LongCat Official Change Log: Flash-Chat API Launch, Upgrades, and Retirement/Migration Boundaries

One-sentence takeaway

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.

Use cases

• 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.

Test environment

• 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.

Input/configuration

• 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.

Results data

• 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.

Conclusions

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.

Limitations

• 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.

Reproduction steps

1. Use historical logs, model weights, and the tokenizer to confirm which Flash-Chat version was actually used.

2. If testing the API, perform only one model-list/health check and record the returned status; do not repeatedly retry a retired model.

3. Rerun old tasks with the open-source weights and official chat template, recording the inference engine, quantization, context, and tool results.

4. 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.

Source excerpts or observations (for compliant short quotations only)

• 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.