[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. Aligns input_tokens semantics with the Anthropic contract.

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.
• Populate cache_creation_input_tokens = 0 when cache info is available. The OpenAI usage protocol exposes only cached_tokens (cache hits, mapped to cache_read_input_tokens); there is no analog for cache creation, so we report 0 to satisfy the Anthropic invariant above.
• Treat "cache info unknown" as field absence, not null. When prompt_tokens_details is not attached (e.g. --enable-prompt-tokens-details is off, or — for streaming — the current chunk has not yet carried cache info), cache_read_input_tokens and cache_creation_input_tokens are omitted from the JSON entirely rather than emitted as null. Clients can distinguish "unknown" (key absent) from "zero" (0).
• Add _get_cached_tokens() and _compute_cache_usage() helpers to centralize the mapping across the three AnthropicUsage construction sites (non-streaming response, streaming message_start, streaming message_delta).
• Handle cached_tokens=0 correctly: returns 0 instead of None, so cache_read_input_tokens is reported as 0 rather than omitted on a cache miss.

Opt-in via --enable-prompt-tokens-details

AnthropicServingMessages now passes through args.enable_prompt_tokens_details like the other serving objects, instead of forcing it to True. Users who want cache fields populated in the Anthropic response opt in via the same --enable-prompt-tokens-details CLI flag the OpenAI side already requires. This avoids silently overriding the user's CLI configuration (per review feedback from @tlrmchlsmth).

Streaming behavior (deliberate consistency with vLLM OpenAI)

vLLM's OpenAI chat completion streaming attaches prompt_tokens_details only on the terminal include_usage chunk (choices == []), not on per-token chunks (see OpenAIServingChat.chat_completion_stream_generator). Consequently:

• message_start.usage (sourced from the first chunk): cache fields omitted (info not yet available).
• message_delta.usage (sourced from the terminal chunk): cache fields populated with the authoritative cumulative count.

This is deliberately consistent with vLLM's OpenAI streaming, not with upstream Anthropic (which does populate cache fields on message_start). Closing that gap requires plumbing prompt_tokens_details into the first stream chunk at the OpenAI serving layer — a separate, broader change out of scope here. The asymmetry is documented on _compute_cache_usage and locked in by tests.

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
cached_tokens=0 treated as None (gemini review)	Fixed	Fixed
Code duplication across 3 sites	Inline in each	Extracted to _compute_cache_usage
Streaming message_start / message_delta semantics documented	No	Yes (docstring + tests)
Unit tests	None	13 new tests

Test Plan

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

Test Result

13 passed (5 TestGetCachedTokens + 5 TestComputeCacheUsage + 3 TestStreamingCacheUsageSemantics)

Full test file (52 tests including pre-existing image / tool_result / thinking-block / inline-system / stream-converter tests) also passes locally.

---

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

[claude]

Claude Code 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.

[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

[tlrmchlsmth]

From what I understand, setting this is consistent with the Anthropic API, however setting this to True, silently overrides the user's --enable-prompt-tokens-details setting. I think we should revert this change

[zhangshuoming990105]

Reverted in 60ff79984 — enable_prompt_tokens_details is now sourced from args.enable_prompt_tokens_details like the other serving objects, so the CLI flag is no longer silently overridden.

A side effect: users who want the cache fields populated in the Anthropic response now have to opt in via --enable-prompt-tokens-details, which is what the OpenAI side already requires. I'll add a note to the PR description.

[tlrmchlsmth]

I think this a little weird for the streaming case since the cached will always be None on the first chunk, and the behavior will be implicitly different on subsequent ones. Not sure exactly how this should be handled but we should at least make sure this is documented + explicit

[zhangshuoming990105]

Good catch — addressed in 60ff79984. The asymmetry is intentional, and now both documented and made explicit in code.

Root cause. vLLM's OpenAI chat completion streaming attaches prompt_tokens_details (and therefore cached_tokens) only on the terminal include_usage chunk (choices == []), not on the per-token include_continuous_usage chunks (see OpenAIServingChat.chat_completion_stream_generator, the two _make_prompt_tokens_details call sites). The Anthropic layer sources message_start.usage from the first chunk and message_delta.usage from that terminal chunk, so today the cache info is only available at message_delta time.

What the PR now does. Treat "cache info unknown" as field absence, not null:

• _compute_cache_usage still returns (prompt_tokens, None, None) when cache info isn't available, but callers now omit cache_read_input_tokens / cache_creation_input_tokens from the emitted AnthropicUsage instead of emitting null. This is applied uniformly across the three sites (non-streaming response, message_start, message_delta).
• The docstring on _compute_cache_usage now spells this out, including the deliberate inconsistency with upstream Anthropic (which does populate cache fields on message_start). Closing that gap requires plumbing prompt_tokens_details into the first stream chunk at the OpenAI serving layer, which is a separate, broader change I'd rather not bundle into this PR.
• TestStreamingCacheUsageSemantics covers the three usage states (cache hit, cache miss with details, no details at all) for both message_start and message_delta, locking the contract in.

Concretely, the wire format is now:

// message_start (cache info unknown — fields absent, not null)
{"type":"message_start","message":{...,"usage":{"input_tokens":100,"output_tokens":0}}}

// message_delta (cache hit — fields populated)
{"type":"message_delta","delta":{...},"usage":{"input_tokens":20,"output_tokens":5,"cache_creation_input_tokens":0,"cache_read_input_tokens":80}}

Clients distinguish "unknown" (key absent) from "zero" (0), which I think is cleaner than the previous null everywhere.

[zhangshuoming990105]

@tlrmchlsmth gentle ping for re-review when you have a moment — both of your review comments are addressed in 60ff79984:

1. api_router.py:173 (silent CLI override): reverted to enable_prompt_tokens_details=args.enable_prompt_tokens_details. Users now opt in via the same CLI flag the OpenAI side already requires. See this reply.

2. _compute_cache_usage streaming asymmetry: explicit in code + documented. Cache fields are now omitted from message_start.usage (key absence signals "unknown") and populated on message_delta.usage (terminal chunk). This mirrors vLLM's OpenAI streaming behavior (cache info only on the terminal include_usage chunk); the inconsistency with upstream Anthropic is intentional and out-of-scope to close in this PR. Three new tests under TestStreamingCacheUsageSemantics lock the contract in. See this reply for the full rationale and wire-format examples.

PR description has been updated to reflect both changes. Happy to discuss further if anything is still unclear.

[bbartels]

@zhangshuoming990105 DCO is failing btw

[zhangshuoming990105]

@bbartels thanks for the heads-up — DCO is fixed. Amended commit 60ff79984 to include the Signed-off-by trailer and force-pushed (b722a922a → 77648ab26). No content change, just the trailer.

[tlrmchlsmth]

Thank you for the contribution!

[mergify]

Hi @zhangshuoming990105, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.