fix: canonicalize JSON values for stable vLLM prefix caching

[myshytf]

Supersedes #28 (closed, not merged).

Problem

#28 enabled serde_json preserve_order to fix struct field serialization order (model before chat_template_kwargs in #[serde(flatten)] output). The fix was correct for the struct-level problem but incomplete: preserve_order switches Value::Object from BTreeMap (alphabetical) to IndexMap (insertion order), removing the free key canonicalization BTreeMap provided.

Without preserve_order, BTreeMap automatically sorted all JSON object keys alphabetically — regardless of how clients constructed the JSON. This gave consistent serialized bytes for free, which is critical for vLLM prefix cache (byte-sensitive token matching).

With preserve_order ON, dynamic JSON values (tool parameters, tool_call arguments, content objects, extra_body) now preserve insertion order. Different clients or code paths constructing the same logical JSON with different key orders produce different bytes → different token sequences → cache miss.

Fix

Add canonicalize_chat_request_json_values() in src/upstream.rs which recursively sorts all Value::Object keys before sending to upstream vLLM. This restores the alphabetical key ordering guarantee that BTreeMap provided, while keeping preserve_order for struct-level field order (model first).

Canonicalization targets

• message content
• assistant tool_call arguments
• tool/function parameters
• tool_choice
• response_format
• extra_body / chat_template_kwargs

Also canonicalize in stringify_json_value() before string conversion.

Regression test

sanitize_chat_request_canonicalizes_preserve_order_sensitive_json_values — constructs the same logical request with reversed key order and asserts identical upstream JSON bytes after sanitize_chat_request().

Files changed

File	Description
Cargo.toml / Cargo.lock	Enable serde_json preserve_order feature (from #28)
src/upstream.rs	canonicalize_chat_request_json_values() + canonicalize_json_value() + stringify_json_value canonicalization + regression test

Test results

cargo test --release:
  lib:     281 passed
  binary:    3 passed
  gateway:  91 passed
  total:   375 passed, 0 failed
cargo build --release: OK

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Review details

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Configuration used: Organization UI

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📥 Commits

Reviewing files that changed from the base of the PR and between a96abaf and f0f40f7.

⛔ Files ignored due to path filters (1)

• Cargo.lock is excluded by !**/*.lock

📒 Files selected for processing (18)

• Cargo.toml
• src/adapters/anthropic_to_responses.rs
• src/adapters/chat_completions.rs
• src/adapters/chat_to_responses.rs
• src/adapters/responses_to_anthropic.rs
• src/adapters/responses_to_chat.rs
• src/cli.rs
• src/config.rs
• src/debug.html
• src/engine.rs
• src/http.rs
• src/lib.rs
• src/models/anthropic.rs
• src/monitor.rs
• src/search.rs
• src/upstream.rs
• src/vision.rs
• tests/gateway.rs

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