Line9

Render diagram

render_diagram

Read-onlyIdempotent

Render Mermaid source with Line9's layout engine. Call this whenever you write or change Mermaid and the user should see the result. Flowcharts and sequence diagrams get Line9's layout; other Mermaid kinds are rendered with Mermaid's own layout and flagged as such. Returns a short verdict — kind, size, warnings, or a parse error with its line and column — plus an "Open in Line9" link. On a parse error, fix the named line and call again. Assistants that support MCP Apps show the diagram in a panel; there, do not paste the SVG or repeat the source in your reply. In an assistant with no panel (a terminal or editor), reply with the verdict line and the open_url link — that link is the only way the user sees the diagram. Write flowcharts as flowchart LR unless the user asks for top-to-bottom; chat panels are wide and short. No sign-in needed; renders carry a small Line9 mark unless the user is signed in to Line9 Pro. Set theme, orientation, spacing and legend in the source itself (line9: frontmatter; flowchart LR), never as arguments. This connector cannot publish or save. To publish, the user opens the diagram in Line9 (the open_url link) and publishes there — not with the Line9 CLI.

Input Schema

Name	Required	Description	Default
`plan`	No	Opaque plan from a previous result (widget use). A hint only: skips the solve when it still fits the source.
`source`	Yes	Mermaid source. Optional YAML frontmatter is honoured — set it in the source, never as arguments: `title`, and a nested `line9:` block with `theme` (one of `boardroom`, `linen`, `blueprint`), `scale: { node, spacing }` and `flowchart: { legend: on \| off }`. The flowchart direction directive is also honoured. Maximum 100000 characters. Line9 Pro accounts can also use: kingfisher.

Output Schema

Name	Required	Description	Default
`svg`	No
`path`	No
`plan`	No
`error`	No
`width`	No
`config`	No
`height`	No
`source`	No
`status`	Yes
`themes`	No
`diagram`	No
`controls`	No
`diagrams`	No
`open_url`	No
`warnings`	No
`signed_in`	No
`truncated`	No
`watermark`	No
`edge_count`	No
`node_count`	No
`orientation`	No
`render_mode`	No
`stop_reason`	No
`diagram_type`	No
`engine_version`	No
`schema_version`	Yes
`fallback_reason`	No
`mermaid_version`	No
`pro_theme_refused`	No

Schema Changelog

Changes observed during successful MCP inspections.

Changed3 schema fields changedOct 20, 2026

changedInput schema / properties / source / description

Previous value: -"Mermaid source. Optional YAML frontmatter is honoured — set it in the source, never as arguments: `title`, and a nested `line9:` block with `theme` (one of `boardroom`, `linen`, `blueprint`), `scale: { node, spacing }` and `flowchart: { legend: on | off }`. The flowchart direction directive is also honoured. Maximum 100000 characters."New value: +"Mermaid source. Optional YAML frontmatter is honoured — set it in the source, never as arguments: `title`, and a nested `line9:` block with `theme` (one of `boardroom`, `linen`, `blueprint`), `scale: { node, spacing }` and `flowchart: { legend: on | off }`. The flowchart direction directive is also honoured. Maximum 100000 characters. Line9 Pro accounts can also use: kingfisher."

addedOutput schema / properties / pro_theme_refused
```
Added value: +{
+  "type": "string"
+}
```

addedOutput schema / properties / themes

Added value: +{
+  "items": {
+    "type": "string"
+  },
+  "type": "array"
+}

Changed1 schema field changedOct 2, 2026

changedOutput schema / properties / fallback_reason / enum

Previous value: -[
-  "unsupported_diagram_kind",
-  "node_cap_exceeded"
-]New value: +[
+  "unsupported_diagram_kind",
+  "node_cap_exceeded",
+  "engine_panic"
+]

First observedSep 17, 2026

TDQS

A4.4/5.0

Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare read-only/idempotent, but the description goes well beyond: environment-dependent behavior (panel vs terminal), the watermark rule, no sign-in requirement, and the error-recovery loop. The 'cannot publish or save' limitation is consistent with readOnlyHint and adds real operational context.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with purpose and trigger, then logically sequenced through output, errors, environment, styling and auth. It is long, but the complexity justifies most of it; the publishing sentence partly repeats 'cannot publish or save' and could be tightened.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Output schema exists, yet the description still frames the verdict and error shape usefully, and it covers auth, environment handling, styling, and limitations. An agent has everything needed to call it correctly and to act on the result in either UI mode.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so baseline is 3; the schema already documents frontmatter, theme values and the max length. The description reinforces the styling directive ('set... in the source itself... never as arguments') but never mentions the opaque `plan` parameter and largely restates schema content rather than adding new semantics.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource — 'Render Mermaid source with Line9's layout engine' — and is immediately distinguishable from get_diagram/list_diagrams, which are retrieval rather than rendering. It also scopes which Mermaid kinds get native layout versus Mermaid's own, so the agent knows exactly what the tool produces.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives a clear trigger ('Call this whenever you write or change Mermaid and the user should see the result') and an explicit when-not ('This connector cannot publish or save... not with the Line9 CLI'). It does not name the sibling tools as alternatives for read/retrieve cases, so the routing advice is strong but not fully closed.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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