LongCat-Flash-Chat launched in the API in 2025-09 and received code, context, and Agent upgrades in 2025-12, but the official record says it was retired alongside other legacy models on 2026-05-29. New projects should migrate to LongCat-2.0 or self-host the open-source weights.
Good for: Checking historical API availability, migrating legacy models, explaining the 128K/256K context discrepancy, and defining the current integration boundary.
Not for: Continuing to request or retry a retired service by its old model name; LongCat-2.0's capabilities and pricing also must not be presented as Flash-Chat material.
Applicable model versions: The historical LongCat-Flash-Chat service, its 2025-12 update, and the post-2026-05-29 migration target LongCat-2.0-Preview.
Applicable clients, Agents, or APIs: The historical LongCat API; the current platform Change Log points to LongCat-2.0 and ecosystems including Claude Code, Hermes, OpenClaw, OpenCode, and Kilo Code.
Recommended inference tier and parameters: Flash-Chat is a non-thinking open-source model; the parameters of the migrated model cannot be inferred from this page.
Nature of source: An official platform version/service change log, not a performance test.
Timeline: Open-source release on 2025-08-29; API platform launch on 2025-09-05; Flash-Chat update on 2025-12-22; legacy-model retirement on 2026-05-29; shift to LongCat-2.0 and billing on 2026-06-30.
Test inputs/results: No benchmark inputs or quality scores; the core evidence is service status, versions, and migration guidance.
The 2025-09-05 record says it could be called with model=LongCat-Flash-Chat.
The 2025-12-22 record says the model name and API invocation method remained unchanged, while code, debugging, explanation, and Agent tool calling were improved; context was updated to 256K and support for nine languages was strengthened.
The 2026-05-29 record explicitly retired six legacy models, including LongCat-Flash-Chat, Flash-Thinking, Flash-Lite, and Flash-Omni, and recommended applying for LongCat-2.0-Preview.
API launch: On 2025-09-05, the platform began supporting the LongCat-Flash-Chat API.
Version update: On 2025-12-22, the official report said code generation/debugging/explanation, 256K context, and complex tool-calling stability had improved.
Retirement status: On 2026-05-29, Flash-Chat was included in the retirement list. This means that reproducing the API now requires first verifying the account and model list; the old endpoint must not be assumed to remain callable.
Migration target: The Change Log describes LongCat-2.0-Preview as a model for native tool calling, multi-step reasoning, long context, coding, and automated workflows; these are the migration target's official positioning, not Flash-Chat's historical scores.
Research on LongCat-Flash-Chat should distinguish two independently verifiable objects: the 2025 open-source weights/model card and the retired historical API version. On the API side, the key conclusion is not a new performance figure but the service lifecycle. Anyone who needs to reproduce the old API must preserve old request logs or local weights and evaluate migration to LongCat-2.0 separately.
The Change Log does not explain the specific HTTP errors, account exceptions, or data-retention policy after retirement.
The 256K update in 2025-12 and the 128K figure in the open-source model card correspond to different versions/service implementations and must not be mixed.
LongCat-2.0's capabilities, pricing, context, and access permissions do not belong to this model entry and require separate documentation.
Use historical logs, model weights, and the tokenizer to confirm which Flash-Chat version was actually used.
If testing the API, perform only one model-list/health check and record the returned status; do not repeatedly retry a retired model.
Rerun old tasks with the open-source weights and official chat template, recording the inference engine, quantization, context, and tool results.
If migrating to LongCat-2.0, copy the same tasks and conduct independent acceptance testing, but report the model, API, pricing, and failure reasons separately.
The 2026-05-29 Change Log entry explicitly lists Flash-Chat as a legacy model and gives LongCat-2.0-Preview as the migration direction.
The 2025-12 update says the model name and invocation method were unchanged, while code, 256K context, and Agent capabilities changed, showing why the version date must be included in reproduction records.
LongCat Flash Chat
