kebab/tasks/p3/p3-1-embedder-trait.md

phase, component, task_id, title, status, depends_on, unblocks, contract_source, contract_sections

phase	component	task_id	title	status	depends_on	unblocks	contract_source	contract_sections
P3	kb-embed (trait crate)	p3-1	Embedder trait + EmbeddingInput/Kind validation	planned	p0-1	p3-2 p3-3 p3-4	../../docs/superpowers/specs/2026-04-27-kb-final-form-design.md	§3.7 SearchHit.embedding_model §7.1 EmbeddingInput/Kind §7.2 Embedder §11 LLM/embedding split

p3-1 — Embedder trait crate

Goal

Provide the kb-embed crate that re-exports Embedder trait, EmbeddingInput/EmbeddingKind, and offers a mock implementation for downstream tests. This task is trait-only; concrete adapters live in p3-2.

Why now / why this size

Concrete adapters (fastembed, ollama-embed, candle) need a stable trait surface. Owning the trait + a mock implementation in a tiny crate keeps kb-store-vector and kb-search testable without touching real models.

Allowed dependencies

• kb-core
• kb-config
• serde
• thiserror
• tracing

Forbidden dependencies

• fastembed, ort, tokenizers, kb-source-fs, kb-parse-md, kb-normalize, kb-chunk, kb-store-*, kb-search, kb-llm*, kb-rag, kb-tui, kb-desktop

Inputs

input	type	source
EmbeddingInput	kb_core::EmbeddingInput<'_>	callers (parser-side or query-side)
model identity	(EmbeddingModelId, EmbeddingVersion, dimensions)	adapter at construction

Outputs

output	type	downstream
Vec<Vec<f32>>	row-aligned with input	kb-store-vector, kb-search (vector mode)

Public surface (signatures only — no new types)

pub use kb_core::{EmbeddingInput, EmbeddingKind, EmbeddingModelId, EmbeddingVersion, Embedder};

/// Test-only mock that produces deterministic vectors.
pub struct MockEmbedder { /* internal: model_id, dims, seed */ }
impl MockEmbedder {
    pub fn new(model_id: kb_core::EmbeddingModelId, version: kb_core::EmbeddingVersion, dimensions: usize) -> Self;
}
impl kb_core::Embedder for MockEmbedder { /* per §7.2 */ }

Behavior contract

• MockEmbedder::embed produces vectors deterministically from (text, kind): e.g., vector[i] = hash_to_unit_float(text, kind, i, seed) so two identical inputs produce identical vectors and different inputs produce nearly-orthogonal vectors. Used by downstream tests.
• MockEmbedder must respect EmbeddingKind::Document vs Query — different prefix mixed into the hash so query embeddings differ from document embeddings of the same text (mirrors real e5 behavior).
• dimensions() returns the value passed at construction; callers must trust it.
• Real adapters (p3-2) MUST NOT implement Embedder here.
• The crate may expose a tiny helper pub fn assert_vector_shape(vecs: &[Vec<f32>], expected_dims: usize) for downstream tests.

Storage / wire effects

• None.

Test plan

kind	description	fixture / data
unit	trait dyn dispatch via Box<dyn Embedder> works	inline
unit	MockEmbedder produces identical vector for identical input	inline
unit	EmbeddingKind::Document vs Query for same text yield different vectors	inline
unit	dimensions match construction-time value	inline
contract	property test: 100 random inputs, each vector has length == dimensions, all finite floats	inline (proptest)

All tests under cargo test -p kb-embed.

Definition of Done

• cargo check -p kb-embed passes
• cargo test -p kb-embed passes
• No external embedding dep present
• PR links design §7.2 Embedder, §11

Out of scope

• Real adapter (kb-embed-local is p3-2).
• Reranker traits (P+).

Risks / notes

• MockEmbedder is for tests; do not let it leak into release builds via default features. Gate behind cfg(test) or a mock feature flag.
• Trait re-exports keep the call site stable even if kb-core reorganizes; downstream crates should use kb_embed::Embedder rather than use kb_core::Embedder.