diff --git a/.gitignore b/.gitignore
index 6c4bf1a6..694735b6 100644
--- a/.gitignore
+++ b/.gitignore
@@ -69,6 +69,9 @@ web_modules/
 # Output of 'npm pack'
 *.tgz
 
+# Output of 'npm run fetch:spec-types'
+spec.types.ts
+
 # Yarn Integrity file
 .yarn-integrity
 
diff --git a/package.json b/package.json
index 24ba826b..894081d7 100644
--- a/package.json
+++ b/package.json
@@ -35,6 +35,7 @@
     "dist"
   ],
   "scripts": {
+    "fetch:spec-types": "curl -o spec.types.ts https://raw.githubusercontent.com/modelcontextprotocol/modelcontextprotocol/refs/heads/main/schema/draft/schema.ts",
     "build": "npm run build:esm && npm run build:cjs",
     "build:esm": "mkdir -p dist/esm && echo '{\"type\": \"module\"}' > dist/esm/package.json && tsc -p tsconfig.prod.json",
     "build:esm:w": "npm run build:esm -- -w",
@@ -43,7 +44,7 @@
     "examples:simple-server:w": "tsx --watch src/examples/server/simpleStreamableHttp.ts --oauth",
     "prepack": "npm run build:esm && npm run build:cjs",
     "lint": "eslint src/",
-    "test": "jest",
+    "test": "npm run fetch:spec-types && jest",
     "start": "npm run server",
     "server": "tsx watch --clear-screen=false src/cli.ts server",
     "client": "tsx src/cli.ts client"
diff --git a/src/spec.types.test.ts b/src/spec.types.test.ts
new file mode 100644
index 00000000..09cd6c2d
--- /dev/null
+++ b/src/spec.types.test.ts
@@ -0,0 +1,705 @@
+/**
+ * This contains:
+ * - Static type checks to verify the Spec's types are compatible with the SDK's types
+ *   (mutually assignable, w/ slight affordances to get rid of ZodObject.passthrough() index signatures, etc)
+ * - Runtime checks to verify each Spec type has a static check
+ *   (note: a few don't have SDK types, see MISSING_SDK_TYPES below)
+ */
+import * as SDKTypes from "./types.js";
+import * as SpecTypes from "../spec.types.js";
+import fs from "node:fs";
+
+/* eslint-disable @typescript-eslint/no-unused-vars */
+/* eslint-disable @typescript-eslint/no-unsafe-function-type */
+
+// Removes index signatures added by ZodObject.passthrough().
+type RemovePassthrough<T> = T extends object
+  ? T extends Array<infer U>
+    ? Array<RemovePassthrough<U>>
+    : T extends Function
+        ? T
+        : {[K in keyof T as string extends K ? never : K]: RemovePassthrough<T[K]>}
+    : T;
+
+type IsUnknown<T> = [unknown] extends [T] ? [T] extends [unknown] ? true : false : false;
+
+// Turns {x?: unknown} into {x: unknown} but keeps {_meta?: unknown} unchanged (and leaves other optional properties unchanged, e.g. {x?: string}).
+// This works around an apparent quirk of ZodObject.unknown() (makes fields optional)
+type MakeUnknownsNotOptional<T> =
+  IsUnknown<T> extends true
+    ? unknown
+    : (T extends object
+      ? (T extends Array<infer U>
+        ? Array<MakeUnknownsNotOptional<U>>
+        : (T extends Function
+          ? T
+          : Pick<T, never> & {
+            // Start with empty object to avoid duplicates
+            // Make unknown properties required (except _meta)
+            [K in keyof T as '_meta' extends K ? never : IsUnknown<T[K]> extends true ? K : never]-?: unknown;
+          } &
+          Pick<T, {
+            // Pick all _meta and non-unknown properties with original modifiers
+            [K in keyof T]: '_meta' extends K ? K : IsUnknown<T[K]> extends true ? never : K
+          }[keyof T]> & {
+            // Recurse on the picked properties
+            [K in keyof Pick<T, {[K in keyof T]: '_meta' extends K ? K : IsUnknown<T[K]> extends true ? never : K}[keyof T]>]: MakeUnknownsNotOptional<T[K]>
+          }))
+      : T);
+
+function checkCancelledNotification(
+  sdk: SDKTypes.CancelledNotification,
+  spec: SpecTypes.CancelledNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkBaseMetadata(
+  sdk: RemovePassthrough<SDKTypes.BaseMetadata>,
+  spec: SpecTypes.BaseMetadata
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkImplementation(
+  sdk: RemovePassthrough<SDKTypes.Implementation>,
+  spec: SpecTypes.Implementation
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkProgressNotification(
+  sdk: SDKTypes.ProgressNotification,
+  spec: SpecTypes.ProgressNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+
+function checkSubscribeRequest(
+  sdk: SDKTypes.SubscribeRequest,
+  spec: SpecTypes.SubscribeRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkUnsubscribeRequest(
+  sdk: SDKTypes.UnsubscribeRequest,
+  spec: SpecTypes.UnsubscribeRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkPaginatedRequest(
+  sdk: SDKTypes.PaginatedRequest,
+  spec: SpecTypes.PaginatedRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkPaginatedResult(
+  sdk: SDKTypes.PaginatedResult,
+  spec: SpecTypes.PaginatedResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListRootsRequest(
+  sdk: SDKTypes.ListRootsRequest,
+  spec: SpecTypes.ListRootsRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListRootsResult(
+  sdk: RemovePassthrough<SDKTypes.ListRootsResult>,
+  spec: SpecTypes.ListRootsResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkRoot(
+  sdk: RemovePassthrough<SDKTypes.Root>,
+  spec: SpecTypes.Root
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkElicitRequest(
+  sdk: RemovePassthrough<SDKTypes.ElicitRequest>,
+  spec: SpecTypes.ElicitRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkElicitResult(
+  sdk: RemovePassthrough<SDKTypes.ElicitResult>,
+  spec: SpecTypes.ElicitResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkCompleteRequest(
+  sdk: RemovePassthrough<SDKTypes.CompleteRequest>,
+  spec: SpecTypes.CompleteRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkCompleteResult(
+  sdk: SDKTypes.CompleteResult,
+  spec: SpecTypes.CompleteResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkProgressToken(
+  sdk: SDKTypes.ProgressToken,
+  spec: SpecTypes.ProgressToken
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkCursor(
+  sdk: SDKTypes.Cursor,
+  spec: SpecTypes.Cursor
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkRequest(
+  sdk: SDKTypes.Request,
+  spec: SpecTypes.Request
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkResult(
+  sdk: SDKTypes.Result,
+  spec: SpecTypes.Result
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkRequestId(
+  sdk: SDKTypes.RequestId,
+  spec: SpecTypes.RequestId
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkJSONRPCRequest(
+  sdk: SDKTypes.JSONRPCRequest,
+  spec: SpecTypes.JSONRPCRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkJSONRPCNotification(
+  sdk: SDKTypes.JSONRPCNotification,
+  spec: SpecTypes.JSONRPCNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkJSONRPCResponse(
+  sdk: SDKTypes.JSONRPCResponse,
+  spec: SpecTypes.JSONRPCResponse
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkEmptyResult(
+  sdk: SDKTypes.EmptyResult,
+  spec: SpecTypes.EmptyResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkNotification(
+  sdk: SDKTypes.Notification,
+  spec: SpecTypes.Notification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkClientResult(
+  sdk: SDKTypes.ClientResult,
+  spec: SpecTypes.ClientResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkClientNotification(
+  sdk: SDKTypes.ClientNotification,
+  spec: SpecTypes.ClientNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkServerResult(
+  sdk: SDKTypes.ServerResult,
+  spec: SpecTypes.ServerResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkResourceTemplateReference(
+  sdk: RemovePassthrough<SDKTypes.ResourceTemplateReference>,
+  spec: SpecTypes.ResourceTemplateReference
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkPromptReference(
+  sdk: RemovePassthrough<SDKTypes.PromptReference>,
+  spec: SpecTypes.PromptReference
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkToolAnnotations(
+  sdk: RemovePassthrough<SDKTypes.ToolAnnotations>,
+  spec: SpecTypes.ToolAnnotations
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkTool(
+  sdk: RemovePassthrough<SDKTypes.Tool>,
+  spec: SpecTypes.Tool
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListToolsRequest(
+  sdk: SDKTypes.ListToolsRequest,
+  spec: SpecTypes.ListToolsRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListToolsResult(
+  sdk: RemovePassthrough<SDKTypes.ListToolsResult>,
+  spec: SpecTypes.ListToolsResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkCallToolResult(
+  sdk: RemovePassthrough<SDKTypes.CallToolResult>,
+  spec: SpecTypes.CallToolResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkCallToolRequest(
+  sdk: SDKTypes.CallToolRequest,
+  spec: SpecTypes.CallToolRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkToolListChangedNotification(
+  sdk: SDKTypes.ToolListChangedNotification,
+  spec: SpecTypes.ToolListChangedNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkResourceListChangedNotification(
+  sdk: SDKTypes.ResourceListChangedNotification,
+  spec: SpecTypes.ResourceListChangedNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkPromptListChangedNotification(
+  sdk: SDKTypes.PromptListChangedNotification,
+  spec: SpecTypes.PromptListChangedNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkRootsListChangedNotification(
+  sdk: SDKTypes.RootsListChangedNotification,
+  spec: SpecTypes.RootsListChangedNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkResourceUpdatedNotification(
+  sdk: SDKTypes.ResourceUpdatedNotification,
+  spec: SpecTypes.ResourceUpdatedNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkSamplingMessage(
+  sdk: RemovePassthrough<SDKTypes.SamplingMessage>,
+  spec: SpecTypes.SamplingMessage
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkCreateMessageResult(
+  sdk: RemovePassthrough<SDKTypes.CreateMessageResult>,
+  spec: SpecTypes.CreateMessageResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkSetLevelRequest(
+  sdk: SDKTypes.SetLevelRequest,
+  spec: SpecTypes.SetLevelRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkPingRequest(
+  sdk: SDKTypes.PingRequest,
+  spec: SpecTypes.PingRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkInitializedNotification(
+  sdk: SDKTypes.InitializedNotification,
+  spec: SpecTypes.InitializedNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListResourcesRequest(
+  sdk: SDKTypes.ListResourcesRequest,
+  spec: SpecTypes.ListResourcesRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListResourcesResult(
+  sdk: RemovePassthrough<SDKTypes.ListResourcesResult>,
+  spec: SpecTypes.ListResourcesResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListResourceTemplatesRequest(
+  sdk: SDKTypes.ListResourceTemplatesRequest,
+  spec: SpecTypes.ListResourceTemplatesRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListResourceTemplatesResult(
+  sdk: RemovePassthrough<SDKTypes.ListResourceTemplatesResult>,
+  spec: SpecTypes.ListResourceTemplatesResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkReadResourceRequest(
+  sdk: SDKTypes.ReadResourceRequest,
+  spec: SpecTypes.ReadResourceRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkReadResourceResult(
+  sdk: RemovePassthrough<SDKTypes.ReadResourceResult>,
+  spec: SpecTypes.ReadResourceResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkResourceContents(
+  sdk: RemovePassthrough<SDKTypes.ResourceContents>,
+  spec: SpecTypes.ResourceContents
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkTextResourceContents(
+  sdk: RemovePassthrough<SDKTypes.TextResourceContents>,
+  spec: SpecTypes.TextResourceContents
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkBlobResourceContents(
+  sdk: RemovePassthrough<SDKTypes.BlobResourceContents>,
+  spec: SpecTypes.BlobResourceContents
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkResource(
+  sdk: RemovePassthrough<SDKTypes.Resource>,
+  spec: SpecTypes.Resource
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkResourceTemplate(
+  sdk: RemovePassthrough<SDKTypes.ResourceTemplate>,
+  spec: SpecTypes.ResourceTemplate
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkPromptArgument(
+  sdk: RemovePassthrough<SDKTypes.PromptArgument>,
+  spec: SpecTypes.PromptArgument
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkPrompt(
+  sdk: RemovePassthrough<SDKTypes.Prompt>,
+  spec: SpecTypes.Prompt
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListPromptsRequest(
+  sdk: SDKTypes.ListPromptsRequest,
+  spec: SpecTypes.ListPromptsRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkListPromptsResult(
+  sdk: RemovePassthrough<SDKTypes.ListPromptsResult>,
+  spec: SpecTypes.ListPromptsResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkGetPromptRequest(
+  sdk: SDKTypes.GetPromptRequest,
+  spec: SpecTypes.GetPromptRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkTextContent(
+  sdk: RemovePassthrough<SDKTypes.TextContent>,
+  spec: SpecTypes.TextContent
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkImageContent(
+  sdk: RemovePassthrough<SDKTypes.ImageContent>,
+  spec: SpecTypes.ImageContent
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkAudioContent(
+  sdk: RemovePassthrough<SDKTypes.AudioContent>,
+  spec: SpecTypes.AudioContent
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkEmbeddedResource(
+  sdk: RemovePassthrough<SDKTypes.EmbeddedResource>,
+  spec: SpecTypes.EmbeddedResource
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkResourceLink(
+  sdk: RemovePassthrough<SDKTypes.ResourceLink>,
+  spec: SpecTypes.ResourceLink
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkContentBlock(
+  sdk: RemovePassthrough<SDKTypes.ContentBlock>,
+  spec: SpecTypes.ContentBlock
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkPromptMessage(
+  sdk: RemovePassthrough<SDKTypes.PromptMessage>,
+  spec: SpecTypes.PromptMessage
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkGetPromptResult(
+  sdk: RemovePassthrough<SDKTypes.GetPromptResult>,
+  spec: SpecTypes.GetPromptResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkBooleanSchema(
+  sdk: RemovePassthrough<SDKTypes.BooleanSchema>,
+  spec: SpecTypes.BooleanSchema
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkStringSchema(
+  sdk: RemovePassthrough<SDKTypes.StringSchema>,
+  spec: SpecTypes.StringSchema
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkNumberSchema(
+  sdk: RemovePassthrough<SDKTypes.NumberSchema>,
+  spec: SpecTypes.NumberSchema
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkEnumSchema(
+  sdk: RemovePassthrough<SDKTypes.EnumSchema>,
+  spec: SpecTypes.EnumSchema
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkPrimitiveSchemaDefinition(
+  sdk: RemovePassthrough<SDKTypes.PrimitiveSchemaDefinition>,
+  spec: SpecTypes.PrimitiveSchemaDefinition
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkJSONRPCError(
+  sdk: SDKTypes.JSONRPCError,
+  spec: SpecTypes.JSONRPCError
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkJSONRPCMessage(
+  sdk: SDKTypes.JSONRPCMessage,
+  spec: SpecTypes.JSONRPCMessage
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkCreateMessageRequest(
+  sdk: RemovePassthrough<SDKTypes.CreateMessageRequest>,
+  spec: SpecTypes.CreateMessageRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkInitializeRequest(
+  sdk: RemovePassthrough<SDKTypes.InitializeRequest>,
+  spec: SpecTypes.InitializeRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkInitializeResult(
+  sdk: RemovePassthrough<SDKTypes.InitializeResult>,
+  spec: SpecTypes.InitializeResult
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkClientCapabilities(
+  sdk: RemovePassthrough<SDKTypes.ClientCapabilities>,
+  spec: SpecTypes.ClientCapabilities
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkServerCapabilities(
+  sdk: RemovePassthrough<SDKTypes.ServerCapabilities>,
+  spec: SpecTypes.ServerCapabilities
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkClientRequest(
+  sdk: RemovePassthrough<SDKTypes.ClientRequest>,
+  spec: SpecTypes.ClientRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkServerRequest(
+  sdk: RemovePassthrough<SDKTypes.ServerRequest>,
+  spec: SpecTypes.ServerRequest
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkLoggingMessageNotification(
+  sdk: MakeUnknownsNotOptional<SDKTypes.LoggingMessageNotification>,
+  spec: SpecTypes.LoggingMessageNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkServerNotification(
+  sdk: MakeUnknownsNotOptional<SDKTypes.ServerNotification>,
+  spec: SpecTypes.ServerNotification
+) {
+  sdk = spec;
+  spec = sdk;
+}
+function checkLoggingLevel(
+  sdk: SDKTypes.LoggingLevel,
+  spec: SpecTypes.LoggingLevel
+) {
+  sdk = spec;
+  spec = sdk;
+}
+
+// This file is .gitignore'd, and fetched by `npm run fetch:spec-types` (called by `npm run test`)
+const SPEC_TYPES_FILE  = 'spec.types.ts';
+const SDK_TYPES_FILE  = 'src/types.ts';
+
+const MISSING_SDK_TYPES = [
+  // These are inlined in the SDK:
+  'Role',
+
+  // These aren't supported by the SDK yet:
+  // TODO: Add definitions to the SDK
+  'Annotations',
+  'ModelHint',
+  'ModelPreferences',
+]
+
+function extractExportedTypes(source: string): string[] {
+  return [...source.matchAll(/export\s+(?:interface|class|type)\s+(\w+)\b/g)].map(m => m[1]);
+}
+
+describe('Spec Types', () => {
+  const specTypes = extractExportedTypes(fs.readFileSync(SPEC_TYPES_FILE, 'utf-8'));
+  const sdkTypes = extractExportedTypes(fs.readFileSync(SDK_TYPES_FILE, 'utf-8'));
+  const testSource = fs.readFileSync(__filename, 'utf-8');
+
+  it('should define some expected types', () => {
+    expect(specTypes).toContain('JSONRPCNotification');
+    expect(specTypes).toContain('ElicitResult');
+    expect(specTypes).toHaveLength(91);
+  });
+
+  it('should have up to date list of missing sdk types', () => {
+    for (const typeName of MISSING_SDK_TYPES) {
+      expect(sdkTypes).not.toContain(typeName);
+    }
+  });
+
+  for (const type of specTypes) {
+    if (MISSING_SDK_TYPES.includes(type)) {
+      continue; // Skip missing SDK types
+    }
+    it(`${type} should have a compatibility test`, () => {
+      expect(testSource).toContain(`function check${type}(`);
+    });
+  }
+});