diff --git a/.changeset/fix-duplicate-tool-parts.md b/.changeset/fix-duplicate-tool-parts.md
new file mode 100644
index 000000000000..48b1acbffec7
--- /dev/null
+++ b/.changeset/fix-duplicate-tool-parts.md
@@ -0,0 +1,5 @@
+---
+'ai': patch
+---
+
+Fixed duplicate tool parts when model calls an unavailable tool. The `tool-input-start` chunk creates a static part without a `dynamic` flag, but the subsequent `tool-input-error` chunk arrives with `dynamic: true`, causing a second `dynamic-tool` part to be created with the same `toolCallId`. This led to providers like Amazon Bedrock rejecting the message for duplicate tool use IDs.
diff --git a/packages/ai/src/ui/process-ui-message-stream.ts b/packages/ai/src/ui/process-ui-message-stream.ts
index be5476768cf6..dac822b877de 100644
--- a/packages/ai/src/ui/process-ui-message-stream.ts
+++ b/packages/ai/src/ui/process-ui-message-stream.ts
@@ -558,7 +558,12 @@ export function processUIMessageStream<UI_MESSAGE extends UIMessage>({
             }
 
             case 'tool-input-error': {
-              if (chunk.dynamic) {
+              const partialToolCall = state.partialToolCalls[chunk.toolCallId];
+              const dynamic = partialToolCall
+                ? partialToolCall.dynamic
+                : chunk.dynamic;
+
+              if (dynamic) {
                 updateDynamicToolPart({
                   toolCallId: chunk.toolCallId,
                   toolName: chunk.toolName,