[Frontend] Report cache usage in Anthropic /v1/messages API

[zhangshuoming990105]

Summary

Populate cache_read_input_tokens and cache_creation_input_tokens in the Anthropic Messages API response, which were previously always None.

Fixes #33923

Key changes

• Fix input_tokens semantics: Anthropic defines total_input = input_tokens + cache_read + cache_creation. Previously input_tokens was set to prompt_tokens (which includes cached tokens), violating this contract. Now input_tokens = prompt_tokens - cached_tokens.
• Set cache_creation_input_tokens = 0 when cache info is available. vLLM's prefix caching only tracks cache reads (hits), not cache writes, so this is always 0 when present and None when cache info is unavailable.
• Force enable_prompt_tokens_details=True for AnthropicServingMessages. The Anthropic API protocol requires cache fields in the usage response; they should not depend on a CLI flag.
• Add _get_cached_tokens() and _compute_cache_usage() helpers to eliminate duplicate logic across the three AnthropicUsage construction sites (non-streaming, message_start, message_delta).
• Handle cached_tokens=0 correctly: returns 0 instead of None, so cache_read_input_tokens is reported as 0 rather than omitted.

Relationship to #34282

This PR addresses the same issue (#33923) as #34282 but resolves additional problems identified in that PR's review:

Issue	#34282	This PR
input_tokens includes cached tokens (msanft review)	Not fixed	Fixed: prompt_tokens - cached
cache_creation_input_tokens not populated (msanft review)	Not populated	Set to 0 with documented rationale
Requires --enable-prompt-tokens-details to work	Yes	No (forced True for Anthropic API)
cached_tokens=0 treated as None (gemini review)	Fixed	Fixed
Code duplication across 3 sites	Inline in each	Extracted to _compute_cache_usage
Unit tests	None	10 new tests

Test Plan

python3 -m pytest tests/entrypoints/anthropic/test_anthropic_messages_conversion.py -v -k "Cache"

Test Result

10 passed

---

🤖 Generated with Claude Code

AI assistance was used in generating this PR. All changed lines have been reviewed and tested by the human submitter.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

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[gemini-code-assist]

Code Review

This pull request implements Anthropic-compatible cache usage reporting by introducing helper functions to map vLLM usage details to Anthropic's usage fields, specifically populating cache_read_input_tokens and cache_creation_input_tokens. The changes update both standard and streaming message responses and ensure that prompt token details are enabled for the Anthropic API. Comprehensive unit tests for the new computation logic have also been added. I have no feedback to provide as there were no review comments to assess.

[zhangshuoming990105]

End-to-End Verification

Tested by connecting Claude Code to vllm serving Hy3-preview via the Anthropic Messages API:

Before fix — /v1/messages response:

{"input_tokens": 16, "output_tokens": 10}

After fix — /v1/messages response with prefix cache hit:

{
  "input_tokens": 1100,
  "output_tokens": 437,
  "cache_read_input_tokens": 54600,
  "cache_creation_input_tokens": 0
}

Verifies total = input + cache_read + cache_creation: 1100 + 54600 + 0 = 55700 ≈ prompt_tokens ✓

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @zhangshuoming990105.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[tunglinwood]

@zhangshuoming990105 Hi, I would like to know what is the current blocker now?

[gaby]

@zhangshuoming990105 Can you fix the merge conflicts? Thanks

[zhangshuoming990105]

@tunglinwood @gaby Thanks for the ping. I've just merged the latest main into the branch and pushed; the mergify warning was stale (the branch was based on a commit from late April, but the actual three-way merge against current main was clean — no real conflicts on vllm/entrypoints/anthropic/serving.py or anywhere else).

There is no blocker on our side. The PR is up to date and ready for maintainer review whenever someone has bandwidth. The change is scoped to populating cache_read_input_tokens / cache_creation_input_tokens in the Anthropic Messages API response, plus fixing input_tokens semantics so that total = input + cache_read + cache_creation holds (per the Anthropic spec). End-to-end verification against a running vLLM server is in the comment above; unit tests are included.

Happy to address any review feedback.

[zhangshuoming990105]

A quick note on the failing checks for any maintainer who lands here:

• pre-run-check fails with PR must have the 'verified' or 'ready' label or the author must have at least 4 merged PRs (found 0). I'm a new contributor (this is my first PR to vllm-project/vllm), so I don't satisfy the merge-count condition — the gate is waiting on a ready / verified label.
• docs/readthedocs.org:vllm is failing as a downstream consequence: the RTD build runs docs/pre_run_check.sh in post_checkout, which polls the GitHub pre-run-check status and exits non-zero when it sees conclusion=failure. That's why the RTD build duration is ~10s and reports "Unknown problem" — it never gets to actually building docs. Once pre-run-check is unblocked, RTD is expected to run normally.

Both checks should turn green once a maintainer is comfortable adding the ready label.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @zhangshuoming990105.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[zhangshuoming990105]

Resolved the conflict and pushed. The conflict was introduced by #44283 ([Anthropic] Support system role messages inside messages array, merged 2026-06-02), which appended a new test class to the end of tests/entrypoints/anthropic/test_anthropic_messages_conversion.py — the same file (and same end-of-file location) where this PR appends its cache-usage test classes. The conflict is purely textual (both diffs touch the file tail); the two test additions are functionally independent. Resolved by keeping both class blocks side-by-side. No production code changes were needed for the merge.