feat(router): route /v1/messages/count_tokens and forward Anthropic headers by chromecast56 · Pull Request #2638 · smg-project/smg

chromecast56 · 2026-09-22T21:50:02Z

PR stack (merge bottom-up; each PR is based on main and includes the PRs below it)

Review this PR's changes relative to #2637; the earlier changes belong to the PRs below.

Description

Problem

Anthropic clients (Claude Code, SDKs) call POST /v1/messages/count_tokens before a request, and they choose protocol behavior with the anthropic-version and anthropic-beta headers. The gateway has neither piece:

There is no count_tokens route, so the call gets a 404.
The request-header allowlist (should_forward_request_header) drops both headers, so engines serving the native Messages API never see them. This affects the regular HTTP path and PD (feat(pd): route Anthropic Messages and Responses API through dual dispatch #2250) alike.

Solution

Implement GenerationRequest for the existing (previously unused) CountMessageTokensRequest: the prompt-side Messages fields, with no max_tokens. Its routing text comes from a routing_text helper shared with CreateMessageRequest.
Route /v1/messages/count_tokens through RouterTrait::route_messages_count_tokens. Support is HTTP-only (regular and PD); gRPC keeps an explicit 501 response.
- HTTP router: the regular typed-request path (policy selection, retries, load guard, circuit breaker, alias rewrite).
- PD router: picks one prefill worker by policy and forwards the request there, with no decode leg and no bootstrap fields. It applies the overload check, holds a load guard through the body read, uses client authorization or the configured worker key, and records the final breaker outcome. It returns 503 when no prefill worker is available.
Preserve additional count-tokens fields, including context_management and mcp_servers, when forwarding to the backend.
Add anthropic-version and anthropic-beta to the forward allowlist.

Ported from sgl-project/sglang#39712.

Changes

crates/protocols/src/messages.rs: GenerationRequest for CountMessageTokensRequest; routing_text factored out of CreateMessageRequest.
crates/external_router/src/header_utils.rs: allowlist the two headers, with tests.
model_gateway/src/routers/{mod.rs,gateway.rs,http/router.rs,http/pd_router.rs}, server.rs: the route and its dispatch.

Test Plan

cargo test -p smg --lib -- count_tokens
cargo test -p smg-external-router --lib -- should_forward_request_header

test routers::http::pd_router::tests::messages_count_tokens_uses_one_prefill_worker_and_no_bootstrap ... ok
test routers::http::router::tests::messages_count_tokens_forwards_to_the_selected_worker ... ok
test routers::http::router::tests::messages_count_tokens_without_workers_is_not_forwarded ... ok
test header_utils::tests::test_should_forward_request_header_whitelist ... ok

On main, the new tests don't compile (no route or trait method). With only the allowlist change reverted, the header assertions fail:

test routers::http::router::tests::messages_count_tokens_forwards_to_the_selected_worker ... FAILED
test header_utils::tests::test_should_forward_request_header_whitelist ... FAILED

Checklist

cargo +nightly fmt passes
cargo clippy --all-targets --all-features -- -D warnings passes (run without OpenCV: cargo clippy --workspace --all-targets -- -D warnings)
(Optional) Documentation updated
(Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

coderabbitai · 2026-09-22T21:50:20Z

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📝 Summary

Summary by CodeRabbit

New Features
- Added support for Anthropic Messages token counting at /v1/messages/count_tokens, with requests routed to available HTTP workers.
- Preserved system and message text for routing decisions and accepted backend-specific token-counting options.
- Forwarded the anthropic-version and anthropic-beta headers to workers, including in cross-origin requests.
Bug Fixes
- Improved handling of token-counting requests when no suitable HTTP worker is available.

Walkthrough

The gateway adds a protected /v1/messages/count_tokens endpoint. It routes requests to HTTP workers and forwards them to a prefill worker. The changes also add Anthropic header forwarding and bounded JSON serialization in existing PD dispatch paths.

Changes

Anthropic message token counting

Layer / File(s)	Summary
Token-count request contract `crates/protocols/src/messages.rs`	`CountMessageTokensRequest` captures additional backend options and implements `GenerationRequest`. Its routing text uses a shared helper that also serves `CreateMessageRequest`.
Endpoint and protocol headers `model_gateway/src/routers/mod.rs`, `model_gateway/src/server.rs`, `crates/external_router/src/header_utils.rs`	The protected endpoint passes typed requests to the router trait. CORS and the header allowlist include `anthropic-version` and `anthropic-beta`.
Gateway selection and HTTP routing `model_gateway/src/routers/gateway.rs`, `model_gateway/src/routers/http/router.rs`	The gateway selects an HTTP router for token-count requests. The HTTP router forwards requests to `/v1/messages/count_tokens`. Tests cover worker selection, forwarded fields, and the no-worker response.
Prefill worker forwarding `model_gateway/src/routers/http/pd_router.rs`	The PD router selects one prefill worker, forwards the request, and tracks worker load and upstream outcomes. Tests cover authorization, load tracking, and body-read failures.

PD request serialization

Layer / File(s)	Summary
Bounded serialization in PD dispatch `model_gateway/src/routers/http/pd_router.rs`	Two PD dispatch paths build JSON values using the raw request body length bound. The NIXL relay test checks that `top_p` is forwarded to both prefill and decode legs.

Priority: ➖ Normal

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

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant Server
  participant Gateway
  participant PDRouter
  participant PrefillWorker
  Client->>Server: POST /v1/messages/count_tokens
  Server->>Gateway: route_messages_count_tokens
  Gateway->>PDRouter: route typed token-count request
  PDRouter->>PrefillWorker: POST /v1/messages/count_tokens
  PrefillWorker-->>PDRouter: response status and body
  PDRouter-->>Client: routed response

Merge Risk: ⚪ Minimal · up to 90872

This change adds Anthropic message token counting and routes each request to a single prefill worker, with load tracking and correct failure accounting. Credentials reach workers the same way they already do for other HTTP routes, so the change does not add new exposure. No merge-blocking issues remain.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 59.68% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 62 functions across 11 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the new count_tokens route and Anthropic header forwarding, which are the main changes.
Description check	✅ Passed	The description explains the routing changes, header forwarding, implementation details, and test plan.
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.

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Inline comments:
In `@model_gateway/src/routers/gateway.rs`:
- Line 495: Update the router selection in the dispatch closure to consider only
HTTP_PD and HTTP_REGULAR, excluding gRPC routers. Preserve the existing behavior
of routing HTTP_PD requests to a prefill worker and HTTP_REGULAR requests to a
regular worker’s token-count endpoint.

In `@model_gateway/src/routers/http/pd_router.rs`:
- Line 2047: In the token-count path around send_with_stale_conn_retry, retain
the prefill worker’s load with a WorkerLoadGuard from selection through reading
the upstream response. Match the guard lifetime used by the existing typed HTTP
dispatch so concurrent token-count requests are reflected in load-aware
selection.
- Line 2059: Move worker.record_outcome out of the pre-body-read path and update
the response handling so it records the final status after res.bytes().await; if
reading the body fails and the handler returns 502, record 502 for the worker.

In `@model_gateway/src/server.rs`:
- Line 307: Update CountMessageTokensRequest to retain the supported top-level
output_config and context_management fields, and ensure the typed request
forwards them to the worker so token counting uses the client’s supplied
request.
- Line 878: Update create_cors_layer’s configured-origin request-header
allowlist to permit anthropic-version and anthropic-beta alongside Content-Type
and Authorization, so preflight requests for the /v1/messages/count_tokens route
pass.

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crates/external_router/src/header_utils.rs
crates/protocols/src/messages.rs
model_gateway/src/routers/common/mod.rs
model_gateway/src/routers/gateway.rs
model_gateway/src/routers/http/pd_router.rs
model_gateway/src/routers/http/request_body.rs
model_gateway/src/routers/http/router.rs
model_gateway/src/routers/mod.rs
model_gateway/src/server.rs
model_gateway/src/worker/metrics_aggregator.rs
model_gateway/tests/metrics_aggregator_test.rs

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chromecast56 · 2026-09-22T22:30:42Z

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In `@model_gateway/src/routers/http/pd_router.rs`:
- Line 2056: Validate the selected worker URL scheme in the prefill dispatch
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…eaders Anthropic clients call /v1/messages/count_tokens before sending a request, and select protocol behavior with the anthropic-version and anthropic-beta headers. The gateway had no count_tokens route (404), and its forward allowlist dropped both headers, so engines serving the native Messages API never saw them. - Route the existing CountMessageTokensRequest (the prompt-side Messages fields, no max_tokens) through worker selection: the HTTP router uses the regular typed-request path (policy, retries, load guard, breaker), the PD router picks one prefill worker by policy and sends no decode leg or bootstrap fields, answering 503 when no prefill worker is available. - Add anthropic-version and anthropic-beta to the request header allowlist. Ported from sgl-project/sglang#39712. Signed-off-by: James Liu <jamesl@modal.com>

Signed-off-by: James Liu <jamesl@modal.com>

slin1237 · 2026-09-23T03:50:39Z

Rebased onto main (b5de046) now that #2636 and #2637 are merged: the two merged commits were dropped from the branch and your four commits were replayed unchanged, with no conflicts (head is now 908721d). Locally on the rebased branch: nightly fmt, stable clippy -D warnings, the count-tokens and header allowlist tests, and the full cargo test -p smg --lib suite (1943 tests) pass.

chromecast56 requested review from CatherineSue, key4ng and slin1237 as code owners September 22, 2026 21:50

This was referenced Sep 22, 2026

fix(metrics): preserve colons in /engine_metrics metric names #2636

Merged

fix(router): forward client JSON numbers exactly on HTTP and PD paths #2637

Merged

perf(pd): commit the client stream on decode's 2xx, not the prefill body #2639

Merged

github-actions Bot added tests Test changes protocols Protocols crate changes model-gateway Model gateway crate changes labels Sep 22, 2026

chromecast56 mentioned this pull request Sep 22, 2026

[SMG] Route native Messages requests without Chat Completions conversion sgl-project/sglang#39712

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Comment thread model_gateway/src/routers/gateway.rs Outdated

Comment thread model_gateway/src/routers/http/pd_router.rs

Comment thread model_gateway/src/routers/http/pd_router.rs Outdated

Comment thread model_gateway/src/server.rs

Comment thread model_gateway/src/server.rs

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Comment thread model_gateway/src/routers/http/pd_router.rs

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chromecast56 added 4 commits September 22, 2026 20:47

fix(router): preserve count-tokens forwarding and worker accounting

f8be0e3

Signed-off-by: James Liu <jamesl@modal.com>

fix(router): select HTTP token counters and allow Anthropic CORS headers

61ab091

Signed-off-by: James Liu <jamesl@modal.com>

test(router): fix CORS regression test imports

908721d

Signed-off-by: James Liu <jamesl@modal.com>

hello-alexmcc force-pushed the smg/messages-count-tokens-headers branch from b7fd9d5 to 908721d Compare September 23, 2026 03:50

github-actions Bot removed the tests Test changes label Sep 23, 2026

slin1237 merged commit 1ae2b79 into smg-project:main Sep 23, 2026
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slin1237 mentioned this pull request Sep 23, 2026

fix(reasoning): arm the reasoning parser from the rendered prompt #2538

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slin1237 mentioned this pull request Sep 24, 2026

docs(api): document Messages count_tokens and forwarded Anthropic headers smg-project/smg-docs#51

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github-actions Bot mentioned this pull request Oct 12, 2026

docs: /v1/messages/count_tokens now works on gRPC and ZMQ workers; the docs say they answer 501 smg-project/smg-docs#162

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feat(router): route /v1/messages/count_tokens and forward Anthropic headers - #2638

feat(router): route /v1/messages/count_tokens and forward Anthropic headers#2638
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