feat(dev-757): set MCP user agent for API log attribution

AGENTS.md

@@ -30,7 +30,7 @@ surrounding code before making structural changes.
 - `internal/tools/`, `internal/resources/`, `internal/prompts/` — the MCP surface
   behind `spur mcp`: five tools, schema/doc resources, and the `assess_ip` prompt,
   all wired to the shared `internal/spur` client.
-- `internal/buildinfo/` — ldflags-injected version, consumed by `spur version` and the API User-Agent.
+- `internal/buildinfo/` — ldflags-injected version, consumed by `spur version` and the API User-Agent. The User-Agent carries a `surface/<name>` token identifying the request's origin (set via `Client.Surface`); `spur mcp` sets `mcp`, plain CLI commands leave it empty.
 - `scripts/mcp-smoke.sh` — end-to-end stdio smoke test for `spur mcp`; run against
   a built binary. `SPUR_LIVE=1` adds authenticated calls.
 

internal/app/mcp.go

@@ -68,7 +68,9 @@ func newMCPCmd(cf *configFlags) *cobra.Command {
 			if err != nil {
 				return err
 			}
-			return runMCPServer(cmd, cf.newClient(token))
+			client := cf.newClient(token)
+			client.Surface = "mcp"
+			return runMCPServer(cmd, client)
 		},
 	}
 }

internal/spur/client.go

@@ -19,10 +19,6 @@ const BaseURL = "https://api.spur.us"
 
 const maxResponseBodyBytes int64 = 1 << 20
 
-func userAgent() string {
-	return fmt.Sprintf("spur/%s (+https://spur.us)", buildinfo.Version)
-}
-
 // Client is a stateless-ish wrapper around net/http. Its only mutable state is
 // the embedded *http.Client; each API method is safe for concurrent use.
 type Client struct {
@@ -37,6 +33,21 @@ type Client struct {
 	// distinct host from Base and FeedsBase, overridable independently via
 	// config, and serves the dynamically filtered feed subsets.
 	ExportsBase string
+	// Surface identifies the calling surface for API-log attribution. Empty
+	// means the CLI surface; set to a short token like "mcp" to tag requests
+	// from a different caller (appended to the User-Agent as " surface/<x>").
+	Surface string
+}
+
+// userAgent builds the User-Agent header value. The CLI-default form is
+// unchanged; a non-empty Surface appends " surface/<surface>" so API logs can
+// attribute requests to a specific caller.
+func (c *Client) userAgent() string {
+	ua := fmt.Sprintf("spur/%s (+https://spur.us)", buildinfo.Version)
+	if c.Surface != "" {
+		ua += " surface/" + c.Surface
+	}
+	return ua
 }
 
 // NewClient constructs a Client with a sensible default timeout. The redirect
@@ -123,7 +134,7 @@ func (c *Client) doJSON(ctx context.Context, endpoint string, dst any) (Meta, er
 	}
 	req.Header.Set("Token", c.Token)
 	req.Header.Set("Accept", "application/json")
-	req.Header.Set("User-Agent", userAgent())
+	req.Header.Set("User-Agent", c.userAgent())
 
 	resp, err := c.HTTP.Do(req)
 	if err != nil {

internal/spur/client_test.go

@@ -4,12 +4,62 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 	"net/http"
 	"net/http/httptest"
 	"strings"
 	"testing"
+
+	"github.com/spurintel/cli/internal/buildinfo"
 )
 
+func TestUserAgentDefaultSurface(t *testing.T) {
+	t.Parallel()
+
+	c := &Client{}
+	want := fmt.Sprintf("spur/%s (+https://spur.us)", buildinfo.Version)
+	if got := c.userAgent(); got != want {
+		t.Fatalf("userAgent() = %q, want %q", got, want)
+	}
+}
+
+func TestUserAgentWithSurface(t *testing.T) {
+	t.Parallel()
+
+	c := &Client{Surface: "mcp"}
+	def := fmt.Sprintf("spur/%s (+https://spur.us)", buildinfo.Version)
+	got := c.userAgent()
+	if !strings.HasPrefix(got, def) {
+		t.Fatalf("userAgent() = %q, want prefix %q", got, def)
+	}
+	if !strings.HasSuffix(got, " surface/mcp") {
+		t.Fatalf("userAgent() = %q, want suffix %q", got, " surface/mcp")
+	}
+}
+
+func TestDoJSONSendsSurfaceInUserAgent(t *testing.T) {
+	t.Parallel()
+
+	var gotUA string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotUA = r.Header.Get("User-Agent")
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = w.Write([]byte(`{}`))
+	}))
+	defer srv.Close()
+
+	client := NewClient("token")
+	client.Base = srv.URL
+	client.Surface = "mcp"
+
+	if _, err := client.GetStatus(context.Background()); err != nil {
+		t.Fatalf("GetStatus: %v", err)
+	}
+	if !strings.Contains(gotUA, "surface/mcp") {
+		t.Fatalf("User-Agent = %q, want to contain %q", gotUA, "surface/mcp")
+	}
+}
+
 func TestReadBodyRejectsOversizeResponses(t *testing.T) {
 	t.Parallel()
 

internal/spur/feeds.go

@@ -163,7 +163,7 @@ func (c *Client) doStream(ctx context.Context, endpoint string) (io.ReadCloser,
 		return nil, 0, err
 	}
 	req.Header.Set("Token", c.Token)
-	req.Header.Set("User-Agent", userAgent())
+	req.Header.Set("User-Agent", c.userAgent())
 
 	resp, err := c.downloadHTTPClient().Do(req)
 	if err != nil {

internal/spur/feeds_test.go

@@ -207,6 +207,37 @@ func TestDownloadFeedStreamsBody(t *testing.T) {
 	}
 }
 
+func TestDownloadFeedSendsSurfaceInUserAgent(t *testing.T) {
+	t.Parallel()
+
+	payload := gzipBytes(t, `{"ip":"1.2.3.4"}`+"\n")
+
+	var gotUA string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotUA = r.Header.Get("User-Agent")
+		w.Header().Set("Content-Type", "application/gzip")
+		_, _ = w.Write(payload)
+	}))
+	defer srv.Close()
+
+	client := NewClient("secret")
+	client.FeedsBase = srv.URL
+	client.Surface = "mcp"
+
+	body, _, err := client.DownloadFeed(context.Background(), "anonymous", "", FeedJSONGzip)
+	if err != nil {
+		t.Fatalf("DownloadFeed: %v", err)
+	}
+	defer func() { _ = body.Close() }()
+	if _, err := io.ReadAll(body); err != nil {
+		t.Fatalf("read body: %v", err)
+	}
+
+	if !strings.Contains(gotUA, "surface/mcp") {
+		t.Fatalf("User-Agent = %q, want to contain %q", gotUA, "surface/mcp")
+	}
+}
+
 // The date and format together select the artifact path: latest vs historical,
 // json.gz vs mmdb; a nested slug keeps its slash.
 func TestDownloadFeedPathSelection(t *testing.T) {