Struct ArmSpec 

pub struct ArmSpec {
    pub engine: ArmEngine,
    pub model: Option<String>,
}

A fully-specified arm: which engine, on which backbone.

The backbone lives here rather than once per report because the interesting experiments need the two arms to differ on it:

• same-backbone (native@gpt-5.5 vs codex@gpt-5.5) — the harness delta, the original claim;
• cross-tier diagonal (native@gpt-5.4 vs codex@gpt-5.5) — what the runtime is worth in model tiers, which is the economically legible claim (see docs/proposals/coder-ab-value-surface.md);
• ablation (native@gpt-5.5 vs native@gpt-5.5 with a mechanism off) — isolates one mechanism, which a CAR-vs-Codex comparison can never do since it confounds the harness with everything else about Codex.

The old shape (an Arm enum plus one report-level backbone) could express only the first, and even then it did not enforce it: the pin reached the native loop alone, so the external arm silently ran on its CLI’s configured default. model here is threaded to whichever engine runs, so the invariant is now the runtime’s job, not the operator’s memory.

Fields

engine: ArmEngine

model: Option<String>

The pinned backbone. None = the engine’s own default (adaptive routing for native; the CLI’s config for external) — unpinned, so a same-backbone claim is unverifiable.

Implementations

impl ArmSpec

pub fn native(model: Option<String>) -> Self

CAR’s native loop on model.

pub fn external(id: impl Into<String>, model: Option<String>) -> Self

An external CLI (codex) on model.

pub fn label(&self) -> String

Report label: native@gpt-5.4, codex@gpt-5.5, or bare native when unpinned. Two arms in one run can share an engine (ablation) or a model (the harness delta), so the label carries both to stay unambiguous.

pub fn slug(&self) -> String

Filename-safe label: native@openai_gpt-5.4_latest.

Model ids carry / and : — a path separator, and illegal in a Windows filename — so the label must never be used as one directly. It was: the A/B wrote <task>-<label>.log and silently lost every native transcript (the / made it a path into a nonexistent dir), which are the exact records ab_learnings proposals tell a human to go read.

Trait Implementations

impl Clone for ArmSpec

fn clone(&self) -> Self

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Debug for ArmSpec

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl<'de> Deserialize<'de> for ArmSpec

fn deserialize<__D>(__deserializer: __D) -> Result<Self, __D::Error>

where __D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl Eq for ArmSpec

impl PartialEq for ArmSpec

fn eq(&self, other: &Self) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl Serialize for ArmSpec

fn serialize<__S>(&self, __serializer: __S) -> Result<__S::Ok, __S::Error>

where __S: Serializer,

Serialize this value into the given Serde serializer.

impl StructuralPartialEq for ArmSpec