Échec des appels d'outils MCP : déboguer arguments invalides et erreurs de validation

Fix MCP tool call failures caused by invalid arguments, wrong IDs, missing fields, or server validation errors. Learn the read-only first strategy and systematic debugging workflow.

Understanding Tool Call Failures

When a tool call fails, the request reached the server and was processed, but something about the input or operation was invalid. This is different from connection or authentication errors.

Error Categories

Input errors: Invalid arguments, wrong types, missing required fields
ID errors: Wrong task/project/board IDs, non-existent resources
Validation errors: Server-side validation failures (business logic)
Permission errors: Valid input but insufficient permissions (see 403 guide)

Debugging Strategy: Read-Only First

Start with read-only operations to verify basic access before attempting writes:

Recommended Debugging Workflow

Test read operations: Try list_tasks, list_projects, get_task (read-only)
Verify IDs: Use IDs from read operations in subsequent calls
Test simple writes: Try add_task_comment (simpler than create_task)
Test complex writes: Finally try create_task, update_task (more validation)

Why Read-Only First?

Fewer validation rules - read operations have simpler requirements
Verify access - confirms you can reach the resource
Get correct IDs - read operations return valid IDs to use in writes
Isolate issues - if reads work but writes fail, it's a write-specific problem

Common Error Types

1. Invalid Arguments

Arguments that don't match the expected type or format:

Wrong Type

❌ Incorrect:

{ "task_id": "123" }  // String

✅ Correct:

{ "task_id": 123 }  // Integer

Invalid Enum Value

❌ Incorrect:

{ "status": "active" }

✅ Correct:

{ "status": "in_progress" }
// Valid: open, in_progress, done, blocked

Invalid Date Format

❌ Incorrect:

{ "due_date": "2026/05/30" }

✅ Correct:

{ "due_date": "2026-05-30" }
// ISO 8601 format

2. Missing Required Fields

Some tool calls require specific fields that must be provided:

Required Fields by Tool

get_task: Requires task_id
create_task: Requires title (and often project_id)
update_task: Requires task_id
add_task_comment: Requires task_id and comment
start_time_tracking: Requires task_id

Example: Missing Required Field

❌ Invalid (Missing title):

{
  "project_id": 123,
  "description": "Task description"
}

✅ Valid (Has title):

{
  "title": "Task title",
  "project_id": 123,
  "description": "Task description"
}

3. Wrong IDs

Using IDs that don't exist or belong to different resources:

Common ID Mistakes

Non-existent ID: Using task_id 99999 that doesn't exist
Wrong resource type: Using project_id where task_id is expected
ID from different workspace: Using task ID from workspace A in workspace B

How to Get Correct IDs

Call list_tasks to see available task IDs
Call list_projects to see available project IDs
Call list_boards to see available board IDs
Use IDs from these lists in subsequent tool calls

4. Server-Side Validation

Even with valid arguments, the server may reject the operation due to business logic:

Common Validation Errors

Cannot delete a task that has dependencies
Cannot update a task that's already completed
Cannot create a task in a closed project
Date constraints (due_date before start_date)
Cannot start time tracking if already tracking another task

Interpreting Error Payloads

Error Response Structure

Typical error response format:

{
  "error": {
    "code": "validation_error",
    "message": "Task ID 99999 not found",
    "field": "task_id",
    "details": "The specified task does not exist"
  }
}

Common Error Codes

validation_error: Input validation failed (wrong type, missing field, invalid value)
not_found: Resource with specified ID doesn't exist
forbidden: Valid input but insufficient permissions
conflict: Operation conflicts with current state

Step-by-Step Debugging Process

Systematic Debugging Workflow

Read the error message: Check the error code and message for clues
Verify IDs exist: Use list operations to confirm resource IDs are valid
Check required fields: Review tool documentation for required parameters
Test with read-only: Try get_task or list_tasks to verify basic access
Simplify the call: Remove optional parameters and test with minimal required fields
Check field types: Ensure strings, integers, dates are correct format
Review error details: Look at the "field" and "details" in error response