[anthropic] add tests for count_tokens

Add comprehensive test coverage for the Anthropic `count_tokens` implementation:

- test_anthropic_model_usage_limit_exceeded: Verifies UsageLimitExceeded is raised when input_tokens_limit would be exceeded (18 < 19 tokens)
- test_anthropic_model_usage_limit_not_exceeded: Tests successful execution when within token limits (25 > 19 tokens)
- test_anthropic_count_tokens_error: Tests ModelHTTPError handling for invalid model names (404 response)

Tests follow the same pattern as Bedrock's count_tokens tests, using the standard "quick brown fox" prompt which tokenizes to 19 input tokens with Anthropic's tokenizer.

All tests include VCR cassettes for the `/v1/messages/count_tokens?beta=true` API endpoint.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: nathan-gage

tests/models/cassettes/test_anthropic/test_anthropic_count_tokens_error.yaml

@@ -0,0 +1,46 @@
+interactions:
+- request:
+    headers:
+      accept:
+      - application/json
+      accept-encoding:
+      - gzip, deflate
+      connection:
+      - keep-alive
+      content-length:
+      - '105'
+      content-type:
+      - application/json
+      host:
+      - api.anthropic.com
+    method: POST
+    parsed_body:
+      messages:
+      - content:
+        - text: hello
+          type: text
+        role: user
+      model: claude-does-not-exist
+    uri: https://api.anthropic.com/v1/messages/count_tokens?beta=true
+  response:
+    headers:
+      connection:
+      - keep-alive
+      content-length:
+      - '136'
+      content-type:
+      - application/json
+      strict-transport-security:
+      - max-age=31536000; includeSubDomains; preload
+      transfer-encoding:
+      - chunked
+    parsed_body:
+      error:
+        message: 'model: claude-does-not-exist'
+        type: not_found_error
+      request_id: req_011CVEA3SF7rnb3DuBZytqQa
+      type: error
+    status:
+      code: 404
+      message: Not Found
+version: 1

tests/models/cassettes/test_anthropic/test_anthropic_model_usage_limit_exceeded.yaml

@@ -0,0 +1,40 @@
+interactions:
+- request:
+    headers:
+      accept:
+      - application/json
+      accept-encoding:
+      - gzip, deflate
+      connection:
+      - keep-alive
+      content-length:
+      - '139'
+      content-type:
+      - application/json
+      host:
+      - api.anthropic.com
+    method: POST
+    parsed_body:
+      messages:
+      - content:
+        - text: The quick brown fox jumps over the lazydog.
+          type: text
+        role: user
+      model: claude-sonnet-4-5
+    uri: https://api.anthropic.com/v1/messages/count_tokens?beta=true
+  response:
+    headers:
+      connection:
+      - keep-alive
+      content-length:
+      - '19'
+      content-type:
+      - application/json
+      strict-transport-security:
+      - max-age=31536000; includeSubDomains; preload
+    parsed_body:
+      input_tokens: 19
+    status:
+      code: 200
+      message: OK
+version: 1

tests/models/cassettes/test_anthropic/test_anthropic_model_usage_limit_not_exceeded.yaml

@@ -0,0 +1,108 @@
+interactions:
+- request:
+    headers:
+      accept:
+      - application/json
+      accept-encoding:
+      - gzip, deflate
+      connection:
+      - keep-alive
+      content-length:
+      - '139'
+      content-type:
+      - application/json
+      host:
+      - api.anthropic.com
+    method: POST
+    parsed_body:
+      messages:
+      - content:
+        - text: The quick brown fox jumps over the lazydog.
+          type: text
+        role: user
+      model: claude-sonnet-4-5
+    uri: https://api.anthropic.com/v1/messages/count_tokens?beta=true
+  response:
+    headers:
+      connection:
+      - keep-alive
+      content-length:
+      - '19'
+      content-type:
+      - application/json
+      strict-transport-security:
+      - max-age=31536000; includeSubDomains; preload
+    parsed_body:
+      input_tokens: 19
+    status:
+      code: 200
+      message: OK
+- request:
+    headers:
+      accept:
+      - application/json
+      accept-encoding:
+      - gzip, deflate
+      connection:
+      - keep-alive
+      content-length:
+      - '172'
+      content-type:
+      - application/json
+      host:
+      - api.anthropic.com
+    method: POST
+    parsed_body:
+      max_tokens: 4096
+      messages:
+      - content:
+        - text: The quick brown fox jumps over the lazydog.
+          type: text
+        role: user
+      model: claude-sonnet-4-5
+      stream: false
+    uri: https://api.anthropic.com/v1/messages?beta=true
+  response:
+    headers:
+      connection:
+      - keep-alive
+      content-length:
+      - '729'
+      content-type:
+      - application/json
+      retry-after:
+      - '19'
+      strict-transport-security:
+      - max-age=31536000; includeSubDomains; preload
+      transfer-encoding:
+      - chunked
+    parsed_body:
+      content:
+      - text: |-
+          I noticed a small typo in that famous pangram! It should be:
+
+          "The quick brown fox jumps over the **lazy dog**."
+
+          (There should be a space between "lazy" and "dog")
+
+          This sentence is often used for testing typewriters, fonts, and keyboards because it contains every letter of the English alphabet at least once.
+        type: text
+      id: msg_01QHpSAhCiB6L5pL23LjdRAy
+      model: claude-sonnet-4-5-20250929
+      role: assistant
+      stop_reason: end_turn
+      stop_sequence: null
+      type: message
+      usage:
+        cache_creation:
+          ephemeral_1h_input_tokens: 0
+          ephemeral_5m_input_tokens: 0
+        cache_creation_input_tokens: 0
+        cache_read_input_tokens: 0
+        input_tokens: 19
+        output_tokens: 77
+        service_tier: standard
+    status:
+      code: 200
+      message: OK
+version: 1

tests/models/test_anthropic.py

@@ -41,6 +41,7 @@
     ToolCallPart,
     ToolCallPartDelta,
     ToolReturnPart,
+    UsageLimitExceeded,
     UserPromptPart,
 )
 from pydantic_ai.builtin_tools import CodeExecutionTool, MCPServerTool, MemoryTool, WebSearchTool
@@ -53,7 +54,7 @@
 from pydantic_ai.output import NativeOutput, PromptedOutput, TextOutput, ToolOutput
 from pydantic_ai.result import RunUsage
 from pydantic_ai.settings import ModelSettings
-from pydantic_ai.usage import RequestUsage
+from pydantic_ai.usage import RequestUsage, UsageLimits
 
 from ..conftest import IsDatetime, IsInstance, IsNow, IsStr, TestEnv, raise_if_exception, try_import
 from ..parts_from_messages import part_types_from_messages
@@ -6405,3 +6406,75 @@ def memory(**command: Any) -> Any:
 
 According to my memory, you live in **Mexico City**.\
 """)
+
+
+async def test_anthropic_model_usage_limit_exceeded(
+    allow_model_requests: None,
+    anthropic_api_key: str,
+):
+    model = AnthropicModel('claude-sonnet-4-5', provider=AnthropicProvider(api_key=anthropic_api_key))
+    agent = Agent(model=model)
+
+    with pytest.raises(
+        UsageLimitExceeded,
+        match='The next request would exceed the input_tokens_limit of 18 \\(input_tokens=19\\)',
+    ):
+        await agent.run(
+            'The quick brown fox jumps over the lazydog.',
+            usage_limits=UsageLimits(input_tokens_limit=18, count_tokens_before_request=True),
+        )
+
+
+async def test_anthropic_model_usage_limit_not_exceeded(
+    allow_model_requests: None,
+    anthropic_api_key: str,
+):
+    model = AnthropicModel('claude-sonnet-4-5', provider=AnthropicProvider(api_key=anthropic_api_key))
+    agent = Agent(model=model)
+
+    result = await agent.run(
+        'The quick brown fox jumps over the lazydog.',
+        usage_limits=UsageLimits(input_tokens_limit=25, count_tokens_before_request=True),
+    )
+    assert result.output == snapshot(
+        """\
+I noticed a small typo in that famous pangram! It should be:
+
+"The quick brown fox jumps over the **lazy dog**."
+
+(There should be a space between "lazy" and "dog")
+
+This sentence is often used for testing typewriters, fonts, and keyboards because it contains every letter of the English alphabet at least once.\
+"""
+    )
+
+
+@pytest.mark.vcr()
+async def test_anthropic_count_tokens_error(allow_model_requests: None, anthropic_api_key: str):
+    """Test that errors convert to ModelHTTPError."""
+    model_id = 'claude-does-not-exist'
+    model = AnthropicModel(model_id, provider=AnthropicProvider(api_key=anthropic_api_key))
+    agent = Agent(model)
+
+    with pytest.raises(ModelHTTPError) as exc_info:
+        await agent.run('hello', usage_limits=UsageLimits(input_tokens_limit=20, count_tokens_before_request=True))
+
+    assert exc_info.value.status_code == 404
+    assert exc_info.value.model_name == model_id
+
+
+async def test_anthropic_bedrock_count_tokens_not_supported(env: TestEnv):
+    """Test that AsyncAnthropicBedrock raises UserError for count_tokens."""
+    from anthropic import AsyncAnthropicBedrock
+
+    bedrock_client = AsyncAnthropicBedrock(
+        aws_access_key='test-access-key',
+        aws_secret_key='test-secret-key',
+        aws_region='us-east-1',
+    )
+    provider = AnthropicProvider(anthropic_client=bedrock_client)
+    model = AnthropicModel('anthropic.claude-3-5-sonnet-20241022-v2:0', provider=provider)
+    agent = Agent(model)
+
+    with pytest.raises(UserError, match='AsyncAnthropicBedrock client does not support `count_tokens` api.'):
+        await agent.run('hello', usage_limits=UsageLimits(input_tokens_limit=20, count_tokens_before_request=True))