r/hermesagent PSA: LongCat 2.0 Reasoning-Tier Bug and Model Positioning (Between DeepSeek V4 Flash/Pro)

One-sentence takeaway

Testing confirmed that LongCat-2.0's API accepts only three reasoning-effort tiers, low/med/high. When it receives another tier (such as xhigh from the DeepSeek family), it returns a malformed 200 response instead of an error, causing Hermes Agent to silently fall back to a fallback provider. The same user positioned its capabilities between DeepSeek-V4-Flash and DeepSeek-V4-Pro, with good caching and cheap PAYG pricing.

Original report (key points from the original)

Model positioning (user's subjective view, based on actual use)

• "After using it for a while, my feeling is that it's (capability-wise) between Deepseek-4-Flash and Deepseek-4-Pro, and the pricing is too."

• They like that it offers a PAYG (pay-as-you-go) plan; caching works as well as Deepseek's, and the API is "extremely cheap" in actual use.

• "Definitely going to be in my regular model rotation."

Bug details (core of the PSA)

• Scenario: the Agent was switched from DS4P to LongCat-2.0 and immediately hit the fallback provider.

• Error (agent.log, excerpt from the original):

agent.conversation_loop: API call failed (attempt 1/3) error_type=RuntimeError
provider=custom base_url=https://api.longcat.chat/openai/v1 model=LongCat-2.0
summary=Provider returned an empty stream with no finish_reason
(possible upstream error or malformed SSE response).

• Root cause: LongCat accepts only low, med, high reasoning levels; the user had previously set xhigh on DeepSeek-4-Pro to obtain maximum thinking. When Hermes switched models, it did not change the reasoning effort, so it sent xhigh to LongCat; LongCat returned a malformed 200 response (an empty stream with no finish_reason) instead of an error response.

• The issue was reported to LongCat, which acknowledged it ("They appear to have acknowledged the issue").

• Lesson: when frequently switching models in Hermes and different models use different reasoning-effort tiers, change the effort back to low/med/high before switching to LongCat.

Review and scope

• This is a single-point report from an anonymous user, with the error log quoted from the original. It differs from the thinking: {"type":"enabled"/"disabled"} field in the official API reference (Prompt Directory 01) — this report concerns the reasoning-effort concept on the Hermes side, and the official documentation does not document how the two map to each other. Take care when switching models.

• The capability positioning (between DS4-Flash and DS4-Pro) is subjective and complements AlphaSignal's claim that DeepSeek V4-Pro is cheaper (Review 04) and the OpenRouter third-party index (Review 03); these can be cross-checked against one another.

• Task fit: suitable for cache-friendly Agent loops and PAYG cost-sensitive scenarios; Hermes users who switch models frequently should watch for the configuration pitfall above.