official endpoint
AWS Bedrock — Anthropic partnership, similar pricing
Google Vertex AI — Claude models available via partnership
OpenAI-compatible aggregators — TokenMix.ai, OpenRouter, and similar
Through TokenMix.ai, you can access both claude-sonnet-4-20250514 and claude-sonnet-4-5-20250929 alongside the current claude-sonnet-4-6, claude-opus-4-7, GPT-5.5, DeepSeek V4-Pro, Kimi K2.6, Gemini 3.1 Pro, and 300+ other models through a single API key. Useful for comparing output across versions or across providers on the same prompts.
from openai import OpenAI
client = OpenAI(
api_key="your-tokenmix-key",
base_url="https://api.tokenmix.ai/v1",
)
# A/B across versions
for model in ["claude-sonnet-4-20250514", "claude-sonnet-4-5-20250929", "claude-sonnet-4-6"]:
response = client.chat.completions.create(
model=model,
messages=[{"role": "user", "content": test_prompt}],
)
print(f"{model}: {response.choices[0].message.content[:200]}")
Should You Just Use Sonnet 4.6?
Almost certainly yes.
Sonnet 4.6 (released early 2026) is:
Same pricing as 4 and 4.5 ($3/
5 per MTok)
Broadly better on all benchmarks
200K context (some variants up to 1M)
Still API-compatible with Sonnet 4 and 4.5
Unless you have specific reproducibility requirements, migrate straight to 4.6. No reason to intermediate through 4.5.
For newest projects: Sonnet 4.6 is the right default.
For legacy maintenance: match the version in production when troubleshooting; upgrade when convenient.
When Specific Version Pinning Matters
Three scenarios where date-stamped IDs are critical:
1. Research reproducibility. If you publish benchmark results or analysis based on specific model behavior, pin the date-stamped ID. Un-pinned identifiers (claude-sonnet-4 without date) may point to different snapshots over time.
2. A/B testing infrastructure. If you're running experiments comparing model versions, date-stamped IDs guarantee you're comparing what you think you're comparing.
3. Regulatory / compliance contexts. Some regulated environments (finance, healthcare) require audit trails of exactly which model version generated which output. Date-stamped IDs provide this.
For most production apps, you don't need date-stamped pinning. Use the latest version identifier and enjoy automatic improvements.
Known Limitations
Sonnet 4 specifically:
Older knowledge cutoff than 4.5
Weaker on reasoning benchmarks than 4.5 or 4.6
Still API-supported as of April 2026, but may be deprecated within 12-18 months
Sonnet 4.5 specifically:
Superseded by 4.6 for all new workloads
Expected to remain API-supported for 12+ months
Won't receive further improvements
Both versions:
Output token max is 64K — long structured outputs may need to be chunked
Context window is 200K (less than Sonnet 4.6's potential 1M)
Zero native audio/video input (vision only)
FAQ
Is there a price difference between Sonnet 4 and Sonnet 4.5?
No. Both are $3/
5 per million tokens. Anthropic kept pricing identical across the point release.
Do my prompts need to change when migrating?
Almost never. Both models follow instructions similarly. For rare cases where output style differs, minor prompt tweaks may help, but the API contract is unchanged.
What about pinning to claude-sonnet-4-5 (without date)?
Un-dated identifiers (like claude-sonnet-4-5) may resolve to whichever snapshot Anthropic considers current. For reproducibility, use the full date-stamped ID.
When will these versions be deprecated?
Anthropic hasn't announced specific end-of-life dates. Based on typical lifecycle (12-24 months for legacy models), expect API removal within 2026-2027. Plan migration to Sonnet 4.6 or later.
Is Sonnet 4.5 strictly better than Sonnet 4?
On benchmarks yes. In practice, for routine workloads, the difference is often not visible to users. For reasoning-heavy or agent-heavy tasks, 4.5 is noticeably better.
What's the best migration path?
If on Sonnet 4: jump directly to Sonnet 4.6 (skip 4.5). If stuck on Sonnet 4 for compliance/reproducibility: stay, upgrade later. Both are price-identical to 4.6.
Does vision work the same in both versions?
Yes. Same image input format, same resolution support, same quality tier.
Can I use both versions in the same app?
Yes. Route different paths through different versions if needed. Through TokenMix.ai, switching versions is a model identifier change in the request.
What about Sonnet 3.5 / 3.7 / earlier versions?
Those are Claude 3 family, separate generation. Sonnet 3.5 remains available; performance is meaningfully lower than Sonnet 4+. Don't use for new work.
