defmodule LangChain.MessageDelta do
@moduledoc """
Models a "delta" message from a chat LLM. A delta is a small chunk, or piece
of a much larger complete message. A series of deltas are used to construct
the complete message.
Delta messages must be applied in order for them to be valid. Delta messages
can be combined and transformed into a `LangChain.Message` once the final
piece is received.
## Roles
* `:unknown` - The role data is missing for the delta.
* `:assistant` - Responses coming back from the LLM.
## Tool Usage
Tools can be used or called by the assistant (LLM). A tool call is also split
across many message deltas and must be fully assembled before it can be
executed.
## Metadata
The `metadata` field is a map that can contain any additional information
about the message delta. It is used to store token usage, model, and other
LLM-specific information.
## Content Fields
The `content` field may contain:
- A string (for backward compatibility)
- A `LangChain.Message.ContentPart` struct
- An empty list `[]` that is received from some services like Anthropic, which
is a signal that the content will be a list of content parts
The module uses two content-related fields:
* `content` - The raw content received from the LLM. This can be either a
string (for backward compatibility), a `LangChain.Message.ContentPart`
struct, or a `[]` indicating it will be a list of content parts. This field
is cleared (set to `nil`) after merging into `merged_content`.
* `merged_content` - The accumulated list of `ContentPart`s after merging
deltas. This is the source of truth for the message content and is used when
converting to a `LangChain.Message`. When merging deltas:
- For string content, it's converted to a `ContentPart` of type `:text`
- For `ContentPart` content, it's merged based on the `index` field
- Multiple content parts can be maintained in the list to support
multi-modal responses (text, images, audio) or separate thinking content
from final text
"""
use Ecto.Schema
import Ecto.Changeset
require Logger
alias __MODULE__
alias LangChain.LangChainError
alias LangChain.Message
alias LangChain.Message.ContentPart
alias LangChain.Message.ToolCall
alias LangChain.Utils
alias LangChain.TokenUsage
@primary_key false
embedded_schema do
field :content, :any, virtual: true
# The accumulated list of ContentParts after merging deltas
field :merged_content, :any, virtual: true, default: []
# Marks if the delta completes the message.
field :status, Ecto.Enum, values: [:incomplete, :complete, :length], default: :incomplete
# When requesting multiple choices for a response, the `index` represents
# which choice it is. It is a 0 based list.
field :index, :integer
field :role, Ecto.Enum, values: [:unknown, :assistant], default: :unknown
field :tool_calls, :any, virtual: true
# Additional metadata about the message.
field :metadata, :map
end
@type t :: %MessageDelta{}
@create_fields [:role, :content, :index, :status, :tool_calls, :metadata, :merged_content]
@required_fields []
@doc """
Create a new `MessageDelta` that represents a message chunk.
"""
@spec new(attrs :: map()) :: {:ok, t()} | {:error, Ecto.Changeset.t()}
def new(attrs \\ %{}) do
%MessageDelta{}
|> cast(attrs, @create_fields)
|> Utils.assign_string_value(:content, attrs)
|> validate_required(@required_fields)
|> apply_action(:insert)
end
@doc """
Create a new `MessageDelta` that represents a message chunk and return it or
raise an error if invalid.
"""
@spec new!(attrs :: map()) :: t() | no_return()
def new!(attrs \\ %{}) do
case new(attrs) do
{:ok, message} ->
message
{:error, changeset} ->
raise LangChainError, changeset
end
end
@doc """
Merge two `MessageDelta` structs. The first `MessageDelta` is the `primary`
one that smaller deltas are merged into.
The merging process:
1. Migrates any string content to `ContentPart`s for backward compatibility
2. Merges the content into `merged_content` based on the `index` field
3. Clears the `content` field (sets to `nil`) after merging
4. Updates other fields (tool_calls, status, etc.)
## Examples
iex> delta_1 =
...> %LangChain.MessageDelta{
...> content: nil,
...> index: 0,
...> tool_calls: [],
...> role: :assistant,
...> status: :incomplete
...> }
iex> delta_2 =
...> %LangChain.MessageDelta{
...> content: "Hello",
...> index: 0,
...> tool_calls: [],
...> role: :unknown,
...> status: :incomplete
...> }
iex> LangChain.MessageDelta.merge_delta(delta_1, delta_2)
%LangChain.MessageDelta{
content: nil,
merged_content: [%LangChain.Message.ContentPart{type: :text, content: "Hello"}],
status: :incomplete,
index: 0,
role: :assistant,
tool_calls: []
}
A set of deltas can be easily merged like this:
MessageDelta.merge_deltas(list_of_delta_messages)
"""
@spec merge_delta(nil | t(), t() | {:error, any()}) :: t()
def merge_delta(nil, %MessageDelta{} = delta_part) do
merge_delta(%MessageDelta{role: :assistant}, delta_part)
end
# Skip error tuples that may arrive mid-stream (e.g. content filtering).
# Return the accumulated delta unchanged so callers like LiveView don't crash.
def merge_delta(primary, {:error, _reason}) do
primary || %MessageDelta{role: :assistant}
end
def merge_delta(%MessageDelta{role: :assistant} = primary, %MessageDelta{} = delta_part) do
new_delta = migrate_to_content_parts(delta_part)
primary
|> migrate_to_content_parts()
|> append_to_merged_content(new_delta)
|> merge_tool_calls(new_delta)
|> update_index(new_delta)
|> update_status(new_delta)
|> accumulate_token_usage(new_delta)
|> clear_content()
end
@doc """
Merges a list of `MessageDelta`s into an accumulated `MessageDelta`. This is
useful for UI applications that accumulate deltas as they are received from
a streaming LLM response.
The first argument is the accumulated delta (or `nil` if no deltas have been
processed yet). The second argument is a list of new deltas to merge in.
## Examples
iex> accumulated = nil
iex> batch_1 = [
...> %LangChain.MessageDelta{content: "Hello", role: :assistant},
...> %LangChain.MessageDelta{content: " world", role: :assistant}
...> ]
iex> accumulated = LangChain.MessageDelta.merge_deltas(accumulated, batch_1)
iex> batch_2 = [
...> %LangChain.MessageDelta{content: "!", role: :assistant, status: :complete}
...> ]
iex> result = LangChain.MessageDelta.merge_deltas(accumulated, batch_2)
iex> result
%LangChain.MessageDelta{
content: nil,
merged_content: [%LangChain.Message.ContentPart{type: :text, content: "Hello world!"}],
status: :complete,
role: :assistant
}
"""
@spec merge_deltas(nil | t(), [t()]) :: t()
def merge_deltas(accumulated_delta, deltas) when is_list(deltas) do
deltas
|> List.flatten()
|> Enum.reduce(accumulated_delta, &merge_delta(&2, &1))
end
@doc """
Merges a list of `MessageDelta`s into a single `MessageDelta`. The deltas
are merged in order, with each delta being merged into the result of the
previous merge.
## Examples
iex> deltas = [
...> %LangChain.MessageDelta{content: "Hello", role: :assistant},
...> %LangChain.MessageDelta{content: " world", role: :assistant},
...> %LangChain.MessageDelta{content: "!", role: :assistant, status: :complete}
...> ]
iex> LangChain.MessageDelta.merge_deltas(deltas)
%LangChain.MessageDelta{
content: nil,
merged_content: [%LangChain.Message.ContentPart{type: :text, content: "Hello world!"}],
status: :complete,
role: :assistant
}
"""
@spec merge_deltas([t()]) :: t()
def merge_deltas(deltas) when is_list(deltas) do
merge_deltas(nil, deltas)
end
# Clear the content field after merging into merged_content
defp clear_content(%MessageDelta{} = delta) do
%MessageDelta{delta | content: nil}
end
# Empty content - nothing to merge
defp append_to_merged_content(%MessageDelta{} = primary, %MessageDelta{content: content})
when content in [nil, []],
do: primary
# String content - convert to ContentPart and merge
defp append_to_merged_content(%MessageDelta{} = primary, %MessageDelta{content: content})
when is_binary(content) do
append_to_merged_content(primary, %MessageDelta{content: ContentPart.text!(content)})
end
# ContentPart from merged_content (already processed delta)
defp append_to_merged_content(
%MessageDelta{} = primary,
%MessageDelta{content: nil, merged_content: %ContentPart{} = part}
) do
merge_content_part_into(primary, part)
end
# Single ContentPart content
defp append_to_merged_content(
%MessageDelta{} = primary,
%MessageDelta{content: %ContentPart{} = part, index: index}
) do
merge_content_part_at_index(primary, part, index)
end
# List of content items (e.g., from Mistral with reference types)
defp append_to_merged_content(
%MessageDelta{merged_content: parts_list} = primary,
%MessageDelta{content: content_list, index: index}
)
when is_list(parts_list) and is_list(content_list) do
Enum.reduce(content_list, primary, &merge_content_item(&2, &1, index))
end
# Catch-all - no content to merge
defp append_to_merged_content(%MessageDelta{} = primary, %MessageDelta{}), do: primary
# Merge a ContentPart directly into merged_content (for already-merged deltas)
@spec merge_content_part_into(t(), ContentPart.t()) :: t()
defp merge_content_part_into(%MessageDelta{merged_content: content} = primary, part)
when content in [nil, []] do
%MessageDelta{primary | merged_content: [part]}
end
defp merge_content_part_into(
%MessageDelta{merged_content: %ContentPart{} = existing} = primary,
part
) do
%MessageDelta{primary | merged_content: [ContentPart.merge_part(existing, part)]}
end
defp merge_content_part_into(%MessageDelta{merged_content: [_ | _]} = primary, part) do
merge_content_part_at_index(primary, part, 0)
end
# Merge a single content item from a list into the primary delta
@spec merge_content_item(t(), ContentPart.t() | map(), integer() | nil) :: t()
defp merge_content_item(primary, %ContentPart{} = part, index) do
merge_content_part_at_index(primary, part, index)
end
defp merge_content_item(primary, %{"type" => "text", "text" => text}, index)
when is_binary(text) do
merge_content_part_at_index(primary, ContentPart.text!(text), index)
end
defp merge_content_item(primary, _unrecognized, _index), do: primary
# Helper function to merge a content part at a specific index
defp merge_content_part_at_index(
%MessageDelta{} = primary,
%ContentPart{type: :text, content: ""} = _new_content_part,
_index
) do
# Skip merging empty text content parts to avoid type conflicts
primary
end
defp merge_content_part_at_index(
%MessageDelta{} = primary,
%ContentPart{} = new_content_part,
index
) do
parts_list = primary.merged_content
# If index is nil, assume position 0 for backward compatibility with some chat models
position = index || 0
# Compute the length once to avoid multiple calculations
list_length = length(parts_list)
# If the index is beyond the current list length, pad with nil values
padded_list =
if position >= list_length do
parts_list ++ List.duplicate(nil, position - list_length + 1)
else
parts_list
end
# Get the content part at the specified index from the primary's content list
primary_part = Enum.at(padded_list, position)
# Merge the parts if we have an existing part, otherwise use the new part
merged_part =
if primary_part do
ContentPart.merge_part(primary_part, new_content_part)
else
new_content_part
end
# Replace the part at the specified index
updated_list = List.replace_at(padded_list, position, merged_part)
%MessageDelta{primary | merged_content: updated_list}
end
# Merge tool call delta by matching on index value (not list position).
# Anthropic's index differentiates calls but doesn't correspond to list offset.
@spec merge_tool_calls(t(), t()) :: t()
defp merge_tool_calls(%MessageDelta{tool_calls: primary_calls} = primary, %MessageDelta{
tool_calls: [delta_call]
}) do
calls = primary_calls || []
initial = Enum.find(calls, &(&1.index == delta_call.index))
merged_call = ToolCall.merge(initial, delta_call)
%MessageDelta{primary | tool_calls: upsert_by_index(calls, merged_call)}
end
defp merge_tool_calls(%MessageDelta{} = primary, %MessageDelta{}), do: primary
@spec update_index(t(), t()) :: t()
defp update_index(%MessageDelta{} = primary, %MessageDelta{index: idx}) when is_number(idx) do
%MessageDelta{primary | index: idx}
end
defp update_index(%MessageDelta{} = primary, %MessageDelta{}), do: primary
# Only update status from :incomplete to a terminal state
@spec update_status(t(), t()) :: t()
defp update_status(%MessageDelta{status: :incomplete} = primary, %MessageDelta{status: status})
when status in [:complete, :length] do
%MessageDelta{primary | status: status}
end
defp update_status(%MessageDelta{} = primary, %MessageDelta{}), do: primary
# Insert or update tool call in list by matching on index field
@spec upsert_by_index([ToolCall.t()], ToolCall.t()) :: [ToolCall.t()]
defp upsert_by_index(calls, call) do
case Enum.find_index(calls, &(&1.index == call.index)) do
nil -> calls ++ [call]
pos -> List.replace_at(calls, pos, call)
end
end
@doc """
Convert the MessageDelta's merged content to a string. Specify the type of
content to convert so it can return just the text parts or thinking parts,
etc. Defaults to `:text`.
"""
@spec content_to_string(t(), type :: atom()) :: nil | String.t()
def content_to_string(delta, type \\ :text)
def content_to_string(nil, _type), do: nil
def content_to_string(%MessageDelta{merged_content: merged_content}, type) do
ContentPart.content_to_string(merged_content, type)
end
@doc """
Convert the MessageDelta to a Message. Can only convert a fully complete
MessageDelta.
This is assumed to be the result of merging all the received `MessageDelta`s.
An error is returned if the `status` is `:incomplete`.
If the `MessageDelta` fails to convert to a `LangChain.Message`, an error is
returned with the reason.
"""
@spec to_message(t()) :: {:ok, Message.t()} | {:error, String.t()}
def to_message(%MessageDelta{status: :incomplete} = _delta) do
{:error, "Cannot convert incomplete message"}
end
def to_message(%MessageDelta{merged_content: merged_content} = delta) do
content = reject_nil(merged_content)
attrs =
delta
|> Map.from_struct()
|> Map.put(:content, content)
with :ok <- validate_not_empty(delta),
{:ok, message} <- Message.new(attrs) do
{:ok, message}
else
{:error, %Ecto.Changeset{} = changeset} ->
{:error, Utils.changeset_error_to_string(changeset)}
{:error, _reason} = error ->
error
end
end
# Validate that assistant message is not empty (no content and no tool_calls).
# Empty assistant messages violate conversation flow rules in most LLM APIs.
defp validate_not_empty(%MessageDelta{
role: :assistant,
merged_content: merged_content,
tool_calls: tool_calls,
status: :complete
})
when (merged_content == [] or merged_content == nil) and
(tool_calls == [] or tool_calls == nil) do
Logger.warning("Received empty assistant message with no content and no tool_calls")
{:error, "Empty assistant message: no content and no tool_calls"}
end
defp validate_not_empty(_delta), do: :ok
# Filter nil values from list (from index padding during delta merging)
@spec reject_nil(list() | any()) :: list() | any()
defp reject_nil(list) when is_list(list), do: Enum.reject(list, &is_nil/1)
defp reject_nil(other), do: other
@doc """
Accumulates token usage from delta messages. Uses `LangChain.TokenUsage.add/2` to combine
the usage data from both deltas.
## Example
iex> alias LangChain.TokenUsage
iex> alias LangChain.MessageDelta
iex> delta1 = %MessageDelta{
...> metadata: %{
...> usage: %TokenUsage{input: 10, output: 5}
...> }
...> }
iex> delta2 = %MessageDelta{
...> metadata: %{
...> usage: %TokenUsage{input: 5, output: 15}
...> }
...> }
iex> result = MessageDelta.accumulate_token_usage(delta1, delta2)
iex> result.metadata.usage.input
15
iex> result.metadata.usage.output
20
"""
@spec accumulate_token_usage(t(), t()) :: t()
def accumulate_token_usage(
%MessageDelta{} = primary,
%MessageDelta{metadata: %{usage: new_usage}} = _delta_part
)
when not is_nil(new_usage) do
current_usage = TokenUsage.get(primary)
combined_usage = TokenUsage.add(current_usage, new_usage)
%MessageDelta{primary | metadata: Map.put(primary.metadata || %{}, :usage, combined_usage)}
end
def accumulate_token_usage(%MessageDelta{} = primary, %MessageDelta{} = _delta_part) do
# No usage data to accumulate
primary
end
@doc """
Migrates a MessageDelta's string content to use `LangChain.Message.ContentPart`.
This is for backward compatibility with models that don't yet support ContentPart streaming.
## Examples
iex> delta = %LangChain.MessageDelta{content: "Hello world"}
iex> upgraded = migrate_to_content_parts(delta)
iex> upgraded.content
%LangChain.Message.ContentPart{type: :text, content: "Hello world"}
"""
@spec migrate_to_content_parts(t()) :: t()
def migrate_to_content_parts(%MessageDelta{content: ""} = delta),
do: %MessageDelta{delta | content: nil}
def migrate_to_content_parts(%MessageDelta{content: content} = delta) when is_binary(content) do
%MessageDelta{delta | content: ContentPart.text!(content)}
end
def migrate_to_content_parts(%MessageDelta{} = delta) do
delta
end
end
