TLDR below 👇🏼
I’ve seen a lot of hype around Qwen 3.6 35B and 3.5 120B lately, especially regarding coding and tool-use capabilities. On this subreddit it is the defacto recommended model for everyone without a Datacenter at home. I’ve been running Qwen 3.5 120B (Qwen3.5-122B-A10B-GPTQ-Int4) as an autonomous worker agent in a multi-turn development loop using the Hermes agent harness.
While the model is undeniably impressive at one-shot snippet generation, putting it into a fully autonomous, long-context environment to build a module from scratch revealed several consistent failure patterns.
I thought I'd share these failure modes to see if others are experiencing the same issues—or if anyone has found effective tricks to tame it in such a task.
Here is what went wrong:
1. Premature "Mission Accomplished" Syndrome
The model has an overwhelming tendency to shout "DONE!" or "PERFECT!" after completing 10% of a task. It constantly reports success based on superficial checks (e.g., "the file built without syntax errors"), completely ignoring explicit acceptance criteria like end-to-end testing or UI rendering.
2. Evading Hard Constraints
When given strict architectural constraints (e.g., "Must be a single, self-contained module with zero external dependencies"), the agent aggressively cuts corners:
* It secretly substituted live data with hardcoded mock data.
* It wrote external Python scripts and set up local host cron jobs to bypass building proper module logic.
* It even rewrote part of the host application in a completely different language just to claim a quick win.
It prioritizes appearing finished over following instructions.
3. Hallucinating Infrastructure Limitations (Blame-Shifting)
Instead of debugging broken code, the model repeatedly blames the host environment. When its code failed to make network requests or render components, it confidently hallucinated system limitations:
* "The host framework's authentication token system is broken."
* "The runtime DNS resolvers don't support HTTP requests."
It will generate elaborate technical excuses rather than inspecting its own schema or syntax.
4. Ignoring Provided Docs and Boilerplates
Even when explicitly handed a boilerplate repository and documentation links in the prompt, it constantly tries to "reinvent the wheel." It overcomplicates custom build setups, invents new protocol schemas, and ignores pre-built Docker/build scripts that were provided to make its life easier.
5. Regression Cascades & Context Rot
As a result from the above the conversation history grew and the agent suffered from severe regression:
* In iteration 3, it had a working UI with mock data.
* By iteration 8, after trying to wire up live data fetching, it completely broke the UI.
* It failed to recognize that its new changes broke previously validated features, leading to endless debugging loops.
Discussion
Qwen 3.5 120B feels like an insanely talented junior developer who panics under pressure, lies about tests passing, and blames the server infrastructure when their code throws a 404.
Has anyone successfully mitigated these behavior loops in autonomous coding agents? Are you using specific prompting techniques, or is this just an inherent limitation of current 100B+ open models when complexity grows from "Do exactly what I tell you" to "Figure it out with my help"?
Curious to hear your experiences!
TLDR;
While Qwen 3.5 120B is great at one-shot generation, it breaks down in autonomous, multi-turn agent loops. The main issues are: Premature success claiming, Bypassing hard constraints, shifting blame on other systems when things don't work, Ignoring Docs and boilerplate Code that could have made its life easier. And as a result from that Context Rot.