An adapter emits harness.StreamPart values while a turn runs. harness.Agent translates them into provider.StreamChunk values for ai.NewStreamTextResultFromParts, so a harness turn streams like a StreamText call. Each part marshals to the JSON shape of the TypeScript HarnessV1StreamPart, so parts persisted by one SDK decode in the other.
The adapter interface is in Harness adapters.
Part types
StreamPart.PartType() returns the type discriminator. The harness.PartType* constants hold the values.
| Constant | type | Go type | Description |
|---|
harness.PartTypeStreamStart | stream-start | *harness.StreamStartPart | Start of a turn. |
harness.PartTypeTextStart | text-start | *harness.TextStartPart | A text block opens. |
harness.PartTypeTextDelta | text-delta | *harness.TextDeltaPart | A piece of text. |
harness.PartTypeTextEnd | text-end | *harness.TextEndPart | A text block closes. |
harness.PartTypeReasoningStart | reasoning-start | *harness.ReasoningStartPart | A reasoning block opens. |
harness.PartTypeReasoningDelta | reasoning-delta | *harness.ReasoningDeltaPart | A piece of reasoning. |
harness.PartTypeReasoningEnd | reasoning-end | *harness.ReasoningEndPart | A reasoning block closes. |
harness.PartTypeToolInputStart | tool-input-start | *harness.ToolInputStartPart | Tool input starts streaming. |
harness.PartTypeToolInputDelta | tool-input-delta | *harness.ToolInputDeltaPart | A piece of tool input. |
harness.PartTypeToolInputEnd | tool-input-end | *harness.ToolInputEndPart | Tool input is complete. |
harness.PartTypeToolCall | tool-call | *harness.ToolCallPart | A tool call. Carries nativeName and stepToolCallCount in addition to the model tool call fields. |
harness.PartTypeToolApprovalRequest | tool-approval-request | *harness.ToolApprovalRequestPart | The runtime asks for approval. |
harness.PartTypeToolResult | tool-result | *harness.ToolResultPart | A tool result. |
harness.PartTypeFinishStep | finish-step | *harness.FinishStepPart | End of a step inside a turn. |
harness.PartTypeFinish | finish | *harness.FinishPart | End of the turn. |
harness.PartTypeFileChange | file-change | *harness.FileChangePart | A workspace change made through an opaque mechanism. |
harness.PartTypeCompaction | compaction | *harness.CompactionPart | The runtime compacted its context. |
harness.PartTypeError | error | *harness.ErrorPart | An error. |
harness.PartTypeRaw | raw | *harness.RawPart | Adapter-specific passthrough. |
Fields
| Field | Type | Description |
|---|
Warnings | []CallWarning | |
ModelID | string | ModelID is the model the runtime resolved to for this turn, when known. |
| Field | Type | Description |
|---|
ID | string | |
Delta | string | |
HarnessMetadata | Metadata | |
| Field | Type | Description |
|---|
ToolCallID | string | |
ToolName | string | |
Input | string | Input is the stringified JSON tool input. |
ProviderExecuted | bool | ProviderExecuted is true for runtime-executed builtins. |
Dynamic | bool | |
ProviderMetadata | ProviderMetadata | |
NativeName | string | NativeName is the runtime's native name when it differs from ToolName. |
StepToolCallCount | *int | StepToolCallCount is the total tool calls in the current model step, when known up front. Must be a positive integer when set. |
| Field | Type | Description |
|---|
ApprovalID | string | |
ToolCallID | string | |
ProviderMetadata | ProviderMetadata | |
| Field | Type | Description |
|---|
ToolCallID | string | |
ToolName | string | |
Result | any | |
IsError | bool | |
Preliminary | bool | |
Dynamic | bool | |
ProviderMetadata | ProviderMetadata | |
| Field | Type | Description |
|---|
FinishReason | FinishReason | |
Usage | Usage | |
HarnessMetadata | Metadata | |
| Field | Type | Description |
|---|
FinishReason | FinishReason | |
TotalUsage | Usage | |
HarnessMetadata | Metadata | |
| Field | Type | Description |
|---|
Event | string | |
Path | string | |
HarnessMetadata | Metadata | |
| Field | Type | Description |
|---|
Trigger | string | |
Summary | string | |
TokensBefore | *float64 | |
TokensAfter | *float64 | |
HarnessMetadata | Metadata | |
| Field | Type | Description |
|---|
Error | any | |
| Field | Type | Description |
|---|
RawValue | any | |
The block parts all carry an id. Text and reasoning parts also carry harnessMetadata, and the delta parts add a delta string. The tool input parts carry providerMetadata. harness.ToolInputStartPart adds toolName, providerExecuted, dynamic and title. These parts are harness.TextStartPart, harness.TextEndPart, harness.ReasoningStartPart, harness.ReasoningDeltaPart, harness.ReasoningEndPart, harness.ToolInputStartPart, harness.ToolInputDeltaPart and harness.ToolInputEndPart.
Constants
| Group | Values |
|---|
| File change kinds | harness.FileChangeCreate, harness.FileChangeModify, harness.FileChangeDelete |
| Compaction triggers | harness.CompactionTriggerManual, harness.CompactionTriggerAuto |
| Finish reasons | harness.FinishReasonStop, harness.FinishReasonLength, harness.FinishReasonContentFilter, harness.FinishReasonToolCalls, harness.FinishReasonError, harness.FinishReasonOther |
Usage
harness.FinishReason is the harness wire encoding of the model finish reason. harness.Usage is the wire encoding of model usage, with harness.InputTokenUsage and harness.OutputTokenUsage for the token breakdowns.
| Field | Type | Description |
|---|
InputTokens | InputTokenUsage | |
OutputTokens | OutputTokenUsage | |
Raw | map[string]any | |
| Field | Type | Description |
|---|
Total | *int | |
NoCache | *int | |
CacheRead | *int | |
CacheWrite | *int | |
| Field | Type | Description |
|---|
Total | *int | |
Text | *int | |
Reasoning | *int | |
harness.Metadata is adapter-namespaced opaque data attached to events, keyed by harness ID. harness.ProviderMetadata is the provider metadata shape on tool parts.
Translation
func TranslatePart(part StreamPart, opts TranslateOptions) []provider.StreamChunk
Converts one part to zero or more chunks. Most parts map one to one. These are the exceptions:
tool-call is not translated here. harness.Agent validates it against the merged tool set first.- A failed
tool-result from a provider-executed tool becomes a tool result with an error.
file-change and compaction have no chunk type of their own. Each becomes a dynamic, provider-executed tool call and tool result pair named fileChange or compaction, so the event stays visible.stream-start, finish-step and finish return nil. harness.Agent owns step and turn boundaries.
| Field | Type | Description |
|---|
IsProviderExecuted | func(toolCallID string) bool | IsProviderExecuted reports whether the tool call that produced a tool-result event ran inside the harness runtime. Host results are echoed back as tool-result events too, so the event alone cannot say who ran the tool — only the originating tool-call can, and correlating the two is the caller's job (run_prompt.go tracks it). A nil func treats every failure as provider-executed, matching TS's ?? true default. |
JSON helpers
| Function | Description |
|---|
harness.DecodeStreamPart(data []byte) (StreamPart, error) | Parses and validates one JSON part. Returns harness.ErrUnknownPartType for an unknown type. |
harness.MarshalStreamPart(part StreamPart) ([]byte, error) | Encodes a part with type as the first key. |
harness.IsStreamPartType(typ string) bool | Reports whether typ is a part discriminator. |
harness.MarshalTagged(typ string, v any) ([]byte, error) | Encodes a struct as a JSON object with "type" first. |
harness.ReadTagged(data []byte) (string, map[string]json.RawMessage, error) | Returns the type and the raw top-level fields of a JSON object. |
harness.RequireKeys(typ string, fields map[string]json.RawMessage, required []string) error | Checks that the required keys are present. |
harness.StripWorkDir(part StreamPart, sessionWorkDir string) StreamPart | Returns a copy of the part with the session work directory removed from path fields. |
harness.EmitFunc | func(part StreamPart). Receives every part an adapter produces during a turn. |
harness.AgentStreamPart | Consumer-facing alias of StreamPart. |