diff --git a/providers/openai/language_model.go b/providers/openai/language_model.go
index 2907008de13b0ff05150a18aadf01b54d99213ed..06610dedb9ff303f676925785e459c274e3f05e9 100644
--- a/providers/openai/language_model.go
+++ b/providers/openai/language_model.go
@@ -517,7 +517,10 @@ func (o languageModel) Stream(ctx context.Context, call fantasy.Call) (fantasy.S
 			}
 
 			// Handle tool calls that finish with empty arguments (e.g., Copilot).
-			// Normalize empty args to "{}" and emit the tool call if valid.
+			// Normalize empty args to "{}" and emit the tool call.
+			// If the arguments are invalid JSON, we still yield the tool call
+			// so the consumer (agent) can handle the error rather than
+			// silently dropping it.
 			for idx, tc := range toolCalls {
 				if tc.hasFinished {
 					continue
@@ -526,16 +529,14 @@ func (o languageModel) Stream(ctx context.Context, call fantasy.Call) (fantasy.S
 					tc.arguments = "{}"
 					toolCalls[idx] = tc
 				}
-				if xjson.IsValid(tc.arguments) {
-					if !yield(fantasy.StreamPart{Type: fantasy.StreamPartTypeToolInputEnd, ID: tc.id}) {
-						return
-					}
-					if !yield(fantasy.StreamPart{Type: fantasy.StreamPartTypeToolCall, ID: tc.id, ToolCallName: tc.name, ToolCallInput: tc.arguments}) {
-						return
-					}
-					tc.hasFinished = true
-					toolCalls[idx] = tc
+				if !yield(fantasy.StreamPart{Type: fantasy.StreamPartTypeToolInputEnd, ID: tc.id}) {
+					return
+				}
+				if !yield(fantasy.StreamPart{Type: fantasy.StreamPartTypeToolCall, ID: tc.id, ToolCallName: tc.name, ToolCallInput: tc.arguments}) {
+					return
 				}
+				tc.hasFinished = true
+				toolCalls[idx] = tc
 			}
 
 			if len(acc.Choices) > 0 {
diff --git a/providers/openai/openai_test.go b/providers/openai/openai_test.go
index 2230deae43e26c63435aa0f1235933a661e4b5db..058c8fc3d04828b24a0a2517ab5d62bedb4beb1c 100644
--- a/providers/openai/openai_test.go
+++ b/providers/openai/openai_test.go
@@ -2296,6 +2296,23 @@ func (sms *streamingMockServer) prepareToolStreamResponseWithEmptyArgs() {
 	sms.chunks = chunks
 }
 
+func (sms *streamingMockServer) prepareToolStreamResponseWithInvalidJSON() {
+	chunks := []string{
+		// Tool call start
+		`data: {"id":"chatcmpl-invalid","object":"chat.completion.chunk","created":1711357598,"model":"gpt-3.5-turbo-0125","system_fingerprint":"fp_3bc1b5746c","choices":[{"index":0,"delta":{"role":"assistant","content":null,"tool_calls":[{"index":0,"id":"call_invalid_json","type":"function","function":{"name":"test-tool","arguments":""}}]},"logprobs":null,"finish_reason":null}]}` + "\n\n",
+		// Arguments delta containing \x00 which is not a valid JSON escape
+		`data: {"id":"chatcmpl-invalid","object":"chat.completion.chunk","created":1711357598,"model":"gpt-3.5-turbo-0125","system_fingerprint":"fp_3bc1b5746c","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"{\"old_string\":\"hello\\x00"}}]},"logprobs":null,"finish_reason":null}]}` + "\n\n",
+		// Remaining arguments — combined is {"old_string":"hello\x00world"} which is invalid JSON
+		`data: {"id":"chatcmpl-invalid","object":"chat.completion.chunk","created":1711357598,"model":"gpt-3.5-turbo-0125","system_fingerprint":"fp_3bc1b5746c","choices":[{"index":0,"delta":{"tool_calls":[{"index":0,"function":{"arguments":"world\"}"}}]},"logprobs":null,"finish_reason":null}]}` + "\n\n",
+		// Finish with tool_calls
+		`data: {"id":"chatcmpl-invalid","object":"chat.completion.chunk","created":1711357598,"model":"gpt-3.5-turbo-0125","system_fingerprint":"fp_3bc1b5746c","choices":[{"index":0,"delta":{},"logprobs":null,"finish_reason":"tool_calls"}]}` + "\n\n",
+		// Usage
+		`data: {"id":"chatcmpl-invalid","object":"chat.completion.chunk","created":1711357598,"model":"gpt-3.5-turbo-0125","system_fingerprint":"fp_3bc1b5746c","choices":[],"usage":{"prompt_tokens":53,"completion_tokens":17,"total_tokens":70}}` + "\n\n",
+		"data: [DONE]\n\n",
+	}
+	sms.chunks = chunks
+}
+
 func collectStreamParts(stream fantasy.StreamResponse) ([]fantasy.StreamPart, error) {
 	var parts []fantasy.StreamPart
 	for part := range stream {
@@ -3022,6 +3039,90 @@ func TestDoStream(t *testing.T) {
 		require.Equal(t, int64(35), finishPart.Usage.TotalTokens)
 		require.Equal(t, int64(10), finishPart.Usage.ReasoningTokens)
 	})
+
+	t.Run("should drop tool calls with invalid JSON arguments", func(t *testing.T) {
+		t.Parallel()
+
+		server := newStreamingMockServer()
+		defer server.close()
+
+		server.prepareToolStreamResponseWithInvalidJSON()
+
+		provider, err := New(
+			WithAPIKey("test-api-key"),
+			WithBaseURL(server.server.URL),
+		)
+		require.NoError(t, err)
+		model, _ := provider.LanguageModel(t.Context(), "gpt-3.5-turbo")
+
+		stream, err := model.Stream(context.Background(), fantasy.Call{
+			Prompt: testPrompt,
+			Tools: []fantasy.Tool{
+				fantasy.FunctionTool{
+					Name: "test-tool",
+					InputSchema: map[string]any{
+						"type": "object",
+						"properties": map[string]any{
+							"old_string": map[string]any{
+								"type": "string",
+							},
+							"new_string": map[string]any{
+								"type": "string",
+							},
+						},
+						"required":             []string{"old_string", "new_string"},
+						"additionalProperties": false,
+						"$schema":              "http://json-schema.org/draft-07/schema#",
+					},
+				},
+			},
+		})
+
+		require.NoError(t, err)
+
+		parts, err := collectStreamParts(stream)
+		require.NoError(t, err)
+
+		// Find tool-related parts
+		toolInputStart, toolInputEnd, toolCall := -1, -1, -1
+		var toolDeltas []string
+		var finishPart *fantasy.StreamPart
+
+		for i, part := range parts {
+			switch part.Type {
+			case fantasy.StreamPartTypeToolInputStart:
+				toolInputStart = i
+				require.Equal(t, "call_invalid_json", part.ID)
+				require.Equal(t, "test-tool", part.ToolCallName)
+			case fantasy.StreamPartTypeToolInputDelta:
+				toolDeltas = append(toolDeltas, part.Delta)
+			case fantasy.StreamPartTypeToolInputEnd:
+				toolInputEnd = i
+				require.Equal(t, "call_invalid_json", part.ID)
+			case fantasy.StreamPartTypeToolCall:
+				toolCall = i
+				require.Equal(t, "call_invalid_json", part.ID)
+				require.Equal(t, "test-tool", part.ToolCallName)
+			case fantasy.StreamPartTypeFinish:
+				finishPart = &part
+			}
+		}
+
+		require.NotEqual(t, -1, toolInputStart, "expected ToolInputStart part")
+		require.NotEqual(t, -1, toolInputEnd, "expected ToolInputEnd part")
+		require.NotEqual(t, -1, toolCall, "expected ToolCall part")
+
+		// Verify tool deltas combine to the complete input with \x00
+		var fullInput strings.Builder
+		for _, delta := range toolDeltas {
+			fullInput.WriteString(delta)
+		}
+		require.Equal(t, `{"old_string":"hello\x00world"}`, fullInput.String())
+
+		// Finish reason is ToolCalls since the tool call was yielded
+		require.NotNil(t, finishPart)
+		require.Equal(t, fantasy.FinishReasonToolCalls, finishPart.FinishReason)
+	})
 }
 
 func TestDefaultToPrompt_DropsEmptyMessages(t *testing.T) {