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Architecture at a glance

Veyyon ships as the veyyon CLI (Bun + TypeScript, Rust helpers). This chapter maps subsystems; each has a handbook page and matching engineering notes under docs/.

The request path

            ┌──────────────────────────────────────────────────────────────┐
prompt ──► │  veyyon (packages/coding-agent)                              │
            │    │                                                          │
            │    ▼                                                          │
            │  AgentSession turn loop                                      │
            │    │   model stream → tools (read, bash, edit, …)            │
            │    ▼                                                          │
            │  hashline / handlers ──► filesystem + sandbox                │
            └──────────────────────────────────────────────────────────────┘

Subsystems that matter

SubsystemResponsibilityChapter
Edit engine (@veyyon/hashline)Default hashline edit pathEdit engine
SessionsSession trees, compactionCompaction & memory
MCPMCP client integrationMCP
ConfigSettings and profilesConfig
Memorymnemopi / local memoryMemory
GoalsGoal cards and budgetsGoal state

Design rules

  1. Prefer one obvious path. Hashline is the default edit surface; alternate edit.mode values exist for compatibility.
  2. Fail loud. Invalid config, stale hashline tags, and sandbox violations surface actionable errors. No silent fallback to weaker behavior.

Spec — not shipped: the full ordered repair rule cascade (alias maps, strict unknown-key rejection, per-(model,tool,shape) telemetry), an app-server or exec-server daemon, and a Tier-B backends.toml catalog as a separate subsystem. Provider and model configuration is documented in Providers against the shipped provider registry. Basic schema repair on tool calls is shipped today as a TypeScript module at the tool-dispatch seam (packages/coding-agent/src/repair/schema-repair.ts) — see Repair.