Component Development

Components are the unit of work in Langflow. They render as nodes on the canvas, persist into flow JSON, and become tools for Agents. Get the contract right and the rest of the system follows; get it wrong and you orphan every saved flow that referenced your component.

Components live in src/lfx/src/lfx/components/<category>/. The mirror tree under src/backend/base/langflow/components/ is legacy stubs — do not add files there.

Before you add a component (decision checklist)

Stop and check, in order. If any answer is "yes," do not create a new component.

1. Does one already exist? Search:
   bashCopy

   rg -l 'display_name = ".*<keyword>.*"' src/lfx/src/lfx/components/
   

   and check the category folder. The Calculator regret (two of them, see tools/calculator.py legacy = True, replacement = ["helpers.CalculatorComponent"]) is what happens when this step is skipped.
2. Is this a thin wrapper around an existing tool/SDK that an Agent could call directly via tool_mode=True? If yes, add tool_mode=True to the relevant input on the existing component instead of building a new one.
3. Does it belong in a vendor bundle (e.g., openai/, anthropic/, datastax/) rather than in generic tools/ or helpers/? Vendor logic goes in vendor folders so bundles can ship/version independently.
4. Is this an lfx-runtime concern (graph execution, schema, IO primitives) or a Langflow-app concern (auth, DB, tracing)? Components are lfx; never import langflow.services.* from a component.
5. Does an existing base class cover this category? Use it:
   • LLMs → LCModelComponent (see openai/openai_chat_model.py)
   • Tools → LCToolComponent (see tools/calculator.py)
   • Vector stores → LCVectorStoreComponent + @check_cached_vector_store (see vectorstores/local_db.py)
   • Chat IO → ChatComponent (see input_output/chat.py)

Scope rules (one component, one job)

• One job per component. If the description has " and " in it, split it.
• Inputs budget: aim for ≤8 visible inputs; push the rest behind advanced=True. If you need a mode switch (Ingest/Retrieve etc.), use TabInput + update_build_config to hide irrelevant fields — see vectorstores/local_db.py.
• Outputs: one primary Output(method=...) is the default. Add more only if downstream nodes genuinely need different shapes (Message vs DataFrame vs Data). Don't add an output you don't use.
• Mark inputs an Agent should be able to fill with tool_mode=True (e.g., local_db.py) — this is how a component becomes an agent tool.

Breaking-change list (locked once shipped)

Saved flows reference components by string identifiers. Once a component is merged, the following are frozen — change them and existing user flows break silently:

• The class name (already in our rules).
• The name = "..." class attribute. This is the flow-JSON identifier, not the class name. See openai_chat_model.py, chat.py.
• Each input name=.... Renaming input_value → prompt orphans every edge pointing at it.
• Each output name=... and its declared return type. Downstream nodes match on both.
• Default values and default behavior of existing inputs (a flow saved with the default re-loads with the new default).

To remove or rework a component, do not edit it in place. Add the replacement under the right category, then on the old one set:

legacy = True
replacement = ["<category>.<NewClassName>"]

(see tools/calculator.py, flow_controls/sub_flow.py). The UI shows "Updates Available" and migrates users; the old class stays importable.

See CONTRACTS.md for the full user-facing surface this protects.

Conventions agents miss

These appear all over the codebase but rarely in old docs.

• name (class attr): flow-JSON identifier. Always set it explicitly; do not rely on __class__.__name__. Example: chat.py name = "ChatInput".
• legacy = True + replacement = [...]: deprecation pair. Always together. Example: tools/calculator.py.
• tool_mode=True on an input: exposes the component as an Agent tool with that input as the tool argument. Example: vectorstores/local_db.py.
• real_time_refresh=True + update_build_config(self, build_config, value, name): dynamic form. Use for mode switches and dependent-field hide/show. Examples: openai/openai_chat_model.py, vectorstores/local_db.py.
• metadata = {"keywords": [...]}: extra search terms for the component picker. Example: data_source/sql_executor.py.
• minimized = True: render collapsed by default. Example: input_output/chat.py.
• documentation = "https://docs.langflow.org/...": deep-link from the node UI. Example: input_output/chat.py.

Placement rules

• Vendor-specific code → vendor folder (openai/, anthropic/, datastax/, cohere/, …). Not tools/, not models/.
• Generic, vendor-agnostic helpers → helpers/, processing/, logic/, flow_controls/.
• IO primitives → input_output/.
• Add the import to the category __init__.py in alphabetical order.
• Do not invent a new top-level category. If you think you need one, that is a signal to discuss with the team first — categories are visible in the UI sidebar and are part of the product surface.

Icons

Every component needs an icon. Use a Lucide icon when one fits; create a custom SVG only for vendor logos.

Lucide icon (default)

icon = "calculator"  # any Lucide icon name, lowercase

See https://lucide.dev/icons for the catalog.

Custom vendor icon

For brand logos you need a frontend SVG component. The Python icon string and the frontend mapping key must match exactly (case-sensitive).

1. Python: set icon = "AstraDB" on the component.
2. Frontend SVG component at src/frontend/src/icons/AstraDB/AstraDB.jsx:
   jsxCopy

   const AstraSVG = (props) => (
     <svg {...props}>
       <path fill={props.isDark ? "#ffffff" : "#0A0A0A"} d="..." />
     </svg>
   );
   

3. Wrapper at src/frontend/src/icons/AstraDB/index.tsx:
   tsxCopy

   import React, { forwardRef } from "react";
   import AstraSVG from "./AstraDB";
   
   export const AstraDBIcon = forwardRef<SVGSVGElement, React.PropsWithChildren<{}>>(
     (props, ref) => <AstraSVG ref={ref} isDark={isDark} {...props} />
   );
   

4. Lazy import in src/frontend/src/icons/lazyIconImports.ts:
   tsCopy

   AstraDB: () =>
     import("@/icons/AstraDB").then((mod) => ({ default: mod.AstraDBIcon })),
   

5. Verify in the UI in both light and dark mode.

Icon checklist

• Python icon = "..." set.
• If custom: SVG component with isDark prop, wrapper with forwardRef, entry in lazyIconImports.ts.
• Light and dark mode both verified.

Component testing

See TESTING.md for the full testing contract. Quick reference:

• Inherit from ComponentTestBaseWithClient (needs API) or ComponentTestBaseWithoutClient (pure logic).
• Provide three fixtures: component_class, default_kwargs, file_names_mapping.
• Use MockLanguageModel for pure-logic LLM paths; use @pytest.mark.api_key_required for real-API tests.
• For graph behavior, use the Graph test pattern: build, .set(), async_start, validate. Do not poke graph internals.

Canonical examples

When in doubt, read these files before starting:

• LLM with dynamic provider switching: src/lfx/src/lfx/components/openai/openai_chat_model.py
• Tool with tool_mode and legacy/replacement: src/lfx/src/lfx/components/tools/calculator.py
• Vector store with cache + tab inputs: src/lfx/src/lfx/components/vectorstores/local_db.py
• Chat input with minimized: src/lfx/src/lfx/components/input_output/chat.py
• Component with searchable metadata: src/lfx/src/lfx/components/data_source/sql_executor.py
• Legacy + replacement pattern: src/lfx/src/lfx/components/flow_controls/sub_flow.py