# How BeamWeaver Works An agent and a graph share the same execution foundation. The difference is who chooses the next action: a model inside an agent, or the workflow your application defines. Start with the [quickstart](quickstart.md) if you want to run code before reading the concepts. ## Follow one agent request A caller supplies messages and trusted context. BeamWeaver evaluates the agent's declarations, prepares instructions/tools and invokes the model. The model either answers or requests actions. Requested tools execute in Elixir; their results return to the model. The loop ends with an answer, an interrupt or an error, subject to the configured limits. A model cannot read your database merely because its prompt mentions orders. It needs a tool or supplied data. A tool declaration makes an action available; it does not prove the model called it. Inspect successful results when the action matters. ## Separate the five kinds of data | Data | Who supplies it | Lifetime/use | | --- | --- | --- | | Instructions | Application and configured capabilities | Guide each model call | | Messages | User, model and tool results | Conversation history | | State | Input plus node/tool/middleware updates | Working data, optionally checkpointed | | Context | Trusted application caller | One invocation; resend on resume | | Config | Application runtime settings | Select thread/checkpoint and execution options | `messages` is a state key with ID-aware merging. Other keys can replace, accumulate or use specialized channels. Context is not automatically a model message and is not saved as conversational state. See [Messages](messages.md), [Runtime Context](runtime_context.md) and [State](graph_state.md). ## Understand the DSL lifecycle Agent and graph declarations describe how to build a run and are evaluated when invoked. They can read application configuration. Tool and schema modules describe their contracts at compilation. Resource creation needs a separate owner. Repeated declarations must not create a fresh ETS table or container every call. Pass existing resources through options or reference supervised services. A database adapter struct is a handle to a repo, not the process that owns its connections. ## Agents, tools and schemas An agent combines a model, instructions, tools and policy. A tool describes an action's arguments and executes application code. A response schema constrains the final answer consumed by your application. These are separate contracts even when all use field declarations. Tool and response fields are required by default. Agent context fields are optional unless explicitly required. Model-supplied JSON arguments use string keys; injected runtime values use their declared atom keys. ## Graphs and channels A graph node reads state and returns changed keys. Edges select subsequent nodes. A superstep runs currently ready nodes in parallel, then merges their updates. Several writers to one key need a reducer or another suitable channel. A join waits for its prerequisite branches. Graphs can include agents as nested workflows. Dynamic sends distribute work known only at runtime. Use explicit graph structure where order and routing must be controlled by application code. ## Threads and checkpoints A thread ID identifies saved workflow/conversation history. A checkpoint records state plus what remains to run. Reusing storage and thread identity continues that history; fresh calls without storage have no past to load. ETS data disappears with its owner process. Database storage can survive restarts, but your application must schedule continuation. External actions may repeat after an ambiguous failure: checkpointing is not an exactly-once payment or messaging guarantee. See [Durable Execution](durable_execution.md). ## Memory and context management Conversation history belongs to a thread. Long-term memory stores namespaced records across threads. Memory files load short Markdown notes into prompts. Saving a record does not automatically expose it to the model; a tool or prompt function must retrieve it. Context management changes the material sent to a finite model window: request trimming, summarization, file offloading and delegation. Some policies change saved history; others only change the request. Prompt caching can reduce repeated prefix processing but does not make the context window larger. ## Middleware and human review Middleware surrounds model/tool calls and state transitions. It can adjust requests, select actions, retry failures or impose limits. Before hooks run forward, after hooks backward, and wrappers nest. Ordering is part of behavior. An interrupt records a pause. Your application displays the request, obtains an authorized decision and resumes the correct saved run. Context must be supplied again. Tool review and dynamic graph input have different resume APIs. ## Streams and task ownership A live stream starts work when consumed. Reading it twice can execute the workflow twice. The consumer owns the run; stopping consumption cancels it. Tokens are provisional until the corresponding model attempt completes. Handle errors and interrupts as well as text. For UI work, consume in a task and send selected events to the view. For work that must outlive the view, use a durable job owner. See [Streaming](streaming.md). ## Deep agents and delegation A deep agent adds planning, workspace files, skills, memory or subagents to an ordinary agent. Capabilities add tools and instructions; they do not replace application permissions. Children can inherit selected context/state/tools, so define their boundaries explicitly. ## Providers and verification A `provider:model` identifier selects an adapter and model. Local model profiles validate known capabilities and parameters. They do not establish account access or live availability. Actual calls establish the provider integration; application assertions establish that the requested work happened. Use fake models for deterministic application tests, real embeddings for search quality and live provider calls for endpoint behavior. [Testing](testing.md) explains these boundaries.