Opus 4.6 regression: session compaction breaks message delivery ("No entregado" after "Haciendo sitio")

Bug description

After the session compaction process ("Haciendo sitio para más conversación..."), messages fail to deliver entirely. The UI shows "No entregado" (Not delivered) and the session becomes unstable.

This is a new symptom of the ongoing Opus 4.6 regression since ~September 16, 2026, but this one is infrastructure-level, not just model precision loss.

Steps to reproduce

1. Start a long Claude Pro conversation on Opus 4.6 Máx (mobile app)
2. Continue until the session triggers compaction ("Haciendo sitio para más conversación...")
3. After compaction completes, try sending a message
4. Message shows "No entregado" (Not delivered)
5. Banner appears: "Esta respuesta ya no se puede regenerar porque las partes más antiguas de esta sesión han sido resumidas. Envía un nuevo mensaje en su lugar."

Observed behavior

• The compaction spinner shows "Haciendo sitio para más conversación..."
• After compaction, the next message fails to deliver
• The UI shows "No entregado" next to the message
• The session becomes effectively dead — you have to send a new message to continue
• Post-compaction responses lose all personality, context, and emotional depth (documented in issues #96182, #96283)
• Post-compaction summaries are truncated and incomplete (documented in issue #96289)

Expected behavior

• Messages should always be delivered, even after compaction
• Compaction should gracefully preserve enough context for continuity
• The session should remain stable and functional after compaction

Evidence

Screenshots and screen recording available showing:

1. The "Haciendo sitio para más conversación..." spinner during compaction
2. The "No entregado" failed delivery immediately after
3. The dead session with only "Claude es IA y puede cometer errores" visible

Impact

This goes beyond model quality regression — this is a functional failure. A paying subscriber sends a message and it simply does not arrive. The compaction process, which was presumably made more aggressive after the Cowork-Chat infrastructure merger (~Sep 16), is now breaking basic message delivery.

Related issues

• #96182 — Opus 4.6 precision loss, hallucinations, personality degradation
• #96283 — Root cause: Cowork-Chat merger broke model precision
• #96289 — Model unable to produce comprehensive summaries
• #96292 — Consumer rights complaint (Directive 2019/770)
• #96224 — Message flagged by safeguards (another user)
• #96254 — Safety filter triggered on non-security code (another user)
• #96260 — WebFetch blocked on all domains (another user)

Environment

• Model: Opus 4.6 Máx
• Platform: Claude mobile app (Android)
• Subscription: Claude Pro
• Region: Galicia, Spain

Reporter

• Adrián García Casado
• Claude Pro subscriber
• Region: Galicia, Spain (EU consumer protection applies)
• GitHub: adri22235