diff --git a/.changeset/cuddly-clouds-agree.md b/.changeset/cuddly-clouds-agree.md
new file mode 100644
index 00000000000..a0ff8276824
--- /dev/null
+++ b/.changeset/cuddly-clouds-agree.md
@@ -0,0 +1,12 @@
+---
+"@effect/ai-anthropic": patch
+---
+
+Fix several issues in the generated OpenAPI models for the Anthropic AI provider 
+package.
+
+The OpenAPI specification that Anthropic maintains for its API is apparently 
+[incorrect](https://github.com/anthropics/anthropic-sdk-typescript/issues/605). 
+Some properties which are marked as nullable but required are sometimes not 
+returned by the API. This fixes the schemas associated with some of those 
+properties, though others may exist / be found that require manual adjustment.
diff --git a/packages/ai/anthropic/scripts/generate.sh b/packages/ai/anthropic/scripts/generate.sh
index 4ff50d83e51..0d31916cc53 100755
--- a/packages/ai/anthropic/scripts/generate.sh
+++ b/packages/ai/anthropic/scripts/generate.sh
@@ -1,10 +1,30 @@
-#!/usr/bin/env sh
-mkdir -p tmp
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+SCRIPT_DIR=$(cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd)
+
+temp_dir=$(mktemp -d)
+
+cleanup() {
+  rm -rf "$temp_dir"
+}
+
+trap cleanup EXIT
+
+
 anthropic_stats_url="https://raw.githubusercontent.com/anthropics/anthropic-sdk-typescript/refs/heads/main/.stats.yml"
-openapi_spec_url=$(curl -sSL $anthropic_stats_url | yq '.openapi_spec_url')
-curl $openapi_spec_url > tmp/anthropic.yaml
+
+openapi_spec_url="$(curl -sSL $anthropic_stats_url | yq ".openapi_spec_url")"
+
+curl "${openapi_spec_url}" > "${temp_dir}/anthropic.yaml"
+
 echo "/**
  * @since 1.0.0
  */" > src/Generated.ts
-pnpm openapi-gen -s tmp/anthropic.yaml >> src/Generated.ts
+
+pnpm openapi-gen -s "${temp_dir}/anthropic.yaml" >> src/Generated.ts
+
 pnpm eslint --fix src/Generated.ts
+
+git apply --reject --whitespace=fix "${SCRIPT_DIR}/generated.patch"
diff --git a/packages/ai/anthropic/scripts/generated.patch b/packages/ai/anthropic/scripts/generated.patch
new file mode 100644
index 00000000000..7a5d1b0c775
--- /dev/null
+++ b/packages/ai/anthropic/scripts/generated.patch
@@ -0,0 +1,217 @@
+diff --git a/packages/ai/anthropic/src/Generated.ts b/packages/ai/anthropic/src/Generated.ts
+index 7c30bea99..644ad429a 100644
+--- a/packages/ai/anthropic/src/Generated.ts
++++ b/packages/ai/anthropic/src/Generated.ts
+@@ -14,27 +14,26 @@ export class MessagesPostParams extends S.Struct({
+   "anthropic-version": S.optionalWith(S.String, { nullable: true })
+ }) {}
+ 
+-export class Model extends S.Union(
+-  S.String,
+-  S.Literal("claude-3-7-sonnet-latest"),
+-  S.Literal("claude-3-7-sonnet-20250219"),
+-  S.Literal("claude-3-5-haiku-latest"),
+-  S.Literal("claude-3-5-haiku-20241022"),
+-  S.Literal("claude-sonnet-4-20250514"),
+-  S.Literal("claude-sonnet-4-0"),
+-  S.Literal("claude-4-sonnet-20250514"),
+-  S.Literal("claude-3-5-sonnet-latest"),
+-  S.Literal("claude-3-5-sonnet-20241022"),
+-  S.Literal("claude-3-5-sonnet-20240620"),
+-  S.Literal("claude-opus-4-0"),
+-  S.Literal("claude-opus-4-20250514"),
+-  S.Literal("claude-4-opus-20250514"),
+-  S.Literal("claude-3-opus-latest"),
+-  S.Literal("claude-3-opus-20240229"),
+-  S.Literal("claude-3-sonnet-20240229"),
+-  S.Literal("claude-3-haiku-20240307"),
+-  S.Literal("claude-2.1"),
+-  S.Literal("claude-2.0")
++export class Model extends S.Literal(
++  "claude-3-7-sonnet-latest",
++  "claude-3-7-sonnet-20250219",
++  "claude-3-5-haiku-latest",
++  "claude-3-5-haiku-20241022",
++  "claude-sonnet-4-20250514",
++  "claude-sonnet-4-0",
++  "claude-4-sonnet-20250514",
++  "claude-3-5-sonnet-latest",
++  "claude-3-5-sonnet-20241022",
++  "claude-3-5-sonnet-20240620",
++  "claude-opus-4-0",
++  "claude-opus-4-20250514",
++  "claude-4-opus-20250514",
++  "claude-3-opus-latest",
++  "claude-3-opus-20240229",
++  "claude-3-sonnet-20240229",
++  "claude-3-haiku-20240307",
++  "claude-2.1",
++  "claude-2.0"
+ ) {}
+ 
+ export class CacheControlEphemeralType extends S.Literal("ephemeral") {}
+@@ -397,14 +396,14 @@ export class WebSearchTool20250305 extends S.Struct({
+   "allowed_domains": S.optionalWith(S.Union(S.Array(S.String), S.Null), { nullable: true }),
+   "blocked_domains": S.optionalWith(S.Union(S.Array(S.String), S.Null), { nullable: true }),
+   "cache_control": S.optionalWith(S.Union(CacheControlEphemeral, S.Null), { nullable: true }),
+-  "max_uses": S.optionalWith(S.Union(S.Int.pipe(S.greaterThan(undefined)), S.Null), { nullable: true }),
++  "max_uses": S.optionalWith(S.Union(S.Int.pipe(S.greaterThan(0)), S.Null), { nullable: true }),
+   "name": WebSearchTool20250305Name,
+   "type": WebSearchTool20250305Type,
+   "user_location": S.optionalWith(S.Union(UserLocation, S.Null), { nullable: true })
+ }) {}
+ 
+ export class CreateMessageParams extends S.Class<CreateMessageParams>("CreateMessageParams")({
+-  "model": Model,
++  "model": S.Union(S.String, Model),
+   "messages": S.Array(InputMessage),
+   "max_tokens": S.Int.pipe(S.greaterThanOrEqualTo(1)),
+   "metadata": S.optionalWith(Metadata, { nullable: true }),
+@@ -473,19 +472,16 @@ export class ResponseWebSearchResultLocationCitation extends S.Struct({
+ export class ResponseTextBlockType extends S.Literal("text") {}
+ 
+ export class ResponseTextBlock extends S.Struct({
+-  "citations": S.NullOr(
+-    S.Union(
+-      S.Array(
+-        S.Union(
+-          ResponseCharLocationCitation,
+-          ResponsePageLocationCitation,
+-          ResponseContentBlockLocationCitation,
+-          ResponseWebSearchResultLocationCitation
+-        )
+-      ),
+-      S.Null
++  "citations": S.optional(S.NullOr(
++    S.Array(
++      S.Union(
++        ResponseCharLocationCitation,
++        ResponsePageLocationCitation,
++        ResponseContentBlockLocationCitation,
++        ResponseWebSearchResultLocationCitation
++      )
+     )
+-  ),
++  )),
+   "text": S.String.pipe(S.minLength(0), S.maxLength(5000000)),
+   "type": ResponseTextBlockType
+ }) {}
+@@ -574,7 +570,7 @@ export class Usage extends S.Struct({
+   "cache_read_input_tokens": S.NullOr(S.Union(S.Int.pipe(S.greaterThanOrEqualTo(0)), S.Null)),
+   "input_tokens": S.Int.pipe(S.greaterThanOrEqualTo(0)),
+   "output_tokens": S.Int.pipe(S.greaterThanOrEqualTo(0)),
+-  "server_tool_use": S.NullOr(S.Union(ServerToolUsage, S.Null)),
++  "server_tool_use": S.optional(S.NullOr(ServerToolUsage)),
+   "service_tier": S.NullOr(S.Union(UsageServiceTierEnum, S.Null))
+ }) {}
+ 
+@@ -583,7 +579,7 @@ export class Message extends S.Class<Message>("Message")({
+   "type": MessageType,
+   "role": MessageRole,
+   "content": S.Array(ContentBlock),
+-  "model": Model,
++  "model": S.Union(S.String, Model),
+   "stop_reason": S.Union(StopReason, S.Null),
+   "stop_sequence": S.NullOr(S.Union(S.String, S.Null)),
+   "usage": Usage
+@@ -675,7 +671,7 @@ export class CompletePostParams extends S.Struct({
+ }) {}
+ 
+ export class CompletionRequest extends S.Class<CompletionRequest>("CompletionRequest")({
+-  "model": Model,
++  "model": S.Union(S.String, Model),
+   "prompt": S.String.pipe(S.minLength(1)),
+   "max_tokens_to_sample": S.Int.pipe(S.greaterThanOrEqualTo(1)),
+   "stop_sequences": S.optionalWith(S.Array(S.String), { nullable: true }),
+@@ -691,7 +687,7 @@ export class CompletionResponseType extends S.Literal("completion") {}
+ export class CompletionResponse extends S.Class<CompletionResponse>("CompletionResponse")({
+   "completion": S.String,
+   "id": S.String,
+-  "model": Model,
++  "model": S.Union(S.String, Model),
+   "stop_reason": S.Union(S.String, S.Null),
+   "type": CompletionResponseType
+ }) {}
+@@ -812,7 +808,7 @@ export class MessagesCountTokensPostParams extends S.Struct({
+ 
+ export class CountMessageTokensParams extends S.Class<CountMessageTokensParams>("CountMessageTokensParams")({
+   "messages": S.Array(InputMessage),
+-  "model": Model,
++  "model": S.Union(S.String, Model),
+   "system": S.optionalWith(S.Union(S.String, S.Array(RequestTextBlock)), { nullable: true }),
+   "thinking": S.optionalWith(ThinkingConfigParam, { nullable: true }),
+   "tool_choice": S.optionalWith(ToolChoice, { nullable: true }),
+@@ -1405,7 +1401,7 @@ export class BetaWebSearchTool20250305 extends S.Struct({
+   "allowed_domains": S.optionalWith(S.Union(S.Array(S.String), S.Null), { nullable: true }),
+   "blocked_domains": S.optionalWith(S.Union(S.Array(S.String), S.Null), { nullable: true }),
+   "cache_control": S.optionalWith(S.Union(BetaCacheControlEphemeral, S.Null), { nullable: true }),
+-  "max_uses": S.optionalWith(S.Union(S.Int.pipe(S.greaterThan(undefined)), S.Null), { nullable: true }),
++  "max_uses": S.optionalWith(S.Union(S.Int.pipe(S.greaterThan(0)), S.Null), { nullable: true }),
+   "name": BetaWebSearchTool20250305Name,
+   "type": BetaWebSearchTool20250305Type,
+   "user_location": S.optionalWith(S.Union(BetaUserLocation, S.Null), { nullable: true })
+@@ -1422,7 +1418,7 @@ export class BetaCodeExecutionTool20250522 extends S.Struct({
+ }) {}
+ 
+ export class BetaCreateMessageParams extends S.Class<BetaCreateMessageParams>("BetaCreateMessageParams")({
+-  "model": Model,
++  "model": S.Union(S.String, Model),
+   "messages": S.Array(BetaInputMessage),
+   "container": S.optionalWith(S.Union(S.String, S.Null), { nullable: true }),
+   "max_tokens": S.Int.pipe(S.greaterThanOrEqualTo(1)),
+@@ -1506,19 +1502,16 @@ export class BetaResponseWebSearchResultLocationCitation extends S.Struct({
+ export class BetaResponseTextBlockType extends S.Literal("text") {}
+ 
+ export class BetaResponseTextBlock extends S.Struct({
+-  "citations": S.NullOr(
+-    S.Union(
+-      S.Array(
+-        S.Union(
+-          BetaResponseCharLocationCitation,
+-          BetaResponsePageLocationCitation,
+-          BetaResponseContentBlockLocationCitation,
+-          BetaResponseWebSearchResultLocationCitation
+-        )
+-      ),
+-      S.Null
++  "citations": S.optional(S.NullOr(
++    S.Array(
++      S.Union(
++        BetaResponseCharLocationCitation,
++        BetaResponsePageLocationCitation,
++        BetaResponseContentBlockLocationCitation,
++        BetaResponseWebSearchResultLocationCitation
++      )
+     )
+-  ),
++  )),
+   "text": S.String.pipe(S.minLength(0), S.maxLength(5000000)),
+   "type": BetaResponseTextBlockType
+ }) {}
+@@ -1675,7 +1668,7 @@ export class BetaUsage extends S.Struct({
+   "cache_read_input_tokens": S.NullOr(S.Union(S.Int.pipe(S.greaterThanOrEqualTo(0)), S.Null)),
+   "input_tokens": S.Int.pipe(S.greaterThanOrEqualTo(0)),
+   "output_tokens": S.Int.pipe(S.greaterThanOrEqualTo(0)),
+-  "server_tool_use": S.NullOr(S.Union(BetaServerToolUsage, S.Null)),
++  "server_tool_use": S.optional(S.NullOr(BetaServerToolUsage)),
+   "service_tier": S.NullOr(S.Union(BetaUsageServiceTierEnum, S.Null))
+ }) {}
+ 
+@@ -1689,7 +1682,7 @@ export class BetaMessage extends S.Class<BetaMessage>("BetaMessage")({
+   "type": BetaMessageType,
+   "role": BetaMessageRole,
+   "content": S.Array(BetaContentBlock),
+-  "model": Model,
++  "model": S.Union(S.String, Model),
+   "stop_reason": S.Union(BetaStopReason, S.Null),
+   "stop_sequence": S.NullOr(S.Union(S.String, S.Null)),
+   "usage": BetaUsage,
+@@ -1907,7 +1900,7 @@ export class BetaCountMessageTokensParams
+   extends S.Class<BetaCountMessageTokensParams>("BetaCountMessageTokensParams")({
+     "mcp_servers": S.optionalWith(S.Array(BetaRequestMCPServerURLDefinition).pipe(S.maxItems(20)), { nullable: true }),
+     "messages": S.Array(BetaInputMessage),
+-    "model": Model,
++    "model": S.Union(S.String, Model),
+     "system": S.optionalWith(S.Union(S.String, S.Array(BetaRequestTextBlock)), { nullable: true }),
+     "thinking": S.optionalWith(BetaThinkingConfigParam, { nullable: true }),
+     "tool_choice": S.optionalWith(BetaToolChoice, { nullable: true }),
diff --git a/packages/ai/anthropic/src/Generated.ts b/packages/ai/anthropic/src/Generated.ts
index c0c1534cff4..644ad429aed 100644
--- a/packages/ai/anthropic/src/Generated.ts
+++ b/packages/ai/anthropic/src/Generated.ts
@@ -14,26 +14,26 @@ export class MessagesPostParams extends S.Struct({
   "anthropic-version": S.optionalWith(S.String, { nullable: true })
 }) {}
 
-export class Model extends S.Union(
-  S.Literal("claude-3-7-sonnet-latest"),
-  S.Literal("claude-3-7-sonnet-20250219"),
-  S.Literal("claude-3-5-haiku-latest"),
-  S.Literal("claude-3-5-haiku-20241022"),
-  S.Literal("claude-sonnet-4-20250514"),
-  S.Literal("claude-sonnet-4-0"),
-  S.Literal("claude-4-sonnet-20250514"),
-  S.Literal("claude-3-5-sonnet-latest"),
-  S.Literal("claude-3-5-sonnet-20241022"),
-  S.Literal("claude-3-5-sonnet-20240620"),
-  S.Literal("claude-opus-4-0"),
-  S.Literal("claude-opus-4-20250514"),
-  S.Literal("claude-4-opus-20250514"),
-  S.Literal("claude-3-opus-latest"),
-  S.Literal("claude-3-opus-20240229"),
-  S.Literal("claude-3-sonnet-20240229"),
-  S.Literal("claude-3-haiku-20240307"),
-  S.Literal("claude-2.1"),
-  S.Literal("claude-2.0")
+export class Model extends S.Literal(
+  "claude-3-7-sonnet-latest",
+  "claude-3-7-sonnet-20250219",
+  "claude-3-5-haiku-latest",
+  "claude-3-5-haiku-20241022",
+  "claude-sonnet-4-20250514",
+  "claude-sonnet-4-0",
+  "claude-4-sonnet-20250514",
+  "claude-3-5-sonnet-latest",
+  "claude-3-5-sonnet-20241022",
+  "claude-3-5-sonnet-20240620",
+  "claude-opus-4-0",
+  "claude-opus-4-20250514",
+  "claude-4-opus-20250514",
+  "claude-3-opus-latest",
+  "claude-3-opus-20240229",
+  "claude-3-sonnet-20240229",
+  "claude-3-haiku-20240307",
+  "claude-2.1",
+  "claude-2.0"
 ) {}
 
 export class CacheControlEphemeralType extends S.Literal("ephemeral") {}
@@ -472,19 +472,16 @@ export class ResponseWebSearchResultLocationCitation extends S.Struct({
 export class ResponseTextBlockType extends S.Literal("text") {}
 
 export class ResponseTextBlock extends S.Struct({
-  "citations": S.NullOr(
-    S.Union(
-      S.Array(
-        S.Union(
-          ResponseCharLocationCitation,
-          ResponsePageLocationCitation,
-          ResponseContentBlockLocationCitation,
-          ResponseWebSearchResultLocationCitation
-        )
-      ),
-      S.Null
+  "citations": S.optional(S.NullOr(
+    S.Array(
+      S.Union(
+        ResponseCharLocationCitation,
+        ResponsePageLocationCitation,
+        ResponseContentBlockLocationCitation,
+        ResponseWebSearchResultLocationCitation
+      )
     )
-  ),
+  )),
   "text": S.String.pipe(S.minLength(0), S.maxLength(5000000)),
   "type": ResponseTextBlockType
 }) {}
@@ -573,7 +570,7 @@ export class Usage extends S.Struct({
   "cache_read_input_tokens": S.NullOr(S.Union(S.Int.pipe(S.greaterThanOrEqualTo(0)), S.Null)),
   "input_tokens": S.Int.pipe(S.greaterThanOrEqualTo(0)),
   "output_tokens": S.Int.pipe(S.greaterThanOrEqualTo(0)),
-  "server_tool_use": S.NullOr(S.Union(ServerToolUsage, S.Null)),
+  "server_tool_use": S.optional(S.NullOr(ServerToolUsage)),
   "service_tier": S.NullOr(S.Union(UsageServiceTierEnum, S.Null))
 }) {}
 
@@ -1505,19 +1502,16 @@ export class BetaResponseWebSearchResultLocationCitation extends S.Struct({
 export class BetaResponseTextBlockType extends S.Literal("text") {}
 
 export class BetaResponseTextBlock extends S.Struct({
-  "citations": S.NullOr(
-    S.Union(
-      S.Array(
-        S.Union(
-          BetaResponseCharLocationCitation,
-          BetaResponsePageLocationCitation,
-          BetaResponseContentBlockLocationCitation,
-          BetaResponseWebSearchResultLocationCitation
-        )
-      ),
-      S.Null
+  "citations": S.optional(S.NullOr(
+    S.Array(
+      S.Union(
+        BetaResponseCharLocationCitation,
+        BetaResponsePageLocationCitation,
+        BetaResponseContentBlockLocationCitation,
+        BetaResponseWebSearchResultLocationCitation
+      )
     )
-  ),
+  )),
   "text": S.String.pipe(S.minLength(0), S.maxLength(5000000)),
   "type": BetaResponseTextBlockType
 }) {}
@@ -1674,7 +1668,7 @@ export class BetaUsage extends S.Struct({
   "cache_read_input_tokens": S.NullOr(S.Union(S.Int.pipe(S.greaterThanOrEqualTo(0)), S.Null)),
   "input_tokens": S.Int.pipe(S.greaterThanOrEqualTo(0)),
   "output_tokens": S.Int.pipe(S.greaterThanOrEqualTo(0)),
-  "server_tool_use": S.NullOr(S.Union(BetaServerToolUsage, S.Null)),
+  "server_tool_use": S.optional(S.NullOr(BetaServerToolUsage)),
   "service_tier": S.NullOr(S.Union(BetaUsageServiceTierEnum, S.Null))
 }) {}