diff --git a/.gitignore b/.gitignore
index aff78ef6..ce234077 100644
--- a/.gitignore
+++ b/.gitignore
@@ -3,6 +3,9 @@
 .idea
 **/*.swp
 
+# AI tools
+.claude
+
 # Project specific
 example/aibridge.db
 build/
diff --git a/example/main.go b/example/main.go
index b6011416..0a54aace 100644
--- a/example/main.go
+++ b/example/main.go
@@ -46,7 +46,8 @@ func main() {
 	// Configure providers.
 	providers := []aibridge.Provider{
 		aibridge.NewAnthropicProvider(aibridge.AnthropicConfig{
-			Key: os.Getenv("ANTHROPIC_API_KEY"),
+			Key:     os.Getenv("ANTHROPIC_API_KEY"),
+			BaseURL: os.Getenv("ANTHROPIC_BASE_URL"),
 		}, nil),
 		aibridge.NewOpenAIProvider(aibridge.OpenAIConfig{
 			Key: os.Getenv("OPENAI_API_KEY"),
diff --git a/intercept/apidump/apidump.go b/intercept/apidump/apidump.go
index 9f6f5ec3..63fccf7d 100644
--- a/intercept/apidump/apidump.go
+++ b/intercept/apidump/apidump.go
@@ -26,6 +26,8 @@ const (
 	SuffixRequest = ".req.txt"
 	// SuffixResponse is the file suffix for response dump files.
 	SuffixResponse = ".resp.txt"
+	// SuffixError is the file suffix for error dump files written when a request fails.
+	SuffixError = ".req_error.txt"
 )
 
 // MiddlewareNext is the function to call the next middleware or the actual request.
@@ -51,9 +53,11 @@ func NewBridgeMiddleware(baseDir string, provider string, model string, intercep
 			logger.Named("apidump").Warn(req.Context(), "failed to dump request", slog.Error(err))
 		}
 
-		// TODO: https://github.com/coder/aibridge/issues/129
 		resp, err := next(req)
 		if err != nil {
+			if dumpErr := d.dumpError(err); dumpErr != nil {
+				logger.Named("apidump").Warn(req.Context(), "failed to dump request error", slog.Error(dumpErr))
+			}
 			return resp, err
 		}
 
@@ -113,6 +117,14 @@ func (d *dumper) dumpRequest(req *http.Request) error {
 	return os.WriteFile(dumpPath, buf.Bytes(), 0o644) //nolint:gosec // https://github.com/coder/aibridge/pull/256#discussion_r3072143983
 }
 
+func (d *dumper) dumpError(reqErr error) error {
+	dumpPath := d.dumpPath + SuffixError
+	if err := os.MkdirAll(filepath.Dir(dumpPath), 0o755); err != nil {
+		return xerrors.Errorf("create dump dir: %w", err)
+	}
+	return os.WriteFile(dumpPath, []byte(reqErr.Error()+"\n"), 0o644) //nolint:gosec // same rationale as other dump files
+}
+
 func (d *dumper) dumpResponse(resp *http.Response) error {
 	dumpPath := d.dumpPath + SuffixResponse
 
@@ -248,6 +260,9 @@ func (rt *dumpRoundTripper) RoundTrip(req *http.Request) (*http.Response, error)
 
 	resp, err := rt.inner.RoundTrip(req)
 	if err != nil {
+		if dumpErr := dumper.dumpError(err); dumpErr != nil {
+			dumper.logger.Named("apidump").Warn(req.Context(), "failed to dump passthrough request error", slog.Error(dumpErr))
+		}
 		return resp, err
 	}
 
diff --git a/intercept/apidump/apidump_test.go b/intercept/apidump/apidump_test.go
index 1aaf56c0..f3fb30e6 100644
--- a/intercept/apidump/apidump_test.go
+++ b/intercept/apidump/apidump_test.go
@@ -147,6 +147,34 @@ func TestBridgedMiddleware_RedactsSensitiveResponseHeaders(t *testing.T) {
 	require.Contains(t, content, "X-Request-Id: req-123")
 }
 
+func TestBridgedMiddleware_WritesErrorFile_WhenNextFails(t *testing.T) {
+	t.Parallel()
+
+	tmpDir := t.TempDir()
+	logger := slogtest.Make(t, &slogtest.Options{IgnoreErrors: false}).Leveled(slog.LevelDebug)
+	clk := quartz.NewMock(t)
+	interceptionID := uuid.New()
+
+	middleware := NewBridgeMiddleware(tmpDir, "openai", "gpt-4", interceptionID, logger, clk)
+	require.NotNil(t, middleware)
+
+	req, err := http.NewRequestWithContext(t.Context(), http.MethodPost, "https://api.openai.com/v1/chat/completions", bytes.NewReader([]byte(`{}`)))
+	require.NoError(t, err)
+
+	upstreamErr := io.ErrUnexpectedEOF
+	resp, err := middleware(req, func(_ *http.Request) (*http.Response, error) { //nolint:bodyclose // resp is nil on error
+		return nil, upstreamErr
+	})
+	require.ErrorIs(t, err, upstreamErr)
+	require.Nil(t, resp)
+
+	modelDir := filepath.Join(tmpDir, "openai", "gpt-4")
+	errDumpPath := findDumpFile(t, modelDir, SuffixError)
+	content, readErr := os.ReadFile(errDumpPath)
+	require.NoError(t, readErr)
+	require.Contains(t, string(content), upstreamErr.Error())
+}
+
 func TestBridgedMiddleware_EmptyBaseDir_ReturnsNil(t *testing.T) {
 	t.Parallel()
 
@@ -365,6 +393,12 @@ func TestPassthroughMiddleware(t *testing.T) {
 		resp, err := rt.RoundTrip(req) //nolint:bodyclose // resp is nil on error
 		require.ErrorIs(t, err, innerErr)
 		require.Nil(t, resp)
+
+		passthroughDir := filepath.Join(tmpDir, "openai", "passthrough")
+		errDumpPath := findDumpFile(t, passthroughDir, SuffixError)
+		content, readErr := os.ReadFile(errDumpPath)
+		require.NoError(t, readErr)
+		require.Contains(t, string(content), innerErr.Error())
 	})
 
 	t.Run("dumps_request_and_response", func(t *testing.T) {