fix(llm-gateway): say a gated model is unavailable, not an auth failure

[evanmarshall]

This is an Ito QA mirror of PostHog#93611.

Original author: @PostHog[bot]
Pinned upstream head: e1cb533

This draft is for QA and build-environment experimentation and will not be merged.

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Problem

• Picking a preview model (DeepSeek V4 Flash, GLM 5.3 Flash, Kimi K3) ends the whole task with a red "Failed to authenticate" banner, after a sandbox has already launched. There is no stated next step.
• The gateway denies the model in its auth dependency with a 403, but that denial reads as three different wrong things downstream:

Signal the gateway sends	What Desktop concludes	What is actually true
code: "model_gate"	ignored — the classifier matches only the free-tier wording	a permission denial
message ends in (rate_limit)	retryable	permanent until the flag is on
model listed with allowed: true	safe to offer in the picker	every request with it 403s

• The (rate_limit) suffix is a documented compat shim for the free-tier gate. The rollout gate copied it, and a rollout flag never clears on a retry.

Changes

• A blocked preview model now says so: "This model isn't available — it's in preview and isn't turned on for your account yet. Pick another model to keep going." No payment prompt, because no payment unlocks it.
• The gated models stop appearing in the picker at all. The gateway's model listing drops any model whose access flag the caller does not hold, and fails closed on a flag-evaluation outage, matching what enforcement does.
• The gateway's rollout denial carries a machine-readable reason, and no longer wears the free-tier shim:

-"message": "Model '<id>' is not available. Choose another model. (rate_limit)",
+"message": "Model '<id>' is not available for your account. Choose another model.",
 "code": "model_gate",
+"reason": "model_not_available",

• code: "model_gate" stays on that body, so builds that read only the code keep reaching a model prompt rather than session teardown.
• Mechanical: the classifier matches the structured fields by regex because the ACP layer delivers the error body as text; a new model_unavailable cause threads through the limit copy, the failure title, and the analytics cause union.

Why drop the model from the listing instead of marking it allowed: false

allowed: false already exists for the free-tier gate, and a marked model renders as a locked upsell row. That is right when a payment method unlocks it. A rollout flag has no user action behind it, so the row would be a dead end.

How did you test this code?

Automated only — the agent did not run the desktop app or reproduce the banner.

• services/llm-gateway: full suite passes. New cases pin that the rollout denial carries reason and no (rate_limit) suffix, and that a flag-gated model is listed only when its flag returns true (off and unavailable both drop it).
• @posthog/shared, @posthog/core, @posthog/ui: suites pass. New cases pin classification from the structured code/reason, and that the wrapped "Internal error: … 403" body is not treated as a fatal session error.
• @posthog/agent fails 312 tests on this branch and the same 312 on master — pre-existing, unrelated.
• Typecheck passes for shared, core, ui, agent. ruff check and ruff format clean.

Automatic notifications

• Publish to changelog?

Docs update

None — error-body shape and in-app copy.

🤖 Agent context

Autonomy: Fully autonomous

• Written by PostHog Desktop from an inbox report. No repo skills were loaded through the skill tool; the copy, comment, PR-description, and simplification conventions were applied from AGENTS.md directly.
• The report proposed reusing the existing model_gate cause on the client. That was rejected once the copy behind it turned out to be a billing upsell: routing a rollout gate there would tell users to add a payment method for a model no payment unlocks. Hence the separate model_unavailable cause, with code: "model_gate" retained on the wire for older builds.
• Public artifact: nothing here carries session material. The model ids, flag keys, and error bodies all come from this repository.

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Created with PostHog Desktop from this inbox report.