diff --git a/packages/provider/openai.go b/packages/provider/openai.go
index 22c0d6e..79a9f2f 100644
--- a/packages/provider/openai.go
+++ b/packages/provider/openai.go
@@ -8,12 +8,32 @@ import (
 	"fmt"
 	"io"
 	"net/http"
+	"regexp"
 	"strings"
 	"time"
 )
 
 const openaiDefaultBaseURL = "https://api.openai.com"
 
+// versionSegmentSuffix matches a trailing API version segment such as
+// "/v1" or Z.AI's "/v4".
+var versionSegmentSuffix = regexp.MustCompile(`/v\d+$`)
+
+// chatCompletionsURL builds the chat-completions endpoint for an
+// OpenAI-compatible base URL. A base that already carries an API
+// version segment gets "/chat/completions" appended directly; a bare
+// host (e.g. api.openai.com) gets the conventional "/v1/chat/completions".
+//
+// Matching any "/vN" segment (not just "/v1") keeps Z.AI's coding-plan
+// base, which ends in "/paas/v4", from getting a spurious "/v1" that
+// yields ".../paas/v4/v1/chat/completions" and a 404.
+func chatCompletionsURL(baseURL string) string {
+	if versionSegmentSuffix.MatchString(baseURL) {
+		return baseURL + "/chat/completions"
+	}
+	return baseURL + "/v1/chat/completions"
+}
+
 type openaiClient struct {
 	apiKey  string
 	baseURL string
@@ -412,10 +432,7 @@ func buildOAIContentBlocks(blocks []Content, isError bool) []interface{} {
 // ---- streaming ----
 
 func (c *openaiClient) Stream(ctx context.Context, req Request) (<-chan Event, error) {
-	apiPath := "/v1/chat/completions"
-	if strings.HasSuffix(c.baseURL, "/v1") {
-		apiPath = "/chat/completions"
-	}
+	endpoint := chatCompletionsURL(c.baseURL)
 	wire, err := c.buildRequest(req)
 	if err != nil {
 		return nil, err
@@ -425,7 +442,7 @@ func (c *openaiClient) Stream(ctx context.Context, req Request) (<-chan Event, e
 		return nil, err
 	}
 	newReq := func() (*http.Request, error) {
-		httpReq, err := http.NewRequestWithContext(ctx, "POST", c.baseURL+apiPath, bytes.NewReader(body))
+		httpReq, err := http.NewRequestWithContext(ctx, "POST", endpoint, bytes.NewReader(body))
 		if err != nil {
 			return nil, err
 		}
diff --git a/packages/provider/openai_url_test.go b/packages/provider/openai_url_test.go
new file mode 100644
index 0000000..ef89bd3
--- /dev/null
+++ b/packages/provider/openai_url_test.go
@@ -0,0 +1,40 @@
+package provider
+
+import "testing"
+
+// TestChatCompletionsURL pins the endpoint built for OpenAI-compatible
+// providers. The regression that motivated it: Z.AI's coding-plan base
+// carries a "/v4" version segment, so blindly appending
+// "/v1/chat/completions" produced ".../paas/v4/v1/chat/completions"
+// and a 404. Any base that already ends in a version segment must get
+// "/chat/completions" appended directly.
+func TestChatCompletionsURL(t *testing.T) {
+	cases := []struct {
+		name    string
+		baseURL string
+		want    string
+	}{
+		{
+			name:    "bare host gets conventional /v1 path",
+			baseURL: "https://api.openai.com",
+			want:    "https://api.openai.com/v1/chat/completions",
+		},
+		{
+			name:    "v1 base is not doubled",
+			baseURL: "https://api.moonshot.ai/v1",
+			want:    "https://api.moonshot.ai/v1/chat/completions",
+		},
+		{
+			name:    "zai coding plan v4 base is not given a spurious /v1",
+			baseURL: "https://api.z.ai/api/coding/paas/v4",
+			want:    "https://api.z.ai/api/coding/paas/v4/chat/completions",
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := chatCompletionsURL(tc.baseURL); got != tc.want {
+				t.Errorf("chatCompletionsURL(%q) = %q, want %q", tc.baseURL, got, tc.want)
+			}
+		})
+	}
+}