diff --git a/internal/api/modules/amp/response_rewriter.go b/internal/api/modules/amp/response_rewriter.go
index 390f301d..707fe576 100644
--- a/internal/api/modules/amp/response_rewriter.go
+++ b/internal/api/modules/amp/response_rewriter.go
@@ -2,6 +2,7 @@ package amp
 
 import (
 	"bytes"
+	"encoding/json"
 	"fmt"
 	"net/http"
 	"strings"
@@ -290,8 +291,10 @@ func (rw *ResponseRewriter) rewriteStreamEvent(data []byte) []byte {
 }
 
 // SanitizeAmpRequestBody removes thinking blocks with empty/missing/invalid signatures
-// from the messages array in a request body before forwarding to the upstream API.
-// This prevents 400 errors from the API which requires valid signatures on thinking blocks.
+// and strips the proxy-injected "signature" field from tool_use blocks in the messages
+// array before forwarding to the upstream API.
+// This prevents 400 errors from the API which requires valid signatures on thinking
+// blocks and does not accept a signature field on tool_use blocks.
 func SanitizeAmpRequestBody(body []byte) []byte {
 	messages := gjson.GetBytes(body, "messages")
 	if !messages.Exists() || !messages.IsArray() {
@@ -309,21 +312,30 @@ func SanitizeAmpRequestBody(body []byte) []byte {
 		}
 
 		var keepBlocks []interface{}
-		removedCount := 0
+		contentModified := false
 
 		for _, block := range content.Array() {
 			blockType := block.Get("type").String()
 			if blockType == "thinking" {
 				sig := block.Get("signature")
 				if !sig.Exists() || sig.Type != gjson.String || strings.TrimSpace(sig.String()) == "" {
-					removedCount++
+					contentModified = true
 					continue
 				}
 			}
-			keepBlocks = append(keepBlocks, block.Value())
+
+			// Use raw JSON to prevent float64 rounding of large integers in tool_use inputs
+			blockRaw := []byte(block.Raw)
+			if blockType == "tool_use" && block.Get("signature").Exists() {
+				blockRaw, _ = sjson.DeleteBytes(blockRaw, "signature")
+				contentModified = true
+			}
+
+			// sjson.SetBytes supports raw JSON strings if wrapped in gjson.Raw
+			keepBlocks = append(keepBlocks, json.RawMessage(blockRaw))
 		}
 
-		if removedCount > 0 {
+		if contentModified {
 			contentPath := fmt.Sprintf("messages.%d.content", msgIdx)
 			var err error
 			if len(keepBlocks) == 0 {
@@ -332,11 +344,10 @@ func SanitizeAmpRequestBody(body []byte) []byte {
 				body, err = sjson.SetBytes(body, contentPath, keepBlocks)
 			}
 			if err != nil {
-				log.Warnf("Amp RequestSanitizer: failed to remove thinking blocks from message %d: %v", msgIdx, err)
+				log.Warnf("Amp RequestSanitizer: failed to sanitize message %d: %v", msgIdx, err)
 				continue
 			}
 			modified = true
-			log.Debugf("Amp RequestSanitizer: removed %d thinking blocks with invalid signatures from message %d", removedCount, msgIdx)
 		}
 	}
 
diff --git a/internal/api/modules/amp/response_rewriter_test.go b/internal/api/modules/amp/response_rewriter_test.go
index 31ba56bd..ac95dfc6 100644
--- a/internal/api/modules/amp/response_rewriter_test.go
+++ b/internal/api/modules/amp/response_rewriter_test.go
@@ -145,6 +145,36 @@ func TestSanitizeAmpRequestBody_RemovesWhitespaceAndNonStringSignatures(t *testi
 	}
 }
 
+func TestSanitizeAmpRequestBody_StripsSignatureFromToolUseBlocks(t *testing.T) {
+	input := []byte(`{"messages":[{"role":"assistant","content":[{"type":"thinking","thinking":"thought","signature":"valid-sig"},{"type":"tool_use","id":"toolu_01","name":"Bash","input":{"cmd":"ls"},"signature":""}]}]}`)
+	result := SanitizeAmpRequestBody(input)
+
+	if contains(result, []byte(`"signature":""`)) {
+		t.Fatalf("expected signature to be stripped from tool_use block, got %s", string(result))
+	}
+	if !contains(result, []byte(`"valid-sig"`)) {
+		t.Fatalf("expected thinking signature to remain, got %s", string(result))
+	}
+	if !contains(result, []byte(`"tool_use"`)) {
+		t.Fatalf("expected tool_use block to remain, got %s", string(result))
+	}
+}
+
+func TestSanitizeAmpRequestBody_MixedInvalidThinkingAndToolUseSignature(t *testing.T) {
+	input := []byte(`{"messages":[{"role":"assistant","content":[{"type":"thinking","thinking":"drop-me","signature":""},{"type":"tool_use","id":"toolu_01","name":"Bash","input":{"cmd":"ls"},"signature":""}]}]}`)
+	result := SanitizeAmpRequestBody(input)
+
+	if contains(result, []byte("drop-me")) {
+		t.Fatalf("expected invalid thinking block to be removed, got %s", string(result))
+	}
+	if contains(result, []byte(`"signature"`)) {
+		t.Fatalf("expected signature to be stripped from tool_use block, got %s", string(result))
+	}
+	if !contains(result, []byte(`"tool_use"`)) {
+		t.Fatalf("expected tool_use block to remain, got %s", string(result))
+	}
+}
+
 func contains(data, substr []byte) bool {
 	for i := 0; i <= len(data)-len(substr); i++ {
 		if string(data[i:i+len(substr)]) == string(substr) {