Third of four; stacked on #38047. Two small changes, both about the same thing:
the record holds the operator's input, and nothing else should be true of it.
## `/server_info` can answer what was actually typed
It reports `resolved_dict()` -- what resolution decided. There was no way to ask
the other question, and the two are not derivable from each other: a field
nobody set reads the same as one set to the value resolution would have picked
anyway.
The launcher stores the arguments it parsed and the in-process `Engine` stores
the call that built the record. All three readbacks report it beside the
resolved values, so both surfaces come back in one request: HTTP `/server_info`,
`Engine.get_server_info`, and the gRPC bridge's -- the last one builds from
`resolved_dict()` and would otherwise have been the one surface of the three
that answers only "what resolution decided".
It rides on the record rather than in a field -- it describes how the
configuration was asked for, so it is not part of the configuration: no CLI
flag, no namespace, not in the bags. Being on the record is what lets a
subprocess copy answer the same question the launcher can, and
`replace_resolved` carries it because a copy was launched by whatever launched
its parent.
The crash dump already collected all four surfaces (`server_args`,
`config_updates`, `resolved_config`, `launch_command`); this is the one that
`/server_info` was missing.
## The record is sealed for the length of resolution
The read-only guard armed on `_resolution_finished`, so for the whole run of the
pipeline nothing stopped a resolver from assigning a field. Nothing in `srt/`
does -- 0 assignments statically, and 0 writes observed across the launch-shape
matrix with a watching `__setattr__` -- but that was a convention, and the
defect it permits is invisible: a value a resolver wrote onto the record is
indistinguishable from a value the operator typed, which is the one distinction
the record exists to preserve.
It now arms when resolution starts. A resolver that assigns a field fails at
boot with a message naming `declare_resolution`, which is where the decision
belongs: the stash carries a source and leaves the input intact.
`declare_direct_writes` asks for the seal by name through `record_writable`. It
hands the record to an out-of-tree platform plugin that sets fields on it; those
implementations cannot be converted by editing a resolver here, so the write
stays and the diff is captured into the stash afterwards. Naming the exception
is the point -- an in-tree resolver reaching for it is doing something it should
be declaring.
## Verification
Costs nothing: the 211 test-side assignments all happen before `resolve_once`,
which a post-resolution write already refused. A full registered-unit sweep
(648 files) against the stack's merge-base: 19 failures on both sides, the same
19, none of them config. Driving a
deliberate write into a real handler produces the new error, so the seal is
tested by more than its own unit test.
First of five. The stack continues a series that moved configuration out of
`ServerArgs` and into the runtime context's namespace bags. This one fixes
something that was actually broken, and gives the fix its other half.
## "Unset" gets its own spelling on two ratio fields
`swa_full_tokens_ratio` and `mamba_full_memory_ratio` carried real values as
their class defaults (0.8, 0.9), so a model family with an opinion had to ask
"is this field still equal to the class default?" to find out whether the
operator had set it. That question has two wrong answers: it says "the operator
set it" as soon as any earlier pass declares the field, and it says "the
operator did not set it" when the operator types the default value.
Both become `Optional[float] = None`. The record carries what the operator typed
and nothing else, and the family test becomes `is None`.
`mamba_radix_cache_strategy` keeps `"auto"`: unlike the ratios it already has a
spelling for "unset" that an operator can type and that means exactly that --
only its comparison changes, from the class default to the token itself, which
is the fix the comment at that site already prescribed. With that, neither
family module imports `ServerArgs` any more.
## And the declaration says what the field means when nobody answers
Making the default `None` leaves a hole: something has to supply the generic
value. `Arg(fallback=...)` supplies it from the declaration.
```python
swa_full_tokens_ratio: A[
Optional[float],
Arg(help="...", resolvable=True, fallback=0.8),
NS("schedule"),
] = None
```
The dataclass default stays `None`. A fallback is not a default: the record is
the wire format, and a child process has to keep being able to tell "unset" from
"set to the value resolution would have picked anyway".
### Which surface it lives on is the whole design
Precedence becomes **override -> decision -> input -> fallback**, applied in
`resolution_result` -- which the projection, `/server_info` and every config bag
read through.
Deliberately **not** in `resolving_view` / `resolved_view`. Those are the
decision-over-input surface a pass reads *while it is deciding*, and two model
families branch on exactly this:
```python
# model_overrides/inkling.py, and the same shape in deepseek_v4.py
if cfg.swa_full_tokens_ratio is None:
overrides["swa_full_tokens_ratio"] = 0.1
```
A fallback answering there is not "the generic value, later" -- a `__getattr__`
layer is read-time, so there is no later. Every read during resolution would
already get 0.8 and the branch would never fire. Running `_inkling_overrides`
against both versions:
```
--- fallback on the effective surface only (this PR) ---
cfg.swa_full_tokens_ratio during resolution = None
family declared swa = 0.1 mamba = 0.1
--- fallback also on the view a pass reads ---
cfg.swa_full_tokens_ratio during resolution = 0.8
family declared swa = None mamba = None <- the key never lands
```
So "resolution first, then the fallback" holds -- not because a step is appended
to the pipeline, but because of which surface the value lives on. Exactly one
reader consults the effective surface during resolution: the range check on the
ratio, which wants the value the pools will be sized against. It asks
`resolution_result` directly -- what its comment already claimed it was doing --
and it runs after the model families.
### The alternative, and why not
A pass that fills the field in when nothing claimed it needs a slot (after the
families, or it beats them), a second call site (the dummy-model short circuit
returns long before that slot), an idempotence requirement so the second call is
harmless, and the value written twice -- once as a literal, once as prose in the
help (`"Unset means 0.8"`). An earlier revision of this series did exactly that
and deleted it four PRs later. A declaration needs none of it, and `pipeline.py`
is untouched by the whole series as a result.
### What may be declared this way, and what may not
Across every hook, `if x is None: x = ...` appears at **55 sites over 29
fields**. They are not one thing:
| | count | examples | declarable |
|---|---|---|---|
| unconditional constant | 5 | the two ratios, `grammar_backend="xgrammar"`, `mm_process_config={}`, `custom_weight_loader=[]` | **yes** |
| unconditional, computed from another field | 4 | `tokenizer_path=model_path`, `device=get_device()`, `served_model_name`, `speculative_draft_model_quantization` | needs a `fallback="dotted.path"` form; not here |
| **conditional decision** | ~20 | `chunked_prefill_size` across seven memory tiers, `max_bs` across eight, `max_running_requests` at 48 or 256 by model family | **no, and it should not be** |
Only a value fixed for the life of the configuration belongs in a declaration.
One that depends on the machine, on another field, or on anything impure
(`random_seed = random.randint(...)`) is a decision, and decisions stay in a hook
where their order is visible. This PR converts the two ratios only.
## Verification
- `resolve_once` ends with the same effective values: the resolution result is
identical across 24 launch shapes x 489 fields except for the two intended
ratio changes. Separately, 16 launch shapes resolved on both sides, real model
and dummy: 7,904 field readings, and the only difference is `random_seed`, a
fresh `random.randint` per process.
- The CLI registers the same 507 options with the same choices and actions; only
the two defaults move.
- `test_declared_fallbacks.py`, 17 cases. One pins the inverse of the dead branch
above: what a pass sees while deciding is still `None`.
- The whole series was swept over all 648 registered unit-test files against its
merge-base: 19 failures on both sides, the same 19, none of them config.
---
### CI States
Latest PR Test (Base): <!-- slot:pr-test:start -->❌ [Run #34083705463](https://github.com/sgl-project/sglang/actions/runs/34083705463)<!-- slot:pr-test:end -->
Latest PR Test (Extra): <!-- slot:pr-test-extra:start -->❌ [Run #34083705284](https://github.com/sgl-project/sglang/actions/runs/34083705284)<!-- slot:pr-test-extra:end -->
Latest PR Test (AMD ROCm 7.2): <!-- slot:pr-test-amd-rocm720:start -->❌ [Run #34083705383](https://github.com/sgl-project/sglang/actions/runs/34083705383)<!-- slot:pr-test-amd-rocm720:end -->
<!-- pr-states:end -->
Cheng Wan