Build an agent map of subagents from configurations.

Build a registry of subagents, raising on error.

Build descriptions map for subagents.

Check if SubAgent can be executed.

Only SubAgents with status :idle can be executed.

Check if SubAgent can be resumed.

Only SubAgents with status :interrupted can be resumed.

Compose the child agent's base_system_prompt from a Config.

Composition rule:

• Header = system_prompt_override (if set) else the built-in boilerplate.
• Body = instructions (if set) else system_prompt (legacy) else "".

The header and body are joined with a blank line. Middleware-contributed prompt fragments are appended later by the normal Sagents.Agent compile path — this function does not handle those.

Execute the SubAgent.

Runs the LLMChain until:

• Natural completion (no more tool calls)
• HITL interrupt (tool needs approval)
• Error

Returns updated SubAgent struct with new status.

Options

• :callbacks - Map of LLMChain callbacks (e.g., %{on_message_processed: fn...})

Examples

case SubAgent.execute(subagent) do
  {:ok, completed_subagent} ->
    result = SubAgent.extract_result(completed_subagent)

  {:interrupt, interrupted_subagent} ->
    # interrupted_subagent.interrupt_data contains action_requests

  {:error, error_subagent} ->
    # error_subagent.error contains the error
end

# With callbacks for real-time message broadcasting
callbacks = %{
  on_message_processed: fn _chain, message ->
    broadcast_message(message)
  end
}
SubAgent.execute(subagent, callbacks: callbacks)

@spec extract_middleware_module(any()) :: module() | nil

Extract the module from any middleware format.

Returns the middleware module regardless of whether the input is:

• A raw module atom
• A {module, opts} tuple
• A MiddlewareEntry struct

Returns nil for unrecognized formats.

Extract result from completed SubAgent.

For completed SubAgents, extracts the final message content as a string. This is the default extraction - middleware or custom logic can provide different extraction.

An until_tool run terminates the moment the target tool's result lands, so its final message is a tool message and the result the caller wants is that tool's content — not assistant prose. Those runs extract the matched tool result directly; every other completion still goes through LangChain.Utils.ChainResult.to_string/1. A tool result flagged is_error is returned as {:error, content} so a failed submission does not read as a successful one.

Returns {:ok, string} on success or {:error, reason} on failure.

Examples

{:ok, completed_subagent} = SubAgent.execute(subagent)
{:ok, result} = SubAgent.extract_result(completed_subagent)
# => {:ok, "Research complete: Solar energy has shown..."}

@spec is_subagent_middleware?(any()) :: boolean()

Check if a middleware spec refers to the SubAgent middleware.

Handles all middleware formats: raw modules, tuples, and MiddlewareEntry structs.

Check if SubAgent is in a terminal state.

Terminal states are :completed and :error.

Create a SubAgent from compiled agent (pre-built).

A compiled subagent is pre-defined by the application with complete control over configuration.

Options

• :parent_agent_id - Parent's agent ID (required)
• :instructions - Task description (required)
• :compiled_agent - Pre-built Agent struct (required)
• :initial_messages - Optional initial message sequence (default: [])
• :parent_tool_context - Parent's tool_context map to inherit (optional). Static, caller-supplied data (e.g., user_id, feature flags) that tools access as flat top-level keys on context. Defaults to %{}.
• :parent_metadata - Parent's state metadata map to inherit (optional). Dynamic, middleware-managed data (e.g., conversation_title) that tools access via context.state.metadata. Defaults to %{}.
• :parent_runtime - Parent's state.runtime map to inherit (optional). Process-local middleware state such as captured ProcessContext snapshots. Inherited so sub-agents continue to see the parent's propagated tenant / OTel / Sentry context. Defaults to %{}.
• :scope - Scope struct to inherit from the parent (optional). Propagated to the SubAgent's custom_context.scope so sub-agent tools and persistence callbacks see the same tenant context as the parent. Defaults to compiled_agent.scope.

Examples

subagent = SubAgent.new_from_compiled(
  parent_agent_id: "main-agent",
  instructions: "Extract structured data",
  compiled_agent: data_extractor_agent,
  initial_messages: [prep_message],
  parent_tool_context: %{user_id: 42},
  parent_metadata: %{"conversation_title" => "Extraction"}
)

Create a SubAgent from configuration (dynamic subagent).

The main agent configures a subagent through the task tool by providing:

• Instructions (becomes user message)
• Agent configuration (Agent struct)

Options

• :parent_agent_id - Parent's agent ID (required)
• :instructions - Task description (required)
• :agent_config - Agent struct with tools, model, middleware (required)
• :initial_messages - Optional list of %LangChain.Message{} structs to slot between the system messages and the user instruction. Use this to inject established context (e.g. a <references> synthetic preamble) for the sub-agent's first turn. Defaults to [].
• :parent_tool_context - Parent's tool_context map to inherit (optional). Static, caller-supplied data (e.g., user_id, feature flags) that tools access as flat top-level keys on context. Defaults to %{}.
• :parent_metadata - Parent's state metadata map to inherit (optional). Dynamic, middleware-managed data (e.g., conversation_title) that tools access via context.state.metadata. Defaults to %{}.
• :parent_runtime - Parent's state.runtime map to inherit (optional). Process-local middleware state such as captured ProcessContext snapshots. Inherited so sub-agents continue to see the parent's propagated tenant / OTel / Sentry context. Defaults to %{}.
• :scope - Scope struct to inherit from the parent (optional). Propagated to the SubAgent's custom_context.scope so sub-agent tools and persistence callbacks see the same tenant context as the parent. Defaults to agent_config.scope.

Examples

subagent = SubAgent.new_from_config(
  parent_agent_id: "main-agent",
  instructions: "Research renewable energy impacts",
  agent_config: agent_struct,
  parent_tool_context: %{user_id: 42, tenant: "acme"},
  parent_metadata: %{"conversation_title" => "Research"},
  scope: %MyApp.Accounts.Scope{user: user}
)

Resume the SubAgent after HITL interrupt.

Takes human decisions and continues execution from where it left off. This is the magic - the agent state is already paused at the right spot.

Parameters

• subagent - SubAgent with status :interrupted
• decisions - List of decision maps from human reviewer
• opts - Optional keyword list with:
  • :callbacks - Map of LLMChain callbacks (e.g., %{on_message_processed: fn...})

Returns

• {:ok, completed_subagent} - Execution completed
• {:interrupt, interrupted_subagent} - Another interrupt (multiple HITL tools)
• {:error, error_subagent} - Resume ran but failed; the SubAgent carries the reason in its :error field
• {:error, {:unsupported_interrupt, :tool_raised}} - Interrupted on a shape this function cannot apply decisions to (see below)
• {:error, {:invalid_status, status, :expected_interrupted}} - Not interrupted

Supported interrupt shapes

Only the pre-tool HITL interrupt is resumable. That shape carries :action_requests and :hitl_tool_call_ids, and names tool calls that have not run yet, so the decisions can be applied by executing them.

An interrupt raised from inside a tool body (is_interrupt: true on a ToolResult, surfaced by check_tool_interrupts/2) is not resumable here and returns {:error, {:unsupported_interrupt, :tool_raised}}. Its tool has already executed, so resolving it means replacing the recorded tool result rather than executing a pending call — the job of the middleware that owns the interrupt type, working on Sagents.State via State.replace_tool_result/3. A sub-agent runs a bare chain with no handle_resume/5 dispatch, so it has no owner to hand the interrupt to.

Callers resolve these at the parent instead: Sagents.Middleware.SubAgent turns the error into a tool result for the parent chain and stops the sub-agent process.

Examples

decisions = [
  %{type: :approve},
  %{type: :edit, arguments: %{"path" => "safe.txt"}}
]

case SubAgent.resume(interrupted_subagent, decisions) do
  {:ok, completed_subagent} ->
    result = SubAgent.extract_result(completed_subagent)

  {:interrupt, interrupted_again} ->
    # Another interrupt - repeat the process

  {:error, error_subagent} ->
    # Handle error
end

@spec subagent_middleware_stack(list(), list(), keyword()) :: list()

Build a middleware stack for subagents, filtering out blocked middleware.

Two middleware modules are excluded from inheritance by default, without needing to appear in :block_middleware:

• Sagents.Middleware.SubAgent, to prevent recursive subagent nesting.
• Sagents.Middleware.AskUserQuestion, because a sub-agent has no user to ask. compose_child_system_prompt/1 already tells every task sub-agent "You cannot ask the user questions. There is no user in this conversation.", so inheriting the ask_user tool handed the sub-agent a capability the prompt then had to police.

Only inherited middleware is filtered. Middleware passed explicitly as additional_middleware (a SubAgent.Config's own :middleware list) is always kept, so a sub-agent that genuinely needs one of these can still opt back in.

This function handles middleware in all formats:

• Raw module atoms (e.g., Sagents.Middleware.SubAgent)
• Raw tuples (e.g., {Sagents.Middleware.SubAgent, opts})
• Initialized MiddlewareEntry structs (from agent.middleware)

Options

• :block_middleware - List of middleware modules to exclude from inheritance. These modules will not be passed to general-purpose subagents. Defaults to [].

Examples

# Default behavior: SubAgent and AskUserQuestion middleware are filtered
subagent_middleware_stack(parent_middleware)

# Block additional middleware from being inherited
subagent_middleware_stack(parent_middleware, [],
  block_middleware: [ConversationTitle, Summarization]
)

# Add additional middleware while blocking others
subagent_middleware_stack(parent_middleware, [CustomMiddleware],
  block_middleware: [ConversationTitle]
)

Returns the universal framing prompt prepended to every task-style sub-agent's system prompt.

Encodes the "no user, complete-or-fail, no clarifying questions" contract so that Sagents.SubAgent.Task authors can focus their instructions/0 on the substantive procedure. Host compilers combine this with the task's instructions/0 to form the child agent's system prompt; it can be replaced per-sub-agent via Sagents.SubAgent.Config's system_prompt_override.