diff --git a/crates/codegraph-core/src/domain/graph/builder/stages/build_edges.rs b/crates/codegraph-core/src/domain/graph/builder/stages/build_edges.rs
index 910203e6..8eb1d7f4 100644
--- a/crates/codegraph-core/src/domain/graph/builder/stages/build_edges.rs
+++ b/crates/codegraph-core/src/domain/graph/builder/stages/build_edges.rs
@@ -1342,9 +1342,13 @@ fn emit_receiver_edge(
         samefile_candidates
     } else {
         // Fall back to any cross-file class/struct/interface candidate.
+        // Cross-language candidates are never legitimate receiver targets
+        // (#1783) — a `new Foo()` in one language can't statically resolve to
+        // an unrelated same-named class in another. Mirrors JS resolveReceiverEdge.
         ctx.nodes_by_name.get(effective_receiver).cloned().unwrap_or_default()
             .into_iter()
-            .filter(|n| ctx.receiver_kinds.contains(n.kind.as_str()))
+            .filter(|n| ctx.receiver_kinds.contains(n.kind.as_str())
+                && resolve::is_same_language_family(rel_path, &n.file))
             .collect()
     };
 
@@ -2393,6 +2397,65 @@ mod call_edge_tests {
         );
     }
 
+    /// Issue #1783: the global (cross-file) receiver fallback had no
+    /// language-consistency check at all, so `Widget.render()` in a Python
+    /// caller with no same-file `Widget` definition could resolve to an
+    /// unrelated same-named class declared in a JS file purely by name.
+    #[test]
+    fn receiver_edge_rejects_cross_language_match() {
+        let all_nodes = vec![
+            node(1, "main",   "function", "widget.py", 3),
+            node(2, "Widget", "class",    "widget.js", 1),
+        ];
+
+        let files = vec![make_file(
+            "widget.py",
+            10,
+            vec![def("main", "function", 3, 8)],
+            vec![call("render", 7, Some("Widget"))],
+            vec![],
+            vec![],
+        )];
+
+        let edges = build_call_edges(files, all_nodes, vec![], MAX_SOLVER_ITERATIONS);
+
+        let receiver_edge = edges.iter().find(|e| e.kind == "receiver");
+        assert!(
+            receiver_edge.is_none(),
+            "a Python caller must not resolve a receiver edge to an unrelated same-named JS class; got: {:?}",
+            edges.iter().map(|e| (&e.kind, e.source_id, e.target_id)).collect::<Vec<_>>()
+        );
+    }
+
+    /// Same-language global receiver fallback must still work after the
+    /// #1783 language-scoping fix — only cross-language candidates are rejected.
+    #[test]
+    fn receiver_edge_still_resolves_same_language_cross_file_match() {
+        let all_nodes = vec![
+            node(1, "main",   "function", "widget.py", 3),
+            node(2, "Widget", "class",    "widget_impl.py", 1),
+        ];
+
+        let files = vec![make_file(
+            "widget.py",
+            10,
+            vec![def("main", "function", 3, 8)],
+            vec![call("render", 7, Some("Widget"))],
+            vec![],
+            vec![],
+        )];
+
+        let edges = build_call_edges(files, all_nodes, vec![], MAX_SOLVER_ITERATIONS);
+
+        let receiver_edge = edges.iter().find(|e| e.kind == "receiver");
+        assert!(
+            receiver_edge.is_some(),
+            "same-language cross-file receiver fallback must still resolve; got: {:?}",
+            edges.iter().map(|e| (&e.kind, e.source_id, e.target_id)).collect::<Vec<_>>()
+        );
+        assert_eq!(receiver_edge.unwrap().target_id, 2);
+    }
+
     /// Issue #1453: `this.logger.error()` inside `UserService.create` where the
     /// constructor seeded the class-scoped key `UserService.logger → Logger`.
     /// The resolver must fall back to the `ClassName.prop` typeMap key (#1323).
diff --git a/crates/codegraph-core/src/domain/graph/resolve.rs b/crates/codegraph-core/src/domain/graph/resolve.rs
index eb23378d..6cece574 100644
--- a/crates/codegraph-core/src/domain/graph/resolve.rs
+++ b/crates/codegraph-core/src/domain/graph/resolve.rs
@@ -3,6 +3,7 @@ use std::path::{Path, PathBuf};
 
 use rayon::prelude::*;
 
+use crate::domain::parser::LanguageKind;
 use crate::types::{ImportResolutionInput, PathAliases, ResolvedImport};
 
 /// Check file existence using known_files set when available, falling back to FS.
@@ -307,6 +308,53 @@ fn directory_distance(a: &str, b: &str) -> usize {
     dist
 }
 
+/// Coarse "language family" for a file, derived from its extension via
+/// `LanguageKind::from_extension`. Collapses TypeScript/Tsx into the same
+/// family as JavaScript: despite being distinct `LanguageKind` variants (one
+/// per tree-sitter grammar), `.ts`/`.tsx` files routinely import from and
+/// call into `.js` files and vice versa within the same project (this
+/// codebase's own `src/` tree does this throughout) — treating them as
+/// separate families would reject huge amounts of legitimate same-project
+/// resolution. Every other `LanguageKind` variant keeps its own family,
+/// preserving `from_extension`'s existing per-language extension groupings
+/// (e.g. C's `.c`+`.h`, C++'s `.cpp`/`.cc`/`.cxx`/`.hpp`) — EXCEPT `.h`,
+/// treated as ambiguous (returns `None`) rather than inheriting
+/// `from_extension`'s C-only mapping: `from_extension` needs one canonical
+/// grammar per extension, but a `.h` header is real-world ambiguous between
+/// C and C++, and the extremely common case of a `.cpp` file calling into
+/// its own project's `.h` header would otherwise be misclassified as
+/// cross-language and rejected outright — a real regression from the
+/// pre-#1783 same-directory score of 0.7 (Greptile review). This keeps the
+/// C/C++-header case working without merging C and C++ source-file families
+/// wholesale (`.c` vs `.cpp` intentionally do NOT merge — see
+/// is_same_language_family_does_not_merge_c_and_cpp).
+fn language_family(file: &str) -> Option<LanguageKind> {
+    if file.to_ascii_lowercase().ends_with(".h") {
+        return None;
+    }
+    match LanguageKind::from_extension(file) {
+        Some(LanguageKind::TypeScript) | Some(LanguageKind::Tsx) => Some(LanguageKind::JavaScript),
+        other => other,
+    }
+}
+
+/// True when `file_a` and `file_b` belong to the same language family, or
+/// when either extension is unrecognised (ambiguous cases are not rejected —
+/// they fall through to normal scoring). False only when both extensions are
+/// recognised AND resolve to different families.
+///
+/// Guards the global-by-name call-resolution fallback against matching a
+/// same-named symbol across unrelated languages — e.g. a Ruby file's bare
+/// `load` call has no static relationship to a same-named `load` export in a
+/// JS file, even when both happen to live in the same directory (issue
+/// #1783). Mirrors `isSameLanguageFamily()` in resolve.ts.
+pub fn is_same_language_family(file_a: &str, file_b: &str) -> bool {
+    match (language_family(file_a), language_family(file_b)) {
+        (Some(a), Some(b)) => a == b,
+        _ => true,
+    }
+}
+
 /// Compute proximity-based confidence for call resolution.
 /// Mirrors `computeConfidence()` in resolve.ts.
 pub fn compute_confidence(
@@ -325,6 +373,12 @@ pub fn compute_confidence(
             return 1.0;
         }
     }
+    // Cross-language candidates are never legitimate call targets (#1783) —
+    // reject before scoring proximity so a same-directory, same-named symbol
+    // in an unrelated language can never pass the resolver's 0.5 threshold.
+    if !is_same_language_family(caller_file, target_file) {
+        return 0.0;
+    }
 
     let caller_dir = Path::new(caller_file)
         .parent()
@@ -558,4 +612,95 @@ mod tests {
         // not a direct sibling pair despite sharing the `src` ancestor.
         assert_eq!(directory_distance("src/graph/algorithms", "src/features"), 3);
     }
+
+    // Regression tests for #1783: the global-by-name call-resolution fallback
+    // had no language-consistency check at all, so a bare-name call with no
+    // import/receiver match could resolve against a same-named symbol in a
+    // completely unrelated language — e.g. a Ruby file's builtin `Kernel#load`
+    // call matched a JS ESM loader hook's unrelated `load` export purely
+    // because both files sat in the same directory (confidence 0.7 from
+    // proximity alone, well above the resolver's 0.5 threshold).
+
+    #[test]
+    fn is_same_language_family_rejects_ruby_and_js() {
+        assert!(!is_same_language_family("tracer/ruby-tracer.rb", "tracer/loader-hooks.mjs"));
+    }
+
+    #[test]
+    fn is_same_language_family_rejects_python_and_go() {
+        assert!(!is_same_language_family("src/main.py", "src/main.go"));
+    }
+
+    #[test]
+    fn is_same_language_family_accepts_same_extension() {
+        assert!(is_same_language_family("src/a.rb", "lib/b.rb"));
+    }
+
+    #[test]
+    fn is_same_language_family_merges_javascript_and_typescript() {
+        assert!(is_same_language_family("src/a.ts", "src/b.js"));
+        assert!(is_same_language_family("src/a.tsx", "src/b.mjs"));
+        assert!(is_same_language_family("src/a.cjs", "src/b.jsx"));
+    }
+
+    #[test]
+    fn is_same_language_family_merges_c_source_and_header() {
+        assert!(is_same_language_family("src/a.c", "src/a.h"));
+    }
+
+    #[test]
+    fn is_same_language_family_treats_h_as_ambiguous_with_cpp() {
+        // Greptile follow-up to #1783: `.h` is real-world ambiguous between C
+        // and C++ (LANGUAGE_REGISTRY/from_extension assigns it to C alone for
+        // grammar-selection purposes), so a `.cpp` file calling into its own
+        // project's `.h` header must not be rejected as cross-language.
+        assert!(is_same_language_family("src/widget.cpp", "src/widget.h"));
+    }
+
+    #[test]
+    fn is_same_language_family_merges_cpp_source_and_header_variants() {
+        assert!(is_same_language_family("src/a.cpp", "src/a.hpp"));
+        assert!(is_same_language_family("src/a.cc", "src/a.cxx"));
+    }
+
+    #[test]
+    fn is_same_language_family_does_not_merge_c_and_cpp() {
+        assert!(!is_same_language_family("src/a.c", "src/a.cpp"));
+    }
+
+    #[test]
+    fn is_same_language_family_does_not_reject_unrecognised_extensions() {
+        // Ambiguous (unrecognised) extensions fall through rather than being rejected.
+        assert!(is_same_language_family("README", "src/b.rb"));
+        assert!(is_same_language_family("src/a.rb", "Makefile"));
+    }
+
+    #[test]
+    fn compute_confidence_rejects_cross_language_same_directory_match() {
+        // The exact #1783 repro shape: same directory, different languages.
+        let conf = compute_confidence(
+            "tests/benchmarks/resolution/tracer/ruby-tracer.rb",
+            "tests/benchmarks/resolution/tracer/loader-hooks.mjs",
+            None,
+        );
+        assert_eq!(conf, 0.0);
+    }
+
+    #[test]
+    fn compute_confidence_still_scores_same_language_same_directory_pair() {
+        let conf = compute_confidence(
+            "tests/benchmarks/resolution/tracer/ruby-tracer.rb",
+            "tests/benchmarks/resolution/tracer/other-tracer.rb",
+            None,
+        );
+        assert_eq!(conf, 0.7);
+    }
+
+    #[test]
+    fn compute_confidence_does_not_regress_same_project_js_ts_resolution() {
+        // A .ts caller resolving a same-directory .js target must be unaffected —
+        // TS/JS are one family despite being different LanguageKind variants.
+        let conf = compute_confidence("src/graph/a.ts", "src/graph/b.js", None);
+        assert_eq!(conf, 0.7);
+    }
 }
diff --git a/src/domain/graph/builder/call-resolver.ts b/src/domain/graph/builder/call-resolver.ts
index dde9fb23..fb97377f 100644
--- a/src/domain/graph/builder/call-resolver.ts
+++ b/src/domain/graph/builder/call-resolver.ts
@@ -9,7 +9,7 @@
  * `resolveByMethodOrGlobal` delegates its two branches to strategy helpers
  * in `../resolver/strategy.ts` to keep per-strategy complexity manageable.
  */
-import { computeConfidence } from '../resolve.js';
+import { computeConfidence, isSameLanguageFamily } from '../resolve.js';
 import {
   isModuleScopedLanguage,
   resolveByGlobal,
@@ -347,9 +347,15 @@ export function resolveReceiverEdge(
   const sameFileAll = lookup.byNameAndFile(effectiveReceiver, relPath);
   const isLocalDefinition = sameFileAll.length > 0 && !importedNames?.has(effectiveReceiver);
   const sameFileCandidates = sameFileAll.filter((n) => RECEIVER_KINDS.has(n.kind ?? ''));
+  // Cross-language candidates are never legitimate receiver targets (#1783) —
+  // a `new Foo()` in one language can't statically resolve to an unrelated
+  // same-named class in another. Only the global (cross-file) branch needs
+  // the check: sameFileCandidates are already scoped to relPath itself.
   const candidates = isLocalDefinition
     ? sameFileCandidates
-    : lookup.byName(effectiveReceiver).filter((n) => RECEIVER_KINDS.has(n.kind ?? ''));
+    : lookup
+        .byName(effectiveReceiver)
+        .filter((n) => RECEIVER_KINDS.has(n.kind ?? '') && isSameLanguageFamily(relPath, n.file));
   if (candidates.length === 0) return null;
   const recvTarget = candidates[0]!;
   const recvKey = `recv|${caller.id}|${recvTarget.id}`;
diff --git a/src/domain/graph/resolve.ts b/src/domain/graph/resolve.ts
index 98a07d75..d8e2a81b 100644
--- a/src/domain/graph/resolve.ts
+++ b/src/domain/graph/resolve.ts
@@ -5,6 +5,7 @@ import { loadNative } from '../../infrastructure/native.js';
 import { normalizePath } from '../../shared/constants.js';
 import { toErrorMessage } from '../../shared/errors.js';
 import type { BareSpecifier, BatchResolvedMap, ImportBatchItem, PathAliases } from '../../types.js';
+import { LANGUAGE_REGISTRY } from '../parser.js';
 
 // ── package.json exports resolution ─────────────────────────────────
 
@@ -518,6 +519,87 @@ function directoryDistance(a: string, b: string): number {
   return dist;
 }
 
+// ── Language-family scoping for global-by-name fallback resolution ─────────
+
+/**
+ * LANGUAGE_REGISTRY ids that must be treated as one family for cross-file
+ * call resolution. TypeScript/TSX compile to and interoperate directly with
+ * JavaScript — a `.ts` file routinely imports from and calls into a `.js`
+ * file and vice versa (this codebase's own `src/` tree does this
+ * throughout) — despite being three separate grammar entries in
+ * LANGUAGE_REGISTRY. Every other registry id keeps its own family, which
+ * preserves LANGUAGE_REGISTRY's existing per-language extension groupings
+ * (e.g. C's `.c`+`.h`, C++'s `.cpp`/`.cc`/`.cxx`/`.hpp`).
+ */
+const JS_FAMILY_REGISTRY_IDS = new Set(['javascript', 'typescript', 'tsx']);
+
+/**
+ * Extensions excluded from the family map entirely, so `languageFamily`
+ * returns null for them and they fall through to the ambiguous-extension
+ * path (ordinary distance-based scoring, never rejected outright).
+ *
+ * `.h` is real-world ambiguous between C and C++: LANGUAGE_REGISTRY assigns
+ * it to the `c` entry alone (grammar selection needs one canonical parser),
+ * but the extremely common case of a `.cpp` file calling into its own
+ * project's `.h` header would otherwise be misclassified as cross-language
+ * and rejected outright (confidence 0) — a real regression from the
+ * pre-#1783 same-directory score of 0.7 (Greptile review). Treating `.h` as
+ * ambiguous — like an unrecognised extension — keeps the C/C++-header case
+ * working without merging C and C++ source-file families wholesale (`.c`
+ * vs `.cpp` intentionally do NOT merge — see
+ * is_same_language_family_does_not_merge_c_and_cpp).
+ */
+const AMBIGUOUS_EXTENSIONS = new Set(['.h']);
+
+/**
+ * extension → language-family lookup, derived from LANGUAGE_REGISTRY (the
+ * single source of truth for language definitions) so newly-added languages
+ * are automatically covered without a second hand-maintained extension list.
+ */
+const _extToLanguageFamily: Map<string, string> = new Map();
+for (const entry of LANGUAGE_REGISTRY) {
+  const family = JS_FAMILY_REGISTRY_IDS.has(entry.id) ? 'javascript' : entry.id;
+  for (const ext of entry.extensions) {
+    if (AMBIGUOUS_EXTENSIONS.has(ext)) continue;
+    _extToLanguageFamily.set(ext, family);
+  }
+}
+
+/**
+ * Resolve a file's coarse language family from its extension. Returns null
+ * for extensionless or unrecognised files so ambiguous cases fall through to
+ * ordinary distance-based scoring rather than being rejected outright.
+ */
+function languageFamily(file: string): string | null {
+  const dot = file.lastIndexOf('.');
+  if (dot === -1) return null;
+  return _extToLanguageFamily.get(file.slice(dot).toLowerCase()) ?? null;
+}
+
+/**
+ * True when `fileA` and `fileB` belong to the same language family, or when
+ * either extension is unrecognised (ambiguous cases are not rejected — they
+ * fall through to normal scoring). False only when both extensions are
+ * recognised AND resolve to different families.
+ *
+ * Guards the global-by-name call-resolution fallback against matching a
+ * same-named symbol across unrelated languages — e.g. a Ruby file's bare
+ * `load` call has no static relationship to a same-named `load` export in a
+ * JS file, even when both happen to live in the same directory (issue
+ * #1783). This codebase has no cross-language static-call mechanism its
+ * resolvers legitimately model (the `dead-ffi` role classifier only
+ * suppresses false dead-code flags for compiled-language files consumed via
+ * FFI — it never creates call edges), so rejecting cross-family candidates
+ * is a strict precision improvement with no legitimate resolution to
+ * regress.
+ */
+export function isSameLanguageFamily(fileA: string, fileB: string): boolean {
+  const famA = languageFamily(fileA);
+  const famB = languageFamily(fileB);
+  if (!famA || !famB) return true;
+  return famA === famB;
+}
+
 function computeConfidenceJS(
   callerFile: string,
   targetFile: string,
@@ -528,6 +610,10 @@ function computeConfidenceJS(
   if (importedFrom === targetFile) return 1.0;
   // Workspace-resolved imports get high confidence even across package boundaries
   if (importedFrom && _workspaceResolvedPaths.has(importedFrom)) return 0.95;
+  // Cross-language candidates are never legitimate call targets (#1783) — reject
+  // before scoring proximity so a same-directory, same-named symbol in an
+  // unrelated language can never pass the resolver's 0.5 confidence threshold.
+  if (!isSameLanguageFamily(callerFile, targetFile)) return 0;
   const dist = directoryDistance(path.dirname(callerFile), path.dirname(targetFile));
   if (dist === 0) return 0.7; // same directory
   if (dist === 1) return 0.6; // direct parent/child directory
diff --git a/tests/unit/call-resolver.test.ts b/tests/unit/call-resolver.test.ts
index a50a1045..0ee65d17 100644
--- a/tests/unit/call-resolver.test.ts
+++ b/tests/unit/call-resolver.test.ts
@@ -213,6 +213,50 @@ describe('resolveByMethodOrGlobal — bare-call JS/TS module-scope guard (#1407)
   });
 });
 
+describe('resolveByMethodOrGlobal — cross-language global fallback rejection (#1783)', () => {
+  // Mirrors the #1783 repro: ruby-tracer.rb's bare `Kernel#load` call has no
+  // static relationship to loader-hooks.mjs's unrelated `load` export, even
+  // though both files live in the same directory (which would otherwise
+  // score confidence 0.7 — well above the resolver's 0.5 threshold).
+  it('does not resolve a bare call to a same-directory, same-named symbol in a different language', () => {
+    const jsExport = { id: 1, file: 'tracer/loader-hooks.mjs', kind: 'function' };
+    const lookup = makeLookup({ load: [jsExport] });
+    const result = resolveByMethodOrGlobal(
+      lookup,
+      { name: 'load', receiver: null },
+      'tracer/ruby-tracer.rb',
+      new Map(),
+    );
+    expect(result).toEqual([]);
+  });
+
+  it('still resolves a bare call to a same-directory, same-named symbol in the SAME language', () => {
+    const rbTarget = { id: 2, file: 'tracer/other-tracer.rb', kind: 'function' };
+    const lookup = makeLookup({ load: [rbTarget] });
+    const result = resolveByMethodOrGlobal(
+      lookup,
+      { name: 'load', receiver: null },
+      'tracer/ruby-tracer.rb',
+      new Map(),
+    );
+    expect(result).toEqual([rbTarget]);
+  });
+
+  it('does not resolve a typed-method lookup to a same-named type in a different language', () => {
+    // receiver 'w' typed to 'Widget' via typeMap; a same-directory JS 'Widget.render'
+    // method must not satisfy a Python caller's 'w.render()' call.
+    const jsMethod = { id: 3, file: 'lib/widget.js', kind: 'method' };
+    const lookup = makeLookup({ 'Widget.render': [jsMethod] });
+    const result = resolveByMethodOrGlobal(
+      lookup,
+      { name: 'render', receiver: 'w' },
+      'lib/widget.py',
+      new Map([['w', 'Widget']]),
+    );
+    expect(result).toEqual([]);
+  });
+});
+
 // ── resolveReceiverEdge ──────────────────────────────────────────────────────
 
 /**
@@ -290,6 +334,44 @@ describe('resolveReceiverEdge — local function constructor blocks global class
   });
 });
 
+describe('resolveReceiverEdge — cross-language global fallback rejection (#1783)', () => {
+  // The global (cross-file) receiver-resolution branch used no confidence or
+  // language check at all, so `new Widget()` in one language could resolve
+  // to an unrelated same-named class declared in a completely different
+  // language. Only the global branch needs the check — sameFileCandidates
+  // are already scoped to the caller's own file (trivially same-language).
+  it('does not resolve a receiver to a same-named class in a different language', () => {
+    const jsClass = { id: 1, file: 'lib/Widget.js', kind: 'class' };
+    const lookup = makeReceiverLookup({}, { Widget: [jsClass] });
+    const result = resolveReceiverEdge(
+      lookup,
+      { name: 'render', receiver: 'Widget' },
+      { id: 99 },
+      'lib/widget.py',
+      new Map(),
+      new Set(),
+      new Map(),
+    );
+    expect(result).toBeNull();
+  });
+
+  it('still resolves a receiver to a same-named class in the SAME language', () => {
+    const pyClass = { id: 2, file: 'lib/widget_impl.py', kind: 'class' };
+    const lookup = makeReceiverLookup({}, { Widget: [pyClass] });
+    const result = resolveReceiverEdge(
+      lookup,
+      { name: 'render', receiver: 'Widget' },
+      { id: 99 },
+      'lib/widget.py',
+      new Map(),
+      new Set(),
+      new Map(),
+    );
+    expect(result).not.toBeNull();
+    expect(result?.receiverId).toBe(2);
+  });
+});
+
 // ── resolveDefinePropertyAccessorTarget ───────────────────────────────────
 
 /**
diff --git a/tests/unit/resolve.test.ts b/tests/unit/resolve.test.ts
index 7ad38e4d..2f9a6a56 100644
--- a/tests/unit/resolve.test.ts
+++ b/tests/unit/resolve.test.ts
@@ -15,6 +15,7 @@ import {
   computeConfidence,
   computeConfidenceJS,
   convertAliasesForNative,
+  isSameLanguageFamily,
   isWorkspaceResolved,
   parseBareSpecifier,
   resolveImportPathJS,
@@ -728,3 +729,97 @@ describe('computeConfidenceJS workspace confidence', () => {
     expect(conf).toBeLessThan(0.95);
   });
 });
+
+// ─── Cross-language fallback rejection (#1783) ───────────────────────
+
+// Regression tests for #1783: the global-by-name call-resolution fallback
+// had no language-consistency check at all, so a bare-name call with no
+// import/receiver match could resolve against a same-named symbol in a
+// completely unrelated language — e.g. a Ruby file's builtin `Kernel#load`
+// call matched a JS ESM loader hook's unrelated `load` export purely because
+// both files sat in the same directory (confidence 0.7 from proximity alone).
+describe('isSameLanguageFamily (#1783)', () => {
+  it('returns false for a Ruby file and a JS file', () => {
+    expect(isSameLanguageFamily('tracer/ruby-tracer.rb', 'tracer/loader-hooks.mjs')).toBe(false);
+  });
+
+  it('returns false for a Python file and a Go file', () => {
+    expect(isSameLanguageFamily('src/main.py', 'src/main.go')).toBe(false);
+  });
+
+  it('returns true for two files with the same extension', () => {
+    expect(isSameLanguageFamily('src/a.rb', 'lib/b.rb')).toBe(true);
+  });
+
+  it('treats JavaScript and TypeScript as the same family', () => {
+    expect(isSameLanguageFamily('src/a.ts', 'src/b.js')).toBe(true);
+    expect(isSameLanguageFamily('src/a.tsx', 'src/b.mjs')).toBe(true);
+    expect(isSameLanguageFamily('src/a.cjs', 'src/b.jsx')).toBe(true);
+  });
+
+  it('treats C and its own header extension as the same family', () => {
+    expect(isSameLanguageFamily('src/a.c', 'src/a.h')).toBe(true);
+  });
+
+  it('treats .h as ambiguous with C++ (Greptile follow-up)', () => {
+    // `.h` is real-world ambiguous between C and C++ (LANGUAGE_REGISTRY
+    // assigns it to C alone for grammar-selection purposes), so a `.cpp`
+    // file calling into its own project's `.h` header must not be rejected
+    // as cross-language.
+    expect(isSameLanguageFamily('src/widget.cpp', 'src/widget.h')).toBe(true);
+  });
+
+  it('treats C++ source and header extensions as the same family', () => {
+    expect(isSameLanguageFamily('src/a.cpp', 'src/a.hpp')).toBe(true);
+    expect(isSameLanguageFamily('src/a.cc', 'src/a.cxx')).toBe(true);
+  });
+
+  it('does not treat C and C++ as the same family', () => {
+    expect(isSameLanguageFamily('src/a.c', 'src/a.cpp')).toBe(false);
+  });
+
+  it('returns true (does not reject) when either extension is unrecognised', () => {
+    expect(isSameLanguageFamily('README', 'src/b.rb')).toBe(true);
+    expect(isSameLanguageFamily('src/a.rb', 'Makefile')).toBe(true);
+  });
+});
+
+describe('computeConfidenceJS — cross-language rejection (#1783)', () => {
+  it('returns 0 for same-directory files in different languages (the #1783 repro shape)', () => {
+    // ruby-tracer.rb's bare `load` call must never match loader-hooks.mjs's
+    // `load` export just because both files live in the same directory.
+    const conf = computeConfidenceJS(
+      'tests/benchmarks/resolution/tracer/ruby-tracer.rb',
+      'tests/benchmarks/resolution/tracer/loader-hooks.mjs',
+      undefined,
+    );
+    expect(conf).toBe(0);
+  });
+
+  it('still returns same-directory confidence for a same-language pair', () => {
+    const conf = computeConfidenceJS(
+      'tests/benchmarks/resolution/tracer/ruby-tracer.rb',
+      'tests/benchmarks/resolution/tracer/other-tracer.rb',
+      undefined,
+    );
+    expect(conf).toBe(0.7);
+  });
+
+  it('does not regress same-project JS/TS cross-file resolution', () => {
+    // A .ts caller resolving a same-directory .js target must be unaffected —
+    // TS/JS are one family despite being different LANGUAGE_REGISTRY entries.
+    const conf = computeConfidenceJS('src/graph/a.ts', 'src/graph/b.js', undefined);
+    expect(conf).toBe(0.7);
+  });
+
+  it('rejects a cross-language match even when same-file/importedFrom shortcuts do not apply', () => {
+    const conf = computeConfidenceJS('src/main.py', 'src/main.go', undefined);
+    expect(conf).toBeLessThan(0.5);
+    expect(conf).toBe(0);
+  });
+
+  it('does not reject when the target extension is unrecognised (falls through to distance scoring)', () => {
+    const conf = computeConfidenceJS('src/a.rb', 'src/README', undefined);
+    expect(conf).toBeGreaterThan(0);
+  });
+});