diff --git a/packages/ai/src/protocols/anthropic-messages.ts b/packages/ai/src/protocols/anthropic-messages.ts
index 2dc1cecd6a7..98a17b4370c 100644
--- a/packages/ai/src/protocols/anthropic-messages.ts
+++ b/packages/ai/src/protocols/anthropic-messages.ts
@@ -1031,8 +1031,8 @@ const fromRequest = Effect.fn("AnthropicMessages.fromRequest")(function* (reques
   // messages. Tools live highest in the cache hierarchy, so when callers
   // over-mark we keep their tool hints and shed the message-tail ones first.
   const breakpoints = Cache.newBreakpoints(ANTHROPIC_BREAKPOINT_CAP)
-  const flattened = ProviderShared.flattenToolRequest(updates.request)
-  const tools = flattened.tools.length === 0 ? undefined : flattened.tools.map((tool) => lowerTool(breakpoints, tool))
+  const definitions = ProviderShared.toolDefinitions(request.tools)
+  const tools = definitions.length === 0 ? undefined : definitions.map((tool) => lowerTool(breakpoints, tool))
   // Anthropic rejects tool_choice when tools are absent; "none" is only meaningful with tools present.
   const toolChoice = tools === undefined || !request.toolChoice ? undefined : yield* lowerToolChoice(request.toolChoice)
   const systemParts = request.system.filter((part) => part.text.length > 0)
@@ -1044,7 +1044,7 @@ const fromRequest = Effect.fn("AnthropicMessages.fromRequest")(function* (reques
           text: part.text,
           cache_control: cacheControl(breakpoints, part.cache),
         }))
-  const messages = yield* lowerMessages(flattened.request, breakpoints)
+  const messages = yield* lowerMessages(updates.request, breakpoints)
   if (breakpoints.dropped > 0) {
     yield* Effect.logWarning(
       `Anthropic Messages: dropped ${breakpoints.dropped} cache breakpoint(s); the API allows at most ${ANTHROPIC_BREAKPOINT_CAP} per request.`,
diff --git a/packages/ai/src/protocols/bedrock-converse.ts b/packages/ai/src/protocols/bedrock-converse.ts
index 52a411e966e..9a153a7121c 100644
--- a/packages/ai/src/protocols/bedrock-converse.ts
+++ b/packages/ai/src/protocols/bedrock-converse.ts
@@ -463,7 +463,7 @@ const applyThinkingBindingDefault = (request: LLMRequest, thinking: Readonly<Rec
 
 const fromRequest = Effect.fn("BedrockConverse.fromRequest")(function* (request: LLMRequest) {
   const toolChoice = request.toolChoice ? yield* lowerToolChoice(request.toolChoice) : undefined
-  const flattened = ProviderShared.flattenToolRequest(request)
+  const tools = ProviderShared.toolDefinitions(request.tools)
   const generation = request.generation
   const options = yield* decodeOptions(request.providerOptions ?? {})
   const maxTokens =
@@ -485,16 +485,16 @@ const fromRequest = Effect.fn("BedrockConverse.fromRequest")(function* (request:
   // tools → system → messages order to favour the highest-impact prefixes.
   const breakpoints = BedrockCache.breakpoints(request.model.id)
   const toolConfig = (() => {
-    if (flattened.tools.length === 0) return undefined
+    if (tools.length === 0) return undefined
     return {
-      tools: lowerTools(breakpoints, flattened.tools),
+      tools: lowerTools(breakpoints, tools),
       // Converse has no native "none". Keep definitions stable for prompt
       // caching and omit only the unsupported choice.
       toolChoice,
     }
   })()
   const system = lowerSystem(breakpoints, request.system)
-  const messages = yield* lowerMessages(flattened.request, breakpoints)
+  const messages = yield* lowerMessages(request, breakpoints)
   if (breakpoints.dropped > 0) {
     yield* Effect.logWarning(
       `Bedrock Converse: dropped ${breakpoints.dropped} cache breakpoint(s); the API allows at most ${BedrockCache.BEDROCK_BREAKPOINT_CAP} per request.`,
diff --git a/packages/ai/src/protocols/gemini.ts b/packages/ai/src/protocols/gemini.ts
index 8c02f11cc9f..dd8555c86be 100644
--- a/packages/ai/src/protocols/gemini.ts
+++ b/packages/ai/src/protocols/gemini.ts
@@ -438,8 +438,8 @@ const lowerMessages = Effect.fn("Gemini.lowerMessages")(function* (request: LLMR
 })
 
 const fromRequest = Effect.fn("Gemini.fromRequest")(function* (request: LLMRequest) {
-  const flattened = ProviderShared.flattenToolRequest(request)
-  const hasTools = flattened.tools.length > 0
+  const tools = ProviderShared.toolDefinitions(request.tools)
+  const hasTools = tools.length > 0
   const generation = request.generation
   const options = yield* decodeOptions(request.providerOptions ?? {})
   const generationConfig = {
@@ -471,7 +471,7 @@ const fromRequest = Effect.fn("Gemini.fromRequest")(function* (request: LLMReque
 
   return {
     cachedContent: options.cachedContent,
-    contents: yield* lowerMessages(flattened.request),
+    contents: yield* lowerMessages(request),
     safetySettings: options.safetySettings,
     serviceTier: options.serviceTier,
     systemInstruction:
@@ -479,7 +479,7 @@ const fromRequest = Effect.fn("Gemini.fromRequest")(function* (request: LLMReque
     tools: hasTools
       ? [
           {
-            functionDeclarations: flattened.tools.map(lowerTool),
+            functionDeclarations: tools.map(lowerTool),
           },
         ]
       : undefined,
diff --git a/packages/ai/src/protocols/meta-messages.ts b/packages/ai/src/protocols/meta-messages.ts
index 0b9f6bd3243..663380c6bb1 100644
--- a/packages/ai/src/protocols/meta-messages.ts
+++ b/packages/ai/src/protocols/meta-messages.ts
@@ -26,8 +26,8 @@ const Body = Schema.Struct({
 })
 
 const fromRequest = Effect.fn("MetaMessages.fromRequest")(function* (request: LLMRequest) {
-  const projected = ProviderShared.flattenToolRequest(request)
-  const body = yield* AnthropicMessages.protocol.body.from(projected.request)
+  const tools = ProviderShared.toolDefinitions(request.tools)
+  const body = yield* AnthropicMessages.protocol.body.from(request)
   return {
     ...body,
     tools:
@@ -35,7 +35,7 @@ const fromRequest = Effect.fn("MetaMessages.fromRequest")(function* (request: LL
         ? undefined
         : yield* Effect.forEach(body.tools, (tool, index) =>
             Effect.gen(function* () {
-              const native = projected.tools[index]?.native
+              const native = tools[index]?.native
               if (native === undefined) return tool
               const search = yield* ProviderShared.validateWith(Schema.decodeUnknownEffect(MetaResponses.WebSearch))(
                 native.meta,
diff --git a/packages/ai/src/protocols/meta-responses.ts b/packages/ai/src/protocols/meta-responses.ts
index 79827fa3178..b11dfdb9a80 100644
--- a/packages/ai/src/protocols/meta-responses.ts
+++ b/packages/ai/src/protocols/meta-responses.ts
@@ -67,40 +67,39 @@ const adapter = {
 
 const fromRequest = Effect.fn("MetaResponses.fromRequest")(function* (request: LLMRequest) {
   const key = request.model.route.providerMetadataKey ?? String(request.model.provider)
-  const projected = ProviderShared.flattenToolRequest(
-    LLMRequest.update(request, {
-      messages: request.messages.map((message) =>
-        Message.make({
-          ...message,
-          content: message.content.map((part) => {
-            if (
-              part.type !== "tool-result" ||
-              !part.providerExecuted ||
-              part.name !== "image_generation" ||
-              part.result.type !== "content" ||
-              part.providerMetadata?.[key]?.itemId !== part.id
-            )
-              return part
-            // Meta's signed image ID carries edit state; replay the handle, not the image bytes as a user message.
-            return ToolResultPart.make({
-              ...part,
-              result: {
-                type: "json",
-                value: { type: "image_generation_call", id: part.id, status: "completed", result: null },
-              },
-            })
-          }),
+  const tools = ProviderShared.toolDefinitions(request.tools)
+  const projected = LLMRequest.update(request, {
+    messages: request.messages.map((message) =>
+      Message.make({
+        ...message,
+        content: message.content.map((part) => {
+          if (
+            part.type !== "tool-result" ||
+            !part.providerExecuted ||
+            part.name !== "image_generation" ||
+            part.result.type !== "content" ||
+            part.providerMetadata?.[key]?.itemId !== part.id
+          )
+            return part
+          // Meta's signed image ID carries edit state; replay the handle, not the image bytes as a user message.
+          return ToolResultPart.make({
+            ...part,
+            result: {
+              type: "json",
+              value: { type: "image_generation_call", id: part.id, status: "completed", result: null },
+            },
+          })
         }),
-      ),
-    }),
-  )
+      }),
+    ),
+  })
   return yield* ProviderShared.validateWith(Schema.decodeUnknownEffect(Body))({
-    ...(yield* OpenResponses.lowerConversation(projected.request, adapter)),
+    ...(yield* OpenResponses.lowerConversation(projected, adapter)),
     ...OpenResponses.lowerGeneration(request),
     tools:
-      projected.tools.length === 0
+      tools.length === 0
         ? undefined
-        : yield* Effect.forEach(projected.tools, (tool) =>
+        : yield* Effect.forEach(tools, (tool) =>
             Effect.gen(function* () {
               if (tool.native === undefined) return yield* OpenResponses.lowerTool(NAME, tool)
               return yield* ProviderShared.validateWith(Schema.decodeUnknownEffect(NativeTool))(tool.native.meta)
diff --git a/packages/ai/src/protocols/mistral-chat.ts b/packages/ai/src/protocols/mistral-chat.ts
index 99f20487341..0e226c3f5f2 100644
--- a/packages/ai/src/protocols/mistral-chat.ts
+++ b/packages/ai/src/protocols/mistral-chat.ts
@@ -390,11 +390,11 @@ export const fromRequest = Effect.fn("MistralChat.fromRequest")(function* (reque
         tool: (name) => ({ type: "function" as const, function: { name } }),
       })
     : undefined
-  const flattened = ProviderShared.flattenToolRequest(request)
+  const tools = ProviderShared.toolDefinitions(request.tools)
   return {
     model: request.model.id,
-    messages: yield* lowerMessages(flattened.request),
-    tools: flattened.tools.length > 0 ? flattened.tools.map(lowerTool) : undefined,
+    messages: yield* lowerMessages(request),
+    tools: tools.length > 0 ? tools.map(lowerTool) : undefined,
     tool_choice: toolChoice,
     stream: true as const,
     max_tokens: request.generation?.maxTokens,
diff --git a/packages/ai/src/protocols/open-responses.ts b/packages/ai/src/protocols/open-responses.ts
index 129cc1441d0..4fff347c5b8 100644
--- a/packages/ai/src/protocols/open-responses.ts
+++ b/packages/ai/src/protocols/open-responses.ts
@@ -816,11 +816,11 @@ export const fromRequestWithAdapter = Effect.fn("OpenResponses.fromRequestWithAd
   request: LLMRequest,
   adapter: ProviderAdapter,
 ) {
-  const projected = ProviderShared.flattenToolRequest(request)
+  const tools = ProviderShared.toolDefinitions(request.tools)
   return {
-    ...(yield* lowerConversation(projected.request, adapter)),
+    ...(yield* lowerConversation(request, adapter)),
     ...lowerGeneration(request),
-    tools: projected.tools.length === 0 ? undefined : yield* lowerTools(projected.tools, adapter),
+    tools: tools.length === 0 ? undefined : yield* lowerTools(tools, adapter),
     tool_choice:
       allowedToolChoice(request) ??
       (request.toolChoice ? yield* lowerToolChoice(adapter.name, request.toolChoice) : undefined),
diff --git a/packages/ai/src/protocols/openai-chat.ts b/packages/ai/src/protocols/openai-chat.ts
index 8f1794b1e34..ea08b228784 100644
--- a/packages/ai/src/protocols/openai-chat.ts
+++ b/packages/ai/src/protocols/openai-chat.ts
@@ -814,7 +814,6 @@ export const fromRequest = Effect.fn("OpenAIChat.fromRequest")(function* (
       `OpenAI Chat reasoning field conflicts with reserved field ${reasoningField}`,
     )
   const generation = request.generation
-  const flattened = ProviderShared.flattenToolRequest(request)
   const provider = String(request.model.provider)
   const baseURL = request.model.route.endpoint.baseURL
   const detectedMaxTokensField = detectMaxTokensField(provider, baseURL)
@@ -827,16 +826,16 @@ export const fromRequest = Effect.fn("OpenAIChat.fromRequest")(function* (
   const zaiToolStream =
     request.model.compatibility?.zaiToolStream ?? detectZaiToolStream(provider, baseURL, request.model.id)
   const hasHistory = hasToolHistory(request.messages)
-  const hasActiveTools = flattened.tools.length > 0
+  const tools = ProviderShared.toolDefinitions(request.tools)
+  const hasActiveTools = tools.length > 0
   return {
     model: request.model.id,
-    messages: yield* lowerMessages(flattened.request, options),
-    tools:
-      flattened.tools.length === 0
-        ? hasHistory
-          ? []
-          : undefined
-        : flattened.tools.map((tool) => lowerTool(tool, options, supportsStrictMode)),
+    messages: yield* lowerMessages(request, options),
+    tools: !hasActiveTools
+      ? hasHistory
+        ? []
+        : undefined
+      : tools.map((tool) => lowerTool(tool, options, supportsStrictMode)),
     tool_choice: hasActiveTools && request.toolChoice ? yield* lowerToolChoice(request.toolChoice) : undefined,
     stream: true as const,
     ...(supportsUsageInStreaming ? { stream_options: { include_usage: true } } : {}),
diff --git a/packages/ai/src/protocols/openai-responses.ts b/packages/ai/src/protocols/openai-responses.ts
index e36496954b9..e352f0f1d94 100644
--- a/packages/ai/src/protocols/openai-responses.ts
+++ b/packages/ai/src/protocols/openai-responses.ts
@@ -172,8 +172,7 @@ const lowerTool = Effect.fn("OpenAIResponses.lowerTool")(function* (tool: ToolDe
   return yield* OpenResponses.lowerTool(NAME, tool)
 })
 
-// Native namespaces hold only function tools, so deeper levels flatten into
-// the leaf names the same way non-native protocols flatten the whole tree.
+// The route has already joined namespace levels below the first into leaf names.
 const lowerToolEntry = Effect.fn("OpenAIResponses.lowerToolEntry")(function* (tool: ToolEntry) {
   if (tool.type === "tool") return yield* lowerTool(tool)
   // OpenAI requires a namespace description; fall back to a generic one so a
@@ -182,7 +181,7 @@ const lowerToolEntry = Effect.fn("OpenAIResponses.lowerToolEntry")(function* (to
     type: "namespace" as const,
     name: tool.name,
     description: tool.description ?? `Tools in the ${tool.name} namespace.`,
-    tools: yield* Effect.forEach(ProviderShared.flattenTools(tool.tools), (leaf) =>
+    tools: yield* Effect.forEach(ProviderShared.toolDefinitions(tool.tools), (leaf) =>
       OpenResponses.lowerTool(NAME, leaf),
     ),
   }
@@ -308,6 +307,7 @@ export const protocol = Protocol.make({
     terminal: OpenResponses.terminal,
   },
   supportsEffortUpdates,
+  namespaces: "native",
 })
 
 const endpoint = Endpoint.path<OpenAIResponsesBody>(PATH, { baseURL: DEFAULT_BASE_URL })
diff --git a/packages/ai/src/protocols/shared.ts b/packages/ai/src/protocols/shared.ts
index bd8d2538d6a..a0a1bd8bb06 100644
--- a/packages/ai/src/protocols/shared.ts
+++ b/packages/ai/src/protocols/shared.ts
@@ -288,36 +288,11 @@ export const unsupportedOperation = (input: {
   })
 
 /**
- * Lower namespaces to flat definitions for protocols without a native
- * namespace construct. Leaf names join their namespace path with `_` because
- * `.` is not broadly accepted in provider tool names.
+ * Tool definitions of a route-lowered request. The route flattens namespaces
+ * before a flat protocol's `body.from`, so no namespace reaches this point.
  */
-export const flattenTools = (tools: ReadonlyArray<ToolEntry>, path: ReadonlyArray<string> = []) => {
-  const flat = tools.flatMap((tool): ReadonlyArray<ToolDefinition> => {
-    if (tool.type === "namespace") return flattenTools(tool.tools, [...path, tool.name])
-    if (path.length === 0) return [tool]
-    return [new ToolDefinition({ ...tool, name: [...path, tool.name].join("_") })]
-  })
-  return [...new Map(flat.map((tool) => [tool.name, tool])).values()]
-}
-
-export const flattenToolRequest = (request: LLMRequest) => {
-  const messages = request.messages.map((message) => {
-    const content = message.content.map((part) => {
-      if ((part.type !== "tool-call" && part.type !== "tool-result") || part.namespace === undefined) return part
-      return { ...part, name: `${part.namespace}_${part.name}`, namespace: undefined }
-    })
-    return content.every((part, index) => part === message.content[index])
-      ? message
-      : new Message({ ...message, content })
-  })
-  return {
-    tools: flattenTools(request.tools),
-    request: messages.every((message, index) => message === request.messages[index])
-      ? request
-      : LLMRequest.update(request, { messages }),
-  }
-}
+export const toolDefinitions = (tools: ReadonlyArray<ToolEntry>) =>
+  tools.filter((tool): tool is ToolDefinition => tool.type === "tool")
 
 export const matchToolChoice = <Auto, None, Required, Tool>(
   route: string,
diff --git a/packages/ai/src/protocols/xai-responses.ts b/packages/ai/src/protocols/xai-responses.ts
index bb19ebd843a..da564cc318e 100644
--- a/packages/ai/src/protocols/xai-responses.ts
+++ b/packages/ai/src/protocols/xai-responses.ts
@@ -95,7 +95,7 @@ export const protocol = Protocol.make({
 })
 
 export const compact = ResponsesCompaction.make(adapter, (request) =>
-  OpenResponses.lowerTools(ProviderShared.flattenTools(request.tools), adapter),
+  OpenResponses.lowerTools(ProviderShared.toolDefinitions(request.tools), adapter),
 )
 
 export * as XAIResponses from "./xai-responses.js"
diff --git a/packages/ai/src/route/client.ts b/packages/ai/src/route/client.ts
index 83e762160a1..bbaf46515da 100644
--- a/packages/ai/src/route/client.ts
+++ b/packages/ai/src/route/client.ts
@@ -10,6 +10,7 @@ import { applyCachePolicy } from "../cache-policy.js"
 import { applyEffortUpdates } from "../effort-updates.js"
 import { normalizeToolHistory } from "../tool-history.js"
 import { sanitizeSurrogates } from "../utils/sanitize.js"
+import { ToolNames, type NamespaceStyle } from "../tool-names.js"
 import * as ProviderShared from "../protocols/shared.js"
 import { ToolSchemaProjection } from "../protocols/utils/tool-schema.js"
 import type { LanguageModelSanitizerCompatibility, ProtocolID, ProviderOptions } from "../schema/index.js"
@@ -59,6 +60,7 @@ export interface Route<
   readonly body: RouteBody<Body>
   readonly supportsEffortUpdates?: (request: LLMRequest) => boolean
   readonly sanitizer?: LanguageModelSanitizerCompatibility
+  readonly namespaces: NamespaceStyle
   readonly with: {
     <Next extends CompactionOperations | undefined>(
       patch: RoutePatch<Body, Prepared> & { readonly compact: Next },
@@ -391,6 +393,7 @@ function makeFromTransport<Body, Prepared, Frame, Event, State>(
       body: protocol.body,
       supportsEffortUpdates: protocol.supportsEffortUpdates,
       sanitizer: protocol.sanitizer,
+      namespaces: protocol.namespaces ?? "flat",
       with: (patch: RoutePatch<Body, Prepared>) => {
         const { compact, id, provider, providerMetadataKey, auth, transport, endpoint, ...defaults } = patch
         return build({
@@ -572,8 +575,12 @@ const prepareRequest = (request: LLMRequest) => {
     : LLMRequest.update(resolved, { http: mergeHttpOptions(new HttpOptions({ headers }), resolved.http) })
 }
 
+// Protocols receive wire tool names; callers keep declared names in both directions.
+const lowerRequest = (request: LLMRequest) => ToolNames.lower(prepareRequest(request), request.model.route.namespaces)
+
 const compile = Effect.fn("LLM.compile")(function* (request: LLMRequest, options?: StreamOptions) {
-  const resolved = prepareRequest(request)
+  const lowered = yield* lowerRequest(request)
+  const resolved = lowered.request
   const route = resolved.model.route
 
   const body = yield* route.body
@@ -586,6 +593,7 @@ const compile = Effect.fn("LLM.compile")(function* (request: LLMRequest, options
     route,
     body,
     prepared,
+    raise: lowered.raise,
   }
 })
 
@@ -606,7 +614,9 @@ const streamRequestWith = (runtime: TransportRuntime) => (request: LLMRequest, o
   Stream.unwrap(
     Effect.gen(function* () {
       const compiled = yield* compile(request, options)
-      return compiled.route.streamPrepared(compiled.prepared, compiled.request, runtime, options)
+      return compiled.route
+        .streamPrepared(compiled.prepared, compiled.request, runtime, options)
+        .pipe(Stream.map(compiled.raise))
     }),
   )
 
@@ -672,10 +682,14 @@ export const layer: Layer.Layer<Service, never, RequestExecutor.Service> = Layer
     ): Effect.Effect<CompactionCheckpointResponse, AIError>
     function compact(request: LLMRequest, options?: EndpointCompactOptions | TriggerCompactOptions) {
       return Effect.suspend((): Effect.Effect<CompactionResponse | CompactionCheckpointResponse, AIError> => {
-        if (options?.mechanism === "trigger" && canCompact(request, options))
-          return request.model.route.compact.trigger(prepareRequest(request), executor, options)
-        if ((options?.mechanism === undefined || options.mechanism === "endpoint") && canCompact(request))
-          return request.model.route.compact.endpoint(prepareRequest(request), executor, options)
+        if (options?.mechanism === "trigger" && canCompact(request, options)) {
+          const trigger = request.model.route.compact.trigger
+          return lowerRequest(request).pipe(Effect.flatMap((lowered) => trigger(lowered.request, executor, options)))
+        }
+        if ((options?.mechanism === undefined || options.mechanism === "endpoint") && canCompact(request)) {
+          const endpoint = request.model.route.compact.endpoint
+          return lowerRequest(request).pipe(Effect.flatMap((lowered) => endpoint(lowered.request, executor, options)))
+        }
         return unsupportedCompaction(request, options?.mechanism)
       })
     }
diff --git a/packages/ai/src/route/protocol.ts b/packages/ai/src/route/protocol.ts
index 89c8d536108..23911098f9e 100644
--- a/packages/ai/src/route/protocol.ts
+++ b/packages/ai/src/route/protocol.ts
@@ -1,5 +1,6 @@
 import { Schema, type Effect } from "effect"
 import type { AIError, LanguageModelSanitizerCompatibility, LLMEvent, LLMRequest, ProtocolID } from "../schema/index.js"
+import type { NamespaceStyle } from "../tool-names.js"
 
 /**
  * The semantic API contract of one model server family.
@@ -45,6 +46,8 @@ export interface Protocol<Body, Frame, Event, State> {
   readonly supportsEffortUpdates?: (request: LLMRequest) => boolean
   /** Tool schema sanitizer for every model on this protocol unless the model's compatibility sets one; wrappers around another `body.from` must forward it. */
   readonly sanitizer?: LanguageModelSanitizerCompatibility
+  /** How `body.from` receives tool namespaces; defaults to `"flat"`. Wrappers around another `body.from` must forward it. */
+  readonly namespaces?: NamespaceStyle
 }
 
 export interface ProtocolBody<Body> {
diff --git a/packages/ai/src/tool-names.ts b/packages/ai/src/tool-names.ts
new file mode 100644
index 00000000000..f2f79ccb179
--- /dev/null
+++ b/packages/ai/src/tool-names.ts
@@ -0,0 +1,114 @@
+import { Effect } from "effect"
+import {
+  AIError,
+  LLMRequest,
+  Message,
+  ToolDefinition,
+  type LLMEvent,
+  type ToolEntry,
+  type ToolNamespace,
+} from "./schema/index.js"
+import { ProviderShared } from "./protocols/shared.js"
+
+/**
+ * How a protocol represents tool namespaces on the wire.
+ *
+ * Callers always name a namespaced tool by its declared path, for example
+ * `{ namespace: "crm.orders", name: "list" }`. Flat protocols see
+ * `crm_orders_list`; native protocols keep the outer namespace and see
+ * `{ namespace: "crm", name: "orders_list" }`.
+ */
+export type NamespaceStyle = "flat" | "native"
+
+interface ToolName {
+  readonly namespace?: string
+  readonly name: string
+}
+
+interface Leaf {
+  readonly namespace?: string
+  readonly tool: ToolDefinition
+}
+
+/**
+ * Lower declared tool names in a request to the protocol's wire names, and
+ * return the inverse for the protocol's tool events.
+ */
+export const lower = Effect.fn("ToolNames.lower")(function* (request: LLMRequest, style: NamespaceStyle) {
+  const leaves = yield* collect(request.tools, [])
+  const names = leaves.map((leaf) => {
+    const declared = { namespace: leaf.namespace, name: leaf.tool.name }
+    return { declared, wire: key(wire(declared, style)) }
+  })
+  const collision = names.flatMap((entry, index) =>
+    names
+      .slice(0, index)
+      .filter((other) => other.wire === entry.wire)
+      .map((other) => [other, entry] as const),
+  )[0]
+  if (collision !== undefined)
+    return yield* ProviderShared.invalidRequest(
+      `Tools "${key(collision[0].declared)}" and "${key(collision[1].declared)}" both use the provider tool name "${collision[1].wire}"`,
+    )
+  const declared = new Map(names.map((entry) => [entry.wire, entry.declared]))
+  return {
+    request: LLMRequest.update(request, {
+      tools: style === "flat" ? leaves.map((leaf) => rename(leaf, style)) : yield* nativeTools(request.tools),
+      messages: lowerMessages(request.messages, style),
+    }),
+    raise: (event: LLMEvent): LLMEvent => {
+      if (!("name" in event)) return event
+      const name = declared.get(key(event))
+      return name === undefined ? event : { ...event, namespace: name.namespace, name: name.name }
+    },
+  }
+})
+
+const collect = (tools: ReadonlyArray<ToolEntry>, path: ReadonlyArray<string>): Effect.Effect<Leaf[], AIError> =>
+  Effect.forEach(tools, (tool) => {
+    if (tool.type === "tool")
+      return Effect.succeed([{ namespace: path.length === 0 ? undefined : path.join("."), tool }])
+    if (tool.name.includes("."))
+      return ProviderShared.invalidRequest(`Tool namespace "${tool.name}" must not contain "."`)
+    return collect(tool.tools, [...path, tool.name])
+  }).pipe(Effect.map((groups) => groups.flat()))
+
+// Native namespaces are one level deep, so deeper levels join into the leaf name.
+const nativeTools = (tools: ReadonlyArray<ToolEntry>) =>
+  Effect.forEach(
+    tools,
+    (tool): Effect.Effect<ToolEntry, AIError> =>
+      tool.type === "tool"
+        ? Effect.succeed(tool)
+        : collect(tool.tools, [tool.name]).pipe(
+            Effect.map((leaves): ToolNamespace => ({ ...tool, tools: leaves.map((leaf) => rename(leaf, "native")) })),
+          ),
+  )
+
+const lowerMessages = (messages: ReadonlyArray<Message>, style: NamespaceStyle) =>
+  messages.map((message) => {
+    const content = message.content.map((part) => {
+      if ((part.type !== "tool-call" && part.type !== "tool-result") || part.namespace === undefined) return part
+      const name = wire(part, style)
+      return { ...part, namespace: name.namespace, name: name.name }
+    })
+    return content.every((part, index) => part === message.content[index])
+      ? message
+      : new Message({ ...message, content })
+  })
+
+const rename = (leaf: Leaf, style: NamespaceStyle) =>
+  leaf.namespace === undefined
+    ? leaf.tool
+    : new ToolDefinition({ ...leaf.tool, name: wire({ namespace: leaf.namespace, name: leaf.tool.name }, style).name })
+
+const wire = (tool: ToolName, style: NamespaceStyle): ToolName => {
+  if (tool.namespace === undefined) return { name: tool.name }
+  const path = tool.namespace.split(".")
+  if (style === "flat") return { name: [...path, tool.name].join("_") }
+  return { namespace: path[0], name: [...path.slice(1), tool.name].join("_") }
+}
+
+const key = (tool: ToolName) => (tool.namespace === undefined ? tool.name : `${tool.namespace}.${tool.name}`)
+
+export * as ToolNames from "./tool-names.js"
diff --git a/packages/ai/src/tool.ts b/packages/ai/src/tool.ts
index c04b666887c..f54b16af21b 100644
--- a/packages/ai/src/tool.ts
+++ b/packages/ai/src/tool.ts
@@ -3,6 +3,7 @@ import { Tool } from "@opencode/schema/tool"
 import type {
   ToolCallPart,
   ToolDefinition as ToolDefinitionClass,
+  ToolEntry,
   ToolOutput as ToolOutputType,
 } from "./schema/index.js"
 import { ToolDefinition, ToolFailure, ToolOutput } from "./schema/index.js"
@@ -207,29 +208,50 @@ export function make(config: TypedToolConfig | DynamicToolConfig): AnyTool {
 }
 
 /**
- * A record of named tools. The record key becomes the tool name on the wire.
+ * A record of named tools. A dotted key such as `crm.lookup` declares `lookup`
+ * inside the `crm` namespace, matching the `namespace.name` identity that
+ * `ToolRuntime.dispatch` resolves.
  */
 export type Tools = Record<string, AnyTool>
 
 /**
- * Convert a tools record into the `ToolDefinition[]` shape that
- * `LLMRequest.tools` expects.
+ * Convert a tools record into the tool entries that `LLMRequest.tools`
+ * expects, nesting dotted keys into namespaces.
  *
  * Tool names come from the record keys, so the per-tool cached
  * `_definition` is rebuilt with the correct name here. The JSON Schema body
  * is reused.
  */
-export const toDefinitions = (tools: Tools): ReadonlyArray<ToolDefinitionClass> =>
-  Object.entries(tools).map(
-    ([name, item]) =>
-      new ToolDefinition({
-        name,
-        description: item._definition.description,
-        inputSchema: item._definition.inputSchema,
-        outputSchema: item._definition.outputSchema,
-      }),
+export const toDefinitions = (tools: Tools): ReadonlyArray<ToolEntry> =>
+  nest(
+    Object.entries(tools).map(([key, item]) => ({
+      path: key.split("."),
+      definition: item._definition,
+    })),
   )
 
+const nest = (
+  entries: ReadonlyArray<{ readonly path: ReadonlyArray<string>; readonly definition: ToolDefinitionClass }>,
+): ReadonlyArray<ToolEntry> =>
+  [...new Set(entries.map((entry) => (entry.path.length === 1 ? entry : entry.path[0])))].map((group) => {
+    if (typeof group !== "string")
+      return new ToolDefinition({
+        name: group.path[0],
+        description: group.definition.description,
+        inputSchema: group.definition.inputSchema,
+        outputSchema: group.definition.outputSchema,
+      })
+    return {
+      type: "namespace" as const,
+      name: group,
+      tools: nest(
+        entries
+          .filter((entry) => entry.path.length > 1 && entry.path[0] === group)
+          .map((entry) => ({ ...entry, path: entry.path.slice(1) })),
+      ),
+    }
+  })
+
 const toJsonSchema = (schema: Schema.Top): JsonSchema.JsonSchema => {
   const document = Schema.toJsonSchemaDocument(schema)
   if (Object.keys(document.definitions).length === 0) return document.schema
diff --git a/packages/ai/test/provider/openai-compatible-responses.test.ts b/packages/ai/test/provider/openai-compatible-responses.test.ts
index 81860398d6d..baf0f1e8e33 100644
--- a/packages/ai/test/provider/openai-compatible-responses.test.ts
+++ b/packages/ai/test/provider/openai-compatible-responses.test.ts
@@ -213,26 +213,83 @@ describe("Open Responses-compatible route", () => {
           strict: false,
         },
       ])
+
+      const collision = yield* compileRequest(
+        LLM.request({
+          model,
+          tools: [
+            ToolDefinition.make({ name: "acme_users", description: "Top-level users", inputSchema: {} }),
+            {
+              type: "namespace",
+              name: "acme",
+              tools: [ToolDefinition.make({ name: "users", description: "Lookup users", inputSchema: {} })],
+            },
+          ],
+        }),
+      ).pipe(Effect.flip)
+      expect(collision.message).toContain(
+        'Tools "acme_users" and "acme.users" both use the provider tool name "acme_users"',
+      )
     }),
   )
 
-  it.effect("flattens tool namespaces in history", () =>
+  it.effect("keeps declared tool names through flattened streaming and history replay", () =>
     Effect.gen(function* () {
       const model = configure({ apiKey: "test-key", baseURL: "https://responses.example.test/v1" }).model(
         "example-model",
       )
+      const tools = [
+        {
+          type: "namespace" as const,
+          name: "crm",
+          tools: [
+            {
+              type: "namespace" as const,
+              name: "orders",
+              tools: [ToolDefinition.make({ name: "list", description: "List orders", inputSchema: {} })],
+            },
+          ],
+        },
+      ]
+      const item = { type: "function_call", id: "fc_1", call_id: "call_1", name: "crm_orders_list", arguments: "" }
+      const response = yield* LLMClient.generate(LLM.request({ model, prompt: "List orders.", tools })).pipe(
+        Effect.provide(
+          fixedResponse(
+            sseEvents(
+              { type: "response.output_item.added", output_index: 0, item },
+              { type: "response.output_item.done", output_index: 0, item: { ...item, arguments: "{}" } },
+              { type: "response.completed", response: { id: "resp_1" } },
+            ),
+          ),
+        ),
+      )
+
+      expect(response.events.filter((event) => event.type.startsWith("tool-"))).toEqual([
+        expect.objectContaining({ type: "tool-input-start", name: "list", namespace: "crm.orders" }),
+        expect.objectContaining({ type: "tool-input-end", name: "list", namespace: "crm.orders" }),
+        expect.objectContaining({ type: "tool-call", name: "list", namespace: "crm.orders", input: {} }),
+      ])
+
       const prepared = yield* compileRequest(
         LLM.request({
           model,
+          tools,
           messages: [
-            Message.assistant({ type: "tool-call", id: "call_1", name: "lookup", namespace: "crm", input: {} }),
-            Message.tool({ id: "call_1", name: "lookup", namespace: "crm", result: "done", resultType: "text" }),
+            response.message,
+            Message.tool({ id: "call_1", name: "list", namespace: "crm.orders", result: "done", resultType: "text" }),
           ],
         }),
       )
 
       expect(prepared.body.input).toEqual([
-        { type: "function_call", call_id: "call_1", name: "crm_lookup", namespace: undefined, arguments: "{}" },
+        {
+          type: "function_call",
+          id: "fc_1",
+          call_id: "call_1",
+          name: "crm_orders_list",
+          namespace: undefined,
+          arguments: "{}",
+        },
         { type: "function_call_output", call_id: "call_1", output: "done" },
       ])
     }),
diff --git a/packages/ai/test/provider/openai-responses.test.ts b/packages/ai/test/provider/openai-responses.test.ts
index 280271a31e8..e19f898e293 100644
--- a/packages/ai/test/provider/openai-responses.test.ts
+++ b/packages/ai/test/provider/openai-responses.test.ts
@@ -2432,17 +2432,28 @@ describe("OpenAI Responses route", () => {
     }),
   )
 
-  it.effect("preserves tool namespaces through streaming and history replay", () =>
+  it.effect("keeps declared tool names through native streaming and history replay", () =>
     Effect.gen(function* () {
+      const tools = [
+        ToolNamespace.make({
+          name: "crm",
+          tools: [
+            ToolNamespace.make({
+              name: "orders",
+              tools: [ToolDefinition.make({ name: "list", description: "List orders", inputSchema: {} })],
+            }),
+          ],
+        }),
+      ]
       const item = {
         type: "function_call",
         id: "fc_1",
         call_id: "call_1",
         namespace: "crm",
-        name: "lookup",
+        name: "orders_list",
         arguments: "",
       }
-      const response = yield* LLMClient.generate(request).pipe(
+      const response = yield* LLMClient.generate(LLM.request({ model, prompt: "List orders.", tools })).pipe(
         Effect.provide(
           fixedResponse(
             sseEvents(
@@ -2466,21 +2477,22 @@ describe("OpenAI Responses route", () => {
 
       const toolEvents = response.events.filter((event) => event.type.startsWith("tool-"))
       expect(toolEvents).toEqual([
-        expect.objectContaining({ type: "tool-input-start", name: "lookup", namespace: "crm" }),
-        expect.objectContaining({ type: "tool-input-delta", name: "lookup", namespace: "crm" }),
-        expect.objectContaining({ type: "tool-input-end", name: "lookup", namespace: "crm" }),
-        expect.objectContaining({ type: "tool-call", name: "lookup", namespace: "crm", input: { id: "123" } }),
+        expect.objectContaining({ type: "tool-input-start", name: "list", namespace: "crm.orders" }),
+        expect.objectContaining({ type: "tool-input-delta", name: "list", namespace: "crm.orders" }),
+        expect.objectContaining({ type: "tool-input-end", name: "list", namespace: "crm.orders" }),
+        expect.objectContaining({ type: "tool-call", name: "list", namespace: "crm.orders", input: { id: "123" } }),
       ])
       expect(response.message.content).toEqual([
-        expect.objectContaining({ type: "tool-call", name: "lookup", namespace: "crm", input: { id: "123" } }),
+        expect.objectContaining({ type: "tool-call", name: "list", namespace: "crm.orders", input: { id: "123" } }),
       ])
 
       const prepared = yield* compileRequest(
         LLM.request({
           model,
+          tools,
           messages: [
             response.message,
-            Message.tool({ id: "call_1", name: "lookup", namespace: "crm", result: { customer: "Ada" } }),
+            Message.tool({ id: "call_1", name: "list", namespace: "crm.orders", result: { customer: "Ada" } }),
           ],
         }),
       )
@@ -2490,7 +2502,7 @@ describe("OpenAI Responses route", () => {
           id: "fc_1",
           call_id: "call_1",
           namespace: "crm",
-          name: "lookup",
+          name: "orders_list",
           arguments: '{"id":"123"}',
         },
         { type: "function_call_output", call_id: "call_1", output: '{"customer":"Ada"}' },
@@ -4998,11 +5010,11 @@ describe("OpenAI Responses route", () => {
 
   it.effect("retains the token-sharing HTTP 429 body for retry hooks", () =>
     Effect.gen(function* () {
-      const body = JSON.stringify({ error: { code: "subscription_sharing_usage_limit_exceeded", message: "Rate limit exceeded" } })
+      const body = JSON.stringify({
+        error: { code: "subscription_sharing_usage_limit_exceeded", message: "Rate limit exceeded" },
+      })
       const error = yield* LLMClient.generate(request).pipe(
-        Effect.provide(
-          fixedResponse(body, { status: 429, headers: { "content-type": "application/json" } }),
-        ),
+        Effect.provide(fixedResponse(body, { status: 429, headers: { "content-type": "application/json" } })),
         Effect.flip,
       )
 
diff --git a/packages/core/src/aisdk.ts b/packages/core/src/aisdk.ts
index cb27a55584c..36339b7b96e 100644
--- a/packages/core/src/aisdk.ts
+++ b/packages/core/src/aisdk.ts
@@ -368,6 +368,7 @@ function modelFromLanguage(info: RuntimeInfo, language: LanguageModelV3) {
     protocol: "ai-sdk",
     // AI SDK providers convert tool schemas themselves, so model-family sanitizers stay off here.
     sanitizer: "none",
+    namespaces: "flat",
     endpoint: Endpoint.path("/", { baseURL: "https://ai-sdk.local" }),
     auth: Auth.none,
     transport: {
@@ -477,9 +478,8 @@ function callOptions(
   modelID: ID,
   optionKey: string,
 ): LanguageModelV3CallOptions {
-  const flattened = ProviderShared.flattenToolRequest(request)
   return {
-    prompt: prompt(flattened.request),
+    prompt: prompt(request),
     maxOutputTokens: request.generation?.maxTokens,
     temperature: request.generation?.temperature,
     stopSequences: request.generation?.stop === undefined ? undefined : [...request.generation.stop],
@@ -488,7 +488,7 @@ function callOptions(
     presencePenalty: request.generation?.presencePenalty,
     frequencyPenalty: request.generation?.frequencyPenalty,
     seed: request.generation?.seed,
-    tools: flattened.tools.map(tool),
+    tools: ProviderShared.toolDefinitions(request.tools).map(tool),
     toolChoice: toolChoice(request.toolChoice),
     headers: request.http?.headers,
     providerOptions: requestProviderOptions(request.providerOptions, packageName, modelID, optionKey),