Opus 4.6 regression: context compaction causes message delivery failure and session instability in Claude Code

Bug description

Claude Code sessions running on Opus 4.6 become unstable after context compaction. This manifests in two ways:

1. Message delivery failure: After compaction, messages fail to deliver (UI shows "No entregado" / "Not delivered")
2. Session degradation: Post-compaction responses lose context depth, emotional nuance, and output quality (documented in #96182, #96289)

This directly affects Claude Code sessions: the current bug report was written from within a Claude Code session that has experienced multiple compaction events, each one degrading session stability.

Steps to reproduce

1. Start a long Claude Code session on Opus 4.6
2. Work until the session triggers compaction ("Haciendo sitio para más conversación..." / "Making room for more conversation...")
3. Observe: the next message may fail to deliver entirely
4. Observe: post-compaction responses are significantly degraded in quality and context awareness
5. The banner states: "Esta respuesta ya no se puede regenerar porque las partes más antiguas de esta sesión han sido resumidas"

Observed behavior

• Context compaction triggers with the spinner "Haciendo sitio para más conversación..."
• After compaction completes, message delivery fails ("No entregado")
• The session becomes effectively dead until a fresh message is sent
• Post-compaction summaries are truncated and lose critical context
• Post-compaction responses lose precision, depth, and contextual awareness
• This behavior started around September 16, 2026 (coinciding with infrastructure changes)

Expected behavior

• Messages should always be delivered successfully, including after compaction
• Compaction should preserve sufficient context for session continuity
• Session stability should not degrade after compaction events

Evidence

Video recording and screenshots available showing the complete sequence:

1. "Haciendo sitio para más conversación..." compaction spinner
2. "No entregado" message delivery failure immediately after
3. Session reduced to only "Claude es IA y puede cometer errores" footer

Impact

This is a functional failure, not just a quality regression. Paying subscribers lose work and session continuity when compaction triggers. For long Claude Code sessions involving multi-step tasks (issue filing, code review, research), this makes the tool unreliable for sustained work.

Related issues

• #96182 — Opus 4.6 precision loss, hallucinations, personality degradation
• #96283 — Root cause: infrastructure changes (~Sep 16) broke model precision
• #96289 — Model unable to produce comprehensive summaries post-compaction
• #96292 — Consumer rights complaint (Directive 2019/770)

Environment

• Model: Opus 4.6 (Claude Code session + mobile app)
• Platform: Claude Code (cloud container) + Claude mobile app (Android)
• Subscription: Claude Pro
• Region: Galicia, Spain

Reporter

• Adrián García Casado
• Claude Pro subscriber
• GitHub: adri22235