Imp v0.12.3

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Imp is a framework for typed, optimizable language-model programs on the BEAM. This release adds three things a caller opts into. An MCP tool can return the text its server wrote for a model while a host still reads its structured result (result_mode: :content, Imp.MCP.call/3). A ReActV2 call can give its own request options (config). A ReActV2 text answer says why the provider stopped its reply (:finish_reason). Calls and connections that do not use the new options send and receive what they did in 0.12.2, apart from one new metadata key on such predictions. Under Imp's versioning rule this is a patch release.

Install

{:imp, "~> 0.12.0"}

~> 0.12.0 takes later 0.12 patch releases and nothing newer. The two-part form, ~> 0.12, means >= 0.12.0 and < 1.0.0 in Mix, so mix deps.update imp under it takes every later 0.x release, including minor releases that change what callers receive. Earlier install lines used the two-part form.

Every dependency comes from Hex. Use a path dependency only while developing against a local checkout.

ExMCP and erlexec are declared runtime: false, so an OTP release that uses Imp.ACP or Imp.MCP must list applications: [ex_mcp: :load, erlexec: :load] in its release definition; see releases that use MCP or ACP. Ordinary Imp startup starts no protocol endpoint.

Changes and upgrading from 0.12.2

Under {:imp, "~> 0.12.0"}, run mix deps.update imp and commit mix.lock. No code or data migration is needed. A caller that uses any of the additions below should require {:imp, "~> 0.12.3"}, since earlier 0.12 releases do not have them.

  • Added: Imp.MCP.connect(..., result_mode: :content). An imported tool returns what the server wrote in content for a model: its text, or its text and typed images as :multimodal returns them. Where that text only repeats structuredContent as JSON, or there is no content, it returns structuredContent, as :multimodal does. Imp.MCP.call(tool, arguments, result_mode: :structured) calls the same tool with its result read in another mode. Existing modes are unchanged.
  • Added: an Imp.Predict.ReActV2 call can pass config beside its inputs, as it can max_iters and last_request_note: request options merged over the program's :config for every request of that call only, such as an output budget for one last request on a history. A value that is not a keyword list, or that names :tools or :tool_choice, is refused with {:error, {:invalid_react_v2_config, value}} before any model call.
  • Added: a ReActV2 prediction whose answer is a reply's text (termination_reason: :answered or :last_text) carries that reply's :finish_reason in its metadata, as the LM client reports it (:stop, :length, ...). :length means the provider's output limit cut the answer short. The termination itself is unchanged: a cut answer is still the text.

Rollback means restoring 0.12.2 in the lock. A call that passes config then has it read as an extra input and ignored, with a warning; a caller that reads :finish_reason finds it absent; result_mode: :content is refused at connect. Provider-free tests check the request options a call sends, the refusal of a bad config, the finish reason on both kinds of text answer, and an HTTP MCP server read in :content and :structured modes; this release does not establish what any particular provider reports as a finish reason beyond what its LM client passes through. See the changelog for every change.

Known limits

  • The usage map (:usage on the :model_response event, Imp.Usage, Imp.Prediction.get_lm_usage/1) is ReqLLM's, unchanged. Its :cost and :total_cost are ReqLLM's catalog estimate, not a charge, for streamed and non-streamed calls alike, and OpenRouter's charge is its "cost". Read money from cost and estimated_cost.
  • An error the provider sends inside a stream has status nil and retryable: true, because ReqLLM's stream decoder keeps only its message; the same error in a non-streamed response may carry a status that says not to retry.
  • A flat two-element name-first list that is not itself an element of a list (at the top level, in a tuple, or under a key that is not a credential name), such as ["password", "hunter2"], is not redacted, where 0.5.0 redacted it. Such a list is read as a key and value only as an element of a list, so that lists of names such as with_inputs([:api_key, :question]) survive saving.
  • The GEPA, SIMBA, MIPROv2, InferRules and random-search checkpoints redact their failure reasons but otherwise hold the resume state as it is: GEPA's candidates, evaluation cache, proposed instructions and reflection data, and the others' instructions, demos and scores. Fast-Slow and Playbook checkpoints likewise hold their prompts, rollouts and playbooks. Treat checkpoint files as sensitive.
  • A ReActV2 step whose prose quotes its own field names (a JSON object with next_thought or tool_calls keys, a [[ ## field ## ]] line, a <next_thought> tag) is not read as prose and goes to the JSON fallback, which usually costs one more call. If the model repeats the same reply, the step ends :incomplete. Beside native tool calls, such a thought becomes nil; the calls still run.
  • Tool images reach the model as user messages after the turn's tool responses, labelled with the call, not nested in the tool result, even for providers that accept images in tool results. Only MCP text and image blocks are converted; other block types stay text, image URLs in text are never fetched, and the selected model must accept image input of that format and size.
  • ATIF's extra.seen_by_model and the read-as content come from matching a tool message in the next captured request. A result that request does not carry, including one that ended the run, keeps the tool's own output and is marked unseen. agent.tool_definitions comes only from :tools_sent events, and final_metrics totals are left out when any response lacks that figure.
  • A streamed completion that the provider's content filter stops still ends as a completion: the stream's finish_reason is :content_filter and the filter's text has already arrived as chunks. Only non-streamed requests turn it into an Imp.LMError.
  • When a streamed ReqLLM request fails, ReqLLM's own warning log prints the error with its response headers. Imp strips them from the error it returns, but cannot change that log line.
  • In command output, a Bearer token of 12 to 15 characters, or one with no digit, is shown when more text or a newline follows it. 0.5.0's command pipeline hid any such token of 12 or more characters.
  • A GEPA checkpoint taken between preparing and starting a full validation resumes by rejecting that candidate, as before.

The CHANGELOG records every user-visible change in this release. Generated module documentation is the complete API reference. Start with Imp, Imp.Signature, Imp.Module, Imp.Evaluate, Imp.Optimizer, Imp.ACP, Imp.MCP, and Imp.Telemetry.