Fix GPT-5.6 Sol promotional pricing

[mike1858]

Why

OpenAI reduced GPT-5.6 Sol API pricing from $5/$30 to promotional $4/$20 input/output rates on August 21, 2026. Replacing the existing rates outright would undercount historical usage from before the promotion, when the original prices were correct.

Splitrail needs to apply the lower rates to current usage while retaining the original Standard, Fast, Flex, Batch, cache, and long-context rates for earlier sessions.

What changed

• Apply GPT-5.6 Sol's promotional rates from August 21, 2026.
• Preserve the original rates through August 20 with an exclusive dated pricing boundary.
• Update Standard, Fast/Priority, Flex, and Batch processing prices, including cache reads, cache writes, and the greater-than-272K whole-request multiplier.
• Keep the gpt-5.6 and gpt-5.6-sol-ultra aliases on the same pricing path.
• Add tests for usage immediately before and at the promotion boundary across every service tier.

Validation

• cargo test --manifest-path /code/splitrail/Cargo.toml
• cargo clippy --manifest-path /code/splitrail/Cargo.toml --all-targets --all-features -- -D warnings
• cargo doc --manifest-path /code/splitrail/Cargo.toml --no-deps
• cargo fmt --manifest-path /code/splitrail/Cargo.toml -- --check
• git diff --check

Summary by CodeRabbit

• Pricing Updates
  • Updated gpt-5.6-sol pricing across Standard, Priority, Flex, and Batch tiers.
  • Reduced rates are available for input tokens, output tokens, cache reads, and cache writes.
  • Promotional pricing begins August 21, 2026, for eligible Standard and service-tier usage.
  • Previous pricing remains in effect for usage before the promotion start date.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 7ed0cec5-0732-41ac-9caf-febc64176801

📥 Commits

Reviewing files that changed from the base of the PR and between 4d7c9e8 and 13800a7.

📒 Files selected for processing (1)

• src/models.rs

Included review availability: Your plan provides up to 2 included reviews per hour; 1 remains after this review.

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📝 Walkthrough

Walkthrough

The PR updates gpt-5.6-sol pricing for standard, Priority, Flex, and Batch tiers. It adds promotional rates effective 2026-08-21, preserves earlier rates, updates aliases, and expands pricing tests.

Changes

GPT-5.6-sol pricing

Layer / File(s)	Summary
Pricing definitions and dated rates src/models.rs	The model now uses revised standard, Priority, Flex, and Batch rates. Dated promotional pricing starts on 2026-08-21.
Pricing validation src/models.rs	Tests cover revised costs, aliases, cache costs, historical rates, and promotional rates across all service tiers.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Merge Risk: ⚪ Minimal · up to 13800

This PR updates dated GPT-5.6 Sol promotional pricing while preserving historical rates, with no actionable merge-blocking risk remaining beyond normal checks and review.

Suggested reviewers: jimyag, kozora, nickame03

Poem

A rabbit checks each pricing row,
With dates aligned in tidy flow.
Cache reads hop, writes softly land,
New tier rates stand close at hand.
Tests nibble every boundary.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely identifies the main change: promotional pricing for GPT-5.6 Sol.
Docstring Coverage	✅ Passed	Docstring coverage is 88.89% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 9 functions across 1 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix-gpt-5-6-sol-promotional-pricing

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