@@ -33,6 +33,7 @@ All paths are relative to `crates/openshell-sandbox/src/`.
 | `l7/relay.rs` | Protocol-aware bidirectional relay with per-request OPA evaluation |
 | `l7/rest.rs` | HTTP/1.1 request/response parsing, body framing (Content-Length, chunked), deny response generation |
 | `l7/provider.rs` | `L7Provider` trait and `L7Request`/`BodyLength` types |
+| `credential_injector.rs` | L7 proxy credential injection for non-inference providers -- extracts injection configs from policy, resolves against provider env, injects credentials at the proxy layer |
 
 ## Startup and Orchestration
 
@@ -81,11 +82,13 @@ flowchart TD
    - Priority 1: `--policy-rules` + `--policy-data` provided -- load OPA engine from local Rego file and YAML data file via `OpaEngine::from_files()`. Query `query_sandbox_config()` for filesystem/landlock/process settings. Network mode forced to `Proxy`.
    - Priority 2: `--sandbox-id` + `--openshell-endpoint` provided -- fetch typed proto policy via `grpc_client::fetch_policy()`. Create OPA engine via `OpaEngine::from_proto()` using baked-in Rego rules. Convert proto to `SandboxPolicy` via `TryFrom`, which always forces `NetworkMode::Proxy` so that all egress passes through the proxy and the `inference.local` virtual host is always addressable.
    - Neither present: return fatal error.
-   - Output: `(SandboxPolicy, Option<Arc<OpaEngine>>)`
+   - Output: `(SandboxPolicy, Option<Arc<OpaEngine>>, proto::SandboxPolicy)`
 
 2. **Provider environment fetching**: If sandbox ID and endpoint are available, call `grpc_client::fetch_provider_environment()` to get a `HashMap<String, String>` of credential environment variables. On failure, log a warning and continue with an empty map.
 
-3. **Binary identity cache**: If OPA engine is active, create `Arc<BinaryIdentityCache::new()>` for SHA256 TOFU enforcement.
+3. **Credential injection extraction**: Scan the proto policy's network endpoints for `credential_injection` configs. For each match, look up the referenced credential in the provider environment, remove it from the env map (so it is not exposed to the sandbox process), and build a `CredentialInjector` that the L7 proxy will use to inject credentials at the network layer. See [Credential Injection](#credential-injection).
+
+4. **Binary identity cache**: If OPA engine is active, create `Arc<BinaryIdentityCache::new()>` for SHA256 TOFU enforcement.
 
 4. **Filesystem preparation** (`prepare_filesystem()`): For each path in `filesystem.read_write`, create the directory if it does not exist and `chown` to the configured `run_as_user`/`run_as_group`. Runs as the supervisor (root) before forking.
 
@@ -1001,6 +1004,56 @@ Implements `L7Provider` for HTTP/1.1:
 5. If allowed (or audit mode): relay request to upstream and response back to client, then loop
 6. If denied in enforce mode: send 403 and close the connection
 
+## Credential Injection
+
+**File:** `crates/openshell-sandbox/src/credential_injector.rs`
+
+Credential injection extends the L7 proxy to inject API credentials at the network layer for arbitrary REST endpoints. This generalizes the `inference.local` credential injection pattern to any service in `network_policies`.
+
+### Problem
+
+When provider credentials are injected as environment variables, the agent process can read raw API keys from `process.env`. A prompt injection attack, malicious skill, or compromised dependency can read and exfiltrate these values. The network policy limits where a leaked key can be sent, but does not prevent the agent from reading it.
+
+### Architecture
+
+When an endpoint has a `credential_injection` configuration in the policy YAML:
+
+1. **Sandbox startup** (`lib.rs`): `CredentialInjector::extract_from_policy()` scans the proto policy for `credential_injection` entries, cross-references them with the provider environment, removes the matched credentials from the child env map, and builds a `CredentialInjector` keyed by `(host, port)`.
+2. **Proxy startup**: The `CredentialInjector` is passed through to `L7EvalContext` alongside the existing `SecretResolver`.
+3. **Request relay** (`l7/rest.rs`): After the OPA policy allows a request, `relay_http_request_with_resolver()` applies credential injection:
+   - For header injection: strips any existing header with the same name (case-insensitive) and appends the injected header with the real credential.
+   - For query parameter injection: appends the credential as a URL query parameter.
+4. **Agent process**: never sees the credential. It is not in `process.env` and not in any placeholder form.
+
+### Injection types
+
+| Type | YAML fields | Example |
+|---|---|---|
+| Header | `header: x-api-key` | `x-api-key: <value>` |
+| Header + prefix | `header: Authorization`, `value_prefix: "Bearer "` | `Authorization: Bearer <value>` |
+| Query parameter | `query_param: key` | URL appended with `?key=<value>` |
+
+### Relationship to SecretResolver
+
+`SecretResolver` and `CredentialInjector` serve different purposes:
+
+| | SecretResolver | CredentialInjector |
+|---|---|---|
+| **Mechanism** | Placeholder rewriting | Direct injection |
+| **Agent visibility** | Agent sees placeholder env vars | Agent sees nothing |
+| **When applied** | All provider credentials (default) | Only credentials with `credential_injection` |
+| **Auth header source** | Agent constructs the header using placeholder | Proxy adds the header from scratch |
+| **Spoofing risk** | Agent could send placeholders to wrong endpoint | Proxy strips any existing header first |
+
+Both are applied in `relay_http_request_with_resolver()`: `SecretResolver` rewrites first, then `CredentialInjector` injects.
+
+### Validation rules
+
+- `credential_injection` requires `protocol: rest` and `tls: terminate`
+- Exactly one of `header` or `query_param` must be set
+- `credential` and `provider` are required
+- `value_prefix` is only valid with `header`
+
 ## Process Identity
 
 ### SHA256 TOFU (Trust-On-First-Use)
 
@@ -25,10 +25,12 @@ url = { workspace = true }
 ## Off by default so production builds have an empty registry.
 ## Enabled by e2e tests and during development.
 dev-settings = []
+## Use bundled protoc from protobuf-src instead of system protoc.
+bundled-protoc = ["protobuf-src"]
 
 [build-dependencies]
 tonic-build = { workspace = true }
-protobuf-src = { workspace = true }
+protobuf-src = { workspace = true, optional = true }
 
 [dev-dependencies]
 tempfile = "3"
 
@@ -17,15 +17,18 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     }
 
     // --- Protobuf compilation ---
-    // Use bundled protoc from protobuf-src.  The system protoc (from apt-get)
-    // does not bundle the well-known type includes (google/protobuf/struct.proto
-    // etc.), so we must use protobuf-src which ships both the binary and the
-    // include tree.
-    // SAFETY: This is run at build time in a single-threaded build script context.
-    // No other threads are reading environment variables concurrently.
-    #[allow(unsafe_code)]
-    unsafe {
-        env::set_var("PROTOC", protobuf_src::protoc());
+    // Prefer PROTOC env var (e.g., from mise or system install) when available.
+    // Fall back to bundled protoc from protobuf-src if the feature is enabled.
+    if env::var("PROTOC").is_err() {
+        #[cfg(feature = "bundled-protoc")]
+        {
+            // SAFETY: This is run at build time in a single-threaded build script context.
+            // No other threads are reading environment variables concurrently.
+            #[allow(unsafe_code)]
+            unsafe {
+                env::set_var("PROTOC", protobuf_src::protoc());
+            }
+        }
     }
 
     let proto_files = [
 
@@ -15,8 +15,8 @@ use std::path::Path;
 
 use miette::{IntoDiagnostic, Result, WrapErr};
 use openshell_core::proto::{
-    FilesystemPolicy, L7Allow, L7Rule, LandlockPolicy, NetworkBinary, NetworkEndpoint,
-    NetworkPolicyRule, ProcessPolicy, SandboxPolicy,
+    CredentialInjection, FilesystemPolicy, L7Allow, L7Rule, LandlockPolicy, NetworkBinary,
+    NetworkEndpoint, NetworkPolicyRule, ProcessPolicy, SandboxPolicy,
 };
 use serde::{Deserialize, Serialize};
 
@@ -99,6 +99,11 @@ struct NetworkEndpointDef {
     rules: Vec<L7RuleDef>,
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     allowed_ips: Vec<String>,
+    /// Optional credential injection. When set, the referenced provider
+    /// credential is withheld from the sandbox environment and injected
+    /// at the L7 proxy layer instead.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    credential_injection: Option<CredentialInjectionDef>,
 }
 
 fn is_zero(v: &u32) -> bool {
@@ -132,6 +137,34 @@ struct NetworkBinaryDef {
     harness: bool,
 }
 
+/// Credential injection configuration for an L7 endpoint.
+///
+/// When attached to an endpoint, the referenced provider credential is not
+/// injected as an environment variable. Instead, the L7 proxy injects it
+/// into outbound requests at the network layer.
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(deny_unknown_fields)]
+struct CredentialInjectionDef {
+    /// HTTP header name (e.g., "x-api-key", "Authorization").
+    /// Mutually exclusive with `query_param`.
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    header: String,
+    /// Optional prefix prepended to the credential value (e.g., "Bearer ").
+    /// Only valid when `header` is set.
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    value_prefix: String,
+    /// URL query parameter name (e.g., "key").
+    /// Mutually exclusive with `header`.
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    query_param: String,
+    /// Provider name that holds the credential.
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    provider: String,
+    /// Credential key within the provider (e.g., "EXA_API_KEY").
+    #[serde(default, skip_serializing_if = "String::is_empty")]
+    credential: String,
+}
+
 // ---------------------------------------------------------------------------
 // YAML → proto conversion
 // ---------------------------------------------------------------------------
@@ -180,6 +213,15 @@ fn to_proto(raw: PolicyFile) -> SandboxPolicy {
                                 })
                                 .collect(),
                             allowed_ips: e.allowed_ips,
+                            credential_injection: e.credential_injection.map(
+                                |ci| CredentialInjection {
+                                    header: ci.header,
+                                    value_prefix: ci.value_prefix,
+                                    query_param: ci.query_param,
+                                    provider: ci.provider,
+                                    credential: ci.credential,
+                                },
+                            ),
                         }
                     })
                     .collect(),
@@ -280,6 +322,15 @@ fn from_proto(policy: &SandboxPolicy) -> PolicyFile {
                                 })
                                 .collect(),
                             allowed_ips: e.allowed_ips.clone(),
+                            credential_injection: e.credential_injection.as_ref().map(
+                                |ci| CredentialInjectionDef {
+                                    header: ci.header.clone(),
+                                    value_prefix: ci.value_prefix.clone(),
+                                    query_param: ci.query_param.clone(),
+                                    provider: ci.provider.clone(),
+                                    credential: ci.credential.clone(),
+                                },
+                            ),
                         }
                     })
                     .collect(),
@@ -1117,4 +1168,157 @@ network_policies:
             proto2.network_policies["test"].endpoints[0].host
         );
     }
+
+    #[test]
+    fn round_trip_preserves_credential_injection_header() {
+        let yaml = r#"
+version: 1
+network_policies:
+  exa_api:
+    name: exa-search-api
+    endpoints:
+      - host: api.exa.ai
+        port: 443
+        protocol: rest
+        tls: terminate
+        enforcement: enforce
+        credential_injection:
+          header: x-api-key
+          provider: exa
+          credential: EXA_API_KEY
+        rules:
+          - allow:
+              method: POST
+              path: /search
+    binaries:
+      - path: /usr/bin/node
+"#;
+        let proto1 = parse_sandbox_policy(yaml).expect("parse failed");
+        let ci1 = proto1.network_policies["exa_api"].endpoints[0]
+            .credential_injection
+            .as_ref()
+            .expect("credential_injection missing");
+        assert_eq!(ci1.header, "x-api-key");
+        assert_eq!(ci1.provider, "exa");
+        assert_eq!(ci1.credential, "EXA_API_KEY");
+        assert!(ci1.value_prefix.is_empty());
+        assert!(ci1.query_param.is_empty());
+
+        let yaml_out = serialize_sandbox_policy(&proto1).expect("serialize failed");
+        let proto2 = parse_sandbox_policy(&yaml_out).expect("re-parse failed");
+        let ci2 = proto2.network_policies["exa_api"].endpoints[0]
+            .credential_injection
+            .as_ref()
+            .expect("credential_injection lost in round-trip");
+        assert_eq!(ci1.header, ci2.header);
+        assert_eq!(ci1.provider, ci2.provider);
+        assert_eq!(ci1.credential, ci2.credential);
+    }
+
+    #[test]
+    fn round_trip_preserves_credential_injection_bearer() {
+        let yaml = r#"
+version: 1
+network_policies:
+  perplexity_api:
+    name: perplexity-api
+    endpoints:
+      - host: api.perplexity.ai
+        port: 443
+        protocol: rest
+        tls: terminate
+        enforcement: enforce
+        credential_injection:
+          header: Authorization
+          value_prefix: "Bearer "
+          provider: perplexity
+          credential: PERPLEXITY_API_KEY
+        rules:
+          - allow:
+              method: POST
+              path: /chat/completions
+    binaries:
+      - path: /usr/bin/node
+"#;
+        let proto1 = parse_sandbox_policy(yaml).expect("parse failed");
+        let ci1 = proto1.network_policies["perplexity_api"].endpoints[0]
+            .credential_injection
+            .as_ref()
+            .expect("credential_injection missing");
+        assert_eq!(ci1.header, "Authorization");
+        assert_eq!(ci1.value_prefix, "Bearer ");
+        assert_eq!(ci1.credential, "PERPLEXITY_API_KEY");
+
+        let yaml_out = serialize_sandbox_policy(&proto1).expect("serialize failed");
+        let proto2 = parse_sandbox_policy(&yaml_out).expect("re-parse failed");
+        let ci2 = proto2.network_policies["perplexity_api"].endpoints[0]
+            .credential_injection
+            .as_ref()
+            .expect("credential_injection lost in round-trip");
+        assert_eq!(ci1.value_prefix, ci2.value_prefix);
+    }
+
+    #[test]
+    fn round_trip_preserves_credential_injection_query_param() {
+        let yaml = r#"
+version: 1
+network_policies:
+  youtube_api:
+    name: youtube-data-api
+    endpoints:
+      - host: www.googleapis.com
+        port: 443
+        protocol: rest
+        tls: terminate
+        credential_injection:
+          query_param: key
+          provider: youtube
+          credential: YOUTUBE_API_KEY
+    binaries:
+      - path: /usr/bin/node
+"#;
+        let proto1 = parse_sandbox_policy(yaml).expect("parse failed");
+        let ci1 = proto1.network_policies["youtube_api"].endpoints[0]
+            .credential_injection
+            .as_ref()
+            .expect("credential_injection missing");
+        assert_eq!(ci1.query_param, "key");
+        assert_eq!(ci1.credential, "YOUTUBE_API_KEY");
+        assert!(ci1.header.is_empty());
+
+        let yaml_out = serialize_sandbox_policy(&proto1).expect("serialize failed");
+        let proto2 = parse_sandbox_policy(&yaml_out).expect("re-parse failed");
+        let ci2 = proto2.network_policies["youtube_api"].endpoints[0]
+            .credential_injection
+            .as_ref()
+            .expect("credential_injection lost in round-trip");
+        assert_eq!(ci1.query_param, ci2.query_param);
+        assert_eq!(ci1.credential, ci2.credential);
+    }
+
+    #[test]
+    fn no_credential_injection_preserves_none() {
+        let yaml = r#"
+version: 1
+network_policies:
+  test:
+    endpoints:
+      - host: example.com
+        port: 443
+    binaries:
+      - path: /usr/bin/curl
+"#;
+        let proto = parse_sandbox_policy(yaml).expect("parse failed");
+        assert!(
+            proto.network_policies["test"].endpoints[0]
+                .credential_injection
+                .is_none()
+        );
+
+        let yaml_out = serialize_sandbox_policy(&proto).expect("serialize failed");
+        assert!(
+            !yaml_out.contains("credential_injection"),
+            "credential_injection should not appear in output when not set"
+        );
+    }
 }