kb/openspec/specs/engine-api/spec.md

Engine API

Purpose

The engine API provides an HTTP interface for knowledge base operations including search, document ingestion, document management, tag management, and system status.

Requirements

Requirement: Engine startup and model loading

The engine SHALL load the embedding model eagerly at startup before accepting HTTP requests. The engine SHALL expose a health endpoint that returns unhealthy until the model is fully loaded and the database is initialised.

Scenario: Cold start with model download

• WHEN the engine starts for the first time with no cached model
• THEN it SHALL download the configured embedding model, load it into memory (GPU if available, CPU otherwise), enable WAL mode on the SQLite database, and begin accepting requests only after all initialisation completes

Scenario: Health check during startup

• WHEN a client sends GET /api/v1/health before the model is loaded
• THEN the engine SHALL respond with HTTP 503 and {"status": "starting"}

Scenario: Health check after startup

• WHEN a client sends GET /api/v1/health after initialisation completes
• THEN the engine SHALL respond with HTTP 200 and {"status": "healthy"}

---

Requirement: Hybrid search

The engine SHALL provide hybrid search combining BM25 full-text search (via FTS5) and vector similarity search (via sqlite-vec), merged using Reciprocal Rank Fusion with a top-rank bonus (+0.05 for rank 1, +0.02 for ranks 2-3 in either arm) that preserves exact matches. Search SHALL complete in under 100ms when the model is warm and reranking is disabled; reranked searches SHALL complete in under 500ms on GPU. The engine SHALL sanitize user query strings to prevent FTS5 syntax errors for any input.

Scenario: Hybrid search with results

• WHEN a client sends POST /api/v1/search with body {"query": "how to change oil", "top": 5}
• THEN the engine SHALL embed the query using the resident model, run both FTS5 and vector searches, merge results via RRF with top-rank bonus, and return a JSON response with matched chunks including scores, document_id, document metadata, tags, and tag_contexts

Scenario: Explain traces

• WHEN a client sends POST /api/v1/search with "explain": true
• THEN each result SHALL include an explain object with per-arm raw scores and ranks (fts_score, fts_rank, vec_score, vec_rank), RRF contributions (rrf_fts, rrf_vec), the top-rank bonus, rerank blend fields when reranking ran (pre_rerank_rank, retrieval_norm, rerank_score, blend_weight), and the final_score; fields for an arm that did not match SHALL be null

Scenario: Search with filters

• WHEN a client sends POST /api/v1/search with body {"query": "brakes", "tags": ["maintenance"], "doc_type": "pdf", "top": 3}
• THEN the engine SHALL apply tag and document type filters to both FTS5 and vector results before merging

Scenario: Search with mode override

• WHEN a client sends POST /api/v1/search with body {"query": "error log", "fts_only": true}
• THEN the engine SHALL return only FTS5 results without running vector search

Scenario: Empty knowledge base

• WHEN a client searches against an empty database
• THEN the engine SHALL return HTTP 200 with {"query": "...", "results": [], "total_matches": 0}

Scenario: Search with special characters

• WHEN a client sends POST /api/v1/search with body {"query": "what color is grass?"}
• THEN the engine SHALL sanitize the query for FTS5, execute the search successfully, and return results (not a 500 error)

Scenario: Search with FTS5 operators in query

• WHEN a client sends POST /api/v1/search with body {"query": "NOT something OR (other)"}
• THEN the engine SHALL treat the input as literal search terms, not FTS5 operators, and return matching results

Scenario: Search with only special characters

• WHEN a client sends POST /api/v1/search with body {"query": "??!@#"}
• THEN the engine SHALL return HTTP 200 with an empty result set (not a 500 error)

Scenario: Search with quotes in query

• WHEN a client sends POST /api/v1/search with body {"query": "the \"quick\" fox"}
• THEN the engine SHALL sanitize embedded quotes and return results normally

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Requirement: Cross-encoder reranking

The engine SHALL support optional server-side reranking of hybrid search results using a local cross-encoder model, enabled via KB_RERANK_ENABLED (default false) with the model set by KB_RERANKER_MODEL (default BAAI/bge-reranker-v2-m3). When active, the engine SHALL over-fetch candidates (KB_RERANK_CANDIDATES, default 40), score each (query, chunk) pair, and blend retrieval and rerank scores position-aware: 75% retrieval weight for pre-rerank ranks 1-3, 60% for 4-10, 40% for 11+, with retrieval scores min-max normalised over the candidate set. Blended scores are on a 0-1 scale distinct from RRF scores; the score threshold SHALL be applied before reranking only. Every search response SHALL include a top-level "reranked" boolean.

Scenario: Reranked search

• WHEN reranking is enabled with a loaded model and a client sends a hybrid search
• THEN the engine SHALL rerank the top candidates and return results ordered by blended score with "reranked": true

Scenario: Per-request opt-out

• WHEN a client sends POST /api/v1/search with "rerank": false
• THEN the engine SHALL skip reranking and return plain hybrid results with "reranked": false

Scenario: Graceful degradation

• WHEN reranking is requested but the model is disabled or failed to load
• THEN the engine SHALL return plain hybrid results with "reranked": false and no error

Scenario: Single-arm searches never rerank

• WHEN a client sends a search with fts_only or vec_only set
• THEN the engine SHALL NOT rerank, keeping single-arm results pure for benchmarking

---

Requirement: Async ingestion via job queue

The engine SHALL accept file uploads and text notes for ingestion asynchronously. Uploaded content SHALL be written to a staging area and a job record created in the database. The engine SHALL return HTTP 202 immediately. A background worker SHALL process queued jobs sequentially. Before staging, the engine SHALL compute a SHA256 hash of the uploaded content and reject duplicates immediately.

Scenario: Upload a PDF file

• WHEN a client sends POST /api/v1/jobs with a multipart form containing a PDF file and optional fields (tags, doc_type)
• THEN the engine SHALL compute the SHA256 hash of the file bytes, verify no existing document has the same hash, write the file to the staging directory, create a job record with status queued, and return HTTP 202 with {"job_id": "<id>", "status": "queued", "filename": "report.pdf"}

Scenario: Upload a text note

• WHEN a client sends POST /api/v1/jobs with a multipart form containing a note text field and optional title field
• THEN the engine SHALL compute the SHA256 hash of the note text (UTF-8 encoded), verify no existing document has the same hash, write the note content to a staging file, create a job record with status queued, and return HTTP 202 with the job ID

Scenario: Upload multiple files in sequence

• WHEN a client sends multiple POST /api/v1/jobs requests in quick succession
• THEN the engine SHALL queue each job independently and the background worker SHALL process them in FIFO order

Scenario: Duplicate file detected at upload time (already ingested)

• WHEN a client uploads a file whose SHA256 content hash matches an already-ingested document
• THEN the engine SHALL NOT stage the file or create a job record, and SHALL return HTTP 409 with {"error": "duplicate", "document_id": <id>, "title": "<title>"}

Scenario: Duplicate file detected at upload time (in-flight job)

• WHEN a client uploads a file whose SHA256 content hash matches a queued or processing job
• THEN the engine SHALL NOT stage the file or create a job record, and SHALL return HTTP 409 with {"error": "duplicate", "job_id": <id>, "title": "<filename>"}

Scenario: Duplicate note detected at upload time (already ingested)

• WHEN a client submits a note whose SHA256 content hash matches an already-ingested document
• THEN the engine SHALL NOT stage the note or create a job record, and SHALL return HTTP 409 with {"error": "duplicate", "document_id": <id>, "title": "<title>"}

Scenario: Duplicate note detected at upload time (in-flight job)

• WHEN a client submits a note whose SHA256 content hash matches a queued or processing job
• THEN the engine SHALL NOT stage the note or create a job record, and SHALL return HTTP 409 with {"error": "duplicate", "job_id": <id>, "title": "<filename>"}

Scenario: Duplicate uploaded during concurrent request handling

• WHEN two identical files are uploaded in the same instant, both passing the API hash check before either job is committed
• THEN both jobs SHALL be queued, and the background worker SHALL process the first normally and mark the second as skipped (worker-side safety net via hash_exists() and UNIQUE constraint)

Scenario: Upload failure due to unsupported file type

• WHEN a client uploads a file with an unsupported extension
• THEN the engine SHALL return HTTP 422 with an error message listing supported types

---

Requirement: Job status tracking

The engine SHALL maintain job records in SQLite with status tracking. Jobs SHALL transition through states: queued → processing → done | failed | skipped.

Scenario: List all jobs

• WHEN a client sends GET /api/v1/jobs
• THEN the engine SHALL return a JSON array of job records ordered by creation time (newest first), each including job_id, filename, status, created_at, and completed_at

Scenario: Filter jobs by status

• WHEN a client sends GET /api/v1/jobs?status=failed
• THEN the engine SHALL return only jobs with the specified status

Scenario: Get job details

• WHEN a client sends GET /api/v1/jobs/{id}
• THEN the engine SHALL return the full job record including status, filename, error message (if failed), document_id (if done), chunk count, and timing information

Scenario: Job not found

• WHEN a client sends GET /api/v1/jobs/{id} with a non-existent ID
• THEN the engine SHALL return HTTP 404

---

Requirement: Background ingestion worker

The engine SHALL run a background worker that processes queued jobs. The worker SHALL process one job at a time. For each job, it SHALL: detect document type, run the appropriate chunking pipeline (Docling for PDFs, header-based for Markdown, AST-based for code, whole-text for notes, fixed-size text chunking for data files with minified JSON pretty-printed first), build enriched text by prepending the document title (and section header when present) to each chunk's text, generate embeddings using the enriched text and the resident model, insert chunks (with both raw text and enriched text) and vectors into the database, and move the original file to persistent storage.

Scenario: Successful PDF ingestion

• WHEN the background worker picks up a queued PDF job
• THEN it SHALL update the job status to processing, run Docling conversion and chunking, build enriched text for each chunk by prepending the document title, embed all chunks using enriched text, insert document and chunks into the database, move the staged file to {data_dir}/documents/{content_hash}.pdf, update documents.stored_path with the permanent path, store the original filename in documents.original_filename, update the job status to done with the resulting document_id and chunk count, and clean up the staging entry

Scenario: Ingestion failure

• WHEN the background worker encounters an error during processing (e.g., corrupt PDF)
• THEN it SHALL update the job status to failed with the error message, delete the staged file, and continue processing the next queued job

Scenario: Search during active ingestion

• WHEN a search request arrives while the background worker is processing a job
• THEN the search SHALL execute without blocking (SQLite WAL mode) and return results from already-ingested documents

---

Requirement: Document management

The engine SHALL provide endpoints to list, inspect, remove, and download original files for ingested documents.

Scenario: List documents

• WHEN a client sends GET /api/v1/documents
• THEN the engine SHALL return a JSON array of documents with id, title, doc_type, tags, chunk_count, created_at, and updated_at

Scenario: List documents with filters

• WHEN a client sends GET /api/v1/documents?type=pdf&tags=manual
• THEN the engine SHALL return only documents matching all specified filters

Scenario: List documents sorted by most recent

• WHEN a client requests documents sorted by date
• THEN the engine SHALL use COALESCE(updated_at, created_at) for ordering, so un-mutated documents sort by creation time and mutated documents sort by their last update

Scenario: Get document details

• WHEN a client sends GET /api/v1/documents/{id}
• THEN the engine SHALL return the full document record including all chunks, their text content, updated_at, and whether the original file is available (has_file: true/false)

Scenario: Download original file

• WHEN a client sends GET /api/v1/documents/{id}/file
• THEN the engine SHALL return the original file with appropriate Content-Type and Content-Disposition: attachment; filename="{original_filename}" headers, or HTTP 404 if the file is not available

Scenario: Remove a document

• WHEN a client sends DELETE /api/v1/documents/{id}
• THEN the engine SHALL delete the document, all its chunks, associated embeddings, tag associations, and the stored original file from disk, and return HTTP 200 with a confirmation

Scenario: Remove non-existent document

• WHEN a client sends DELETE /api/v1/documents/{id} with a non-existent ID
• THEN the engine SHALL return HTTP 404

---

Requirement: Note mutation endpoint

The engine SHALL provide a PATCH /api/v1/notes/{id} endpoint for updating existing notes in place. See the note-mutation spec for full details.

Scenario: Note update endpoint exists

• WHEN a client sends PATCH /api/v1/notes/42 with body {"text": "new content"}
• THEN the engine SHALL process the update synchronously and return the updated document

---

Requirement: Document updated_at tracking

The engine SHALL track when documents are modified via an updated_at column. This column SHALL be NULL for documents that have never been updated.

Scenario: New document has no updated_at

• WHEN a document is first ingested
• THEN updated_at SHALL be NULL and created_at SHALL be set to the ingestion timestamp

Scenario: Note update sets updated_at

• WHEN a note is updated via PATCH /api/v1/notes/{id}
• THEN updated_at SHALL be set to the current timestamp

Scenario: Tag change sets updated_at

• WHEN tags are modified via PUT /api/v1/documents/{id}/tags
• THEN updated_at SHALL be set to the current timestamp

Scenario: Schema migration for updated_at

• WHEN the engine starts against a v2 database without an updated_at column
• THEN the engine SHALL automatically add ALTER TABLE documents ADD COLUMN updated_at TEXT and all existing documents SHALL have updated_at = NULL

---

Requirement: Tag management

The engine SHALL provide endpoints to list all tags and manage tags on documents.

Scenario: List all tags

• WHEN a client sends GET /api/v1/tags
• THEN the engine SHALL return a JSON array of tags with name, document count, and description (null when unset)

Scenario: Add tags to a document

• WHEN a client sends PUT /api/v1/documents/{id}/tags with body {"add": ["manual", "v2"]}
• THEN the engine SHALL add the specified tags to the document and return the updated tag list

Scenario: Remove tags from a document

• WHEN a client sends PUT /api/v1/documents/{id}/tags with body {"remove": ["draft"]}
• THEN the engine SHALL remove the specified tags from the document and return the updated tag list

---

Requirement: Tag context descriptions

The engine SHALL support a one-line context description per tag, stored in a description column on the tags table (added via idempotent migration). Search results SHALL include a tag_contexts object mapping each of the document's described tags to its description, so consumers can judge which similar-scoring chunks answer the question.

Scenario: Set a tag description

• WHEN a client sends PUT /api/v1/tags/{name}/description with body {"description": "Lab operations runbooks"}
• THEN the engine SHALL store the description (matching the tag name case-insensitively) and return {"name": "<name>", "description": "<description>"}

Scenario: Clear a tag description

• WHEN a client sends PUT /api/v1/tags/{name}/description with a null or empty description
• THEN the engine SHALL clear the stored description

Scenario: Unknown tag

• WHEN a client sets a description for a tag that does not exist
• THEN the engine SHALL return HTTP 404

Scenario: Descriptions in search results

• WHEN a search result's document carries tags and at least one tag has a description
• THEN the result SHALL include tag_contexts with only the described tags; results with no described tags SHALL include an empty tag_contexts object

---

Requirement: Engine status and reindex

The engine SHALL provide status information and support re-embedding all chunks. The version field in the status response SHALL always be present and SHALL reflect the engine's release version as read from the VERSION file. This field is the contract used by clients for compatibility checking.

Scenario: Get engine status

• WHEN a client sends GET /api/v1/status
• THEN the engine SHALL return JSON with version (string, from VERSION file), model_name, embedding_dim, GPU device info, database stats (document count by type, total chunks, DB size), queue stats (queued/processing job count), and a rerank object with enabled, model, loaded, and candidates

Scenario: Trigger reindex

• WHEN a client sends POST /api/v1/reindex
• THEN the engine SHALL re-embed all existing chunks using the enriched_text column and the currently loaded model, and return progress information. This operation SHALL NOT block search queries.

---

Requirement: API authentication

The engine SHALL support optional API key authentication via Bearer token. When KB_API_KEY is set, all requests MUST include a matching Authorization: Bearer <key> header. When KB_API_KEY is not set, authentication SHALL be disabled.

Scenario: Valid API key

• WHEN KB_API_KEY is set and a request includes a matching Bearer token
• THEN the engine SHALL process the request normally

Scenario: Missing API key when required

• WHEN KB_API_KEY is set and a request has no Authorization header
• THEN the engine SHALL return HTTP 401 {"error": "authentication required"}

Scenario: Invalid API key

• WHEN KB_API_KEY is set and a request includes a non-matching Bearer token
• THEN the engine SHALL return HTTP 401 {"error": "invalid api key"}

Scenario: Auth disabled

• WHEN KB_API_KEY is not set
• THEN the engine SHALL process all requests without requiring authentication

---

Requirement: Engine configuration via environment variables

The engine SHALL be configured via environment variables. No config file is read by the engine — all configuration comes from the environment (set via compose.yaml or Docker run).

Scenario: Default configuration

• WHEN the engine starts with no environment variables set
• THEN it SHALL use defaults: data directory /data, model all-MiniLM-L6-v2, device auto, no API key required. It SHALL create staging/ and documents/ subdirectories under the data directory.

Scenario: Custom model

• WHEN KB_MODEL is set to BAAI/bge-small-en-v1.5
• THEN the engine SHALL download and load that model instead of the default