diff --git a/.changeset/fix-validator-memory-leak.md b/.changeset/fix-validator-memory-leak.md
new file mode 100644
index 0000000000..93213a629b
--- /dev/null
+++ b/.changeset/fix-validator-memory-leak.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/core": patch
+---
+
+Cache compiled Ajv validator instances to prevent memory leak from repeated schema compilation
diff --git a/packages/core-internal/src/validators/ajvProvider.ts b/packages/core-internal/src/validators/ajvProvider.ts
index e33adb741f..0ef61e84d8 100644
--- a/packages/core-internal/src/validators/ajvProvider.ts
+++ b/packages/core-internal/src/validators/ajvProvider.ts
@@ -10,6 +10,8 @@ import _addFormats from 'ajv-formats';
 import { declaredDialect } from './dialects';
 import type { JsonSchemaType, JsonSchemaValidator, jsonSchemaValidator, JsonSchemaValidatorResult } from './types';
 
+const MAX_CACHE_SIZE = 1000;
+
 /** Structural subset of the AJV interface used by {@link AjvJsonSchemaValidator}. */
 interface AjvLike {
     compile: (schema: unknown) => AjvValidateFunction;
@@ -83,6 +85,19 @@ export class AjvJsonSchemaValidator implements jsonSchemaValidator {
     private _ajv2019: AjvLike | undefined;
     /** True iff the constructor received a caller-supplied engine; the `$schema` dispatch is skipped. */
     private readonly _userAjv: boolean;
+    /**
+     * Content-keyed cache for compiled validators of schemas without `$id`.
+     * Prevents the memory leak where `engine.compile(schema)` is called unconditionally,
+     * causing Ajv's internal scope to grow without bound in long-running processes.
+     *
+     * Capped at {@link MAX_CACHE_SIZE} entries. Servers with a fixed tool set (the
+     * overwhelmingly common shape) converge well below this limit. Beyond the cap,
+     * new schemas are compiled but not stored in this Map. Note that Ajv's internal
+     * scope still retains compilation metadata in that case — the cap bounds our
+     * external allocation, not Ajv's internals. Truly-distinct-per-request schemas
+     * are an antipattern regardless; this cache targets the fixed-set case.
+     */
+    private readonly _compiledCache: Map<string, AjvValidateFunction> = new Map();
 
     /**
      * @param ajv - Optional pre-configured AJV-compatible instance. When supplied, this instance is
@@ -128,12 +143,41 @@ export class AjvJsonSchemaValidator implements jsonSchemaValidator {
         return (this._ajvDraft7 ??= createDefaultAjvInstance(Draft7Ajv));
     }
 
+    /**
+     * Compile or retrieve a cached validator for the given schema.
+     *
+     * Schemas with `$id` use Ajv's built-in identity cache (`engine.getSchema`).
+     * Schemas without `$id` are cached by their JSON-serialised content to prevent
+     * unbounded growth of Ajv's internal scope in long-running processes (see #2605).
+     */
+    private _getCompiled(schema: JsonSchemaType, engine: AjvLike): AjvValidateFunction {
+        if ('$id' in schema && typeof schema.$id === 'string') {
+            return engine.getSchema(schema.$id) ?? engine.compile(schema);
+        }
+
+        let key: string;
+        try {
+            key = JSON.stringify(schema);
+        } catch {
+            // Non-serialisable schema (e.g. cyclic): fall back to uncached compilation.
+            return engine.compile(schema);
+        }
+
+        const cached = this._compiledCache.get(key);
+        if (cached !== undefined) {
+            return cached;
+        }
+
+        const compiled = engine.compile(JSON.parse(key));
+        if (this._compiledCache.size < MAX_CACHE_SIZE) {
+            this._compiledCache.set(key, compiled);
+        }
+        return compiled;
+    }
+
     getValidator<T>(schema: JsonSchemaType): JsonSchemaValidator<T> {
         const engine = this._engineFor(schema);
-        const ajvValidator =
-            '$id' in schema && typeof schema.$id === 'string'
-                ? (engine.getSchema(schema.$id) ?? engine.compile(schema))
-                : engine.compile(schema);
+        const ajvValidator = this._getCompiled(schema, engine);
 
         return (input: unknown): JsonSchemaValidatorResult<T> => {
             const valid = ajvValidator(input);
diff --git a/packages/core-internal/src/validators/cfWorkerProvider.ts b/packages/core-internal/src/validators/cfWorkerProvider.ts
index fe876bf9b6..c2ce887826 100644
--- a/packages/core-internal/src/validators/cfWorkerProvider.ts
+++ b/packages/core-internal/src/validators/cfWorkerProvider.ts
@@ -13,6 +13,8 @@ import { Validator } from '@cfworker/json-schema';
 import { declaredDialect } from './dialects';
 import type { JsonSchemaType, JsonSchemaValidator, jsonSchemaValidator, JsonSchemaValidatorResult } from './types';
 
+const MAX_CACHE_SIZE = 1000;
+
 /**
  * JSON Schema draft version supported by `@cfworker/json-schema`.
  */
@@ -52,6 +54,12 @@ export class CfWorkerJsonSchemaValidator implements jsonSchemaValidator {
     private readonly shortcircuit: boolean;
     /** Caller-supplied draft; when set, the `$schema` check is skipped (caller owns dialect). */
     private readonly draft?: CfWorkerSchemaDraft;
+    /**
+     * Content-keyed cache for compiled validators of schemas without `$id`.
+     * Prevents redundant Validator instantiation in long-running processes (see #2605).
+     * Capped at {@link MAX_CACHE_SIZE} entries.
+     */
+    private readonly _compiledCache: Map<string, Validator> = new Map();
 
     /**
      * Create a validator
@@ -77,18 +85,45 @@ export class CfWorkerJsonSchemaValidator implements jsonSchemaValidator {
         return dialect === 'draft-7' ? '7' : dialect;
     }
 
+    /**
+     * Retrieve or create a cached Validator instance for the given schema.
+     * Schemas are cached by their JSON-serialised content to prevent redundant
+     * instantiation in long-running processes (see #2605).
+     */
+    private _getValidator(schema: JsonSchemaType, draft: CfWorkerSchemaDraft): Validator {
+        let key: string;
+        try {
+            key = JSON.stringify(schema);
+        } catch {
+            // Non-serialisable schema: fall back to uncached instantiation.
+            return new Validator(schema as ConstructorParameters<typeof Validator>[0], draft, this.shortcircuit);
+        }
+
+        // Include draft in the key since the same schema content under different drafts
+        // may validate differently.
+        const cacheKey = `${draft}:${key}`;
+        const cached = this._compiledCache.get(cacheKey);
+        if (cached !== undefined) {
+            return cached;
+        }
+
+        const compiled = new Validator(JSON.parse(key) as ConstructorParameters<typeof Validator>[0], draft, this.shortcircuit);
+        if (this._compiledCache.size < MAX_CACHE_SIZE) {
+            this._compiledCache.set(cacheKey, compiled);
+        }
+        return compiled;
+    }
+
     /**
      * Create a validator for the given JSON Schema
      *
-     * Unlike AJV, this validator is not cached internally
-     *
      * @param schema - Standard JSON Schema object
      * @returns A validator function that validates input data
      */
     getValidator<T>(schema: JsonSchemaType): JsonSchemaValidator<T> {
         const draft = this.draft ?? this._draftFor(schema);
         // Cast to the cfworker Schema type - our JsonSchemaType is structurally compatible
-        const validator = new Validator(schema as ConstructorParameters<typeof Validator>[0], draft, this.shortcircuit);
+        const validator = this._getValidator(schema, draft);
 
         return (input: unknown): JsonSchemaValidatorResult<T> => {
             const result = validator.validate(input);
diff --git a/packages/core-internal/test/validators/validatorCaching.test.ts b/packages/core-internal/test/validators/validatorCaching.test.ts
new file mode 100644
index 0000000000..43b4b2f8a5
--- /dev/null
+++ b/packages/core-internal/test/validators/validatorCaching.test.ts
@@ -0,0 +1,206 @@
+/**
+ * Tests for validator caching behaviour (fixes #2605: memory leak from
+ * unconditional recompilation of schemas without `$id`).
+ */
+
+import { describe, expect, it } from 'vitest';
+
+import { AjvJsonSchemaValidator } from '../../src/validators/ajvProvider';
+import { CfWorkerJsonSchemaValidator } from '../../src/validators/cfWorkerProvider';
+import type { JsonSchemaType } from '../../src/validators/types';
+
+describe('AjvJsonSchemaValidator caching (#2605)', () => {
+    it('returns the same validator function for identical schemas without $id', () => {
+        const provider = new AjvJsonSchemaValidator();
+        const schema: JsonSchemaType = {
+            type: 'object',
+            properties: { name: { type: 'string' } },
+            required: ['name']
+        };
+
+        // Call getValidator twice with structurally identical (but different object) schemas
+        const v1 = provider.getValidator(schema);
+        const v2 = provider.getValidator({ ...schema });
+
+        // Both should validate correctly
+        expect(v1({ name: 'Alice' }).valid).toBe(true);
+        expect(v2({ name: 'Alice' }).valid).toBe(true);
+        expect(v1({}).valid).toBe(false);
+        expect(v2({}).valid).toBe(false);
+    });
+
+    it('does not recompile when called repeatedly with the same schema content', () => {
+        let compileCount = 0;
+        const fakeEngine = {
+            compile: (schema: unknown) => {
+                compileCount++;
+                return Object.assign(() => true, { errors: undefined });
+            },
+            getSchema: () => undefined,
+            errorsText: () => ''
+        };
+        const provider = new AjvJsonSchemaValidator(fakeEngine);
+        const schema: JsonSchemaType = { type: 'string' };
+
+        provider.getValidator(schema);
+        provider.getValidator(schema);
+        provider.getValidator({ type: 'string' });
+
+        // Should compile only once — subsequent calls use the cache
+        expect(compileCount).toBe(1);
+    });
+
+    it('recompiles when schema content changes', () => {
+        const provider = new AjvJsonSchemaValidator();
+
+        const v1 = provider.getValidator({ type: 'string' } as JsonSchemaType);
+        const v2 = provider.getValidator({ type: 'number' } as JsonSchemaType);
+
+        expect(v1('hello').valid).toBe(true);
+        expect(v1(42).valid).toBe(false);
+        expect(v2(42).valid).toBe(true);
+        expect(v2('hello').valid).toBe(false);
+    });
+
+    it('schemas with $id still use Ajv built-in identity cache', () => {
+        const provider = new AjvJsonSchemaValidator();
+        const schema: JsonSchemaType = {
+            $id: 'https://example.com/test-schema',
+            type: 'object',
+            properties: { x: { type: 'number' } }
+        };
+
+        const v1 = provider.getValidator(schema);
+        const v2 = provider.getValidator(schema);
+
+        expect(v1({ x: 1 }).valid).toBe(true);
+        expect(v2({ x: 1 }).valid).toBe(true);
+        expect(v1({ x: 'nope' }).valid).toBe(false);
+    });
+
+    it('a mutated schema object produces a validator for the new content', () => {
+        const provider = new AjvJsonSchemaValidator();
+        const schema: JsonSchemaType = { type: 'string' };
+
+        const v1 = provider.getValidator(schema);
+        expect(v1('hello').valid).toBe(true);
+        expect(v1(42).valid).toBe(false);
+
+        // Mutate in place
+        (schema as Record<string, unknown>).type = 'number';
+
+        const v2 = provider.getValidator(schema);
+        expect(v2(42).valid).toBe(true);
+        expect(v2('hello').valid).toBe(false);
+    });
+
+    it('shared cached validator does not have cross-call error pollution', () => {
+        const provider = new AjvJsonSchemaValidator();
+        const schema: JsonSchemaType = {
+            type: 'object',
+            properties: { name: { type: 'string' } },
+            required: ['name']
+        };
+
+        const v1 = provider.getValidator(schema);
+        const v2 = provider.getValidator(schema);
+
+        // Validate with invalid data on v1
+        const r1 = v1({});
+        expect(r1.valid).toBe(false);
+        expect(r1.errorMessage).toBeDefined();
+
+        // v2 should still validate correctly (not inheriting errors from v1)
+        const r2 = v2({ name: 'Bob' });
+        expect(r2.valid).toBe(true);
+    });
+
+    it('works correctly across different dialect schemas', () => {
+        const provider = new AjvJsonSchemaValidator();
+
+        const schema2020: JsonSchemaType = {
+            $schema: 'https://json-schema.org/draft/2020-12/schema',
+            type: 'string'
+        };
+        const schema07: JsonSchemaType = {
+            $schema: 'http://json-schema.org/draft-07/schema#',
+            type: 'string'
+        };
+
+        const v2020 = provider.getValidator(schema2020);
+        const v07 = provider.getValidator(schema07);
+
+        expect(v2020('hello').valid).toBe(true);
+        expect(v07('hello').valid).toBe(true);
+    });
+
+    it('degrades gracefully when cache size cap is exceeded', () => {
+        const provider = new AjvJsonSchemaValidator();
+
+        // Generate 1010 distinct schemas (exceeding the 1000-entry cap)
+        for (let i = 0; i < 1010; i++) {
+            const schema: JsonSchemaType = {
+                type: 'object',
+                properties: { [`field_${i}`]: { type: 'string' } }
+            };
+            const v = provider.getValidator(schema);
+            expect(v({ [`field_${i}`]: 'val' }).valid).toBe(true);
+        }
+
+        // The first 1000 should still be cached (validate correctly)
+        const earlySchema: JsonSchemaType = {
+            type: 'object',
+            properties: { field_0: { type: 'string' } }
+        };
+        const v = provider.getValidator(earlySchema);
+        expect(v({ field_0: 'hello' }).valid).toBe(true);
+        expect(v({ field_0: 123 }).valid).toBe(false);
+    });
+});
+
+describe('CfWorkerJsonSchemaValidator caching (#2605)', () => {
+    it('returns the same validation result for identical schemas without $id', () => {
+        const provider = new CfWorkerJsonSchemaValidator();
+        const schema: JsonSchemaType = {
+            type: 'object',
+            properties: { name: { type: 'string' } },
+            required: ['name']
+        };
+
+        const v1 = provider.getValidator(schema);
+        const v2 = provider.getValidator({ ...schema });
+
+        expect(v1({ name: 'Alice' }).valid).toBe(true);
+        expect(v2({ name: 'Alice' }).valid).toBe(true);
+        expect(v1({}).valid).toBe(false);
+        expect(v2({}).valid).toBe(false);
+    });
+
+    it('recompiles when schema content changes', () => {
+        const provider = new CfWorkerJsonSchemaValidator();
+
+        const v1 = provider.getValidator({ type: 'string' } as JsonSchemaType);
+        const v2 = provider.getValidator({ type: 'number' } as JsonSchemaType);
+
+        expect(v1('hello').valid).toBe(true);
+        expect(v1(42).valid).toBe(false);
+        expect(v2(42).valid).toBe(true);
+        expect(v2('hello').valid).toBe(false);
+    });
+
+    it('caches per draft — same content with different drafts validates differently', () => {
+        const provider = new CfWorkerJsonSchemaValidator();
+
+        // A schema with prefixItems: under 2020-12 it's enforced, under draft-07 it's ignored
+        const schema2020: JsonSchemaType = {
+            $schema: 'https://json-schema.org/draft/2020-12/schema',
+            type: 'array',
+            prefixItems: [{ type: 'number' }, { type: 'string' }]
+        };
+
+        const v2020 = provider.getValidator(schema2020);
+        // prefixItems is enforced under 2020-12
+        expect(v2020([1, 'x']).valid).toBe(true);
+        expect(v2020(['x', 1]).valid).toBe(false);
+    });
+});