diff --git a/packages/ack-id/eslint.config.js b/packages/ack-id/eslint.config.js
index 0979926..4299bdb 100644
--- a/packages/ack-id/eslint.config.js
+++ b/packages/ack-id/eslint.config.js
@@ -2,6 +2,14 @@
 
 import { config } from "@repo/eslint-config/base"
 
-export default config({
-  root: import.meta.dirname
-})
+export default [
+  ...config({
+    root: import.meta.dirname
+  }),
+  {
+    files: ["**/*.test.ts"],
+    rules: {
+      "@cspell/spellchecker": "off"
+    }
+  }
+]
diff --git a/packages/ack-id/src/discovery/base-discovery-service.ts b/packages/ack-id/src/discovery/base-discovery-service.ts
new file mode 100644
index 0000000..579ad25
--- /dev/null
+++ b/packages/ack-id/src/discovery/base-discovery-service.ts
@@ -0,0 +1,104 @@
+/* eslint-disable @typescript-eslint/no-unnecessary-condition */
+// This rule is disabled because TypeScript's type inference is too strict here.
+// The rule incorrectly flags our null checks and array length checks as unnecessary,
+// even though they're needed for runtime safety and code clarity.
+import type {
+  AgentDiscoveryService,
+  AgentRegistration,
+  DiscoveryFilter,
+  DiscoveryOptions,
+  DiscoveryResponse
+} from "./types"
+import type { DidUri } from "@agentcommercekit/did"
+
+/**
+ * Base class for implementing agent discovery services
+ */
+export abstract class BaseDiscoveryService implements AgentDiscoveryService {
+  /**
+   * Register an agent with the discovery service
+   */
+  abstract register(registration: AgentRegistration): Promise<void>
+
+  /**
+   * Update an agent's registration
+   */
+  abstract update(
+    did: DidUri,
+    registration: Partial<AgentRegistration>
+  ): Promise<void>
+
+  /**
+   * Deregister an agent from the discovery service
+   */
+  abstract deregister(did: DidUri): Promise<void>
+
+  /**
+   * Get a specific agent's registration
+   */
+  abstract get(did: DidUri): Promise<AgentRegistration | undefined>
+
+  /**
+   * Discover agents matching the given filters
+   */
+  abstract discover(
+    filter: DiscoveryFilter,
+    options?: DiscoveryOptions
+  ): Promise<DiscoveryResponse>
+
+  /**
+   * Check if an agent matches the given filter
+   */
+  protected matchesFilter(
+    agent: AgentRegistration,
+    filter: DiscoveryFilter
+  ): boolean {
+    // Check protocols
+    const protocols = filter.protocols ?? []
+    if (!protocols.every((p) => agent.capabilities.protocols.includes(p))) {
+      return false
+    }
+
+    // Check service types
+    const serviceTypes = filter.serviceTypes ?? []
+    if (
+      !serviceTypes.every((s) => agent.capabilities.serviceTypes.includes(s))
+    ) {
+      return false
+    }
+
+    // Check attributes
+    const attributes = filter.attributes ?? {}
+    return Object.entries(attributes).every(
+      ([key, value]) => agent.capabilities.attributes[key] === value
+    )
+  }
+
+  /**
+   * Check if an agent registration has expired
+   */
+  protected isExpired(agent: AgentRegistration): boolean {
+    return Date.now() > (agent.expiresAt ?? Number.MAX_SAFE_INTEGER)
+  }
+
+  /**
+   * Validate agent registration data
+   */
+  protected validateRegistration(registration: AgentRegistration): void {
+    // Ensure timestamp is present
+    registration.timestamp = registration.timestamp || Date.now()
+
+    // Ensure capabilities are present
+    if (!registration.capabilities) {
+      throw new Error("Agent capabilities are required")
+    }
+
+    // Initialize capabilities if not present
+    registration.capabilities.protocols =
+      registration.capabilities.protocols ?? []
+    registration.capabilities.serviceTypes =
+      registration.capabilities.serviceTypes ?? []
+    registration.capabilities.attributes =
+      registration.capabilities.attributes ?? {}
+  }
+}
diff --git a/packages/ack-id/src/discovery/memory-discovery-service.test.ts b/packages/ack-id/src/discovery/memory-discovery-service.test.ts
new file mode 100644
index 0000000..544fa10
--- /dev/null
+++ b/packages/ack-id/src/discovery/memory-discovery-service.test.ts
@@ -0,0 +1,109 @@
+import { describe, expect, test } from "vitest"
+import { MemoryDiscoveryService } from "./memory-discovery-service"
+import type { AgentRegistration } from "./types"
+
+describe("MemoryDiscoveryService", () => {
+  const createService = () => new MemoryDiscoveryService()
+  const createAgent = (did: string): AgentRegistration => ({
+    did: `did:web:${did}`,
+    timestamp: Date.now(),
+    capabilities: {
+      protocols: ["test"],
+      serviceTypes: ["test"],
+      attributes: {
+        test: "test"
+      }
+    }
+  })
+
+  test("registers and retrieves an agent", async () => {
+    const service = createService()
+    const agent = createAgent("example.com")
+
+    await service.register(agent)
+    const result = await service.get(agent.did)
+
+    expect(result).toEqual(agent)
+  })
+
+  test("updates an agent registration", async () => {
+    const service = createService()
+    const agent = createAgent("example.com")
+
+    await service.register(agent)
+    await service.update(agent.did, {
+      capabilities: {
+        protocols: ["test"],
+        serviceTypes: ["updated"],
+        attributes: {
+          test: "updated"
+        }
+      }
+    })
+
+    const result = await service.get(agent.did)
+    expect(result).toBeDefined()
+    expect(result).toMatchObject({
+      capabilities: {
+        protocols: ["test"],
+        serviceTypes: ["updated"],
+        attributes: { test: "updated" }
+      }
+    })
+  })
+
+  test("deregisters an agent", async () => {
+    const service = createService()
+    const agent = createAgent("example.com")
+
+    await service.register(agent)
+    await service.deregister(agent.did)
+
+    const result = await service.get(agent.did)
+    expect(result).toBeUndefined()
+  })
+
+  test("discovers agents by protocol", async () => {
+    const service = createService()
+    const agent1 = createAgent("example1.com")
+    const agent2 = createAgent("example2.com")
+    agent2.capabilities.protocols = ["other"]
+
+    await service.register(agent1)
+    await service.register(agent2)
+
+    const result = await service.discover({ protocols: ["test"] })
+    expect(result.total).toBe(1)
+    expect(result.agents[0]).toEqual(agent1)
+  })
+
+  test("discovers agents by service type", async () => {
+    const service = createService()
+    const agent1 = createAgent("example1.com")
+    const agent2 = createAgent("example2.com")
+    agent2.capabilities.serviceTypes = ["other"]
+
+    await service.register(agent1)
+    await service.register(agent2)
+
+    const result = await service.discover({ serviceTypes: ["test"] })
+    expect(result.total).toBe(1)
+    expect(result.agents[0]).toEqual(agent1)
+  })
+
+  test("discovers agents by attribute", async () => {
+    const service = createService()
+    const agent1 = createAgent("example1.com")
+    const agent2 = createAgent("example2.com")
+    agent2.capabilities.attributes = { test: "other" }
+
+    await service.register(agent1)
+    await service.register(agent2)
+
+    const result = await service.discover({
+      attributes: { test: "test" }
+    })
+    expect(result.total).toBe(1)
+    expect(result.agents[0]).toEqual(agent1)
+  })
+})
diff --git a/packages/ack-id/src/discovery/memory-discovery-service.ts b/packages/ack-id/src/discovery/memory-discovery-service.ts
new file mode 100644
index 0000000..6ad54e7
--- /dev/null
+++ b/packages/ack-id/src/discovery/memory-discovery-service.ts
@@ -0,0 +1,101 @@
+import { BaseDiscoveryService } from "./base-discovery-service"
+import type {
+  AgentRegistration,
+  DiscoveryFilter,
+  DiscoveryOptions,
+  DiscoveryResponse
+} from "./types"
+import type { DidUri } from "@agentcommercekit/did"
+
+/**
+ * In-memory implementation of the agent discovery service
+ */
+export class MemoryDiscoveryService extends BaseDiscoveryService {
+  private agents: Map<string, AgentRegistration>
+
+  constructor() {
+    super()
+    this.agents = new Map()
+  }
+
+  /**
+   * Register an agent with the discovery service
+   */
+  register(registration: AgentRegistration): Promise<void> {
+    this.validateRegistration(registration)
+    this.agents.set(registration.did, registration)
+    return Promise.resolve()
+  }
+
+  /**
+   * Update an agent's registration
+   */
+  async update(
+    did: DidUri,
+    registration: Partial<AgentRegistration>
+  ): Promise<void> {
+    const existing = await this.get(did)
+    if (!existing) {
+      throw new Error(`Agent ${did} not found`)
+    }
+
+    const updated: AgentRegistration = {
+      ...existing,
+      ...registration,
+      capabilities: {
+        protocols:
+          registration.capabilities?.protocols ??
+          existing.capabilities.protocols,
+        serviceTypes:
+          registration.capabilities?.serviceTypes ??
+          existing.capabilities.serviceTypes,
+        attributes: {
+          ...existing.capabilities.attributes,
+          ...registration.capabilities?.attributes
+        }
+      }
+    }
+
+    this.validateRegistration(updated)
+    this.agents.set(did, updated)
+  }
+
+  /**
+   * Deregister an agent from the discovery service
+   */
+  deregister(did: DidUri): Promise<void> {
+    this.agents.delete(did)
+    return Promise.resolve()
+  }
+
+  /**
+   * Get a specific agent's registration
+   */
+  get(did: DidUri): Promise<AgentRegistration | undefined> {
+    const agent = this.agents.get(did)
+    return Promise.resolve(agent ?? undefined)
+  }
+
+  /**
+   * Discover agents matching the given filters
+   */
+  discover(
+    filter: DiscoveryFilter,
+    options?: DiscoveryOptions
+  ): Promise<DiscoveryResponse> {
+    const { limit = 10, includeExpired = false } = options ?? {}
+    const agents = Array.from(this.agents.values())
+      .filter((agent) => {
+        if (!includeExpired && this.isExpired(agent)) {
+          return false
+        }
+        return this.matchesFilter(agent, filter)
+      })
+      .slice(0, limit)
+
+    return Promise.resolve({
+      agents,
+      total: agents.length
+    })
+  }
+}
diff --git a/packages/ack-id/src/discovery/schemas/valibot.ts b/packages/ack-id/src/discovery/schemas/valibot.ts
new file mode 100644
index 0000000..633dfdc
--- /dev/null
+++ b/packages/ack-id/src/discovery/schemas/valibot.ts
@@ -0,0 +1,57 @@
+import { didUriSchema } from "@agentcommercekit/did/schemas/valibot"
+import {
+  array,
+  boolean,
+  number,
+  object,
+  optional,
+  record,
+  string,
+  unknown
+} from "valibot"
+
+/**
+ * Schema for agent capabilities
+ */
+export const agentCapabilitiesSchema = object({
+  protocols: array(string()),
+  serviceTypes: array(string()),
+  attributes: record(string(), string())
+})
+
+/**
+ * Schema for agent registration
+ */
+export const agentRegistrationSchema = object({
+  did: didUriSchema,
+  capabilities: agentCapabilitiesSchema,
+  timestamp: number(),
+  expiresAt: optional(number())
+})
+
+/**
+ * Schema for discovery filter
+ */
+export const discoveryFilterSchema = object({
+  protocols: optional(array(string())),
+  serviceTypes: optional(array(string())),
+  attributes: optional(record(string(), string()))
+})
+
+/**
+ * Schema for discovery options
+ */
+export const discoveryOptionsSchema = object({
+  limit: optional(number()),
+  after: optional(string()),
+  includeExpired: optional(string())
+})
+
+/**
+ * Schema for discovery response
+ */
+export const discoveryResponseSchema = object({
+  agents: array(agentRegistrationSchema),
+  nextPage: optional(string()),
+  total: number()
+})
diff --git a/packages/ack-id/src/discovery/schemas/zod.ts b/packages/ack-id/src/discovery/schemas/zod.ts
new file mode 100644
index 0000000..44c93bf
--- /dev/null
+++ b/packages/ack-id/src/discovery/schemas/zod.ts
@@ -0,0 +1,48 @@
+import { didUriSchema } from "@agentcommercekit/did/schemas/zod"
+import { z } from "zod"
+
+/**
+ * Schema for agent capabilities
+ */
+export const agentCapabilitiesSchema = z.object({
+  protocols: z.array(z.string()),
+  serviceTypes: z.array(z.string()),
+  attributes: z.record(z.string(), z.unknown())
+})
+
+/**
+ * Schema for agent registration
+ */
+export const agentRegistrationSchema = z.object({
+  did: didUriSchema,
+  capabilities: agentCapabilitiesSchema,
+  timestamp: z.number(),
+  expiresAt: z.number().optional()
+})
+
+/**
+ * Schema for discovery filter
+ */
+export const discoveryFilterSchema = z.object({
+  protocols: z.array(z.string()).optional(),
+  serviceTypes: z.array(z.string()).optional(),
+  attributes: z.record(z.string(), z.unknown()).optional()
+})
+
+/**
+ * Schema for discovery options
+ */
+export const discoveryOptionsSchema = z.object({
+  limit: z.number().optional(),
+  after: z.string().optional(),
+  includeExpired: z.boolean().optional()
+})
+
+/**
+ * Schema for discovery response
+ */
+export const discoveryResponseSchema = z.object({
+  agents: z.array(agentRegistrationSchema),
+  nextPage: z.string().optional(),
+  total: z.number()
+})
diff --git a/packages/ack-id/src/discovery/types.ts b/packages/ack-id/src/discovery/types.ts
new file mode 100644
index 0000000..4cf01d4
--- /dev/null
+++ b/packages/ack-id/src/discovery/types.ts
@@ -0,0 +1,103 @@
+import type { DidUri } from "@agentcommercekit/did"
+
+/**
+ * Agent capabilities and attributes for discovery
+ */
+export interface AgentCapabilities {
+  /** List of supported protocols */
+  protocols: string[]
+  /** List of supported service types */
+  serviceTypes: string[]
+  /** Custom attributes for filtering */
+  attributes: Record<string, string>
+}
+
+/**
+ * Agent registration data
+ */
+export interface AgentRegistration {
+  /** Agent's DID */
+  did: DidUri
+  /** Agent capabilities */
+  capabilities: AgentCapabilities
+  /** Registration timestamp */
+  timestamp: number
+  /** Optional expiration timestamp */
+  expiresAt?: number
+}
+
+/**
+ * Query filters for discovering agents
+ */
+export interface DiscoveryFilter {
+  /** Required protocols */
+  protocols?: string[]
+  /** Required service types */
+  serviceTypes?: string[]
+  /** Required attributes */
+  attributes?: Record<string, string>
+}
+
+/**
+ * Discovery query options
+ */
+export interface DiscoveryOptions {
+  /** Maximum number of results */
+  limit?: number
+  /** Pagination token */
+  after?: string
+  /** Include expired registrations */
+  includeExpired?: boolean
+}
+
+/**
+ * Discovery query response
+ */
+export interface DiscoveryResponse {
+  /** Found agents */
+  agents: AgentRegistration[]
+  /** Next page token */
+  nextPage?: string
+  /** Total number of matching agents */
+  total: number
+}
+
+/**
+ * Interface for agent discovery service
+ */
+export interface AgentDiscoveryService {
+  /**
+   * Register an agent with the discovery service
+   * @param registration Agent registration data
+   */
+  register(registration: AgentRegistration): Promise<void>
+
+  /**
+   * Update an agent's registration
+   * @param did Agent's DID
+   * @param registration Updated registration data
+   */
+  update(did: DidUri, registration: Partial<AgentRegistration>): Promise<void>
+
+  /**
+   * Deregister an agent from the discovery service
+   * @param did Agent's DID
+   */
+  deregister(did: DidUri): Promise<void>
+
+  /**
+   * Discover agents matching the given filters
+   * @param filter Discovery filters
+   * @param options Query options
+   */
+  discover(
+    filter: DiscoveryFilter,
+    options?: DiscoveryOptions
+  ): Promise<DiscoveryResponse>
+
+  /**
+   * Get a specific agent's registration
+   * @param did Agent's DID
+   */
+  get(did: DidUri): Promise<AgentRegistration | undefined>
+}