LangChain ACP Bridges

Capability bridges are the main extension seam in langchain-acp.

They let the adapter expose ACP-visible behavior without hard-coding every product policy into the runtime core.

Built-in Bridges

The built-in bridge set is:

• ModelSelectionBridge
• ModeSelectionBridge
• ConfigOptionsBridge
• ToolSurfaceBridge
• DeepAgentsCompatibilityBridge

What Bridges Can Influence

On the LangChain side, bridges contribute through graph build aggregation rather than through Pydantic capability objects.

That means bridges can influence:

• model and mode state
• config options
• tool classification
• approval policy keys
• session metadata
• plan extraction
• graph build contributions

Graph Build Contributions

GraphBridgeBuilder and GraphBuildContributions aggregate bridge output into one graph-shaped contribution object.

That contribution seam can affect:

• middleware
• tools
• system prompt parts
• response format
• interrupt configuration
• graph metadata

This is the main reason langchain-acp does not collapse into one monolithic runtime file.

Example: Build A Bridge Stack

from langchain_acp import (
    AdapterConfig,
    ConfigOptionsBridge,
    DeepAgentsCompatibilityBridge,
    GraphBridgeBuilder,
    ToolSurfaceBridge,
)

config = AdapterConfig(
    capability_bridges=[
        ConfigOptionsBridge(),
        ToolSurfaceBridge(
            tool_kinds={
                "read_file": "read",
                "execute": "execute",
            },
            approval_policy_keys={
                "execute": "shell",
            },
        ),
        DeepAgentsCompatibilityBridge(),
    ]
)

builder = GraphBridgeBuilder.from_config(config)

That is the normal shape when the runtime owns a real graph but ACP should see more than the raw upstream callbacks.

Example: Tool Classification

from langchain_acp import AdapterConfig, ToolSurfaceBridge

config = AdapterConfig(
    capability_bridges=[
        ToolSurfaceBridge(
            tool_kinds={
                "read_file": "read",
                "execute": "execute",
            },
            approval_policy_keys={
                "execute": "shell",
            },
        )
    ]
)

DeepAgents Compatibility Bridge

DeepAgentsCompatibilityBridge is intentionally narrow.

Use it when you want:

• DeepAgents-flavored session metadata
• write_todos plan extraction
• predictable projection defaults for the DeepAgents example runtime

Do not use it as a general-purpose replacement for native ACP plan ownership.

When To Write A Custom Bridge

Write one when:

• the host already has product policy that should stay outside the adapter core
• ACP metadata should expose a runtime concern the graph already owns
• tool classification needs product-specific semantics

Do not write one just to repackage static config that AdapterConfig can already represent directly.