fix(copilot): forward role-specific model config to launch and restore

backend/internal/adapters/agent/copilot/copilot.go

@@ -72,6 +72,23 @@ func (p *Plugin) Manifest() adapters.Manifest {
 	}
 }
 
+// GetConfigSpec reports the per-project agent config keys the GitHub Copilot
+// CLI understands: a model override passed via --model.
+func (p *Plugin) GetConfigSpec(ctx context.Context) (ports.ConfigSpec, error) {
+	if err := ctx.Err(); err != nil {
+		return ports.ConfigSpec{}, err
+	}
+	return ports.ConfigSpec{
+		Fields: []ports.ConfigField{
+			{
+				Key:         "model",
+				Type:        ports.ConfigFieldString,
+				Description: "Model override passed to `copilot --model` (e.g. claude-sonnet-4.5).",
+			},
+		},
+	}, nil
+}
+
 // GetLaunchCommand builds the argv to start a new interactive Copilot session:
 //
 //	copilot [permission flags] [--agent ao-<session>] [--interactive <prompt>]
@@ -89,6 +106,7 @@ func (p *Plugin) GetLaunchCommand(ctx context.Context, cfg ports.LaunchConfig) (
 
 	cmd = append(cmd, binary)
 	appendApprovalFlags(&cmd, cfg.Permissions)
+	appendModelFlag(&cmd, cfg.Config)
 	if agentName := copilotAgentName(cfg.SessionID, cfg.SystemPrompt, cfg.SystemPromptFile); agentName != "" {
 		cmd = append(cmd, "--agent="+agentName)
 	}
@@ -133,6 +151,10 @@ func (p *Plugin) GetRestoreCommand(ctx context.Context, cfg ports.RestoreConfig)
 
 	cmd = append(cmd, binary)
 	appendApprovalFlags(&cmd, cfg.Permissions)
+	// Deliberately does not forward cfg.Config.Model: --model + --resume
+	// composition is unverified against a real copilot install (see #2895
+	// and appendModelFlag). Pinned by
+	// TestGetRestoreCommandDoesNotForwardModelOverride.
 	if agentName := copilotAgentName(cfg.Session.ID, cfg.SystemPrompt, cfg.SystemPromptFile); agentName != "" {
 		cmd = append(cmd, "--agent="+agentName)
 	}
@@ -318,3 +340,24 @@ func appendApprovalFlags(cmd *[]string, permissions ports.PermissionMode) {
 		*cmd = append(*cmd, "--allow-all")
 	}
 }
+
+// appendModelFlag appends `--model <trimmed>` when cfg.Model is set,
+// mirroring the Codex adapter's pattern (see #2869) and taking the full
+// ports.AgentConfig for consistency with the sibling adapters. A blank or
+// whitespace-only value is omitted so Copilot falls back to its own default
+// resolution (COPILOT_MODEL env, or its configured default) exactly as an
+// unconfigured launch would.
+//
+// Restore-path note: this is intentionally NOT called from
+// GetRestoreCommand. An attempt to verify `--model` + `--resume`
+// composition against a real copilot install was inconclusive: the test
+// account is on the Copilot Free plan, which only grants access to "auto"
+// regardless of --model, on both launch and resume. That makes the plan
+// itself a confound, not evidence either way about whether --resume
+// honors --model. Left as a fast-follow pending verification from an
+// account with paid-plan model access (see #2895).
+func appendModelFlag(cmd *[]string, cfg ports.AgentConfig) {
+	if trimmed := strings.TrimSpace(cfg.Model); trimmed != "" {
+		*cmd = append(*cmd, "--model", trimmed)
+	}
+}

backend/internal/adapters/agent/copilot/copilot_test.go

@@ -190,15 +190,95 @@ func TestGetLaunchCommandSelectsSessionCustomAgent(t *testing.T) {
 	}
 }
 
-func TestGetConfigSpecHasNoCustomFieldsYet(t *testing.T) {
+func TestGetConfigSpecReportsModelField(t *testing.T) {
 	plugin := &Plugin{}
 
 	spec, err := plugin.GetConfigSpec(context.Background())
+	if err != nil {
+		t.Fatalf("GetConfigSpec: %v", err)
+	}
+
+	var found bool
+	for _, f := range spec.Fields {
+		if f.Key != "model" {
+			continue
+		}
+		found = true
+		if f.Type != ports.ConfigFieldString {
+			t.Errorf("model field Type = %v, want %v", f.Type, ports.ConfigFieldString)
+		}
+		if f.Description == "" {
+			t.Error("model field Description is empty")
+		}
+	}
+	if !found {
+		t.Fatalf("GetConfigSpec did not report a \"model\" field: %#v", spec.Fields)
+	}
+}
+
+func TestGetConfigSpecRespectsCanceledContext(t *testing.T) {
+	plugin := &Plugin{}
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+
+	if _, err := plugin.GetConfigSpec(ctx); err == nil {
+		t.Fatal("GetConfigSpec with canceled context: err = nil, want non-nil")
+	}
+}
+
+func TestGetLaunchCommandAppendsModelFlag(t *testing.T) {
+	plugin := &Plugin{resolvedBinary: "copilot"}
+
+	cmd, err := plugin.GetLaunchCommand(context.Background(), ports.LaunchConfig{
+		Config: ports.AgentConfig{Model: "claude-sonnet-4.5"},
+	})
 	if err != nil {
 		t.Fatal(err)
 	}
-	if len(spec.Fields) != 0 {
-		t.Fatalf("unexpected config fields: %#v", spec.Fields)
+	want := []string{"copilot", "--model", "claude-sonnet-4.5"}
+	if !reflect.DeepEqual(cmd, want) {
+		t.Fatalf("cmd = %#v, want %#v", cmd, want)
+	}
+}
+
+func TestGetLaunchCommandOmitsBlankModel(t *testing.T) {
+	plugin := &Plugin{resolvedBinary: "copilot"}
+
+	cmd, err := plugin.GetLaunchCommand(context.Background(), ports.LaunchConfig{
+		Config: ports.AgentConfig{Model: "   "},
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if contains(cmd, "--model") {
+		t.Fatalf("command %#v unexpectedly contains --model for blank config", cmd)
+	}
+}
+
+// Restore path intentionally does not forward a configured model override —
+// see appendModelFlag's doc comment (issue #2895; --model + --resume
+// composition could not be verified — the test account's Copilot Free plan
+// only grants "auto" regardless of --model). This test locks that decision
+// in so a future "just mirror the launch path" edit doesn't silently wire
+// it without re-verifying.
+func TestGetRestoreCommandDoesNotForwardModelOverride(t *testing.T) {
+	plugin := &Plugin{resolvedBinary: "copilot"}
+
+	cmd, ok, err := plugin.GetRestoreCommand(context.Background(), ports.RestoreConfig{
+		Config: ports.AgentConfig{Model: "claude-sonnet-4.5"},
+		Session: ports.SessionRef{
+			Metadata: map[string]string{ports.MetadataKeyAgentSessionID: "uuid-123"},
+		},
+	})
+	if err != nil {
+		t.Fatalf("err = %v, want nil", err)
+	}
+	if !ok {
+		t.Fatal("ok = false, want true")
+	}
+	want := []string{"copilot", "--resume", "uuid-123"}
+	if !reflect.DeepEqual(cmd, want) {
+		t.Fatalf("cmd = %#v, want %#v (model override must not be forwarded on restore)", cmd, want)
 	}
 }