diff --git a/core/providers/openai/responses.go b/core/providers/openai/responses.go
index 0f07167aaa2..3ab57be9658 100644
--- a/core/providers/openai/responses.go
+++ b/core/providers/openai/responses.go
@@ -342,7 +342,14 @@ func (resp *OpenAIResponsesRequest) filterUnsupportedTools() {
 	// Filter tools to only include supported types
 	filteredTools := make([]schemas.ResponsesTool, 0, len(resp.Tools))
 	for _, tool := range resp.Tools {
-		if supportedTypes[tool.Type] {
+		// OpenRouter exposes server-side tools under the "openrouter:" namespace
+		// (web_search, web_fetch, datetime, image_generation, apply_patch, subagent, ...).
+		// They are native to OpenRouter and must not be stripped by the
+		// OpenAI-oriented whitelist. Match the whole namespace so future tools are
+		// covered without per-tool additions.
+		isOpenRouterServerTool := resp.Provider == schemas.OpenRouter &&
+			strings.HasPrefix(string(tool.Type), schemas.ResponsesToolTypeOpenRouterPrefix)
+		if supportedTypes[tool.Type] || isOpenRouterServerTool {
 			// check for computer use preview
 			if tool.Type == schemas.ResponsesToolTypeComputerUsePreview && tool.ResponsesToolComputerUsePreview != nil && tool.ResponsesToolComputerUsePreview.EnableZoom != nil {
 				newTool := tool
diff --git a/core/providers/openai/responses_test.go b/core/providers/openai/responses_test.go
index 9dc241bcf5c..cc9aee8e072 100644
--- a/core/providers/openai/responses_test.go
+++ b/core/providers/openai/responses_test.go
@@ -1958,3 +1958,45 @@ func valuesEqual(v1, v2 interface{}) bool {
 		return v1 == v2
 	}
 }
+
+func TestToOpenAIResponsesRequest_OpenRouterServerToolsPreserved(t *testing.T) {
+	makeReq := func(provider schemas.ModelProvider, toolType schemas.ResponsesToolType) *schemas.BifrostResponsesRequest {
+		return &schemas.BifrostResponsesRequest{
+			Provider: provider,
+			Model:    "anthropic/claude-haiku-4.5",
+			Input: []schemas.ResponsesMessage{
+				{
+					Role:    schemas.Ptr(schemas.ResponsesInputMessageRoleUser),
+					Content: &schemas.ResponsesMessageContent{ContentStr: schemas.Ptr("hi")},
+				},
+			},
+			Params: &schemas.ResponsesParameters{
+				Tools: []schemas.ResponsesTool{{Type: toolType}},
+			},
+		}
+	}
+
+	// Any tool under the "openrouter:" namespace must survive for the OpenRouter
+	// provider (web_search, web_fetch, and any future server tool).
+	for _, toolType := range []schemas.ResponsesToolType{"openrouter:web_search", "openrouter:web_fetch"} {
+		t.Run("openrouter keeps "+string(toolType), func(t *testing.T) {
+			result := ToOpenAIResponsesRequest(nil, makeReq(schemas.OpenRouter, toolType))
+			if result == nil {
+				t.Fatal("ToOpenAIResponsesRequest returned nil")
+			}
+			if len(result.Tools) != 1 || result.Tools[0].Type != toolType {
+				t.Fatalf("expected %s to be preserved for OpenRouter, got %+v", toolType, result.Tools)
+			}
+		})
+	}
+
+	t.Run("openai strips openrouter: namespace tools", func(t *testing.T) {
+		result := ToOpenAIResponsesRequest(nil, makeReq(schemas.OpenAI, "openrouter:web_search"))
+		if result == nil {
+			t.Fatal("ToOpenAIResponsesRequest returned nil")
+		}
+		if len(result.Tools) != 0 {
+			t.Fatalf("expected openrouter: tools to be stripped for OpenAI, got %+v", result.Tools)
+		}
+	})
+}
diff --git a/core/schemas/responses.go b/core/schemas/responses.go
index fb765d36e8d..50b4c1cdd77 100644
--- a/core/schemas/responses.go
+++ b/core/schemas/responses.go
@@ -1750,6 +1750,12 @@ const (
 	ResponsesToolTypeAdvisor            ResponsesToolType = "advisor"
 )
 
+// ResponsesToolTypeOpenRouterPrefix is the namespace prefix for OpenRouter server
+// tools (e.g. "openrouter:web_search", "openrouter:web_fetch", "openrouter:datetime",
+// "openrouter:image_generation", "openrouter:apply_patch", "openrouter:subagent").
+// These are executed server-side by OpenRouter and are not part of the OpenAI spec.
+const ResponsesToolTypeOpenRouterPrefix = "openrouter:"
+
 // normalizeResponsesToolType maps versioned/provider-specific tool type strings
 // to their canonical ResponsesToolType. For example, "web_search_20250305" → "web_search".
 // Returns the input unchanged if it's already canonical or unrecognized.