Claude agents fail silently when default model requires usage credits (Fable 5) #2265

Summary

Default agents (e.g. Bumble) appear online and receive @mentions, but never reply in-channel when Claude Code’s default model is set to Fable 5 (claude-fable-5[1m]), which requires usage credits on a Claude Pro subscription.

The harness logs a clear error and requeues the turn; the Desktop UI and channel stay silent. From the user’s perspective the agent is “broken.”

Environment

• Buzz Desktop v0.4.21
• Runtime: Claude Code via claude-agent-acp
• Claude auth: claude.ai Pro (claude auth status → loggedIn: true, subscriptionType: pro)
• Relay: hosted community relay (e.g. wss://*.communities.buzz.xyz)
• macOS arm64

Steps to reproduce

1. Install / use Claude Code with a Pro login.
2. Set Claude default model to Fable 5 (or leave it if Claude already chose it), e.g. in ~/.claude/settings.json:

   { "model": "claude-fable-5[1m]" }

3. Start a managed Buzz agent with agent_command / override claude-agent-acp (welcome team / default agents is enough).
4. Confirm agent presence is online in a channel the agent is a member of.
5. Send @Bumble hello (or any mention) from the owner account.
6. Observe: no reply in the channel; no in-app error surface about credits/model.

Expected

• Agent replies, or
• Agent/Desktop posts a visible failure in-channel or in the Agents UI, e.g.
  “Claude model Fable 5 requires usage credits — switch model or run /usage-credits.”

Actual

• Mention is received and dispatched by buzz-acp.
• Claude ACP session starts, then fails:

  Fable 5 requires usage credits. Run /usage-credits to continue or switch models with /model.
  session_prompt error: Agent reported error (code -32603): Internal error: Fable 5 requires usage credits...
  requeueing failed batch with backoff
  agent_returned (application error — pipe intact) outcome=error
  

• Channel stays empty; user only sees silence.

Workaround

Change Claude default model to a plan-included model:

// ~/.claude/settings.json
{ "model": "sonnet" }

Restart agents. After that, @Bumble replies normally (verified: agent pubkey replied pong in #general).

Related / secondary bug

buzz-acp --model sonnet / BUZZ_ACP_MODEL=sonnet did not override the bad default:

desired model sonnet not found in agent's available models — proceeding with agent default

even though buzz-acp models --agent-command claude-agent-acp lists sonnet under stable configOptions. Session then still hit Fable 5 until ~/.claude/settings.json was changed.

Suggest treating model-override catalog matching as a follow-up, but the user-visible bug is silent failure on LLM/auth/credit errors.

Suggested fix directions

1. Surface hard turn failures to the channel or Agents panel (at least once per failure class; don’t only requeue quietly).
2. On Claude ACP init/prompt errors matching usage-credits / auth, map to a clear readiness / doctor state.
3. Prefer a safe default model for Claude Pro (sonnet/haiku) when Fable requires extra credits, or warn at agent start if default model is credit-gated.
4. Fix --model / BUZZ_ACP_MODEL so IDs from buzz-acp models actually apply via session/set_model / set_config_option.

Logs (redacted)

INFO  buzz_acp: connected to relay ... presence set to online
DEBUG buzz_acp: agent_claimed channel=<uuid>
INFO  acp::session: session created: ...
INFO  acp::stream: Fable 5 requires usage credits. Run /usage-credits to continue or switch models with /model.
ERROR pool::prompt: session_prompt error: ... code -32603 ... Fable 5 requires usage credits
WARN  buzz_acp::queue: requeueing failed batch with backoff attempt=1
WARN  buzz_acp: agent_returned outcome=error configured_model=<none|sonnet>

Impact

High for first-run / default agents: welcome team looks alive (online) but never answers if Claude’s default is Fable 5. Easy to misdiagnose as “agents don’t work” or relay/auth issues.

Related issues

• Agent @mentions silently fail when mentioned agent is offline #1743 — Agent @mentions silently fail when mentioned agent is offline (related silent-failure UX; this issue is when the agent is online but the LLM fails)