diff --git a/packages/core/src/mcp/instructions.ts b/packages/core/src/mcp/instructions.ts
index e5865de69f3b..6c0be1dc737f 100644
--- a/packages/core/src/mcp/instructions.ts
+++ b/packages/core/src/mcp/instructions.ts
@@ -17,7 +17,7 @@ type Summary = typeof Summary.Type
 const entries = (servers: ReadonlyArray<Summary>) =>
   servers.flatMap((server) => [
     `  <server name="${server.server}">`,
-    `    Use tools from this server through \`execute\` under \`tools[${JSON.stringify(McpTool.group(server.server))}]\`.`,
+    `    Use tools from this server through \`execute\` under \`tools[${JSON.stringify(McpTool.namespace(server.server))}]\`.`,
     ...server.instructions.split("\n").map((line) => `    ${line}`),
     "  </server>",
   ])
diff --git a/packages/core/src/tool/AGENTS.md b/packages/core/src/tool/AGENTS.md
index 8e74bd5bb3af..beb838a567e6 100644
--- a/packages/core/src/tool/AGENTS.md
+++ b/packages/core/src/tool/AGENTS.md
@@ -29,7 +29,7 @@ Leaves own resolution, permission, and side-effect ordering. Translate only expe
 ## Registration
 
 Built-ins and plugin tools register through `Tools.Service.register({ [name]: tool })`. Registrations may provide a
-group, which flattens direct model names to `<group>_<tool>`, and default into CodeMode (`codemode` defaults true;
+namespace, which flattens direct model names to `<namespace>_<tool>`, and default into CodeMode (`codemode` defaults true;
 `codemode: false` keeps the tool on the provider's native tool list).
 
 Registrations are scoped:
diff --git a/packages/core/src/tool/execute.ts b/packages/core/src/tool/execute.ts
index 0a6222377090..e47adbba4caf 100644
--- a/packages/core/src/tool/execute.ts
+++ b/packages/core/src/tool/execute.ts
@@ -39,7 +39,7 @@ type CollectedFiles = {
 interface Registration {
   readonly tool: AnyTool
   readonly name: string
-  readonly group?: string
+  readonly namespace?: string
 }
 
 export const create = (registrations: ReadonlyMap<string, Registration>) => {
@@ -47,7 +47,7 @@ export const create = (registrations: ReadonlyMap<string, Registration>) => {
     invoke: (name: string, registration: Registration, input: unknown) => Effect.Effect<unknown, unknown>,
     hooks?: CodeMode.ToolCallHooks,
   ) => {
-    const tools: Record<string, Tool.Definition<never> | Record<string, Tool.Definition<never>>> = {}
+    const tools: Record<string, Tool.Definition<never>> = {}
     for (const [name, registration] of registrations) {
       const child = definition(name, registration.tool)
       const value = Tool.make({
@@ -56,24 +56,8 @@ export const create = (registrations: ReadonlyMap<string, Registration>) => {
         output: child.outputSchema,
         run: (input) => invoke(name, registration, input),
       })
-      if (registration.group === undefined) {
-        const path = registration.name
-        if (Object.hasOwn(tools, path)) throw new TypeError(`CodeMode tool namespace conflict: ${path}`)
-        tools[path] = value
-        continue
-      }
-      const path = registration.name
-      const namespace = registration.group
-      const group = tools[namespace]
-      if (group && Tool.isDefinition(group)) throw new TypeError(`CodeMode tool namespace conflict: ${namespace}`)
-      if (group) {
-        if (Object.hasOwn(group, path)) throw new TypeError(`CodeMode tool namespace conflict: ${namespace}.${path}`)
-        group[path] = value
-        continue
-      }
-      const entries: Record<string, Tool.Definition<never>> = {}
-      entries[path] = value
-      tools[namespace] = entries
+      const path = registration.namespace === undefined ? registration.name : `${registration.namespace}.${registration.name}`
+      tools[path] = value
     }
     return CodeMode.make<typeof tools>({ tools, ...hooks })
   }
diff --git a/packages/core/src/tool/mcp.ts b/packages/core/src/tool/mcp.ts
index a636671f6077..3263ee41f9cf 100644
--- a/packages/core/src/tool/mcp.ts
+++ b/packages/core/src/tool/mcp.ts
@@ -13,10 +13,11 @@ import { Tools } from "./tools"
 import { ToolRegistry } from "./registry"
 
 /**
- * Registry group and permission action names for MCP tools.
+ * Registry namespace and permission action names for MCP tools.
  */
-export const group = (server: string) => server.replace(/[^a-zA-Z0-9_-]/g, "_")
-export const name = (server: string, tool: string) => `${group(server)}_${tool.replace(/[^a-zA-Z0-9_-]/g, "_")}`
+export const namespace = (server: string) => server.replace(/[^a-zA-Z0-9_-]/g, "_")
+export const name = (server: string, tool: string) =>
+  `${namespace(server)}_${tool.replace(/[^a-zA-Z0-9_-]/g, "_")}`
 
 export const layer = Layer.effectDiscard(
   Effect.gen(function* () {
@@ -107,7 +108,7 @@ export const layer = Layer.effectDiscard(
         const next = yield* Scope.fork(scope)
         yield* Effect.forEach(
           groups,
-          ([group, record]) => tools.register(record, { group }),
+          ([server, record]) => tools.register(record, { namespace: namespace(server) }),
           {
             discard: true,
           },
diff --git a/packages/core/src/tool/registry.ts b/packages/core/src/tool/registry.ts
index 8d95a95eafd5..844e4896db39 100644
--- a/packages/core/src/tool/registry.ts
+++ b/packages/core/src/tool/registry.ts
@@ -10,7 +10,15 @@ import { SessionSchema } from "../session/schema"
 import { ToolOutputStore } from "../tool-output-store"
 import { Wildcard } from "../util/wildcard"
 import { ExecuteTool } from "./execute"
-import { definition, permission, registrationEntries, RegistrationError, settle, type AnyTool } from "./tool"
+import {
+  definition,
+  permission,
+  registrationEntries,
+  RegistrationError,
+  settle,
+  validateNamespace,
+  type AnyTool,
+} from "./tool"
 import { Tools } from "./tools"
 import { ToolHooks } from "./hooks"
 import { makeLocationNode } from "../effect/app-node"
@@ -95,7 +103,7 @@ const registryLayer = Layer.effect(
     type Registration = {
       readonly tool: AnyTool
       readonly name: string
-      readonly group?: string
+      readonly namespace?: string
       readonly codemode: boolean
     }
     const local = new Map<string, Array<{ readonly token: object; readonly registration: Registration }>>()
@@ -186,7 +194,8 @@ const registryLayer = Layer.effect(
 
     return Service.of({
       register: Effect.fn("ToolRegistry.register")(function* (tools, options) {
-        const entries = registrationEntries(tools, options?.group)
+        if (options?.namespace !== undefined) yield* validateNamespace(options.namespace)
+        const entries = registrationEntries(tools, options?.namespace)
         if (entries.length === 0) return
         const codemode = options?.codemode ?? true
         const reserved = codemode ? undefined : entries.find((entry) => entry.key === "execute")
@@ -205,7 +214,7 @@ const registryLayer = Layer.effect(
                   registration: {
                     tool: entry.tool,
                     name: entry.name,
-                    group: entry.group,
+                    namespace: entry.namespace,
                     codemode,
                   },
                 },
diff --git a/packages/core/test/mcp.test.ts b/packages/core/test/mcp.test.ts
index fd6f99f53848..015bd0e541ff 100644
--- a/packages/core/test/mcp.test.ts
+++ b/packages/core/test/mcp.test.ts
@@ -268,6 +268,7 @@ describe("MCP errors", () => {
 })
 
 test("MCP tool names match V1 sanitization", () => {
+  expect(McpTool.namespace("context 7")).toBe("context_7")
   expect(McpTool.name("context 7", "resolve.library/id")).toBe("context_7_resolve_library_id")
 })
 
diff --git a/packages/core/test/plugin.test.ts b/packages/core/test/plugin.test.ts
index 9ff717d66576..0d19e4e74b96 100644
--- a/packages/core/test/plugin.test.ts
+++ b/packages/core/test/plugin.test.ts
@@ -274,7 +274,7 @@ describe("PluginV2", () => {
     }),
   )
 
-  it.effect("groups tool names and routes codemode registrations through execute", () =>
+  it.effect("namespaces tool names and routes codemode registrations through execute", () =>
     Effect.gen(function* () {
       const plugins = yield* PluginV2.Service
       const registry = yield* ToolRegistry.Service
@@ -291,8 +291,8 @@ describe("PluginV2", () => {
           ctx.tool
             .transform((draft) => {
               draft.add("plain", tool("Plain"), { codemode: false })
-              draft.add("look/up", tool("Lookup"), { group: "context 7", codemode: false })
-              draft.add("search", tool("Search"), { group: "context 7" })
+              draft.add("look/up", tool("Lookup"), { namespace: "context7", codemode: false })
+              draft.add("search", tool("Search"), { namespace: "context7" })
             })
             .pipe(Effect.orDie),
       })
@@ -301,7 +301,7 @@ describe("PluginV2", () => {
 
       expect((yield* registry.materialize()).definitions.map((tool) => tool.name)).toEqual([
         "plain",
-        "context_7_look_up",
+        "context7_look_up",
         "execute",
       ])
     }),
diff --git a/packages/core/test/session-runner-tool-registry.test.ts b/packages/core/test/session-runner-tool-registry.test.ts
index 6cb012312b93..6f7b13f6e14a 100644
--- a/packages/core/test/session-runner-tool-registry.test.ts
+++ b/packages/core/test/session-runner-tool-registry.test.ts
@@ -84,6 +84,17 @@ const constant = (text: string) =>
   })
 
 describe("ToolRegistry", () => {
+  it.effect("rejects invalid dotted namespaces", () =>
+    Effect.gen(function* () {
+      const service = yield* ToolRegistry.Service
+      const error = yield* service.register({ echo: make() }, { namespace: "slack..admin" }).pipe(Effect.flip)
+
+      expect(error).toBeInstanceOf(Tool.RegistrationError)
+      expect(error.message).toBe('Invalid tool namespace: "slack..admin"')
+      expect((yield* service.materialize()).definitions).toEqual([])
+    }),
+  )
+
   it.effect("filters disabled tools with edit aliases and ordered wildcard precedence", () =>
     Effect.gen(function* () {
       const service = yield* ToolRegistry.Service
diff --git a/packages/core/test/tool-execute.test.ts b/packages/core/test/tool-execute.test.ts
index 3eedc6fbab1a..66254ec408db 100644
--- a/packages/core/test/tool-execute.test.ts
+++ b/packages/core/test/tool-execute.test.ts
@@ -46,3 +46,50 @@ test("execute preserves successful results with visible unhandled rejections", a
     },
   ])
 })
+
+test("execute supports callable namespace tools", async () => {
+  const callable = Tool.make({
+    description: "Administer Slack",
+    input: Schema.Struct({}),
+    output: Schema.String,
+    execute: () => Effect.succeed("admin"),
+  })
+  const child = Tool.make({
+    description: "Create a Slack resource",
+    input: Schema.Struct({}),
+    output: Schema.String,
+    execute: () => Effect.succeed("created"),
+  })
+  const execute = ExecuteTool.create(
+    new Map([
+      ["slack_admin", { tool: callable, name: "admin", namespace: "slack" }],
+      ["slack_admin_create", { tool: child, name: "create", namespace: "slack.admin" }],
+    ]),
+  )
+  const result = await Effect.runPromise(
+    Tool.settle(
+      execute,
+      {
+        type: "tool-call",
+        id: "call_execute",
+        name: "execute",
+        input: { code: "return [await tools.slack.admin({}), await tools.slack.admin.create({})]" },
+      },
+      {
+        sessionID: Session.ID.make("ses_execute"),
+        agent: Agent.ID.make("build"),
+        messageID: SessionMessage.ID.make("msg_execute"),
+        callID: "call_execute",
+        progress: () => Effect.void,
+      },
+    ),
+  )
+
+  expect(result.structured).toEqual({
+    toolCalls: [
+      { tool: "slack.admin", status: "completed" },
+      { tool: "slack.admin.create", status: "completed" },
+    ],
+  })
+  expect(result.content).toEqual([{ type: "text", text: '[\n  "admin",\n  "created"\n]' }])
+})
diff --git a/packages/plugin/src/v2/effect/tool.ts b/packages/plugin/src/v2/effect/tool.ts
index 59ef488433b3..879e5badef2e 100644
--- a/packages/plugin/src/v2/effect/tool.ts
+++ b/packages/plugin/src/v2/effect/tool.ts
@@ -150,18 +150,24 @@ export const validateName = (name: string) =>
     ? Effect.void
     : Effect.fail(new RegistrationError({ name, message: `Invalid tool name: ${name}` }))
 
-export const registrationEntries = (tools: Readonly<Record<string, AnyTool>>, group?: string) =>
+export const registrationEntries = (tools: Readonly<Record<string, AnyTool>>, namespace?: string) =>
   Object.entries(tools).map(([name, tool]) => {
     const normalized = name.replace(/[^a-zA-Z0-9_-]/g, "_")
-    const parent = group?.replace(/[^a-zA-Z0-9_-]/g, "_")
     return {
-      key: parent === undefined ? normalized : `${parent}_${normalized}`,
+      key: namespace === undefined ? normalized : `${namespace.replaceAll(".", "_")}_${normalized}`,
       name: normalized,
-      group: parent,
+      namespace,
       tool,
     }
   })
 
+export const validateNamespace = (namespace: string) =>
+  namespace.split(".").every((segment) => /^[A-Za-z][A-Za-z0-9_-]{0,63}$/.test(segment))
+    ? Effect.void
+    : Effect.fail(
+        new RegistrationError({ name: namespace, message: `Invalid tool namespace: ${JSON.stringify(namespace)}` }),
+      )
+
 export const withPermission = <T extends AnyTool>(
   tool: T,
   permission: string,
@@ -288,7 +294,7 @@ export interface ToolExecuteAfterEvent {
 }
 
 export interface RegisterOptions {
-  readonly group?: string
+  readonly namespace?: string
   /** Defaults to true. False exposes the tool directly to the provider. */
   readonly codemode?: boolean
 }