Add JSON Schema for Prompt API Format

Closes #8899

- schemas/prompt.json: Draft-07 JSON Schema documenting the prompt format
  - Node objects with class_type (required), inputs (required), _meta (optional)
  - Node links as [source_id, output_index] arrays
  - Self-validating examples included in schema
- docs/api/prompt-schema.md: Documentation with validation rules, examples,
  and common error types

Based on analysis of execution.py:validate_prompt() and server.py

docs/api/prompt-schema.md

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+# ComfyUI Prompt API Schema
+
+This document describes the JSON format used by the `/prompt` endpoint in ComfyUI.
+
+## Overview
+
+A ComfyUI prompt is a JSON object where:
+- **Keys** are unique node IDs (arbitrary strings, typically numeric)
+- **Values** are node objects containing `class_type`, `inputs`, and optional `_meta`
+
+## Format Specification
+
+### Node Object
+
+```json
+{
+  "class_type": "KSampler",
+  "inputs": {
+    "seed": 123456,
+    "steps": 20,
+    "model": ["4", 0]
+  },
+  "_meta": {
+    "title": "My KSampler"
+  }
+}
+```
+
+| Field | Type | Required | Description |
+|-------|------|----------|-------------|
+| `class_type` | string | ✅ | Must match a registered node class in `NODE_CLASS_MAPPINGS` |
+| `inputs` | object | ✅ | Input parameters for the node |
+| `_meta` | object | ❌ | UI metadata (display title, position, etc.) |
+
+### Input Values
+
+Each input can be one of two types:
+
+#### Direct Value
+Any JSON-serializable value:
+```json
+{"seed": 123456, "steps": 20, "cfg": 7.0, "width": 512}
+```
+
+#### Node Link (Output Reference)
+A 2-element array `[source_node_id, output_index]`:
+```json
+{"model": ["4", 0], "positive": ["6", 0]}
+```
+
+### Validation Rules
+
+Based on `execution.py:validate_prompt()`:
+
+1. Every node **must** have a `class_type` field
+2. `class_type` **must** reference a registered node (existing in `NODE_CLASS_MAPPINGS`)
+3. The prompt **must** contain at least one `OUTPUT_NODE`
+4. All linked inputs must reference valid node IDs and output indices
+5. Required inputs for each node class must be provided
+
+### Common Error Types
+
+| Type | Description |
+|------|-------------|
+| `missing_node_type` | Node has no `class_type` or class not found |
+| `prompt_no_outputs` | No OUTPUT_NODE found in the prompt |
+
+## Validation
+
+Use the JSON Schema to validate prompts:
+
+```bash
+# Using ajv-cli
+npx ajv validate -s schemas/prompt.json -d my_prompt.json
+
+# Using Python
+pip install jsonschema
+python -c "
+import json, jsonschema
+with open('schemas/prompt.json') as f:
+    schema = json.load(f)
+with open('my_prompt.json') as f:
+    prompt = json.load(f)
+jsonschema.validate(prompt, schema)
+print('Valid!')
+"
+```
+
+## Minimal Example
+
+```json
+{
+  "1": {
+    "class_type": "CheckpointLoaderSimple",
+    "inputs": {"ckpt_name": "model.safetensors"}
+  },
+  "2": {
+    "class_type": "CLIPTextEncode",
+    "inputs": {"text": "hello world", "clip": ["1", 1]}
+  },
+  "3": {
+    "class_type": "EmptyLatentImage",
+    "inputs": {"width": 512, "height": 512, "batch_size": 1}
+  },
+  "4": {
+    "class_type": "KSampler",
+    "inputs": {
+      "seed": 0, "steps": 20, "cfg": 7,
+      "sampler_name": "euler", "scheduler": "normal",
+      "denoise": 1, "model": ["1", 0],
+      "positive": ["2", 0], "negative": ["2", 0],
+      "latent_image": ["3", 0]
+    }
+  },
+  "5": {
+    "class_type": "VAEDecode",
+    "inputs": {"samples": ["4", 0], "vae": ["1", 2]}
+  },
+  "6": {
+    "class_type": "SaveImage",
+    "inputs": {"filename_prefix": "test", "images": ["5", 0]}
+  }
+}
+```
+
+## Workflow vs Prompt Format
+
+The **prompt format** (this schema) is the compact API format used by `/prompt`.
+
+The **workflow format** is the UI serialization format that includes additional layout data, group information, and node positions. Workflows are converted to prompts before execution.
+
+## Future Enhancements
+
+This schema documents the current static format. Future versions may include:
+- Per-node schemas with required/optional input definitions
+- Dynamic schema generation based on installed custom nodes
+- LLM-friendly structured output definitions

schemas/prompt.json

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+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "$id": "https://github.com/Comfy-Org/ComfyUI/blob/main/schemas/prompt.json",
+  "title": "ComfyUI Prompt Format",
+  "description": "JSON Schema for the ComfyUI /prompt endpoint. Each key is a unique node ID, and each value describes a node with its class_type, inputs, and optional metadata.",
+  "type": "object",
+  "patternProperties": {
+    "^[a-zA-Z0-9_-]+$": {
+      "$ref": "#/definitions/node"
+    }
+  },
+  "additionalProperties": false,
+  "definitions": {
+    "node": {
+      "type": "object",
+      "required": ["class_type", "inputs"],
+      "properties": {
+        "class_type": {
+          "type": "string",
+          "description": "The node class to instantiate. Must match a key in NODE_CLASS_MAPPINGS."
+        },
+        "inputs": {
+          "$ref": "#/definitions/inputs"
+        },
+        "_meta": {
+          "type": "object",
+          "description": "Optional UI metadata for a node.",
+          "properties": {
+            "title": {
+              "type": "string",
+              "description": "Display title for the node in the UI."
+            }
+          }
+        }
+      },
+      "additionalProperties": false
+    },
+    "inputs": {
+      "type": "object",
+      "description": "Node inputs. Keys are parameter names. Values are either direct values or node links [source_id, output_index].",
+      "additionalProperties": true
+    },
+    "node_link": {
+      "type": "array",
+      "description": "A link to another node's output.",
+      "items": [
+        {"type": "string", "description": "Source node ID"},
+        {"type": "integer", "description": "Output index of the source node"}
+      ],
+      "minItems": 2,
+      "maxItems": 2
+    }
+  },
+  "examples": [
+    {
+      "4": {
+        "class_type": "CheckpointLoaderSimple",
+        "inputs": {"ckpt_name": "v1-5-pruned-emaonly.safetensors"}
+      },
+      "5": {
+        "class_type": "EmptyLatentImage",
+        "inputs": {"width": 512, "height": 512, "batch_size": 1}
+      },
+      "6": {
+        "class_type": "CLIPTextEncode",
+        "inputs": {"text": "a beautiful landscape", "clip": ["4", 1]}
+      },
+      "7": {
+        "class_type": "CLIPTextEncode",
+        "inputs": {"text": "ugly, blurry", "clip": ["4", 1]}
+      },
+      "3": {
+        "class_type": "KSampler",
+        "inputs": {
+          "seed": 123456, "steps": 20, "cfg": 7.0,
+          "sampler_name": "euler", "scheduler": "normal", "denoise": 1.0,
+          "model": ["4", 0], "positive": ["6", 0], "negative": ["7", 0],
+          "latent_image": ["5", 0]
+        }
+      },
+      "8": {
+        "class_type": "VAEDecode",
+        "inputs": {"samples": ["3", 0], "vae": ["4", 2]},
+        "_meta": {"title": "VAE Decode"}
+      },
+      "9": {
+        "class_type": "SaveImage",
+        "inputs": {"filename_prefix": "ComfyUI", "images": ["8", 0]}
+      }
+    }
+  ]
+}