fix: set Bedrock streaming accept headers (#24776)

> Mux is working on behalf of Mike.

## Summary
- Bump `github.com/coder/anthropic-sdk-go` to the clean Bedrock
streaming header fix from coder/anthropic-sdk-go#10.
- Add chatd regression coverage that verifies Bedrock streaming requests
use AWS event stream headers and include `X-Amzn-Bedrock-Accept` in the
SigV4 signed headers.

## SDK follow-up
- Reverted the bad coder/anthropic-sdk-go#8 merge with
coder/anthropic-sdk-go#9.
- Re-applied only the intended Bedrock streaming header change in
coder/anthropic-sdk-go#10.

## Validation
- `go test ./coderd/x/chatd/chatprovider -run
'TestModelFromConfig_Bedrock(StreamingHeaders|StripsAnthropicHeaders)?$'
-count=1`
- `go test ./coderd/x/chatd/chatprovider -count=1`
- `go mod tidy -diff`
- `make lint`
- pre-commit hook during `git commit`

coderd/x/chatd/chatprovider/chatprovider_test.go

@@ -1049,6 +1049,87 @@ func TestModelFromConfig_BedrockStripsAnthropicHeaders(t *testing.T) {
 	require.Contains(t, got.Body, `"anthropic_version":"bedrock-2023-05-31"`)
 }
 
+func TestModelFromConfig_BedrockStreamingHeaders(t *testing.T) {
+	ctx := testutil.Context(t, testutil.WaitShort)
+
+	t.Setenv("ANTHROPIC_API_KEY", "anthropic-env-key")
+	t.Setenv("AWS_REGION", "us-east-2")
+	t.Setenv("AWS_ACCESS_KEY_ID", "test-access-key")
+	t.Setenv("AWS_SECRET_ACCESS_KEY", "test-secret-key")
+	t.Setenv("AWS_SESSION_TOKEN", "test-session-token")
+
+	type requestCapture struct {
+		Path          string
+		Accept        string
+		BedrockAccept string
+		Authorization string
+		Body          string
+		ReadError     error
+	}
+
+	requests := make(chan requestCapture, 1)
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		body, err := io.ReadAll(r.Body)
+
+		requests <- requestCapture{
+			Path:          r.URL.Path,
+			Accept:        r.Header.Get("Accept"),
+			BedrockAccept: r.Header.Get("X-Amzn-Bedrock-Accept"),
+			Authorization: r.Header.Get("Authorization"),
+			Body:          string(body),
+			ReadError:     err,
+		}
+
+		w.Header().Set("Content-Type", "application/vnd.amazon.eventstream")
+		w.WriteHeader(http.StatusOK)
+	}))
+	defer server.Close()
+
+	model, err := chatprovider.ModelFromConfig(
+		fantasybedrock.Name,
+		"anthropic.claude-opus-4-6-v1",
+		chatprovider.ProviderAPIKeys{
+			ByProvider: map[string]string{
+				fantasybedrock.Name: "",
+			},
+			BaseURLByProvider: map[string]string{
+				fantasybedrock.Name: server.URL,
+			},
+		},
+		chatprovider.UserAgent(),
+		nil,
+		nil,
+	)
+	require.NoError(t, err)
+	require.NotNil(t, model)
+
+	stream, err := model.Stream(ctx, fantasy.Call{
+		Prompt: []fantasy.Message{
+			{
+				Role: fantasy.MessageRoleUser,
+				Content: []fantasy.MessagePart{
+					fantasy.TextPart{Text: "hello"},
+				},
+			},
+		},
+	})
+	require.NoError(t, err)
+
+	for part := range stream {
+		require.NotEqual(t, fantasy.StreamPartTypeError, part.Type)
+		break
+	}
+
+	got := testutil.TryReceive(ctx, t, requests)
+	require.NoError(t, got.ReadError)
+	require.Equal(t, "/model/us.anthropic.claude-opus-4-6-v1/invoke-with-response-stream", got.Path)
+	require.Equal(t, "application/vnd.amazon.eventstream", got.Accept)
+	require.Equal(t, "application/json", got.BedrockAccept)
+	require.Contains(t, got.Authorization, "AWS4-HMAC-SHA256")
+	require.Contains(t, got.Authorization, "x-amzn-bedrock-accept")
+	require.Contains(t, got.Body, `"anthropic_version":"bedrock-2023-05-31"`)
+}
+
 func bedrockNonStreamingResponse() map[string]any {
 	return map[string]any{
 		"id":    "msg_01Test",

go.mod

@@ -91,8 +91,8 @@ replace charm.land/fantasy => github.com/coder/fantasy v0.0.0-20260426185602-951
 
 // coder/coder uses a fork of charmbracelet's fork of the Anthropic Go SDK
 // with performance improvements and Bedrock header cleanup.
-// See: https://github.com/coder/anthropic-sdk-go/commits/3be8e193ec89
-replace github.com/charmbracelet/anthropic-sdk-go => github.com/coder/anthropic-sdk-go v0.0.0-20260424230212-3be8e193ec89
+// See: https://github.com/coder/anthropic-sdk-go/commits/67514346e0fb
+replace github.com/charmbracelet/anthropic-sdk-go => github.com/coder/anthropic-sdk-go v0.0.0-20260428111347-67514346e0fb
 
 // Replace sdks with our own optimized forks until relevant upstream PRs are merged.
 // https://github.com/anthropics/anthropic-sdk-go/pull/262

go.sum

@@ -314,8 +314,8 @@ github.com/coder/agentapi-sdk-go v0.0.0-20250505131810-560d1d88d225 h1:tRIViZ5JR
 github.com/coder/agentapi-sdk-go v0.0.0-20250505131810-560d1d88d225/go.mod h1:rNLVpYgEVeu1Zk29K64z6Od8RBP9DwqCu9OfCzh8MR4=
 github.com/coder/aisdk-go v0.0.9 h1:Vzo/k2qwVGLTR10ESDeP2Ecek1SdPfZlEjtTfMveiVo=
 github.com/coder/aisdk-go v0.0.9/go.mod h1:KF6/Vkono0FJJOtWtveh5j7yfNrSctVTpwgweYWSp5M=
-github.com/coder/anthropic-sdk-go v0.0.0-20260424230212-3be8e193ec89 h1:IVJutHfU944mb4D66K7XdPwKMAJrNC9FOq6JB4bveuI=
-github.com/coder/anthropic-sdk-go v0.0.0-20260424230212-3be8e193ec89/go.mod h1:hqlYqR7uPKOKfnNeicUbZp0Ps0GeYFlKYtwh5HGDCx8=
+github.com/coder/anthropic-sdk-go v0.0.0-20260428111347-67514346e0fb h1:/qJzrmYz+ii329dgoQ1BxtlrhdJvtTkk7UwhrroSohU=
+github.com/coder/anthropic-sdk-go v0.0.0-20260428111347-67514346e0fb/go.mod h1:hqlYqR7uPKOKfnNeicUbZp0Ps0GeYFlKYtwh5HGDCx8=
 github.com/coder/boundary v0.8.4-0.20260304164748-566aeea939ab h1:HrlxyTmMQpOHfSKzRU1vf5TxrmV6vL5OiWq+Dvn5qh0=
 github.com/coder/boundary v0.8.4-0.20260304164748-566aeea939ab/go.mod h1:BhJhyKW/+zZQzaGZ3vn27if2k0Vx5xLXzq7ZCQx5gPk=
 github.com/coder/bubbletea v1.2.2-0.20241212190825-007a1cdb2c41 h1:SBN/DA63+ZHwuWwPHPYoCZ/KLAjHv5g4h2MS4f2/MTI=