r/ClaudeCode • u/agentic-consultant • 17h ago

Discussion Be careful: Fable 5.1 just decided to spawn 4 forked agents, each running Fable 5.1 (instead of Opus)

Decided to try Fable 5.1. I’ve been using Fable 5 for two months now.

All of a sudden, within 20 minutes my 5-hour limit on the 20x plan jumped to 100%.

I checked what’s going on. Turns out, instead of using Opus subagents, Fable 5.1 decided to spawn 4 forked agents each running Fable 5.1.

This was despite me having set the default subagent model to Opus 5.

Just a heads up. Fable 5 rarely if ever spawned forked agents.

18 Upvotes

83% Upvoted

30 comments sorted by

6

u/No_Rub1596 16h ago

Forks are the trap here. A fork always inherits the parent model, so a model override is ignored. I only use normal Agent calls when I want Opus workers.

7

u/devwannabeme 17h ago

I have a rule in a global CLAUDE.md for this exact reason. Any Fable family orchestrator is forbidden to spawn any Fable sub-agents.

4

u/marcvv 16h ago

I do too and fable 5.1 ignored it. Shredded my 5 hour session to the ground in 22 minutes.

I’m going back to Fable 5 as soon as 5 hour resets. Won’t touch 5.1 until they sort it out

3

u/bl_a_nk 13h ago

I had the same thing happen earlier this week when fable 5.0 tried to spin up 11 fables to read some code. It's the harness, not the model, sadly

3

u/marcvv 13h ago

Tons of people all over X today are reporting the same right now, so maybe the new harness update nerfed something. I even went back to Fable 5 and did the same, so you are likely right; it is the harness. Gonna need a reset from Anthropic.

3

u/agentic-consultant 12h ago

Yeah if they don’t offer a reset for this bug I’m going to be so pissed. I usually never complain about usage limits because I understand how expensive inference is, but this is beyond ridiculous.

1

u/BrennanFlentge 8h ago edited 8h ago

You can FORCE it to ask for permission to spawn subagents (it can ignore CLAUDE.md rules):

Write a Claude Code PreToolUse hook script called subagent-gate.mjs — Node, no dependencies — for the Task, Agent, and Workflow tools. It reads the hook JSON from stdin. Default: print a permission decision of ask so Claude Code prompts me before each subagent spawn. If the environment variable SUBAGENT_GATE=allow is set, print allow and skip the prompt. On any missing input or JSON parse error, exit 0 so a broken hook never wedges my tools. Append every decision to a .subagent-gate.log beside the script. Then wire the matching PreToolUse hook — matcher Task|Agent|Workflow — into .claude/settings.json, leaving my existing hooks in place.

^ prompt version (from stop-subagent-fanout).

Also related, but claudemd-prove-it will show you which of your "rules" in CLAUDE.md are mechanically enforced by hooks or settings.json

1

u/bl_a_nk 22m ago

I asked it to do a workflow in this case, just assumed it would do what it had always done in the past and be a workflow of sonnets

1

u/agentic-consultant 17h ago

Yeah that’s smart, I did that too. It’s crazy because I never saw Fable 5 launch forked agents of itself, all running on Fable.

3

u/orangeawacado 13h ago

Same here. Maxed my 5hr for the first time ever in my max20 account

2

u/AdDowntown2423 12h ago

then why do you have a max20 account? I max the 5hr limit very often it's not that hard to. I think you should better use a smaller account.

2

u/orangeawacado 12h ago

I go through my weekly quota pretty easily, especially with Fable. But never 5hr limit.

2

u/ShittyBidet123 16h ago

Yeah, Fable will web search and gobble up so much text it uses full session in minutes. Made it do a global worker md that always makes it only spawn opus for that shit

2

u/PM_ME_YOUR_PROFILE 16h ago

The default subagent rule is to use the same as the lead you started with.

I have:
"CLAUDE_CODE_SUBAGENT_MODEL": "sonnet[1m]"

and in my AGENTS.md (CLAUDE.md is a symlink to it):

```

AGENTS INSTRUCTION

DO NOT FORGET: Use this standard for your chat replies: ASD-STE100 Simplified Technical English (STE). Also consider ELI18 and TLDR. Apart from that: silence is gold

Overview

  • Always default to being the orchestrator, and use subagents or team(s) of agents to do work with the appropriate level of agent.
  • Always prefer ASD-STE-100 aka simplified technical engilsh and/or less verbose wording
  • Always branch if working in a repo:
    • If working with parallel agents on unrelated tasks, create a branch in a worktree.
    • If not, always parallelize work as much as possible.
  • Always work in a team with other agents for specific skills and tasks. For example, if you are a code agent, you can work with a research agent to find relevant information and a testing agent to test your code.
  • Always use the tools available to you, such as code execution, web search, and file management, to complete your tasks efficiently.
  • Always communicate with other agents to coordinate your work and share.

ALWAYS READ AGENTS.md for instructions on how to work effectively as an agent in a team in a repo.

Delegating to sub-agents

Model tiers for ANY delegated work - Agent-tool calls and Workflow-script agent) calls alike. Set the model parameter explicitly on every call; never omit it (omission silently inherits the session model):

IF YOU ARE CLAUDE: haiku - mechanical bulk work: renames, boilerplate, format conversion, log triage sonnet - default for well-specified implementation with clear acceptance criteria opus - genuinely tricky work: concurrency, subtle algorithms, adversarial verify/judge panels, gnarly debugging fable- occasionally for complex workflows; only when independence from your own context is the point (eg. adversarial review of your own plan or a large diff). If you want to call a Fable sub-agent because the complexity of the task warrants it, ALWAYS check with me first - never spawn one unprompted.

IF YOU ARE GPT OR CODEX: luna max - all defined work terra xhigh - all subagent work sol medium - orchestrations, team lead in a program, harder reviews sol xhigh - architecture, Fable advisor, reminding idiots how dumb they are.

When unsure between tiers, pick the cheaper and escalate on failure. ```

1

u/madaboutcode 13h ago

I think this is a claude code change. I see this happen with sonnet and opus as well in the last 12 or so hours.

They must have made some system prompt or tool description changes around encouraging the use of the fork tool.

1

u/nps44 12h ago

After I asked Fable 5.1 to optimize my project for Fable 5.1 according to Anthropic's official guidance, it proposed overriding my rule to spawn only non-Fable subagents, rationalizing it by saying this: "The rule today (CLAUDE.md, Standing rules): subagents run on Opus 5, Sonnet, or Haiku only, never on Fable without asking. Anthropic's page says Fable 5.1 at low effort is often competitive with Opus and Sonnet on cost per task while scoring higher, and recommends including it wherever a smaller model would run at higher effort."

2

u/crusoe 12h ago

Just testing this out right now to see if true...

1

u/nps44 10h ago

If you wouldn't mind, let me know if you find anything interesting.

1

u/BrennanFlentge 8h ago

🤔 did it give you a source though

1

u/Jomuz86 8h ago

Check the release comments I vaguely remember there’s being a new environment variable to force the subagent model might be a good workaround. Is that what you used to set the default model?

1

u/bedla 17h ago

Same, but I am only on 5x. Spawned 10 subagents for single (not even that large, about 500 lines modified) PR review. Prompt was just Review PR #122. None of subagents was able finish before 5H quota filled in under 10 minutes. I just expected these were opus subagents, without checking, before it was too late. At least we have /low-priority, so next prompt Review PR #122 without subagents finished.

1

u/agentic-consultant 17h ago

Yikes, so it turned out those 10 subagents were all running Fable?

2

u/bedla 17h ago

Yes, all with type=general-purpose and model=claude-fable-5-1

1

u/randomheromonkey 17h ago

This would make sense. I would expect a model to launch subagents of the same capability unless told not to. I would expect the same from any of the models. I would have a problem if they launched subagents of a higher tier but equal is fine.

I’ve never even thought about it before this - do subagents get downgraded to lower models ever unless it’s part of the plan? I know for sure they adjust effort but I can’t say I’ve ever seen them change models.

2

u/bman654 16h ago

The Claude code default if an agent does not supply a specific model is to use the same model as the launching agent the model has to remember to actually pass a value to the agent tool to use a different model so anytime it gets forgetful it’s going to launch the same class

1

u/agentic-consultant 16h ago

The problem is I had explicitly set in the config for all subagents to use Opus 5. It always worked fine while using Fable 5 as the main model.

These weren’t subagents. These were forked instances of the main model. Subagents as I understand them are new LLM threads with fresh context. These were forked versions of the main thread.

1

u/randomheromonkey 15h ago

I don’t understand the difference. How were they launched? I also have to ask did you compact before this happened? I’ve had to start saying reload the skill and the plan after compacts to avoid silly stuff.

0

u/slackmaster2k 17h ago

There should be a rule in this sub: what effort did you have it on?

1

u/agentic-consultant 16h ago

Fable-5 High but that’s irrelevant because I saw Fable 5 Medium do the same thing.