diff --git a/src/adapters/openai-responses.ts b/src/adapters/openai-responses.ts
index bfb9dcce1..a650eba72 100644
--- a/src/adapters/openai-responses.ts
+++ b/src/adapters/openai-responses.ts
@@ -708,10 +708,13 @@ function preferConfiguredHostedTools(
     if (strippedTopLevelImageGenTool && Array.isArray(tools)) {
       tools = [...tools, { type: HOSTED_IMAGE_GENERATION_TOOL }];
     } else if (strippedAdditionalToolsIndices.size > 0 && Array.isArray(input)) {
-      // Restore in EVERY container we stripped, not just the first: a request carrying
-      // two `additional_tools` groups would otherwise leave the later ones with no image
-      // capability at all. Raised by the automated review on #924.
-      input = input.map((item, index) => strippedAdditionalToolsIndices.has(index)
+      // Restore into the FIRST stripped container only. Tool declarations are
+      // request-scoped, not container-scoped — the containers are separate carriers for
+      // one tool set, so a single hosted declaration covers the request. An earlier
+      // revision restored into every stripped container and put `image_generation` on
+      // the wire twice; review caught it.
+      const firstStripped = Math.min(...strippedAdditionalToolsIndices);
+      input = input.map((item, index) => index === firstStripped
         && isPlainObject(item)
         && Array.isArray(item.tools)
         ? { ...item, tools: [...item.tools, { type: HOSTED_IMAGE_GENERATION_TOOL }] }
diff --git a/src/config.ts b/src/config.ts
index 005b44ae3..fe523323a 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -724,7 +724,22 @@ export function modelPreferHostedToolsConfigError(
   if (prototype !== Object.prototype && prototype !== null) return `${field} must be a plain object with own properties`;
   const entries = Object.entries(value);
   const registry = getProviderRegistryEntry(providerName);
-  if (entries.length > 0 && (registry?.authKind === "forward" || (!registry && provider.authMode === "forward"))) {
+  // Effective transport: a `preserveCustomDestination` registry row reused under a
+  // different endpoint keeps its own adapter AND its own auth at runtime, because
+  // `routedProviderConfig()` honors `providerMatchesRegistryTransport()`. Both the
+  // wire check below and the forward-auth check here have to start from the same
+  // decision, or validation accepts a preference the adapter never applies —
+  // `preferConfiguredHostedTools()` runs only on the non-forward branch.
+  const registryTransportMatches = typeof provider.baseUrl === "string"
+    && providerMatchesRegistryTransport(providerName, {
+      baseUrl: provider.baseUrl,
+      adapter: provider.adapter as OcxProviderConfig["adapter"],
+      ...(typeof provider.authMode === "string" ? { authMode: provider.authMode as OcxProviderConfig["authMode"] } : {}),
+    });
+  const effectiveForwardAuth = registryTransportMatches
+    ? registry?.authKind === "forward"
+    : provider.authMode === "forward";
+  if (entries.length > 0 && effectiveForwardAuth) {
     return `${field} is not supported on forward-auth Responses providers`;
   }
   const requestedWireFor = (modelId: string): unknown => provider.modelAdapters
@@ -773,18 +788,9 @@ export function modelPreferHostedToolsConfigError(
         return `${field}.${key} cannot prefer ${tool}: the model does not support it`;
       }
     }
-    // Start from the registry adapter only when this config still points at the registry's
-    // documented transport. A `preserveCustomDestination` row reused under a different
-    // endpoint keeps its own adapter at runtime (`routedProviderConfig()` honors
-    // `providerMatchesRegistryTransport()`), so trusting `registry.adapter` there would
-    // accept a preference the Responses adapter never sees. Raised by the automated
-    // review on #924.
-    const registryTransportMatches = typeof provider.baseUrl === "string"
-      && providerMatchesRegistryTransport(providerName, {
-        baseUrl: provider.baseUrl,
-        adapter: provider.adapter as OcxProviderConfig["adapter"],
-        ...(typeof provider.authMode === "string" ? { authMode: provider.authMode as OcxProviderConfig["authMode"] } : {}),
-      });
+    // Same `registryTransportMatches` decision the forward-auth check above uses:
+    // start from the registry adapter only when this config still points at the
+    // registry's documented transport.
     const baseWire = registryTransportMatches ? registry?.adapter ?? provider.adapter : provider.adapter;
     let effectiveWire = resolveEffectiveWire(key, baseWire);
     const virtualWireModel = resolveOpenAiVirtualModel(providerName, key)?.wireModelId;
diff --git a/tests/config.test.ts b/tests/config.test.ts
index e687d3b9d..e5855aa49 100644
--- a/tests/config.test.ts
+++ b/tests/config.test.ts
@@ -1070,6 +1070,27 @@ describe("opencodex config defaults", () => {
     expect(readConfigDiagnostics().source).toBe("fallback");
     expect(readConfigDiagnostics().error).toContain("requires the openai-responses wire");
 
+    // The forward-auth half of the same effective-transport question. A
+    // `preserveCustomDestination` registry row reused under a different endpoint keeps
+    // its OWN auth at runtime, not the registry's, because `routedProviderConfig()`
+    // honors `providerMatchesRegistryTransport()`. Deciding forward-auth from
+    // `registry.authKind` alone accepted a preference the adapter never applies:
+    // `preferConfiguredHostedTools()` runs only on the non-forward branch.
+    writeConfig({
+      port: 12345,
+      providers: {
+        "volcengine-agent-plan": {
+          adapter: "openai-responses",
+          authMode: "forward",
+          baseUrl: "https://custom.example.test/v1",
+          modelPreferHostedTools: { "some-model": ["image_generation"] },
+        },
+      },
+      defaultProvider: "volcengine-agent-plan",
+    });
+    expect(readConfigDiagnostics().source).toBe("fallback");
+    expect(readConfigDiagnostics().error).toContain("not supported on forward-auth");
+
     // Registry providers route through their registry wire, not this persisted adapter.
     writeConfig({
       port: 12345,
diff --git a/tests/openai-responses-passthrough.test.ts b/tests/openai-responses-passthrough.test.ts
index 9b7040c0a..34b281842 100644
--- a/tests/openai-responses-passthrough.test.ts
+++ b/tests/openai-responses-passthrough.test.ts
@@ -1147,9 +1147,12 @@ describe("OpenAI Responses hosted-tool name conflicts", () => {
     }
   });
 
-  test("every stripped additional_tools container gets hosted image generation restored", () => {
-    // Stripping runs over all containers, but restoration originally targeted only the
-    // first stripped index, so a second container lost its image capability entirely.
+  test("multi-container stripping restores hosted image generation exactly once", () => {
+    // Stripping runs over every container. Restoration must not: tool declarations are
+    // request-scoped, and `hasHostedImageGenDeclaration` treats a declaration in any
+    // container as covering the request. #924 briefly restored into each stripped
+    // container and put `image_generation` on the wire twice; this asserts against both
+    // that and the original defect of losing the capability entirely.
     const adapter = createResponsesPassthroughAdapter({
       ...keyedProvider,
       modelPreferHostedTools: { "provider-image-model": ["image_generation"] },
@@ -1181,9 +1184,12 @@ describe("OpenAI Responses hosted-tool name conflicts", () => {
     const containers = body.input.filter(item => item.type === "additional_tools");
 
     expect(containers).toHaveLength(2);
-    for (const container of containers) {
-      expect(container.tools).toContainEqual({ type: "image_generation" });
-    }
+    const hostedDeclarations = containers.flatMap(container =>
+      (container.tools ?? []).filter(tool => tool.type === "image_generation"));
+    // Exactly one hosted declaration on the wire, riding the first stripped container
+    // so the capability is neither lost nor duplicated.
+    expect(hostedDeclarations).toEqual([{ type: "image_generation" }]);
+    expect(containers[0].tools).toContainEqual({ type: "image_generation" });
   });
 
   test("configured model rewrites a custom image-gen selector", () => {