Building Tools

Building Tools

A tool is Celigo's first-class reusable building block. It encapsulates logic -- lookups, imports, transforms, branching -- behind a defined input and output contract. Build it once, use it everywhere: from Flows, APIs, AI Agents, MCP Servers, and other Tools.

Tool Concepts

Why tools exist: Without tools, users build the same lookup-transform-import patterns repeatedly across Flows, APIs, and Agents. Tools solve this by providing a governed, composable abstraction: one definition, many consumers, consistent behavior.

When to build a tool:

• You're building an MCP server -- MCP servers expose tools as endpoints; every piece of logic an MCP server offers must be a tool
• The same logic is needed by 2+ consumers (flows, APIs, agents, MCP servers) -- build once, call everywhere
• You want connection flexibility -- callers can pass different connections to the same tool definition
• You're composing smaller pieces -- tools can call other tools for nested orchestration

When NOT to build a tool:

• The logic is specific to one flow or API and won't be reused -- inline it as a lookup/import processor directly
• You need abstract/instance templating for multi-tenant patterns -- use abstract flows

Architecture:

Tool
+-- input         -- JSON Schema defining what callers send
+-- routers[]     -- processing pipeline
|   +-- branches[]
|       +-- pageProcessors[]  -- lookups (exports), imports, transfers
+-- output        -- schema, mappings, lookups, hooks defining what the tool returns

Routers hold branches, branches hold page processors. Use multiple branches when different inputs need different processing paths. Chain routers via nextRouterId for sequential processing stages. The special nextRouterId: "outputRouter" exits the tool and returns results.

Tool Execution Pipeline

When a tool is invoked:

1. Input received -- input data validated against the tool's JSON Schema (input)
2. Router evaluation -- routeRecordsUsing evaluates branch conditions (if multiple branches exist)
3. Branch selection -- matching branch processes the input
4. Page processors -- each processor executes sequentially (export lookups, import writes)
5. Response mapping -- responseMapping on each processor carries data to the next processor
6. Output mapping -- final output mapped according to the tool's output configuration (schema, mappings, lookups, hooks)
7. Return -- output returned to the caller (flow, API, AI agent, MCP server)

Execution mode depends on the caller:

• Called from a Flow -- runs in flow mode (batch-oriented, run console, error management)
• Called from an API -- runs in API mode (single request/response)
• Called from an AI Agent -- runs with agent context; agent maps connections to the tool
• Called from an MCP Server -- exposed as an MCP tool endpoint for external AI clients
• Called from another Tool -- nested execution within the parent tool's context

Connection model: Always bring-your-own-keys. The caller (flow, API, agent, MCP server) maps connections to the tool at configuration time. MCP Server overrides can swap connections per-server without modifying the tool.

Build order: Connection --> Export + Import --> Tool --> (consumer: Flow / API / Agent / MCP Server)

Concerns beyond the build steps:

• Response mapping -- extract fields from processor responses back into the record. Configured on pageProcessors[] entries within branches, but planned when building the processors. For lookups the response has data[] and errors[] (use data[0].fieldName for single results); for imports use _json.fieldName. Uses Transformation 1.0 syntax (extract/generate pairs)
• postResponseMap hook -- JavaScript processing after response mapping. Also on pageProcessors[] entries, but planned when building the processors

Quick Reference

Decision Matrix

| You need to... | Build a tool? | Instead use | |---|---|---| | Reuse logic across 2+ flows/APIs/agents | Yes | -- | | Expose logic via MCP server | Yes | -- | | Allow callers to swap connections | Yes | -- | | Nest orchestration (tool calls tool) | Yes | -- | | One-off logic for a single flow | No | Inline lookup/import in flow | | Multi-tenant templating | No | Abstract/instance flows |

Minimum Required Fields

Every tool needs at minimum: name, _integrationId, and an input.schema.

Which Schemas to Read

1. Always: [request.yml](references/schemas/request.yml) (base fields for create/update)
2. Input config: [input.yml](references/schemas/input.yml) (schema, transform, mockInput)
3. Output config: [output.yml](references/schemas/output.yml) (mappings, lookups, hooks)
4. Pipeline config: [router.yml](references/schemas/router.yml) (routing strategy, branches, page processors, response mapping)
5. Response shape: [response.yml](references/schemas/response.yml)

Schema Index

All schemas are in [references/schemas/](references/schemas/):

• Base fields (create/update): [request.yml](references/schemas/request.yml)
• Response shape: [response.yml](references/schemas/response.yml)
• Input configuration: [input.yml](references/schemas/input.yml) -- schema, transform, mockInput
• Output configuration: [output.yml](references/schemas/output.yml) -- mappings, lookups, hooks
• Router and branch configuration: [router.yml](references/schemas/router.yml) -- routing strategy, branches, page processors, response mapping

Related Skills

• [configuring-exports > Quick Reference](../configuring-exports/SKILL.md#quick-reference) -- building lookup exports used as steps in the tool pipeline
• [configuring-imports > Quick Reference](../configuring-imports/SKILL.md#quick-reference) -- building imports used as action steps in the tool pipeline
• [building-flows > How to Build a Flow](../building-flows/SKILL.md#how-to-build-a-flow) -- wiring tools into flow pipelines
• [building-apis > Quick Reference](../building-apis/SKILL.md#quick-reference) -- exposing tools via API endpoints
• [writing-scripts > Quick Reference](../writing-scripts/SKILL.md#quick-reference) -- script hooks on tool page processors
• [writing-handlebars > Quick Reference](../writing-handlebars/SKILL.md#quick-reference) -- dynamic expressions in request bodies, URIs, and field mappings
• [configuring-filters > Quick Reference](../configuring-filters/SKILL.md#quick-reference) -- input filters on tool steps and router branches

How to Build a Tool

1. Determine the tool's purpose

What should this tool do when called? Define the inputs it expects and the processing steps it needs.

2. Identify the integration

Every tool belongs to an integration. Find or create the integration first.

celigo integrations list

3. Check for existing patterns

Before building from scratch, look at what already exists:

# Search your account (fast, uses local index)
celigo account search ""

# Show what an existing tool uses (exports, imports, connections)
celigo account dependencies tool 

# Find orphaned resources that could be reused
celigo account lint

# Check if similar tools already exist
celigo tools list

# Search marketplace for pre-built integration templates
celigo templates marketplace

The account index auto-refreshes when stale (>4 hours). Force a fresh snapshot with celigo account snapshot.

Existing tools in the account are the best reference -- they show proven patterns for that specific customer's setup. Marketplace templates may provide a complete pre-built integration you can install rather than building from scratch.

4. Build the connections, exports, and imports

Tools reference exports and imports as page processors in router branches. Build bottom-up: connections first, then exports and imports that use those connections, then the tool that wires them together. See configuring-exports and configuring-imports.

5. Define the input schema

The input schema is a JSON Schema object describing what data the tool accepts. For MCP compatibility, the root schema must have type: "object".

6. Configure routing (if needed)

• No routing -- all inputs processed the same way; skip routers entirely
• Filter-based routing (routeRecordsUsing: "input_filters") -- declarative expression rules on each branch
• Script-based routing (routeRecordsUsing: "script") -- custom JavaScript function returns the branch name

7. Wire page processors into branches

Each branch contains pageProcessors[] -- an ordered list of exports (lookups) and imports (actions). Each processor has:

• type: "export" or "import"
• _exportId or _importId: reference to the resource
• responseMapping: extract fields from the processor response back into the record
• hooks.postResponseMap: optional script for post-processing
• proceedOnFailure: whether to continue if this step fails

8. Configure output

Output mappings transform the processed data into the tool's return value. Supports:

• mappings[] -- extract/generate field pairs (Celigo standard mapping format)
• lookups[] -- static key-value enrichment tables
• hooks.preMap / hooks.postMap -- script hooks before and after mapping

9. Build the JSON

Reference the [Schema Index](#schema-index) for the exact fields needed. Use the [Which Schemas to Read](#which-schemas-to-read) decision rule to determine which files to consult.

CLI Commands

# CRUD
celigo tools list
celigo tools get 
celigo tools create   key=value [key2=value2 ...]
celigo tools delete 

# Manage page processors
celigo tools add-processor   [--router ] [--branch ] [-y]
celigo tools remove-processor   [--router ] [--branch ] [-y]

# Test run
celigo tools test-run 
celigo tools test-run-step-results   
celigo tools test-run-step-logs   

# Debug (requires debug enabled on the underlying export/import)
celigo tools debug-requests   [--since ]
celigo tools debug-request-detail   

# Discovery
celigo account search ""
celigo templates marketplace

Pre-Submit Checklist

Required (all tools)

• [ ] name is set
• [ ] _integrationId references a valid integration
• [ ] input.schema is defined with type: "object" at root (required for MCP compatibility)

Pipeline

• [ ] All _exportId / _importId references in page processors point to existing resources
• [ ] Connections for referenced exports/imports are online
• [ ] Router IDs are unique within the tool
• [ ] All branches merge back to output node (no dangling branches)
• [ ] Last branch in chain uses nextRouterId: "outputRouter" to exit the tool

Cross-resource consistency

• [ ] If response mapping needed: configured on pageProcessors[] entries within branches
• [ ] If routing used: routeRecordsUsing and branch filters/scripts are configured correctly
• [ ] If tool is for MCP: tool name is unique across all tool and API entries in the MCP server

Gotchas

1. PUT erases omitted fields. Always GET first, modify, then PUT. The set command handles this.
2. Router IDs must be unique within the tool. Branch nextRouterId must reference a real router id or the special "outputRouter" terminal value.
3. No dangling branches. All branches must merge back to the output node. A dangling branch causes a configuration error at execution time, even if no record takes that path.
4. Do not include routeRecordsTo or routeRecordsUsing on routers unless needed. If either is present, the API may also require a top-level dataType field, triggering validation errors. Omit both for simple tools -- the API defaults correctly.
5. Tools cannot be deleted while in use. Check "Used by" dependencies first (flows, APIs, agents, MCP servers referencing the tool).
6. add-processor auto-creates a router. If the tool has no routers, the command creates a default router with one branch. Otherwise it targets the first router's first branch by default -- use --router and --branch to target a specific location.
7. Tool names must be unique in the MCP Server. The name field in tools[] on the MCP Server must be unique across all tool AND api entries in that server.
8. Debug logging is on the export/import, not the tool. Use celigo exports enable-debug or celigo imports enable-debug on the resources referenced by page processors, then use celigo tools debug-requests to view the logs scoped to the tool.
9. Test run results may be base64-encoded. The CLI auto-decodes these, but raw API responses need manual decoding.

Common Errors

| Error | Cause | Fix | |-------|-------|-----| | 422 _integrationId required | Missing integration | Set _integrationId to a valid integration ID | | 422 input.schema invalid | Bad JSON Schema | Ensure root schema has type: "object" and valid JSON Schema syntax | | 422 router id not unique | Duplicate router IDs | Each router id must be unique within the tool | | 422 dangling branch | Branch missing exit | Set nextRouterId to a valid router ID or "outputRouter" on every branch | | 422 _exportId not found / _importId not found | Deleted or invalid resource | Verify the referenced export/import exists and has not been deleted | | 409 tool in use | Tool referenced by consumers | Remove tool from all flows, APIs, agents, and MCP servers before deleting | | 422 dataType required | Unnecessary routing fields | Remove routeRecordsTo / routeRecordsUsing from simple tools; the API defaults correctly |

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: celigo
• Source: celigo/ai
• License: MIT

Install and usage instructions live in the source repository linked above.