> ## Documentation Index > Fetch the complete documentation index at: https://docs.ag-ui.com/llms.txt > Use this file to discover all available pages before exploring further. # Key Changes > What 1.0 changes against the 0.x line — 1.0 This page lists the behavioural changes 1.0 makes against the 0.x line of the protocol, for reviewers who know 0.x and want the differences rather than the whole document. It is informative: the linked pages are the specification. ## Major changes 1. **The specification exists.** 0.x defined shapes; behaviour lived in the TypeScript client. The rules on these pages — sequencing, unknown versus malformed, warnings, attribution — are now normative, written in [BCP 14](https://datatracker.ietf.org/doc/html/bcp14) language, with the [schema](/spec/1.0/schema) authoritative for structure and this document authoritative for behaviour. 2. **Runs report how they ended.** `RUN_FINISHED` carries an optional `outcome`: absent or success means success, the interrupt outcome carries what the run is waiting for, and the [cancelled outcome](/spec/1.0/events/lifecycle#cancelled-runs) marks a run stopped on purpose before it completed. With it come `Interrupt`, resume entries on the [run input](/spec/1.0/basic/run-input), and the [interrupt–resume pattern](/spec/1.0/basic/patterns/interrupt-resume). Every producer written before outcomes existed is already conformant. 3. **Subagents.** Delegated work is attributed with `subagentRunId` on the events that can carry it, optionally announced by `SUBAGENT_STARTED` and closed by `SUBAGENT_FINISHED` or `SUBAGENT_ERROR`, with [rules for ownership, nesting, parallelism and termination](/spec/1.0/events/subagents). 4. **Reasoning replaces thinking.** The 0.x `THINKING_*` events are retired in favour of the [reasoning family](/spec/1.0/events/reasoning): spans, streamed reasoning messages matched by `messageId`, and `REASONING_ENCRYPTED_VALUE` for provider artefacts a consumer stores and returns without reading. Retired shapes are translated at the [compatibility boundary](/spec/1.0/basic/versioning#retired-shapes), not dropped. 5. **Activity events.** `ACTIVITY_SNAPSHOT` and `ACTIVITY_DELTA` carry [structured progress](/spec/1.0/events/activity) as messages whose content is an object, amended by JSON Patch. 6. **Unknown versus malformed, normatively.** Unrecognised events, fields and union members survive translation to enforcement, which drops an unknown event and strips unknown members, warning as it goes; a malformed known value is fatal. Nothing is removed before the translators have had their chance, and both transports feed one [processing pipeline](/spec/1.0/basic/processing), which also states the pipeline's one admitted narrowing for chunk fields. 7. **The chunked form has rules.** First chunks MUST carry what opening requires (`messageId` and role semantics for text, `toolCallId` and `toolCallName` for tool calls); later chunks MAY omit them; a continuation repeating an opener field with a conflicting value is fatal ([streaming pattern](/spec/1.0/basic/patterns/streaming)). 8. **A binary wire.** The [HTTP + Protobuf binding](/spec/1.0/basic/transports/http-protobuf) is specified — negotiated by media type, 4-byte length-prefixed frames, generated from the same schema, with cross-implementation parity pinned by a shared byte corpus. 9. **Capabilities are in the schema.** The `AgentCapabilities` declaration the SDKs carried as three hand-written copies is now defined once, in the schema, and generated for every SDK. Its semantics are stated: omitted means undeclared, declarations are informative and the stream is authoritative, and retrieval is deliberately left to the implementation ([Capabilities](/spec/1.0/basic/capabilities)). The subagent list is spelled `subagents`, matching `subagentRunId`; the earlier `subAgents` key is not read. 10. **Tool results carry content parts.** `TOOL_CALL_RESULT.content` and the tool message it mints accept a string or an ordered list of the same parts a user message carries, so a tool can return a document, an image or a search hit without encoding it into a string ([Tool calls](/spec/1.0/events/tool-calls#result-content)). The parts are renamed for it — `InputContent` → `ContentPart`, `TextInputContent` → `TextPart`, `ImageInputContent` → `ImagePart` and so on, `InputContentSource` → `PartSource` with `DataSource` and `UrlSource` — because a part named by direction is misnamed the moment it travels the other way. The wire is unchanged: every `type` value is the same, only the definition names and anchors move, and the SDKs keep the old names as aliases. Every part gains an optional `id`, and the text part gains the `metadata` the media parts already had. `ReasoningPart`, `ToolCallPart` and `AssistantPart` are reserved for the release in which assistant messages carry parts too. ## Minor changes 1. `tools` and `context` on the run input are optional: absent and empty mean the same thing ([Run input](/spec/1.0/basic/run-input)). 2. Metadata merge semantics are normative: per key, last write wins, no recursion, per-family merge targets; the `ag-ui` key is reserved ([Metadata](/spec/1.0/basic/metadata)). 3. Absent means absent: optional fields are omitted, never `null` ([The event model](/spec/1.0/basic#absent-means-absent)). 4. `RUN_FINISHED` and `RUN_ERROR` MAY carry per-provider token usage, in one accounting: the input and output counts are totals, cached and reasoning tokens are parts of them, cache reads and cache writes are reported separately, and usage follows the run boundary — subagents' calls in, child and resumed runs' calls out ([Runs and steps](/spec/1.0/events/lifecycle#token-usage)). 5. A run that stops on a frontend tool call finishes as success, never as an interrupt, and the success outcome MAY name the calls it left unanswered in `pendingToolCallIds` ([Tool calls](/spec/1.0/events/tool-calls#frontend-tools)). 6. Input messages MAY carry multimodal content parts (text, image, audio, video, document), by URL, as inline data or as a provider file handle — the `file` source, new in 1.0, for bytes already uploaded to the model provider ([Run input](/spec/1.0/basic/run-input#provider-file-handles)). 7. `TOOL_CALL_RESULT` is a message in its own right and does not reopen the call it answers ([Tool calls](/spec/1.0/events/tool-calls)). 8. A late `RUN_ERROR` after `RUN_FINISHED` is admitted, reporting a failure that surfaced after success was already reported ([Runs and steps](/spec/1.0/events/lifecycle)). 9. A consumer keeps a stream it rejects apart from a run that reports its own failure, and "treat the run as failed" is defined: surface it, never report success, with the API shape left to the implementation ([Runs and steps](/spec/1.0/events/lifecycle#error-handling)). 10. The protocol version travels in-band: consumers declare theirs on `RunAgentInput.protocolVersion`, producers answer with their own on `RUN_STARTED.protocolVersion`, and absence identifies a pre-versioning peer ([Versioning](/spec/1.0/basic/versioning#version-negotiation)).