Codec: token-native binary transport for /v1/completions + /v1/chat/completions streaming

[wdunn001]

Summary

Adds a binary streaming wire format (MessagePack and length-prefixed Protocol Buffers) for /v1/completions and /v1/chat/completions. Opt-in via the request-body field stream_format. Default behaviour byte-identical to current vllm.

Why

Real cross-stack measurements (run id 2026-05-15T20-00-00Z, 2,000-token completion on Qwen2.5-0.5B-Instruct; full matrix at https://github.com/wdunn001/Codec/blob/main/packages/bench/results/2026-05-15T20-00-00Z/MATRIX.md):

Engine	JSON-SSE	Best Codec	Reduction
sglang	485.2 KB	291 B (msgpack + dict-zstd)	1,707×
vllm	517.8 KB	3.9 KB (msgpack + gzip)	137×
llama.cpp	528.8 KB	140 B (msgpack + dict-zstd, fp16)	3,868×

vllm's 137× headline is gzip-only because the model's output at temp=0 is content-bound, not protocol-bound. With a dict-zstd path (Codec v0.5's discoverable .well-known/codec/dicts/), vllm reaches the same multi-thousand-× range as the other two.

The savings come from:

1. Not detokenizing at the serving server (no UTF-8 / JSON envelope per chunk).
2. Letting the client opt into HTTP-level compression (gzip, br, zstd with pre-trained dictionaries) on the binary stream.
3. Skipping the re-tokenize round-trip in agent-to-agent and tool-dispatch hops — the consumer reads uint32 IDs directly and feeds them to the next model.

What this PR adds (additive only)

13 files, +2,032 / -16 lines. All under vllm/entrypoints/.

New modules:

• codec_frame.py — MessagePack + Protocol Buffers encoders for CodecFrame {ids[], done, finish_reason?, tool_calls?}. Hand-rolled protobuf (no codegen step). Accepts Union[Sequence[int], np.ndarray[uint32], array.array('I'), bytes] on the ids parameter so future CODEC_OPENAI_BYPASS=1 work (skip the PyLong-list allocation per token in tokenizer_manager) can be wired in without further encoder churn.
• codec_compression.py — Accept-Encoding negotiation for zstd (with pre-trained dictionary), br, gzip, identity. Emits Codec-Zstd-Dict: sha256:<hex> on every zstd response so clients can verify the matching dict is loaded before decompressing.
• codec_agent.py — ToolWatcher: uint32-compare state machine that detects delimited regions (tool calls, reasoning blocks, multimodal spans) in the raw token stream without detokenizing. ~100× faster than detokenize+regex in the steady state. Per-request opt-in via the tool_watcher field on the request types.
• codec_dispatcher.py — bolt-on tool dispatch (default off, CODEC_BOLT_ON_DISPATCH=1). Reads tool manifests from CODEC_TOOL_MANIFEST_URLS at boot, hash-validates each manifest's tokenizerHash against the active model's tokenizer, POSTs CodecToolCall (msgpack-framed) to registered tools when ToolWatcher fires, reinjects response_ids into the generation stream.
• codec_version.py — protocol-version negotiation (Codec-Client-Version, Codec-Min-Version headers + 426 Upgrade Required + VERSION_INCOMPATIBLE frame).

Modified existing files:

• openai/completion/serving.py + openai/chat_completion/serving.py — dispatch to binary generator when stream_format != "json"; preserve JSON-SSE path byte-for-byte when unset or "json".
• openai/completion/protocol.py + openai/chat_completion/protocol.py — stream_format, tool_watcher, tool_watcher_start, tool_watcher_end fields.
• openai/completion/api_router.py + openai/chat_completion/api_router.py — route registration.
• openai/server_utils.py — codec-aware response helpers.

Plus 1 new test file: test_codec_compression.py.

Trust posture

• Default behaviour byte-identical to current vllm. Client that does NOT set stream_format gets JSON-SSE exactly as today.
• No new mandatory dependencies. Wire emit uses msgspec (already in vllm's reqs); compression uses brotli + zstandard (missing → graceful fallthrough to identity per Accept-Encoding negotiation rules).
• Engine boot unchanged unless opt-in. CODEC_BOLT_ON_DISPATCH=1 loads manifests lazily; without it nothing in the codec path runs at boot.

Test plan

• pytest vllm/entrypoints/test_codec_compression.py green (new in this PR).
• stream_format=msgpack round-trip via curl against a Qwen2.5-0.5B-Instruct engine; response Content-Type application/x-msgpack, decode reproduces token IDs end-to-end.
• Existing stream_format unset / stream_format=json requests produce byte-identical JSON-SSE to current main.
• Accept-Encoding: zstd, br, gzip negotiates correctly per RFC 7231 §5.3.4 preference order; Codec-Zstd-Dict header present on zstd responses.

Cross-stack reference + spec

• Wire format spec: https://github.com/wdunn001/Codec/blob/main/spec/versions/v0.5.md
• .well-known/codec/ discovery surface: https://github.com/wdunn001/Codec/blob/main/spec/WELL_KNOWN_DISCOVERY.md
• 6 client-language reference implementations (TS / Python / Rust / .NET / Java / C) consume this wire byte-equally: https://github.com/wdunn001/Codec/tree/main/packages
• Cross-stack bench (the table above + per-cell breakdown): https://github.com/wdunn001/Codec/blob/main/packages/bench/RESULTS.md

Companion PR against sgl-project/sglang filed in parallel: sgl-project/sglang#25544. The wdunn001/vllm fork has been carrying this code in production-grade wdunn001/codec-vllm Docker images for two releases (v0.4 and v0.5); cross-client byte-equality is 24/24 cells unanimous as of v0.4.1.

🤖 Generated with Claude Code

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🚀

[gemini-code-assist]

Code Review

This pull request introduces "Codec," a token-native binary transport protocol for vLLM that utilizes MessagePack and Protobuf framing to reduce wire overhead. The implementation includes a server-side tool call detection state machine, an agentic tool dispatcher, negotiated transport compression (supporting Zstandard with pre-trained dictionaries, Brotli, and Gzip), and a version negotiation system for graceful client downgrades. Review feedback identifies critical performance bottlenecks caused by synchronous, blocking HTTP requests and manifest fetching within asynchronous contexts and token generation loops. Additionally, improvements are suggested for the custom Protobuf decoder to include bounds checking and varint size limits to prevent potential crashes or infinite loops.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @wdunn001.

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

[depthfirst-app]

⚪ Severity: LOW

dispatch_call sends HTTP requests to tool.endpoint which is read from external manifest JSON (not directly from an env var). No URL validation (scheme, hostname, IP range) is enforced, so a compromised manifest source can direct the server to reach internal services, cloud metadata endpoints, or local files via file://.
Helpful? Add 👍 / 👎

💡 Fix Suggestion

Suggestion: Add a URL validation helper that restricts both _fetch_manifest and dispatch_call to only http:// and https:// schemes, and blocks requests to private/link-local IP ranges (e.g., 169.254.x.x, 127.x.x.x, 10.x.x.x, 192.168.x.x, etc.).

1. Add a _validate_endpoint_url(url: str) function near the top of the module that:

   • Parses the URL with urllib.parse.urlparse
   • Rejects any scheme other than http or https (blocks file://, ftp://, gopher://, etc.)
   • Resolves the hostname and checks the IP address is not in private, loopback, or link-local ranges (using ipaddress.ip_address(addr).is_private or explicit CIDR checks)
   • Raises ValueError on violations

2. Call _validate_endpoint_url(tool.endpoint) at the top of dispatch_call (before line 241)

3. Call _validate_endpoint_url(url) at the top of _fetch_manifest (before line 201)

4. Additionally, validate manifest["endpoint"] at registration time in ToolRegistry.from_env (around line 170) so malformed endpoints are caught at boot rather than at dispatch time.

Example helper:

import ipaddress
from urllib.parse import urlparse

_ALLOWED_SCHEMES = frozenset({"http", "https"})

def _validate_endpoint_url(url: str) -> None:
    parsed = urlparse(url)
    if parsed.scheme not in _ALLOWED_SCHEMES:
        raise ValueError(
            f"Unsupported URL scheme {parsed.scheme!r}; "
            f"only {_ALLOWED_SCHEMES} are allowed"
        )
    if not parsed.hostname:
        raise ValueError(f"URL {url!r} has no hostname")
    import socket
    try:
        addr = socket.getaddrinfo(parsed.hostname, None)[0][4][0]
        ip = ipaddress.ip_address(addr)
        if ip.is_private or ip.is_loopback or ip.is_link_local:
            raise ValueError(
                f"Tool endpoint resolves to private/loopback address {ip}"
            )
    except socket.gaierror as e:
        raise ValueError(f"Cannot resolve hostname {parsed.hostname!r}: {e}") from e