fix(bedrock): inference profile prefix resolution

[meowgorithm]

This PR resolves Bedrock model IDs using AWS region-group prefixes with AWS_DEFAULT_REGION fallback.

Related: charmbracelet/catwalk#224

[Copilot]

Pull request overview

Resolves AWS Bedrock model ID prefixing by mapping AWS regions to Bedrock “region-group” inference profile prefixes, with an added AWS_DEFAULT_REGION fallback.

Changes:

• Updated Bedrock auth/config region selection to use AWS_REGION with AWS_DEFAULT_REGION fallback.
• Reworked Bedrock model ID prefixing to use region-group prefixes (us, eu, jp, au, global) instead of the prior 2-letter region-derived prefix.
• Added unit tests covering prefixing behavior and region-to-prefix mapping.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
providers/anthropic/bedrock.go	Adds region resolution helper, region→prefix mapping, and prefix detection to correctly form Bedrock inference profile model IDs.
providers/anthropic/anthropic_test.go	Adds tests verifying prefixing behavior, env var precedence, and region→prefix mapping.

---

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[Copilot]

awsRegion() only checks AWS_REGION/AWS_DEFAULT_REGION. When Bedrock is used with default AWS auth, config.LoadDefaultConfig can resolve the region from shared config/IMDS even when both env vars are unset; in that case model prefixing will still default to us-east-1 ("us") and can generate the wrong inference-profile prefix. Consider passing the resolved AWS config region into the prefixing logic (or changing this helper to accept a region parameter) so the model ID prefix matches the actual region the SDK will use.

[puffitos]

I'm not 100% sure about this one. Adding the awsRegion() function seems to do 2 things:

• duplicate the logic of which region is supposed to be selected, which is done by aws' config.LoadDefaultConfig
• could eventually lead to a situation where if no AWS_REGION / AWS_DEFAULT_REGION is set we run into models with the wrong prefix, because we're prefixing the model before resolving the region by aws' default helper (check resolveConfigLoaders in config.go in the config@v1.32.12)

Maybe we can take a safer path and just do the following:

• get the default aws config from its config library
• if an API key is present/ no-auth was passed, we update the config with an API key
• if not, we move on with the parsed config
• if an error happened during parsing - we fallback to the default region
• BONUS: no if-else :)

// keep the model prefix enrichment method, but also add the resolved region instead of using the awsRegion helper
  func bedrockPrefixModelWithRegion(modelID, region string) string {
     if hasBedrockInferenceProfilePrefix(modelID) {
          return modelID
      }
      return bedrockRegionPrefix(cmp.Or(region, "us-east-1")) + "." + modelID
  }

// in anthropic.go

if a.options.useBedrock {
          cfg, err := config.LoadDefaultConfig(ctx)
          if err != nil {
              cfg = aws.Config{}
          }

          // we inline the logic of the bedrockBasicAuthConfig function (or just call it)
          // if the user has no region configured, we fallback to us-east 
          cfg.Region = cmp.Or(cfg.Region, "us-east-1") 
          if a.options.skipAuth || a.options.apiKey != "" { 
              cfg.BearerAuthTokenProvider = bearer.StaticTokenProvider{
                  Token: bearer.Token{Value: a.options.apiKey},
              }
          }

          // now we update the model prefix, as we're sure about the region
          // and update the client options
          modelID = bedrockPrefixModelWithRegion(modelID, cfg.Region)
          clientOptions = append(clientOptions, bedrock.WithConfig(cfg))

          if a.options.baseURL != "" {
              clientOptions = append(clientOptions, option.WithBaseURL(a.options.baseURL)
          }
      }

      return languageModel{
          modelID:  modelID,
          provider: a.options.name,
          options:  a.options,
          client:   anthropic.NewClient(clientOptions...),
      }, nil
  }

I hope that makes sense. We get the win of relying just on AWS "stdlib" to get the configuration and we still get to overlay the bedrock API key, if needed.

[puffitos]

Suggested change

	require.Equal(t, "global", bedrockRegionPrefix("ap-south-1"))
	require.Equal(t, "global", bedrockRegionPrefix("sa-east-1"))
	require.Equal(t, "apac", bedrockRegionPrefix("ap-south-1"))
	require.Equal(t, "apac", bedrockRegionPrefix("ap-northeast-2"))
	require.Equal(t, "global", bedrockRegionPrefix("ap-south-1"))
	require.Equal(t, "global", bedrockRegionPrefix("sa-east-1"))

[puffitos]

Let's add the missing case again, to make sure we support the apac region as well:

Suggested change

	case region == "ap-southeast-2":
	return "au"
	case region == "ap-southeast-2":
	return "au"
	case strings.HasPrefix(region, "ap-"):
	return "apac"

[puffitos]

And finally, the missing apac prefix should be added here as well:

Suggested change

	for _, prefix := range []string{"us.", "eu.", "jp.", "au.", "global."} {
	for _, prefix := range []string{"us.", "eu.", "jp.", "au.", "apac.", "global."} {

[puffitos]

I'm not 100% sure about this one. Adding the awsRegion() function seems to do 2 things:

• duplicate the logic of which region is supposed to be selected, which is done by aws' config.LoadDefaultConfig
• could eventually lead to a situation where if no AWS_REGION / AWS_DEFAULT_REGION is set we run into models with the wrong prefix, because we're prefixing the model before resolving the region by aws' default helper (check resolveConfigLoaders in config.go in the config@v1.32.12)

Maybe we can take a safer path and just do the following:

• get the default aws config from its config library
• if an API key is present/ no-auth was passed, we update the config with an API key
• if not, we move on with the parsed config
• if an error happened during parsing - we fallback to the default region
• BONUS: no if-else :)

// keep the model prefix enrichment method, but also add the resolved region instead of using the awsRegion helper
  func bedrockPrefixModelWithRegion(modelID, region string) string {
     if hasBedrockInferenceProfilePrefix(modelID) {
          return modelID
      }
      return bedrockRegionPrefix(cmp.Or(region, "us-east-1")) + "." + modelID
  }

// in anthropic.go

if a.options.useBedrock {
          cfg, err := config.LoadDefaultConfig(ctx)
          if err != nil {
              cfg = aws.Config{}
          }

          // we inline the logic of the bedrockBasicAuthConfig function (or just call it)
          // if the user has no region configured, we fallback to us-east 
          cfg.Region = cmp.Or(cfg.Region, "us-east-1") 
          if a.options.skipAuth || a.options.apiKey != "" { 
              cfg.BearerAuthTokenProvider = bearer.StaticTokenProvider{
                  Token: bearer.Token{Value: a.options.apiKey},
              }
          }

          // now we update the model prefix, as we're sure about the region
          // and update the client options
          modelID = bedrockPrefixModelWithRegion(modelID, cfg.Region)
          clientOptions = append(clientOptions, bedrock.WithConfig(cfg))

          if a.options.baseURL != "" {
              clientOptions = append(clientOptions, option.WithBaseURL(a.options.baseURL)
          }
      }

      return languageModel{
          modelID:  modelID,
          provider: a.options.name,
          options:  a.options,
          client:   anthropic.NewClient(clientOptions...),
      }, nil
  }

I hope that makes sense. We get the win of relying just on AWS "stdlib" to get the configuration and we still get to overlay the bedrock API key, if needed.