SWE-Marathon-Ext C

Introducing the Mercor Research team. We’re building a world-class research organization, including @edwardjhu (first author of LoRA), @MinyangTian1 (SciCode, CritPt), Victor Barres (τ²-bench, τ-voice), @hrdkbhatnagar (PostTrainBench), @bertievidgen (APEX-Agents) and many others. As models get better, the questions get harder: → How do you measure a model on a job that AI itself is changing? → What data helps when a model fails at real work? → How much can a small amount of expert data improve a model? Mercor Research covers the full AI flywheel. APEX shows where a model struggles. Experts help us understand why and create data for those weak points. We post-train the model and measure it again. Learn more about our research mission:

— mercor (@mercor) Oct 8, 2026

Claude Haiku 5.5 debuts at #7 on APEX-SWE. 55.1% Pass@1, ahead of last generation's Opus and Sonnet models. It achieves this score using 1/20th of Sonnet 5.5's input and output cost. Congrats @AnthropicAI on the launch. 𝗖𝗼𝗱𝗶𝗻𝗴 Anthropic positions Haiku 5.5 as a coding subagent next to Opus 5.5 and Sonnet 5.5. Its results on APEX-SWE suggest this model is a strong coding partner. APEX-SWE: 55.1% (#7) Integration: 63.0% (#15) Observability: 47.2% (#7) APEX-SWE covers two kinds of work. - 𝗢𝗯𝘀𝗲𝗿𝘃𝗮𝗯𝗶𝗹𝗶𝘁𝘆 asks the agent to debug a production-style system from logs and telemetry. Haiku 5.5 excels here and ranks #7, behind only six models. It still trails Opus 5.5 (69.8%) by 22.6 pts, and a typical attempt uses about 2.5M tokens, 4.5x an Integration attempt. - 𝗜𝗻𝘁𝗲𝗴𝗿𝗮𝘁𝗶𝗼𝗻 asks the agent to build and deploy systems across services. Haiku 5.5 finishes 2.5 pts behind Opus 5.5 (65.5%), but the field is bunched, and it ranks #15. Across 4 runs per task, Haiku 5.5 solved 62.5% of APEX-SWE tasks at least once. 75 of 200 tasks were never solved. It also passes last generation's Anthropic models on APEX-SWE: Opus 4.8: 43.9% Opus 4.7: 47.6% Sonnet 5: 46.4% 𝗣𝗿𝗼𝗳𝗲𝘀𝘀𝗶𝗼𝗻𝗮𝗹 𝘄𝗼𝗿𝗸 APEX-Agents is our flagship benchmark for long-horizon, agentic professional services work. It tests workflows across management consulting, corporate law, and investment banking. APEX-Agents: 56.1% (#19) Management consulting: 50.9% (#22) Corporate law: 60.0% (#26) Investment banking: 57.5% (#13) That puts it 19.4 pts behind Sonnet 5.5 (75.5%) and 17.4 pts behind Opus 5.5 (73.5%). This matches Anthropic's own advice. Sonnet and Opus are for complex agentic work, Haiku is for scoped, high-volume tasks. 𝗖𝗼𝘀𝘁 𝗮𝗻𝗱 𝗰𝗼𝗻𝘀𝗶𝘀𝘁𝗲𝗻𝗰𝘆 Haiku 5.5 lists at $0.10 input and $0.50 output per 1M tokens. That is 1/20th of Sonnet 5.5's input and output cost for prompts up to 100K tokens, which Anthropic says covers about 90% of Haiku requests. It runs about 75% cheaper than Haiku 4.5 on average, and it is the first Haiku with an adjustable effort setting. We tested all three 5.5 models at Max effort. On APEX-SWE, Haiku 5.5's output token cost was about 35x lower than Sonnet 5.5 and about 60x lower than Opus 5.5. It also finished a typical task about 3x faster. On APEX-Agents, a typical Haiku 5.5 task used more output tokens than Sonnet 5.5 and Opus 5.5 combined. It still costs about 14x to 16x less on output tokens. Across 4 runs per task on APEX-Agents, Haiku 5.5 solved 74.6% of tasks at least once, but only 37.9% on every run. See full leaderboard:

— mercor (@mercor) Oct 8, 2026

Watch Mercor's Head of AI modeling @edwardjhu to learn about AI data, evaluation, and model training. Find out why "the closer we are to how AI actually gets deployed, the more realistic the benchmarks will be."

— mercor (@mercor) Oct 7, 2026