diff --git a/src/responses/code-mode-helper-compat.ts b/src/responses/code-mode-helper-compat.ts
index 726c5dcf19..e0c71ee87f 100644
--- a/src/responses/code-mode-helper-compat.ts
+++ b/src/responses/code-mode-helper-compat.ts
@@ -3,7 +3,7 @@ import {
   normalizeApplyPatchDelimiters,
   unwrapFreeformToolInput,
 } from "./apply-patch-envelope";
-import { declaresCodeModeExec } from "../types/tools";
+import { declaresCodeModeExec, stripDefaultNamespacePrefix } from "../types/tools";
 
 function isPlainObject(value: unknown): value is Record<string, unknown> {
   return !!value && typeof value === "object" && !Array.isArray(value);
@@ -80,7 +80,7 @@ export function resolveCodeModeHelperName(
   namespace?: string,
   declaredNames?: ReadonlySet<string>,
 ): string | undefined {
-  if (codeModeHelperName) return codeModeHelperName;
+  if (codeModeHelperName) return stripDefaultNamespacePrefix(codeModeHelperName, declaredNames);
   if (toolName !== "exec" || namespace !== undefined) return undefined;
   // `exec` is a name, not a guarantee. Without a catalog that is genuinely code mode, a
   // caller-defined `exec` could legitimately take patch text, and handing it generated
diff --git a/src/server/responses-undeclared-tool-guard.ts b/src/server/responses-undeclared-tool-guard.ts
index cad1b39011..ea5e101e35 100644
--- a/src/server/responses-undeclared-tool-guard.ts
+++ b/src/server/responses-undeclared-tool-guard.ts
@@ -5,7 +5,8 @@ import {
   namespacedToolName,
   normalizeDeclaredToolName,
 } from "../types";
-import { sseDataPayload, type SseBlockRewrite } from "./sse-payload-rewrite";
+import { hasDeclaredNamespaceTools, stripDefaultNamespacePrefix } from "../types/tools";
+import { sseDataPayload, type SseBlockRewrite, type SsePayloadRewrite } from "./sse-payload-rewrite";
 
 /** Item types the client executes through a request-declared wire name. */
 const CLIENT_EXECUTED_CALL_TYPES = new Set(["function_call", "custom_tool_call"]);
@@ -319,6 +320,12 @@ function undeclaredNameInItem(
       dottedAliasIsUnambiguous(item.namespace, name)
       && declared.has(dottedToolName(item.namespace, name))
     ) return undefined;
+    // A routed provider may attach a default namespace to a bare tool, e.g. a
+    // 'default' namespace around 'view_image' when only bare 'view_image' was
+    // declared. Fold it only while that namespace is not genuinely in use.
+    if ((item.namespace === 'default' || item.namespace === 'functions')
+      && declared.has(name)
+      && !hasDeclaredNamespaceTools(declared, item.namespace)) return undefined;
     return name;
   }
   const effectiveName = normalizeDeclaredToolName(name, declared);
@@ -412,3 +419,70 @@ export function createUndeclaredToolCallGuardBlockRewrite(
     return failedBlocks(name, block.includes("\r\n") ? "\r\n" : "\n");
   };
 }
+
+const DEFAULT_STRIP_CALL_TYPES = new Set(['function_call', 'custom_tool_call']);
+const DEFAULT_STRIP_NAMESPACES = new Set(['default', 'functions']);
+
+function normalizeDefaultPrefixedToolCallNode(
+  node: unknown,
+  declared: ReadonlySet<string>,
+): { value: unknown; changed: boolean } {
+  if (Array.isArray(node)) {
+    let changed = false;
+    const out = node.map((entry) => {
+      const result = normalizeDefaultPrefixedToolCallNode(entry, declared);
+      changed = changed || result.changed;
+      return result.value;
+    });
+    return changed ? { value: out, changed: true } : { value: node, changed: false };
+  }
+  if (!isPlainObject(node)) return { value: node, changed: false };
+  let changed = false;
+  const out: Record<string, unknown> = {};
+  for (const key of Object.keys(node)) {
+    const result = normalizeDefaultPrefixedToolCallNode(node[key], declared);
+    out[key] = result.value;
+    changed = changed || result.changed;
+  }
+  if (DEFAULT_STRIP_CALL_TYPES.has(out.type as string) && typeof out.name === 'string') {
+    if (typeof out.namespace === 'string' && DEFAULT_STRIP_NAMESPACES.has(out.namespace)) {
+      const namespace = out.namespace;
+      const bare = out.name;
+      if (!declared.has(namespace + '__' + bare) && !declared.has(namespace + '.' + bare)
+        && declared.has(bare)
+        && !hasDeclaredNamespaceTools(declared, namespace)) {
+        delete out.namespace;
+        changed = true;
+      }
+    } else if (!('namespace' in out)) {
+      const stripped = stripDefaultNamespacePrefix(out.name, declared);
+      if (stripped !== out.name && declared.has(stripped)) {
+        out.name = stripped;
+        changed = true;
+      }
+    }
+  }
+  return changed ? { value: out, changed: true } : { value: node, changed: false };
+}
+
+export function normalizeDefaultNamespacePrefixInJson(
+  text: string,
+  declared: ReadonlySet<string>,
+): string {
+  // No substring fast path here: JSON unicode escapes can hide a default-namespace
+  // prefix from byte matching, so every payload goes through the parser below.
+  let payload: unknown;
+  try {
+    payload = JSON.parse(text);
+  } catch {
+    return text;
+  }
+  const result = normalizeDefaultPrefixedToolCallNode(payload, declared);
+  return result.changed ? JSON.stringify(result.value) : text;
+}
+
+export function createDefaultNamespacePrefixStripRewrite(
+  declared: ReadonlySet<string>,
+): SsePayloadRewrite {
+  return (payload) => normalizeDefaultNamespacePrefixInJson(payload, declared);
+}
diff --git a/src/server/responses/core.ts b/src/server/responses/core.ts
index 3f4cfe4345..253bbeecf3 100644
--- a/src/server/responses/core.ts
+++ b/src/server/responses/core.ts
@@ -430,6 +430,8 @@ import {
   createUndeclaredToolCallGuardBlockRewrite,
   currentTurnWireToolCatalogBody,
   hasExplicitWireToolCatalog,
+  createDefaultNamespacePrefixStripRewrite,
+  normalizeDefaultNamespacePrefixInJson,
   undeclaredToolCallMessage,
   undeclaredToolCallName,
   undeclaredToolCallNameInResponse,
@@ -5938,6 +5940,10 @@ async function handleResponsesInner(
         : undefined;
       // Compose opt-in payload rewrites into one parse/stringify pass (image-gen restore first).
       const payloadRewrites = [
+        // Fold a provider-added default namespace ('default.view_image' for a
+        // declared bare 'view_image') back to bare before the undeclared-tool
+        // guard compares names the client will actually receive.
+        createDefaultNamespacePrefixStripRewrite(new Set(declaredWireToolNames)),
         createImageGenCallRestoreRewrite(imageGenCallAliases),
         // #3217: a call whose namespace repeats its own name is unroutable in codex-rs.
         createSelfNamedToolCallNamespaceScrubRewrite(selfNamedNamespaceScrubAuthorization),
@@ -6220,7 +6226,11 @@ async function handleResponsesInner(
           restored,
           routedToolSearchNames,
         );
-        const repaired = normalizeFunctionCompletionJson(restoredToolSearch);
+        const restoredDefaultPrefix = normalizeDefaultNamespacePrefixInJson(
+          restoredToolSearch,
+          declaredWireToolNames,
+        );
+        const repaired = normalizeFunctionCompletionJson(restoredDefaultPrefix);
         const modelRewritten = parsed._responseModelId !== undefined && parsed._responseModelId !== parsed.modelId
           ? rewriteResponsesModelJson(repaired, parsed._responseModelId)
           : repaired;
diff --git a/src/types/tools.ts b/src/types/tools.ts
index bb91ebe63f..1085825ecc 100644
--- a/src/types/tools.ts
+++ b/src/types/tools.ts
@@ -66,22 +66,69 @@ const CODE_MODE_HELPER_TOOL_NAMES = [
  */
 export const CODE_MODE_EXEC_TOOL_NAME = "exec";
 
+/**
+ * Wire prefixes that mean 'no namespace'.
+ *
+ * Some routed providers qualify a bare tool with a default namespace on the
+ * way back (observed: muse-spark via opencode-go emits 'default.view_image'
+ * for the Codex client bare 'view_image'). 'default' carries no tool identity
+ * of its own, and the reserved 'functions' group likewise has no wire prefix,
+ * so fold that spelling back to the bare name, but only while the catalog
+ * declares no tools under that namespace. A genuinely declared full name
+ * always wins over the fold, so a real namespaced identity can never be
+ * misrouted to bare.
+ */
+const DEFAULT_NAMESPACE_PREFIXES = [
+  { namespace: 'default', prefixes: ['default.', 'default__'] },
+  { namespace: 'functions', prefixes: ['functions.', 'functions__'] },
+] as const;
+
+export function hasDeclaredNamespaceTools(
+  declared: ReadonlySet<string>,
+  namespace: string,
+): boolean {
+  const dot = namespace + '.';
+  const flat = namespace + '__';
+  for (const name of declared) {
+    if (name.startsWith(dot) || name.startsWith(flat)) return true;
+  }
+  return false;
+}
+
+export function stripDefaultNamespacePrefix(
+  name: string,
+  declared: ReadonlySet<string> | undefined,
+): string {
+  if (!declared || declared.has(name)) return name;
+  for (const entry of DEFAULT_NAMESPACE_PREFIXES) {
+    for (const prefix of entry.prefixes) {
+      if (name.startsWith(prefix) && name.length > prefix.length) {
+        if (hasDeclaredNamespaceTools(declared, entry.namespace)) return name;
+        return name.slice(prefix.length);
+      }
+    }
+  }
+  return name;
+}
+
 export function normalizeDeclaredToolName(
   name: string,
   declared: ReadonlySet<string> | undefined,
 ): string {
-  if (!declared || !declared.has(CODE_MODE_EXEC_TOOL_NAME)) return name;
-  if (declared.has(name)) return name;
-  if (name === "apply_patch") return CODE_MODE_EXEC_TOOL_NAME;
+  if (!declared) return name;
+  const unprefixed = stripDefaultNamespacePrefix(name, declared);
+  if (!declared.has(CODE_MODE_EXEC_TOOL_NAME)) return unprefixed;
+  if (declared.has(unprefixed)) return unprefixed;
+  if (unprefixed === 'apply_patch') return CODE_MODE_EXEC_TOOL_NAME;
   // When the catalog explicitly declares any legacy shell bridge name, the environment
   // genuinely exposes that tool — turn normalization off so a call is never mis-routed
   // to `exec`.
   if ((LEGACY_SHELL_BRIDGE_TOOL_NAMES as readonly string[]).some(legacy => declared.has(legacy))) {
-    return name;
+    return unprefixed;
   }
-  return (CODE_MODE_HELPER_TOOL_NAMES as readonly string[]).includes(name)
+  return (CODE_MODE_HELPER_TOOL_NAMES as readonly string[]).includes(unprefixed)
     ? CODE_MODE_EXEC_TOOL_NAME
-    : name;
+    : unprefixed;
 }
 
 /**
diff --git a/tests/responses/responses-undeclared-tool-guard.test.ts b/tests/responses/responses-undeclared-tool-guard.test.ts
index 41d8383a6c..c1b3b306bf 100644
--- a/tests/responses/responses-undeclared-tool-guard.test.ts
+++ b/tests/responses/responses-undeclared-tool-guard.test.ts
@@ -12,10 +12,13 @@ import {
   createUndeclaredToolCallGuardBlockRewrite,
   currentTurnWireToolCatalogBody,
   hasExplicitWireToolCatalog,
+  normalizeDefaultNamespacePrefixInJson,
+  undeclaredToolCallName,
   undeclaredToolCallNameInResponse,
   UNDECLARED_TOOL_CALL_ERROR_CODE,
   type ProviderExecutedCallType,
 } from "../../src/server/responses-undeclared-tool-guard";
+import { normalizeDeclaredToolName } from "../../src/types/tools";
 import { relaySseWithBlockRewrite } from "../../src/server/sse-payload-rewrite";
 import { handleResponses } from "../../src/server/responses";
 import { expandPreviousResponseInput } from "../../src/responses/state";
@@ -1840,3 +1843,102 @@ describe("xAI hosted-call authorization through handleResponses", () => {
     expect(body.error.message).toContain('undeclared client tool "x_keyword_search"');
   });
 });
+
+describe("provider-added default namespace prefix", () => {
+  const viewImageCatalog = {
+    tools: [{ type: "function", name: "view_image" }],
+  };
+
+  function declaredBareViewImage(): Set<string> {
+    return collectDeclaredWireToolNames(viewImageCatalog);
+  }
+
+  test("dotted default prefix resolves to the declared bare tool", () => {
+    const declared = declaredBareViewImage();
+    expect(declared.has("view_image")).toBe(true);
+    expect(
+      undeclaredToolCallName(
+        {
+          type: "response.output_item.added",
+          item: { type: "function_call", name: "default.view_image", call_id: "call_1" },
+        },
+        declared,
+      ),
+    ).toBeUndefined();
+    expect(normalizeDeclaredToolName("default.view_image", declared)).toBe("view_image");
+    expect(normalizeDeclaredToolName("default__view_image", declared)).toBe("view_image");
+    expect(normalizeDeclaredToolName("functions.view_image", declared)).toBe("view_image");
+    expect(normalizeDeclaredToolName("functions__view_image", declared)).toBe("view_image");
+  });
+
+  test("namespaced default shape resolves to the declared bare tool", () => {
+    const declared = declaredBareViewImage();
+    expect(
+      undeclaredToolCallName(
+        {
+          type: "response.output_item.added",
+          item: { type: "function_call", name: "view_image", namespace: "default", call_id: "call_2" },
+        },
+        declared,
+      ),
+    ).toBeUndefined();
+  });
+
+  test("genuinely undeclared default-prefixed tools still fail closed", () => {
+    const declared = declaredBareViewImage();
+    expect(
+      undeclaredToolCallName(
+        {
+          type: "response.output_item.added",
+          item: { type: "function_call", name: "default.apply_patch", call_id: "call_3" },
+        },
+        declared,
+      ),
+    ).toBe("default.apply_patch");
+    expect(
+      normalizeDefaultNamespacePrefixInJson(JSON.stringify({ name: "default.apply_patch" }), declared),
+    ).toBe(JSON.stringify({ name: "default.apply_patch" }));
+  });
+
+  test("no fold while the default namespace is genuinely declared", () => {
+    const declared = collectDeclaredWireToolNames({
+      tools: [{ type: "namespace", name: "default", tools: [{ type: "function", name: "view_image" }] }],
+    });
+    expect(declared.has("default__view_image")).toBe(true);
+    expect(normalizeDeclaredToolName("default.other", declared)).toBe("default.other");
+  });
+
+  test("payload rewrite restores the bare name before relay", () => {
+    const declared = declaredBareViewImage();
+    const rewritten = normalizeDefaultNamespacePrefixInJson(
+      JSON.stringify({
+        type: "response.output_item.added",
+        item: { type: "function_call", name: "default.view_image", call_id: "call_1" },
+      }),
+      declared,
+    );
+    const parsed = JSON.parse(rewritten) as { item: { name: string } };
+    expect(parsed.item.name).toBe("view_image");
+  });
+
+  test("guard block rewrite relays the folded call without failing", () => {
+    const declared = declaredBareViewImage();
+    const guard = createUndeclaredToolCallGuardBlockRewrite(declared);
+    const blocks = guard(
+      frame("response.output_item.added", {
+        output_index: 0,
+        item: {
+          type: "function_call",
+          id: "fc_1",
+          call_id: "call_1",
+          name: "default.view_image",
+          arguments: "{}",
+          status: "in_progress",
+        },
+      }),
+    );
+    expect(blocks).toHaveLength(1);
+    expect(blocks[0]).toContain("response.output_item.added");
+    expect(blocks[0]).not.toContain("response.failed");
+  });
+});