docs: Add Python session identity ADR

docs/decisions/0029-python-agent-session-identity.md

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+---
+status: proposed
+contact: eavanvalkenburg
+date: 2026-06-19
+deciders: eavanvalkenburg, moonbox3, TaoChenOSU, chetantoshnival
+consulted: westey-m
+informed:
+---
+
+# Python identity lifetimes for sessions, tasks, and continuation
+
+## Context and Problem Statement
+
+Python `AgentSession` currently carries a local `session_id`, an optional opaque service continuation
+`service_session_id`, and provider state. `service_session_id` is any service-owned value that lets that service continue
+a conversation, session, or thread; chat clients happen to map it through the abstract `conversation_id` option, but
+other agent types can use it differently. It is not a generic correlation field, and generic correlation should not
+require parsing or understanding that opaque service-owned value.
+
+The related issues mix values with different lifetimes:
+
+- **Session / conversation identity**: values that group a multi-turn interaction. Examples: A2A `context_id`, OpenAI
+  Responses `conversation` (`conv_*`) or response-chain continuation (`previous_response_id`).
+- **Task identity**: values that identify a protocol task and may affect future protocol calls. Example: A2A `task_id`.
+- **Message / response identity**: values that identify an output message or response. Examples: A2A `message_id` /
+  `artifact_id`, OpenAI Responses response id (`resp_*`).
+- **Continuation token**: a framework resume payload for in-progress work. It may contain the same underlying value as a
+  protocol id, such as A2A `task_id`, but it only exists when there is an unfinished operation to resume.
+
+These values should not automatically live in the same object just because they all help "continue" something. A value
+belongs in `AgentSession` only when it is needed to continue future calls across turns. A value that identifies one
+result belongs on the response or message. A value that resumes in-progress work belongs in a `ContinuationToken`.
+
+An `AgentSession` created for one agent is not expected to be guaranteed to work against another agent. When a session is
+used with an incompatible agent, protocol, or service, the framework should still help users understand what is wrong as
+early as possible, preferably before calling out to the remote service.
+
+For #4673, native conversation identity propagation should be based on `AgentSession` where the value is durable session
+state. For #4893, A2A `context_id` and `task_id` need a coherent Agent Framework mapping.
+
+AG-UI is out of scope for the decision. Its `thread_id` already maps to `AgentSession.session_id` in the normal wrapper
+path, and `run_id` is wrapper-owned event correlation. If AG-UI run correlation needs framework telemetry integration
+later, that should be handled as a run-context/telemetry design, not as session identity.
+
+## Current implementation notes
+
+- A2A currently has `A2AAgentSession`, but `A2AAgent.create_session(...)` does not automatically return it.
+- A2A currently mirrors `context_id` into `service_session_id`; that is current behavior, not necessarily the target
+  abstraction.
+- A2A `task_id` is not just cosmetic correlation. It is used for `task_id` when a task is `INPUT_REQUIRED`, for
+  `reference_task_ids` when refining a previous task, and inside `A2AContinuationToken` for in-progress tasks.
+- `RawAgent._prepare_run_context(...)` currently forwards `active_session.service_session_id` as chat `conversation_id`,
+  so any non-string or formatted value affects existing chat-client paths.
+- `OpenAIChatClient` maps chat options `conversation_id` to the Responses API as `previous_response_id` for `resp_*`,
+  `conversation` for `conv_*`, and defaults unrecognized strings to `previous_response_id`. When `store` is not `False`,
+  it returns `response.conversation.id` when available, otherwise `response.id`, as the next service continuation value.
+- For Responses API, the response id (`resp_*`) is also the response/message identity surfaced as
+  `ChatResponse.response_id`; when used for continuation on the next request, it becomes the `previous_response_id`
+  value.
+- Python A2A has not been released as stable yet, so its session factory or session shape can still be adjusted before
+  release.
+
+## Decision Drivers
+
+- Preserve `AgentSession.session_id` as the local/client conversation identity.
+- Preserve `AgentSession.service_session_id` as an opaque service-owned continuation handle.
+- Keep `AgentSession` for durable state needed across turns, not per-run bookkeeping.
+- Store values needed by future calls in durable session state; keep values that only resume in-progress work in
+  `ContinuationToken`.
+- Fix the current confusion where session, task, response, and continuation values can be treated as interchangeable
+  because they all participate in "continuing" something.
+- Make the implementation following this ADR preserve the lifetime split clearly: future-call state, in-progress resume
+  tokens, response/message ids, and protocol event correlation must not be silently mixed.
+- Expose durable continuation state in a typed way when future calls depend on it.
+- Let telemetry correlate runs without parsing opaque service continuation handles.
+- Reuse existing run/context surfaces before introducing a new identity abstraction.
+- Keep MCP and other remote tool boundaries safe: framework identity must not be forwarded to remote tools unless an
+  existing explicit opt-in mechanism says so.
+- Keep existing `AgentSession.to_dict()` / `from_dict()` migration and compatibility straightforward.
+- Stay close to .NET where there is already behavior to match, especially A2A's `ContextId`, `TaskId`, and `TaskState`.
+- Detect incompatible session identity shapes as early as practical, preferably before a remote service call.
+
+## Non-goals
+
+- Do not design a provider-agnostic conversation creation API here. That is tracked separately in #6622.
+- Do not make `service_session_id` a generic telemetry or run-correlation field.
+- Do not introduce a new identity object if existing run/context objects can carry the selected per-run correlation value.
+- Do not make a session from one agent guaranteed to work against another agent.
+- Do not optimize the public `agent.run(...)` API for protocol-wrapper internals.
+
+## Remaining question: durable shape for additional continuation state
+
+- Option A: Use protocol-specific `AgentSession` subclasses.
+- Option B: Store additional durable state inside `AgentSession.state`.
+- Option C: Extend `service_session_id` with richer service-owned values.
+
+### Option A: Use protocol-specific `AgentSession` subclasses
+
+Each protocol or agent type that needs additional durable state keeps a specialized `AgentSession` subclass. For A2A,
+that means keeping `A2AAgentSession` for A2A-specific durable state and changing `A2AAgent.create_session(...)` to return
+that type.
+
+Example:
+
+```python
+# First call returns a task that future A2A messages may need to reference.
+session = await a2a_agent.create_session()
+
+response = await a2a_agent.run(
+    message,
+    session=session,
+)
+
+# A2AAgent updates durable A2A protocol state from the returned task/status payload.
+# The user does not set these manually.
+assert isinstance(session, A2AAgentSession)
+assert session.task_id is not None
+assert session.task_state is not None
+
+# Later call reuses the durable A2A session state. A2AAgent decides whether to send task_id
+# for INPUT_REQUIRED or reference_task_ids for task refinement.
+next_response = await a2a_agent.run(
+    next_message,
+    session=session,
+)
+```
+
+- Good, because protocol-specific state stays in a protocol-specific type.
+- Good, because it aligns with .NET A2A's `A2AAgentSession` shape.
+- Good, because Python A2A can still make this pre-release session factory adjustment.
+- Good, because `task_state` does not get promoted to a base `AgentSession` concept.
+- Bad, because generic consumers cannot read protocol-specific state without knowing about the subclass or a helper API.
+- Bad, because it depends on each subclass consistently setting shared session fields such as `service_session_id` where
+  those are part of the shared abstraction.
+
+### Option B: Store additional durable state inside `AgentSession.state`
+
+Keep base `AgentSession` unchanged and store additional durable continuation/protocol state under namespaced keys in
+`session.state`.
+
+Example:
+
+```python
+session = AgentSession(session_id="ctx_123")
+
+session.state["a2a"] = {
+    "task_id": "task_456",
+    "task_state": TaskState.TASK_STATE_WORKING,
+}
+```
+
+- Good, because it avoids new public fields and avoids a subclass requirement.
+- Good, because `AgentSession.state` already exists for provider/session state.
+- Neutral, because helper APIs can hide the raw dictionary access.
+- Bad, because stringly typed state is easier to corrupt and harder to validate.
+- Bad, because generic consumers need helper APIs anyway; directly reading nested dictionaries is not a good abstraction.
+- Bad, because users may accidentally overwrite or persist invalid protocol state.
+
+### Option C: Extend `service_session_id` with richer service-owned values
+
+Some services may need `service_session_id` to carry richer service-owned continuation values. That can be addressed with
+a structured value or service-owned formatted string while keeping the common case as a plain string. This option remains
+about service-owned continuation only; it is not the mechanism for generic run/task correlation.
+
+Examples:
+
+```python
+simple_session = AgentSession(
+    service_session_id="resp_123",
+)
+
+structured_session = AgentSession(
+    service_session_id=ServiceSessionIdentity(
+        continuation_id="ctx_123",
+        task_id="task_789",
+        task_state=TaskState.TASK_STATE_WORKING,
+    ),
+)
+```
+
+- Good, because the common case remains a plain string and stays simple.
+- Good, because richer service-owned continuation state stays under the existing continuation property.
+- Good, because a structured value can make framework-side validation possible before a value is sent back to a service.
+- Neutral, because service-specific formats or structured values could be hidden behind helpers.
+- Neutral, because Python A2A would likely need a pre-release adjustment to reshape or wrap `A2AAgentSession`.
+- Bad, because changing the field type is a broad compatibility risk for users, providers, serialization, and tests.
+- Bad, because existing chat-client code treats `service_session_id` as the opaque value to pass through the
+  `conversation_id` option, so every such path must consistently extract/adapt the service-owned continuation component.
+- Bad, because this does not solve generic run/task correlation unless the structured fields become framework-owned,
+  which is not the purpose of `service_session_id`.
+
+## Decision
+
+Chosen direction: **split identity by lifecycle**.
+
+Values that are needed by future calls must be durable session state. Values that only identify one response stay on the
+response/message. Values that resume unfinished work stay in `ContinuationToken`. Values that only correlate a protocol
+run should stay in the protocol wrapper or run/telemetry context, not in `AgentSession`.
+
+Durable-state option decision: **TBD**.
+
+## Appendix: A2A `task_id` and `reference_task_ids` implementation check
+
+The A2A protocol distinguishes a message's `task_id` from `reference_task_ids`:
+
+- `task_id` associates the message with a specific task.
+- `reference_task_ids` provides additional task context, for example when a new task refines or follows up on the result
+  of a previous task.
+
+The protocol does not appear to prescribe that `task_id` and `reference_task_ids` are mutually exclusive. If both are
+present, the natural reading is that the message is associated with one task while also referencing other tasks for
+context. The serving agent decides how to interpret that context.
+
+The Python implementation should check and likely adjust the current behavior:
+
+- `task_id` should be updated by the current run when the remote A2A service returns a task/status payload.
+- `task_id` should remain durable A2A session state when needed for future calls, for example when a task is
+  `INPUT_REQUIRED`.
+- `reference_task_ids` should be a run parameter / caller intent for the current request, not implicit durable session
+  continuation state.
+- A follow-up/refinement request should pass explicit `reference_task_ids` when it wants to reference previous tasks.
+- If both session `task_id` and run `reference_task_ids` are present, the wrapper should preserve the protocol
+  distinction rather than treating one as a replacement for the other.
+- If no `reference_task_ids` are supplied, the wrapper should not automatically infer them from the last session task
+  unless we deliberately keep that convenience for compatibility.
+
+## More Information
+
+Related work and issues:
+
+- #4673: native conversation ID propagation.
+- #4893: align A2A protocol concepts with Agent Framework session/continuation concepts.
+- #2931: Foundry-specific conversation creation helper, split into a separate Python PR.
+- #6622: broader provider-agnostic conversation creation API discussion requiring .NET sync.
+- [ADR-0015](0015-agent-run-context.md): AgentRunContext for Agent Run.
+- [ADR-0018](0018-agentthread-serialization.md): AgentSession serialization.
+- [ADR-0026](0026-hosted-session-identity-context.md): hosted session identity context.