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You set up an automated background
subagent, head off to grab a flat white,

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and come back to discover it silently
upgraded itself to Opus 5 mid task.

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Suddenly your API bill looks like a phone
number.

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Oh yeah, that surprise context shift will
get you every time.

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Before we dive into how to lock that down,
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00:00:22,295 --> 00:00:27,375
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00:00:28,250 --> 00:00:34,490
So, Anthropic dropped version 2 point 1
point 251 for Claude Code on August 28,

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2026, and it finally gives us proper
brakes for model jumping.

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Yeah, the big news is two lifecycle hook
events, PreModelSwitch and PostModelSwitch.

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Plus, when you resume a session with
SessionStart, you now get session staleness

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metrics and estimated re cache costs right
in the payload.

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Walk me through PreModelSwitch though.

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How does that stop my agents from spending
all my gold bullion?

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It fires synchronously right before any
model transition happens.

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So if an agent tries to bump up from Haiku
to Opus, platform engineers can catch it,

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demand developer confirmation in the
terminal, or just outright block it in CI CD

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pipelines.

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Ah, that's beautiful mate.

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You can also attach custom audit
annotations to your team telemetry logs so you know

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who requested the upgrade and why.

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Exactly.

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Setting it up is super straightforward.

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In your project, you open up dot claude
slash hooks dot json and point a

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PreModelSwitch handler to a validation
script, say validate model dot py.

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Right, the script checks the target model
ID from the event payload,

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and if it breaks your team policy, you
just exit with code 1 or return a denial JSON

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object.

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Boom, request blocked.

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Just watch out for edge cases though.

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If you block model switches too
aggressively, you will break subagent workflows that

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legitimately need higher reasoning models
to fix a complex bug.

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Yeah, fair call.

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And while SessionStart gives you re cache
cost estimates on resume,

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if your prompt shifted massively while you
were away, you're still hitting a full

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cold cache miss regardless.

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True.

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But there are some slick quality of life
tweaks in this release too.

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The slash cost command now breaks down
your prompt cache hit ratios between warm and

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cold tokens.

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Plus slash effort now remembers your
preference per model, so you don't have to keep

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resetting it.

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And you get direct CLI management commands
like claude attach with session ID,

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stop, respawn, and rm.

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No more hunting down rogue background
processes manually.

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You have full command line governance from
top to bottom.