Allow empty prompts when there are messages and no files to attach.
The last message must be a user or tool message - this ensures the
conversation state is valid for the LLM to respond.

This addresses use cases where:
- Generating from an existing conversation
- Tool results were just returned and we want the LLM to respond

Validation:
- Error if empty prompt with no messages
- Error if empty prompt with files (files need a user message)
- Error if last message is assistant (nothing to respond to)
- Allow if last message is user or tool

Closes #54
Closes #59
Closes #103

Co-authored-by: Arun Barua <arun.barua@onit.com>
Co-authored-by: Ruslan Mayer <r.stupa@ya.ru>

The Anthropic provider's toTools function was returning nil for
anthropicToolChoice when ToolChoiceNone was set, which meant
tool_choice was never included in the API request. The API defaults
to "auto", so the model could still make tool calls.

Now properly constructs ToolChoiceUnionParam with OfNone set using
anthropic.NewToolChoiceNoneParam().

Also adds test coverage for ToolChoiceNone.

* Closes #177

💘 Generated with Crush

Assisted-by: Kimi K2.5 via Crush <crush@charm.land>

Signed-off-by: Calum Murray <cmurray@redhat.com>

OpenAI's API reports prompt_tokens/input_tokens INCLUDING cached tokens,
while also separately reporting cached_tokens in prompt_tokens_details.
This caused double-counting when users summed InputTokens + CacheReadTokens.

For example, if OpenAI reports:
  - prompt_tokens: 1000 (includes 900 cached)
  - cached_tokens: 900

Before this fix, fantasy reported:
  - InputTokens: 1000
  - CacheReadTokens: 900

After this fix, fantasy reports:
  - InputTokens: 100 (non-cached only)
  - CacheReadTokens: 900

This matches the behavior of Vercel AI SDK and prevents billing
miscalculations when pricing input tokens and cache read tokens separately.

See: https://platform.openai.com/docs/guides/prompt-caching#requirements

💘 Generated with Crush

Assisted-by: Kimi K2.5 via Crush <crush@charm.land>

Co-authored-by: Christian Rocha <christian@rocha.is>

Bumps the kronk group with 1 update: [github.com/ardanlabs/kronk](https://github.com/ardanlabs/kronk).


Updates `github.com/ardanlabs/kronk` from 1.20.8 to 1.21.3
- [Release notes](https://github.com/ardanlabs/kronk/releases)
- [Commits](https://github.com/ardanlabs/kronk/compare/v1.20.8...v1.21.3)

---
updated-dependencies:
- dependency-name: github.com/ardanlabs/kronk
  dependency-version: 1.21.3
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: kronk
...

Signed-off-by: dependabot[bot] <support@github.com>

This is currently already covered by `web-search` example.

Add OpenAI web search as a provider-defined tool, mirroring the
Anthropic web search implementation pattern.

New files:
- providers/openai/web_search.go: WebSearchTool() helper with
  SearchContextSize, AllowedDomains, and UserLocation options.
- examples/openai-web-search/main.go: Example using the agent
  pattern with web search.

Changes to existing files:
- responses_options.go: WebSearchCallMetadata, WebSearchAction,
  and WebSearchSource types registered for JSON round-tripping.
- responses_language_model.go:
  - toResponsesTools(): Route ProviderDefinedTool web_search to
    the OpenAI SDK WebSearchToolParam.
  - Generate(): Emit ToolCallContent + ToolResultContent pair for
    web_search_call output items. Source citations come from
    url_citation annotations on message text.
  - Stream(): Emit ToolInputStart/ToolInputEnd/ToolCall/ToolResult
    lifecycle events for web_search_call items.
  - toResponsesPrompt(): Round-trip provider-executed tool calls
    via item_reference; skip SourceContent and provider-executed
    ToolResultPart (already handled by the reference).
- providertests/provider_registry_test.go: Add metadata test case.

Add support for the Anthropic web search tool

Bumps the all group with 4 updates: [github.com/aws/aws-sdk-go-v2](https://github.com/aws/aws-sdk-go-v2), [github.com/aws/aws-sdk-go-v2/config](https://github.com/aws/aws-sdk-go-v2), [github.com/openai/openai-go/v3](https://github.com/openai/openai-go) and [google.golang.org/genai](https://github.com/googleapis/go-genai).


Updates `github.com/aws/aws-sdk-go-v2` from 1.41.3 to 1.41.4
- [Release notes](https://github.com/aws/aws-sdk-go-v2/releases)
- [Commits](https://github.com/aws/aws-sdk-go-v2/compare/v1.41.3...v1.41.4)

Updates `github.com/aws/aws-sdk-go-v2/config` from 1.32.11 to 1.32.12
- [Release notes](https://github.com/aws/aws-sdk-go-v2/releases)
- [Commits](https://github.com/aws/aws-sdk-go-v2/compare/config/v1.32.11...config/v1.32.12)

Updates `github.com/openai/openai-go/v3` from 3.26.0 to 3.28.0
- [Release notes](https://github.com/openai/openai-go/releases)
- [Changelog](https://github.com/openai/openai-go/blob/main/CHANGELOG.md)
- [Commits](https://github.com/openai/openai-go/compare/v3.26.0...v3.28.0)

Updates `google.golang.org/genai` from 1.49.0 to 1.50.0
- [Release notes](https://github.com/googleapis/go-genai/releases)
- [Changelog](https://github.com/googleapis/go-genai/blob/main/CHANGELOG.md)
- [Commits](https://github.com/googleapis/go-genai/compare/v1.49.0...v1.50.0)

---
updated-dependencies:
- dependency-name: github.com/aws/aws-sdk-go-v2
  dependency-version: 1.41.4
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: all
- dependency-name: github.com/aws/aws-sdk-go-v2/config
  dependency-version: 1.32.12
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: all
- dependency-name: github.com/openai/openai-go/v3
  dependency-version: 3.28.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: all
- dependency-name: google.golang.org/genai
  dependency-version: 1.50.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: all
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

This guards against cases when web_search options could be silently
dropped when tool args pass through generic/JSON-shaped data, so domain
filters, max uses, and user location reliably reach Anthropic.

- Handle ProviderDefinedTool with ID "web_search" in toTools(), mapping to
  anthropic.WebSearchTool20250305Param with optional allowed/blocked domains
- Parse "server_tool_use" blocks in Generate() and Stream(), producing
  ToolCallContent with ProviderExecuted=true
- Parse "web_search_tool_result" blocks, extracting SourceContent for each
  search result and a ToolResultContent summary
- Stream server_tool_use via ToolInputStart/ToolInputEnd/ToolCall events
- Stream web_search_tool_result via Source events for each result item
- Skip provider-executed tool results and source content when building
  Anthropic prompts (toPrompt) to avoid sending unrecognized block types
- Add ProviderExecuted field to ToolResultPart with JSON marshal/unmarshal
- Propagate ProviderExecuted from ToolResultContent to ToolResultPart in
  agent's toResponseMessages()

Anthropic sends the reasoning signature chunk after tool_calls have
started. The streaming hook was ending reasoning prematurely on the
first tool_call chunk, so the late-arriving signature was lost. Track
the reasoning format and defer ending for anthropic streams until the
signature arrives. Also fix the agent stream assembler to not overwrite
provider metadata with nil on reasoning deltas.

💘 Generated with Crush

Assisted-by: Claude Opus 4.6 via Crush <crush@charm.land>

Reasoning metadata can exist without encrypted content, causing a nil
pointer panic when building prompt messages for openrouter and vercel.

💘 Generated with Crush

Assisted-by: Claude Opus 4.6 via Crush <crush@charm.land>

This change to be better follow the convention:

* App name should not have spaces: `Charm Fantasy` -> `Charm-Fantasy`.
* Add comment `()` with app site.

💘 Generated with Crush

Assisted-by: Kimi K2.5 via Crush <crush@charm.land>

💘 Generated with Crush

Assisted-by: Kimi K2.5 via Crush <crush@charm.land>

💘 Generated with Crush

Assisted-by: Kimi K2.5 via Crush <crush@charm.land>

💘 Generated with Crush

Assisted-by: Kimi K2.5 via Crush <crush@charm.land>

💘 Generated with Crush

Assisted-by: Kimi K2.5 via Crush <crush@charm.land>

OpenRouter rejects API calls using a custom User Agent. This workaround
skips OpenRouter when setting a user agent.

Note that we've added two new methods accordingly, as OpenRouter uses
the openai package:

openai.WithSkipUserAgent()
openai.WithLanguageModelSkipUserAgent()

* Follow OpenAI's Go SDK.
* Check status code >= 500.
* Check `x-should-retry` header.

Co-authored-by: yuguorui <yuguorui@pku.edu.cn>

Bumps the all group with 4 updates in the / directory: [github.com/aws/aws-sdk-go-v2](https://github.com/aws/aws-sdk-go-v2), [github.com/aws/aws-sdk-go-v2/config](https://github.com/aws/aws-sdk-go-v2), [golang.org/x/oauth2](https://github.com/golang/oauth2) and [google.golang.org/genai](https://github.com/googleapis/go-genai).


Updates `github.com/aws/aws-sdk-go-v2` from 1.41.2 to 1.41.3
- [Release notes](https://github.com/aws/aws-sdk-go-v2/releases)
- [Commits](https://github.com/aws/aws-sdk-go-v2/compare/v1.41.2...v1.41.3)

Updates `github.com/aws/aws-sdk-go-v2/config` from 1.32.10 to 1.32.11
- [Release notes](https://github.com/aws/aws-sdk-go-v2/releases)
- [Commits](https://github.com/aws/aws-sdk-go-v2/compare/config/v1.32.10...config/v1.32.11)

Updates `golang.org/x/oauth2` from 0.35.0 to 0.36.0
- [Commits](https://github.com/golang/oauth2/compare/v0.35.0...v0.36.0)

Updates `google.golang.org/genai` from 1.48.0 to 1.49.0
- [Release notes](https://github.com/googleapis/go-genai/releases)
- [Changelog](https://github.com/googleapis/go-genai/blob/main/CHANGELOG.md)
- [Commits](https://github.com/googleapis/go-genai/compare/v1.48.0...v1.49.0)

---
updated-dependencies:
- dependency-name: github.com/aws/aws-sdk-go-v2
  dependency-version: 1.41.3
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: all
- dependency-name: github.com/aws/aws-sdk-go-v2/config
  dependency-version: 1.32.11
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: all
- dependency-name: golang.org/x/oauth2
  dependency-version: 0.36.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: all
- dependency-name: google.golang.org/genai
  dependency-version: 1.49.0
  dependency-type: direct:production
  update-type: version-update:semver-minor
  dependency-group: all
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Note that the user agent defaults to Charm Fantasy/<version>, which
means we need to maintain a const for the version number.

Bumps the all group with 3 updates: [github.com/ardanlabs/kronk](https://github.com/ardanlabs/kronk), [github.com/aws/smithy-go](https://github.com/aws/smithy-go) and [github.com/kaptinlin/jsonschema](https://github.com/kaptinlin/jsonschema).


Updates `github.com/ardanlabs/kronk` from 1.20.3 to 1.20.8
- [Release notes](https://github.com/ardanlabs/kronk/releases)
- [Commits](https://github.com/ardanlabs/kronk/compare/v1.20.3...v1.20.8)

Updates `github.com/aws/smithy-go` from 1.24.1 to 1.24.2
- [Release notes](https://github.com/aws/smithy-go/releases)
- [Changelog](https://github.com/aws/smithy-go/blob/main/CHANGELOG.md)
- [Commits](https://github.com/aws/smithy-go/compare/v1.24.1...v1.24.2)

Updates `github.com/kaptinlin/jsonschema` from 0.7.3 to 0.7.5
- [Commits](https://github.com/kaptinlin/jsonschema/compare/v0.7.3...v0.7.5)

---
updated-dependencies:
- dependency-name: github.com/ardanlabs/kronk
  dependency-version: 1.20.8
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: all
- dependency-name: github.com/aws/smithy-go
  dependency-version: 1.24.2
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: all
- dependency-name: github.com/kaptinlin/jsonschema
  dependency-version: 0.7.5
  dependency-type: direct:production
  update-type: version-update:semver-patch
  dependency-group: all
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>