diff --git a/aibridge/internal/testutil/mockprovider.go b/aibridge/internal/testutil/mockprovider.go
index 0c56cf2c9eee9..0fd85d2863637 100644
--- a/aibridge/internal/testutil/mockprovider.go
+++ b/aibridge/internal/testutil/mockprovider.go
@@ -13,12 +13,11 @@ import (
 )
 
 type MockProvider struct {
-	NameStr              string
-	URL                  string
-	Bridged              []string
-	Passthrough          []string
-	InterceptorFunc      func(w http.ResponseWriter, r *http.Request, tracer trace.Tracer) (intercept.Interceptor, error)
-	InjectAuthHeaderFunc func(h *http.Header)
+	NameStr         string
+	URL             string
+	Bridged         []string
+	Passthrough     []string
+	InterceptorFunc func(w http.ResponseWriter, r *http.Request, tracer trace.Tracer) (intercept.Interceptor, error)
 }
 
 func (m *MockProvider) Type() string                { return m.NameStr }
@@ -28,11 +27,6 @@ func (m *MockProvider) RoutePrefix() string         { return fmt.Sprintf("/%s",
 func (m *MockProvider) BridgedRoutes() []string     { return m.Bridged }
 func (m *MockProvider) PassthroughRoutes() []string { return m.Passthrough }
 func (*MockProvider) AuthHeader() string            { return "Authorization" }
-func (m *MockProvider) InjectAuthHeader(h *http.Header) {
-	if m.InjectAuthHeaderFunc != nil {
-		m.InjectAuthHeaderFunc(h)
-	}
-}
 
 func (*MockProvider) KeyFailoverConfig(_ slog.Logger) keypool.KeyFailoverConfig {
 	return keypool.KeyFailoverConfig{}
diff --git a/aibridge/passthrough_test.go b/aibridge/passthrough_test.go
index 6679feb932528..246becc0d903e 100644
--- a/aibridge/passthrough_test.go
+++ b/aibridge/passthrough_test.go
@@ -321,6 +321,7 @@ func TestPassthrough_KeyFailover(t *testing.T) {
 		extractKey  func(*http.Request) string
 		setBYOK     func(*http.Request, string)
 		newProvider func(baseURL string, pool *keypool.Pool) provider.Provider
+		byokOnly    bool
 	}{
 		{
 			name: "anthropic",
@@ -353,6 +354,22 @@ func TestPassthrough_KeyFailover(t *testing.T) {
 				return provider.NewOpenAI(cfg)
 			},
 		},
+		{
+			// Copilot is always BYOK and returns an empty KeyFailoverConfig,
+			// which makes the KeyFailoverTransport short-circuit. Only the
+			// BYOK scenario applies, centralized-pool cases are skipped below.
+			name: "copilot",
+			extractKey: func(r *http.Request) string {
+				return strings.TrimPrefix(r.Header.Get("Authorization"), "Bearer ")
+			},
+			setBYOK: func(r *http.Request, key string) {
+				r.Header.Set("Authorization", "Bearer "+key)
+			},
+			newProvider: func(baseURL string, _ *keypool.Pool) provider.Provider {
+				return provider.NewCopilot(config.Copilot{BaseURL: baseURL})
+			},
+			byokOnly: true,
+		},
 	}
 
 	tests := []struct {
@@ -516,6 +533,10 @@ func TestPassthrough_KeyFailover(t *testing.T) {
 
 	for _, prov := range providers {
 		for _, tc := range tests {
+			// BYOK-only providers do not exercise centralized-pool scenarios.
+			if prov.byokOnly && tc.byokKey == "" {
+				continue
+			}
 			t.Run(prov.name+"/"+tc.name, func(t *testing.T) {
 				t.Parallel()
 
diff --git a/aibridge/provider/anthropic.go b/aibridge/provider/anthropic.go
index 64128cee5fab9..c03ba19762f75 100644
--- a/aibridge/provider/anthropic.go
+++ b/aibridge/provider/anthropic.go
@@ -198,29 +198,6 @@ func (*Anthropic) AuthHeader() string {
 	return "X-Api-Key"
 }
 
-func (p *Anthropic) InjectAuthHeader(headers *http.Header) {
-	if headers == nil {
-		headers = &http.Header{}
-	}
-
-	// BYOK: if the request already carries user-supplied credentials,
-	// do not overwrite them with the centralized key.
-	if headers.Get("X-Api-Key") != "" || headers.Get("Authorization") != "" {
-		return
-	}
-
-	// Centralized: pull a single key from the pool. No failover
-	// or exhaustion handling here.
-	// TODO(ssncferreira): replace with RoundTripper-based auth
-	// in the upstack passthrough PR.
-	if p.cfg.KeyPool == nil {
-		return
-	}
-	if key, err := p.cfg.KeyPool.Walker().Next(); err == nil {
-		headers.Set(p.AuthHeader(), key.Value())
-	}
-}
-
 func (p *Anthropic) KeyFailoverConfig(logger slog.Logger) keypool.KeyFailoverConfig {
 	name := p.Name()
 	return keypool.KeyFailoverConfig{
diff --git a/aibridge/provider/anthropic_test.go b/aibridge/provider/anthropic_test.go
index 7ebf4495d9b3d..3c7378ae71081 100644
--- a/aibridge/provider/anthropic_test.go
+++ b/aibridge/provider/anthropic_test.go
@@ -317,60 +317,6 @@ func TestAnthropic_CreateInterceptor_BYOK(t *testing.T) {
 	}
 }
 
-func TestAnthropic_InjectAuthHeader(t *testing.T) {
-	t.Parallel()
-
-	provider := NewAnthropic(config.Anthropic{Key: "centralized-key"}, nil)
-
-	tests := []struct {
-		name              string
-		presetHeaders     map[string]string
-		wantXApiKey       string
-		wantAuthorization string
-	}{
-		{
-			name:          "when no auth headers are provided, inject centralized key",
-			presetHeaders: map[string]string{},
-			wantXApiKey:   "centralized-key",
-		},
-		{
-			name:          "when X-Api-Key header is provided, use it",
-			presetHeaders: map[string]string{"X-Api-Key": "user-api-key"},
-			wantXApiKey:   "user-api-key",
-		},
-		{
-			name:              "when Authorization header is provided, use it",
-			presetHeaders:     map[string]string{"Authorization": "Bearer user-access-token"},
-			wantAuthorization: "Bearer user-access-token",
-		},
-		{
-			name: "when both headers are provided, keep both",
-			presetHeaders: map[string]string{
-				"Authorization": "Bearer user-access-token",
-				"X-Api-Key":     "user-api-key",
-			},
-			wantXApiKey:       "user-api-key",
-			wantAuthorization: "Bearer user-access-token",
-		},
-	}
-
-	for _, tc := range tests {
-		t.Run(tc.name, func(t *testing.T) {
-			t.Parallel()
-
-			headers := http.Header{}
-			for k, v := range tc.presetHeaders {
-				headers.Set(k, v)
-			}
-
-			provider.InjectAuthHeader(&headers)
-
-			assert.Equal(t, tc.wantXApiKey, headers.Get("X-Api-Key"))
-			assert.Equal(t, tc.wantAuthorization, headers.Get("Authorization"))
-		})
-	}
-}
-
 func TestExtractAnthropicHeaders(t *testing.T) {
 	t.Parallel()
 
diff --git a/aibridge/provider/copilot.go b/aibridge/provider/copilot.go
index b68513ecec84a..1186e8b253f6d 100644
--- a/aibridge/provider/copilot.go
+++ b/aibridge/provider/copilot.go
@@ -107,12 +107,6 @@ func (*Copilot) AuthHeader() string {
 	return "Authorization"
 }
 
-// InjectAuthHeader is a no-op for Copilot.
-// Copilot uses per-user tokens passed in the original Authorization header,
-// rather than a global key configured at the provider level.
-// The original Authorization header flows through untouched from the client.
-func (*Copilot) InjectAuthHeader(_ *http.Header) {}
-
 // KeyFailoverConfig returns a config with a nil Pool, which makes
 // the KeyFailoverTransport short-circuit. Copilot is always BYOK.
 func (*Copilot) KeyFailoverConfig(_ slog.Logger) keypool.KeyFailoverConfig {
diff --git a/aibridge/provider/copilot_test.go b/aibridge/provider/copilot_test.go
index cd30a833500d8..3d68881379dc3 100644
--- a/aibridge/provider/copilot_test.go
+++ b/aibridge/provider/copilot_test.go
@@ -51,39 +51,6 @@ func TestCopilot_TypeAndName(t *testing.T) {
 	}
 }
 
-func TestCopilot_InjectAuthHeader(t *testing.T) {
-	t.Parallel()
-
-	// Copilot uses per-user key passed in the Authorization header,
-	// so InjectAuthHeader should not modify any headers.
-	provider := NewCopilot(config.Copilot{})
-
-	t.Run("ExistingHeaders_Unchanged", func(t *testing.T) {
-		t.Parallel()
-
-		headers := http.Header{}
-		headers.Set("Authorization", "Bearer user-token")
-		headers.Set("X-Custom-Header", "custom-value")
-
-		provider.InjectAuthHeader(&headers)
-
-		assert.Equal(t, "Bearer user-token", headers.Get("Authorization"),
-			"Authorization header should remain unchanged")
-		assert.Equal(t, "custom-value", headers.Get("X-Custom-Header"),
-			"other headers should remain unchanged")
-	})
-
-	t.Run("EmptyHeaders_NoneAdded", func(t *testing.T) {
-		t.Parallel()
-
-		headers := http.Header{}
-
-		provider.InjectAuthHeader(&headers)
-
-		assert.Empty(t, headers, "no headers should be added")
-	})
-}
-
 func TestCopilot_CreateInterceptor(t *testing.T) {
 	t.Parallel()
 
diff --git a/aibridge/provider/openai.go b/aibridge/provider/openai.go
index 894dda7194106..3f6f131de6a37 100644
--- a/aibridge/provider/openai.go
+++ b/aibridge/provider/openai.go
@@ -197,29 +197,6 @@ func (*OpenAI) AuthHeader() string {
 	return "Authorization"
 }
 
-func (p *OpenAI) InjectAuthHeader(headers *http.Header) {
-	if headers == nil {
-		headers = &http.Header{}
-	}
-
-	// BYOK: if the request already carries user-supplied credentials,
-	// do not overwrite them with the centralized key.
-	if headers.Get("Authorization") != "" {
-		return
-	}
-
-	// Centralized: pull a single key from the pool. No failover
-	// or exhaustion handling here.
-	// TODO(ssncferreira): replace with RoundTripper-based auth
-	// in the upstack passthrough PR.
-	if p.cfg.KeyPool == nil {
-		return
-	}
-	if key, err := p.cfg.KeyPool.Walker().Next(); err == nil {
-		headers.Set(p.AuthHeader(), "Bearer "+key.Value())
-	}
-}
-
 func (p *OpenAI) KeyFailoverConfig(logger slog.Logger) keypool.KeyFailoverConfig {
 	name := p.Name()
 	return keypool.KeyFailoverConfig{
diff --git a/aibridge/provider/openai_test.go b/aibridge/provider/openai_test.go
index d739a2dc20082..6aaa0b56441a5 100644
--- a/aibridge/provider/openai_test.go
+++ b/aibridge/provider/openai_test.go
@@ -325,44 +325,6 @@ func TestOpenAI_CreateInterceptor(t *testing.T) {
 	}
 }
 
-func TestOpenAI_InjectAuthHeader(t *testing.T) {
-	t.Parallel()
-
-	provider := NewOpenAI(config.OpenAI{Key: "centralized-key"})
-
-	tests := []struct {
-		name              string
-		presetHeaders     map[string]string
-		wantAuthorization string
-	}{
-		{
-			name:              "when no Authorization header is provided, inject centralized key",
-			presetHeaders:     map[string]string{},
-			wantAuthorization: "Bearer centralized-key",
-		},
-		{
-			name:              "when Authorization header is provided, do not overwrite it",
-			presetHeaders:     map[string]string{"Authorization": "Bearer user-token"},
-			wantAuthorization: "Bearer user-token",
-		},
-	}
-
-	for _, tc := range tests {
-		t.Run(tc.name, func(t *testing.T) {
-			t.Parallel()
-
-			headers := http.Header{}
-			for k, v := range tc.presetHeaders {
-				headers.Set(k, v)
-			}
-
-			provider.InjectAuthHeader(&headers)
-
-			assert.Equal(t, tc.wantAuthorization, headers.Get("Authorization"))
-		})
-	}
-}
-
 func BenchmarkOpenAI_CreateInterceptor_ChatCompletions(b *testing.B) {
 	provider := NewOpenAI(config.OpenAI{
 		BaseURL: "https://api.openai.com/v1/",
diff --git a/aibridge/provider/provider.go b/aibridge/provider/provider.go
index 587dfd85ce014..7520333b53b61 100644
--- a/aibridge/provider/provider.go
+++ b/aibridge/provider/provider.go
@@ -77,10 +77,6 @@ type Provider interface {
 	// AuthHeader returns the name of the header which the provider expects to find its authentication
 	// token in.
 	AuthHeader() string
-	// InjectAuthHeader allows [Provider]s to set its authentication header.
-	// TODO(ssncferreira): remove. Auth is now applied per-attempt by
-	// KeyFailoverTransport (see [Provider.KeyFailoverConfig]).
-	InjectAuthHeader(*http.Header)
 	// KeyFailoverConfig returns the per-provider configuration for
 	// automatic key failover on passthrough routes.
 	KeyFailoverConfig(logger slog.Logger) keypool.KeyFailoverConfig