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+# AGENTS.md — stellar-horizon
+
+## Overview
+Horizon is the client-facing HTTP API for the Stellar blockchain network. Ingests ledger data from Stellar Core, serves REST/SSE endpoints, persists to PostgreSQL.
+
+## Architecture
+```
+main.go → cmd/root.go (cobra CLI) → internal/flags.go (NewAppFromFlags)
+    → internal/app.go (App) → internal/httpx/server.go (HTTP server)
+                            → internal/ingest/ (FSM-based ledger processor)
+                            → internal/txsub/ (transaction submission)
+```
+
+### Core Subsystems
+| Package | Purpose | Entry |
+|---------|---------|-------|
+| `internal/ingest/` | FSM ledger ingestion. States: start→build→resume loop | `main.go`, `fsm.go` |
+| `internal/db2/history/` | DB models + queries. `Q` struct wraps all history ops | `main.go` |
+| `internal/actions/` | HTTP handlers. One file per resource type | `account.go`, `transaction.go`, etc. |
+| `internal/txsub/` | Tx submission to Core, result tracking | `submitter.go` |
+| `internal/httpx/` | Router setup, middleware, server lifecycle | `server.go`, `router.go` |
+
+### Key Types
+- `App` (internal/app.go): Main application container. Holds web server, ingester, submitter, DB sessions
+- `Q` (internal/db2/history/main.go): Query builder for all history tables. Wraps squirrel
+- `system` (internal/ingest/main.go): Ingestion coordinator. Runs FSM, holds processors
+
+## Build & Run
+```bash
+# Build
+go build -o stellar-horizon -trimpath -v .
+
+# Local dev (requires Docker)
+./docker/start.sh standalone   # or: testnet, pubnet
+
+# CLI commands
+./stellar-horizon serve        # Start API server
+./stellar-horizon db migrate up
+./stellar-horizon ingest verify-range 1000 2000
+```
+
+## Testing
+```bash
+go test ./...                              # Unit tests (co-located *_test.go)
+HORIZON_INTEGRATION_TESTS_ENABLED=true \   # Integration tests
+  go test -v ./internal/integration/...
+
+# Integration test setup requires:
+# - PostgreSQL (HORIZON_INTEGRATION_TESTS_DOCKER_IMG or local)
+# - Stellar Core binary (STELLAR_CORE_BINARY_PATH)
+```
+DO NOT use `internal/test/integration/` framework — deprecated, will be EOL.
+
+## Code Style
+**Linting**: golangci-lint with `.golangci.yml`
+- Max line length: 140 chars
+- Max function length: 100 lines, 50 statements  
+- Cyclomatic complexity: 15
+- Run: `golangci-lint run` or `./gofmt.sh && ./govet.sh && ./staticcheck.sh`
+
+**Formatting**: gofmt + goimports (local prefix: `github.com/stellar/stellar-horizon`)
+
+## Database
+PostgreSQL. Migrations in `internal/db2/schema/migrations/` (goose-style numbered SQL).
+
+Two table categories:
+1. **History tables**: Time-series data (`history_transactions`, `history_operations`, `history_effects`)
+2. **State tables**: Current ledger state (`accounts`, `offers`, `trust_lines`, `claimable_balances`)
+
+Generated bindata: `internal/db2/schema/bindata.go` — DO NOT EDIT manually.
+
+## Ingest FSM (internal/ingest/)
+State machine for ledger processing:
+```
+start → build (checkpoint) → resume (ledger-by-ledger) ↺
+      → historyRange (backfill gaps)
+      → waitForCheckpoint (if ahead of archives)
+```
+- Checkpoint ledger: `(ledger# + 1) mod 64 == 0`
+- `lastIngestedLedger`: cumulative + time-series data complete
+- `lastHistoryLedger`: time-series only (from `history_ledgers` table)
+
+Critical: Only ONE instance should write to DB at a time globally.
+
+## API Endpoints (internal/actions/)
+Handlers return JSON or SSE streams. Pattern:
+```go
+func GetAccount(w http.ResponseWriter, r *http.Request) {
+    // 1. Parse/validate params via helpers.go
+    // 2. Query via historyQ (db2/history)
+    // 3. Adapt to resource via resourceadapter/
+    // 4. Render JSON or stream SSE
+}
+```
+
+## Dependencies (go.mod)
+| Package | Use |
+|---------|-----|
+| `github.com/stellar/go-stellar-sdk` | XDR, ingest, history archives |
+| `github.com/go-chi/chi` | HTTP router |
+| `github.com/Masterminds/squirrel` | SQL query builder |
+| `github.com/spf13/cobra` | CLI framework |
+| `github.com/prometheus/client_golang` | Metrics |
+
+## Key Files
+| File | Purpose |
+|------|---------|
+| `internal/flags.go` | All CLI flags, env vars. DEPRECATED flags documented inline |
+| `internal/app.go` | App lifecycle: init, serve, tick, shutdown |
+| `internal/ingest/fsm.go` | FSM state definitions and transitions |
+| `internal/db2/history/main.go` | All DB types (1295 lines). Central data model |
+| `internal/httpx/router.go` | Route definitions, middleware chain |
+
+## Conventions
+- Errors: Use `github.com/stellar/go-stellar-sdk/support/errors` for wrapping
+- Logging: `github.com/stellar/go-stellar-sdk/support/log` (structured)
+- Problems: RFC 7807 via `internal/render/problem/` and `support/render/problem`
+- Context: Pass `context.Context` first param, respect cancellation
+
+## Anti-patterns (from codebase)
+- NEVER run `db reingest` on production DB without planning for endpoint unavailability
+- DO NOT change key_value store keys in `internal/db2/history/key_value.go` — migration-sensitive
+- DO NOT modify `bindata.go` files — auto-generated
+
+## See Also
+- `DEVELOPING.md`: Full dev environment setup
+- `CONTRIBUTING.md`: PR workflow
+- `internal/docs/TESTING_NOTES.md`: Test patterns and gotchas
+- `internal/ingest/README.md`: FSM deep dive with state diagram
diff --git a/internal/actions/AGENTS.md b/internal/actions/AGENTS.md
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+# AGENTS.md — internal/actions
+
+## Purpose
+HTTP handlers for Horizon REST API. One file per resource type.
+
+## Handler Pattern
+```go
+// account.go
+func GetAccountByID(w http.ResponseWriter, r *http.Request) {
+    // 1. Parse params
+    accountID, err := getAccountID(r, "account_id")
+    
+    // 2. Query DB
+    historyQ, _ := horizonContext.HistoryQFromRequest(r)
+    account, err := historyQ.Accounts().ForAccounts(ctx, []string{accountID}).Select(ctx)
+    
+    // 3. Adapt to resource
+    resource := resourceadapter.NewAccount(account)
+    
+    // 4. Render
+    httpjson.Render(w, resource, httpjson.HALJSON)
+}
+```
+
+## File Organization
+| File | Endpoints |
+|------|-----------|
+| `account.go` | `/accounts/{id}`, `/accounts` |
+| `transaction.go` | `/transactions/{hash}`, `/transactions` |
+| `operation.go` | `/operations/{id}`, `/operations` |
+| `ledger.go` | `/ledgers/{seq}`, `/ledgers` |
+| `offer.go` | `/offers/{id}`, `/accounts/{id}/offers` |
+| `trade.go` | `/trades`, `/accounts/{id}/trades` |
+| `path.go` | `/paths/strict-receive`, `/paths/strict-send` |
+| `submit_transaction.go` | `POST /transactions` |
+| `submit_transaction_async.go` | `POST /transactions_async` |
+
+## Helper Files
+| File | Purpose |
+|------|---------|
+| `helpers.go` | Param parsing: `getString()`, `getAccountID()`, `getCursor()` |
+| `validators.go` | Input validation |
+| `query_params.go` | URL query struct definitions |
+| `main.go` | Shared types, `ActionContext` |
+
+## Response Formats
+- **JSON**: Standard responses via `httpjson.Render()`
+- **SSE**: Streaming via `sse.Stream()` for `/stream` suffix endpoints
+
+## Pagination Pattern
+```go
+// GetPageQuery returns cursor, order, limit
+pq, _ := GetPageQuery(r, opts...)
+query := q.Transactions().Page(pq)
+```
+
+## Context Access
+```go
+historyQ, _ := horizonContext.HistoryQFromRequest(r)    // DB queries
+ledgerState := r.Context().Value(&ledger.State{})       // Current ledger
+app := r.Context().Value(&App{})                        // Full app access
+```
+
+## Error Handling
+Use `support/render/problem` for RFC 7807 responses:
+```go
+problem.Render(ctx, w, problem.NotFound)
+problem.Render(ctx, w, hProblem.StaleHistory)  // Horizon-specific
+```
+
+## Anti-patterns
+- Raw SQL in handlers → use `historyQ` methods
+- Blocking in SSE handlers → respect context cancellation
+- Ignoring `Latest-Ledger` header → always set via `SetLastLedgerHeader()`
diff --git a/internal/db2/history/AGENTS.md b/internal/db2/history/AGENTS.md
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+# AGENTS.md — internal/db2/history
+
+## Purpose
+Data access layer for all Horizon DB operations. `Q` struct wraps all queries via squirrel builder.
+
+## File Organization
+| Pattern | Purpose |
+|---------|---------|
+| `{entity}.go` | Query methods: `Accounts()`, `Select()`, `InsertX()` |
+| `{entity}_batch_insert_builder.go` | Bulk insert interfaces + impls |
+| `{entity}_loader.go` | Foreign key lookup caching (AccountLoader, AssetLoader) |
+| `mock_*.go` | Test doubles for interfaces |
+| `main.go` | Core types: `Q`, EffectType constants, all DB struct definitions |
+
+## Key Types (main.go)
+- `Q`: Central query struct. Wraps `db.Session` from go-stellar-sdk
+- `EffectType`: 80+ constants for effect categorization (EffectAccountCreated=0, EffectTrade=33, etc.)
+- `*BatchInsertBuilder`: Interfaces for bulk upserts during ingestion
+
+## Query Patterns
+```go
+// Always use Q methods, not raw SQL
+q.Accounts().ForAccounts(ctx, addresses)     // Returns AccountsQuery builder
+q.Transactions().ForAccount(ctx, address)    // Chainable
+q.Select(ctx, &dest, query)                  // Execute and scan
+```
+
+## Table Categories
+| Category | Tables | Query File |
+|----------|--------|------------|
+| History (time-series) | `history_transactions`, `history_operations`, `history_effects` | `transaction.go`, `operation.go`, `effect.go` |
+| State (cumulative) | `accounts`, `offers`, `trust_lines`, `claimable_balances` | `accounts.go`, `offers.go`, etc. |
+| Lookup | `history_accounts`, `history_assets` | `account.go`, `asset.go` |
+
+## Loaders (FK caching)
+Used during ingestion to batch-resolve FKs:
+```go
+loader := history.NewAccountLoader()
+future := loader.GetFuture(address)  // Queue lookup
+loader.Exec(ctx, session)            // Bulk resolve
+id := future.Value()                 // Get cached ID
+```
+
+## Critical Constraints
+- DO NOT change `key_value.go` constants (`exp_ingest_last_ledger`, `exp_ingest_version`) — migration-sensitive, distributed locking relies on exact key names
+- `GetLastLedgerIngest()` uses `SELECT ... FOR UPDATE` — intentional for distributed lock
+- Batch insert builders: call `Add()` then `Exec()` — not calling Exec = data loss
+
+## Anti-patterns
+- Never bypass `Q` methods with raw queries
+- Never modify `bindata.go` — auto-generated from migrations
+- Avoid `Q.Clone()` unless you need independent transaction scope
diff --git a/internal/ingest/AGENTS.md b/internal/ingest/AGENTS.md
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+# AGENTS.md — internal/ingest
+
+## Purpose
+FSM-based ledger ingestion from Stellar Core into Horizon's PostgreSQL.
+
+## Architecture
+```
+system (main.go) → FSM (fsm.go) → states → processor_runner.go → processors/
+                                                              → db2/history/
+```
+
+## FSM States (fsm.go)
+| State | Purpose | Next |
+|-------|---------|------|
+| `start` | Entry point. Checks DB version, determines path | build, resume, historyRange, waitForCheckpoint |
+| `build` | Initial state from checkpoint archive | resume (success), start (failure) |
+| `resume` | Steady-state ledger-by-ledger ingestion | resume (loop), start (error) |
+| `historyRange` | Backfill missing time-series data | start |
+| `waitForCheckpoint` | Sleep until next 64-ledger checkpoint | start |
+
+## Key Definitions
+- **Checkpoint ledger**: `(ledger# + 1) mod 64 == 0` — when history archives publish
+- **lastIngestedLedger**: Both cumulative AND time-series data complete
+- **lastHistoryLedger**: Time-series only (from `history_ledgers`)
+- FSM ensures these stay in sync
+
+## Critical Files
+| File | Purpose |
+|------|---------|
+| `main.go` | `system` struct, `Run()` loop |
+| `fsm.go` | State definitions, `stateMachineNode` interface |
+| `processor_runner.go` | Runs all processors for a ledger range |
+| `verify_range_state.go` | Data integrity verification |
+
+## Running Processors
+```go
+// processor_runner.go orchestrates
+runner.RunAllProcessorsOnLedger(ledger)  // Change + Transaction processors
+runner.RunTransactionProcessorsOnLedger(ledger)  // Time-series only
+```
+
+## Concurrency Warning
+**Only ONE Horizon instance should write to DB globally.** FSM uses `SELECT FOR UPDATE` on `key_value` table for distributed locking.
+
+## Reingestion
+```bash
+# Backfill range (offline operation)
+./stellar-horizon db reingest range 1000 2000
+
+# DO NOT run on production while serving traffic
+# Endpoints become unavailable until rebuild complete
+```
+
+## Anti-patterns
+- Never run multiple ingestion instances against same DB
+- Never skip `verify-range` after reingest
+- State machine errors → fix root cause, don't restart blindly
diff --git a/internal/ingest/processors/AGENTS.md b/internal/ingest/processors/AGENTS.md
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+# AGENTS.md — internal/ingest/processors
+
+## Purpose
+Transform ledger data into database rows. Two processor types: Change (state) and Transaction (time-series).
+
+## Processor Types
+| Interface | Input | Output Tables |
+|-----------|-------|---------------|
+| `ChangeProcessor` | `ingest.Change` (state delta) | `accounts`, `offers`, `trust_lines`, etc. |
+| `LedgerTransactionProcessor` | `ingest.LedgerTransaction` | `history_*` tables |
+
+## Naming Convention
+| Suffix | Type | Example |
+|--------|------|---------|
+| `*_processor.go` | Core processor impl | `accounts_processor.go` |
+| `*_change_processor.go` | State table processor | `liquidity_pools_change_processor.go` |
+| `*_transaction_processor.go` | History table processor | `claimable_balances_transaction_processor.go` |
+
+## Processor Pattern
+```go
+type FooProcessor struct {
+    batch history.FooBatchInsertBuilder
+}
+
+func (p *FooProcessor) ProcessTransaction(lcm xdr.LedgerCloseMeta, tx ingest.LedgerTransaction) error {
+    // Extract data from tx
+    // p.batch.Add(row)
+    return nil
+}
+
+func (p *FooProcessor) Flush(ctx context.Context, session db.SessionInterface) error {
+    return p.batch.Exec(ctx, session)  // MUST call or data lost
+}
+```
+
+## Key Processors
+| Processor | Tables | Notes |
+|-----------|--------|-------|
+| `effects_processor.go` | `history_effects` | 80+ effect types, largest processor (~1500 lines) |
+| `operations_processor.go` | `history_operations` | One row per operation |
+| `transactions_processor.go` | `history_transactions` | Fee bumps handled specially |
+| `accounts_processor.go` | `accounts` | State table, change processor |
+| `asset_stats_processor.go` | `asset_stats` | Aggregated per-asset metrics |
+
+## Batch Insert Pattern
+All processors use loaders for FK resolution:
+```go
+accountLoader.GetFuture(address)  // Queue
+assetLoader.GetFuture(asset)      // Queue
+// ... after all txs processed ...
+accountLoader.Exec(ctx, session)  // Bulk resolve
+```
+
+## Adding New Processor
+1. Implement `ChangeProcessor` or `LedgerTransactionProcessor`
+2. Register in `processor_runner.go` (change or transaction list)
+3. Ensure `Flush()` calls batch builder's `Exec()`
+
+## Anti-patterns
+- Forgetting to call `Flush()` → silent data loss
+- Processing failed transactions for effects → effects only exist for successful txs
+- Modifying processor order without understanding dependencies