Sglang Runtime Context

SGLang runtime-context architecture

One container owns process-static runtime state: sglang.srt.runtime_context.RuntimeContext (a process singleton reached via get_context()). Everything below is a tier on it.

reset_context() (unit-test teardown) drops the published config and installs fresh flags/resources/forward tiers.

Config: publish + namespace bags

ServerArgs holds the raw input and nothing else. Resolution writes no field: it declares, and the declarations are what the namespace bags are projected from. Business code never reads the record for a decision: a field read there answers with what the operator typed, not with what resolution decided.

• Every publishing process entry calls publish(server_args, role=...) (run_scheduler_process, the Ray SchedulerActor, the DP controller, tokenizer, detokenizer, encoder, weight-cache daemon, the multi-tokenizer worker, the spawned encoder TP/DP workers, the benchmark work functions, ...); constructors do not publish — ModelRunner, TokenizerManager and MMEncoder call assert_published and fail loudly if an entry forgot. The roles are enumerated once, as the keys of ROLE_NAMESPACE_SETS — there is no launcher role, the launch path publishes as tokenizer. The remaining non-publisher is run_multi_detokenizer_router_process: it is handed a ServerArgs, and uses it only for configure_logger(server_args) today, so it has nothing to publish for — a bag read added under that entry needs a publish at the entry first. publish projects the config bags from the declarations over the record's raw fields; the accessors (get_exec() etc.) fail closed before it runs. role records which process type published, and keys per-role namespace enforcement: SGLANG_ROLE_NAMESPACES=record audits which namespaces each role's process actually reads (per-pair persisted via SGLANG_ROLE_NAMESPACES_OUT; reads inside torch.compile-traced code are NOT observed — audit with compilation disabled before restricting a role), and =enforce fails closed on bag reads outside the role's ROLE_NAMESPACE_SETS entry (None = full tree; only audited roles are restricted).
• Bag membership is where the field is declared: one class per namespace under arg_groups/fields/, each carrying the _NS_PATH it stands for, and ServerArgs is assembled from them (collect_input_fields). The per-field NS("path") marker survives only for a class that cannot express this — an ad-hoc dataclass spanning namespaces, which is what the config-bag tests build. Coverage is linted two-way (test_server_args_namespaces.py, test_runtime_context_config_bags.py).
• Reading config: get_<ns>()[.sub].field — e.g. get_exec().moe.moe_a2a_backend, get_schedule().max_running_requests. Bag leaves are plain instance attributes, safe inside torch.compile-traced code.
• Mutating config after publish: the ONLY entry point is get_context().override(source, **fields). It writes the bag leaves in place (namespace readers see the new value) and records provenance in the overrides log. There is no write-through to the ServerArgs instance — it stays pristine. There is no in-place mutation entry on the instance at all: it is read-only after resolution.
• Reading a leaf when the caller holds the field name (a readback endpoint, a control-plane handler): get_context().config_leaf(name) — the read side of override. It resolves the flat name through the same NS map the write side uses and raises on a name that is not a config leaf. Code that knows its field when it is written reads the bag leaf directly; config_leaf is for name-driven code, not a way around the seed ratchet.
• Post-startup control-plane changes — a weight update, a HiCache mirror attach, a parser resolved from the chat template — go through TokenizerManager.record_config_updates(source, **fields), a named wrapper over get_context().override. One process keeps one log: the request dumps ship get_context().overrides_log(), and config_value(name) / resolved_config_dict(base) answer from the bags. The exposure ratchet resolves the wrapper, so a field recorded through it joins the post-publish override surface exactly like a direct override and needs the same ordering judgment against any supplied-instance read of it (test_supplied_instance_exposure_ratchet.py).
• model_path and served_model_name are answered off the manager. Both are NS leaves and override accepts them, but the tokenizer-side weight reload records only load_format and writes the two path fields as TokenizerManager attributes (_MANAGER_OWNED_FIELDS); config_value and resolved_config_dict overlay them on top of the bags. Bags do not cross a process boundary (above), so recording those two in the tokenizer process would leave every other process's bag on the old path while the log claimed a process-wide change. The scheduler rewrites its own copy where the reload happens — ModelRunner.update_model_fields overrides model_path / load_format for the target runner.
• Late launcher-stage resolution (pre-publish): a few rules cannot run inside __post_init__ — LoRA normalization, and the auto-parser detection that needs a tokenizer/chat-template load. They are resolution, not mutation, and they declare via arg_groups.overrides.declare_late_resolution(server_args, source, **fields), which refuses the published instance. The declaration lands in the stash on that very object, so every holder of it carries the decision — the HTTP server, the multi-tokenizer workers it is serialized for, the schedulers it forks — and each of them publishes bags projected from it. The fields stay the operator's input; resolution_result(sa, field) and the bags are what answer for the decision. Returning a variant here is a bug: the launcher rebinds its local and everyone else keeps the unresolved object.
• A value another runner / worker owns is a constructor argument, not a config copy. The draft worker's context_length, load format and attention backend travel as arguments to TpModelWorker / ModelRunner and live on the runner (ModelRunner.draft_attention_backend, kv_cache_dtype_str, …); the encoder DP worker's device is MMEncoder(gpu_id=...). There is no ServerArgs.derive any more — a config object is never copied-and-edited; test doubles that need a modified copy use sglang.test.test_utils.server_args_variant.

Why a bag override cannot stand in for late resolution or per-runner construction. The bags are projected at publish from the declarations over the instance's raw fields, so anything the runtime must read has to be declared before publish — an override afterwards puts instance and bags back out of agreement, and whole-object readers (ModelConfig.from_server_args, build_load_config, MMEncoder's own self.server_args.X) never see it. And bags do not cross a process boundary: a child publishes from the object it receives and re-projects its own bags, so a parent-side override is lost. Values that feed construction before any bag exists (group init reads server_args.tp_size) have no bag to override at all.

Reads that legitimately stay on a ServerArgs instance

• Per-runner values — there is no per-runner ServerArgs any more. The draft-worker config copy is gone: every worker (TpModelWorker, the draft workers in speculative/) is handed the same instance the process published, so a bag leaf is the decision and self.server_args.X is the operator's input — a post-publish override moves only the bag, which is exactly why a field that is process-wide config (attention_backend, skip_tokenizer_init, kv_cache_dtype) reads from the bags like any other, and why a residual instance read on this path is stale the moment someone overrides that leaf. What is genuinely per-runner travels two ways, neither of them a config instance: constructor arguments (ModelRunner(draft_attention_backend=...), MMEncoder(gpu_id=...)) and runner attributes holding the resolved value (model_runner.kv_cache_dtype_str, prefill_attention_backend_str, num_fused_shared_experts, linear_attn_backends) — threaded to consumers as arguments, never backfilled onto a shared object. A per-runner choice also stays out of the bags: recording it there is how a second runner inherits the first one's answer, which is exactly the bug linear_attn_backends replaced. The one sanctioned bend in that rule is scoped: ModelRunner._load_format_scope exposes the draft's --speculative-draft-load-format through get_model().override(load_format=...) for exactly the duration of the draft build, because model construction reads that bag leaf — the override restores on exit, so nothing outlives the scope. When there is a runner in hand, read its stamp; that is a different rule from "read the instance".
• Per-instance boundaries — the tokenizer-manager family, everything under entrypoints/, and the tokenizer-process multimodal processors read the bags. The old justification for keeping them on self.server_args ("several Engines can share one process, bags are last-publish-wins across them") is retracted — owner ruling (2026-08-15): a process holds at most one live config at a time (concurrent multi-Engine is unsupported; sequential rebuild stays legal, unit tests rely on it). Nothing in those files reads the instance any more -- the exposure ratchet's pin set is empty, so the next such read is a new entry that has to argue for itself. What genuinely stays per-instance is what differs per worker within one engine: base_gpu_id travels as a constructor argument (MMEncoder(gpu_id=...); BaseMultimodalProcessor._fast_image_processor_device is the shape to copy).
• Whole-object passes (f(server_args) handing the instance along) keep the supplied-instance contract; don't rewrite the parameter reads unless the field is runtime-mutated (see the elastic-EP ep_size case in eplb/expert_location.py) — or the field is one that resolution fills in and the callee runs in a process that has published. That second case is a decision, not a style question: the record carries the user's raw input, so a resolution-filled field read off it inside a runner-owned constructor answers with the pre-resolution value instead of the effective one. The answer is not automatically a bag read: pick where the value should come from — usually the get_*() bag, sometimes a runner stamp or a constructor argument (the per-mode attention pair and the encode-server gpu_id above are both this). The per-instance boundaries above are not exempt from this unless-clause (the multi-Engine exemption is retracted); each one gets its own disposition. test_supplied_instance_exposure_ratchet.py pins that set (empty today) — three spellings of the read: server_args.field, literal-name getattr(server_args, "field", default), and the parked form (self.x = server_args in a method that takes the parameter, read as self.x.field anywhere in the class) — and fails on a new one, so the disposition gets picked when the read is written. Two shapes stay parameter-form on purpose: a helper the resolution pipeline calls with a resolved_view (its parameter happens to be named server_args), and a factory whose contract is "build X from the record you are handed" (create_kt_config_from_server_args, DllmConfig.from_server_args).

Four ways a config sweep breaks something no test runs

Each of these shipped in a review round and cost a real defect; each now has a guard, named here so the next sweep checks the same four things by hand first.

1. The other implementations of an interface. Dropping a parameter means auditing implementers, not just callers: CustomSpecAlgo is the plugin base for speculative algorithms, and the dispatch calls it with the built-in's argument list. Nothing in the tree implements it, so only a plugin user hits the TypeError. Guard: test_plugin_hook_signatures.py.
2. Publish order inside a process entry, not per file. A file containing a publish says nothing about whether a given read runs before it. Spawned workers (MMEncoder for encoder DP/TP, the Ray scheduler actor) start with an empty context, so a bag read above the publish raises only there. Guard: test_publish_precedes_bag_reads.py.
3. The role namespace a process publishes under. ROLE_NAMESPACE_SETS narrows what each role may read; the DP controller is audited for exec alone. A helper that reaches for another namespace passes every default-mode test and aborts startup under SGLANG_ROLE_NAMESPACES=enforce. Prefer answering from the caller's own namespaces over widening the set.
4. Sibling surfaces of a readback. Changing what one entry point reports means enumerating the others: HTTP, gRPC and in-process Engine each have their own server-info and model-info, and each passes its own tests while its users lose the field. Guard: test_effective_state_surfaces.py.

A fifth, from the same rounds: the accessor name itself. Called as an object member (manager.get_disagg()), or shadowed by a same-named import (from model_loader import get_model next to the model bag, where the later import silently wins and the loader call gets a zero-argument bag), it imports fine and fails only when that path runs. The invariant is one line: the name means the process-wide bag, takes no arguments, and is bound once per module. ruff --select F811 catches the import collision; the member-call shapes are an AttributeError at call time only because RuntimeContext has no bag-named member and no __getattr__ -- a delegating __getattr__ would make them silent, and that is when this needs a guard again rather than a rule.

Write these guards over a derived set, never a hand-kept list: an entry naming a function that no longer exists, or a field list missing the one field nobody migrated, passes green forever. Both happened here -- a _ENTRY_POINTS row for a method the Ray actor does not have, and an effective-field set without load_format -- and both were invisible because the assertion had slack (>= len(...) - 1) or compared key names instead of value sources.

get_parallel(): one spelling per name

There is no .config hop. Ranks and group handles are @property read-through over the canonical getters, so they answer with the live process groups. Everything else — tp_size, pp_size, attn_cp_size, dcp_size, moe_dp_size included, alongside config-only leaves like nccl_port, enable_dp_attention, dp_size, ep_size, dwdp_size — is answered from the published parallel bag. Reading a leaf before publish raises a ValueError naming the namespace; an unknown name is an AttributeError.

A size reads from the configuration because the groups are built at exactly the configured widths — checked at every assignment to _TP / _PP / _ATTN_CP / _DCP / _MOE_DP in parallel_state.py. Three things do not follow that rule:

• initialize_model_parallel aliases _MOE_DP to _ATTN_CP when attn_cp_size > moe_dp_size, so a reader that means the MoE communicator's width calls get_moe_cp_size(), not get_parallel().moe_dp_size.
• patch_tensor_parallel_group runs a scope under a different TP group (draft workers), and declares it by overriding tp_size, tp_rank and tp_group for the scope's duration. Readers inside need no special spelling.
• Elastic EP scales ep_size / dp_size on the published bag while the group coordinators keep their construction width. Those are different names, not two answers to one name.

DCP keeps its own pair: get_parallel().attn_dcp_size / .dcp_enabled answer the effective topology (1 / False with no group installed), while dcp_size is what the launch requested.

A process-global seed field-read of one of these sizes (get_server_args().tp_size, or an alias of it) is a read-ratchet failure. A server_args the object was handed is a different thing and not a ratchet matter — see "Reads that legitimately stay on a ServerArgs instance". Fail-loud is narrower: before dist init, a live rank/group read raises. The six parallel quotients are not live reads at all — attn_tp_size, attn_dp_size, attn_dcp_size, moe_ep_size, moe_tp_size, dcp_enabled are a function of the configured leaves, computed once at publish into bag leaves, and answered override → stamp → published leaf. So dcp_enabled means "the launch configured DCP" (dcp_size > 1), not "a DCP group is installed here"; in a scheduler the stamp makes the two identical, in a process that publishes without dist init they differ. test_a_topology_is_stated_by_naming_the_width and its neighbours in test_runtime_context.py pin this; they replaced test_attn_dcp_defaults_when_group_is_uninitialized. One consequence for tests: overriding a leaf no longer moves its quotient — state a topology by publishing a config, or by naming the width. After init, only the DCP group is optional (_DCP exists only when dcp_size > 1; attn-CP and moe-DP always install, as size-1 aliases if unused). The config hop is deliberately dynamo-traceable (a plain property over a slot, no object.__getattribute__); gate helpers like enable_moe_dense_fully_dp() run inside compiled model forwards (test_parallel_config_leaves_trace_under_torch_compile pins this).

A third surface carries the same names: ParallelState (self.ps / mr.ps), the frozen per-process snapshot built once in Scheduler.__init__ from these configured sizes plus this process's ranks, and handed down (draft runners included). Prefer it where an object was handed one; it is not a global accessor.

Reading config: the seed is off limits

get_server_args().field in business code is a ratchet failure. Read:

• a resolved leaf → its namespace bag (get_exec().moe.moe_runner_backend, get_schedule().chunked_prefill_size, …). Bag-backed reads — a leaf directly, or a bag-derived accessor below — are what see post-publish overrides. Only the instance-derived accessors (the ones with no leaf to read) answer from the startup record and therefore do not.
• a leaf the caller names at runtime (a readback reporting a list of fields) → get_context().config_leaf(name); it resolves the name through NS and raises on a non-leaf. A call site that knows its field reads the bag leaf.
• the live topology → get_parallel() (bare names).
• a value derived from published leaves → an accessor in runtime_context that derives it from the bags. The strongest form of this is a Derived(fn=...) declared beside the leaves it is computed from, in the namespace's own arg_groups/fields/ class: publish computes it once and stores it as an ordinary bag leaf, so the read is a plain attribute load and it sees post-publish overrides. enable_mamba_extra_buffer, is_ep_joiner, is_ep_scale_joiner and is_startup_weight_load_overlap are declared that way now — read them where they are declared: get_exec().mamba.enable_mamba_extra_buffer, get_exec().moe.is_ep_joiner, get_model().is_startup_weight_load_overlap. (The namespace is the class that declares the field, not the namespaces its fn happens to read: the mamba one spans exec.mamba and memory, which is exactly why it could not be a method on either bag.) The old mamba_extra_buffer_enabled() / is_ep_joiner() functions and the same-named ServerArgs members are gone. The pre-publish helpers that remain exist for resolution, which has no bag to read yet. attention_backends() derives the (prefill, decode) pair from the three exec.kernel leaves, and max_speculative_num_draft_tokens() / cutedsl_moe_max_num_tokens() derive theirs from spec / schedule / exec.graph.
• a value only the instance can compute → the named accessor in runtime_context, which is the one module allowed to read the slot: mamba_cache_chunk_size(), mamba_state_chunk_size(), uses_mla_backend(), process_model_config(). These have no leaf to read — they combine several fields, the HF config, or a property with no bag of its own. A new derived member gets an accessor here rather than call sites reaching for the record, and only when the bag-derived shape above cannot express it.
• a parallel size → get_parallel().{tp,pp,moe_dp,attn_cp,dcp}_size, which is the parallel bag's own leaf: it answers with the resolved configuration and follows a post-publish override. Two questions are not that, and have their own spelling: the width of the MoE communicator you are about to collectively operate on is get_moe_cp_size() (the _MOE_DP = _ATTN_CP alias makes it differ), and the effective DCP topology is get_parallel().attn_dcp_size / .dcp_enabled (1 / False when no group is installed), which does not need dist init to answer.
• this runner's resolved value → the runner (prefill_attention_backend_str, kv_cache_dtype_str, draft_attention_backend, num_fused_shared_experts on the model).

self.server_args.field is still right for handed per-instance config (see "Reads that legitimately stay on a ServerArgs instance" above for the full set — per-instance boundaries and whole-object passes; there are no per-runner config copies to read any more). The allow-list is GrammarManager and MMEncoder; what sits beside it is residue, not a family — and not for one single reason:

• the tokenizer-manager family and entrypoints/ read the bags; what is left of them in the exposure ratchet is a handful of individually-dispositioned pairs, not a family awaiting conversion. Read the ratchet for the current set rather than assuming a directory is off-limits;
• GrammarManager is a handed instance for its residual self.server_args reads, but backend selection is not on the instance any more: create_grammar_backend reads get_exec().kernel.grammar_backend, and __init__ calls that factory whenever skip_tokenizer_init is false. In production the scheduler process has published; a test that constructs one without publishing has to keep patching the factory (or publish itself);
• MMEncoder publishes the very instance it is handed (publish(server_args, role="encoder")) and takes its per-worker device as a separate gpu_id argument. Its self.server_args reads are on this list as a construction-path convention, and the residual is real: they answer with the raw input, so a leaf resolution decided and a post-publish override both pass them by.

Their tests are not one story: a GrammarManager built standalone turns the factory's bag read into "config namespace not published" unless the test patches it or publishes, while MMEncoder publishes in its own __init__ and so needs no such arrangement.

Test doubles publish, they do not inject. A stand-in that carries server_args=SimpleNamespace(field=...) stops working the moment production reads the bag; seed the value with override_server_args, which publishes only once it is entered or installed — the bare call just builds the override:

override = get_context().override_server_args(field=...)
override.install()
self.addCleanup(override.restore)      # or: with get_context().override_server_args(...):

Five separate test files learned this the hard way during the sweep.

The rule is about a double standing in for config: a SimpleNamespace that pretends to be server_args. Prefer the context override even where a single-accessor stub would work — override_server_args(...) composed with the scoped bag / get_parallel() overrides expresses the cause (the configuration) rather than pinning one helper's answer, and it keeps working when a reader migrates between the accessor and the leaf. The sweep converted the last two accessor stubs to exactly that shape (test_attention_patching.py publishes the non-lazy strategy; test_kimi_k3_vision.py publishes tp_size and forces the live topology through get_parallel().override), so no test stubs an accessor today. Stubbing one named accessor remains a last resort for a case that isolates one branch of one helper where no published config can reach it — if you do it, say so in the test.

Mid-resolution reads (inside the pipeline only)

Resolution runs in __post_init__ and writes nothing onto the record: a handler declares (self._declare / declare_resolution), the declaration goes into the stash, and the fields keep what the caller passed. So a mid-resolution read of a field answers with the raw input — every reader in the pipeline goes through a view instead:

• resolving_view(server_args) / self._resolved() — the live view (walks the stash per read). This is what handlers and hooks bind, conventionally as cfg = resolving_view(self) at the top of the handler.
• resolved_view(server_args) — snapshots the overlay when built, which is what a post-process pass wants: it reads the state at its slot.

test_resolution_reads_the_declarations pins direct field reads at zero over the two scopes it can derive exactly (every arg_groups function taking a config, every ServerArgs handler the dispatcher reaches). Readers the pipeline calls from elsewhere (ModelConfig, the platform defaults, the spec-algo hook) have moved to the view as well — a field read there is the same bug, just one the derivation cannot enumerate.

One consequence worth knowing: because the fields are the raw input, resolving a bare dataclasses.replace copy lands in the same place as the parent — the pipeline reads only its own input. replace_resolved is the way to copy a resolved record (it carries the declarations and the model_config memo, so the copy does not re-resolve at all).

Adding a model-specific config adjustment

Never assign server_args fields from model code. Declare instead (sglang/srt/arg_groups/overrides.py):

• Constant per-arch values → MODEL_OVERRIDES["MyArchForCausalLM"] = {...}.
• Derived values → @register_model_override("MyArchForCausalLM") returning a dict; the callable receives pristine server_args + hf_config and must not write.
• Normalization that must see earlier declarations → a post-process pass invoked via run_post_process_pass at its slot (reads a view, returns a declaration dict).
• Values only knowable at load time are per-runner state, not declarations: there is no declare_load_time_override any more. A model-family decision that its checkpoint drives (shared-experts fusion) is a question the loader asks the model class — shared_experts_fusion_disable_reason(hf_config, quant_config), a classmethod answering without an instance — at the single model-instantiation point, and install_shared_experts_fusion_decision writes the answer to the ACTIVE moe flag before that model's layers build and read it (is_shared_experts_fusion_disabled, config-intent fallback). draft_model_build_scope brackets every draft build and routes the draft's answer to the speculative leaf, so a draft's decision never overwrites the target's. A process-level load-time fact (the sm80 dtype fallback — device-driven, identical for every runner) records directly via get_context().override.

Declarable fields form a whitelist: Arg(..., resolvable=True) in the ServerArgs dataclass. A declaration against a non-whitelisted field fails at its slot.

Load-time vs resolution-time (critical)

__post_init__ runs in the launcher process before any model/platform import. Logic that consults an extensible registry (e.g. out-of-tree platforms registering attention backends in init_backend(), which runs at model_runner import) must stay at load time (ModelRunner init), writing through get_context().override(). Before moving any load-time logic into resolution, verify everything it reads is already complete at construction time.

Runtime flags (get_flags())

For state that init-time code derives and runtime code reads — parsed enums, platform probes, swappable ACTIVE values. Not for config mirrors (read the bag leaf instead).

• Groups are typed dataclasses on Flags (capture / moe / dp): typo-safe writes, transactional test-only override(**kw) context manager.
• flags.moe is materialized by initialize_moe_config() at scheduler init (it reads exec.moe / spec / model, and takes no record); accessors (get_moe_a2a_backend etc.) are thin shims with lazy defaults. The speculative contexts (speculative_moe_backend_context) swap the ACTIVE leaves around draft forwards.
• flags.dp is materialized by initialize_dp_attention; is_dp_attention_enabled() is a shim over flags.dp.enabled.
• Adding a leaf: declare the dataclass field with a default equal to the pre-init behavior, materialize it at the owning subsystem's init, keep any public accessor as a shim.

Resources (get_resources())

Named slots + two keyed-lazy registries:

• get_stream(name) — get-or-create a named CUDA side stream; set_stream(name, stream) installs explicitly. Name leases by subsystem ROLE: all model alternate streams share "alt"; the offloader's copy stream is "offload"; DP-TBO comm is "dp_tbo_comm"; LoRA side stream is "lora_side". Two call sites may share a name only if their work belongs on one stream — sharing across roles serializes intended overlap.
• get_buffer(name, factory) — get-or-create a named persistent buffer. Grow-only or per-device semantics manage their resources.buffers entries directly (see tokenspeed / SM120 split / Marlin workspace). Buffer names are per-backend today; do not silently share.
• Singletons with manager semantics (EP dispatcher buffers, EPLB recorder/metadata, graph memory pool) keep their owning accessors/classes as facades; only the state lives in a resources entry. Preserve exact semantics in the shim: lazy defaults (the EPLB recorder defaults to a Noop instance, not None), publish-once asserts, event-reuse contracts.
• Stream/buffer creation is a driver call — it must happen outside cuda-graph capture; keep lease points at init/warmup time.

Per-forward flags (get_forward())

Contextvar-backed; a new thread sees the defaults; scoped(**kw) is the regular write path (transactional, restores on exit and on exception); set(name, value) exists for legacy sticky setters (is_extend_in_batch is intentionally sticky within a thread). Use this tier for anything set-per-forward and read-within-forward. Before adding cross-thread state here, prove the readers' thread affinity: contextvars do NOT propagate to already- running or newly spawned threads. Note TBO ("two-batch overlap") interleaves ubatches on ONE thread — do not design for TBO threads that don't exist.

Testing idioms

• Force a code path by overriding causes, not effects: compose get_context().override_server_args(**fields) (publishes a fresh dummy-boundary ServerArgs carrying the overrides AND projects the bags — with-scoped, or install()/restore() + addCleanup for fixture-lifetime use) + get_<ns>().override(...) (scoped override of one bag's own leaves) + get_parallel().override(...) (live topology) + get_flags().<group>.override(...) + get_forward().scoped(...). All are scoped and transactional. Tests control execution through the context — do not hand-build and publish config objects.
• Never monkeypatch import bindings (module.get_x = lambda: ...) and never fake a config source with a SimpleNamespace stand-in: production reads the published bags, so a faked accessor silently stops intercepting after any reader migration. Publish for real (override_server_args(...)), then adjust bag leaves with the scoped bag override where the constructed ServerArgs cannot carry the value (e.g. get_device().override(device="meta")). The one carve-out is the deliberate single-accessor stub for isolating one predicate — the terms and the two sanctioned examples live under "Test doubles publish, they do not inject" above; anything wider than one named accessor is this rule.
• Mocked runners/managers still need the per-runner instance attributes the code under test reads (kv_cache_dtype_str, server_args for whole-object passes) — set them explicitly on the mock; MagicMock(spec=...) raises on attributes that only exist post-__init__, which is the fastest way to find a missed stub.
• reset_context() in teardown when a test publishes outside a scoped override.
• ServerArgs(model_path="dummy") early-returns the pipeline (few declarations, no strict guard) — fine for lightweight fixtures.
• Asserting what resolution decided reads resolution_result(sa, "field"), not sa.field: the field is the raw input. Assert the field only when the point of the case is that the record stayed pristine (the FA4 page-size and waterfill cases do exactly that, and say so).
• Run changed test files per-file (own process), the way CI does: a monolithic local pytest run lets a context published by an earlier file mask a missing-publish bug in a later one.

Guardrails (these fail CI; what to do when they fire)

1. Strict mutation guard (always on): bare server_args.x = ... after resolution raises unconditionally in ServerArgs.__setattr__ — this is the guarantee that no writer can desync the bags, so there is no writer ratchet any more. Change resolved config with get_context().override; hand a per-runner value to its runner as a constructor argument. Projected bags are sealed the same way (leaf assignment raises).
2. Mutation ratchet (test_server_args_mutation_ratchet.py, exact pin 0 over the whole package minus the pipeline / multimodal_gen): textual scan for assignment forms. Never raise the baseline.
3. No-copy contract (test_server_args_no_instance_mutation_entry.py): neither ServerArgs.override nor ServerArgs.derive exists, and nothing in the package calls either form. Rerouting a writer to the bags means flipping all its readers in the same commit (no transitional dual-write).
4. The legacy accessor is retired (test_runtime_context.py): every get_global_server_args() call now raises, because it answered with the record -- a caller reading a field resolution had decided got a stale value and no error. The replacement for a decision is a bag leaf, a named accessor, or the owning runner's stamp — not get_server_args().field, which the read ratchet below pins at zero. runtime_context.get_server_args() is only for the whole-object shapes (dumps, provenance, a hand-off to a callee that takes a config).
5. Global config read ratchet (test_global_config_read_ratchet.py): baselines are 0 for both the direct get_server_args().field and the alias form (function-local — including local copies of an alias, cfg = sa — module-level, or parked on an instance attribute, plus the getattr(..., "field") spelling of each; a name computed at runtime or indirection deeper than a local name copy is census-tool territory, per the test's docstring). The scanner matches get_server_args by its literal name — bare or module-qualified (ctx.get_server_args()) — and TestNoRenamedAccessorImports in the same file bans import ... as renames of it, which is what makes literal-name matching sound. Exempt by owner module only (runtime_context.py, server_args.py, arg_groups/). Two classes, no more: TestGlobalConfigReadRatchet holds the two baselines and TestNoRenamedAccessorImports holds the ban. There is no configured-size registry here any longer — get_parallel() has one spelling per name, so a size read is not a choice between two answers and nothing needs registering.
6. Module-state ratchet (test_module_state_ratchet.py): global statements in the flag-owning layers are pinned by name. A new module-level runtime global belongs on a flags group / resources slot instead; migrating a pinned survivor must shrink the pin.
7. Namespace coverage (test_server_args_namespaces.py, test_runtime_context_config_bags.py): every ServerArgs field resolves to a namespace — from the arg_groups/fields/ class that declares it — and the projected bags must cover the fields exactly (two-way).

Never module-skip a test "until the migration settles" — seed the context instead (the deferral ratchet that once pinned this is retired; the rule stands).

Hard-won pitfalls (check these before/while refactoring)

• Moving code drops first-line guards: early returns (if self.is_draft_worker: return) are the easiest thing to lose when relocating a method body. A draft is built from the target's published config — there is no draft co

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