diff --git a/format.c b/format.c
index 528c777c..cb8761e5 100644
--- a/format.c
+++ b/format.c
@@ -118,6 +118,9 @@ format_job_cmp(struct format_job *fj1, struct format_job *fj2)
 #define FORMAT_REPEAT 0x200000
 #define FORMAT_QUOTE_ARGUMENTS 0x400000
 #define FORMAT_RELATIVE 0x800000
+#define FORMAT_CLIENT_TERMCAP 0x1000000
+#define FORMAT_CLIENT_TERMFEAT 0x2000000
+#define FORMAT_CLIENT_ENVIRON 0x4000000
 
 /* Limit on recursion. */
 #define FORMAT_LOOP_LIMIT 100
@@ -4422,7 +4425,7 @@ format_build_modifiers(struct format_expand_state *es, const char **s,
 
 	/*
 	 * Modifiers are a ; separated list of the forms:
-	 *	l,m,C,a,b,c,d,n,t,w,q,E,T,S,W,P,R,<,>
+	 *	l,m,C,a,b,c,d,I,n,t,w,q,E,T,S,W,P,R,<,>
 	 *	=a
 	 *	=/a
 	 *	=/a/
@@ -4463,7 +4466,7 @@ format_build_modifiers(struct format_expand_state *es, const char **s,
 		}
 
 		/* Now try single character with arguments. */
-		if (strchr("mCLNPSst=pReqW", cp[0]) == NULL)
+		if (strchr("ImCLNPSst=pReqW", cp[0]) == NULL)
 			break;
 		c = cp[0];
 
@@ -5104,6 +5107,7 @@ format_replace(struct format_expand_state *es, const char *key, size_t keylen,
 	struct format_modifier		 *bool_op_n = NULL;
 	u_int				  i, count, nsub = 0, nrep;
 	struct format_expand_state	  next;
+	struct environ_entry		 *envent;
 
 	/* Set sorting defaults. */
 	sc->order = SORT_ORDER;
@@ -5186,6 +5190,16 @@ format_replace(struct format_expand_state *es, const char *key, size_t keylen,
 			case 'n':
 				modifiers |= FORMAT_LENGTH;
 				break;
+			case 'I':
+				if (fm->argc < 1)
+					break;
+				if (strchr(fm->argv[0], 'f') != NULL)
+					modifiers |= FORMAT_CLIENT_TERMFEAT;
+				if (strchr(fm->argv[0], 'c') != NULL)
+					modifiers |= FORMAT_CLIENT_TERMCAP;
+				if (strchr(fm->argv[0], 'e') != NULL)
+					modifiers |= FORMAT_CLIENT_ENVIRON;
+				break;
 			case 't':
 				modifiers |= FORMAT_TIMESTRING;
 				if (fm->argc < 1)
@@ -5313,6 +5327,38 @@ format_replace(struct format_expand_state *es, const char *key, size_t keylen,
 		}
 	}
 
+	/* Look up client capability, feature or environment. */
+	if ((modifiers & FORMAT_CLIENT_TERMCAP) ||
+	    (modifiers & FORMAT_CLIENT_TERMFEAT) ||
+	    (modifiers & FORMAT_CLIENT_ENVIRON)) {
+		if (ft->c == NULL ||
+		    ft->c->tty.term == NULL ||
+		    ft->c->flags & CLIENT_UNATTACHEDFLAGS) {
+			value = xstrdup("");
+			goto done;
+		}
+		if (modifiers & FORMAT_CLIENT_TERMCAP) {
+			if (tty_term_has_name(ft->c->tty.term, copy))
+				value = xstrdup("1");
+			else
+				value = xstrdup("0");
+		}
+		if (modifiers & FORMAT_CLIENT_TERMFEAT) {
+			if (tty_feature_present(ft->c->tty.term, copy))
+				value = xstrdup("1");
+			else
+				value = xstrdup("0");
+		}
+		if (modifiers & FORMAT_CLIENT_ENVIRON) {
+			envent = environ_find(ft->c->environ, copy);
+			if (envent != NULL && envent->value != NULL)
+				value = xstrdup(envent->value);
+			else
+				value = xstrdup("");
+		}
+		goto done;
+	}
+
 	/* Is this a literal string? */
 	if (modifiers & FORMAT_LITERAL) {
 		format_log(es, "literal string is '%s'", copy);
diff --git a/tmux.1 b/tmux.1
index 8f69b6fb..e956955e 100644
--- a/tmux.1
+++ b/tmux.1
@@ -6431,6 +6431,24 @@ see
 .Xr strftime 3 .
 .Pp
 The
+.Ql I:\&
+prefix will interrogate the client.
+.Ql I/f
+will be true if
+.Nm
+believes that the client supports the given terminal feature, for example
+.Ql I/f:RGB ,
+.Ql I/c
+will be true if a client has the given
+.Xr terminfo 3
+capability, for example
+.Ql I/c:smcup ,
+and
+.Ql I/e
+gives the value of a client environment variable, for example
+.Ql #{I/e:FOO} .
+.Pp
+The
 .Ql b:\&
 and
 .Ql d:\&
diff --git a/tmux.h b/tmux.h
index 6cc9c391..eb4f0fd8 100644
--- a/tmux.h
+++ b/tmux.h
@@ -2774,6 +2774,7 @@ int		 tty_term_read_list(const char *, int, char ***, u_int *,
 		     char **);
 void		 tty_term_free_list(char **, u_int);
 int		 tty_term_has(struct tty_term *, enum tty_code_code);
+int		 tty_term_has_name(struct tty_term *, const char *);
 const char	*tty_term_string(struct tty_term *, enum tty_code_code);
 const char	*tty_term_string_i(struct tty_term *, enum tty_code_code, int);
 const char	*tty_term_string_ii(struct tty_term *, enum tty_code_code, int,
@@ -2791,6 +2792,7 @@ const char	*tty_term_describe(struct tty_term *, enum tty_code_code);
 /* tty-features.c */
 void		 tty_add_features(int *, const char *, const char *);
 const char	*tty_get_features(int);
+int		 tty_feature_present(struct tty_term *, const char *);
 int		 tty_apply_features(struct tty_term *, int);
 void		 tty_default_features(int *, const char *, u_int);
 
diff --git a/tty-features.c b/tty-features.c
index b4340e2e..01b9bff3 100644
--- a/tty-features.c
+++ b/tty-features.c
@@ -442,6 +442,45 @@ tty_get_features(int feat)
 	return (s);
 }
 
+int
+tty_feature_present(struct tty_term *term, const char *name)
+{
+	const struct tty_feature	*tf = NULL;
+	const char *const		*capability;
+	u_int				 i;
+	char				*copy;
+
+	for (i = 0; i < nitems(tty_features); i++) {
+		tf = tty_features[i];
+		if (strcmp(tf->name, name) == 0) {
+			if (term->features & (1 << i))
+			    return (1);
+			break;
+		}
+	}
+
+	/*
+	 * We don't just have the feature flag set. Check if the capabilities
+	 * supported by the client are actual set instead.
+	 */
+	if (tf == NULL || strcmp(name, "ignorefkeys") == 0)
+		return (0);
+	if (tf->flags != 0 && (term->flags & tf->flags) != tf->flags)
+		return (0);
+	capability = tf->capabilities;
+	while (*capability != NULL) {
+		copy = xstrdup(*capability);
+		copy[strcspn(copy, "=")] = '\0';
+		if (!tty_term_has_name(term, copy)) {
+			free(copy);
+			return (0);
+		}
+		free(copy);
+		capability++;
+	}
+	return (1);
+}
+
 int
 tty_apply_features(struct tty_term *term, int feat)
 {
diff --git a/tty-term.c b/tty-term.c
index 39bfd9d6..c248aa84 100644
--- a/tty-term.c
+++ b/tty-term.c
@@ -776,6 +776,18 @@ tty_term_has(struct tty_term *term, enum tty_code_code code)
 	return (term->codes[code].type != TTYCODE_NONE);
 }
 
+int
+tty_term_has_name(struct tty_term *term, const char *name)
+{
+	u_int	i;
+
+	for (i = 0; i < tty_term_ncodes(); i++) {
+		if (strcmp(tty_term_codes[i].name, name) == 0)
+			return (tty_term_has(term, i));
+	}
+	return (0);
+}
+
 const char *
 tty_term_string(struct tty_term *term, enum tty_code_code code)
 {