Bill Gemini 4 Argon at standard rates after its introductory period

[mike1858]

Why

Gemini 4 Argon became generally available on 2026-10-13, and Google now publishes when its introductory pricing ends. The Gemini API pricing page bills gemini-4-argon at $2 input / $10 output per 1M tokens ($0.10 cached) "through November 30, 2026", then $4 / $20 ($0.20 cached) "starting December 1, 2026".

When #270 added Argon, Google hadn't given an end date, so the introductory rate went into the base entry and the standard rates were only recorded in a comment. Shipped Splitrail (3.11.0 and 3.11.1) therefore bills Argon at $2 / $10 forever, which would under-report every Argon token from December onward by half. With a real date available, Argon can use the same promo pattern as gemini-3.8-flash.

What changed

• The gemini-4-argon base entry now holds the standard rates: $4.00 input / $20.00 output per 1M tokens, $0.20 cached input.
• The introductory $2.00 / $10.00 / $0.10 rate is an add_dated_pricing! override with valid_until 2026-12-01. valid_until is exclusive, so all of November 30 still bills at the introductory rate. As the earliest period, the override also covers pre-GA Fairwind Program usage back to the 2026-09-30 announcement. Introductory pricing is the only rate Google applied to Argon during that time.
• Pricing stays flat. The pricing page lists no >200K long-context tier for Argon. Batch, Flex, and Priority rates are not modelled, consistent with the other Gemini entries.
• The per-entry // Source: line pointing at the launch blog post is removed. Argon's rates now come from the Google section's default source, the Gemini API pricing page.
• gemini_4_argon_bills_introductory_rates_until_end_date_is_announced is replaced by gemini_4_argon_switches_from_introductory_to_standard_rates. The new test checks input, output, and cache-read costs at three times: the 2026-09-30 announcement, 2026-11-30 23:00 UTC, and 2026-12-01 00:00 UTC. It covers both gemini-4-argon and google/gemini-4-argon, so an off-by-one in the exclusive end date fails it.

Undated cost helpers, which use the base entry, now return the standard rates. Usage with a timestamp keeps the rate that applied on its UTC date.

Lockfile bump for cargo audit

The current RustSec database flags quinn-proto 0.11.15 with five advisories (RUSTSEC-2026-0338 through RUSTSEC-2026-0342, fixed in >= 0.11.18). That fails the cargo audit job for main's lockfile, not just this branch. cargo update -p quinn-proto moves it to 0.11.19, which also pulls its rand dependency from 0.9.2 to 0.10.3. quinn-proto is only reachable through reqwest's optional http3 feature, which Splitrail leaves off, so cargo tree -e normal --target all lists none of these crates: the change affects Cargo.lock only, not the compiled binary. The RUSTSEC-2026-0097 comment in .cargo/audit.toml is updated to match: the ignore is still needed for rand 0.8.5 via phf_generator, but the quinn-proto copy of rand is now a patched version.

Validation

• cargo build --quiet

• cargo test --quiet: 500 passed

• cargo clippy --quiet -- -D warnings: exit 0

• cargo doc --quiet

• cargo fmt --all --check

• Ran the real splitrail stats against a Gemini CLI session fixture (~/.gemini/tmp/<project>/chats/session.json under a scratch HOME). The fixture has two identical gemini-4-argon responses, each with 100K input, 50K output, and 400K cached tokens: one at 2026-11-30T12:00Z and one at 2026-12-01T12:00Z. Daily costCents on each build:

  Build	2026-11-30	2026-12-01
  Released 3.11.1	74	74
  This PR	74 ($0.20 input + $0.50 output + $0.04 cached)	148 ($0.40 input + $1.00 output + $0.08 cached)

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