[aisdk] add Truncation: {auto, default} as an encoding metadata option (#592)

## Summary

In `getModelConfig`, we specify some default settings for the openai
models.
In particular, the RequiredAutoTruncation is hardcoded in there.

Some users may want to override this and manually set the truncation to
auto or disabled, depending on their specific use-case.
This PR allows that.

## How was it tested?

Did not do any rigorous testing aside from compiling, and code
inspection.
We can consider adding some test-cases here in the future.

## Community Contribution License

All community contributions in this pull request are licensed to the
project maintainers under the terms of the [Apache 2
License](https://www.apache.org/licenses/LICENSE-2.0).

By creating this pull request I represent that I have the right to
license the contributions to the project maintainers under the Apache 2
License as stated in the [Community Contribution
License](https://github.com/jetify-com/opensource/blob/main/CONTRIBUTING.md#community-contribution-license).

Author: savil

aisdk/ai/provider/openai/internal/codec/encode.go

@@ -118,10 +118,8 @@ func applyCallOptions(params *responses.ResponseNewParams, opts api.CallOptions,
 	// Apply provider options from metadata
 	applyProviderMetadata(params, opts)
 
-	// Apply model-specific settings
-	if modelConfig.RequiredAutoTruncation {
-		params.Truncation = "auto"
-	}
+	// Apply truncation settings: caller-provided takes precedence over model defaults
+	applyTruncationSettings(params, opts, modelConfig)
 
 	// Apply reasoning settings and handle unsupported options for reasoning models
 	reasoningWarnings := applyReasoningSettings(params, opts, modelConfig)
@@ -185,6 +183,24 @@ func applyProviderMetadata(params *responses.ResponseNewParams, opts api.CallOpt
 	}
 }
 
+// applyTruncationSettings applies truncation settings to the parameters.
+// Caller-provided truncation via metadata takes precedence over model defaults.
+func applyTruncationSettings(params *responses.ResponseNewParams, opts api.CallOptions, modelConfig modelConfig) {
+	// Check if caller provided a truncation setting via metadata
+	if opts.ProviderMetadata != nil {
+		metadata := GetMetadata(&opts)
+		if metadata != nil && metadata.Truncation != "" {
+			params.Truncation = responses.ResponseNewParamsTruncation(metadata.Truncation)
+			return
+		}
+	}
+
+	// Fall back to model-specific defaults
+	if modelConfig.RequiredAutoTruncation {
+		params.Truncation = "auto"
+	}
+}
+
 // applyReasoningSettings applies settings specific to reasoning models
 // and handles unsupported options
 func applyReasoningSettings(params *responses.ResponseNewParams, opts api.CallOptions,

aisdk/ai/provider/openai/internal/codec/metadata.go

@@ -66,6 +66,16 @@ type Metadata struct {
 	// Supported values are `concise` and `detailed`.
 	ReasoningSummary string `json:"reasoning_summary,omitempty"`
 
+	// Truncation specifies the truncation strategy to use for the model input.
+	// When set, this overrides any model-specific default truncation behavior.
+	//
+	// Supported values are `auto` and `disabled` (default).
+	// - `auto`: If the context of this response and previous ones exceeds the
+	//   model's context window size, the model will truncate the response to fit.
+	// - `disabled`: If the context exceeds the model's context window size,
+	//   the request will fail with a 400 error.
+	Truncation string `json:"truncation,omitempty"`
+
 	// --- Used in blocks ---
 
 	// ImageDetail indicates the level of detail that should be used when processing