diff --git a/.release-please-manifest.json b/.release-please-manifest.json
index 0c6b0698..661ffa45 100644
--- a/.release-please-manifest.json
+++ b/.release-please-manifest.json
@@ -1,3 +1,3 @@
 {
-  ".": "1.25.0"
+  ".": "1.25.1"
 }
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 738d8d11..d6765477 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,5 +1,13 @@
 # Changelog
 
+## 1.25.1 (2026-02-19)
+
+Full Changelog: [v1.25.0...v1.25.1](https://github.com/anthropics/anthropic-sdk-go/compare/v1.25.0...v1.25.1)
+
+### Bug Fixes
+
+* **client:** use correct format specifier for header serialization ([9115a61](https://github.com/anthropics/anthropic-sdk-go/commit/9115a6154d0b1ba94370911822986b2ef8584e9a))
+
 ## 1.25.0 (2026-02-18)
 
 Full Changelog: [v1.24.0...v1.25.0](https://github.com/anthropics/anthropic-sdk-go/compare/v1.24.0...v1.25.0)
diff --git a/README.md b/README.md
index 3501609c..437fff68 100644
--- a/README.md
+++ b/README.md
@@ -26,7 +26,7 @@ Or to pin the version:
 <!-- x-release-please-start-version -->
 
 ```sh
-go get -u 'github.com/anthropics/anthropic-sdk-go@v1.25.0'
+go get -u 'github.com/anthropics/anthropic-sdk-go@v1.25.1'
 ```
 
 <!-- x-release-please-end -->
diff --git a/betafile.go b/betafile.go
index 2f1d6477..8d09ee06 100644
--- a/betafile.go
+++ b/betafile.go
@@ -48,7 +48,7 @@ func NewBetaFileService(opts ...option.RequestOption) (r BetaFileService) {
 func (r *BetaFileService) List(ctx context.Context, params BetaFileListParams, opts ...option.RequestOption) (res *pagination.Page[FileMetadata], err error) {
 	var raw *http.Response
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "files-api-2025-04-14"), option.WithResponseInto(&raw)}, opts...)
@@ -73,7 +73,7 @@ func (r *BetaFileService) ListAutoPaging(ctx context.Context, params BetaFileLis
 // Delete File
 func (r *BetaFileService) Delete(ctx context.Context, fileID string, body BetaFileDeleteParams, opts ...option.RequestOption) (res *DeletedFile, err error) {
 	for _, v := range body.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "files-api-2025-04-14")}, opts...)
@@ -89,7 +89,7 @@ func (r *BetaFileService) Delete(ctx context.Context, fileID string, body BetaFi
 // Download File
 func (r *BetaFileService) Download(ctx context.Context, fileID string, query BetaFileDownloadParams, opts ...option.RequestOption) (res *http.Response, err error) {
 	for _, v := range query.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "files-api-2025-04-14"), option.WithHeader("Accept", "application/binary")}, opts...)
@@ -105,7 +105,7 @@ func (r *BetaFileService) Download(ctx context.Context, fileID string, query Bet
 // Get File Metadata
 func (r *BetaFileService) GetMetadata(ctx context.Context, fileID string, query BetaFileGetMetadataParams, opts ...option.RequestOption) (res *FileMetadata, err error) {
 	for _, v := range query.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "files-api-2025-04-14")}, opts...)
@@ -121,7 +121,7 @@ func (r *BetaFileService) GetMetadata(ctx context.Context, fileID string, query
 // Upload File
 func (r *BetaFileService) Upload(ctx context.Context, params BetaFileUploadParams, opts ...option.RequestOption) (res *FileMetadata, err error) {
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "files-api-2025-04-14")}, opts...)
diff --git a/betamessage.go b/betamessage.go
index ae4cc083..0f9b9167 100644
--- a/betamessage.go
+++ b/betamessage.go
@@ -53,7 +53,7 @@ func NewBetaMessageService(opts ...option.RequestOption) (r BetaMessageService)
 // Note: If you choose to set a timeout for this request, we recommend 10 minutes.
 func (r *BetaMessageService) New(ctx context.Context, params BetaMessageNewParams, opts ...option.RequestOption) (res *BetaMessage, err error) {
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 
@@ -86,7 +86,7 @@ func (r *BetaMessageService) NewStreaming(ctx context.Context, params BetaMessag
 		err error
 	)
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append(opts, option.WithJSONSet("stream", true))
@@ -104,7 +104,7 @@ func (r *BetaMessageService) NewStreaming(ctx context.Context, params BetaMessag
 // [user guide](https://docs.claude.com/en/docs/build-with-claude/token-counting)
 func (r *BetaMessageService) CountTokens(ctx context.Context, params BetaMessageCountTokensParams, opts ...option.RequestOption) (res *BetaMessageTokensCount, err error) {
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	path := "v1/messages/count_tokens?beta=true"
diff --git a/betamessagebatch.go b/betamessagebatch.go
index aebf46a4..dfb77f0f 100644
--- a/betamessagebatch.go
+++ b/betamessagebatch.go
@@ -52,7 +52,7 @@ func NewBetaMessageBatchService(opts ...option.RequestOption) (r BetaMessageBatc
 // [user guide](https://docs.claude.com/en/docs/build-with-claude/batch-processing)
 func (r *BetaMessageBatchService) New(ctx context.Context, params BetaMessageBatchNewParams, opts ...option.RequestOption) (res *BetaMessageBatch, err error) {
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "message-batches-2024-09-24")}, opts...)
@@ -69,7 +69,7 @@ func (r *BetaMessageBatchService) New(ctx context.Context, params BetaMessageBat
 // [user guide](https://docs.claude.com/en/docs/build-with-claude/batch-processing)
 func (r *BetaMessageBatchService) Get(ctx context.Context, messageBatchID string, query BetaMessageBatchGetParams, opts ...option.RequestOption) (res *BetaMessageBatch, err error) {
 	for _, v := range query.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "message-batches-2024-09-24")}, opts...)
@@ -90,7 +90,7 @@ func (r *BetaMessageBatchService) Get(ctx context.Context, messageBatchID string
 func (r *BetaMessageBatchService) List(ctx context.Context, params BetaMessageBatchListParams, opts ...option.RequestOption) (res *pagination.Page[BetaMessageBatch], err error) {
 	var raw *http.Response
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "message-batches-2024-09-24"), option.WithResponseInto(&raw)}, opts...)
@@ -125,7 +125,7 @@ func (r *BetaMessageBatchService) ListAutoPaging(ctx context.Context, params Bet
 // [user guide](https://docs.claude.com/en/docs/build-with-claude/batch-processing)
 func (r *BetaMessageBatchService) Delete(ctx context.Context, messageBatchID string, body BetaMessageBatchDeleteParams, opts ...option.RequestOption) (res *BetaDeletedMessageBatch, err error) {
 	for _, v := range body.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "message-batches-2024-09-24")}, opts...)
@@ -152,7 +152,7 @@ func (r *BetaMessageBatchService) Delete(ctx context.Context, messageBatchID str
 // [user guide](https://docs.claude.com/en/docs/build-with-claude/batch-processing)
 func (r *BetaMessageBatchService) Cancel(ctx context.Context, messageBatchID string, body BetaMessageBatchCancelParams, opts ...option.RequestOption) (res *BetaMessageBatch, err error) {
 	for _, v := range body.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "message-batches-2024-09-24")}, opts...)
@@ -179,7 +179,7 @@ func (r *BetaMessageBatchService) ResultsStreaming(ctx context.Context, messageB
 		err error
 	)
 	for _, v := range query.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "message-batches-2024-09-24"), option.WithHeader("Accept", "application/x-jsonl")}, opts...)
diff --git a/betamodel.go b/betamodel.go
index d469795b..863a5a04 100644
--- a/betamodel.go
+++ b/betamodel.go
@@ -46,7 +46,7 @@ func NewBetaModelService(opts ...option.RequestOption) (r BetaModelService) {
 // model or resolve a model alias to a model ID.
 func (r *BetaModelService) Get(ctx context.Context, modelID string, query BetaModelGetParams, opts ...option.RequestOption) (res *BetaModelInfo, err error) {
 	for _, v := range query.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	if modelID == "" {
@@ -65,7 +65,7 @@ func (r *BetaModelService) Get(ctx context.Context, modelID string, query BetaMo
 func (r *BetaModelService) List(ctx context.Context, params BetaModelListParams, opts ...option.RequestOption) (res *pagination.Page[BetaModelInfo], err error) {
 	var raw *http.Response
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithResponseInto(&raw)}, opts...)
diff --git a/betaskill.go b/betaskill.go
index 6191bff5..66f3f043 100644
--- a/betaskill.go
+++ b/betaskill.go
@@ -47,7 +47,7 @@ func NewBetaSkillService(opts ...option.RequestOption) (r BetaSkillService) {
 // Create Skill
 func (r *BetaSkillService) New(ctx context.Context, params BetaSkillNewParams, opts ...option.RequestOption) (res *BetaSkillNewResponse, err error) {
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "skills-2025-10-02")}, opts...)
@@ -59,7 +59,7 @@ func (r *BetaSkillService) New(ctx context.Context, params BetaSkillNewParams, o
 // Get Skill
 func (r *BetaSkillService) Get(ctx context.Context, skillID string, query BetaSkillGetParams, opts ...option.RequestOption) (res *BetaSkillGetResponse, err error) {
 	for _, v := range query.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "skills-2025-10-02")}, opts...)
@@ -76,7 +76,7 @@ func (r *BetaSkillService) Get(ctx context.Context, skillID string, query BetaSk
 func (r *BetaSkillService) List(ctx context.Context, params BetaSkillListParams, opts ...option.RequestOption) (res *pagination.PageCursor[BetaSkillListResponse], err error) {
 	var raw *http.Response
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "skills-2025-10-02"), option.WithResponseInto(&raw)}, opts...)
@@ -101,7 +101,7 @@ func (r *BetaSkillService) ListAutoPaging(ctx context.Context, params BetaSkillL
 // Delete Skill
 func (r *BetaSkillService) Delete(ctx context.Context, skillID string, body BetaSkillDeleteParams, opts ...option.RequestOption) (res *BetaSkillDeleteResponse, err error) {
 	for _, v := range body.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "skills-2025-10-02")}, opts...)
diff --git a/betaskillversion.go b/betaskillversion.go
index 63e52ffc..26115cc7 100644
--- a/betaskillversion.go
+++ b/betaskillversion.go
@@ -45,7 +45,7 @@ func NewBetaSkillVersionService(opts ...option.RequestOption) (r BetaSkillVersio
 // Create Skill Version
 func (r *BetaSkillVersionService) New(ctx context.Context, skillID string, params BetaSkillVersionNewParams, opts ...option.RequestOption) (res *BetaSkillVersionNewResponse, err error) {
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "skills-2025-10-02")}, opts...)
@@ -61,7 +61,7 @@ func (r *BetaSkillVersionService) New(ctx context.Context, skillID string, param
 // Get Skill Version
 func (r *BetaSkillVersionService) Get(ctx context.Context, version string, params BetaSkillVersionGetParams, opts ...option.RequestOption) (res *BetaSkillVersionGetResponse, err error) {
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "skills-2025-10-02")}, opts...)
@@ -82,7 +82,7 @@ func (r *BetaSkillVersionService) Get(ctx context.Context, version string, param
 func (r *BetaSkillVersionService) List(ctx context.Context, skillID string, params BetaSkillVersionListParams, opts ...option.RequestOption) (res *pagination.PageCursor[BetaSkillVersionListResponse], err error) {
 	var raw *http.Response
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "skills-2025-10-02"), option.WithResponseInto(&raw)}, opts...)
@@ -111,7 +111,7 @@ func (r *BetaSkillVersionService) ListAutoPaging(ctx context.Context, skillID st
 // Delete Skill Version
 func (r *BetaSkillVersionService) Delete(ctx context.Context, version string, params BetaSkillVersionDeleteParams, opts ...option.RequestOption) (res *BetaSkillVersionDeleteResponse, err error) {
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithHeader("anthropic-beta", "skills-2025-10-02")}, opts...)
diff --git a/completion.go b/completion.go
index e5d0c8ce..beaf526c 100644
--- a/completion.go
+++ b/completion.go
@@ -48,7 +48,7 @@ func NewCompletionService(opts ...option.RequestOption) (r CompletionService) {
 // Note: If you choose to set a timeout for this request, we recommend 10 minutes.
 func (r *CompletionService) New(ctx context.Context, params CompletionNewParams, opts ...option.RequestOption) (res *Completion, err error) {
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	path := "v1/complete"
@@ -72,7 +72,7 @@ func (r *CompletionService) NewStreaming(ctx context.Context, params CompletionN
 		err error
 	)
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append(opts, option.WithJSONSet("stream", true))
diff --git a/internal/version.go b/internal/version.go
index 25ddc61c..83cb6994 100644
--- a/internal/version.go
+++ b/internal/version.go
@@ -2,4 +2,4 @@
 
 package internal
 
-const PackageVersion = "1.25.0" // x-release-please-version
+const PackageVersion = "1.25.1" // x-release-please-version
diff --git a/model.go b/model.go
index ea7b0f5b..7552020c 100644
--- a/model.go
+++ b/model.go
@@ -46,7 +46,7 @@ func NewModelService(opts ...option.RequestOption) (r ModelService) {
 // model or resolve a model alias to a model ID.
 func (r *ModelService) Get(ctx context.Context, modelID string, query ModelGetParams, opts ...option.RequestOption) (res *ModelInfo, err error) {
 	for _, v := range query.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	if modelID == "" {
@@ -65,7 +65,7 @@ func (r *ModelService) Get(ctx context.Context, modelID string, query ModelGetPa
 func (r *ModelService) List(ctx context.Context, params ModelListParams, opts ...option.RequestOption) (res *pagination.Page[ModelInfo], err error) {
 	var raw *http.Response
 	for _, v := range params.Betas {
-		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%s", v)))
+		opts = append(opts, option.WithHeaderAdd("anthropic-beta", fmt.Sprintf("%v", v)))
 	}
 	opts = slices.Concat(r.Options, opts)
 	opts = append([]option.RequestOption{option.WithResponseInto(&raw)}, opts...)