msgspec.Struct gc=False Safety Check

When to use this skill

• Adding or modifying a class that inherits from msgspec.Struct
• Reviewing or refactoring code that defines or uses msgspec structs
• Deciding whether to add or remove gc=False on a Struct

Why gc=False matters

Setting gc=False on a Struct means instances are never tracked by Python's garbage collector. This reduces GC pressure and can improve performance when many structs are allocated. The only risk: if a reference cycle involves only gc=False structs (or objects not tracked by GC), that cycle will never be collected (memory leak).

Reference: msgspec Structs – Disabling Garbage Collection.

Verified safety constraints

Use these constraints to decide if a Struct can use gc=False. All must hold.

1. No reference cycles

• The struct (and any container it references) must never be part of a reference cycle.
• Multiple variables pointing to the same struct (x = s; y = x) are safe — that is not a cycle. A cycle is A → B → … → A.
• Returning a struct from a function is safe. What matters is whether any reference path leads back to the struct (e.g. struct's list contains the struct or something that holds the struct).

2. No mutation that could create cycles

• Do not mutate struct fields after construction in a way that could introduce a cycle (e.g. set a field to an object that references the struct, or append the struct to its own list/dict).
• Frozen structs (frozen=True) prevent field reassignment; force_setattr in __post_init__ is one-time init only, so that's acceptable.
• Assigning scalars (int, str, bool, float, None) to fields is safe — they cannot form cycles.

3. Mutable containers (list, dict, set) on the struct

• If the struct has list/dict/set fields, either:
  • Never mutate those containers after creation (no .append, .update, [...] = ..., etc.), and never store in them any object that references the struct, or
  • Do not use gc=False (conservative).
• Reading from containers (e.g. x = struct.foobars[i]) does not create cycles and is allowed.

4. Nested structs

• If a struct holds another Struct (or holds containers that hold Structs), the same rules apply to the whole reference graph: no cycles, no mutation that could create cycles. If any nested Struct uses gc=False, the whole graph must still be cycle-free.

5. Generic / mixins

• With gc=False, the type must be compatible with __slots__ (e.g. if using Generic, the mixin must define __slots__ = ()). See msgspec issue #631 / PR #635.

Checklist for "can use gc=False"

• Struct and everything it references can never participate in a reference cycle.
• No mutation of struct fields after construction that could introduce a cycle (frozen or init-only mutation is ok; scalar assignment is ok).
• Any list/dict/set fields are never mutated after creation, or we do not use gc=False.
• No storing the struct (or anything that references it) inside its own container fields.
• If Generic/mixins are used, __slots__ compatibility is satisfied.

Checklist for "must NOT use gc=False"

• Struct is mutated after creation in a way that could create a cycle (e.g. appending self to a list field).
• Container fields are mutated after creation and could hold the struct or back-references.
• Struct is used in a pattern where it's stored in a container that the struct (or its fields) also references.

Quick per-struct analysis steps

1. List all fields and their types (scalars vs containers vs nested Structs).
2. Search the codebase for: assignments to this struct's fields, mutations of its container fields (.append, .update, etc.), and any place the struct instance is stored (e.g. in a list/dict that might be referenced by the struct).
3. If only scalars or immutable types, or frozen with no container mutation → likely safe for gc=False.
4. If mutable containers and they're never mutated (and never made to reference the struct) → likely safe; otherwise → do not use gc=False.

Risky structs: audit and at-risk comment

A struct is risky for gc=False if it has a condition that would normally disallow gc=False (e.g. mutable list/dict/set fields), but that condition might never arise in practice (e.g. the field is only ever read, never mutated after construction).

Auditing a risky struct

1. Identify the at-risk condition (e.g. "has metadata: dict that could be mutated").
2. Search the codebase for all uses of that struct and of the at-risk field:
   • Any assignment to the field: obj.field = ..., obj.field[key] = ..., obj.field.append(...), obj.field.update(...), etc.
   • Any code path that could store the struct (or something holding it) inside that container.
3. If the audit finds no such mutation or cycle-creating storage, the condition never arises and gc=False is acceptable provided you add the at-risk marker so future changes are re-audited.

When audit passes

• Set gc=False on the struct.

• Add an at-risk comment and docstring note:

  • Above the class: a short comment stating why gc=False is used despite the at-risk condition, and when the audit was done (e.g. # gc=False: audit YYYY-MM: <condition> is only read, never mutated.).

  • In the docstring: a line that signals to future readers and to this skill that changes touching this struct must be re-audited. Use this format:

    AT-RISK (gc=False): Has <brief condition>. Any change that <what would violate safety> must be audited; if so, remove gc=False.

• Example (for a struct with a metadata dict that is only ever read):

  # gc=False: audit 2026-03: metadata dict is only ever read, never mutated after construction.
  class QueryResult(msgspec.Struct, ..., gc=False):
      """Result of a completed inference query.
  
      AT-RISK (gc=False): Has mutable container field `metadata`. Any change that
      mutates `metadata` after construction or stores this struct in a container
      referenced by this struct must be audited; if so, remove gc=False.
      ...
  

When touching an at-risk struct

If you are adding or changing code that uses a struct marked AT-RISK (gc=False):

1. Re-run the audit for that struct (searches above).
2. If your change mutates the at-risk field(s) or creates a cycle (e.g. stores the struct in its own container), remove gc=False from the struct and remove the at-risk comment/docstring line.
3. If your change does not touch the at-risk field or create cycles, the existing gc=False and at-risk comment remain; you may add a short note in the at-risk comment if the audit was re-checked (e.g. update the audit date).

References

• msgspec Structs – Disabling Garbage Collection
• msgspec Performance Tips – Use gc=False
• msgspec #631 – Generic structs and gc=False