Opus 5.5 vs Fable - what the model pin in your three wrappers actually buysCapability

Claude Opus 5.5 arrived 9-22 at $4/$20 per million tokens against Fable 5.1's $10/$50, with Anthropic claiming Fable-level output on most work. Your three headless wrappers still pin Fable. Today: what a model pin is, and how to trial the cheaper model on one morning without touching code.

A model pin is the --model flag a script passes to claude -p. Without it the run silently inherits whatever the CLI's default is that week, which is how the nightly briefing changed models under you before 9-11 (the comment at nightly_wrapper.sh:43 records this). With it, the model is a decision you made once and can read back.

Your pins are written as ${PDB_MODEL:-fable}. That shell idiom means: use the PDB_MODEL environment variable if set, else the word fable. So the default lives in the script, but any single run can override it from the command line without an edit. That is the trial mechanism: one morning on a different model, nothing committed.

Opus 5.5 differs from Fable in two ways that matter for your jobs. Price: $4 in / $20 out versus $10 / $50, and cache reads at $0.20. Behaviour: its default effort is medium, one notch below Fable's, and thinking cannot be switched off. Claude Code 2.1.280 made it the default Opus, so the alias opus now resolves to Opus 5.5. The fallback chain (sonnet,haiku) stays the same either way.

Worked example

Read the three pins, then run one plan-day on Opus 5.5 by hand.

grep -n 'PDB_MODEL\|NIGHTLY_MODEL' \
  /Users/tom/Claude/PDB/scripts/run_plan_day.sh \
  /Users/tom/Claude/PDB/scripts/run_weekly_review.sh \
  /Users/tom/Claude/JobHunt/scripts/nightly_wrapper.sh

PDB_MODEL=opus /Users/tom/Claude/PDB/scripts/run_plan_day.sh

tail -5 /Users/tom/Claude/PDB/logs/plan-day.log

• The grep shows the three lines where the model is chosen. Two default to fable; the nightly reads NIGHTLY_MODEL from its own header.
• Setting PDB_MODEL on the same line as the command overrides the default for that one process only. Nothing is written to disk.
• The log's last lines show the exit status and which model answered. Compare the brief it produced against today's earlier one.

Gotchas

• The alias opus means 'current default Opus'. If you want the pin to survive future releases, write claude-opus-5-5 in full.
• Opus 5.5 defaults to medium effort. If a brief reads thinner, the fix is --effort high, not going back to Fable.
• Fallback order still matters: sonnet,haiku catches a usage cap on the primary, whichever primary you choose.