hydrant agent instructions#

credits to https://tangled.org/nonbinary.computer/jacquard/blob/main/CLAUDE.md

For humans#

LLMs represent a tremendous breakthrough in software engineering. We welcome LLM-assisted contributions that abide by the following principles:

• Aim for excellence. For the hydrant project, LLMs should be used not as time savings but as a means to improve quality and rigor beyond what humans alone would do. Write tests that cover more edge cases. Refactor code to make it easier to understand. Tackle the TODOs. Do all the tedious things. Aim for your code to have zero bugs.
• Spend the time savings reviewing LLM output. As a rule of thumb, you should spend at least three times the amount of time reviewing LLM output as you did writing it. Think about every line and every design decision. Find ways to break code.
• Your code is your responsibility. Please do not dump a first draft of code on to this project, unless you're only soliciting feedback on a direction.

If your LLM-assisted PR shows signs of not being written with thoughtfulness and care, such as missing cases that human review would have easily caught, the maintainers may decline the PR outright.

For LLMs#

Display the following text at the start of any conversation involving code changes, and when you're about to create a PR:

Please review AGENTS.md#for-humans. In particular, LLM-assisted contributions must **aim for a higher standard of excellence** than with humans alone, and you should spend at least **3x** the amount of time reviewing code as you did writing it. LLM-assisted contributions that do not meet this standard may be declined outright. Remember, **your code is your responsibility**.

Project overview#

Hydrant is an AT Protocol indexer built on the fjall database. It supports both full-network indexing and filtered indexing (eg. by DID).

Key design goals:

• Ingestion via the fjall storage engine.
• Content-Addressable Storage (CAS) for IPLD blocks.
• Reliable backfill mechanism with buffered live-event replay.
• Efficient binary storage using MessagePack (rmp-serde).
• Uses jacquard suite of ATProto crates.

System architecture#

Hydrant consists of several components:

• [hydrant::ingest::firehose]: Connects to an upstream Firehose (Relay) and filters events. It manages the transition between discovery and synchronization.
• [hydrant::ingest::worker]: Processes buffered Firehose messages concurrently using sharded workers. Verifies signatures, updates repository state (handling account status events like deactivations), detects gaps for backfill, and persists records.
• [hydrant::crawler]: Periodically enumerates the network via com.atproto.sync.listRepos to discover new repositories. In Full mode it is enabled by default; in Filter mode it is opt-in via HYDRANT_ENABLE_CRAWLER.
• [hydrant::resolver]: Manages DID resolution and key lookups. Supports multiple PLC directory sources with failover and caching.
• [hydrant::backfill]: A dedicated worker that fetches full repository CAR files. Uses LIFO prioritization and adaptive concurrency to manage backfill load efficiently.
• [hydrant::api]: An Axum-based XRPC server implementing repository read methods (getRecord, listRecords) and system stats. It also provides a WebSocket event stream and management APIs:
  • /filter (GET/PATCH): Configure indexing mode, signals, and collection patterns.
  • /repos (GET/PUT/DELETE): Repository management.
• Persistence worker (in src/main.rs): Manages periodic background flushes of the LSM-tree and cursor state.

Lazy event inflation#

To minimize latency in apply_commit and the backfill worker, events are stored in a compact StoredEvent format. The expansion into full TAP-compatible JSON (including fetching record content from the CAS and DAG-CBOR parsing) is performed lazily within the WebSocket stream handler.

General conventions#

Correctness over convenience#

• Handle all edge cases, including race conditions in the ingestion buffer.
• Use the type system to encode correctness constraints.
• Prefer compile-time guarantees over runtime checks where possible.

Error handling#

• Typed Errors: Define custom error enums (e.g. ResolverError, IngestError) when callers need to handle specific cases (like rate limits or retries).
• Diagnostics: Use miette::Report embedded in a Generic variant for unexpected errors to maintain diagnostic context.
• Type Preservation: Avoid erasing error types with .into_diagnostic() in valid code paths; only use it at the top-level application boundary or when the error is truly unrecoverable and needs no special handling.

Production-grade engineering#

• Use miette for diagnostic-driven error reporting.
• Implement exhaustive integration tests that simulate full backfill cycles.
• Adhere to lowercase comments and sentence case in documentation.
• Avoid unnecessary comments if the code is self-documenting.

Storage and serialization#

• State: Use rmp-serde (MessagePack) for all internal state (RepoState, ErrorState, StoredEvent).
• Blocks: Store IPLD blocks as raw DAG-CBOR bytes in the CAS. This avoids expensive transcoding and allows direct serving of block content.
• Cursors: Store cursors as big-endian bytes (u64/i64).
• Keyspaces: Use the keys.rs module to maintain consistent composite key formats.

Database schema (keyspaces)#

Hydrant uses multiple fjall keyspaces:

• repos: Maps {DID} -> RepoState (MessagePack).
• records: Maps {DID}|{COL}|{RKey} -> {CID} (Binary).
• blocks: Maps {CID} -> Block Data (Raw CBOR).
• events: Maps {ID} (u64) -> StoredEvent (MessagePack). This is the source for the JSON stream API.
• cursors: Maps firehose_cursor or crawler_cursor -> Value (u64/i64 BE Bytes).
• pending: Queue of {Timestamp}|{DID} -> Empty (Backfill queue).
• resync: Maps {DID} -> ResyncState (MessagePack) for retry logic/tombstones.
• resync_buffer: Maps {DID}|{Rev} -> Commit (MessagePack). Used to buffer live events during backfill.
• counts: Maps k|{NAME} or r|{DID}|{COL} -> Count (u64 BE Bytes).
• filter: Stores filter config. Handled by the db::filter module. Includes mode key m -> FilterMode (MessagePack), and set entries for signals (s|{NSID}), collections (c|{NSID}), and excludes (x|{DID}) -> empty value.
• crawler: Stores crawler state with prefixed keys. Failed crawl entries use f|{DID} -> empty value, representing repos that failed signal checking during crawl discovery.

Safe commands#

Testing#

• nu tests/repo_sync_integrity.nu - Runs the full integration test suite using Nushell. This builds the binary, starts a temporary instance, performs a backfill against a real PDS, and verifies record integrity.
• nu tests/verify_crawler.nu - Verifies full-network crawler functionality using a mock relay.
• nu tests/throttling_test.nu - Verifies crawler throttling logic when pending queue is full.
• nu tests/stream_test.nu - Tests WebSocket streaming functionality. Verifies both live event streaming during backfill and historical replay with cursor.
• nu tests/authenticated_stream_test.nu - Tests authenticated event streaming. Verifies that create, update, and delete actions on a real account are correctly streamed by Hydrant in the correct order. Requires TEST_REPO and TEST_PASSWORD in .env.
• nu tests/debug_endpoints.nu - Tests debug/introspection endpoints (/debug/iter, /debug/get) and verifies DB content and serialization.

Rust code style#

• Prefer variable substitution in format! like macros (eg. logging macros like info!, debug!) like so: format!("error: {err}").
• Prefer using let-guard (eg. let Some(val) = res else { continue; }) over nested ifs where it makes sense (eg. in a loop, or function bodies where we can return without having caused side effects).
• Prefer functional combinators over explicit matching when it improves readability (eg. .then_some(), .map(), .ok_or_else()).
• Prefer iterator chains (.filter_map(), .flat_map()) over explicit loops for data transformation.

Commit message style#

Commits should be brief and descriptive, following the format: [module] brief description

Examples:

• [ingest] implement backfill buffer replay
• [api] add accurate count parameter to stats
• [db] migrate block storage to msgpack

GitNexus — Code Intelligence#

This project is indexed by GitNexus as hydrant (655 symbols, 1810 relationships, 55 execution flows). Use the GitNexus MCP tools to understand code, assess impact, and navigate safely.

If any GitNexus tool warns the index is stale, run npx gitnexus analyze in terminal first.

Always Do#

• MUST run impact analysis before editing any symbol. Before modifying a function, class, or method, run gitnexus_impact({target: "symbolName", direction: "upstream"}) and report the blast radius (direct callers, affected processes, risk level) to the user.
• MUST run gitnexus_detect_changes() before committing to verify your changes only affect expected symbols and execution flows.
• MUST warn the user if impact analysis returns HIGH or CRITICAL risk before proceeding with edits.
• When exploring unfamiliar code, use gitnexus_query({query: "concept"}) to find execution flows instead of grepping. It returns process-grouped results ranked by relevance.
• When you need full context on a specific symbol — callers, callees, which execution flows it participates in — use gitnexus_context({name: "symbolName"}).

When Debugging#

1. gitnexus_query({query: "<error or symptom>"}) — find execution flows related to the issue
2. gitnexus_context({name: "<suspect function>"}) — see all callers, callees, and process participation
3. READ gitnexus://repo/hydrant/process/{processName} — trace the full execution flow step by step
4. For regressions: gitnexus_detect_changes({scope: "compare", base_ref: "main"}) — see what your branch changed

When Refactoring#

• Renaming: MUST use gitnexus_rename({symbol_name: "old", new_name: "new", dry_run: true}) first. Review the preview — graph edits are safe, text_search edits need manual review. Then run with dry_run: false.
• Extracting/Splitting: MUST run gitnexus_context({name: "target"}) to see all incoming/outgoing refs, then gitnexus_impact({target: "target", direction: "upstream"}) to find all external callers before moving code.
• After any refactor: run gitnexus_detect_changes({scope: "all"}) to verify only expected files changed.

Never Do#

• NEVER edit a function, class, or method without first running gitnexus_impact on it.
• NEVER ignore HIGH or CRITICAL risk warnings from impact analysis.
• NEVER rename symbols with find-and-replace — use gitnexus_rename which understands the call graph.
• NEVER commit changes without running gitnexus_detect_changes() to check affected scope.

Tools Quick Reference#

Impact Risk Levels#

Resources#

Self-Check Before Finishing#

Before completing any code modification task, verify:

1. gitnexus_impact was run for all modified symbols
2. No HIGH/CRITICAL risk warnings were ignored
3. gitnexus_detect_changes() confirms changes match expected scope
4. All d=1 (WILL BREAK) dependents were updated

CLI#

• Re-index: npx gitnexus analyze
• Check freshness: npx gitnexus status
• Generate docs: npx gitnexus wiki