MCP tool reference

The worker gives your agent an MCP server with One Horizon tools on every turn. The list below is generated from the same tool catalog the worker registers, so every name, parameter, and availability note matches what your agent receives.

Expand a tool to see its parameters.

Availability

Most tools are registered on every document MCP server the harness starts. A few are conditional:

get_task_details, list_task_comments, and add_task_comment are present on every real turn, even when the task has no linked documents. get_task_details takes no input and always returns the current task, so an agent cannot point it at another task. add_task_comment takes only content and always posts to the current task.
get_parent_details is present when the current task has a parent initiative. It takes no input.
list_linked_documents and read_linked_document are present on every real turn. They cover manually linked documents (a spec, requirement, or similar, not a workflow artifact like plan/research/review/code_summary, which the document-block tools cover). With nothing linked, list_linked_documents returns an empty list. A linked document that was deleted or can't be read shows up with an error (document_not_found or document_access_denied) instead of hiding the others.
report_workflow_verdict is registered only for review-mode workflow task turns. It is absent from plan and code turns.
report_result is present on every real turn, in every task mode. Only Execute requires the call.

See documents, tools, and artifacts for how document blocks, review verdicts, and result summaries fit into a turn, and the ACP contract for how session/new delivers this MCP server to your agent.

MCP tool reference

Availability

Documents & tools

Harness manifest

ACP contract

Capabilities

MCP tool reference

Availability

Documents & tools

Harness manifest

ACP contract

Capabilities

Availability

select_design_doc_diagramsConditionalPick useful plan diagrams (including Wiremark UI). Returns a compact JSON list; empty means prefer prose or a table.

get_design_doc_diagram_detailsConditionalReturn guidance and templates for diagram IDs (includes offline Wiremark rules for wiremark_ui).

validate_design_doc_diagramConditionalValidate Wiremark or Mermaid source before embedding. Pass fence body only (no ```). Returns valid, line errors, and warnings.

lookup_wiremark_componentConditionalLook up a Wiremark component by name or query. Returns Markdown.

get_research_guidanceConditionalReturn deterministic outputs, questions, criteria, and conditional sections for selected research methods.

get_research_graph_guidanceConditionalReturn candidate diagram identifiers for selected research methods. Evidence decides whether any candidate is used.

get_review_guidanceConditionalReturn deterministic target focus, evidence relationships, methodology, questions, and criteria for a Review step.

list_artifact_blocksAlways availableList block metadata from declared workflow artifacts without loading bodies.

load_artifact_blocksAlways availableLoad artifact blocks with their markdown bodies.

set_artifact_titleAlways availableSet the document title for a declared writable workflow artifact. Call before or with the first block write.

append_artifact_blockAlways availableAppend a block to a declared writable artifact; creates the artifact if missing. Ids must be unique per artifact — a repeated id is rejected — except `type: summary`, which upserts in place and moves the block to the top of the document.

replace_artifact_block_bodyAlways availableReplace an existing artifact block's markdown body. Returns the block's new revision.

update_artifact_block_metadataAlways availablePatch YAML metadata on an existing artifact block (status, severity, title, …). Returns the block's new revision.

find_related_blocksAlways availableFollow related refs across declared artifacts (qualified refs: documentId:blockId).

get_task_detailsConditionalFetch this turn's task with full description. No input; always the current task. Use list_task_comments / read_linked_document for comments and linked docs.

get_parent_detailsConditionalFetch this turn's parent initiative with full description. No input; always the linked parent.

list_task_commentsConditionalList comments on this turn's task. Excludes agent_runtime recaps unless includeAgentRecaps is true.

add_task_commentConditionalComment on this turn's task as the agent, for notes a person should read. Not for results (report_result), verdicts (report_workflow_verdict), or blocking questions (request_input). Comments cannot be edited or removed.

read_linked_documentConditionalRead a linked document by documentId from list_linked_documents. Rejects IDs not linked to this task.

request_inputConditionalPause the workflow turn until a human responds. Call when you cannot continue without human judgment, credentials, or a product decision. Do not call for fixable issues the next agent can handle.

report_resultConditionalSave a one- or two-sentence, non-technical owner summary of what was done or concluded and the outcome. Call once when work is done.

Documents & tools

Harness manifest

ACP contract

Capabilities

Availability

select_design_doc_diagramsConditionalPick useful plan diagrams (including Wiremark UI). Returns a compact JSON list; empty means prefer prose or a table.

get_design_doc_diagram_detailsConditionalReturn guidance and templates for diagram IDs (includes offline Wiremark rules for wiremark_ui).

validate_design_doc_diagramConditionalValidate Wiremark or Mermaid source before embedding. Pass fence body only (no ```). Returns valid, line errors, and warnings.

lookup_wiremark_componentConditionalLook up a Wiremark component by name or query. Returns Markdown.

get_research_guidanceConditionalReturn deterministic outputs, questions, criteria, and conditional sections for selected research methods.

get_research_graph_guidanceConditionalReturn candidate diagram identifiers for selected research methods. Evidence decides whether any candidate is used.

get_review_guidanceConditionalReturn deterministic target focus, evidence relationships, methodology, questions, and criteria for a Review step.

list_artifact_blocksAlways availableList block metadata from declared workflow artifacts without loading bodies.

load_artifact_blocksAlways availableLoad artifact blocks with their markdown bodies.

set_artifact_titleAlways availableSet the document title for a declared writable workflow artifact. Call before or with the first block write.

append_artifact_blockAlways availableAppend a block to a declared writable artifact; creates the artifact if missing. Ids must be unique per artifact — a repeated id is rejected — except `type: summary`, which upserts in place and moves the block to the top of the document.

replace_artifact_block_bodyAlways availableReplace an existing artifact block's markdown body. Returns the block's new revision.

update_artifact_block_metadataAlways availablePatch YAML metadata on an existing artifact block (status, severity, title, …). Returns the block's new revision.

find_related_blocksAlways availableFollow related refs across declared artifacts (qualified refs: documentId:blockId).

get_task_detailsConditionalFetch this turn's task with full description. No input; always the current task. Use list_task_comments / read_linked_document for comments and linked docs.

get_parent_detailsConditionalFetch this turn's parent initiative with full description. No input; always the linked parent.

list_task_commentsConditionalList comments on this turn's task. Excludes agent_runtime recaps unless includeAgentRecaps is true.

add_task_commentConditionalComment on this turn's task as the agent, for notes a person should read. Not for results (report_result), verdicts (report_workflow_verdict), or blocking questions (request_input). Comments cannot be edited or removed.

read_linked_documentConditionalRead a linked document by documentId from list_linked_documents. Rejects IDs not linked to this task.

request_inputConditionalPause the workflow turn until a human responds. Call when you cannot continue without human judgment, credentials, or a product decision. Do not call for fixable issues the next agent can handle.

report_resultConditionalSave a one- or two-sentence, non-technical owner summary of what was done or concluded and the outcome. Call once when work is done.

Documents & tools

Harness manifest

ACP contract

Capabilities