[Go] Anthropic tool-calling on Vertex AI broken in v1.5.0

[Dasio]

Disclaimer: This bug was found and investigated by an LLM (Claude). The reproduction steps and analysis have been verified by a human.

Describe the bug
Genkit v1.5.0 unconditionally sets Strict: anthropic.Bool(true) on all tool definitions when using the Vertex AI Model Garden Anthropic plugin. The Vertex AI Anthropic endpoint does
not support the strict field on tools and rejects the request with:

400 Bad Request:

{"type":"error","error":{"type":"invalid_request_error","message":"tools.0.custom.strict: Extra inputs are not permitted"}}

This is a regression from v1.4.0 where tool-calling with Anthropic models on Vertex AI worked correctly.

The problematic line is plugins/internal/anthropic/anthropic.go:355:

  resp = append(resp, anthropic.ToolUnionParam{
      OfTool: &anthropic.ToolParam{
          Name:        t.Name,
          Description: anthropic.String(t.Description),
          InputSchema: schema,
          Strict:      anthropic.Bool(true),  // ← Vertex AI rejects this
      },
  })

To Reproduce
Minimal repro — passes with v1.4.0, fails with v1.5.0 (verified):

  package main

  import (
        "context"
        "fmt"
        "log"

        "github.com/anthropics/anthropic-sdk-go"
        "github.com/firebase/genkit/go/ai"
        "github.com/firebase/genkit/go/genkit"
        "github.com/firebase/genkit/go/plugins/vertexai/modelgarden"
  )

  func main() {
        ctx := context.Background()

        plugin := &modelgarden.Anthropic{
                ProjectID: "your-gcp-project",
                Location:  "us-east5",
        }

        // Register a model (adjust name to one available in your project)
        modelgarden.AnthropicModels["claude-sonnet-4-6"] = ai.ModelOptions{
                Label: "Claude Sonnet 4.6",
                Supports: &ai.ModelSupports{
                        Multiturn:  true,
                        Tools:      true,
                        SystemRole: true,
                        Media:      true,
                },
        }

        g := genkit.Init(ctx, genkit.WithPlugins(plugin))
        model := modelgarden.AnthropicModel(g, "claude-sonnet-4-6")

        greet := genkit.DefineTool(g, "greet", "Says hello to a person",
                func(ctx *ai.ToolContext, input struct{ Name string }) (string, error) {
                        return fmt.Sprintf("Hello, %s!", input.Name), nil
                },
        )

        resp, err := genkit.Generate(ctx, g,
                ai.WithModel(model),
                ai.WithPrompt("Greet Alice using the greet tool"),
                ai.WithTools(greet),
                ai.WithConfig(&anthropic.MessageNewParams{
                        MaxTokens:   1024,
                        Temperature: anthropic.Float(0),
                }),
        )
        if err != nil {
                // v1.4.0: prints "Hello, Alice!"
                // v1.5.0: 400 Bad Request — tools.0.custom.strict: Extra inputs are not permitted
                log.Fatal(err)
        }
        fmt.Println(resp.Text())
  }

v1.4.0 - Hello, Alice! (tool-calling works)
v1.5.0- 400 Bad Request: tools.0.custom.strict: Extra inputs are not permitted
Without tools (removing ai.WithTools(greet)), both versions work fine.

Expected behavior
Tool-calling with Anthropic models via Vertex AI Model Garden should work as it did in v1.4.0.

Runtime (please complete the following information):

• OS: macOS (Darwin 25.3.0 arm64)

Go version

• go1.26.1 darwin/arm64

Additional context

• github.com/firebase/genkit/go v1.5.0
• github.com/anthropics/anthropic-sdk-go v1.23.0
• The Strict field uses param.Opt[bool] with omitzero — simply not setting it would prevent serialization and fix the issue.
• The enforceStrictSchema helper (which adds additionalProperties: false recursively to tool input schemas) is fine — only the Strict field on ToolParam is problematic.
• Suggested fix: skip setting Strict when the provider is "vertexai", or make it opt-in via config.