refactor: extract shared parser runtime to core-parser.ts

Both parser.ts and beta-parser.ts contained an identical parseOutputFormat
implementation (18 lines each): resolve the format object, check for
json_schema type, call format.parse() or JSON.parse(), throw AnthropicError
on failure.

Extract that shared runtime to src/lib/core-parser.ts as
parseAccumulatedFormat(format, content). Both parsers now import and
delegate to it, removing the AnthropicError import from each.

The type-level logic (conditional types, getOutputFormat, deprecated
output_format shim, .parsed getter) stays file-local — it is intentionally
different between the stable and beta paths and has zero runtime cost.

Behaviour preserved exactly: same error message, same fallback to
JSON.parse, same null return when format is absent or non-json_schema.

Author: apoc

src/lib/beta-parser.ts

@@ -1,5 +1,5 @@
 import type { Logger } from '../client';
-import { AnthropicError } from '../core/error';
+import { parseAccumulatedFormat } from './core-parser';
 import {
   BetaContentBlock,
   BetaJSONOutputFormat,
@@ -10,6 +10,7 @@ import {
 } from '../resources/beta/messages/messages';
 
 import { Simplify } from './type-utils';
+
 type AutoParseableBetaOutputConfig = Omit<BetaOutputConfig, 'format'> & {
   format?: BetaJSONOutputFormat | AutoParseableBetaOutputFormat<any> | null;
 };
@@ -129,18 +130,8 @@ function parseBetaOutputFormat<Params extends BetaParseableMessageCreateParams>(
   params: Params,
   content: string,
 ): ExtractParsedContentFromBetaParams<Params> | null {
-  const outputFormat = getOutputFormat(params);
-  if (outputFormat?.type !== 'json_schema') {
-    return null;
-  }
-
-  try {
-    if ('parse' in outputFormat) {
-      return outputFormat.parse(content);
-    }
-
-    return JSON.parse(content);
-  } catch (error) {
-    throw new AnthropicError(`Failed to parse structured output: ${error}`);
-  }
+  return parseAccumulatedFormat(
+    getOutputFormat(params),
+    content,
+  ) as ExtractParsedContentFromBetaParams<Params> | null;
 }

src/lib/core-parser.ts

@@ -0,0 +1,29 @@
+import { AnthropicError } from '../core/error';
+
+/**
+ * Shared parse-step runtime used by both parser.ts (stable) and beta-parser.ts.
+ *
+ * Accepts the resolved format object (already extracted from params by the
+ * caller's getOutputFormat) and the accumulated text content. Returns the
+ * parsed value, or null when the format is absent or non-parseable.
+ *
+ * Throws AnthropicError if the format is json_schema but parsing fails —
+ * same behaviour that lived in each parser previously.
+ */
+export function parseAccumulatedFormat(
+  format: { type: string; parse?: (content: string) => unknown } | null | undefined,
+  content: string,
+): unknown | null {
+  if (format?.type !== 'json_schema') {
+    return null;
+  }
+
+  try {
+    if ('parse' in (format as object)) {
+      return (format as { parse: (c: string) => unknown }).parse(content);
+    }
+    return JSON.parse(content);
+  } catch (error) {
+    throw new AnthropicError(`Failed to parse structured output: ${error}`);
+  }
+}

src/lib/parser.ts

@@ -1,5 +1,5 @@
 import type { Logger } from '../client';
-import { AnthropicError } from '../core/error';
+import { parseAccumulatedFormat } from './core-parser';
 import {
   ContentBlock,
   JSONOutputFormat,
@@ -107,18 +107,8 @@ function parseOutputFormat<Params extends ParseableMessageCreateParams>(
   params: Params,
   content: string,
 ): ExtractParsedContentFromParams<Params> | null {
-  const outputFormat = getOutputFormat(params);
-  if (outputFormat?.type !== 'json_schema') {
-    return null;
-  }
-
-  try {
-    if ('parse' in outputFormat) {
-      return outputFormat.parse(content);
-    }
-
-    return JSON.parse(content);
-  } catch (error) {
-    throw new AnthropicError(`Failed to parse structured output: ${error}`);
-  }
+  return parseAccumulatedFormat(
+    getOutputFormat(params),
+    content,
+  ) as ExtractParsedContentFromParams<Params> | null;
 }