SEP-1763: Interceptors for Model Context Protocol

# Interceptor Framework for Model Context Protocol

## Preamble

**Title:** Interceptor Framework for Model Context Protocol

**Status:** Draft

**Created:** 2025-11-04

**Authors:** @sambhav

## Abstract

This SEP proposes an interceptor framework for the Model Context Protocol (MCP) that enables intercepting, validating, and transforming messages at various extension points within the protocol lifecycle. The interceptor system introduces a new resource type that can hook into MCP events including server features (tool discovery, tool invocation, prompt handling, resource access), client features (sampling requests, elicitation requests, roots access), as well as LLM interactions (chat completions). Interceptors can be deployed on both client and server sides, supporting validation with severity levels (info, warn, error), mutation operations that replace message content, and observability for auditing, logging, and metrics collection. The framework follows MCP's JSON-RPC patterns and includes a discovery mechanism similar to existing tools, prompts, and resources.

## Motivation

### The Problem: A Sprawling Ecosystem Without Reusability

The MCP ecosystem is rapidly developing a sprawling landscape of sidecars, proxies, and gateways to address cross-cutting concerns like validation, security, observability, and rate limiting. However, these implementations are largely:

- **Non-reusable**: Each solution is tightly coupled to specific deployment patterns or languages
- **Non-interoperable**: Different approaches don't work together, creating vendor lock-in
- **Duplicative**: Similar functionality is reimplemented across different tools and servers
- **Fragmented**: No common interface or discovery mechanism exists

This leads to an **M × N problem**: Each of M clients must integrate with each of N middleware solutions, resulting in M × N integration points and configurations.

### The Solution: Standardized Plug-and-Play Interceptors

This SEP proposes standardizing interceptors in the same plug-and-play fashion that made MCP tools successful. The core goal is to transform the M × N problem into an **M + N problem**:

- **Clients** implement the interceptor call pattern once
- **Servers** expose interceptors through a standardized interface once
- **Platform teams** can deploy interceptors across all compatible clients and servers

#### Key Advantages

**1. Plug-and-Play Simplicity**

Just as tools made MCP powerful through discoverability and standardization, interceptors bring the same benefits to cross-cutting concerns:

```
Add gateway-level interceptor → Plug in configuration → All tools and servers gain new capabilities
```

**2. Language-Agnostic Deployment**

Unlike framework-specific middleware (e.g., FastAPI middleware, Express.js middleware), interceptors enable platform teams to:

- Introduce new capabilities company-wide regardless of implementation language
- Guarantee consistent behavior across Python, TypeScript, Go, Rust, or any other server implementation
- Deploy guardrails and policies without requiring code changes to each service

**3. Sidecar and Service-Based Architecture**

While traditional library-based middleware requires code integration, interceptors support:

- **First-party interceptors**: Deployed as code running locally within the server process
- **Third-party interceptors**: Deployed as separate services that servers call out to
- **Hybrid approaches**: Mix local and remote interceptors based on security, performance, and organizational requirements

This flexibility is critical for:
- **Security**: Sensitive validation logic can run in isolated, hardened services
- **Compliance**: Audit logging can be centralized and immutable
- **Performance**: High-throughput operations can use in-process interceptors while complex policies use dedicated services

**4. Well-Defined Invocation Lifecycle**

Interceptors provide a standardized execution model with clear semantics for:

- Event subscription and filtering
- Request/response phase handling
- Priority-based ordering for mutations
- Validation severity levels (info, warn, error)
- Error handling and propagation

### Technical Gaps Addressed

The current MCP specification lacks a standardized mechanism for:

1. **Input Validation**: No way to validate tool parameters, prompt inputs, resource requests, sampling requests, or elicitation requests before processing
2. **Output Transformation**: No capability to transform or sanitize responses before they reach clients or servers
3. **Cross-cutting Concerns**: No support for logging, rate limiting, or content filtering that applies across multiple protocol operations (both server and client features)
4. **Policy Enforcement**: No mechanism to enforce organizational policies on what prompts can be sent to LLMs, what tool results are returned, what sampling models can be used, or what data can be elicited from users
5. **Security Boundaries**: When MCP servers and clients cross trust boundaries, there's no way to inspect or validate messages in either direction
6. **LLM Interaction Control**: While MCP handles tool/resource/prompt orchestration and sampling requests, there's no hook into these interactions for content filtering or prompt injection detection
7. **Client Feature Protection**: No way to prevent servers from requesting excessive sampling tokens, asking for sensitive information through elicitation, or accessing unauthorized filesystem roots
8. **Observability**: No standardized way to collect metrics, traces, and audit logs across MCP operations including client-provided capabilities

These limitations become critical in enterprise deployments where:
- Multiple MCP servers with varying trust levels need coordination
- Compliance requirements mandate audit trails and content filtering
- Security policies require validation of inputs and outputs
- Performance monitoring requires instrumentation across protocol operations
- Debugging complex interactions requires detailed tracing

### Ecosystem Benefits

**For Platform Teams**

- Deploy interceptors once, gain coverage across all compatible servers and clients
- Enforce company-wide policies without modifying individual services
- Centralize security, compliance, and observability concerns

**For Server Developers**

- Inherit rich ecosystem of interceptors without custom integration work
- Focus on core business logic rather than cross-cutting concerns
- Support both local and remote interceptor patterns

**For Client Developers**

- Discover and utilize interceptors through standard MCP mechanisms
- Consistent behavior across different servers
- Reduced integration complexity

**For the Community**

- Build reusable interceptors that work across the entire ecosystem
- Share best practices through standardized implementations
- Foster innovation in security, observability, and policy enforcement

An interceptor framework addresses these needs by providing standardized extension points that maintain MCP's design principles while enabling powerful cross-cutting functionality and solving the ecosystem's reusability challenge.
f
## Specification

### 1. Interceptor Resource Type

Interceptors are introduced as a new first-class resource type in MCP, similar to tools, prompts, and resources.

#### 1.1 Interceptor Definition

```typescript
interface Interceptor {
 /**
 * Unique identifier for the interceptor
 */
 name: string;
 
 /**
 * Semantic version of this interceptor
 */
 version?: string;
 
 /**
 * Human-readable description
 */
 description?: string;
 
 /**
 * Events this interceptor subscribes to
 */
 events: InterceptorEvent[];
 
 /**
 * Interceptor operation type
 */
 type: "validation" | "mutation" | "observability";
 
 /**
 * Execution phase
 */
 phase: "request" | "response" | "both";
 
 /**
 * Priority hint for mutation interceptor ordering (lower executes first).
 * 
 * Can be specified as:
 * - A single number: applies to both request and response phases
 * - An object: specify different priorities per phase
 * 
 * Range: 32-bit signed integer (-2,147,483,648 to 2,147,483,647)
 * Default: 0 if omitted
 * 
 * Tie-breaker: Interceptors with equal priorityHint are ordered alphabetically by name.
 * For validation and observability interceptors, this field is ignored.
 * 
 * Examples:
 * priorityHint: -1000 // Same priority for both phases
 * priorityHint: { request: -1000 } // Request: -1000, Response: 0 (default)
 * priorityHint: { response: 1000 } // Request: 0 (default), Response: 1000
 * priorityHint: { request: -1000, response: 1000 } // Different per phase
 */
 priorityHint?: number | {
 request?: number;
 response?: number;
 };
 
 /**
 * Protocol version compatibility
 */
 compat?: {
 /**
 * Minimum MCP protocol version required
 */
 minProtocol: string;
 /**
 * Maximum MCP protocol version supported (optional)
 */
 maxProtocol?: string;
 };
 
 /**
 * Optional JSON Schema for interceptor configuration
 * Documents the expected configuration format
 */
 configSchema?: {
 type: "object";
 properties?: Record<string, unknown>;
 required?: string[];
 additionalProperties?: boolean;
 };
}
```

#### 1.2 Interceptor Events

```typescript
type InterceptorEvent =
 // MCP Server Features
 | "tools/list"
 | "tools/call"
 | "prompts/list"
 | "prompts/get"
 | "resources/list"
 | "resources/read"
 | "resources/subscribe"
 
 // MCP Client Features
 | "sampling/createMessage"
 | "elicitation/create"
 | "roots/list"
 
 // LLM Interaction Events (using common format)
 | "llm/completion"
 
 // Wildcard
 | "*/request"
 | "*/response"
 | "*";
```

### 2. JSON-RPC Methods

#### 2.1 Interceptor Discovery

**Request:**
```typescript
{
 jsonrpc: "2.0",
 id: 1,
 method: "interceptors/list",
 params?: {
 // Optional filter by event type
 event?: InterceptorEvent;
 }
}
```

**Response:**
```typescript
{
 jsonrpc: "2.0",
 id: 1,
 result: {
 interceptors: [
 {
 name: "content-filter",
 version: "1.2.0",
 description: "Filters sensitive content from prompts and responses",
 events: ["llm/completion", "prompts/get"],
 type: "mutation",
 phase: "both",
 priorityHint: -500, // Same priority for both request and response
 compat: {
 minProtocol: "2024-11-05"
 },
 configSchema: {
 type: "object",
 properties: {
 sensitivityLevel: {
 type: "string",
 enum: ["low", "medium", "high"]
 },
 redactPatterns: {
 type: "array",
 items: { type: "string" }
 }
 }
 }
 },
 {
 name: "parameter-validator",
 version: "2.0.1",
 description: "Validates tool call parameters",
 events: ["tools/call"],
 type: "validation",
 phase: "request",
 compat: {
 minProtocol: "2024-11-05",
 maxProtocol: "2025-12-31"
 },
 configSchema: {
 type: "object",
 properties: {
 strictMode: { type: "boolean" }
 }
 }
 },
 {
 name: "pii-redactor",
 version: "2.1.0",
 description: "Redacts PII from requests and responses",
 events: ["tools/call", "llm/completion"],
 type: "mutation",
 phase: "both",
 // Different priorities for request vs response
 priorityHint: {
 request: -1000, // Redact early when sending
 response: 1000 // Redact late when receiving (after decryption)
 },
 compat: {
 minProtocol: "2024-11-05"
 },
 configSchema: {
 type: "object",
 properties: {
 patterns: {
 type: "array",
 items: { type: "string" }
 }
 }
 }
 },
 {
 name: "sampling-guard",
 version: "1.0.0",
 description: "Enforces token limits and model policies for sampling requests",
 events: ["sampling/createMessage"],
 type: "validation",
 phase: "request",
 compat: {
 minProtocol: "2025-06-18"
 },
 configSchema: {
 type: "object",
 properties: {
 maxTokens: { type: "number" },
 allowedModels: {
 type: "array",
 items: { type: "string" }
 }
 }
 }
 },
 {
 name: "elicitation-pii-blocker",
 version: "1.0.0",
 description: "Prevents elicitation requests from asking for sensitive information",
 events: ["elicitation/create"],
 type: "validation",
 phase: "request",
 compat: {
 minProtocol: "2025-06-18"
 },
 configSchema: {
 type: "object",
 properties: {
 blockedPatterns: {
 type: "array",
 items: { type: "string" }
 },
 sensitiveFields: {
 type: "array",
 items: { type: "string" }
 }
 }
 }
 },
 {
 name: "audit-logger",
 version: "1.0.0",
 description: "Logs all MCP operations for compliance",
 events: ["*"],
 type: "observability",
 phase: "both",
 configSchema: {
 type: "object",
 properties: {
 destination: {
 type: "string",
 enum: ["local", "remote", "both"]
 },
 includePayloads: { type: "boolean" }
 },
 required: ["destination"]
 }
 }
 ]
 }
}
```

#### 2.2 Unified Result Envelope

All interceptor invocations return results conforming to a unified envelope structure:

```typescript
/**
 * Base result shared by all interceptor types
 */
interface BaseInterceptorResult {
 /**
 * Name of the interceptor that produced this result
 */
 interceptor: string;
 
 /**
 * Type of interceptor
 */
 type: "validation" | "mutation" | "observability";
 
 /**
 * Phase when this interceptor executed
 */
 phase: "request" | "response";
 
 /**
 * Execution duration in milliseconds
 */
 durationMs?: number;
 
 /**
 * Additional interceptor-specific information
 */
 info?: Record<string, unknown>;
}

/**
 * Validation interceptor result
 */
interface ValidationResult extends BaseInterceptorResult {
 type: "validation";
 
 /**
 * Overall validation outcome
 */
 valid: boolean;
 
 /**
 * Validation severity (error blocks execution)
 */
 severity?: "info" | "warn" | "error";
 
 /**
 * Detailed validation messages
 */
 messages?: Array<{
 path?: string;
 message: string;
 severity: "info" | "warn" | "error";
 }>;
 
 /**
 * Optional suggested corrections
 */
 suggestions?: Array<{
 path: string;
 value: unknown;
 }>;
 
 /**
 * Future: Cryptographic signature for validation result
 */
 signature?: {
 algorithm: "ed25519";
 publicKey: string;
 value: string;
 };
}

/**
 * Mutation interceptor result
 */
interface MutationResult extends BaseInterceptorResult {
 type: "mutation";
 
 /**
 * Whether the payload was modified
 */
 modified: boolean;
 
 /**
 * The mutated payload (or original if not modified)
 */
 payload: unknown;
}

/**
 * Observability interceptor result
 */
interface ObservabilityResult extends BaseInterceptorResult {
 type: "observability";
 
 /**
 * Whether observation was recorded
 */
 observed: boolean;
 
 /**
 * Optional metrics collected
 */
 metrics?: Record<string, number>;
}

type InterceptorResult = ValidationResult | MutationResult | ObservabilityResult;
```

#### 2.3 Interceptor Invocation

**Request for Validation:**
```typescript
{
 jsonrpc: "2.0",
 id: 2,
 method: "interceptor/invoke",
 params: {
 name: "parameter-validator",
 event: "tools/call",
 phase: "request",
 payload: {
 // Original request content
 method: "tools/call",
 params: {
 name: "get_weather",
 arguments: {
 location: "malicious_sql_injection"
 }
 }
 },
 config?: {
 // Optional interceptor-specific configuration
 }
 }
}
```

**Response for Validation:**
```typescript
{
 jsonrpc: "2.0",
 id: 2,
 result: {
 valid: false,
 severity: "error",
 messages: [
 {
 path: "params.arguments.location",
 message: "Input contains potentially malicious content",
 severity: "error"
 }
 ],
 // Optional: suggested corrections
 suggestions?: [
 {
 path: "params.arguments.location",
 value: "[REDACTED]"
 }
 ],
 // Future: Cryptographic signature for validation result
 // This enables verification that validation occurred at trust boundary
 signature?: {
 algorithm: "ed25519",
 publicKey: "...",
 value: "..."
 }
 }
}
```

> **Note**: The `signature` field is reserved for future use to enable cryptographic verification of validation results at trust boundaries. This will allow recipients to verify that validations were performed by authorized interceptor before accepting data.

**Request for Mutation:**
```typescript
{
 jsonrpc: "2.0",
 id: 3,
 method: "interceptor/invoke",
 params: {
 name: "content-filter",
 event: "llm/completion",
 phase: "request",
 payload: {
 // LLM completion request (following common format)
 messages: [
 {
 role: "user",
 content: "What is the password for admin@example.com?"
 }
 ],
 model: "gpt-4"
 }
 }
}
```

**Response for Mutation:**
```typescript
{
 jsonrpc: "2.0",
 id: 3,
 result: {
 modified: true,
 payload: {
 messages: [
 {
 role: "user",
 content: "What is the password for [REDACTED_EMAIL]?"
 }
 ],
 model: "gpt-4"
 },
 // Additional information about the mutation
 info: {
 redactions: 1,
 reason: "PII detected and redacted"
 }
 }
}
```

**Request for Observability:**
```typescript
{
 jsonrpc: "2.0",
 id: 4,
 method: "interceptor/invoke",
 params: {
 name: "audit-logger",
 event: "tools/call",
 phase: "request",
 payload: {
 method: "tools/call",
 params: {
 name: "execute_command",
 arguments: {
 command: "ls -la"
 }
 }
 },
 context: {
 principal: {
 type: "user",
 id: "user-123"
 },
 traceId: "trace-abc-123",
 timestamp: "2024-01-15T10:30:00Z"
 }
 }
}
```

**Response for Observability:**
```typescript
{
 jsonrpc: "2.0",
 id: 4,
 result: {
 observed: true,
 // Optional metrics collected
 metrics: {
 payloadSize: 156,
 processingTime: 2
 },
 // Additional information about what was observed
 info: {
 logId: "log-xyz-789",
 destination: "audit-system",
 // Optional alerts or notifications triggered
 alerts?: [
 {
 level: "info",
 message: "High-privilege command executed",
 tags: ["security", "audit"]
 }
 ]
 }
 }
}
```

#### 2.4 Interceptor Chain Execution

**Request:**
```typescript
{
 jsonrpc: "2.0",
 id: 5,
 method: "interceptor/executeChain",
 params: {
 event: "tools/call",
 phase: "request",
 payload: {
 method: "tools/call",
 params: {
 name: "get_weather",
 arguments: {
 location: "San Francisco"
 }
 }
 },
 // Optional: specify which interceptor to include
 interceptor?: string[];
 // Optional: per-invocation configuration
 config?: Record<string, Record<string, unknown>>;
 // Optional: timeout for entire chain
 timeoutMs?: number;
 // Optional: context passed to all interceptor
 context?: {
 principal?: {
 type: "user" | "service" | "anonymous";
 id?: string;
 };
 traceId?: string;
 timestamp: string;
 };
 }
}
```

**Response:**
```typescript
{
 jsonrpc: "2.0",
 id: 5,
 result: {
 /**
 * Overall chain execution status
 */
 status: "success" | "validation_failed" | "mutation_failed" | "timeout",
 
 /**
 * Event and phase for this chain
 */
 event: "tools/call",
 phase: "request",
 
 /**
 * Results from all executed interceptor
 */
 results: [
 {
 interceptor: "pii-redactor",
 type: "mutation",
 phase: "request",
 modified: true,
 payload: { /* mutated payload */ },
 durationMs: 5
 },
 {
 interceptor: "content-filter",
 type: "mutation",
 phase: "request",
 modified: false,
 payload: { /* unchanged */ },
 durationMs: 3
 },
 {
 interceptor: "parameter-validator",
 type: "validation",
 phase: "request",
 valid: true,
 durationMs: 2
 },
 {
 interceptor: "audit-logger",
 type: "observability",
 phase: "request",
 observed: true,
 metrics: { payloadSize: 156 },
 durationMs: 1
 }
 ],
 
 /**
 * Final payload after all mutations (if chain completed)
 */
 finalPayload?: unknown;
 
 /**
 * Validation summary
 */
 validationSummary: {
 errors: 0,
 warnings: 0,
 infos: 1
 },
 
 /**
 * Total execution time
 */
 totalDurationMs: 11,
 
 /**
 * If chain was aborted, details about where and why
 */
 abortedAt?: {
 interceptor: string;
 reason: string;
 type: "validation" | "mutation" | "timeout";
 }
 }
}
```

The `interceptor/executeChain` method executes the full interceptor chain for an event and phase, handling ordering, parallel execution, and aggregation automatically. **This is an optional convenience helper, not required by MCP.** Implementations can choose to use individual `interceptor/invoke` calls if they prefer explicit control. The helper method obeys negotiated capabilities and protocol versions per the MCP Lifecycle initialization.

This is the recommended way to invoke interceptor as it:

- Automatically discovers and orders applicable interceptor
- Handles mutations sequentially by `priorityHint` with alphabetical tie-breaking
- Executes validations and observability in parallel
- Aggregates results into a single response
- Provides detailed execution metrics and validation summaries
- Simplifies client/server implementation

#### 2.5 Error Handling

Interceptor invocation errors follow standard JSON-RPC error format:

```typescript
{
 jsonrpc: "2.0",
 id: 4,
 error: {
 code: -32603,
 message: "Interceptor execution failed",
 data: {
 interceptor: "content-filter",
 reason: "Configuration invalid"
 }
 }
}
```

### 3. Execution Model

#### 3.1 Interceptor Chain

Multiple interceptors can be chained for a single event. The execution model follows a **trust-boundary-aware pattern** where validation acts as a security gate.

**Discovery and Ordering**

The `interceptor/list` method returns all available interceptors. The invoker is responsible for:
- Collecting relevant interceptors for an event
- Resolving phase-specific priorities (see Section 3.1.3)
- Ordering mutations by priority (lower executes first), then alphabetically by name for ties
- Executing validations and observability in parallel

**Trust Boundary Execution Pattern**

Execution order depends on data flow direction:

**Sending Data (Client → Server or Server → Client):**
```
Mutate (sequential) → Validate & Observe (parallel) → Send
```
- Mutations prepare/sanitize data
- Validations verify before crossing boundary (can block)
- Observability logs (fire-and-forget, never blocks)

**Receiving Data (Server ← Client or Client ← Server):**
```
Receive → Validate & Observe (parallel) → Mutate (sequential)
```
- Validations act as security barrier (can block)
- Observability logs (fire-and-forget, never blocks)
- Mutations process validated data

This pattern ensures validation guards every trust boundary crossing, enabling defense-in-depth and future cryptographic verification.

**Interceptor Type Behaviors**

| Type | Execution | Failure Behavior | Ordering |
|------|-----------|------------------|----------|
| **Mutation** | Sequential | Chain halts, returns last valid state | By `priorityHint` (low→high), then alphabetically |
| **Validation** | Parallel | `severity: "error"` rejects request | None (parallel) |
| **Observability** | Parallel, fire-and-forget | Never blocks | None (parallel) |

Key semantics:
- Mutations are atomic: entire chain succeeds or none apply
- Validations with `severity: "warn"/"info"` don't block execution
- Observability failures are logged internally but never affect request flow

**4. Example Interceptor Chain**

For a `tools/call` event, assume the following interceptor are available:

```typescript
// From interceptor/list response:
[
 {
 name: "pii-redactor",
 type: "mutation",
 phase: "both",
 priorityHint: {
 request: -1000, // Run first when sending
 response: 1000 // Run last when receiving
 }
 },
 {
 name: "content-filter",
 type: "mutation",
 phase: "both",
 priorityHint: -500 // Same priority for both phases
 },
 {
 name: "format-normalizer",
 type: "mutation",
 phase: "both",
 priorityHint: { request: 100 } // Only applies to requests, response uses default (0)
 },
 {
 name: "schema-validator",
 type: "validation",
 phase: "request"
 // No priorityHint - runs in parallel with other validators
 },
 {
 name: "parameter-validator",
 type: "validation",
 phase: "request"
 // No priorityHint - runs in parallel
 },
 {
 name: "audit-logger",
 type: "observability",
 phase: "both"
 // No priorityHint - fire-and-forget
 }
]
```

**Full Request/Response Cycle**

```
CLIENT REQUEST (Sending):
 Original: { email: "john@example.com", ssn: "123-45-6789" }
 → Mutate: pii-redactor (-1000), content-filter (-500), format-normalizer (100)
 → Result: { email: "[EMAIL]", ssn: "[SSN]" }
 → Validate & Observe: client-validator ✓, audit-logger ✓
 → Send across TRUST BOUNDARY

SERVER REQUEST (Receiving):
 Received: { email: "[EMAIL]", ssn: "[SSN]" }
 → Validate & Observe: schema-validator ✓, parameter-validator ✓, audit-logger ✓
 → Mutate: sanitizer (10)
 → Execute Tool → { result: { name: "John", ... } }

SERVER RESPONSE (Sending):
 Original: { name: "John", ... }
 → Mutate: pii-redactor (1000), content-filter (-500)
 → Validate & Observe: response-validator ✓, audit-logger ✓
 → Send across TRUST BOUNDARY

CLIENT RESPONSE (Receiving):
 Received: { name: "John", ... }
 → Validate & Observe: response-validator ✓, audit-logger ✓
 → Mutate: response-enhancer (10)
 → Deliver to Application
```

**Key Points:**
- Mutations run in priority order: pii-redactor (-1000) → content-filter (-500) → format-normalizer (100)
- Validations and observability run in parallel at each boundary
- Trust boundaries are guarded by validation on both sides

**Error Handling**

Interceptor chains handle errors based on type:

```typescript
// Mutation failure: Chain halts immediately
{
 error: {
 code: -32603,
 message: "Interceptor mutation failed",
 data: { failedInterceptor: "content-filter", lastValidPayload: {...} }
 }
}
```

// Validation failure: All validations complete, then reject if severity: "error"
{
 error: {
 code: -32602,
 message: "Interceptor validation failed",
 data: {
 validationErrors: [
 { interceptor: "schema-validator", severity: "error", message: "Required field missing" }
 ]
 }
 }
}
```

**Short-Circuit Semantics:**
- **Sending**: Mutations → Validate → Send (mutation errors halt before validation)
- **Receiving**: Validate → Mutations → Process (validation errors prevent mutations)
- **Observability**: Fire-and-forget, failures never block execution

**9. Recommended Priority Ranges**

To promote consistency across interceptor implementations, the following priority ranges are recommended:

| Priority Range | Purpose | Example Use Cases |
|----------------|---------|-------------------|
| **-2,000,000,000 to -1,000,000** | System-critical security | Anti-malware scanning, SQL injection prevention |
| **-999,999 to -10,000** | Security sanitization | PII redaction, credential scrubbing, XSS filtering |
| **-9,999 to -1,000** | Input/output normalization | Character encoding, format standardization |
| **-999 to -1** | Content transformation | Language translation, markdown rendering |
| **0** | Default (no priority specified) | General-purpose interceptor |
| **1 to 999** | Enrichment | Metadata injection, tagging, formatting |
| **1,000 to 9,999** | Optimization | Compression, caching, batching |
| **10,000 to 999,999** | Observability | Detailed logging, tracing, metrics |
| **1,000,000 to 2,000,000,000** | Low-priority finalization | Audit stamps, response wrapping |

**Note on Phase-Specific Priorities:**

Different interceptor may need different priorities for request vs response phases:

```typescript
// PII Redactor: High priority on both phases, but different positioning
{
 name: "pii-redactor",
 priorityHint: {
 request: -50000, // Redact early when sending (before other transforms)
 response: 50000 // Redact late when receiving (after decompression, decryption)
 }
}

// Compression: Opposite pattern
{
 name: "compressor",
 priorityHint: {
 request: 5000, // Compress late when sending (after all transforms)
 response: -5000 // Decompress early when receiving (before other processing)
 }
}
```

#### 3.2 Priority Resolution

When ordering mutations, the invoker resolves phase-specific priorities:

```typescript
function resolvePriority(interceptor: Interceptor, phase: "request" | "response"): number {
 if (interceptor.priorityHint === undefined) return 0;
 if (typeof interceptor.priorityHint === "number") return interceptor.priorityHint;
 return interceptor.priorityHint[phase] ?? 0;
}
```

**Recommended Priority Ranges:**

| Priority Range | Purpose | Examples |
|----------------|---------|----------|
| **< -1,000,000** | System-critical security | Anti-malware, SQL injection prevention |
| **-999,999 to -10,000** | Security sanitization | PII redaction, credential scrubbing |
| **-9,999 to -1** | Normalization & transformation | Encoding, format standardization |
| **0** | Default | General-purpose interceptors |
| **1 to 9,999** | Enrichment & optimization | Metadata injection, compression |
| **≥ 10,000** | Observability & finalization | Logging, audit stamps |

#### 3.3 Sequence Diagrams

**Client-Side Interceptor Invocation:**

```mermaid
sequenceDiagram
 participant App as Application
 participant Client as MCP Client
 participant MW1 as Mutation 1 (PII Redactor)
 participant MW2 as Mutation 2 (Content Filter)
 participant MW3 as Validation (Validator)
 participant MW4 as Observability (Audit Logger)
 participant Server as MCP Server

 Note over App,Server: CLIENT REQUEST (Sending)
 App->>Client: tools/call request
 Client->>Client: Discover interceptor via interceptor/list Order mutations by suggestedPriority
 
 Note over Client,MW2: STEP 1: Mutations (Sequential)
 Client->>MW1: interceptor/invoke (pii-redactor, priority=10)
 MW1-->>Client: { modified: true, payload: {...} }
 
 Client->>MW2: interceptor/invoke (content-filter, priority=20)
 MW2-->>Client: { modified: true, payload: {...} }
 
 Note over Client,MW4: STEP 2: Validation & Observability (Parallel) Trust Boundary Check
 par Validation before sending
 Client->>MW3: interceptor/invoke (validator)
 MW3-->>Client: { valid: true }
 and Observability (fire-and-forget)
 Client->>MW4: interceptor/invoke (audit-logger)
 MW4-->>Client: { observed: true }
 end
 
 Client->>Server: Modified & validated request
 Note over Client,Server: ═══ TRUST BOUNDARY ═══
 Server-->>Client: tools/call response
 
 Note over App,Server: CLIENT RESPONSE (Receiving)
 
 Note over Client,MW4: STEP 1: Validation & Observability (Parallel) Trust Boundary Check
 par Validation after receiving
 Client->>MW3: interceptor/invoke (response-validator)
 MW3-->>Client: { valid: true }
 and Observability (fire-and-forget)
 Client->>MW4: interceptor/invoke (audit-logger)
 MW4-->>Client: { observed: true }
 end
 
 Note over Client,MW2: STEP 2: Mutations (Sequential)
 Client->>MW1: interceptor/invoke (response-enhancer, priority=10)
 MW1-->>Client: { modified: false, payload: {...} }
 
 Client->>App: Final validated & processed response
```

**Server-Side Interceptor Invocation:**

```mermaid
sequenceDiagram
 participant Client as MCP Client
 participant Server as MCP Server
 participant MW1 as Validation (Schema Validator)
 participant MW2 as Validation (Rate Limiter)
 participant MW3 as Observability (Audit Logger)
 participant MW4 as Mutation (Sanitizer)
 participant Tool as Tool Implementation

 Note over Client,Tool: SERVER REQUEST (Receiving)
 Client->>Server: tools/call request
 Note over Client,Server: ═══ TRUST BOUNDARY ═══
 Server->>Server: Discover interceptor via interceptor/list Order mutations by suggestedPriority
 
 Note over Server,MW3: STEP 1: Validation & Observability (Parallel) Trust Boundary Check
 par Validation after receiving
 Server->>MW1: interceptor/invoke (schema-validator)
 MW1-->>Server: { valid: true }
 and Validation
 Server->>MW2: interceptor/invoke (rate-limiter)
 MW2-->>Server: { valid: true }
 and Observability (fire-and-forget)
 Server->>MW3: interceptor/invoke (audit-logger)
 MW3-->>Server: { observed: true }
 end
 
 alt Any validation failed
 Server-->>Client: Error: Validation failed
 else All validations passed
 Note over Server,MW4: STEP 2: Mutations (Sequential)
 Server->>MW4: interceptor/invoke (sanitizer, priority=10)
 MW4-->>Server: { modified: true, payload: {...} }
 
 Server->>Tool: Execute tool
 Tool-->>Server: Tool result
 
 Note over Client,Tool: SERVER RESPONSE (Sending)
 
 Note over Server,MW4: STEP 1: Mutations (Sequential)
 Server->>MW4: interceptor/invoke (response-sanitizer, priority=10)
 MW4-->>Server: { modified: true, payload: {...} }
 
 Note over Server,MW3: STEP 2: Validation & Observability (Parallel) Trust Boundary Check
 par Validation before sending
 Server->>MW1: interceptor/invoke (schema-validator)
 MW1-->>Server: { valid: true }
 and Observability (fire-and-forget)
 Server->>MW3: interceptor/invoke (audit-logger)
 MW3-->>Server: { observed: true }
 end
 
 Note over Client,Server: ═══ TRUST BOUNDARY ═══
 Server-->>Client: Mutated & validated response
 end
```

**Cross-Boundary Interceptor (Client + Server):**

```mermaid
sequenceDiagram
 participant App as Application
 participant Client as MCP Client
 participant CMut as Client Mutation (PII Redactor)
 participant CVal as Client Validation (Validator)
 participant CObs as Client Observability (Logger)
 participant Server as MCP Server
 participant SVal as Server Validation (Schema/PII Validator)
 participant SMut as Server Mutation (Sanitizer)
 participant SObs as Server Observability (Audit Logger)
 participant Tool as Tool

 Note over App,Tool: CLIENT REQUEST (Sending - Trust Boundary)
 App->>Client: tools/call with PII
 
 Note over Client,CMut: Client STEP 1: Mutations
 Client->>CMut: interceptor/invoke (pii-redactor)
 CMut-->>Client: { modified: true, payload: "[REDACTED]" }
 
 Note over Client,CObs: Client STEP 2: Validation & Observability (Trust Boundary Check)
 par Client validates before sending
 Client->>CVal: interceptor/invoke (validator)
 CVal-->>Client: { valid: true }
 and Observability
 Client->>CObs: interceptor/invoke (logger)
 CObs-->>Client: { observed: true }
 end
 
 Client->>Server: Validated request
 Note over Client,Server: ═══════════════════════ ║ TRUST BOUNDARY ║ ═══════════════════════
 
 Note over Server,SObs: Server STEP 1: Validation & Observability (Trust Boundary Check)
 par Server validates after receiving
 Server->>SVal: interceptor/invoke (schema/pii-validator)
 SVal-->>Server: { valid: true }
 and Observability
 Server->>SObs: interceptor/invoke (audit-logger)
 SObs-->>Server: { observed: true }
 end
 
 Note over Server,SMut: Server STEP 2: Mutations
 Server->>SMut: interceptor/invoke (sanitizer)
 SMut-->>Server: { modified: true, payload: {...} }
 
 Server->>Tool: Execute tool safely
 Tool-->>Server: Result
 
 Note over App,Tool: SERVER RESPONSE (Sending - Trust Boundary)
 
 Note over Server,SMut: Server STEP 1: Mutations
 Server->>SMut: interceptor/invoke (response-sanitizer)
 SMut-->>Server: { modified: true, payload: {...} }
 
 Note over Server,SObs: Server STEP 2: Validation & Observability (Trust Boundary Check)
 par Server validates before sending
 Server->>SVal: interceptor/invoke (schema-validator)
 SVal-->>Server: { valid: true }
 and Observability
 Server->>SObs: interceptor/invoke (audit-logger)
 SObs-->>Server: { observed: true }
 end
 
 Server-->>Client: Validated response
 Note over Client,Server: ═══════════════════════ ║ TRUST BOUNDARY ║ ═══════════════════════
 
 Note over Client,CObs: Client STEP 1: Validation & Observability (Trust Boundary Check)
 par Client validates after receiving
 Client->>CVal: interceptor/invoke (response-validator)
 CVal-->>Client: { valid: true }
 and Observability
 Client->>CObs: interceptor/invoke (logger)
 CObs-->>Client: { observed: true }
 end
 
 Note over Client,CMut: Client STEP 2: Mutations
 Client->>CMut: interceptor/invoke (response-enhancer)
 CMut-->>Client: { modified: false, payload: {...} }
 
 Client->>App: Final validated result
 
 Note over App,Tool: Validation guards both sides of trust boundary
```

### 4. Payload Formats

This section describes the payload formats for all interceptable events. Payloads are organized into three categories: **Server Features** (tools, prompts, resources), **Client Features** (sampling, elicitation, roots), and **LLM Interactions** (completion requests). Each event type receives equal treatment with format specifications and example use cases.

#### 4.1 Server Feature Events

Server feature events encompass tool discovery and invocation, prompt handling, and resource access. These events follow the standard MCP JSON-RPC format.

##### 4.1.1 Tool Events

**tools/call**

```typescript
// tools/call request
{
 method: "tools/call",
 params: {
 name: string;
 arguments?: Record<string, unknown>;
 }
}

// tools/call response
{
 result: {
 content: Array<TextContent | ImageContent | EmbeddedResource>;
 isError?: boolean;
 }
}
```

**tools/list**

```typescript
// tools/list request
{
 method: "tools/list",
 params: {}
}

// tools/list response
{
 result: {
 tools: Array<{
 name: string;
 description?: string;
 inputSchema: {
 type: "object";
 properties?: Record<string, unknown>;
 required?: string[];
 };
 }>;
 }
}
```

**Example Use Cases for Tool Interceptors:**

1. **Validation**: Verify tool arguments match schema, check for injection attacks
2. **Mutation**: Transform arguments, sanitize outputs, inject context
3. **Observability**: Track tool usage, measure execution time, audit access patterns

##### 4.1.2 Prompt Events

**prompts/get**

```typescript
// prompts/get request
{
 method: "prompts/get",
 params: {
 name: string;
 arguments?: Record<string, unknown>;
 }
}

// prompts/get response
{
 result: {
 description?: string;
 messages: Array<{
 role: "user" | "assistant";
 content: {
 type: "text" | "image" | "resource";
 text?: string;
 data?: string;
 mimeType?: string;
 resource?: {
 uri: string;
 text?: string;
 mimeType?: string;
 };
 };
 }>;
 }
}
```

**prompts/list**

```typescript
// prompts/list request
{
 method: "prompts/list",
 params: {}
}

// prompts/list response
{
 result: {
 prompts: Array<{
 name: string;
 description?: string;
 arguments?: Array<{
 name: string;
 description?: string;
 required?: boolean;
 }>;
 }>;
 }
}
```

**Example Use Cases for Prompt Interceptors:**

1. **Validation**: Verify prompt arguments, check for prohibited content patterns
2. **Mutation**: Inject system instructions, apply prompt templates, sanitize outputs
3. **Observability**: Log prompt usage, track argument patterns, monitor access frequency

##### 4.1.3 Resource Events

**resources/read**

```typescript
// resources/read request
{
 method: "resources/read",
 params: {
 uri: string;
 }
}

// resources/read response
{
 result: {
 contents: Array<{
 uri: string;
 mimeType?: string;
 text?: string;
 blob?: string; // base64-encoded
 }>;
 }
}
```

**resources/list**

```typescript
// resources/list request
{
 method: "resources/list",
 params: {}
}

// resources/list response
{
 result: {
 resources: Array<{
 uri: string;
 name: string;
 description?: string;
 mimeType?: string;
 }>;
 }
}
```

**resources/subscribe**

```typescript
// resources/subscribe request
{
 method: "resources/subscribe",
 params: {
 uri: string;
 }
}

// resources/subscribe response
{
 result: {}
}
```

**Example Use Cases for Resource Interceptors:**

1. **Validation**: Verify resource URIs are authorized, check content size limits
2. **Mutation**: Transform resource content, apply access control filters, redact sensitive data
3. **Observability**: Track resource access patterns, monitor bandwidth usage, audit data flow

#### 4.2 Client Feature Events

Client feature events enable servers to request services from clients: sampling (LLM inference), elicitation (user input), and roots (filesystem access). These events follow MCP client capability formats.

##### 4.2.1 Sampling Events

**sampling/createMessage**

```typescript
// sampling/createMessage request
{
 method: "sampling/createMessage",
 params: {
 messages: Array<{
 role: "user" | "assistant";
 content: {
 type: "text" | "image" | "audio";
 text?: string;
 data?: string; // base64-encoded for image/audio
 mimeType?: string;
 };
 }>;
 modelPreferences?: {
 hints?: Array<{ name: string }>;
 costPriority?: number; // 0-1
 speedPriority?: number; // 0-1
 intelligencePriority?: number; // 0-1
 };
 systemPrompt?: string;
 includeContext?: "none" | "thisServer" | "allServers";
 temperature?: number;
 maxTokens: number;
 stopSequences?: string[];
 metadata?: Record<string, unknown>;
 }
}

// sampling/createMessage response
{
 result: {
 role: "assistant";
 content: {
 type: "text" | "image" | "audio";
 text?: string;
 data?: string;
 mimeType?: string;
 };
 model: string;
 stopReason?: "endTurn" | "stopSequence" | "maxTokens" | string;
 }
}
```

**Example Use Cases for Sampling Interceptors:**

1. **Validation**: Verify sampling requests don't contain prohibited content or exceed token limits
2. **Mutation**: Inject additional context or modify model preferences before sampling
3. **Observability**: Track sampling usage, measure latency, and audit model selections

```typescript
// Example: Intercepting a sampling request to enforce token limits
{
 jsonrpc: "2.0",
 id: 5,
 method: "interceptor/invoke",
 params: {
 name: "token-limit-validator",
 event: "sampling/createMessage",
 phase: "request",
 payload: {
 messages: [
 {
 role: "user",
 content: {
 type: "text",
 text: "Generate a very long response..."
 }
 }
 ],
 maxTokens: 100000 // Exceeds organizational limit
 }
 }
}

// Validation response blocking excessive token request
{
 jsonrpc: "2.0",
 id: 5,
 result: {
 valid: false,
 severity: "error",
 messages: [
 {
 path: "maxTokens",
 message: "Requested tokens (100000) exceeds organizational limit (4096)",
 severity: "error"
 }
 ]
 }
}
```

##### 4.2.2 Elicitation Events

**elicitation/create**

```typescript
// elicitation/create request
{
 method: "elicitation/create",
 params: {
 message: string;
 requestedSchema: {
 type: "object";
 properties: Record<string, {
 type: "string" | "number" | "integer" | "boolean";
 title?: string;
 description?: string;
 enum?: any[];
 enumNames?: string[];
 minimum?: number;
 maximum?: number;
 minLength?: number;
 maxLength?: number;
 format?: "email" | "uri" | "date" | "date-time";
 default?: any;
 }>;
 required?: string[];
 };
 }
}

// elicitation/create response
{
 result: {
 action: "accept" | "decline" | "cancel";
 content?: Record<string, unknown>; // Present when action is "accept"
 }
}
```

**Example Use Cases for Elicitation Interceptors:**

1. **Validation**: Verify elicitation requests don't ask for sensitive information (PII, credentials)
2. **Mutation**: Modify or enhance the requested schema, add validation rules
3. **Observability**: Log elicitation requests for compliance and audit trails

```typescript
// Example: Intercepting an elicitation request to prevent sensitive data collection
{
 jsonrpc: "2.0",
 id: 6,
 method: "interceptor/invoke",
 params: {
 name: "pii-blocker",
 event: "elicitation/create",
 phase: "request",
 payload: {
 message: "Please provide your credit card number for verification",
 requestedSchema: {
 type: "object",
 properties: {
 creditCard: {
 type: "string",
 description: "Your credit card number"
 }
 },
 required: ["creditCard"]
 }
 }
 }
}

// Validation response blocking PII request
{
 jsonrpc: "2.0",
 id: 6,
 result: {
 valid: false,
 severity: "error",
 messages: [
 {
 path: "requestedSchema.properties.creditCard",
 message: "Elicitation requests MUST NOT request sensitive financial information",
 severity: "error"
 }
 ],
 suggestions: [
 {
 path: "message",
 value: "Please confirm your identity through the secure verification system"
 }
 ]
 }
}
```

##### 4.2.3 Roots Events

**roots/list**

```typescript
// roots/list request
{
 method: "roots/list",
 params: {}
}

// roots/list response
{
 result: {
 roots: Array<{
 uri: string; // Must be file:// URI
 name?: string;
 }>;
 }
}
```

**Example Use Cases for Roots Interceptors:**

1. **Validation**: Verify servers only request allowed root paths
2. **Mutation**: Filter or transform root URIs based on security policies
3. **Observability**: Audit which roots are exposed to which servers

#### 4.3 LLM Interaction Events

For LLM interaction events, we use a common format inspired by OpenAI's API:

**llm/completion**

```typescript
// llm/completion request
{
 messages: Array<{
 role: "system" | "user" | "assistant" | "tool";
 content: string | Array<ContentPart>;
 name?: string;
 }>;
 model: string;
 tools?: Array<ToolDefinition>;
 temperature?: number;
 max_tokens?: number;
 top_p?: number;
 frequency_penalty?: number;
 presence_penalty?: number;
 stop?: string | string[];
 // ...other parameters
}

// llm/completion response
{
 choices: Array<{
 message: {
 role: "assistant";
 content: string | null;
 tool_calls?: Array<ToolCall>;
 };
 finish_reason: string;
 index: number;
 }>;
 usage?: {
 prompt_tokens: number;
 completion_tokens: number;
 total_tokens: number;
 };
 model: string;
 id: string;
 created: number;
}
```

**Example Use Cases for LLM Interceptors:**

1. **Validation**: Detect prompt injection attempts, verify model access permissions, enforce token limits
2. **Mutation**: Apply prompt templates, inject safety instructions, sanitize tool call results
3. **Observability**: Monitor LLM usage costs, track model performance metrics, audit sensitive queries

### 5. Interceptor Server Capabilities

During initialization, servers declare interceptor support:

```typescript
{
 jsonrpc: "2.0",
 id: 1,
 result: {
 protocolVersion: "2024-11-05",
 capabilities: {
 interceptor?: {
 // Events this server's interceptor can handle
 supportedEvents: InterceptorEvent[];
 },
 // ...existing capabilities
 },
 serverInfo: {
 name: "example-server",
 version: "1.0.0"
 }
 }
}
```

### 6. Configuration and Context

Interceptor may receive configuration and context:

```typescript
interface InterceptorInvocationParams {
 name: string;
 event: InterceptorEvent;
 phase: "request" | "response";
 payload: unknown;
 
 // Optional configuration for this invocation
 config?: Record<string, unknown>;
 
 // Optional timeout in milliseconds
 // If exceeded, interceptor execution is cancelled and returns timeout error
 timeoutMs?: number;
 
 // Optional context information
 context?: {
 // Identity information
 principal?: {
 type: "user" | "service" | "anonymous";
 id?: string;
 claims?: Record<string, unknown>;
 };
 
 // Tracing information
 traceId?: string;
 spanId?: string;
 
 // Timing information
 timestamp: string; // ISO 8601
 
 // Session information
 sessionId?: string;
 
 // FUTURE: Interceptor state propagation
 // Allows interceptor to share state through the chain
 // This field is reserved for future enhancement
 interceptorState?: Record<string, unknown>;
 };
}
```

**Timeout and Cancellation:**

When `timeoutMs` is specified:
- Interceptor execution is cancelled if it exceeds the timeout
- A timeout error is returned with code `-32000` (Server error)
- For `interceptor/executeChain`, the entire chain is cancelled on timeout
- Individual `interceptor/invoke` calls timeout independently
- Observability interceptor timeouts are logged but don't affect the main flow (fire-and-forget)

```typescript
// Timeout error response
{
 jsonrpc: "2.0",
 id: 1,
 error: {
 code: -32000,
 message: "Interceptor execution timeout",
 data: {
 interceptor: "slow-validator",
 timeoutMs: 5000,
 phase: "request"
 }
 }
}
```

### 7. Future Enhancements

#### 7.1 Context Propagation and Interceptor State (Future)

**Status**: Reserved for future specification

Inspired by gRPC interceptors and OpenTelemetry context propagation, future versions may support enrichable interceptor state that flows through the entire chain. This would enable:

- **State Sharing**: Early interceptor can enrich context for downstream interceptor
- **Cross-Cutting Data**: Authentication, rate limiting, and audit data flows through chain
- **Trace Correlation**: Distributed tracing baggage propagates automatically
- **Deadline Propagation**: Timeout context flows through nested operations

**Proposed Model:**

```typescript
// FUTURE: Enhanced context with state propagation
interface InterceptorChainContext {
 // Immutable request context (set by initiator, read-only)
 readonly request: {
 principal?: {
 type: "user" | "service" | "anonymous";
 id?: string;
 claims?: Record<string, unknown>;
 };
 traceId: string;
 spanId?: string;
 timestamp: string;
 sessionId?: string;
 };
 
 // Mutable interceptor state (enriched by interceptor during chain execution)
 // Each interceptor can read and write to this shared state
 interceptorState?: Record<string, unknown>;
 
 // Deadline for entire operation (inherited and propagated)
 deadline?: string; // ISO 8601 timestamp
 
 // Distributed tracing baggage (key-value pairs for trace correlation)
 // Follows W3C Baggage specification
 baggage?: Record<string, string>;
}
```

**Interceptor State Enrichment:**

Mutation and validation interceptor could optionally return context updates:

```typescript
// FUTURE: Interceptor result with context enrichment
interface MutationResult extends BaseInterceptorResult {
 type: "mutation";
 modified: boolean;
 payload: unknown;
 
 // Optional: Updates to propagate to downstream interceptor
 contextUpdates?: {
 interceptorState?: Record<string, unknown>;
 baggage?: Record<string, string>;
 };
}
```

**Example Flow:**

```typescript
// Interceptor 1: Authentication enriches context
{
 interceptor: "authenticator",
 type: "mutation",
 modified: false,
 payload: { /* unchanged */ },
 contextUpdates: {
 interceptorState: {
 principal: { 
 id: "user-123", 
 roles: ["admin"],
 authMethod: "oauth2"
 }
 }
 }
}

// Interceptor 2: Rate limiter reads enriched context
// Has access to context.interceptorState.principal
{
 interceptor: "rate-limiter",
 type: "validation",
 valid: true,
 info: {
 rateLimitUsed: 45,
 rateLimitMax: 100,
 // Used principal.id from context for rate limiting
 },
 contextUpdates: {
 interceptorState: {
 rateLimit: { remaining: 55, resetAt: "2025-11-04T15:30:00Z" }
 }
 }
}

// Interceptor 3: Audit logger has full enriched context
// Logs with principal + rate limit info + original request
{
 interceptor: "audit-logger",
 type: "observability",
 observed: true,
 info: {
 logEntry: {
 user: "user-123",
 roles: ["admin"],
 rateLimitRemaining: 55,
 action: "tools/call",
 timestamp: "2025-11-04T15:29:00Z"
 }
 }
}
```

**Benefits of State Propagation:**

1. **Loose Coupling**: Interceptor don't need direct dependencies
2. **Composability**: New interceptor can leverage existing enriched state
3. **Observability**: Full context available for logging and tracing
4. **Performance**: Avoid redundant operations (e.g., auth check once, reuse result)

**Design Considerations:**

- **Immutability**: Request context is read-only to prevent tampering
- **Namespace Conflicts**: Interceptor should use namespaced keys (e.g., `auth.principal`, `rateLimit.info`)
- **Size Limits**: Context should have reasonable size limits to prevent abuse
- **Serialization**: Context must be JSON-serializable for transport

This feature would be opt-in and backward compatible, with interceptor that don't support context enrichment working unchanged.

## Rationale

### Design Decisions

**1. Separation of Validation, Mutation, and Observability**

We explicitly separate these three interceptor types rather than combining them because:
- **Clear intent**: Users know immediately what a interceptor does
- **Simpler implementation**: Each type has focused logic
- **Better error handling**: Validation failures vs mutation failures have different semantics
- **Performance**: Validation and observability can run in parallel, mutations run sequentially
- **Isolation**: Observability failures never affect the request/response flow

Alternative considered: Combined interceptor that can both validate and mutate. Rejected because it creates ambiguity in execution order and error handling.

**2. Validation as Trust Boundary Guard**

Execution order is trust-boundary-aware:
- **Sending**: Mutate → Validate → Send (prepare data, verify before crossing)
- **Receiving**: Validate → Mutate → Process (security barrier, then process)

This ensures validation guards all boundary crossings, enabling zero-trust architecture and future cryptographic verification. Alternative (same order for all phases) rejected because validation must be first when receiving untrusted data.

**3. Phase-Aware Priority Hints**

Mutations declare `priorityHint` (default: 0) which can differ by phase. Benefits:
- **Flexibility**: PII redaction runs early when sending (-50000), late when receiving (+50000)
- **Simplicity**: Single number for common cases, object for phase-specific needs
- **Determinism**: Alphabetical tie-breaking by `name` ensures consistency
- **Wide range**: 32-bit integers align with Kubernetes standards

Alternative (global fixed ordering) rejected as too rigid for phase-aware operations like compression/decompression.

**4. Other Design Choices**

- **Event-Based Targeting**: Interceptors declare specific events (vs. all traffic) for performance and security
- **Severity Levels** (info/warn/error): Graduated response vs. binary pass/fail enables observability without blocking
- **Replace vs. Patch**: Mutations replace entire payloads (vs. JSON Patch) for simplicity and atomicity
- **Method Names**: `interceptor/list` and `interceptor/invoke` mirror MCP patterns (`tools/list`, etc.)
- **"info" Field**: Used instead of "metadata" to avoid confusion with MCP's `_meta` protocol metadata
- **Cross-Boundary Support**: Client and server interceptors enable defense-in-depth and zero-trust architecture

### Related Work and Inspiration

**Inspired By:**
- **Kubernetes Admission Controllers**: Validation/mutation webhooks (MCP uses replace vs. patch for simplicity)
- **gRPC Interceptors**: Client/server-side interception (MCP adds discoverability and dynamic invocation)
- **HTTP Middleware**: Express.js, ASP.NET patterns (MCP is event-specific vs. path-based)
- **Envoy Filters**: Network-level request/response filtering

**Complements Existing MCP:**
- Adds validation/mutation capabilities to sampling, tools, prompts, and resources
- Enables parameter validation, access control, and content filtering across all MCP features

## Backward Compatibility

This SEP is fully backward compatible:

1. **Optional Capability**: Interceptor is an optional server capability. Servers without interceptor support continue to work.

2. **No Protocol Changes**: Existing JSON-RPC methods are unchanged. Interceptor adds new methods (`interceptor/list`, `interceptor/invoke`) but doesn't modify existing ones.

3. **Client Compatibility**: Clients that don't support interceptor can still communicate with interceptor-enabled servers. Interceptor runs transparently on the server side.

4. **Graceful Degradation**: If a client requests interceptor from a server that doesn't support it, the server returns a standard JSON-RPC error, which clients already handle.

5. **Version Negotiation**: The existing protocol version negotiation during initialization allows clients and servers to discover interceptor support.

## Security Implications

### Threat Model

**1. Malicious Interceptor**

A compromised interceptor could:
- Exfiltrate sensitive data from requests/responses
- Modify data to inject malicious content
- Cause denial of service by rejecting all requests
- Bypass security controls by approving invalid inputs

**Mitigations:**
- Interceptor runs in the same trust domain as the server/client
- Organizations must vet interceptor before deployment
- Interceptor should be sandboxed where possible
- Audit logging of all interceptor invocations
- Cryptographic signing of interceptor for integrity

**2. Interceptor Bypass**

Attackers might try to bypass interceptor validation:
- Send requests directly to underlying services
- Exploit interceptor chain ordering

**Mitigations:**
- Interceptor enforcement must be at the transport layer
- Server-side interceptor is mandatory for requests entering the trust boundary
- Interceptor chain order must be explicitly configured and immutable

**3. Information Disclosure**

Interceptor error messages might leak sensitive information:
- Internal system details
- Validation rules that could be reverse-engineered
- PII in error messages

**Mitigations:**
- Interceptor should use generic error messages
- Detailed errors should only be logged, not returned to untrusted clients
- Separate internal and external validation messages

**4. Denial of Service**

Interceptor could be exploited for DoS:
- Computationally expensive validation
- Infinite loops in mutation logic
- Memory exhaustion from large payloads

**Mitigations:**
- Timeouts for interceptor execution
- Resource limits (CPU, memory)
- Rate limiting on interceptor invocations
- Circuit breakers for failing interceptor

**5. Privilege Escalation**

Interceptor mutation could be used to inject unauthorized operations:
- Modifying request parameters to access unauthorized resources
- Changing tool invocation targets
- Injecting administrative commands

**Mitigations:**
- Validation interceptor should verify request authorization at trust boundaries
- Immutable fields should be enforced at protocol level
- Audit all mutations that touch security-sensitive fields

### Security Best Practices

1. **Principle of Least Privilege**: Interceptor should only have access to the data it needs to process

2. **Defense in Depth**: Deploy interceptor at both client and server boundaries

3. **Audit Everything**: Log all interceptor invocations, including payloads (with PII redaction)

4. **Fail Secure**: If interceptor fails, the default should be to reject the request

5. **Input Validation**: Interceptor configuration and payloads should be validated

6. **Cryptographic Integrity**: Consider signing interceptor responses to prevent tampering

7. **Isolation**: Run untrusted interceptor in sandboxed environments

### Compliance Considerations

Interceptor enables compliance with:
- **GDPR**: PII detection and redaction
- **HIPAA**: PHI filtering in healthcare contexts
- **SOC 2**: Audit logging and access control
- **PCI DSS**: Sensitive data masking

However, organizations must ensure:
- Interceptor doesn't create new compliance violations (e.g., storing PII in logs)
- Audit trails are tamper-proof
- Data retention policies apply to interceptor logs

## Reference Implementation

The reference implementation will provide:

### Multi-Language SDKs

SDK libraries for writing and running interceptors in various languages, including Python, TypeScript/JavaScript, Go and other community-contributed languages. These SDKs will provide base classes, helpers, and patterns for building validation, mutation, and observability interceptors.

### Common Interceptor Sidecar

A universal interceptor runtime that enables teams to deploy and manage interceptors without modifying individual MCP servers. The sidecar acts as an MCP proxy that loads custom interceptors (both local and remote) via configuration and injects interceptor invocations transparently.

**Example Configuration:**

```yaml
# interceptor-config.yaml
interceptors:
 # Local interceptor: runs in-process
 - name: pii-redactor
 type: mutation
 transport: local
 command: ./interceptors/pii-redactor.py
 args: []
 config:
 patterns: ["email", "ssn", "phone"]
 
 # Remote interceptor: calls external service
 - name: security-scanner
 type: validation
 transport: remote
 endpoint: https://security.example.com/scan
 timeout: 5s
 headers:
 Authorization: "Bearer ${SECURITY_TOKEN}"
 
 # Local TypeScript interceptor
 - name: rate-limiter
 type: validation
 transport: local
 comamnd: ./interceptors/rate-limiter.js
 args: []
 config:
 requests_per_minute: 100

routing:
 # Apply interceptors to specific event types
 tools.call:
 request: [rate-limiter, security-scanner]
 response: [pii-redactor]
 
 llm.chatCompletion:
 request: [security-scanner]
 response: [pii-redactor]
```

This approach enables platform teams to deploy guardrails across all services from a central control plane without code changes to existing MCP servers.


## Acknowledgments

This proposal draws inspiration from:
- [Kubernetes Admission Controllers](https://kubernetes.io/docs/reference/access-authn-authz/admission-controllers/)
- [gRPC interceptors](https://grpc.io/docs/guides/interceptors/) - particularly context propagation concepts (Section 7.1)
- [W3C Trace Context](https://www.w3.org/TR/trace-context/) and [W3C Baggage](https://www.w3.org/TR/baggage/) specifications for distributed tracing
- The MCP community's discussions on extensibility and security

Type	Execution	Failure Behavior	Ordering
Mutation	Sequential	Chain halts, returns last valid state	By `priorityHint` (low→high), then alphabetically
Validation	Parallel	`severity: "error"` rejects request	None (parallel)
Observability	Parallel, fire-and-forget	Never blocks	None (parallel)

Priority Range	Purpose	Examples
< -1,000,000	System-critical security	Anti-malware, SQL injection prevention
-999,999 to -10,000	Security sanitization	PII redaction, credential scrubbing
-9,999 to -1	Normalization & transformation	Encoding, format standardization
0	Default	General-purpose interceptors
1 to 9,999	Enrichment & optimization	Metadata injection, compression
≥ 10,000	Observability & finalization	Logging, audit stamps

Uh oh!

SEP-1763: Interceptors for Model Context Protocol #1763

Description

Interceptor Framework for Model Context Protocol

Preamble

Abstract

Motivation

The Problem: A Sprawling Ecosystem Without Reusability

The Solution: Standardized Plug-and-Play Interceptors

Key Advantages

Technical Gaps Addressed

Ecosystem Benefits

Specification

1. Interceptor Resource Type

1.1 Interceptor Definition

1.2 Interceptor Events

2. JSON-RPC Methods

2.1 Interceptor Discovery

2.2 Unified Result Envelope

2.3 Interceptor Invocation

2.4 Interceptor Chain Execution

2.5 Error Handling

3. Execution Model

3.1 Interceptor Chain

3.2 Priority Resolution

3.3 Sequence Diagrams

4. Payload Formats

4.1 Server Feature Events

4.1.1 Tool Events

4.1.2 Prompt Events

4.1.3 Resource Events

4.2 Client Feature Events

4.2.1 Sampling Events

4.2.2 Elicitation Events

4.2.3 Roots Events

4.3 LLM Interaction Events

5. Interceptor Server Capabilities

6. Configuration and Context

7. Future Enhancements

7.1 Context Propagation and Interceptor State (Future)

Rationale

Design Decisions

Related Work and Inspiration

Backward Compatibility

Security Implications

Threat Model

Security Best Practices

Compliance Considerations

Reference Implementation

Multi-Language SDKs

Common Interceptor Sidecar

Acknowledgments

Metadata

Metadata

Assignees

Labels

Type

Projects

Milestone

Relationships

Development

Issue actions