diff --git a/sast-engine/graph/callgraph/builder/c_builder.go b/sast-engine/graph/callgraph/builder/c_builder.go
index 1cf58a8c..02970623 100644
--- a/sast-engine/graph/callgraph/builder/c_builder.go
+++ b/sast-engine/graph/callgraph/builder/c_builder.go
@@ -304,18 +304,45 @@ func resolveCCallTarget(
 	return "", false, nil
 }
 
-// lookupCStdlib walks the caller file's `#include <...>` list and asks the
-// stdlib registry for a function with the requested name in each header.
-// First match wins; ties are unlikely (stdlib symbols are uniquely owned by
-// one header) but if they do happen, the include order in source decides.
+// lookupCStdlib resolves a free function against the C stdlib registry.
+//
+// The lookup runs in two stages:
+//
+//  1. Direct system includes — walk the caller file's own `#include <…>`
+//     list. This is the fast path and covers the case where the calling
+//     file directly pulls in the header that owns the symbol.
+//  2. Manifest-wide fallback — when the direct walk doesn't yield a hit,
+//     scan every header in the manifest. Real-world C codebases routinely
+//     route stdlib pulls through a project-internal "common.h" header
+//     (redis does this with `server.h`), so without this fallback every
+//     `.c` file that doesn't redundantly `#include <string.h>` itself
+//     would lose strlen / strcmp / memcpy / etc.
+//
+// First hit wins. Stdlib symbols are uniquely owned by one header in the
+// real world, so first-hit-wins is effectively the same as best-match.
+// The fallback is bounded to the manifest's loaded headers — symbols not
+// in the manifest can never resolve, so there's no risk of binding a
+// project-internal symbol to a stdlib FQN.
 func lookupCStdlib(callerFile, funcName string, registry *core.CModuleRegistry) (string, *core.CStdlibFunction) {
-	prefix, ok := registry.FileToPrefix[callerFile]
-	if !ok {
+	loader := registry.StdlibRegistry
+	if loader == nil {
 		return "", nil
 	}
-	for _, header := range registry.SystemIncludes[prefix] {
-		fn, err := registry.StdlibRegistry.GetFunction(header, funcName)
-		if err == nil && fn != nil {
+
+	// Stage 1: direct system includes from the caller file.
+	if prefix, ok := registry.FileToPrefix[callerFile]; ok {
+		for _, header := range registry.SystemIncludes[prefix] {
+			if fn, err := loader.GetFunction(header, funcName); err == nil && fn != nil {
+				return fn.FQN, fn
+			}
+		}
+	}
+
+	// Stage 2: transitive-include fallback. Walk every manifest header.
+	// O(headers) per unresolved call — ~1900 headers on Linux glibc;
+	// bounded and predictable. Mirrors the PR-04 C++ resolver fallback.
+	for _, header := range loader.ListHeaders() {
+		if fn, err := loader.GetFunction(header, funcName); err == nil && fn != nil {
 			return fn.FQN, fn
 		}
 	}
diff --git a/sast-engine/graph/callgraph/builder/c_builder_stdlib_test.go b/sast-engine/graph/callgraph/builder/c_builder_stdlib_test.go
index 59747d0b..3f19007a 100644
--- a/sast-engine/graph/callgraph/builder/c_builder_stdlib_test.go
+++ b/sast-engine/graph/callgraph/builder/c_builder_stdlib_test.go
@@ -143,10 +143,13 @@ func TestBuildCCallGraph_StdlibFallback_NotConsultedWhenProjectDefinitionExists(
 	assert.Empty(t, sites[0].SecurityTag, "project resolution must not pick up stdlib SecurityTag")
 }
 
-// TestBuildCCallGraph_StdlibFallback_NoIncludesLeavesUnresolved verifies
-// that a call to an unknown function with no matching system include
-// stays unresolved — the registry must not return arbitrary symbols.
-func TestBuildCCallGraph_StdlibFallback_NoIncludesLeavesUnresolved(t *testing.T) {
+// TestBuildCCallGraph_StdlibFallback_TransitiveIncludeResolves verifies
+// the PR-05 transitive-include fallback. Real C codebases route stdlib
+// pulls through a project-internal "common.h" header, so files that
+// call printf without a direct `#include <stdio.h>` still need to
+// resolve. The resolver scans every manifest header when the file's
+// direct system-include list doesn't yield a hit.
+func TestBuildCCallGraph_StdlibFallback_TransitiveIncludeResolves(t *testing.T) {
 	root := fixtureRoot
 	mainC := root + "/src/main.c"
 
@@ -154,7 +157,9 @@ func TestBuildCCallGraph_StdlibFallback_NoIncludesLeavesUnresolved(t *testing.T)
 	mainFn := f.addFunction(t, mainC, "src/main.c", "main", "int", false)
 	f.addCall(t, mainFn, "printf", nil)
 
-	// No SystemIncludes entry for this file → stdlib lookup is a no-op.
+	// Notice: no SystemIncludes entry for src/main.c. Pre-PR-05 this
+	// stayed unresolved; with the manifest-wide fallback the resolver
+	// finds printf via the (only) loaded header.
 	f.registry.StdlibRegistry = newFakeCStdlibLoader(map[string]map[string]*core.CStdlibFunction{
 		"stdio.h": {"printf": {FQN: "c::stdio::printf", ReturnType: "int"}},
 	})
@@ -163,5 +168,29 @@ func TestBuildCCallGraph_StdlibFallback_NoIncludesLeavesUnresolved(t *testing.T)
 
 	sites := cg.CallSites["src/main.c::main"]
 	require.Len(t, sites, 1)
-	assert.False(t, sites[0].Resolved, "no matching include => stdlib must not be consulted")
+	assert.True(t, sites[0].Resolved, "transitive fallback must resolve printf")
+	assert.Equal(t, "c::stdio::printf", sites[0].TargetFQN)
+}
+
+// TestBuildCCallGraph_StdlibFallback_UnknownSymbolStaysUnresolved is the
+// safety guard: a call whose name doesn't appear anywhere in the
+// manifest must not bind to an arbitrary entry. Bounds the fallback to
+// "names actually in the manifest".
+func TestBuildCCallGraph_StdlibFallback_UnknownSymbolStaysUnresolved(t *testing.T) {
+	root := fixtureRoot
+	mainC := root + "/src/main.c"
+
+	f := newCFixture(t)
+	mainFn := f.addFunction(t, mainC, "src/main.c", "main", "int", false)
+	f.addCall(t, mainFn, "totally_made_up_function", nil)
+
+	f.registry.StdlibRegistry = newFakeCStdlibLoader(map[string]map[string]*core.CStdlibFunction{
+		"stdio.h": {"printf": {FQN: "c::stdio::printf", ReturnType: "int"}},
+	})
+
+	cg, _ := f.build(t)
+
+	sites := cg.CallSites["src/main.c::main"]
+	require.Len(t, sites, 1)
+	assert.False(t, sites[0].Resolved, "unknown symbol must stay unresolved")
 }