diff --git a/internal/protocol/ops/request_anthropic_server_tool_use.go b/internal/protocol/ops/request_anthropic_server_tool_use.go
new file mode 100644
index 000000000..978f79403
--- /dev/null
+++ b/internal/protocol/ops/request_anthropic_server_tool_use.go
@@ -0,0 +1,117 @@
+package ops
+
+import (
+	"fmt"
+	"regexp"
+
+	"github.com/anthropics/anthropic-sdk-go"
+)
+
+// serverToolUseIDPattern is the ID format api.anthropic.com enforces on
+// server_tool_use blocks (messages.N.content.M.server_tool_use.id).
+var serverToolUseIDPattern = regexp.MustCompile(`^srvtoolu_[a-zA-Z0-9_]+$`)
+
+// serverToolUseIDInvalidChars matches every character the pattern above rejects.
+var serverToolUseIDInvalidChars = regexp.MustCompile(`[^a-zA-Z0-9_]`)
+
+// SanitizeAnthropicV1ServerToolUseIDs rewrites server_tool_use block IDs in the
+// replayed message history so they satisfy Anthropic's ^srvtoolu_[a-zA-Z0-9_]+$
+// requirement. History that passed through another provider (an OpenAI-converted
+// response or a third-party Anthropic-compatible endpoint) can carry IDs minted
+// in that provider's format; forwarding them verbatim makes api.anthropic.com
+// reject the whole request with a 400. Result blocks referencing a rewritten ID
+// via tool_use_id are remapped to keep the pairing intact.
+func SanitizeAnthropicV1ServerToolUseIDs(req *anthropic.MessageNewParams) {
+	if req == nil {
+		return
+	}
+
+	remap := make(map[string]string)
+	unnamed := 0
+	for mi := range req.Messages {
+		for bi := range req.Messages[mi].Content {
+			stu := req.Messages[mi].Content[bi].OfServerToolUse
+			if stu == nil || serverToolUseIDPattern.MatchString(stu.ID) {
+				continue
+			}
+			newID, ok := remap[stu.ID]
+			if !ok {
+				newID = rewriteServerToolUseID(stu.ID, &unnamed)
+				if stu.ID != "" {
+					remap[stu.ID] = newID
+				}
+			}
+			stu.ID = newID
+		}
+	}
+
+	if len(remap) == 0 {
+		return
+	}
+	for mi := range req.Messages {
+		for bi := range req.Messages[mi].Content {
+			if idRef := req.Messages[mi].Content[bi].GetToolUseID(); idRef != nil {
+				if newID, ok := remap[*idRef]; ok {
+					*idRef = newID
+				}
+			}
+		}
+	}
+}
+
+// SanitizeAnthropicBetaServerToolUseIDs is the Beta-variant of
+// SanitizeAnthropicV1ServerToolUseIDs.
+func SanitizeAnthropicBetaServerToolUseIDs(req *anthropic.BetaMessageNewParams) {
+	if req == nil {
+		return
+	}
+
+	remap := make(map[string]string)
+	unnamed := 0
+	for mi := range req.Messages {
+		for bi := range req.Messages[mi].Content {
+			stu := req.Messages[mi].Content[bi].OfServerToolUse
+			if stu == nil || serverToolUseIDPattern.MatchString(stu.ID) {
+				continue
+			}
+			newID, ok := remap[stu.ID]
+			if !ok {
+				newID = rewriteServerToolUseID(stu.ID, &unnamed)
+				if stu.ID != "" {
+					remap[stu.ID] = newID
+				}
+			}
+			stu.ID = newID
+		}
+	}
+
+	if len(remap) == 0 {
+		return
+	}
+	for mi := range req.Messages {
+		for bi := range req.Messages[mi].Content {
+			if idRef := req.Messages[mi].Content[bi].GetToolUseID(); idRef != nil {
+				if newID, ok := remap[*idRef]; ok {
+					*idRef = newID
+				}
+			}
+		}
+	}
+}
+
+// rewriteServerToolUseID derives a conforming ID from a foreign one. The
+// original ID is kept (with rejected characters replaced by "_") so the model
+// can still correlate the block across turns, and the mapping stays
+// deterministic for identical input. Empty IDs get a per-request counter since
+// they carry nothing to correlate on.
+func rewriteServerToolUseID(id string, unnamed *int) string {
+	if id == "" {
+		*unnamed++
+		return fmt.Sprintf("srvtoolu_missing_%d", *unnamed)
+	}
+	sanitized := serverToolUseIDInvalidChars.ReplaceAllString(id, "_")
+	if serverToolUseIDPattern.MatchString(sanitized) {
+		return sanitized
+	}
+	return "srvtoolu_" + sanitized
+}
diff --git a/internal/protocol/ops/request_anthropic_server_tool_use_test.go b/internal/protocol/ops/request_anthropic_server_tool_use_test.go
new file mode 100644
index 000000000..ab4210a04
--- /dev/null
+++ b/internal/protocol/ops/request_anthropic_server_tool_use_test.go
@@ -0,0 +1,248 @@
+package ops
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+
+	"github.com/anthropics/anthropic-sdk-go"
+)
+
+const betaServerToolUseHistoryJSON = `{
+	"model": "claude-sonnet-5",
+	"max_tokens": 1024,
+	"messages": [
+		{"role": "user", "content": [{"type": "text", "text": "search something"}]},
+		{"role": "assistant", "content": [
+			{"type": "text", "text": "searching"},
+			{"type": "thinking", "thinking": "t", "signature": "s"},
+			{"type": "server_tool_use", "id": "call_abc-123", "name": "web_search", "input": {"query": "q"}}
+		]},
+		{"role": "user", "content": [
+			{"type": "web_search_tool_result", "tool_use_id": "call_abc-123", "content": []}
+		]}
+	]
+}`
+
+func TestSanitizeAnthropicBetaServerToolUseIDs_RewritesInvalidIDAndRemapsResult(t *testing.T) {
+	var req anthropic.BetaMessageNewParams
+	if err := json.Unmarshal([]byte(betaServerToolUseHistoryJSON), &req); err != nil {
+		t.Fatalf("unmarshal request: %v", err)
+	}
+
+	SanitizeAnthropicBetaServerToolUseIDs(&req)
+
+	stu := req.Messages[1].Content[2].OfServerToolUse
+	if stu == nil {
+		t.Fatalf("server_tool_use block lost after sanitize")
+	}
+	if !serverToolUseIDPattern.MatchString(stu.ID) {
+		t.Fatalf("server_tool_use id %q still invalid", stu.ID)
+	}
+	if !strings.Contains(stu.ID, "call_abc") {
+		t.Fatalf("rewritten id %q lost the original correlation hint", stu.ID)
+	}
+
+	result := req.Messages[2].Content[0].OfWebSearchToolResult
+	if result == nil {
+		t.Fatalf("web_search_tool_result block lost after sanitize")
+	}
+	if result.ToolUseID != stu.ID {
+		t.Fatalf("tool_use_id %q not remapped to %q", result.ToolUseID, stu.ID)
+	}
+
+	// The sanitized request must serialize with the new IDs.
+	b, err := json.Marshal(req)
+	if err != nil {
+		t.Fatalf("marshal request: %v", err)
+	}
+	if strings.Contains(string(b), "call_abc-123") {
+		t.Fatalf("serialized request still contains the invalid id: %s", b)
+	}
+}
+
+func TestSanitizeAnthropicBetaServerToolUseIDs_KeepsValidIDs(t *testing.T) {
+	req := anthropic.BetaMessageNewParams{
+		Messages: []anthropic.BetaMessageParam{
+			betaAssistantMessage(
+				anthropic.NewBetaServerToolUseBlock("srvtoolu_01AbC", map[string]any{"query": "q"}, "web_search"),
+			),
+		},
+	}
+
+	SanitizeAnthropicBetaServerToolUseIDs(&req)
+
+	if got := req.Messages[0].Content[0].OfServerToolUse.ID; got != "srvtoolu_01AbC" {
+		t.Fatalf("valid id was rewritten to %q", got)
+	}
+}
+
+func TestSanitizeAnthropicBetaServerToolUseIDs_EmptyIDsGetDistinctIDs(t *testing.T) {
+	req := anthropic.BetaMessageNewParams{
+		Messages: []anthropic.BetaMessageParam{
+			betaAssistantMessage(
+				anthropic.NewBetaServerToolUseBlock("", nil, "web_search"),
+				anthropic.NewBetaServerToolUseBlock("", nil, "web_search"),
+			),
+		},
+	}
+
+	SanitizeAnthropicBetaServerToolUseIDs(&req)
+
+	first := req.Messages[0].Content[0].OfServerToolUse.ID
+	second := req.Messages[0].Content[1].OfServerToolUse.ID
+	if !serverToolUseIDPattern.MatchString(first) || !serverToolUseIDPattern.MatchString(second) {
+		t.Fatalf("empty ids not rewritten: %q, %q", first, second)
+	}
+	if first == second {
+		t.Fatalf("empty ids collided: %q", first)
+	}
+}
+
+func TestSanitizeAnthropicV1ServerToolUseIDs_RewritesInvalidIDAndRemapsResult(t *testing.T) {
+	req := anthropic.MessageNewParams{
+		Messages: []anthropic.MessageParam{
+			anthropic.NewAssistantMessage(
+				anthropic.NewServerToolUseBlock("toolu_xyz.9", map[string]any{"query": "q"}, "web_search"),
+			),
+			{
+				Role: anthropic.MessageParamRoleUser,
+				Content: []anthropic.ContentBlockParamUnion{
+					{OfWebSearchToolResult: &anthropic.WebSearchToolResultBlockParam{ToolUseID: "toolu_xyz.9"}},
+				},
+			},
+		},
+	}
+
+	SanitizeAnthropicV1ServerToolUseIDs(&req)
+
+	stu := req.Messages[0].Content[0].OfServerToolUse
+	if !serverToolUseIDPattern.MatchString(stu.ID) {
+		t.Fatalf("server_tool_use id %q still invalid", stu.ID)
+	}
+	if got := req.Messages[1].Content[0].OfWebSearchToolResult.ToolUseID; got != stu.ID {
+		t.Fatalf("tool_use_id %q not remapped to %q", got, stu.ID)
+	}
+}
+
+func TestSanitizeAnthropicV1ServerToolUseIDs_SameOldIDMapsToSameNewID(t *testing.T) {
+	req := anthropic.MessageNewParams{
+		Messages: []anthropic.MessageParam{
+			anthropic.NewAssistantMessage(
+				anthropic.NewServerToolUseBlock("call_1", nil, "web_search"),
+			),
+			anthropic.NewAssistantMessage(
+				anthropic.NewServerToolUseBlock("call_1", nil, "web_search"),
+			),
+		},
+	}
+
+	SanitizeAnthropicV1ServerToolUseIDs(&req)
+
+	first := req.Messages[0].Content[0].OfServerToolUse.ID
+	second := req.Messages[1].Content[0].OfServerToolUse.ID
+	if first != second {
+		t.Fatalf("same old id mapped to different new ids: %q vs %q", first, second)
+	}
+}
+
+func TestSanitizeAnthropicBetaServerToolUseIDs_PrefixedButInvalidCharsNotDoublePrefixed(t *testing.T) {
+	req := anthropic.BetaMessageNewParams{
+		Messages: []anthropic.BetaMessageParam{
+			betaAssistantMessage(
+				anthropic.NewBetaServerToolUseBlock("srvtoolu_abc-def", nil, "web_search"),
+			),
+		},
+	}
+
+	SanitizeAnthropicBetaServerToolUseIDs(&req)
+
+	if got := req.Messages[0].Content[0].OfServerToolUse.ID; got != "srvtoolu_abc_def" {
+		t.Fatalf("expected srvtoolu_abc_def, got %q", got)
+	}
+}
+
+func TestSanitizeAnthropicBetaServerToolUseIDs_MixedValidAndInvalid(t *testing.T) {
+	req := anthropic.BetaMessageNewParams{
+		Messages: []anthropic.BetaMessageParam{
+			betaAssistantMessage(
+				anthropic.NewBetaServerToolUseBlock("srvtoolu_ok1", nil, "web_search"),
+				anthropic.NewBetaServerToolUseBlock("call_bad-1", nil, "web_search"),
+			),
+			anthropic.NewBetaUserMessage(
+				anthropic.BetaContentBlockParamUnion{OfWebSearchToolResult: &anthropic.BetaWebSearchToolResultBlockParam{ToolUseID: "srvtoolu_ok1"}},
+				anthropic.BetaContentBlockParamUnion{OfWebSearchToolResult: &anthropic.BetaWebSearchToolResultBlockParam{ToolUseID: "call_bad-1"}},
+			),
+		},
+	}
+
+	SanitizeAnthropicBetaServerToolUseIDs(&req)
+
+	if got := req.Messages[0].Content[0].OfServerToolUse.ID; got != "srvtoolu_ok1" {
+		t.Fatalf("valid id was rewritten to %q", got)
+	}
+	if got := req.Messages[1].Content[0].OfWebSearchToolResult.ToolUseID; got != "srvtoolu_ok1" {
+		t.Fatalf("result referencing valid id was remapped to %q", got)
+	}
+	rewritten := req.Messages[0].Content[1].OfServerToolUse.ID
+	if !serverToolUseIDPattern.MatchString(rewritten) {
+		t.Fatalf("invalid id not rewritten: %q", rewritten)
+	}
+	if got := req.Messages[1].Content[1].OfWebSearchToolResult.ToolUseID; got != rewritten {
+		t.Fatalf("result referencing invalid id not remapped: %q vs %q", got, rewritten)
+	}
+}
+
+func TestSanitizeAnthropicBetaServerToolUseIDs_PlainToolResultRemapped(t *testing.T) {
+	req := anthropic.BetaMessageNewParams{
+		Messages: []anthropic.BetaMessageParam{
+			betaAssistantMessage(
+				anthropic.NewBetaServerToolUseBlock("ws-42", nil, "web_search"),
+			),
+			anthropic.NewBetaUserMessage(
+				anthropic.NewBetaToolResultBlock("ws-42", "result", false),
+			),
+		},
+	}
+
+	SanitizeAnthropicBetaServerToolUseIDs(&req)
+
+	rewritten := req.Messages[0].Content[0].OfServerToolUse.ID
+	if got := req.Messages[1].Content[0].OfToolResult.ToolUseID; got != rewritten {
+		t.Fatalf("plain tool_result not remapped: %q vs %q", got, rewritten)
+	}
+}
+
+func TestSanitizeAnthropicBetaServerToolUseIDs_RegularToolUseUntouched(t *testing.T) {
+	req := anthropic.BetaMessageNewParams{
+		Messages: []anthropic.BetaMessageParam{
+			betaAssistantMessage(
+				anthropic.NewBetaToolUseBlock("call_regular-1", map[string]any{}, "get_weather"),
+			),
+			anthropic.NewBetaUserMessage(
+				anthropic.NewBetaToolResultBlock("call_regular-1", "sunny", false),
+			),
+		},
+	}
+
+	SanitizeAnthropicBetaServerToolUseIDs(&req)
+
+	if got := req.Messages[0].Content[0].OfToolUse.ID; got != "call_regular-1" {
+		t.Fatalf("regular tool_use id was rewritten to %q", got)
+	}
+	if got := req.Messages[1].Content[0].OfToolResult.ToolUseID; got != "call_regular-1" {
+		t.Fatalf("regular tool_result id was rewritten to %q", got)
+	}
+}
+
+func TestSanitizeAnthropicServerToolUseIDs_NilRequestNoPanic(t *testing.T) {
+	SanitizeAnthropicV1ServerToolUseIDs(nil)
+	SanitizeAnthropicBetaServerToolUseIDs(nil)
+}
+
+func betaAssistantMessage(blocks ...anthropic.BetaContentBlockParamUnion) anthropic.BetaMessageParam {
+	return anthropic.BetaMessageParam{
+		Role:    anthropic.BetaMessageParamRoleAssistant,
+		Content: blocks,
+	}
+}
diff --git a/internal/protocol/transform/vendor.go b/internal/protocol/transform/vendor.go
index cf699e89e..75149435e 100644
--- a/internal/protocol/transform/vendor.go
+++ b/internal/protocol/transform/vendor.go
@@ -74,6 +74,7 @@ func (t *VendorTransform) applyAnthropicV1(ctx *TransformContext, req *anthropic
 	case strings.Contains(url, "api.anthropic.com"), strings.Contains(url, "claude.ai"):
 		req = ops.ApplyAnthropicV1ModelTransform(req, string(req.Model))
 		req = ops.ApplyAnthropicV1MetadataTransform(req, ctx.configExtraForMetadata())
+		ops.SanitizeAnthropicV1ServerToolUseIDs(req)
 	case strings.Contains(url, "api.deepseek.com"):
 		ops.SanitizeAnthropicV1ThinkingConfig(req)
 		ops.ApplyAnthropicV1DeepSeekThinkingPatch(req)
@@ -89,6 +90,7 @@ func (t *VendorTransform) applyAnthropicBeta(ctx *TransformContext, req *anthrop
 	case strings.Contains(url, "api.anthropic.com"), strings.Contains(url, "claude.ai"):
 		req = ops.ApplyAnthropicBetaModelTransform(req, string(req.Model))
 		req = ops.ApplyAnthropicBetaMetadataTransform(req, ctx.configExtraForMetadata())
+		ops.SanitizeAnthropicBetaServerToolUseIDs(req)
 	case strings.Contains(url, "api.deepseek.com"):
 		ops.SanitizeAnthropicBetaThinkingConfig(req)
 		ops.ApplyAnthropicBetaDeepSeekThinkingPatch(req)
diff --git a/internal/protocol/transform/vendor_test.go b/internal/protocol/transform/vendor_test.go
index a9c997914..46d00ca1b 100644
--- a/internal/protocol/transform/vendor_test.go
+++ b/internal/protocol/transform/vendor_test.go
@@ -944,3 +944,61 @@ func TestVendorTransform_AnthropicV1_ThinkingBlocksFiltered(t *testing.T) {
 	}
 	assert.True(t, foundText, "text block should be preserved")
 }
+
+func newBetaRequestWithForeignServerToolUseID(model string) *anthropic.BetaMessageNewParams {
+	return &anthropic.BetaMessageNewParams{
+		Model:     anthropic.Model(model),
+		MaxTokens: 1024,
+		Messages: []anthropic.BetaMessageParam{
+			{
+				Role: anthropic.BetaMessageParamRoleAssistant,
+				Content: []anthropic.BetaContentBlockParamUnion{
+					anthropic.NewBetaServerToolUseBlock("call_foreign-1", map[string]any{"query": "q"}, "web_search"),
+				},
+			},
+			anthropic.NewBetaUserMessage(
+				anthropic.BetaContentBlockParamUnion{
+					OfWebSearchToolResult: &anthropic.BetaWebSearchToolResultBlockParam{ToolUseID: "call_foreign-1"},
+				},
+			),
+		},
+	}
+}
+
+func TestVendorTransform_AnthropicBeta_SanitizesServerToolUseIDs(t *testing.T) {
+	vt := NewVendorTransform()
+
+	ctx := &TransformContext{
+		Provider: &typ.Provider{APIBase: "https://api.anthropic.com"},
+		Request:  newBetaRequestWithForeignServerToolUseID("claude-sonnet-5"),
+		Extra:    map[string]interface{}{},
+	}
+
+	require.NoError(t, vt.Apply(ctx))
+
+	req, ok := ctx.Request.(*anthropic.BetaMessageNewParams)
+	require.True(t, ok)
+
+	stu := req.Messages[0].Content[0].OfServerToolUse
+	require.NotNil(t, stu)
+	assert.Regexp(t, `^srvtoolu_[a-zA-Z0-9_]+$`, stu.ID)
+	assert.Equal(t, stu.ID, req.Messages[1].Content[0].OfWebSearchToolResult.ToolUseID,
+		"web_search_tool_result must keep pointing at the rewritten server_tool_use id")
+}
+
+func TestVendorTransform_NonAnthropicTarget_LeavesServerToolUseIDs(t *testing.T) {
+	vt := NewVendorTransform()
+
+	ctx := &TransformContext{
+		Provider: &typ.Provider{APIBase: "https://api.deepseek.com"},
+		Request:  newBetaRequestWithForeignServerToolUseID("deepseek-chat"),
+		Extra:    map[string]interface{}{},
+	}
+
+	require.NoError(t, vt.Apply(ctx))
+
+	req, ok := ctx.Request.(*anthropic.BetaMessageNewParams)
+	require.True(t, ok)
+	assert.Equal(t, "call_foreign-1", req.Messages[0].Content[0].OfServerToolUse.ID,
+		"non-Anthropic targets must not have their ids rewritten")
+}