fix: preserve JSON object key order in proxied payloads

[himorishige]

Fixes #438.

What

Enables serde_json's preserve_order feature for the workspace, so JSON objects in proxied payloads are forwarded with their keys in the order the client sent them instead of alphabetized (the BTreeMap default). One line in Cargo.toml, plus the lockfile update (indexmap was already in the dependency tree).

Why

Object key order is semantic for response_format.json_schema: order-enforcing structured-output backends (vLLM with its default xgrammar backend) use the properties declaration order as the generation order. The router currently re-serializes every object with alphabetized keys, silently changing what downstream models are forced to generate. A model-independent repro is in #438.

With an order-sensitive fine-tuned judge behind an llm_classifier route we measured an 18% judgment-runaway rate (grammar/EOS whitespace deadlock at max_tokens, each occurrence an invalid verdict and a fail-open); restoring key order drops it to zero.

Verification

• cargo test --workspace passes with the feature enabled
• Passthrough probe: a schema sent as zebra/apple/mango reaches the backend in that order (previously apple/mango/zebra)
• E2E: an identical 100-judgment probe through the router against a local vLLM target went from 36/200 runaways — with all 10 request shapes flipping between tiers across reruns — to 0 runaways and 0 flips once key order is preserved

Notes

• preserve_order swaps serde_json's Map backing to IndexMap workspace-wide. Anything relying on alphabetical serialization would change behavior; the test suite passes, and for a proxy, forwarding payloads unmodified seems the safer default.
• Array order was already preserved; only object keys were affected.
• If you'd rather avoid the workspace-wide feature (e.g., carry passthrough JSON as RawValue instead), happy to rework.

Summary by CodeRabbit

• Improvements
  • JSON object fields now retain their original order when processed and displayed, providing more consistent and predictable output.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Enterprise

Run ID: 20036233-9305-4f2a-8d81-409a73be172a

📥 Commits

Reviewing files that changed from the base of the PR and between 9ad6744 and e43d8ea.

⛔ Files ignored due to path filters (1)

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

📒 Files selected for processing (1)

• Cargo.toml

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Walkthrough

The workspace enables serde_json's preserve_order feature. JSON object keys now retain their input order during serialization.

Changes

JSON order preservation

Layer / File(s)	Summary
Enable serde_json order preservation Cargo.toml	The workspace dependency enables serde_json's preserve_order feature while retaining version 1.

Estimated code review effort: 1 (Trivial) | ~2 minutes

Merge Risk: ⚪ Minimal · up to e43d8

This localized dependency configuration change preserves client-supplied JSON object key order, with the stated workspace tests and verification passing; no actionable merge-blocking risk remains beyond normal checks and review.

Poem

I’m a rabbit with keys in a row,
No alphabet shuffle, no overflow.
Zebra, apple, mango stay,
In the order sent, they pass this way.
Structured JSON can now proceed,
With every field in the intended lead. 🐇

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Linked Issues check	❓ Inconclusive	Cargo.toml enables serde_json preserve_order as required, but Cargo.lock is excluded and its update cannot be verified.	Confirm that Cargo.lock contains the dependency resolution changes for the serde_json preserve_order feature.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: preserving JSON object key order in proxied payloads.
Out of Scope Changes check	✅ Passed	The reviewed Cargo.toml change directly supports the linked issue, and no unrelated changes are shown.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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