diff --git a/packages/provider/amazon_bedrock.go b/packages/provider/amazon_bedrock.go
index a3baaf6..7229ec7 100644
--- a/packages/provider/amazon_bedrock.go
+++ b/packages/provider/amazon_bedrock.go
@@ -299,6 +299,24 @@ func bedrockModelSupportsCaching(modelID string) bool {
 	return strings.HasPrefix(modelID, "anthropic.claude-")
 }
 
+// bedrockMessagesHaveToolBlocks reports whether any message in msgs
+// contains a toolUse or toolResult content block. Bedrock's Converse API
+// rejects the request with HTTP 400 if those block types are present but
+// toolConfig is absent.
+func bedrockMessagesHaveToolBlocks(msgs []bedrockMessage) bool {
+	for _, m := range msgs {
+		for _, c := range m.Content {
+			if _, ok := c["toolUse"]; ok {
+				return true
+			}
+			if _, ok := c["toolResult"]; ok {
+				return true
+			}
+		}
+	}
+	return false
+}
+
 func (c *bedrockClient) buildRequest(req Request) (*bedrockRequest, error) {
 	out := &bedrockRequest{}
 
@@ -380,6 +398,21 @@ func (c *bedrockClient) buildRequest(req Request) (*bedrockRequest, error) {
 			tc.Tools = append(tc.Tools, ts)
 		}
 		out.ToolConfig = &tc
+	} else if bedrockMessagesHaveToolBlocks(out.Messages) {
+		// Bedrock requires toolConfig to be present whenever the message
+		// history contains toolUse or toolResult content blocks, even if
+		// the current request does not intend to call any tools (e.g. the
+		// /btw side-chat sends the frozen main transcript which may include
+		// prior tool turns). Inject a stub tool so the API accepts the
+		// request; it will never be invoked since no tool_use stop reason
+		// can be triggered when the real tool list is empty.
+		var stub bedrockToolSpec
+		stub.ToolSpec.Name = "_placeholder"
+		stub.ToolSpec.Description = "placeholder required by Bedrock when tool history is present"
+		stub.ToolSpec.InputSchema.JSON = json.RawMessage(`{"type":"object","properties":{}}`)
+		out.ToolConfig = &struct {
+			Tools []bedrockToolSpec `json:"tools"`
+		}{Tools: []bedrockToolSpec{stub}}
 	}
 
 	if caching {
diff --git a/packages/provider/amazon_bedrock_test.go b/packages/provider/amazon_bedrock_test.go
index 2c0038f..aa13c9a 100644
--- a/packages/provider/amazon_bedrock_test.go
+++ b/packages/provider/amazon_bedrock_test.go
@@ -357,6 +357,75 @@ func TestBedrockBuildRequestCachingWireJSON(t *testing.T) {
 	}
 }
 
+// TestBedrockBuildRequestBtwToolHistory reproduces the /btw HTTP 400 bug:
+// the side-chat sends no tools but the frozen main transcript contains
+// toolUse / toolResult blocks. Bedrock rejects such a request unless
+// toolConfig is present. buildRequest must inject a stub toolConfig.
+func TestBedrockBuildRequestBtwToolHistory(t *testing.T) {
+	client := &bedrockClient{region: "us-east-1"}
+	req, err := client.buildRequest(Request{
+		Model: "amazon.nova-pro-v1:0",
+		// No tools — simulates /btw side-chat.
+		Messages: []Message{
+			// Frozen main transcript: one tool call + result already happened.
+			{Role: RoleAssistant, Content: []Content{
+				ToolCallBlock{ID: "tc-1", Name: "bash", Arguments: json.RawMessage(`{"command":"ls"}`)},
+			}},
+			{Role: RoleTool, Content: []Content{
+				ToolResultBlock{CallID: "tc-1", Content: []Content{TextBlock{Text: "file.go"}}},
+			}},
+			// Side-chat question appended by btwDialog.submit.
+			{Role: RoleUser, Content: []Content{TextBlock{Text: "what does that file do?"}}},
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if req.ToolConfig == nil {
+		t.Fatal("buildRequest should inject a stub toolConfig when history has tool blocks and req.Tools is empty, but ToolConfig is nil")
+	}
+	if len(req.ToolConfig.Tools) == 0 {
+		t.Fatal("stub toolConfig should have at least one tool")
+	}
+	// The stub must have a valid name and a non-nil schema so Bedrock won't
+	// reject it for a different reason.
+	stub := req.ToolConfig.Tools[0]
+	if stub.ToolSpec.Name == "" {
+		t.Error("stub tool name must not be empty")
+	}
+	if stub.ToolSpec.InputSchema.JSON == nil {
+		t.Error("stub tool schema must not be nil")
+	}
+	// Serialised wire payload must include toolConfig.
+	b, err := json.Marshal(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(b), `"toolConfig"`) {
+		t.Errorf("serialised request missing toolConfig: %s", b)
+	}
+}
+
+// TestBedrockBuildRequestNoToolHistoryNoStub ensures that when there are
+// no tool blocks in the history and no tools in the request (ordinary
+// conversational call), we do NOT inject a toolConfig at all — the extra
+// field is unnecessary and some models may behave differently with it.
+func TestBedrockBuildRequestNoToolHistoryNoStub(t *testing.T) {
+	client := &bedrockClient{region: "us-east-1"}
+	req, err := client.buildRequest(Request{
+		Model: "amazon.nova-pro-v1:0",
+		Messages: []Message{
+			{Role: RoleUser, Content: []Content{TextBlock{Text: "hello"}}},
+		},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if req.ToolConfig != nil {
+		t.Errorf("expected nil ToolConfig for plain conversation, got: %+v", req.ToolConfig)
+	}
+}
+
 func TestBedrockBuildRequestSkipsEmptyToolMessages(t *testing.T) {
 	client := &bedrockClient{}
 	req, err := client.buildRequest(Request{Messages: []Message{