What it does
A typical request goes like this:- You create a conversation with a system prompt, a model choice, and a list of tools the model is allowed to use.
- You start a run by sending the model a user message.
- The model decides whether to answer directly or call one of the tools.
- If it picks an MCP-resolved tool, the platform calls it for you and feeds the result back into the next round of reasoning. This can repeat several times.
- If it picks a caller-declared tool, the run pauses and asks you to provide the answer. You start a new run with the answer; the model continues from there.
- Eventually the model produces a final answer matching the structured schema you provided.
The run reaches
completedand you fetch the result.
The shape mirrors the OpenAI Assistants API: a
thread is a conversation, a run is one user-initiated turn, a step is one inference
iteration inside a run. If you already know that mental model, the only new ideas here are
tool aliases and optimistic-concurrency versioning (covered below).
API endpoints
All endpoints live under/agents and require a Bearer token with
agent_conversations read/write permission.
Authentication and malformed-input failures come back before any agent logic runs:
- 401 — the token lacks the
agent_conversationspermission the endpoint needs, or has expired. - 403 — no bearer token, or one the platform does not recognize. The body is empty. A token
with no user attached, such as a company API key, also gets a 403, with an
{error, error_description}body. Conversations belong to a user, so these endpoints need a user token. - 404 with an empty body — a malformed id in the path, or a malformed
since,page, orper_pagequery parameter. - 400 with an
{error, error_description}body — a request body that isn’t valid JSON or doesn’t decode, for example an unknownpayload.kind. Validation failures past that point use the RFC 7807 body described in Run status lifecycle.
Scope: per-user, not per-company. Conversations and runs are keyed on the bearer
token’s
(company_id, user_id) pair. Peers in the same company cannot see each other’s
conversations, runs, or messages — every endpoint returns 404 for cross-user access,
identical to cross-company access. The API does not distinguish “doesn’t exist” from
“owned by another user”.
The first request (
POST /agents/conversations) returns the conversation id you need for
all the others. The fourth (POST .../runs) returns the run id you can poll with the fifth.
Listing your conversations
GET /agents/conversations returns the calling user’s conversations in created_at
descending order, wrapped in the standard pagination envelope:
per_page is 50. Results are filtered by both company_id and user_id —
peers in the same company do not see each other’s conversations (see the scope
callout above).
Core concepts in one paragraph each
Conversation
The long-lived container. Thesystem_prompt is pinned for life — once set at creation
time, it applies to every run that follows and cannot be changed. Everything else in defaults
(model, tools, max iterations, temperature, output schema, etc.) is exactly that — a
default: it applies unless a particular run explicitly replaces it. The conversation also
carries a monotonic version counter that every successful run bumps. The counter doubles as
both a delta cursor (for GET .../messages?since=N) and a compare-and-swap token (more on
that under Troubleshooting).
Run
One user-initiated turn of conversation. Each run kicks off a workflow on the platform that performs zero or more inference rounds plus tool calls. A run is asynchronous: thePOST
returns immediately with status: "pending", and you poll GET /agents/runs/{id} until it
reaches a terminal state (completed, requires_action, or failed).
A run may override the conversation’s defaults for that single turn via config_override —
pick a different model, raise max_iterations for a hard question, swap in a different tool
catalog, tighten temperature for a deterministic answer, etc. The only thing a run cannot
change is the system_prompt (that’s permanently set on the conversation). Two run-only
settings have no conversation-level counterpart: tool_choice (which biases the model toward
a particular tool on the first iteration) and the run’s payload itself.
Tools
A run’s tool catalog comes from two sources, listed side by side in the conversationdefaults:
mcp_servers[]— each entry has analias(1–8 letters/digits, must start with a letter), a URL, and an optional description. Eachurlis validated up front at conversation/run create — it must be an absolutehttpsURL with a host, and the host must be publicly reachable (loopback, private, link-local, IPv6 unique-local, unspecified, and multicast addresses are refused by the outbound egress guard). See Invalid MCP Server URL for the syntax check and MCP Discovery Failed for the reachability check. You do not declare individual tools — the platform discovers each server’s catalog at run start via the JSON-RPCtools/listmethod and stitches the wire names as{alias}-{tool_name}before handing them to the model. When the model emits a call to{alias}-{tool_name}, the workflow makes thetools/callrequest, gets the result, and feeds it back into the next inference round inside the same run.tools[]— caller-declared tools you answer yourself. No alias — the model sees the barename. When the model calls one of these, the run terminates withstatus: "requires_action"; you read the question frompending_tool_calls, decide on an answer, and start a new run withpayload.kind: "tool_outputs"carrying your reply.
Every run also gets the platform’s waiting tools —
job_monitor, wait_for and sleep. You do
not declare them, and a caller-declared tool of the same name cannot replace them. They are what let
a run pause for a long-running job instead of asking whether it has finished: see
Waiting for Work to Finish.Discovery is per-run, not cached. Every run re-fetches
tools/list from each registered
MCP server. There is no inter-run cache — if the server adds, removes, or renames a tool,
the next run picks it up automatically. The trade-off: each new run pays one HTTP roundtrip
per registered server before the first inference. For 1–3 typical servers this is in the
noise (10–100ms total); if you ever register a slow-discovering server, expect it to delay
every run’s first iteration. Discovery failures terminate the run with
MCP Discovery Failed.Narrative-owned MCP servers are authenticated automatically. When an
mcp_servers[].url matches the Data Collaboration MCP Server
(https://mcp.narrative.io/mcp in prod), the platform mints a Default-scoped API token
for the conversation’s user and company and attaches it as
Authorization: Bearer ... on every tools/list and tools/call request to that
server. The token is created server-side, lives only for the duration of the request
chain, and is never persisted to agent_runs.effective_config, Temporal event history,
or the GET /agents/runs/{id} echo. Public or third-party MCP servers (any URL outside
the allowlist) continue to be called without auth — unless you attach a connection_id
(see next note).External OAuth-protected servers use At every
connection_id. For a third-party MCP server
that requires the end user’s own OAuth authorization, first connect it via
POST /mcp-connections — the platform performs Dynamic Client Registration (RFC 7591) and
PKCE against the external authorization server, then persists the tokens KMS-encrypted.
Reference the resulting id from the matching entry in mcp_servers[]:tools/list and tools/call the platform resolves the bearer server-side —
refreshing it if the access token has expired — and attaches it to the outbound request.
The token never appears in the request payload, effective_config, run history, or
GET /agents/runs/{id}. The connection must belong to the calling user and be in
connected status; a connection owned by another user is treated as missing. See the
MCP Connections reference for the full flow.If you register a server with no connection_id and it actually requires authorization,
run-start discovery gets a 401 on tools/list and the run fails with
MCP Discovery Failed telling you to
connect it via POST /mcp-connections — and naming your existing connection_id for
that URL if you already have one, so you can drop it into mcp_servers[] without
re-running the OAuth flow.Tool input schemas are normalized at discovery time. MCP servers can declare
inputSchema shapes that Bedrock Converse and Anthropic Messages don’t accept verbatim
— oneOf, $ref, format validators, additionalProperties: true, and similar JSON
Schema features. The platform runs every discovered tool’s schema through a converter
that translates oneOf to anyOf, inlines non-recursive $ref, replaces recursive
$ref with {} to break cycles, forces additionalProperties: false, and lifts
unsupported keywords (format, pattern, range validators) into the property’s
description. Tools that previously had to be dropped wholesale because they carried
$ref now load successfully; nothing is rejected at the discovery layer. The converter
emits strict: false because Bedrock caps strict-tool counts per request — the model
still gets the schema, and each MCP server validates arguments on its own side at call
time.{alias}-
prefix; caller-declared tool names must not contain a dash. This is what lets the workflow
classify a hallucinated or unknown call without ambiguity: dash + unknown alias →
AgentLoopUnknownToolAlias; dash-free + unknown name → same error. The validation rule “no
dash in tools[].name” is the cornerstone.
Harness tools
On top ofmcp_servers[] and tools[], every run gets a set of harness tools the
platform always provides. You do not declare them and cannot disable them. Their names are
reserved — a caller-declared tool that takes one is refused with
Reserved Tool Name.
A sandboxed Linux computer, one per conversation:
Waiting primitives, so a run does not spend its
max_iterations re-checking whether work has
finished:
sleep and wait_for park the run as a durable Temporal timer, not a worker thread, so a
waiting run survives a worker restart. While a run is paused, live.messages on
GET /agents/runs/{id} shows what it is waiting for, and a second run posted on the same
conversation gets a 409 Run Already In Progress
naming the wait.
Every one of these spends an iteration, like any other tool call: a run that starts a
job, registers it with job_monitor, waits with wait_for, then answers uses four
iterations. That is why max_iterations defaults to 10.
job_monitor is scoped to the caller’s company — a job that does not exist and one owned by
another company both come back without a handle, so the tool cannot be used to probe. Handles
expire after 24 hours; the wait_for ceiling is well under that TTL so a handle cannot expire
beneath a run still waiting on it. A wait_for timeout leaves the underlying tasks running —
the model can call wait_for again on the same handles to keep waiting.
sleep clamps arguments outside its 1 second – 1 hour range rather than refusing them; the
result tells the model the value was clamped. A malformed argument comes back as an isError
tool result the model can correct, never a loop failure.
Tool choice
Per-run policy that nudges the model toward (or away from) using tools on the first iteration. Three options:{"kind": "auto"}— model decides. The default.{"kind": "any"}— model must call some tool (no plain-text answer allowed).{"kind": "specific_tool", "name": "confirm_booking"}— model must call exactly this caller-declared tool. To pin an MCP-resolved tool instead, include the explicitmcp_alias:{"kind": "specific_tool", "mcp_alias": "docs", "name": "search_kb"}. The platform validates the target against the catalog synchronously and returns 400 with Unknown Tool Choice Name or Unknown Tool Choice MCP Alias on a miss.
any and specific_tool — are mutually exclusive with thinking. A run that asks
for both is a 400; a run that forces a call without mentioning thinking runs without reasoning. See
One thing you cannot combine it with.
The choice only applies to iteration 1 of the current run. From iteration 2 onward inside
that same run the model is back on {"kind": "auto"}. There is no carry-over to subsequent
runs — every new run picks its own tool_choice afresh. That means you can absolutely
re-impose a forced tool call on a follow-up run (for example, push the model toward a specific
caller-declared tool every time you need to ask the user something), so long as you set
tool_choice on the run’s request body.
Configuration parameters
When you create a conversation, you pass adefaults object that pins how every run on that
conversation behaves. Each field can be overridden per-run via config_override, except where
noted.
mcp_servers and tools are wholesale-replaced, not merged, when overridden in a run.
If you set them in config_override, you replace the whole list. This keeps tool namespacing
predictable.Thinking
Claude models can reason before they answer. Every Claude model does so by default — omitthinking and the model thinks as hard as it can. Send "disabled" for the cheap, direct answer.
Thinking costs output tokens. On a five-constraint logic puzzle a model spent ~550 completion tokens
reasoning its way to the right answer, against ~70 answering the same prompt with thinking off (and
getting it wrong). That is the trade: better answers on hard problems, more tokens on every problem.
What each model accepts
Models take one of two vocabularies, and it goes by generation. Newer models are told how hard to think; older ones are given a token budget to spend on it.
Reading the table:
- Accepts — the shapes that model will take. Sending the other one is a 400, not a silent downgrade: an effort level on Haiku 4.5 is rejected, as is a token budget on Opus 5. Models that take either accept both.
- Default when you say nothing — what the platform sends for you. The newer models get the top of the effort scale. The budget-only models get the floor, 1024 tokens, because a budget is a hard ceiling and a large default would silently multiply the cost of every call.
- Reasoning text — whether the reasoning arrives as words you can show a user. The newest models return a signature only: proof they reasoned, with the words withheld. That is the model’s behaviour, not a platform choice, and no flag we know of changes it. Build for text being absent.
"disabled" is accepted, because it asks for
what they already do. So a client that turns thinking off across the board needs no per-model special
casing.
Choosing a setting
- Leave it out for anything where a better answer is worth the tokens. This is the default.
"disabled"for short, mechanical, or high-volume work — classification, extraction, titles. A model that thinks by default is otherwise billed for reasoning nobody reads.{"type":"effort","level":"low"}to keep reasoning on but cheap, where the model takes an effort scale. Note thatlowandmediummay produce no reasoning at all on an easy prompt: how hard a model thinks is decided by the prompt, and effort only sets the ceiling.{"type":"budget","budget_tokens":N}where the model takes a budget and you want a firm cap.Nmust be at least 1024 and smaller thanmax_tokens— reasoning is spent from the same output budget as the answer, so a budget that fillsmax_tokensleaves nothing to answer with (400).
config_override.thinking shadows the conversation’s setting, so one expensive turn in a cheap
conversation (or the reverse) needs no new conversation.
One thing you cannot combine it with
Thinking and a forced tool call are mutually exclusive — both forcing forms,tool_choice: {"kind":"any"} as much as {"kind":"specific_tool"}. With a call forced, models either reject the
request or quietly stop reasoning: no reasoning block, a fraction of the tokens, and nothing in the
response to say thinking didn’t happen. {"kind":"auto"} is unaffected.
Which of the two you get depends on who asked for what:
- You asked for both —
thinkingon the run or the conversation, plus a forcingtool_choice— is a 400. Two explicit instructions that cannot both be honoured is worth telling you about rather than resolving on your behalf. - You forced a call and said nothing about thinking runs normally, without reasoning. Thinking by
default is our choice and your
tool_choiceis yours, so the default gives way. Code that pins a tool needs no change. - The model doesn’t accept a forced call at all (Claude Sonnet 5.5 and Claude Opus 5.5) is a 400, with or without thinking.
{"kind":"auto"} and name the tool in the
message. A reasoning model that has been told what to call generally calls it, and you keep the reasoning.
Structured output (output_format_schema) does not conflict, even though it uses a tool internally.
That tool is not a choice of yours, and the platform handles the interaction for you.
What comes back
Reasoning arrives asreasoning content blocks on the assistant turn, ahead of the
answer, and is persisted with the conversation. A turn may hold the model’s working and its reply as
separate blocks — render the reply from final_answer_from, not from every text block.
Content blocks
Every message carries an ordered list of blocks, discriminated bytype. Six variants exist:
Blocks the platform produced also carry an
id (b_<job>_<ordinal>), which is what
final_answer_from points at. Blocks you sent have none.
The last two appear when the run asked for thinking. A reasoning block comes first in the turn,
ahead of the answer:
final_answer_from, not from every text block. That turn has two text
blocks: the model narrating its working, and the answer. Both are real output and both are persisted —
the working is kept so the whole lineage of a turn is inspectable — but only the one named in
final_answer_from is the reply. A reader that renders every text block shows the working and the
answer as two separate messages.
final_answer_from always names text blocks that are present in the same message, and is empty on a
turn that answered nothing: a tool-call turn, a user’s message, or anything stored before ids existed.
On the live view it is empty while a turn is still streaming and fills in when the
run finalizes.
Two things to know before you build against it:
textis optional on a reasoning block. Some models return their working as words; others return only a signature and keep the words to themselves. A block with notextstill means the model reasoned — and the tokens were billed — so don’t treat its absence as “nothing happened”.signatureis opaque, and it’s yours to use or ignore. It is not a hash you can verify or a token you can decode. Send the block back unchanged if you want the model to continue a turn exactly as it left it; drop it if you only want to display the answer. A continuation without reasoning blocks is accepted. Don’t edit one — the signature covers what the model produced.
Run payload shapes
POST /agents/conversations/{id}/runs accepts one of two payload kinds:
client_op_id— a UUID you generate, unique per request. Used as an idempotency key — re-sending the sameclient_op_idreturns the original run unchanged, with a 202 like the first call, whichever conversation that run is on. This lets you retry safely across network blips. The version check runs first, so a retry only gets the original run back whileexpected_versionstill matches. Once that run’s messages land, the conversation’s version moves on and the same retry gets a 409 Version Conflict. Read the original run withGET /agents/runs/{id}instead.expected_version— the conversation’s current version, which you got from the most recentGET /agents/conversations/{id}. The platform rejects the run if the version has moved on since you read it (see Version conflicts).tool_choice— optional, per-run only.config_override— optional, sparse — only the fields you want to change.
Images
A run can send the model images, in the same shape as an Anthropic Messages API image block. Put them in a user message’scontent, alongside text blocks, or in a tool output’s content:
text (a string) or content (a string, or a list of text and image blocks). A message that is a
single text block is read back as text. An MCP tool can return images too: they arrive as image blocks inside
its tool_result.
What the platform accepts, checked when you start the run:
Every Claude model takes images on an AWS data plane. The OpenAI models (
openai.gpt-oss-120b, openai.gpt-4.1,
openai.o4-mini) take none. On a Snowflake data plane, or when an earlier turn’s images reach a model that can’t
read them, the model gets a short note in place of each image and the run goes on.
A run carries at most 1 MB of images in all, counting earlier turns and what MCP tools return. When a conversation
holds more, the model sees the newest images that fit and a note in place of each older one. An image an MCP tool
returns that a model can’t read, of an unsupported type or over the size limit, is replaced with a text block
saying so. Messages read back from the conversation keep every image you sent.
A first message with only images, and no text, leaves the conversation untitled: automatic
titling works from the text.
Run status lifecycle
Every run starts atpending and eventually reaches one of three terminal states:
GET /agents/runs/{id} every few seconds (start with 2–4 seconds; back off if you don’t
care about latency). When you see one of the terminal states you can stop polling.
One active run per conversation
A conversation runs one turn at a time. While a run ispending or running, posting
another run on the same conversation returns a 409
Run Already In Progress. The detail names
the active run’s id — and, when the run has produced a live turn, what it is currently
doing (for example, “waiting on a Narrative job” for a run parked on wait_for).
Recovery: poll GET /agents/runs/{active_run_id} until it reaches a terminal state,
re-read the conversation’s version, then post your run. A run parked on sleep or
wait_for can stay non-terminal for a while; live.messages on the run response shows what
it is waiting for.
The three terminal states do not block a new run — in particular a requires_action run
is done, so the run that resumes it with tool_outputs is accepted as normal.
Re-posting a client_op_id that already has a run on this conversation is not refused: you
get that run’s row back, even while a different run is active. The bypass is per-conversation
while idempotency is per-owner, so a key whose run lives on another conversation gets the 409 —
let the active run finish, then re-post.
The pre-flight guard is best-effort. Under a tight race the finalize-time CAS
(Concurrent Conversation Append)
stays the authority for simultaneous posts.
completed runs populate exactly one of two fields, depending on whether you supplied an
output_format_schema:
- No schema (text mode) →
final_textcarries the model’s reply as a plain string;final_structured_outputisnull. - Schema supplied (structured mode) →
final_structured_outputcarries the parsed JSON object conforming verbatim to your schema;final_textisnull. Your schema does not need a top-leveltextfield — whatever shape you declare is what you get back.
requires_action runs populate pending_tool_calls — an array of caller-declared tool
calls waiting for you to answer. See Example 3 for the resume flow.
failed runs populate error.type (an opaque incident code) and error.message (a
human-readable detail). The response always includes error.title and error.docs_url pointing
at the relevant error catalog page. An
incident code with no catalog entry gets the Unexpected
entry’s title and URL.
A run that was deliberately cancelled also lands in failed, but carries the reserved
error.type: "AgentLoopCancelled" so you can tell an intentional stop from a genuine error.
The run’s in-flight inference job is cancelled too, so the run and its data-plane job end up
consistent. See /errors/cancelled.
One error vocabulary for both surfaces. The agent API exposes errors in two physical shapes
— HTTP 4xx/5xx with an RFC 7807 body for
synchronous failures (request validation, conversation not found, version conflict, etc.), and
HTTP 200 with
status: "failed" and an error object for failures that happen inside the
workflow after the run has been accepted (max iterations exceeded, MCP server unreachable,
invalid effective config, etc.).The two shapes carry the same caller-facing fields:One synchronous error has no catalog page. A company over its credit limit, or with no credit
account, gets a 403 on
POST .../runs with an RFC 7807 body whose type is null. No run is
created.Both shapes link into the same error catalog,
which means a single playbook covers both. Whether your client got a 409 on POST .../runs or a
200 with error.type: "AgentLoopMaxIterationsExceeded" on GET /agents/runs/{id}, the docs URL
in the response is the canonical “how do I recover” entry point.The workflow side itself never knows about caller-facing presentation — it only emits opaque
incident codes ("AgentLoopMaxIterationsExceeded", "UnknownTool", etc.). The translation to
title + docs_url happens at the API boundary against a single catalog kept in sync with the
docs pages.The live view
Every run response (POST .../runs and GET /agents/runs/{id}) carries a live object. Unlike
the status-dependent fields, live is orthogonal to status and is populated on every
read — it reflects the conversation’s current, still-mutating state, so a run you polled
minutes ago can surface newer values on the next read without a separate
GET /agents/conversations/{id} call.
live holds two fields:
current_name— the conversation’s display name, ornullif it has none yet. This is the same value asnameonGET /agents/conversations/{id}; it’s mirrored onto the run so a client that’s already polling a run sees title changes for free.messages— the run’s produced turns so far, streamed while the run is still in flight so you can show tool execution live. See Streaming tool execution below.
Streaming tool execution
While a run is non-terminal (pending / running), live.messages carries the turns the
loop has produced so far — each tool-use assistant turn appears before the platform executes the
call (so you can render “calling docs-search_kb…”), and the matching tool_result turn appears
once it returns. This lets a chat UI animate a multi-iteration tool loop from the same run poll
you’re already doing, with no extra endpoint.
Each entry has a turn_index (per-run ordering, independent of sequence_no), a role, and the
same content_blocks shape as committed messages.
The handoff to the committed log is lossless:
- Render
committed messages (GET .../messages) ++ live.messageswhile the run is non-terminal. - Once the run reaches a terminal status,
live.messagesis empty — the same turns are now committed and authoritative viaGET .../messages?since=…. Drop your live tail and keep the committed rows. - De-dupe is exact: the live
tool_use/tool_resultblocks carry the sametool_use_ids that land in the committedcontent_blocks.
version or the committed message stream (live turns aren’t messages — they carry no
sequence_no and don’t advance your expected_version). A run with no server-side tool calls (a
direct answer) simply shows an empty live.messages and you get the answer via final_text /
final_structured_output at completion.
Automatic titling
When you start the first run on a conversation that has no name, the platform kicks off a small asynchronous job that generates a short title from your first user message and writes it to the conversation’sname. Because it’s asynchronous and independent of your run, the timing
is best-effort:
current_nameis usuallynullon the first read right after the run is created, then becomes the generated title a beat later — keep reading the run (or the conversation) and it appears.- A
nameyou set explicitly at conversation-creation time is never overwritten — auto titling only fills an empty name. - Titling never touches the conversation
versionor the message stream: it’s not a message, doesn’t advance yourexpected_version, and never shows up inGET .../messages.
live is a growth point. It accumulates state that changes after a run was written, so a
single run poll can stand in for several reads (current_name, messages, more over time). Treat
it as “render whatever keys are present” rather than assuming a fixed set.Example 1 — hello world (no tools)
The simplest possible run: one user message, one assistant reply, no tools.Create the conversation
output_format_schema here — this is text mode. The final answer comes back as
final_text. See Example 4 for the structured-mode flow.
Response (abbreviated):
Start the run
Poll until terminal
Read the message stream
current_version: 2. Future runs on this conversation start with expected_version: 2.
Example 2 — searching docs via an MCP server
Now the model has tools. It searches the Narrative docs for “rate limiting”, makes a few attempts to locate the right page, and produces a grounded summary.Create the conversation with mcp_servers
alias: "docs"— the platform discovers the server’s tools at run start, then prefixes each with the alias before showing them to the model. An MCP-side tool namedsearch_narrative_i_o_knowledge_basebecomesdocs-search_narrative_i_o_knowledge_baseon the wire.- No
tools[]here — discovery is automatic. To inspect what the server exposes, call itstools/listdirectly (e.g.curl https://docs.narrative.io/mcp ...). max_iterations: 8— gives the model room to: search, read, refine, then answer.
Start the run
Same shape as Example 1 — just a user message. The model decides what tools to call.What you see while polling
status cycles pending → running → running → … — each running you see corresponds
roughly to one inference iteration. Be patient: MCP roundtrips can take 30–60 seconds total
for multi-step research like this.
Final state:
assistant (with
tool_use blocks) and tool (with tool_result blocks) turns, and a final assistant turn
carrying the answer.
Example 3 — asking the user a question (caller-declared tool)
Sometimes the model needs information only the caller has. Configure atools[] entry,
force the model to use it with tool_choice, and the run will pause at requires_action
until you answer.
Create the conversation with tools[]
Start the run and force the tool call
Poll until requires_action
pending_tool_calls[].tool_use_id is the handle you’ll need to resume.
Resume with tool_outputs
Re-read the conversation to get the new version (it has advanced because the assistant turn is
now persisted):
You can answer with
is_error: true to tell the model the tool failed (e.g. the user
declined). The model decides whether to try another approach, ask a different question, or
fall through to a final answer.Example 4 — structured output
When you need the model’s answer as a typed object rather than free text, supply anoutput_format_schema describing the shape you want. The model is grammar-constrained to
produce JSON matching it, and the parsed value comes back on final_structured_output.
This example records a login event using a discriminated union (anyOf) of login /
logout variants. Note that the schema has no top-level text field — you declare
whatever shape your application needs.
Create the conversation with a structured schema
Start the run
Final state
final_text is null because the run is in structured mode; the answer is on
final_structured_output, conforming verbatim to your schema. The two fields are mutually
exclusive — text-mode runs (Examples 1–3) populate final_text; structured-mode runs
populate final_structured_output.
Troubleshooting
Version conflicts (409)
You get a 409 fromPOST .../runs with error.type pointing at
/errors/version-conflict.
What happened: between the moment you read version: N and the moment you posted a run
with expected_version: N, something else added messages to the conversation (a previous
run’s finalize, or a concurrent caller). The platform refuses to start a run that would
conflict at finalize time.
How to recover:
completed is actually requires_action — the latest assistant turn might be a tool-call
prompt that needs your reply, not a finished answer.
”Bad request” with a tool-alias message (400)
Thedefaults.mcp_servers[].alias you sent isn’t in the allowed shape. MCP aliases must:
- be 1–8 characters total,
- start with a letter,
- contain only ASCII letters and digits (no underscores, no dashes, no other punctuation),
- be unique across
mcp_serversfor the conversation.
tools[]) have no alias — their name must be non-empty and
dash-free. See
/errors/invalid-caller-tool-name.
Full alias rules and examples: /errors/invalid-tool-alias.
”Tool wire name too long” (400)
The combined{alias}-{tool_name} exceeds 64 characters (an underlying Bedrock limit).
Shorten the alias or the underlying tool name. See
/errors/tool-name-too-long.
requires_action and you don’t know what to answer
pending_tool_calls[] lists every tool call awaiting your reply, with the tool name and the
arguments the model produced. When you post tool_outputs, every entry in
pending_tool_calls must have a matching outputs[] entry — no missing, no extras. If you
have nothing useful to say for a particular call (e.g. the user dismissed the prompt), still
post an entry with is_error: true and a short reason.
Common mistakes around tool outputs:
- Unknown
tool_use_id— you sent an id the latest assistant turn never produced. Almost always a typo or a stale resume payload. - Not a client tool call — you sent a
tool_use_idwhosenamehas the{alias}-{tool}shape (an MCP-resolved call). The platform already answered those; only dash-free names belong intool_outputs. - Incomplete tool outputs — you missed one of the pending ids, or sent extras the model didn’t ask for.
Run failed with AgentLoopMaxIterationsExceeded
The model used up its max_iterations budget without producing a final answer. Either:
- raise
max_iterationsindefaults(orconfig_overrideper run), - tighten the system prompt so the model is steered toward an answer sooner,
- inspect the message stream — repeated identical tool calls suggest the model is stuck in a loop because the tool isn’t giving it useful new information.
/errors/max-iterations-exceeded.
Run failed with AgentLoopSchemaDecodeFailed
In text mode (no output_format_schema) this is almost always a truncation issue: the
model’s reply got cut off before the platform could read it. Raise max_tokens.
In structured mode this means the model produced JSON that didn’t match the shape your
output_format_schema declared, or returned prose instead of JSON. Fixes:
- be explicit in the system prompt: “respond with a JSON object matching this schema; no other text.”
- consider widening
max_tokens— sometimes the model is truncated mid-JSON. - simplify the schema. Bedrock’s structured-output sampler enforces a subset of Draft 2020-12; features outside that subset can silently fall through to free-text generation. See the warning under Configuration fields.
- inspect the last assistant turn to see what the model actually said.
/errors/schema-decode-failed.
Run failed with AgentLoopStructuredOutputRetriesExhausted
A structured-output validation failure no longer fails the run on the spot — the platform feeds
the non-conforming output and the validator’s message back to the model and asks it to correct
itself, up to 10 times (a budget separate from max_iterations). This error means every
retry still failed validation. The fixes are the same as for a structured-mode
AgentLoopSchemaDecodeFailed — simplify the schema, reinforce it in the system prompt — and the
last inference job’s failed_structured_output holds the raw output plus validator message from
the final attempt.
Details:
/errors/structured-output-retries-exhausted.
Run failed with AgentLoopCancelled
Not an error — the run was deliberately cancelled before it finished. The run lands in failed
with this reserved code (so it’s distinguishable from a genuine failure), and its in-flight
inference job is cancelled too. Any partial work is preserved on the run row and the message
stream. If you didn’t expect the cancellation, find out who issued it (operations tooling, a
worker drain on deploy, a client abort); the conversation is intact, so start a fresh run to
continue.
Details: /errors/cancelled.
Anything else
Every other failure class has its own page. Start at the error catalog and use the page name that matches theerror.docs_url or type in the response.
Where to go next
- Full error catalog — every failure mode the API can return, with cause and fix.
- Concepts: Model Context Protocol — the open standard
behind
mcp_servers. Helpful if you’re wiring up a tool the platform doesn’t ship by default.

