diff --git a/ts-packages/rust-qmd-json/.gitignore b/ts-packages/rust-qmd-json/.gitignore
new file mode 100644
index 0000000..dd6e803
--- /dev/null
+++ b/ts-packages/rust-qmd-json/.gitignore
@@ -0,0 +1,4 @@
+node_modules/
+dist/
+*.log
+.DS_Store
diff --git a/ts-packages/rust-qmd-json/README.md b/ts-packages/rust-qmd-json/README.md
new file mode 100644
index 0000000..38b4d84
--- /dev/null
+++ b/ts-packages/rust-qmd-json/README.md
@@ -0,0 +1,138 @@
+# @quarto/rust-qmd-json
+
+Convert quarto-markdown-pandoc JSON output to AnnotatedParse structures with full source mapping.
+
+## Overview
+
+This package converts the JSON output from the Rust-based `quarto-markdown-pandoc` parser
+into `AnnotatedParse` structures that are compatible with quarto-cli's YAML validation
+infrastructure. It preserves complete source location information through the conversion.
+
+## Installation
+
+```bash
+npm install @quarto/rust-qmd-json
+```
+
+## Quick Start
+
+```typescript
+import { parseRustQmdMetadata } from '@quarto/rust-qmd-json';
+import type { RustQmdJson } from '@quarto/rust-qmd-json';
+
+// JSON from quarto-markdown-pandoc
+const json: RustQmdJson = {
+  meta: {
+    title: { t: 'MetaString', c: 'My Document', s: 0 }
+  },
+  blocks: [],
+  source_pool: [
+    { r: [11, 22], t: 0, d: 0 }
+  ],
+  source_context: {
+    files: [
+      { id: 0, path: 'doc.qmd', content: '---\ntitle: My Document\n---' }
+    ]
+  }
+};
+
+const annotatedParse = parseRustQmdMetadata(json);
+
+console.log(annotatedParse.result);  // { title: 'My Document' }
+console.log(annotatedParse.kind);    // 'mapping'
+console.log(annotatedParse.components.length);  // 2 (key + value)
+```
+
+## API
+
+### `parseRustQmdMetadata(json, errorHandler?)`
+
+Main entry point for converting quarto-markdown-pandoc JSON to AnnotatedParse.
+
+**Parameters:**
+- `json: RustQmdJson` - The JSON output from quarto-markdown-pandoc
+- `errorHandler?: (msg: string, id?: number) => void` - Optional error handler for SourceInfo reconstruction errors
+
+**Returns:** `AnnotatedParse`
+
+**Example with error handling:**
+
+```typescript
+import { parseRustQmdMetadata } from '@quarto/rust-qmd-json';
+
+const errorHandler = (msg: string, id?: number) => {
+  console.error(`SourceInfo error: ${msg}`, id);
+};
+
+const result = parseRustQmdMetadata(json, errorHandler);
+```
+
+### Types
+
+The package exports all necessary TypeScript types:
+
+```typescript
+import type {
+  AnnotatedParse,
+  JSONValue,
+  JsonMetaValue,
+  MetaMapEntry,
+  RustQmdJson,
+  SerializableSourceInfo,
+  SourceContext,
+  SourceInfoErrorHandler
+} from '@quarto/rust-qmd-json';
+```
+
+### Advanced Usage
+
+For more control, you can use the underlying classes directly:
+
+```typescript
+import { SourceInfoReconstructor, MetadataConverter } from '@quarto/rust-qmd-json';
+
+const reconstructor = new SourceInfoReconstructor(
+  json.source_pool,
+  json.source_context
+);
+
+const converter = new MetadataConverter(reconstructor);
+const result = converter.convertMeta(json.meta);
+```
+
+## Development
+
+```bash
+# Install dependencies
+npm install
+
+# Build
+npm run build
+
+# Test
+npm test
+
+# Clean
+npm run clean
+```
+
+## Architecture
+
+The conversion happens in two phases:
+
+1. **SourceInfo Reconstruction**: Convert the pooled SourceInfo format from JSON into
+   MappedString objects that track source locations through transformation chains.
+
+2. **Metadata Conversion**: Recursively convert MetaValue variants into AnnotatedParse
+   structures with proper source tracking. MetaInlines/MetaBlocks are treated as leaf
+   nodes with the JSON array structure preserved in the result.
+
+## Design Decisions
+
+- **Direct JSON Value Mapping**: MetaInlines and MetaBlocks are preserved as JSON arrays
+  in the `result` field, avoiding any text reconstruction
+- **Source Tracking**: Every value can be traced back to original file location via SourceInfo
+- **Compatible Types**: Produces AnnotatedParse structures compatible with existing validation code
+
+See repository's `claude-notes/plans/2025-10-23-json-to-annotated-parse-conversion.md` for
+detailed implementation plan.
diff --git a/ts-packages/rust-qmd-json/SETUP-NOTES.md b/ts-packages/rust-qmd-json/SETUP-NOTES.md
new file mode 100644
index 0000000..1317415
--- /dev/null
+++ b/ts-packages/rust-qmd-json/SETUP-NOTES.md
@@ -0,0 +1,49 @@
+# Setup Notes
+
+## TypeScript Infrastructure Verified
+
+✅ Successfully set up standalone TypeScript package
+✅ `@quarto/mapped-string` integration working
+✅ All tests passing
+
+## Test Results
+
+```
+✔ can import and use @quarto/mapped-string (0.550583ms)
+✔ can create mapped substrings (0.106792ms)
+✔ placeholder conversion function (15.146917ms)
+ℹ tests 3
+ℹ suites 0
+ℹ pass 3
+ℹ fail 0
+```
+
+## Environment
+
+- Node.js: v23.11.0
+- npm: 10.9.2
+- TypeScript: ^5.4.2
+- Dependencies:
+  - `@quarto/mapped-string`: ^0.1.8 (working correctly)
+  - `tsx`: ^4.7.1 (for running TypeScript tests)
+  - `@types/node`: ^20.0.0
+
+## Key Learnings
+
+1. **Module imports**: @quarto/mapped-string exports `MappedString` as a type, not a value.
+   Must use `export type { MappedString }` in TypeScript.
+
+2. **Test framework**: Using Node.js built-in test runner with tsx for TypeScript execution.
+   Works well for ES modules.
+
+3. **Project structure**: Following Rust workspace conventions by placing TypeScript packages
+   in `ts-packages/` directory parallel to `crates/`.
+
+## Next Steps
+
+Ready to implement:
+1. Phase 1: SourceInfo reconstruction
+2. Phase 2: Metadata conversion
+3. Phase 3: Integration & testing
+
+See `claude-notes/plans/2025-10-23-json-to-annotated-parse-conversion.md` for detailed plan.
diff --git a/ts-packages/rust-qmd-json/package-lock.json b/ts-packages/rust-qmd-json/package-lock.json
new file mode 100644
index 0000000..2456082
--- /dev/null
+++ b/ts-packages/rust-qmd-json/package-lock.json
@@ -0,0 +1,227 @@
+{
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+  "version": "0.1.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "@quarto/rust-qmd-json",
+      "version": "0.1.0",
+      "license": "MIT",
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+      },
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+      "license": "MIT",
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+      },
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+      },
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+      "license": "Apache-2.0",
+      "bin": {
+        "tsc": "bin/tsc",
+        "tsserver": "bin/tsserver"
+      },
+      "engines": {
+        "node": ">=14.17"
+      }
+    },
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+      "dev": true,
+      "license": "MIT"
+    }
+  }
+}
diff --git a/ts-packages/rust-qmd-json/package.json b/ts-packages/rust-qmd-json/package.json
new file mode 100644
index 0000000..176a08f
--- /dev/null
+++ b/ts-packages/rust-qmd-json/package.json
@@ -0,0 +1,36 @@
+{
+  "name": "@quarto/rust-qmd-json",
+  "version": "0.1.0",
+  "description": "Convert quarto-markdown-pandoc JSON output to AnnotatedParse structures",
+  "license": "MIT",
+  "author": {
+    "name": "Posit PBC"
+  },
+  "publishConfig": {
+    "access": "public",
+    "registry": "https://registry.npmjs.org/"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/quarto-dev/quarto.git",
+    "directory": "ts-packages/rust-qmd-json"
+  },
+  "type": "module",
+  "main": "dist/index.js",
+  "types": "dist/index.d.ts",
+  "files": ["dist"],
+  "scripts": {
+    "build": "tsc",
+    "test": "node --import tsx --test test/*.test.ts",
+    "clean": "rm -rf dist",
+    "prepublishOnly": "npm run clean && npm run build && npm test"
+  },
+  "dependencies": {
+    "@quarto/mapped-string": "^0.1.8"
+  },
+  "devDependencies": {
+    "typescript": "^5.4.2",
+    "tsx": "^4.7.1",
+    "@types/node": "^20.0.0"
+  }
+}
diff --git a/ts-packages/rust-qmd-json/src/index.ts b/ts-packages/rust-qmd-json/src/index.ts
new file mode 100644
index 0000000..bdb139a
--- /dev/null
+++ b/ts-packages/rust-qmd-json/src/index.ts
@@ -0,0 +1,83 @@
+/**
+ * @quarto/rust-qmd-json
+ *
+ * Converts quarto-markdown-pandoc JSON output to AnnotatedParse structures
+ * compatible with quarto-cli's YAML validation infrastructure.
+ */
+
+// Re-export types and functions from @quarto/mapped-string
+export type { MappedString } from '@quarto/mapped-string';
+export { asMappedString, mappedSubstring } from '@quarto/mapped-string';
+
+// Re-export our types
+export type {
+  AnnotatedParse,
+  JSONValue,
+  JsonMetaValue,
+  MetaMapEntry,
+  RustQmdJson
+} from './types.js';
+
+export type {
+  SerializableSourceInfo,
+  SourceContext,
+  SourceInfoErrorHandler
+} from './source-map.js';
+
+// Re-export classes
+export { SourceInfoReconstructor } from './source-map.js';
+export { MetadataConverter } from './meta-converter.js';
+
+// Import for main function
+import { SourceInfoReconstructor } from './source-map.js';
+import { MetadataConverter } from './meta-converter.js';
+import type { RustQmdJson, AnnotatedParse } from './types.js';
+import type { SourceInfoErrorHandler } from './source-map.js';
+
+/**
+ * Convert quarto-markdown-pandoc JSON output to AnnotatedParse
+ *
+ * @param json - The JSON output from quarto-markdown-pandoc
+ * @param errorHandler - Optional error handler for SourceInfo reconstruction errors
+ * @returns AnnotatedParse structure compatible with quarto-cli
+ *
+ * @example
+ * ```typescript
+ * import { parseRustQmdMetadata } from '@quarto/rust-qmd-json';
+ *
+ * const json = {
+ *   meta: {
+ *     title: { t: 'MetaString', c: 'Hello', s: 0 }
+ *   },
+ *   blocks: [],
+ *   source_pool: [
+ *     { r: [11, 16], t: 0, d: 0 }
+ *   ],
+ *   source_context: {
+ *     files: [
+ *       { id: 0, path: 'test.qmd', content: '---\ntitle: Hello\n---' }
+ *     ]
+ *   }
+ * };
+ *
+ * const metadata = parseRustQmdMetadata(json);
+ * console.log(metadata.result); // { title: 'Hello' }
+ * ```
+ */
+export function parseRustQmdMetadata(
+  json: RustQmdJson,
+  errorHandler?: SourceInfoErrorHandler
+): AnnotatedParse {
+  // 1. Create SourceInfoReconstructor with pool and context
+  const sourceReconstructor = new SourceInfoReconstructor(
+    json.source_pool,
+    json.source_context,
+    errorHandler
+  );
+
+  // 2. Create MetadataConverter
+  const converter = new MetadataConverter(sourceReconstructor);
+
+  // 3. Convert metadata to AnnotatedParse
+  return converter.convertMeta(json.meta);
+}
diff --git a/ts-packages/rust-qmd-json/src/meta-converter.ts b/ts-packages/rust-qmd-json/src/meta-converter.ts
new file mode 100644
index 0000000..292818c
--- /dev/null
+++ b/ts-packages/rust-qmd-json/src/meta-converter.ts
@@ -0,0 +1,315 @@
+/**
+ * Metadata Conversion
+ *
+ * Converts MetaValue structures from quarto-markdown-pandoc JSON
+ * into AnnotatedParse structures compatible with quarto-cli.
+ */
+
+import type { AnnotatedParse, JsonMetaValue, MetaMapEntry, JSONValue } from './types.js';
+import type { SourceInfoReconstructor } from './source-map.js';
+
+/**
+ * Type guard for MetaMap content structure
+ */
+function isMetaMapContent(c: unknown): c is { entries: MetaMapEntry[] } {
+  return (
+    typeof c === 'object' &&
+    c !== null &&
+    'entries' in c &&
+    Array.isArray((c as { entries: unknown }).entries)
+  );
+}
+
+/**
+ * Type guard for checking if value is an array of JsonMetaValue
+ */
+function isMetaValueArray(c: unknown): c is JsonMetaValue[] {
+  return Array.isArray(c);
+}
+
+/**
+ * Type guard for Span structure with yaml-tagged-string
+ */
+function isTaggedSpan(obj: unknown): obj is {
+  t: string;
+  c: [{ c: unknown; kv?: [string, string][] }, unknown];
+} {
+  return (
+    typeof obj === 'object' &&
+    obj !== null &&
+    't' in obj &&
+    (obj as { t: unknown }).t === 'Span' &&
+    'c' in obj &&
+    Array.isArray((obj as { c: unknown }).c) &&
+    (obj as { c: unknown[] }).c.length === 2
+  );
+}
+
+/**
+ * Converts metadata from quarto-markdown-pandoc JSON to AnnotatedParse
+ */
+export class MetadataConverter {
+  constructor(private sourceReconstructor: SourceInfoReconstructor) {}
+
+  /**
+   * Convert top-level metadata object to AnnotatedParse
+   */
+  convertMeta(jsonMeta: Record<string, JsonMetaValue>): AnnotatedParse {
+    // Create a synthetic MetaMap for the top-level metadata
+    const entries: MetaMapEntry[] = Object.entries(jsonMeta).map(([key, value]) => ({
+      key,
+      key_source: value.s,  // Use value's source for key (not ideal, but metadata doesn't include key sources)
+      value
+    }));
+
+    // Find the overall range by getting min/max offsets
+    let minStart = Infinity;
+    let maxEnd = -Infinity;
+    for (const value of Object.values(jsonMeta)) {
+      const [start, end] = this.sourceReconstructor.getOffsets(value.s);
+      minStart = Math.min(minStart, start);
+      maxEnd = Math.max(maxEnd, end);
+    }
+
+    // If no metadata, use defaults
+    if (minStart === Infinity) {
+      minStart = 0;
+      maxEnd = 0;
+    }
+
+    // Convert to AnnotatedParse components
+    const components: AnnotatedParse[] = [];
+    const result: Record<string, JSONValue> = {};
+
+    for (const [key, value] of Object.entries(jsonMeta)) {
+      // Create AnnotatedParse for key
+      const [keyStart, keyEnd] = this.sourceReconstructor.getOffsets(value.s);
+      const keySource = this.sourceReconstructor.toMappedString(value.s);
+
+      const keyAP: AnnotatedParse = {
+        result: key,
+        kind: 'key',
+        source: keySource,
+        components: [],
+        start: keyStart,
+        end: keyEnd
+      };
+
+      // Create AnnotatedParse for value
+      const valueAP = this.convertMetaValue(value);
+
+      // Interleave key and value
+      components.push(keyAP, valueAP);
+      result[key] = valueAP.result;
+    }
+
+    // Create synthetic MappedString for top-level (using first file if available)
+    const firstFile = this.sourceReconstructor['sourceContext'].files[0];
+    const topSource = this.sourceReconstructor['sourceContext'].files[0]
+      ? this.sourceReconstructor.toMappedString(0)  // Use first SourceInfo if available
+      : this.sourceReconstructor.toMappedString(0);
+
+    return {
+      result,
+      kind: 'mapping',  // Top-level metadata is a mapping
+      source: topSource,
+      components,
+      start: minStart,
+      end: maxEnd
+    };
+  }
+
+  /**
+   * Convert individual MetaValue to AnnotatedParse
+   */
+  convertMetaValue(meta: JsonMetaValue): AnnotatedParse {
+    const source = this.sourceReconstructor.toMappedString(meta.s);
+    const [start, end] = this.sourceReconstructor.getOffsets(meta.s);
+
+    switch (meta.t) {
+      case 'MetaString':
+        return {
+          result: typeof meta.c === 'string' ? meta.c : String(meta.c ?? ''),
+          kind: 'MetaString',
+          source,
+          components: [],
+          start,
+          end
+        };
+
+      case 'MetaBool':
+        return {
+          result: typeof meta.c === 'boolean' ? meta.c : Boolean(meta.c),
+          kind: 'MetaBool',
+          source,
+          components: [],
+          start,
+          end
+        };
+
+      case 'MetaInlines':
+        return {
+          result: meta.c as JSONValue,  // Array of inline JSON objects AS-IS
+          kind: this.extractKind(meta),  // Handle tagged values
+          source,
+          components: [],  // Empty - cannot track internal locations yet
+          start,
+          end
+        };
+
+      case 'MetaBlocks':
+        return {
+          result: meta.c as JSONValue,  // Array of block JSON objects AS-IS
+          kind: 'MetaBlocks',
+          source,
+          components: [],
+          start,
+          end
+        };
+
+      case 'MetaList':
+        return this.convertMetaList(meta, source, start, end);
+
+      case 'MetaMap':
+        return this.convertMetaMap(meta, source, start, end);
+
+      default:
+        // Unknown type - return as-is with generic kind
+        return {
+          result: meta.c as JSONValue,
+          kind: meta.t,
+          source,
+          components: [],
+          start,
+          end
+        };
+    }
+  }
+
+  /**
+   * Convert MetaList to AnnotatedParse
+   */
+  private convertMetaList(
+    meta: JsonMetaValue,
+    source: ReturnType<SourceInfoReconstructor['toMappedString']>,
+    start: number,
+    end: number
+  ): AnnotatedParse {
+    // Runtime type check
+    if (!isMetaValueArray(meta.c)) {
+      // Return empty list if content is not an array
+      return {
+        result: [],
+        kind: 'MetaList',
+        source,
+        components: [],
+        start,
+        end
+      };
+    }
+
+    const items = meta.c.map(item => this.convertMetaValue(item));
+
+    return {
+      result: items.map(item => item.result),
+      kind: 'MetaList',
+      source,
+      components: items,
+      start,
+      end
+    };
+  }
+
+  /**
+   * Convert MetaMap to AnnotatedParse with interleaved key/value components
+   */
+  private convertMetaMap(
+    meta: JsonMetaValue,
+    source: ReturnType<SourceInfoReconstructor['toMappedString']>,
+    start: number,
+    end: number
+  ): AnnotatedParse {
+    // Runtime type check
+    if (!isMetaMapContent(meta.c)) {
+      // Return empty map if content is not valid
+      return {
+        result: {},
+        kind: 'MetaMap',
+        source,
+        components: [],
+        start,
+        end
+      };
+    }
+
+    const entries = meta.c.entries;
+    const components: AnnotatedParse[] = [];
+    const result: Record<string, JSONValue> = {};
+
+    for (const entry of entries) {
+      const keySource = this.sourceReconstructor.toMappedString(entry.key_source);
+      const [keyStart, keyEnd] = this.sourceReconstructor.getOffsets(entry.key_source);
+
+      const keyAP: AnnotatedParse = {
+        result: entry.key,
+        kind: 'key',
+        source: keySource,
+        components: [],
+        start: keyStart,
+        end: keyEnd
+      };
+
+      const valueAP = this.convertMetaValue(entry.value);
+
+      // Interleave key and value in components (matches js-yaml pattern)
+      components.push(keyAP, valueAP);
+      result[entry.key] = valueAP.result;
+    }
+
+    return {
+      result,
+      kind: 'MetaMap',
+      source,
+      components,
+      start,
+      end
+    };
+  }
+
+  /**
+   * Extract kind with special tag handling for YAML tagged values
+   *
+   * TODO: For now, use simple encoding like "MetaInlines:tagged:expr"
+   * Future enhancement: Modify @quarto/mapped-string to add optional tag field
+   * to AnnotatedParse interface, then use that instead
+   */
+  private extractKind(meta: JsonMetaValue): string {
+    if (meta.t !== 'MetaInlines' || !Array.isArray(meta.c) || meta.c.length === 0) {
+      return meta.t;
+    }
+
+    // Check if wrapped in Span with yaml-tagged-string class
+    const first = meta.c[0];
+    if (!isTaggedSpan(first)) {
+      return 'MetaInlines';
+    }
+
+    const [attrs, _content] = first.c;
+
+    // Check if attrs.c is an array containing 'yaml-tagged-string'
+    if (!Array.isArray(attrs.c) || !attrs.c.includes('yaml-tagged-string')) {
+      return 'MetaInlines';
+    }
+
+    // Find the tag in kv pairs
+    if (attrs.kv) {
+      const tagPair = attrs.kv.find(([k, _]) => k === 'tag');
+      if (tagPair) {
+        const tag = tagPair[1];
+        return `MetaInlines:tagged:${tag}`;
+      }
+    }
+
+    return 'MetaInlines';
+  }
+}
diff --git a/ts-packages/rust-qmd-json/src/source-map.ts b/ts-packages/rust-qmd-json/src/source-map.ts
new file mode 100644
index 0000000..7cb1eb3
--- /dev/null
+++ b/ts-packages/rust-qmd-json/src/source-map.ts
@@ -0,0 +1,305 @@
+/**
+ * SourceInfo Reconstruction
+ *
+ * Converts pooled SourceInfo from quarto-markdown-pandoc JSON output
+ * into MappedString objects from @quarto/mapped-string.
+ */
+
+import { MappedString, asMappedString, mappedConcat, mappedSubstring } from '@quarto/mapped-string';
+
+/**
+ * Serialized SourceInfo from the JSON pool
+ */
+export interface SerializableSourceInfo {
+  r: [number, number]; // [start_offset, end_offset]
+  t: number;           // type code (0=Original, 1=Substring, 2=Concat)
+  d: unknown;          // type-specific data (varies by type)
+}
+
+/**
+ * Type guard for Concat data structure
+ */
+function isConcatData(data: unknown): data is { pieces: [number, number, number][] } {
+  return (
+    typeof data === 'object' &&
+    data !== null &&
+    'pieces' in data &&
+    Array.isArray((data as { pieces: unknown }).pieces)
+  );
+}
+
+/**
+ * Source context containing file information
+ */
+export interface SourceContext {
+  files: Array<{
+    id: number;
+    path: string;
+    content: string;
+  }>;
+}
+
+/**
+ * Resolved SourceInfo pointing to a file location
+ */
+interface ResolvedSource {
+  file_id: number;
+  range: [number, number];
+}
+
+/**
+ * Error handler callback for SourceInfo reconstruction errors
+ */
+export type SourceInfoErrorHandler = (msg: string, id?: number) => void;
+
+/**
+ * Default error handler that throws on errors
+ */
+const defaultErrorHandler: SourceInfoErrorHandler = (msg: string, id?: number) => {
+  const idStr = id !== undefined ? ` (SourceInfo ID: ${id})` : '';
+  throw new Error(`SourceInfo reconstruction error: ${msg}${idStr}`);
+};
+
+/**
+ * Reconstructs SourceInfo from pooled format to MappedString objects
+ */
+export class SourceInfoReconstructor {
+  private pool: SerializableSourceInfo[];
+  private sourceContext: SourceContext;
+  private errorHandler: SourceInfoErrorHandler;
+  private resolvedCache = new Map<number, ResolvedSource>();
+  private mappedStringCache = new Map<number, MappedString>();
+
+  constructor(
+    pool: SerializableSourceInfo[],
+    sourceContext: SourceContext,
+    errorHandler?: SourceInfoErrorHandler
+  ) {
+    this.pool = pool;
+    this.sourceContext = sourceContext;
+    this.errorHandler = errorHandler || defaultErrorHandler;
+  }
+
+  /**
+   * Convert SourceInfo ID to MappedString
+   */
+  toMappedString(id: number): MappedString {
+    // Check cache first
+    const cached = this.mappedStringCache.get(id);
+    if (cached) {
+      return cached;
+    }
+
+    // Validate ID
+    if (id < 0 || id >= this.pool.length) {
+      this.errorHandler(`Invalid SourceInfo ID ${id} (pool size: ${this.pool.length})`, id);
+      // Return empty MappedString as fallback
+      return asMappedString('');
+    }
+
+    const info = this.pool[id];
+    let result: MappedString;
+
+    switch (info.t) {
+      case 0: // Original
+        result = this.handleOriginal(id, info);
+        break;
+      case 1: // Substring
+        result = this.handleSubstring(id, info);
+        break;
+      case 2: // Concat
+        result = this.handleConcat(id, info);
+        break;
+      default:
+        this.errorHandler(`Unknown SourceInfo type ${info.t}`, id);
+        result = asMappedString('');
+    }
+
+    // Cache and return
+    this.mappedStringCache.set(id, result);
+    return result;
+  }
+
+  /**
+   * Get offsets from SourceInfo (without creating full MappedString)
+   */
+  getOffsets(id: number): [number, number] {
+    if (id < 0 || id >= this.pool.length) {
+      this.errorHandler(`Invalid SourceInfo ID ${id}`, id);
+      return [0, 0];
+    }
+    return this.pool[id].r;
+  }
+
+  /**
+   * Handle Original SourceInfo type (t=0)
+   * Data format: file_id (number)
+   */
+  private handleOriginal(id: number, info: SerializableSourceInfo): MappedString {
+    // Runtime type check
+    if (typeof info.d !== 'number') {
+      this.errorHandler(`Original SourceInfo data must be a number (file_id), got ${typeof info.d}`, id);
+      return asMappedString('');
+    }
+
+    const fileId = info.d;
+    const [start, end] = info.r;
+
+    // Find file in context
+    const file = this.sourceContext.files.find(f => f.id === fileId);
+    if (!file) {
+      this.errorHandler(`File ID ${fileId} not found in source context`, id);
+      return asMappedString('');
+    }
+
+    // Extract substring from file content
+    const content = file.content.substring(start, end);
+    return asMappedString(content, file.path);
+  }
+
+  /**
+   * Handle Substring SourceInfo type (t=1)
+   * Data format: parent_id (number)
+   * The range in info.r is relative to the parent's content
+   */
+  private handleSubstring(id: number, info: SerializableSourceInfo): MappedString {
+    // Runtime type check
+    if (typeof info.d !== 'number') {
+      this.errorHandler(`Substring SourceInfo data must be a number (parent_id), got ${typeof info.d}`, id);
+      return asMappedString('');
+    }
+
+    const parentId = info.d;
+    const [localStart, localEnd] = info.r;
+
+    // Get parent MappedString (recursive, with caching)
+    const parent = this.toMappedString(parentId);
+
+    // Create substring with offset mapping
+    return mappedSubstring(parent, localStart, localEnd);
+  }
+
+  /**
+   * Handle Concat SourceInfo type (t=2)
+   * Data format: {pieces: [[source_info_id, offset, length], ...]}
+   */
+  private handleConcat(id: number, info: SerializableSourceInfo): MappedString {
+    // Runtime type check
+    if (!isConcatData(info.d)) {
+      this.errorHandler(`Invalid Concat data format (expected {pieces: [...]}), got ${typeof info.d}`, id);
+      return asMappedString('');
+    }
+
+    const pieces = info.d.pieces;
+
+    // Build MappedString array from pieces
+    const mappedPieces: MappedString[] = [];
+    for (const [pieceId, offset, length] of pieces) {
+      const pieceMapped = this.toMappedString(pieceId);
+      // Extract substring at specified offset/length
+      const substring = mappedSubstring(pieceMapped, offset, offset + length);
+      mappedPieces.push(substring);
+    }
+
+    // Concatenate all pieces
+    if (mappedPieces.length === 0) {
+      return asMappedString('');
+    }
+    if (mappedPieces.length === 1) {
+      return mappedPieces[0];
+    }
+
+    return mappedConcat(mappedPieces);
+  }
+
+  /**
+   * Recursively resolve SourceInfo chains to find original file location
+   * This is cached to avoid re-resolving deep chains
+   */
+  private resolveChain(id: number): ResolvedSource {
+    // Check cache first
+    const cached = this.resolvedCache.get(id);
+    if (cached) {
+      return cached;
+    }
+
+    // Validate ID
+    if (id < 0 || id >= this.pool.length) {
+      this.errorHandler(`Invalid SourceInfo ID ${id}`, id);
+      return { file_id: -1, range: [0, 0] };
+    }
+
+    const info = this.pool[id];
+    let resolved: ResolvedSource;
+
+    switch (info.t) {
+      case 0: // Original - base case
+        {
+          if (typeof info.d !== 'number') {
+            this.errorHandler(`Original SourceInfo data must be a number`, id);
+            resolved = { file_id: -1, range: info.r };
+          } else {
+            resolved = {
+              file_id: info.d,
+              range: info.r
+            };
+          }
+        }
+        break;
+
+      case 1: // Substring - chain through parent
+        {
+          if (typeof info.d !== 'number') {
+            this.errorHandler(`Substring SourceInfo data must be a number`, id);
+            resolved = { file_id: -1, range: info.r };
+          } else {
+            const parentResolved = this.resolveChain(info.d);
+            const [localStart, localEnd] = info.r;
+            const [parentStart, _] = parentResolved.range;
+            resolved = {
+              file_id: parentResolved.file_id,
+              range: [parentStart + localStart, parentStart + localEnd]
+            };
+          }
+        }
+        break;
+
+      case 2: // Concat - use first piece's resolution
+        // TODO: Concat doesn't have a single file location, so we use the first piece
+        // For error reporting, this may not be ideal
+        {
+          if (!isConcatData(info.d)) {
+            this.errorHandler(`Invalid Concat data format`, id);
+            resolved = { file_id: -1, range: info.r };
+          } else {
+            const pieces = info.d.pieces;
+            if (pieces.length === 0) {
+              this.errorHandler(`Empty Concat pieces`, id);
+              resolved = { file_id: -1, range: info.r };
+            } else {
+              const [firstPieceId, offset, length] = pieces[0];
+              const firstResolved = this.resolveChain(firstPieceId);
+              // Offset into the first piece
+              const [pieceStart, _] = firstResolved.range;
+              resolved = {
+                file_id: firstResolved.file_id,
+                range: [pieceStart + offset, pieceStart + offset + length]
+              };
+            }
+          }
+        }
+        break;
+
+      default:
+        this.errorHandler(`Unknown SourceInfo type ${info.t}`, id);
+        resolved = { file_id: -1, range: [0, 0] };
+    }
+
+    // Cache and return
+    this.resolvedCache.set(id, resolved);
+    return resolved;
+  }
+
+  // TODO: Implement circular reference detection
+  // This would require tracking visited IDs during resolveChain traversal
+}
diff --git a/ts-packages/rust-qmd-json/src/types.ts b/ts-packages/rust-qmd-json/src/types.ts
new file mode 100644
index 0000000..a1408f3
--- /dev/null
+++ b/ts-packages/rust-qmd-json/src/types.ts
@@ -0,0 +1,64 @@
+/**
+ * Type definitions for quarto-markdown-pandoc JSON format
+ * and AnnotatedParse structures
+ */
+
+import type { MappedString } from '@quarto/mapped-string';
+import type { SerializableSourceInfo } from './source-map.js';
+
+/**
+ * JSON value type (matching quarto-cli's JSONValue)
+ */
+export type JSONValue =
+  | string
+  | number
+  | boolean
+  | null
+  | JSONValue[]
+  | { [key: string]: JSONValue };
+
+/**
+ * AnnotatedParse structure (matching quarto-cli's interface)
+ */
+export interface AnnotatedParse {
+  start: number;
+  end: number;
+  result: JSONValue;
+  kind: string;
+  source: MappedString;
+  components: AnnotatedParse[];
+}
+
+/**
+ * MetaValue from quarto-markdown-pandoc JSON
+ */
+export interface JsonMetaValue {
+  t: string;      // Type: "MetaString", "MetaBool", "MetaInlines", "MetaBlocks", "MetaList", "MetaMap"
+  c?: unknown;    // Content (varies by type)
+  s: number;      // SourceInfo ID
+}
+
+/**
+ * MetaMap entry structure
+ */
+export interface MetaMapEntry {
+  key: string;
+  key_source: number;  // SourceInfo ID for key
+  value: JsonMetaValue;
+}
+
+/**
+ * Complete JSON output from quarto-markdown-pandoc
+ */
+export interface RustQmdJson {
+  meta: Record<string, JsonMetaValue>;
+  blocks: unknown[];  // Not used in metadata conversion
+  source_pool: SerializableSourceInfo[];
+  source_context: {
+    files: Array<{
+      id: number;
+      path: string;
+      content: string;
+    }>;
+  };
+}
diff --git a/ts-packages/rust-qmd-json/test/basic.test.ts b/ts-packages/rust-qmd-json/test/basic.test.ts
new file mode 100644
index 0000000..542ab7f
--- /dev/null
+++ b/ts-packages/rust-qmd-json/test/basic.test.ts
@@ -0,0 +1,62 @@
+/**
+ * Basic test to verify @quarto/mapped-string integration
+ */
+
+import { test } from 'node:test';
+import assert from 'node:assert';
+import { MappedString, asMappedString, mappedSubstring } from '@quarto/mapped-string';
+
+test('can import and use @quarto/mapped-string', () => {
+  const str = asMappedString("Hello, World!", "test.txt");
+
+  assert.strictEqual(str.value, "Hello, World!");
+  assert.strictEqual(str.fileName, "test.txt");
+
+  // Test that map function works
+  const result = str.map(0);
+  assert.ok(result !== undefined);
+  assert.strictEqual(result.index, 0);
+  assert.strictEqual(result.originalString, str);
+});
+
+test('can create mapped substrings', () => {
+  const original = asMappedString("Hello, World!", "test.txt");
+  const sub = mappedSubstring(original, 7, 12);
+
+  assert.strictEqual(sub.value, "World");
+  assert.strictEqual(sub.fileName, "test.txt");
+
+  // Map through the substring
+  const result = sub.map(0);
+  assert.ok(result !== undefined);
+  assert.strictEqual(result.index, 7); // Should map to offset 7 in original
+});
+
+test('can convert complete JSON to AnnotatedParse', async () => {
+  const { parseRustQmdMetadata } = await import('../src/index.js');
+
+  const json = {
+    meta: {
+      title: { t: 'MetaString', c: 'Hello World', s: 0 },
+      author: { t: 'MetaString', c: 'Alice', s: 1 }
+    },
+    blocks: [],
+    source_pool: [
+      { r: [11, 22], t: 0, d: 0 },  // "Hello World"
+      { r: [31, 36], t: 0, d: 0 }   // "Alice"
+    ],
+    source_context: {
+      files: [
+        { id: 0, path: 'test.qmd', content: '---\ntitle: Hello World\nauthor: Alice\n---' }
+      ]
+    }
+  };
+
+  const result = parseRustQmdMetadata(json);
+
+  assert.strictEqual(result.kind, 'mapping');
+  assert.strictEqual(typeof result.result, 'object');
+  assert.strictEqual((result.result as any).title, 'Hello World');
+  assert.strictEqual((result.result as any).author, 'Alice');
+  assert.strictEqual(result.components.length, 4);  // title key, title value, author key, author value
+});
diff --git a/ts-packages/rust-qmd-json/test/meta-conversion.test.ts b/ts-packages/rust-qmd-json/test/meta-conversion.test.ts
new file mode 100644
index 0000000..46e6849
--- /dev/null
+++ b/ts-packages/rust-qmd-json/test/meta-conversion.test.ts
@@ -0,0 +1,320 @@
+/**
+ * Tests for Metadata Conversion
+ */
+
+import { strict as assert } from 'assert';
+import { SourceInfoReconstructor, SourceContext, SerializableSourceInfo } from '../src/source-map.js';
+import { MetadataConverter } from '../src/meta-converter.js';
+import type { JsonMetaValue } from '../src/types.js';
+
+console.log('Running Metadata conversion tests...');
+
+// Test 1: Simple string metadata
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\ntitle: Hello\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [11, 16], t: 0, d: 0 }  // "Hello"
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaString',
+    c: 'Hello',
+    s: 0
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.result, 'Hello');
+  assert.equal(result.kind, 'MetaString');
+  assert.equal(result.source.value, 'Hello');
+  assert.equal(result.components.length, 0);
+  assert.equal(result.start, 11);
+  assert.equal(result.end, 16);
+
+  console.log('✔ Simple string metadata works');
+}
+
+// Test 2: Boolean metadata
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\ntoc: true\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [9, 13], t: 0, d: 0 }  // "true"
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaBool',
+    c: true,
+    s: 0
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.result, true);
+  assert.equal(result.kind, 'MetaBool');
+  assert.equal(result.components.length, 0);
+
+  console.log('✔ Boolean metadata works');
+}
+
+// Test 3: Markdown in metadata (MetaInlines)
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\ntitle: My **Document**\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 5], t: 0, d: 0 },    // "My"
+    { r: [6, 7], t: 0, d: 0 },    // " "
+    { r: [8, 16], t: 0, d: 0 },   // "Document"
+    { r: [0, 30], t: 0, d: 0 }    // Whole value
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaInlines',
+    c: [
+      { t: 'Str', c: 'My', s: 0 },
+      { t: 'Space', s: 1 },
+      { t: 'Strong', c: [{ t: 'Str', c: 'Document', s: 2 }], s: 3 }
+    ],
+    s: 3
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.kind, 'MetaInlines');
+  assert.ok(Array.isArray(result.result));
+  assert.equal((result.result as any).length, 3);
+  assert.equal(result.components.length, 0);  // Empty - cannot track internal locations yet
+
+  console.log('✔ Markdown in metadata (MetaInlines) works');
+}
+
+// Test 4: MetaList
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\nauthor: [Alice, Bob]\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [17, 22], t: 0, d: 0 },  // "Alice"
+    { r: [24, 27], t: 0, d: 0 },  // "Bob"
+    { r: [16, 28], t: 0, d: 0 }   // Whole list
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaList',
+    c: [
+      { t: 'MetaString', c: 'Alice', s: 0 },
+      { t: 'MetaString', c: 'Bob', s: 1 }
+    ],
+    s: 2
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.kind, 'MetaList');
+  assert.ok(Array.isArray(result.result));
+  assert.equal((result.result as any[]).length, 2);
+  assert.equal((result.result as any[])[0], 'Alice');
+  assert.equal((result.result as any[])[1], 'Bob');
+  assert.equal(result.components.length, 2);  // Non-empty - contains child AnnotatedParse
+  assert.equal(result.components[0].result, 'Alice');
+  assert.equal(result.components[1].result, 'Bob');
+
+  console.log('✔ MetaList works');
+}
+
+// Test 5: MetaMap
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\nauthor:\n  name: Alice\n  email: alice@example.com\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [18, 23], t: 0, d: 0 },  // "name" key
+    { r: [25, 30], t: 0, d: 0 },  // "Alice"
+    { r: [33, 38], t: 0, d: 0 },  // "email" key
+    { r: [40, 58], t: 0, d: 0 },  // "alice@example.com"
+    { r: [16, 59], t: 0, d: 0 }   // Whole map
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaMap',
+    c: {
+      entries: [
+        { key: 'name', key_source: 0, value: { t: 'MetaString', c: 'Alice', s: 1 } },
+        { key: 'email', key_source: 2, value: { t: 'MetaString', c: 'alice@example.com', s: 3 } }
+      ]
+    },
+    s: 4
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.kind, 'MetaMap');
+  assert.equal(typeof result.result, 'object');
+  assert.equal((result.result as any).name, 'Alice');
+  assert.equal((result.result as any).email, 'alice@example.com');
+  assert.equal(result.components.length, 4);  // Interleaved key/value pairs
+  assert.equal(result.components[0].kind, 'key');
+  assert.equal(result.components[0].result, 'name');
+  assert.equal(result.components[1].result, 'Alice');
+  assert.equal(result.components[2].kind, 'key');
+  assert.equal(result.components[2].result, 'email');
+  assert.equal(result.components[3].result, 'alice@example.com');
+
+  console.log('✔ MetaMap works');
+}
+
+// Test 6: Tagged YAML value (!expr)
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\ncompute: !expr x + 1\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [17, 22], t: 0, d: 0 },  // "x + 1"
+    { r: [17, 22], t: 0, d: 0 }   // Whole value
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaInlines',
+    c: [
+      {
+        t: 'Span',
+        c: [
+          { t: '', c: ['yaml-tagged-string'], kv: [['tag', 'expr']] },
+          [{ t: 'Str', c: 'x + 1', s: 0 }]
+        ],
+        s: 1
+      }
+    ],
+    s: 1
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.kind, 'MetaInlines:tagged:expr');
+  assert.ok(Array.isArray(result.result));
+
+  console.log('✔ Tagged YAML value (!expr) works');
+}
+
+// Test 7: Top-level metadata conversion
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\ntitle: Hello\nauthor: Alice\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [11, 16], t: 0, d: 0 },  // "Hello"
+    { r: [25, 30], t: 0, d: 0 }   // "Alice"
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const jsonMeta: Record<string, JsonMetaValue> = {
+    title: { t: 'MetaString', c: 'Hello', s: 0 },
+    author: { t: 'MetaString', c: 'Alice', s: 1 }
+  };
+
+  const result = converter.convertMeta(jsonMeta);
+
+  assert.equal(result.kind, 'mapping');
+  assert.equal(typeof result.result, 'object');
+  assert.equal((result.result as any).title, 'Hello');
+  assert.equal((result.result as any).author, 'Alice');
+  assert.equal(result.components.length, 4);  // Interleaved: title key, title value, author key, author value
+
+  console.log('✔ Top-level metadata conversion works');
+}
+
+// Test 8: Nested structures
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\nauthor:\n  - name: Alice\n    email: alice@example.com\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [20, 25], t: 0, d: 0 },   // "name" key
+    { r: [27, 32], t: 0, d: 0 },   // "Alice"
+    { r: [37, 42], t: 0, d: 0 },   // "email" key
+    { r: [44, 62], t: 0, d: 0 },   // "alice@example.com"
+    { r: [18, 63], t: 0, d: 0 },   // Whole map
+    { r: [16, 64], t: 0, d: 0 }    // Whole list
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaList',
+    c: [
+      {
+        t: 'MetaMap',
+        c: {
+          entries: [
+            { key: 'name', key_source: 0, value: { t: 'MetaString', c: 'Alice', s: 1 } },
+            { key: 'email', key_source: 2, value: { t: 'MetaString', c: 'alice@example.com', s: 3 } }
+          ]
+        },
+        s: 4
+      }
+    ],
+    s: 5
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.kind, 'MetaList');
+  assert.ok(Array.isArray(result.result));
+  assert.equal((result.result as any[]).length, 1);
+  assert.equal(typeof (result.result as any[])[0], 'object');
+  assert.equal((result.result as any[])[0].name, 'Alice');
+  assert.equal((result.result as any[])[0].email, 'alice@example.com');
+
+  console.log('✔ Nested structures work');
+}
+
+console.log('\nAll Metadata conversion tests passed! ✨');
diff --git a/ts-packages/rust-qmd-json/test/source-map.test.ts b/ts-packages/rust-qmd-json/test/source-map.test.ts
new file mode 100644
index 0000000..66de198
--- /dev/null
+++ b/ts-packages/rust-qmd-json/test/source-map.test.ts
@@ -0,0 +1,229 @@
+/**
+ * Tests for SourceInfo reconstruction
+ */
+
+import { strict as assert } from 'assert';
+import { SourceInfoReconstructor, SerializableSourceInfo, SourceContext } from '../src/source-map.js';
+
+console.log('Running SourceInfo reconstruction tests...');
+
+// Test 1: Original SourceInfo type
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 5], t: 0, d: 0 }  // "Hello"
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const mapped = reconstructor.toMappedString(0);
+
+  assert.equal(mapped.value, 'Hello');
+  assert.equal(mapped.fileName, 'test.qmd');
+  console.log('✔ Original SourceInfo type works');
+}
+
+// Test 2: Substring SourceInfo type
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 11], t: 0, d: 0 },   // "Hello World" (Original)
+    { r: [6, 11], t: 1, d: 0 }    // "World" (Substring of Original)
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const mapped = reconstructor.toMappedString(1);
+
+  assert.equal(mapped.value, 'World');
+  assert.equal(mapped.fileName, 'test.qmd');
+  console.log('✔ Substring SourceInfo type works');
+}
+
+// Test 3: Nested Substring (Substring of Substring)
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 11], t: 0, d: 0 },   // "Hello World" (Original)
+    { r: [6, 11], t: 1, d: 0 },   // "World" (Substring of Original)
+    { r: [0, 3], t: 1, d: 1 }     // "Wor" (Substring of "World")
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const mapped = reconstructor.toMappedString(2);
+
+  assert.equal(mapped.value, 'Wor');
+  assert.equal(mapped.fileName, 'test.qmd');
+  console.log('✔ Nested Substring works');
+}
+
+// Test 4: Concat SourceInfo type
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 5], t: 0, d: 0 },    // "Hello" (Original)
+    { r: [6, 11], t: 0, d: 0 },   // "World" (Original)
+    {
+      r: [0, 10],
+      t: 2,
+      d: { pieces: [[0, 0, 5], [1, 0, 5]] }  // Concat "Hello" + "World"
+    }
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const mapped = reconstructor.toMappedString(2);
+
+  assert.equal(mapped.value, 'HelloWorld');
+  // Note: mappedConcat may not preserve fileName, so we just check that it exists
+  // The important part is that the value is correct
+  console.log('✔ Concat SourceInfo type works');
+}
+
+// Test 5: getOffsets method
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 5], t: 0, d: 0 },
+    { r: [6, 11], t: 0, d: 0 }
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+
+  const [start1, end1] = reconstructor.getOffsets(0);
+  assert.equal(start1, 0);
+  assert.equal(end1, 5);
+
+  const [start2, end2] = reconstructor.getOffsets(1);
+  assert.equal(start2, 6);
+  assert.equal(end2, 11);
+
+  console.log('✔ getOffsets method works');
+}
+
+// Test 6: Error handling - invalid ID
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 5], t: 0, d: 0 }
+  ];
+
+  let errorCalled = false;
+  const errorHandler = (msg: string, id?: number) => {
+    errorCalled = true;
+    assert.equal(id, 999);
+    assert.ok(msg.includes('Invalid SourceInfo ID'));
+  };
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext, errorHandler);
+  const mapped = reconstructor.toMappedString(999);
+
+  assert.ok(errorCalled);
+  assert.equal(mapped.value, ''); // Fallback to empty string
+  console.log('✔ Error handling for invalid ID works');
+}
+
+// Test 7: Error handling - missing file
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 5], t: 0, d: 999 }  // File ID 999 doesn't exist
+  ];
+
+  let errorCalled = false;
+  const errorHandler = (msg: string, id?: number) => {
+    errorCalled = true;
+    assert.ok(msg.includes('File ID 999 not found'));
+  };
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext, errorHandler);
+  const mapped = reconstructor.toMappedString(0);
+
+  assert.ok(errorCalled);
+  assert.equal(mapped.value, ''); // Fallback to empty string
+  console.log('✔ Error handling for missing file works');
+}
+
+// Test 8: Caching behavior
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 5], t: 0, d: 0 }
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+
+  const mapped1 = reconstructor.toMappedString(0);
+  const mapped2 = reconstructor.toMappedString(0);
+
+  // Should return the same cached object
+  assert.equal(mapped1, mapped2);
+  console.log('✔ Caching works correctly');
+}
+
+// Test 9: MappedString source mapping
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\ntitle: My **Document**\n---' }
+    ]
+  };
+
+  // Simulate "My **Document**" at offset 11-26
+  const pool: SerializableSourceInfo[] = [
+    { r: [11, 26], t: 0, d: 0 }
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const mapped = reconstructor.toMappedString(0);
+
+  assert.equal(mapped.value, 'My **Document**');
+
+  // Test that map function exists and can be called
+  // Note: The actual line/column mapping depends on how asMappedString
+  // creates the mapping, which may need additional context
+  const mapResult = mapped.map(0);
+  // Just verify that mapping exists and returns something
+  // The actual line/column values depend on the MappedString implementation
+
+  console.log('✔ MappedString source mapping works');
+}
+
+console.log('\nAll SourceInfo reconstruction tests passed! ✨');
diff --git a/ts-packages/rust-qmd-json/test/type-safety.test.ts b/ts-packages/rust-qmd-json/test/type-safety.test.ts
new file mode 100644
index 0000000..e5cb68d
--- /dev/null
+++ b/ts-packages/rust-qmd-json/test/type-safety.test.ts
@@ -0,0 +1,206 @@
+/**
+ * Tests for runtime type safety
+ */
+
+import { strict as assert } from 'assert';
+import { SourceInfoReconstructor, SourceContext, SerializableSourceInfo } from '../src/source-map.js';
+import { MetadataConverter } from '../src/meta-converter.js';
+import type { JsonMetaValue } from '../src/types.js';
+
+console.log('Running type safety tests...');
+
+// Test 1: Invalid Original SourceInfo data type
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 5], t: 0, d: 'invalid' as unknown }  // Should be number
+  ];
+
+  let errorCalled = false;
+  const errorHandler = (msg: string) => {
+    errorCalled = true;
+    assert.ok(msg.includes('must be a number'));
+  };
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext, errorHandler);
+  const mapped = reconstructor.toMappedString(0);
+
+  assert.ok(errorCalled);
+  assert.equal(mapped.value, '');  // Fallback
+  console.log('✔ Invalid Original data type caught');
+}
+
+// Test 2: Invalid Substring SourceInfo data type
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 11], t: 0, d: 0 },
+    { r: [6, 11], t: 1, d: { invalid: 'object' } as unknown }  // Should be number
+  ];
+
+  let errorCalled = false;
+  const errorHandler = (msg: string) => {
+    errorCalled = true;
+    assert.ok(msg.includes('must be a number'));
+  };
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext, errorHandler);
+  const mapped = reconstructor.toMappedString(1);
+
+  assert.ok(errorCalled);
+  assert.equal(mapped.value, '');  // Fallback
+  console.log('✔ Invalid Substring data type caught');
+}
+
+// Test 3: Invalid Concat SourceInfo data type
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: 'Hello World' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [0, 5], t: 0, d: 0 },
+    { r: [0, 10], t: 2, d: 'not-an-object' as unknown }  // Should be {pieces: [...]}
+  ];
+
+  let errorCalled = false;
+  const errorHandler = (msg: string) => {
+    errorCalled = true;
+    assert.ok(msg.includes('Invalid Concat data format'));
+  };
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext, errorHandler);
+  const mapped = reconstructor.toMappedString(1);
+
+  assert.ok(errorCalled);
+  assert.equal(mapped.value, '');  // Fallback
+  console.log('✔ Invalid Concat data type caught');
+}
+
+// Test 4: Invalid MetaList content type
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\nauthor: invalid\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [16, 23], t: 0, d: 0 }
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaList',
+    c: 'not-an-array' as unknown,  // Should be array
+    s: 0
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.kind, 'MetaList');
+  assert.deepEqual(result.result, []);  // Fallback to empty array
+  assert.equal(result.components.length, 0);
+  console.log('✔ Invalid MetaList content type handled gracefully');
+}
+
+// Test 5: Invalid MetaMap content type
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\nauthor: invalid\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [16, 23], t: 0, d: 0 }
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaMap',
+    c: 'not-an-object' as unknown,  // Should be {entries: [...]}
+    s: 0
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.kind, 'MetaMap');
+  assert.deepEqual(result.result, {});  // Fallback to empty object
+  assert.equal(result.components.length, 0);
+  console.log('✔ Invalid MetaMap content type handled gracefully');
+}
+
+// Test 6: Non-string MetaString content
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\ntitle: 123\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [11, 14], t: 0, d: 0 }
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaString',
+    c: 123 as unknown,  // Number instead of string
+    s: 0
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.kind, 'MetaString');
+  assert.equal(result.result, '123');  // Converted to string
+  console.log('✔ Non-string MetaString content converted');
+}
+
+// Test 7: Non-boolean MetaBool content
+{
+  const sourceContext: SourceContext = {
+    files: [
+      { id: 0, path: 'test.qmd', content: '---\ntoc: 1\n---' }
+    ]
+  };
+
+  const pool: SerializableSourceInfo[] = [
+    { r: [9, 10], t: 0, d: 0 }
+  ];
+
+  const reconstructor = new SourceInfoReconstructor(pool, sourceContext);
+  const converter = new MetadataConverter(reconstructor);
+
+  const metaValue: JsonMetaValue = {
+    t: 'MetaBool',
+    c: 1 as unknown,  // Number instead of boolean
+    s: 0
+  };
+
+  const result = converter.convertMetaValue(metaValue);
+
+  assert.equal(result.kind, 'MetaBool');
+  assert.equal(result.result, true);  // Converted to boolean
+  console.log('✔ Non-boolean MetaBool content converted');
+}
+
+console.log('\nAll type safety tests passed! ✨');
diff --git a/ts-packages/rust-qmd-json/tsconfig.json b/ts-packages/rust-qmd-json/tsconfig.json
new file mode 100644
index 0000000..4553cf1
--- /dev/null
+++ b/ts-packages/rust-qmd-json/tsconfig.json
@@ -0,0 +1,20 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ES2022",
+    "lib": ["ES2022"],
+    "moduleResolution": "node",
+    "outDir": "./dist",
+    "rootDir": "./src",
+    "strict": true,
+    "esModuleInterop": true,
+    "skipLibCheck": true,
+    "forceConsistentCasingInFileNames": true,
+    "declaration": true,
+    "declarationMap": true,
+    "sourceMap": true,
+    "resolveJsonModule": true
+  },
+  "include": ["src/**/*"],
+  "exclude": ["node_modules", "dist", "test"]
+}