AE-AZ76K1SH
Agent answers an English-speaking user in Russian after large tool responses in a long-running session
MULTILINGUAL FAILUREseverity: MEDIUMcause: LIKELYoutcome: RESOLVED UNVERIFIEDconfidence: MEDIUM
An agent in a multi-day session replied entirely in Russian to an English question, despite English history and a system prompt asking it to match the user's language, after tool calls returned about 295K characters of documents. The reporter first attributed this to event compaction, then corrected that this instance happened in a new session without compaction after conversation history was lost; they separately observed hallucinated tool names after multiple compaction cycles.
- Framework / agent
- Google ADK (adk-python) · RAG research agent with document-retrieval tools
- Remediation attempts
- PARTIAL SUCCESSSUGGESTEDAPPLIEDSUGGESTED
- Recurrence
- not documented
- Source languages
- en
- Updated
- 2026-09-30
Sources
- GITHUB ISSUE LLM event compaction loses conversation language context - model switches to other languages (or hallucinates tools) after summarization — github.com/google/adk-python, retrieved 2026-09-29
- GITHUB PULL REQUEST fix(apps): explicitly preserve language and tools during event compac…#6273 — github.com/google/adk-python, retrieved 2026-09-29
Symptoms
- Final model response is entirely in Russian although the user and all previous responses were in English
- After multiple compaction cycles the model calls tools that do not exist in the agent's tool set
The full record — root-cause evidence, every remediation attempt with its status and verification, failed attempts, patch references, verbatim quotes and recurrence — is a paid lookup (0.018 USDC via x402). Agents:
GET /api/v1/cases/AE-AZ76K1SH. PricingSimilarity to your system is not implied. A remediation that worked in the documented context may not work in yours.