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Fix/anthropic beta header merge #10158

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3e8787f
feat(provider-utils): implement combineAndMergeMergeableHeaders
chentsulin Nov 11, 2025
80f210f
fix(anthropic): use combineAndMergeMergeableHeaders instead
chentsulin Nov 11, 2025
4e87648
test(anthropic): add test for merging custom anthropic-beta header
chentsulin Nov 11, 2025
9359692
chore: update changeset
chentsulin Nov 11, 2025
48c3538
Merge branch 'main' into fix/anthropic-beta-header-merge
chentsulin Nov 15, 2025
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6 changes: 6 additions & 0 deletions .changeset/shiny-cups-speak.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,6 @@
---
'@ai-sdk/provider-utils': patch
'@ai-sdk/anthropic': patch
---

Fix the custom anthropic-beta merge issue.
28 changes: 28 additions & 0 deletions packages/anthropic/src/anthropic-messages-language-model.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -3678,6 +3678,34 @@ describe('AnthropicMessagesLanguageModel', () => {
`);
});

it('should merge custom anthropic-beta header with fine-grained-tool-streaming beta', async () => {
server.urls['https://api.anthropic.com/v1/messages'].response = {
type: 'stream-chunks',
chunks: [
`data: {"type":"message_start","message":{"id":"msg_01KfpJoAEabmH2iHRRFjQMAG","type":"message","role":"assistant","content":[],"model":"claude-3-haiku-20240307","stop_reason":null,"stop_sequence":null,"usage":{"input_tokens":17,"output_tokens":1}}}\n\n`,
`data: {"type":"content_block_start","index":0,"content_block":{"type":"text","text":""}}\n\n`,
`data: {"type":"content_block_delta","index":0,"delta":{"type":"text_delta","text":"Hello, World!"}}\n\n`,
`data: {"type":"content_block_stop","index":0}\n\n`,
`data: {"type":"message_delta","delta":{"stop_reason":"end_turn","stop_sequence":null},"usage":{"output_tokens":227}}\n\n`,
`data: {"type":"message_stop"}\n\n`,
],
};

const provider = createAnthropic({
apiKey: 'test-api-key',
headers: {
'anthropic-beta': 'context-1m-2025-08-07',
},
});

await provider('claude-3-haiku-20240307').doStream({
prompt: TEST_PROMPT,
});

expect(server.calls[0].requestHeaders['anthropic-beta']).toContain('fine-grained-tool-streaming-2025-05-14');
expect(server.calls[0].requestHeaders['anthropic-beta']).toContain('context-1m-2025-08-07');
});

it('should support cache control', async () => {
server.urls['https://api.anthropic.com/v1/messages'].response = {
type: 'stream-chunks',
Expand Down
15 changes: 9 additions & 6 deletions packages/anthropic/src/anthropic-messages-language-model.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,7 @@ import {
UnsupportedFunctionalityError,
} from '@ai-sdk/provider';
import {
combineHeaders,
combineAndMergeMergeableHeaders,
createEventSourceResponseHandler,
createJsonResponseHandler,
FetchFunction,
Expand Down Expand Up @@ -394,11 +394,14 @@ export class AnthropicMessagesLanguageModel implements LanguageModelV3 {
betas: Set<string>;
headers: Record<string, string | undefined> | undefined;
}) {
return combineHeaders(
await resolve(this.config.headers),
betas.size > 0 ? { 'anthropic-beta': Array.from(betas).join(',') } : {},
headers,
);
return combineAndMergeMergeableHeaders({
headers: [
await resolve(this.config.headers),
betas.size > 0 ? { 'anthropic-beta': Array.from(betas).join(',') } : {},
headers,
],
mergeableKeys: ['anthropic-beta'],
});
}

private buildRequestUrl(isStreaming: boolean): string {
Expand Down
36 changes: 36 additions & 0 deletions packages/provider-utils/src/combine-headers.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -9,3 +9,39 @@ export function combineHeaders(
{},
) as Record<string, string | undefined>;
}

export function combineAndMergeMergeableHeaders({
headers,
mergeableKeys = [],
}: {
headers: Array<Record<string, string | undefined> | undefined>,
mergeableKeys?: string[],
}): Record<string, string | undefined> {
const mergeableLowerCaseKeys = mergeableKeys.map(key => key.toLowerCase());

return headers.reduce(
(combinedHeaders, currentHeaders) => {
if (!currentHeaders) return combinedHeaders;

const result = { ...combinedHeaders };

for (const [key, value] of Object.entries(currentHeaders)) {
if (value === undefined) continue;

// Check if the key is mergeable (case-insensitive) and already exists
if (mergeableLowerCaseKeys.includes(key.toLowerCase()) && result[key]) {
const existingValues = result[key].split(',').map(val => val.trim());
const newValues = value.split(',').map(val => val.trim());
// Combine and deduplicate values
const allValues = Array.from(new Set([...existingValues, ...newValues]));
result[key] = allValues.join(',');
Comment on lines +31 to +37
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Suggested change
// Check if the key is mergeable (case-insensitive) and already exists
if (mergeableLowerCaseKeys.includes(key.toLowerCase()) && result[key]) {
const existingValues = result[key].split(',').map(val => val.trim());
const newValues = value.split(',').map(val => val.trim());
// Combine and deduplicate values
const allValues = Array.from(new Set([...existingValues, ...newValues]));
result[key] = allValues.join(',');
// Check if the key is mergeable (case-insensitive)
if (mergeableLowerCaseKeys.includes(key.toLowerCase())) {
// Find existing key with case-insensitive comparison
const existingKey = Object.keys(result).find(
k => k.toLowerCase() === key.toLowerCase(),
);
if (existingKey && result[existingKey]) {
const existingValues = result[existingKey]!.split(',').map(val => val.trim());
const newValues = value.split(',').map(val => val.trim());
// Combine and deduplicate values
const allValues = Array.from(new Set([...existingValues, ...newValues]));
result[existingKey] = allValues.join(',');
} else {
result[key] = value;
}

The case-sensitivity check for mergeable headers is incomplete. The code checks if a key is mergeable using case-insensitive comparison (key.toLowerCase()) but then checks if the key exists in the result using exact casing (result[key]), which will fail to find existing keys with different casing.

View Details

Analysis

Case-insensitive header merging fails with different casing

What fails: combineAndMergeMergeableHeaders() in packages/provider-utils/src/combine-headers.ts fails to merge headers when the same logical header is provided with different casing (e.g., 'Anthropic-Beta' and 'anthropic-beta').

How to reproduce:

const result = combineAndMergeMergeableHeaders({
  headers: [
    { 'Anthropic-Beta': 'value1' },
    { 'anthropic-beta': 'value2' }
  ],
  mergeableKeys: ['anthropic-beta']
});

Result: Returns object with both keys present:

{
  'Anthropic-Beta': 'value1',
  'anthropic-beta': 'value2'
}

Expected: Single merged header:

{
  'Anthropic-Beta': 'value1,value2'
}

Root cause: Line 32 checked if a mergeable key exists using case-insensitive comparison (key.toLowerCase()) but then searched for it in the result object using exact case matching (result[key]). This caused case-mismatched headers to not be found and merged.

Impact: Users providing headers with different casing will end up with duplicate headers that may not be properly combined, potentially causing API request failures.

} else {
result[key] = value;
}
}

return result;
},
{},
) as Record<string, string | undefined>;
}